investigation of instances of low or negative pressures in...

Defra Ref: WT1243/DWI 70/2/245 WRc Ref: DEFRA8356

October 2011

Investigation of Instances of Low or Negative

Pressures in UK Drinking Water Systems -

Final Report

RESTRICTION: This report has the following limited distribution:

External: DEFRA

© WRc plc 2011 The contents of this document are subject to copyright and all rights are reserved. No part of this document may be reproduced, stored in a retrieval system or transmitted, in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of WRc plc.

This document has been produced by WRc plc.

Any enquiries relating to this report should be referred to the Project Manager at the following address:

WRc plc,

Frankland Road, Blagrove,

Swindon, Wiltshire, SN5 8YF

Telephone: + 44 (0) 1793 865000

Fax: + 44 (0) 1793 865001

Website: www.wrcplc.co.uk

Investigation of Instances of Low or Negative

Pressures in UK Drinking Water Systems -

Final Report

Report No.: DEFRA8356

Date: October 2011

Authors: John Creasey, David Garrow

Project Manager: Joanne Hulance

Project No.: 15340-0

Client: DEFRA

Client Manager: Annabelle May

The research was funded by the Department for Environment, Food & Rural Affairs (Defra) under

project DWI 70/2/245. The views expressed here are those of the authors and not necessarily those of

the Department or DWI.

Contents

Summary .................................................................................................................................. 1

1. Introduction .................................................................................................................. 5

1.1 Background ................................................................................................................. 5

1.2 Objectives .................................................................................................................... 5

2. Methodology for Surge Sites ....................................................................................... 9

2.1 Surge in distribution .................................................................................................... 9

2.2 Approach to logging .................................................................................................. 10

2.3 Choice of sites and locations .................................................................................... 10

2.4 Requirements for logging equipment ........................................................................ 11

2.5 Description of equipment .......................................................................................... 11

3. Methodology for Exceptional Demand Sites ............................................................. 15

3.1 The hydraulic effect of exceptional demands ............................................................ 15

3.2 Approach to logging .................................................................................................. 16

3.3 Choice of sites and locations .................................................................................... 16

3.4 Requirements for logging equipment ........................................................................ 18

3.5 Description of equipment .......................................................................................... 18

4. Surge Site A .............................................................................................................. 19

4.1 Choice of site and locations ...................................................................................... 19

4.2 Logging Results ......................................................................................................... 21

4.3 Example events ......................................................................................................... 23

4.4 Discussion and conclusions ...................................................................................... 29

5. Surge Site B .............................................................................................................. 31


5.2 Logging results .......................................................................................................... 33

5.3 Mechanism of pressure surge in Site B .................................................................... 35

5.4 Example events ......................................................................................................... 36

5.5 Summary results ....................................................................................................... 39

5.6 Discussion and conclusion ........................................................................................ 39

6. Surge Site C .............................................................................................................. 41


6.2 Logging Results ......................................................................................................... 42

6.3 Example events ......................................................................................................... 43

6.4 Summary results ....................................................................................................... 46

6.5 Discussion and conclusions ...................................................................................... 46

7. Exceptional Demand Site D ...................................................................................... 47


7.2 Logging results .......................................................................................................... 47

7.3 Example events ......................................................................................................... 50

7.4 Summary results ....................................................................................................... 54


8. Exceptional Demand Site E ...................................................................................... 57


8.2 Logging results .......................................................................................................... 57

8.3 Example events ......................................................................................................... 59

8.4 Summary results ....................................................................................................... 65


9. Exceptional Demand Site F ....................................................................................... 67


9.2 Logging results .......................................................................................................... 69

9.3 Example events ......................................................................................................... 70

9.4 Summary results ....................................................................................................... 78


10. Discussion ................................................................................................................. 79

10.1 Surge ......................................................................................................................... 79

10.2 Exceptional demand .................................................................................................. 80

10.3 Health risk ................................................................................................................. 82

10.4 Operational practice .................................................................................................. 82

10.5 Practical aspects of pressure monitoring .................................................................. 82

11. Conclusions and Recommendations......................................................................... 85

References ............................................................................................................................. 87

List of Tables

Table 4.1 Summary of low pressures in Site A ....................................................... 22

Table 5.1 Summary of low pressures in Site B ....................................................... 33

Table 6.1 Summary of low pressures in Site C ....................................................... 43

Table 7.1 Summary of low pressures in Site D ....................................................... 49

Table 7.2 Minimum pressures recorded at each location during low pressure event on 6

th January 2011, Site D ............................................ 54

Table 8.1 Summary of low pressures in Site E ....................................................... 59

Table 9.1 Summary of low pressures in Site F ........................................................ 70

List of Figures

Figure 3.1 The effect of high demand ....................................................................... 15

Figure 4.1 Location of loggers in relation to pumping station, Site A ....................... 20

Figure 4.2 Location of the loggers within the network, Site A .................................. 21

Figure 4.3 Burst event on 23rd

June 2010, Site A ..................................................... 23

Figure 4.4 Location of major burst on 23rd

June, Site A ........................................... 25

Figure 4.5 Area adjacent to ferrule blow out ............................................................. 26

Figure 4.6 Pressure at logger 5 following ferrule blow-out ....................................... 27

Figure 4.7 Pressure at the booster station on September 29th 2010 ........................ 28

Figure 5.1 DMA and large customer flows ............................................................... 31

Figure 5.2 Location of the loggers within the network, Site B .................................. 32

Figure 5.3 Flow to large user on August 23rd

2010 .................................................. 35

Figure 5.4 Surge event on August 23rd

..................................................................... 37

Figure 5.5 Surge event on August 18th ..................................................................... 38

Figure 6.1 Location of loggers in relation to pumping station, Site C ....................... 41

Figure 6.2 Location of loggers within the DMA, Site C ............................................. 42

Figure 6.3 Surge event on 8th February 2011 ........................................................... 44

Figure 6.4 Effect of hydrant flushing at location 4 on 29th March 2011 .................... 45

Figure 7.1 Location of loggers at Site D ................................................................... 48

Figure 7.2 Pressure trace on 29th November 2010 in DMA1, Site D ........................ 50

Figure 7.3 Location of local booster pumps for location 1, Site D ............................ 51

Figure 7.4 Pressure trace on 6th January 2011 in DMA 1, Site D ............................ 52

Figure 7.5 Pressure trace on 6th January 2011 in DMA 2, Site D ............................ 52

Figure 7.6 Location of loggers in DMA 1, Site D ...................................................... 53

Figure 8.1 Location of loggers at Site E ................................................................... 58

Figure 8.2 Pressure trace on 19th July 2010 in DMA 3, Site E ................................. 60



Figure 8.5 Pressure trace on 24th August 2010 in DMA 5, Site E ............................ 63

Figure 8.6 Location of loggers in DMA 5, Site E ....................................................... 63

Figure 8.7 Pressure trace on 24th August in DMA 4 in Site E .................................. 64

Figure 8.8 Pressure trace on 19th September 2010 in DMA 3 in Site E ................... 65

Figure 9.1 Location of loggers at Site F .................................................................... 68

Figure 9.2 Location of flushing on 2nd

August, Site F ............................................... 71

Figure 9.3 Flushing on 2nd

August 2010, Site F........................................................ 72

Figure 9.4 PRV event on 17th August 2010, Site F ................................................... 73

Figure 9.5 Burst on 3rd

December 2010, Site F ........................................................ 74

Figure 9.6 Mains repair on 8th December 2010, Site F............................................. 75

Figure 9.7 Event on 19th July 2010, Site F ............................................................... 76

Figure 9.8 Location of affected loggers on 19th July event, Site F ........................... 77

List of Photographs

Photograph 2.1 Installed kiosk for pressure logging – surge equipment .......................... 12

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Summary

i Background

In a number of papers, concern had been raised that low pressures may occur in water

distribution systems such that dirty water could be drawn into the pipe with a consequent risk

to health. It was suggested that this could be the result of exceptional demands or unusual

operational events. A review was carried out for Defra by WRc. The review concluded that the

most valuable next step would be to carry out long-term pressure monitoring in distribution

systems. This project was the result of following up this conclusion.

ii Objectives

1. Identify three distribution systems that are likely to be subject to surge effects.

2. Identify three distribution systems that are likely to be subject to longer term

depressurisation caused by exceptional demand including bursts.

3. Within each distribution system identify appropriate vulnerable points for pressure

monitoring.

4. Install appropriate high speed pressure monitoring equipment and recording

instrumentation at the locations identified.

5. Maintain the pressure monitors throughout the study.

6. Download and review data from the pressure monitors at regular periods, gathering

other relevant data to help interpret the results.

7. Carry out final analysis and investigation, draw conclusions and report the findings.

iii Approach

Detailed pressure monitoring was carried out in 6 separate distribution networks for a

combined period of 56 months. The data was reviewed regularly and low pressure events

were investigated. The investigation included reference to the knowledge of water company

staff. Hydraulics theory, low pressure statistics from the fieldwork and detail from example

events were used to draw conclusions.

iv Conclusions

1. Some low and negative pressures were observed during the study.

2. The probability of very low (surge) pressures as a result of a sudden demand is very

low.

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3. The probability of very low pressures as a result of exceptional high demands is very

low.

4. A system is most at risk from low surge pressures if:

a. Pipe junctions are widely spaced

b. Property density is low (low number of service pipe connections)

c. There are very rapid increases in demand.

5. A system is most at risk from exceptional demands if:

a. There is a fall in ground level from the source followed by a substantial rise

b. There is a significant area at low level

c. There is a significant area at high level

d. Normal pressures in the high level area are low.

6. Pressures low enough to cause ingress are more likely as a result of the following than

they are from a demand-driven event:

a. Isolating mains for repair

b. Mains draining down during valving operations

c. Pump failure or rapid pump switching

d. PRV failure or maintenance.

7. To pose a health risk a source of contaminant around the low pressure point and a

pathway to the pipe flow are also necessary.

8. Thorough planning is required to ensure the successful completion of such an

extensive monitoring exercise.

v Recommendations

1. Additional proactive measures are not required to minimise an already low probability of

very low pressures occurring.

2. The practice of designing distribution systems with alternative routes to most customers

(i.e. with loops) should continue.

3. Good practice should be followed with respect to:

a. Opening and closing valves and hydrants slowly

b. Running hydrants at the lowest necessary flow

c. Returning mains to service (disinfection)

d. Disinfecting local mains which have drained as a consequence of work on other

mains

e. Implementing soft start and stop for pumps

f. Maintaining PRVs and maintaining pressures during maintenance.

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4. When carrying out research of this nature:

a. A thorough survey should be carried out of all potential locations for any monitoring

equipment

b. Sufficient alternative locations should be identified.

vi Résumé of Contents

The report gives underlying theory, describes the choice of sites and logging equipment, gives

a summary of all low pressure events and includes example events from each site.

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1. Introduction

1.1 Background

In a number of papers, concern had been raised that low pressures may occur in water

distribution systems such that dirty water could be drawn into the pipe with a consequent risk

to health. It was suggested that this could be the result of exceptional demands or unusual

operational events.

A review was carried out for Defra by WRc (WRc, 2008). It was chiefly concerned with the

circumstances under which low pressure could occur and the probability of such an

occurrence. Clearly, the risk to health depends on the integrity of the pipe, the nature of the

surrounding material, the quantity of water drawn into the pipe and the number of customers

downstream of the low-pressure point. However, the study investigated the hydraulic aspects

of this issue.

The review concluded that:

1. There are two different possible mechanisms: surge caused (for example) by pump

switching and longer-term fluctuations caused by exceptional demands.

2. The probability of ingress is low but the consequences may be significant.

3. The most valuable next step would be to carry out long-term pressure monitoring in

distribution systems.

This project was the result of following up conclusion (3).

1.2 Objectives

It was intended to monitor pressure in six systems. In three cases, the investigation was to be

concerned with the effects of exceptional demand and, in the others, with the surge

mechanism. Pressure was to be logged at a number of locations in each system for up to a

year. When pressure events occurred, background information would be collected in order to

identify a cause. Examples of such possible causes were thought to be pumps stopping or a

large burst at the time of the low-pressure event. The events were to be investigated to

determine if there had been a causal link, why the low pressure occurred at a particular

location and, what the effect would be of a change in the severity of the initiating event.

Sites were provided by a number of water companies, following discussions with WRc. The

participating water companies provided details on possible sites, allowing WRc to select those

sites most vulnerable to very low pressures and most appropriate for a study of this nature.

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The water companies also provided information on activities that might explain the low

pressure events witnessed within the sites.

All water companies, sites and locations within this report are anonymous. However, WRc are

very grateful to the four water companies who provided assistance.

The objectives of the project were:

1. Identify three distribution systems (“sites”) that are likely to be subject to surge effects

based on the factors previously reported.

2. Identify three distribution systems (“sites”) that are likely to be subject to longer term

depressurisation caused by exceptional demand including bursts.

3. Within each distribution system identify appropriate vulnerable points (“locations”) for

pressure monitoring.

4. Install appropriate high speed pressure monitoring equipment and recording

instrumentation at the locations identified.

5. Maintain the pressure monitors throughout the study including periodic calibration and

zeroing and ensure any spurious reading are promptly identified and steps taken to

correct performance.

6. Download and review data from the pressure monitors at regular periods, gathering

other relevant data to help interpret the results including data from the water company‟s

own monitoring and modelling outputs and the water company‟s own knowledge of

events in the network.

7. Carry out final analysis and investigation and draw conclusions.

8. Report the findings.

The final report was to include the following items:

1. Results of the monitoring surveys, identifying low and negative pressure events.

2. Conclusions on the spatial extent of surge pressures.

3. Identification of possible causes by linking low pressure occurrences to network events.

4. Extrapolation of results to demonstrate the possibility of more extreme pressure

excursions.

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5. An estimation of the frequency of these events throughout England and Wales.

6. Suggested improvements in operational practices.

This report contains the results of the monitoring work undertaken at all six selected sites.

The remainder of this report is split into sections. Sections 2 and 3 detail the monitoring

methodology used for the surge and exceptional demand sites, Sections 4 to 9 present

locational details and the results of monitoring pressures at each individual site, Section 10

provides a discussion of these results and the insights gained, and Section 11 presents the

conclusions and recommendations arising from this work.

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2. Methodology for Surge Sites

2.1 Surge in distribution

Whenever the rate of flow of water in a pipe changes there is a related change in pressure.

This change can be very much larger than the normal operating pressure, resulting in very

high or very low (surge) pressure. If a rapid increase in demand takes place, pressure at that

point will fall. This low pressure will travel through the system at high speed (the

“wavespeed”). A useful rule of thumb which gives the potential pressure change is given by

the Joukowski relationship (Wylie and Streeter, 1978):

v.a.p

where Δp change in pressure (N/m2)

ρ density of water (kg/m3)

a wavespeed (m/s)

Δv change in velocity (m/s)

The wavespeed depends (inter alia) on the pipe material. Wavespeeds in metal pipes are

greater than 1000 m/s, whereas wavespeeds in plastic pipes are usually in the range 400 to

500 m/s. As a consequence, the pressure change in a metal pipe for a given velocity change

is greater than that in a plastic pipe. Surge pressure changes can be very large. If the velocity

in a metal pipe changes by 1 m/s, the pressure may change by more than 100 m.

An example is provided by valve closure in a long simple trunk main. As the valve closes, the

pressure rises on the upstream side and falls on the downstream. These changes travel away

from the valve, travelling at the wavespeed.

There are a number of reasons why the change in pressure may be less than the value

predicted by the Joukowski equation. In the context of distribution systems, the important

issue is reflection from pipe junctions. When the surge event reaches a junction, only some of

the change is transmitted along the other pipes from the junction. A reflection of a lesser

magnitude and opposite sign returns along the incident pipe.

This has two effects. The impact of the pressure change decreases with distance from the

initiating event and the reflection modifies the pressure change at the source making it smaller

than the potential would suggest. The first consequence in a distribution system is that, at no

point, is the change as large as might be expected. The second is that the impact reduces

with each junction as the event moves away from the starting point. This latter effect limits the

spatial extent of any low pressure risk.

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Timescale is an important issue. If the nearest junction to the initiating point is distant by

100 m of iron pipe, the first reflection will return in approximately 0.2 seconds (= 2*100/1000).

As a consequence, any pressure change which takes longer than 0.2 seconds will be

modified (i.e. reduced). In this context, a rapid event (surge) is one which takes less than

0.2 seconds.

The pipe pressure at which ingress would be possible depends on the depth of the pipe. It is,

therefore, unlikely to be greater than 0.1 bars since the pipe would have to be more than 1 m

below water for ingress to occur at that pressure.

2.2 Approach to logging

The main concern is with intermittent and (possibly) unusual events. It was not known, in

advance of the logging, when or where (or even if) such an event would occur. This had a

marked effect on the approach taken. In order to increase the likelihood of securing data

during a surge event, it was planned to leave the loggers in situ for up to one year.

For this type of site, the location of the possible event is likely to be known (e.g. power failure

at a specific pumping station). This removes one of the unknowns discussed for exceptional

demands (see Section 3.2). As a result, far fewer loggers were to be deployed (five). One

would be at the event site (e.g. downstream of the non-return valve at the pumping station).

The others would be deployed at increasing distances into the distribution system. (More

accurately, with increasing numbers of junctions between event location and logger location.)

The rapidity of surge events (see Section 2.1) demands that pressures are logged frequently.

It is easy to miss maximum and minimum values of pressure if the logging interval is too

large. In this case, it was necessary to log at 0.1-second intervals. This is the time taken for a

surge pressure to travel 50 to 60 m in an iron pipe and return to the initiating point. It is

therefore short enough to capture the details produced by reflections in a distribution system.

Plastic pipes do not require such a short logging interval.

2.3 Choice of sites and locations

2.3.1 How the choice was made

It is known that maximum surge pressure change can develop in a long un-branched main. It

was argued that the best chance of recording an event with very low pressures would be in a

distribution system fed by a long pumping main. On pump failure, pressure would drop

significantly in the main because sufficient time would be available for the surge to develop.

By deploying loggers within the system, the progress of the low pressure could be tracked.

For these sites, the main parameters of interest are number and spacing of junctions and pipe

material. Ground level is only a secondary consideration, as opposed to being of primary

consideration for exceptional-demand driven low pressures. The plan was, therefore, to site

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one logger at the pumping station, one at the feed to the distribution system and three more at

increasing distances from the pumping main. These three locations would be hydrants chosen

with increasing number of junctions between each and the pumping main. In each case, it

was possible to select alternative logger locations when practical difficulties were found with

the initial choice.

Sites A-C were selected as examples of surge scenarios and are described in Sections 4 to 6

below.

2.3.2 The lessons learnt

In addition to the preferred locations, a number of alternative hydrants were selected for each

site. This was done in order to accommodate the practical issues that may arise which would

prevent a location from being used. These issues can include a leaking or damaged hydrant,

lack of appropriate position for the kiosk housing the logger, or inability to obtain permission

from the land owner/highways authority to install the logger and kiosk.

2.4 Requirements for logging equipment

The logging equipment was required to generate readings at a sufficient frequency to detect

pressure changes of the duration anticipated. The lowest pressure generated in a kilometre of

iron main would be expected within the first two seconds after the initiating event (a pump

stopping). In this case, it would be necessary to log at intervals of 0.1 seconds to capture the

low pressure value. Standard loggers, of a type similar to those used in the exceptional

demand sites, were unable to provide the granularity of logging required.

The selected specialised loggers could log at such intervals and were programmed to monitor

pressures continuously but were only triggered to record data when the pressure fell below a

set low value (dependent on the location). By using a rolling buffer, the logger captured data

from 100 seconds prior to the trigger threshold being surpassed, until the end of the low

pressure event. When the pressure remained below the trigger level for a short period only,

the logger records data until an end point of 400 seconds after the pressure dropped below

the level. It was necessary for the loggers to be capable of being downloaded weekly via a

remote connection. This was needed to allow any events to be investigated with the water

company while they were still recent.

2.5 Description of equipment

The loggers used for surge monitoring were Campbell CR800 loggers complete with modem,

battery and photocell. The pressure sensors were 0-10 bar gauge, 0.25% accuracy and gave

un-calibrated negative readings at negative pressures. The loggers were connected to

pressure sensors which were attached to the hydrants using a hydrant cap via a quick action

coupling. The sensors also had a sealed electrical connection, to protect them in the event of

the hydrant pit flooding. Throughout the monitoring operation the hydrants remained slightly

opened.

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The size of the Campbell CR800 loggers prevented them from being installed in the hydrant

chambers, as was the case with the exceptional demand sites. Each logger was, therefore,

mounted inside a roadside kiosk, with a slot drilled into the road and pavement from the kiosk

to the hydrant to protect the sensor cable inside a 12 mm airline. This kiosk also provided

sufficient storage space for the batteries. A solar panel was mounted upon a pole above the

kiosk to keep the batteries charged.

Where the kiosk was to be installed on a natural surface, such as grass, the kiosk was either

attached to paving slabs first or dug directly into the ground, depending upon the specifics of

the location. Each kiosk measures 1750 mm (height) x 900 mm (width) x 545 mm (depth).

Permission to install these kiosks was obtained from the land owner and highways authorities

for each location and effort was taken to site the kiosks in the least obtrusive way at each

location.

Photograph 2.1 Installed kiosk for pressure logging – surge equipment

An example of an installed kiosk is shown in Photograph 2.1. The slot for the cable can be

seen in the bottom left of the picture.

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Data were downloaded weekly from each site. Downloads were performed remotely, using a

mobile telecommunications link. This link also allowed the loggers‟ performance and status to

be monitored, trigger points to be altered and new programs to be downloaded without the

added expense of sending out personnel. Initially, the communications facility was available

constantly, but could be limited to a period of only (for example) eight hours once a week to

limit the drain on the battery should the solar panel fail to provide enough power to keep the

battery running. This was necessary for one logger, where the solar panel became unable to

provide sufficient power to the batteries for constant communication.

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3. Methodology for Exceptional Demand Sites

3.1 The hydraulic effect of exceptional demands

The mechanism being investigated in these cases is illustrated in Figure 3.1 and is as follows:

1. An unusual high demand occurs at point

2. The resulting high flowrate increases the friction loss between the source (S) and P

3. This reduces the available pressure at P and at other points in the system.

Pressure at points downstream of P (D, E and F) will fall by the same amount as the reduction

at P. Pressure at points upstream of P and on route to P (B) will fall by a smaller amount (the

proportion of extra friction loss which occurs between S and B). Similarly, points fed from

branches on route to point P will suffer only a smaller drop (for example, pressure at C will fall

by the same amount as the pressure at B). There will be no effect on pressure at point A.

Figure 3.1 The effect of high demand

These low pressures will be maintained for as long as the exceptional demand persists.

Although the pressure at point P will drop significantly, it will necessarily remain positive to

drive the large demand at that point. Points vulnerable to low pressure will, therefore, need to

be at markedly higher ground level than point P. A point will also be vulnerable if the pressure

S

C

A

B

P

D

E

F

E

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is normally low and if there is inadequate pipework between S and P. This aspect is

developed further in Section 3.3 on the choice of sites.

The pipe pressure at which ingress would be possible depends on the depth of the pipe. It is,

therefore, unlikely to be greater than 0.1 bars since the pipe would have to be more than 1 m

below water for ingress to occur at that pressure.

3.2 Approach to logging

As with the surge sites, these investigations are concerned with unusual, unpredictable

events. It was not known, in advance of the logging, when or where (or even if) such an event

would occur. This had a marked effect on the approach taken. In order to increase the

likelihood of securing data during an exceptional demand event, it was planned to leave the

loggers in situ for up to one year. This estimate was based on results from a previous

investigation (WRc, 2008).

The loggers need to be sited at vulnerable points in order to record the most useful data. The

position of these points would depend on the position of the demand event (as in Figure 3.1).

Since the position of the event (e.g. burst or fire flow) is not predictable, it is necessary to log

over a wide area. It was planned to install up to 20 loggers on each site, provided that a

sufficient number of appropriate locations could be identified.

In the case of these “exceptional demand” sites, it is envisaged that the high flow will continue

for at least a few minutes and that the defining hydraulics will be “quasi-steady-state” without

any transient pressure effects. This leads to the argument put forward in Section 3.1 and

should be compared with the discussion of “surge” in Section 2.1. As a result it was decided

that an instantaneous value logged every 10 seconds would provide sufficient detail.

Inevitably it is unlikely that the detail of any transient events that occurred in these systems

would be detected.

3.3 Choice of sites and locations

3.3.1 How the choice was made

Three sites needed to be found for this aspect of the work. To maximise the probability of

logging at locations of interest, and to obtain maximum benefit from the data, sites were

chosen bearing in mind the arguments of Section 2.1.

A significant variation in ground level is needed. Potential low-pressure points will be much

higher than the location of the exceptional demand. Normal operating pressures will be at

least 15 m (about 1.5 bars head). To give a possibility of sub-atmospheric pressures at a high

point, and still maintain a driving head at the demand point, a difference in ground level of at

least 20 m is necessary. Sites with at least this variation in ground level were sought.

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Section 3.1 shows that it is possible for a location, vulnerable to low pressure, to lie between

the demand point and the source of supply. However, a further requirement is that significant

head loss will be developed between the source and this location. It is the nature of

distribution networks that pipework close to the source is of larger diameter than that further

into the system. The effect of a given demand on friction in these pipes will usually be small.

Therefore, it is most likely that the vulnerable points will be downstream of the demand point.

Sites were, therefore, sought where a low-lying area lies between the supply source

(e.g. reservoir) and an area which is 20 m or more above this low point. The bigger the low-

lying area, the greater the chance of an exceptional demand in a position to cause low

pressures elsewhere. The bigger the area of high ground, the more positions are suitable for

logging and the greater the chance of insight into this mechanism.

Mains maps with contours and hydrant positions were used to find hydrants at which to site

the loggers. Contours sufficiently high compared to the system low point were highlighted.

Hydrant positions to cover the high ground were chosen. Hydrants in close proximity to each

other were avoided on the grounds that no extra information would be gained by logging at

both. This did, however, leave some alternative locations should any of the first choices be

unusable for practical reasons. A few locations at hydrants unlikely to be significantly affected

by pressure events were chosen for comparison purposes.

Sites D-F were selected as examples of exceptional demand scenarios and are described in

Sections 7 to 9 below.

3.3.2 The lessons learnt

It proved difficult to find distribution systems with the necessary topography to give a risk of

sub-atmospheric pressures. (Sites were sought in four companies. They cover large areas of

Southern England and the Midlands including significant areas of hilly terrain.) This, in itself,

gives an indication that the risk at a national level may be quite low.

Additionally, some sites which fulfilled the topographical requirements had only short pipe

lengths within the low-lying area and/or within the high area. The former issue reduces the

risk of a demand in a critical area; the latter reduces the area where ingress may occur. The

probability that other environmental conditions are such that ingress occurs and that there are

health implications is reduced if the area is small. However, ingress in this small area may still

put at risk downstream customers. The consequences could potentially be large if there are a

large number of these downstream customers.

To obtain maximum value from the logging exercise, systems with a substantial area of high

ground were chosen. This also allows for a number of alternative logging locations to be

identified in order to use when practical issues regarding the installation and maintenance of

loggers prevent the preferred location being used. Issues that can prevent loggers being

installed upon hydrants include a leaking or damaged hydrant, lack of access to the hydrant

DEFRA


© WRc plc 2011 18

chamber (e.g. cars parked over the cover or the hydrant being located in a busy road), too

shallow a hydrant chamber to allow the logger to be installed, or a buried hydrant. A number

of these issues were encountered during installation, requiring the alternative locations to be

utilised on occasion.

3.4 Requirements for logging equipment

Exceptional demand pressure events are relatively slow events, thus the logging equipment

for the exceptional demand sites did not need to log at the very short interval required for

surge monitoring (Section 2.4). The loggers were required to log at ten-second intervals and

to store sufficient data to allow for monthly downloads without data loss.

3.5 Description of equipment

The loggers used were Primayer Primelog 10 bar loggers, with additional memory provided to

ensure sufficient storage.

The locations of all loggers were agreed with the water company and details of final locations

provided for their operational records. The pressure loggers were attached to selected

hydrants throughout the distribution system. The loggers were connected to pressure sensors

attached to a hydrant cap via a quick action coupling, and the logger placed within the hydrant

pit.

Data were collected from the loggers on a monthly basis from each site. Data from each

logger was manually downloaded onto a laptop using bespoke software provided by

Primayer. All installation and data downloading was undertaken by trained WRc personal with

the appropriate “National Hygiene Scheme” accreditation.

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© WRc plc 2011 19

4. Surge Site A

4.1 Choice of site and locations

Site A was identified through discussions with water company representatives. This site has

experienced low pressure problems in the past. However, the implementation of a new soft

start/stop system at the pumping station prior to the logging period was expected to reduce

these problems. Despite this, it was decided that this site was of interest for the purposes of

this project, as the potential existed for low pressures, should there be a fault at the pumping

station.

The system at Site A is fed from a number of boreholes via a pumping station. This pumping

station pumps water along two mains (16” PVC and 12” cast iron (CI)) to a reservoir at the

other end of the system. One of the pressure loggers was installed at this pumping station, to

observe any low pressures that may originate here (location 1, Figure 4.1).

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© WRc plc 2011 20

Figure 4.1 Location of loggers in relation to pumping station, Site A

123123123123123123123123123

999999999999999999

129129129129129129129129129

878787878787878787

848484848484848484

138138138138138138138138138117117117117117117117117117

939393939393939393

909090909090909090

102102102102102102102102102

13

213

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213

2

105105105105105105105105105

96969696

9696

969696

120120120120120120120120120135135135135135135135135135

11

411

411

411

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411

111

111

111

111

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111

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612

612

612

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108108108108108108108108108

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141

818181

8181

818181

81

787878787878787878

144144144144144144144144144

52

3

Trunk Mains

Water mains

3m contours

Hydrants

Meters

Logger locations

Pumps

1

16"

12"

4

DEFRA


© WRc plc 2011 21

The other four loggers were located in a village supplied from the 12” pumping main

(Figure 4.2). A booster station draws water from the pumping main and supplies it to the

village. One of the pressure loggers was installed on the outflow from the booster pumps, to

observe the impact of any low pressure events at the boosters (location 2). The remaining

three pressure loggers were installed at selected locations throughout the village, chosen to

reflect an increasing level of complexity in the system, with a greater number of junctions

reducing the magnitude of any pressure waves as they travel from location 3 to 4 and then to

5.

Figure 4.2 Location of the loggers within the network, Site A

4.2 Logging Results

Pressure loggers and kiosks were installed at the pumping station and booster pumps

(locations 1 and 2) on 17th February 2010, with the remaining three loggers installed later, on

9th June 2010. The loggers and kiosks were removed on 9

th March 2011.

Table 4.1 gives a summary of the low pressures recorded at these locations. It should be

noted that a number of individual low pressure events may have occurred but could have

been overwritten by subsequent low pressure events, before the weekly download took place.

This would only happen if long-term (non-surge) low pressure events lasted long enough to

exceed the memory size. This would only occur in the event of either many pressure drops or

117

117

117

117

117

117

117

117

117

135

135

135

135

135

135

135

135

135

132

132

132

132

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132

132

132

132

126

126

126

126

126

126

126

126

126

141141141141141141141141141

138

138

138

138

138

138

138

138

138

111111111111111111111111111108

108

108

108

108

108

108

108

108

120120120120120120120120120

123123123123123123123123123

129

129

129

129

129

129

129

129

129

114114114114114114114114114

102102102102102102102102102

999999999999999999

969696969696969696

93

93

93

9393

93

93

93

93

52

3

4

Trunk mains

Water mains

3m contours

Hydrants

Meters

Logger locations

Pumps

DEFRA


© WRc plc 2011 22

a prolonged drop occurring. This occurred at all five loggers during an event on 26th July,

which is discussed in more detail below.

Table 4.1 Summary of low pressures in Site A

Location Trigger

(bar)

No of

events

< 1 bar

No of

events

< 0.1 bar

Min Observed Pressure

Pressure

(bars) Date

Duration

below 0.1 bar

1 4.40 1 1 -0.10 26th Jul n/a

2 1.3 84 1 0.06 23rd

June 56 sec

3 1.6 3 1 -0.03 26th Jul n/a

4 1.7 4 1 -0.01 26th Jul n/a

5 2.75 2 1† -0.47 21

st Sept 17.5 min

† Pressure fell below zero three times during this one event.

At four of the locations, there were very few occasions where the pressure fell below one bar.

At location 2, there appears to be a minor pump control problem which frequently causes the

pressure immediately downstream to fall briefly below one bar. The pressure as a result of

this control issue was never observed to fall to negative pressures and, therefore, is of no

consequence from an ingress point of view.

There were only four sub-zero pressures recorded in the nine-month logging period. A major

burst occurred on the 16” main on 26th July. However, on the same day pressures fell below

zero at locations 3 and 4. This was due to the fact that the supply to the village was cut off to

protect the reservoir, which is a critical supply to a nearby military establishment. The

pressure observed at location 2 was very close to zero but did not fall below it, as a result of

the village being cut-off. The pressure stayed low for a prolonged period of time and the data

recording the initial impact of the burst was overwritten at all five locations. This event has not

been included as an example (Section 4.3) due to the fact that the period of logged data

containing the initiating event (burst of 16” main) was overwritten due to the volume of data

recorded during the period when the reservoir supply was isolated. Note that, although the

logger was triggered at location 5 when the village was isolated, pressures below 1.5 bars

were not recorded.

The very low pressure observed at location 5 occurred on 21st September when the actions

taken by the water company to repair a burst main resulted in the temporary isolation of the

small part of the network where the logger was located.

There was also another major burst on the 16” main on 23rd

June, which produced very low

pressures at location 2, but not at the other locations.

DEFRA


© WRc plc 2011 23

There were other minor pressure excursions which were noted during the logging. These

were of no consequence with respect to low pressure. However they did demonstrate the way

in which rapid pressure transients decay across the network.

4.3 Example events

4.3.1 Mains burst on June 23rd 2010

As discussed above, there were two occasions when there was a severe event (a major

burst) on the 16” pumping main which caused a large pressure fluctuation at the pumping

station (location 1). The location of the burst on 23rd

June 2010 is marked “B” in Figure 4.4.

This fluctuation at the pumping station resulted in lesser fluctuations being observed at the

logger locations within the distribution system (locations 2, 3 4 and 5). The pressure variation

at each location is shown in Figure 4.3.

Figure 4.3 Burst event on 23rd

June 2010, Site A

Figure 4.3 shows a pressure range of 8 bars at the pumping station but the range is only 3

bars at the other locations. The pumping mains are long enough to allow the high and low

pressures to develop as a result of the surge event. However, interactions within the

distribution system, where pipe junctions are much closer together, modify the transient. This

mechanism is explained in Section 2.1.

A further example of this aspect is shown by the positive pressure spike initiated at the

booster station (location 2) after about 280 seconds, as shown in Figure 4.3. This is a very

rapid change which is alleviated as the event crosses the system until it is much reduced at

location 5, which is the logger most distant from the booster station.

0

2

4

6

8

10

12

0 100 200 300 400 500 600

Pre

ssu

re (

bar

s)

Time (seconds)

1 2 3 4 5

DEFRA


© WRc plc 2011 24

Although the large pressure swing at location 1 is not reproduced in the distribution network, it

should be noted that the pressure falls close to zero at location 2 (booster station) with some

small possibility of ingress occurring. Pressure remains below 0.1 bars for 56 seconds.

DEFRA


© WRc plc 2011 25

Figure 4.4 Location of major burst on 23rd

June, Site A

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9696969696

999999999999999999

105105105105105105105105105

909090909090909090

114114114114114114114114114

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117

117

14

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714

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713

513

513

513

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513

513

513

5

111111111111111111111111111

848484848484848484

138138138138138138138138138

120120120120120120120120120

10

810

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810

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810

810

810

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810

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210

2

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2

126126126126126126126126126

144

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144

144

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144

141141141141141141141141141

878787878787878787

129129129129129129129129129

939393939393939393

123123123123123123123123123

153153153153153153153153153

81

81

81

8181

81

81

81

81

787878787878787878

52

34

Trunk mains

Water mains

3m contours

Hydrants

Meters

Logger locations

Pumps

1

2B

16"

DEFRA


© WRc plc 2011 26

4.3.2 Mains burst on September 21st 2010

In the early afternoon of 21st September 2010, there was a burst (caused by a ferrule blow-

out) on the 3” main at the position marked “B” on Figure 4.5 resulting in “water shooting into

the air”, as observed by local residents. This is the type of sudden, high-flow-rate burst event

that might be expected to produce low pressures. However, in reality the pressure did not fall

low enough to trigger the loggers. It is clear from the values of normal pressure and of the set

trigger pressures that any resulting pressure drop was less than 1 bar.

Figure 4.5 Area adjacent to ferrule blow out

The explanation lies with the pipe junctions between the burst site and the upstream loggers

at locations 3 and 4 (see Section 2.1). Each of these reflects the pressure change back

towards the burst site as a rise in pressure and reduces the transmitted drop away from the

burst site. As can be seen from Figure 4.2, there are a large number of junctions on the path

through the network.

It is possible that low pressures were generated immediately upstream of the burst but there

were no loggers located in that area. However, this is unlikely since there is a „T‟ junction very

close to where the burst occurred and others within 100 m.

Water company staff proceeded to close valves on each side of the burst to isolate this

section for repair. There are cross-connected twin mains (3” and 6”) in this area and so a

number of valves needed to be closed. Logger 5 position is marked on Figure 4.5 as “5”. The

effect of valve closures at this point is shown in Figure 4.6.

5

102102102102102102102102102

108108108108108108108108108

117117117117117117117117117

12

612

612

612

612

612

612

612

612

6

120120120120120120120120120

114114114114114114114114114

Water mains

3m contours

Valve

Hydrant

Logger location

B6"

3"

3"

6"

3"

3"

6"

DEFRA


© WRc plc 2011 27

Figure 4.6 Pressure at logger 5 following ferrule blow-out

When a valve on the 6” main upstream of the burst was closed, the area where logger 5 was

situated was supplied via the 3” main and a cross connect, with the 3” main also supplying

flow to the burst. This will induce a large headloss in the mains. This valve closure produced

the major fall in pressure seen at approximately 100 seconds, marked as „A‟ on Figure 4.6.

The pressure fell below zero and then recovered to a small positive value. Oscillations caused

by further valve closures caused the pressure at this point to dip below zero again after

300 seconds, marked „B‟. Logger 5 was located on a dead end at a ground level higher than

the valves that were closed. When the final valve was closed (550 seconds, marked „C‟),

pressure fell below zero for a third time and remained there until the main was recharged

following repair.

Although negative pressures occur at this point, it is important to remember that this was not

caused by the catastrophic burst but by the valve closures necessary to deal with the burst.

The point where the negative pressures were seen is some metres higher than the burst site

and therefore translates the zero pressure at the burst to a negative pressure. The burst itself

did not trigger any of the loggers and very low pressures were not measured at any logger

other than number 5.

4.3.3 Pressure events at the booster station

On September 29th

2010, a rapid drop in pressure was observed at the booster station

(location 2). The recorded pressure data from location 2 is shown in Figure 4.7. This was a

frequent event in the months that followed.

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© WRc plc 2011 28

Figure 4.7 Pressure at the booster station on September 29th

2010

This was probably caused by the pump control and not by a sudden demand change. It is

certainly not a cause for concern from an ingress point of view. What it does do is provide

more insight into the behaviour of surge pressures in a distribution system. If the total drop in

pressure seen at around 100 seconds in Figure 4.7 had been transferred to the other logging

locations, it is certain that recording would have been triggered. This has not happened at any

of the loggers on any of the days that the transient was seen at the booster station.

Reflections and interactions at pipe junctions have reduced the pressure change

considerably. The most rapid fall in Figure 4.7 is 1.1 bars in 0.1 seconds: a genuine pressure

transient.

4.3.4 Summary results

1. In the nine-month logging period, pressure below zero was recorded on only four

occasions. There was one other instance when pressure was observed to fall below

0.1 bars.

2. On only one of these occasions (26th July) was the pressure the direct result of high

flow (see Section 4.2). This was recorded at the source pumps (pumping station) rather

than in distribution. It is not known whether pressures at the other locations fell

immediately to very low values because these records (if any) would have been

overwritten by the subsequent valving event. The other low pressure events resulted

from valving operations.

3. The catastrophic burst on 23rd

June on the trunk mains led to large pressure transients

in those mains. These pressure drops were considerably modified in the distribution

0

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150

Ba

r

Secs

DEFRA


© WRc plc 2011 29

system so that only at one location (logger 2) did the pressure fall close to a value

which might cause ingress.

4. A similar transient occurred in the trunk mains following the burst on 26th July.

However, data from the four loggers in distribution were not available to confirm the

immediate effect on the network.

5. A catastrophic burst (21 September) within the distribution system did not trigger

recording at any location (see Section 4.3.2).

6. Negative pressures were recorded on 21st September at one location because a

section of the network was isolated for repair.

7. Other evidence of pressure transients being alleviated within the system was found.

4.4 Discussion and conclusions

During 9 months logging at 5 locations, there were just 5 occasions when pressures fell to a

point where the possibility of ingress was increased. On one occasion this was caused as a

direct result of high flow. The other low pressures were caused in isolated mains by closing

valves. Some data were overwritten during one long timescale event

One incident, due to a sudden flow change causing low pressure, occurred just downstream

of the source pumps. There was a possibility of ingress occurring at this point. It is significant

that this point is on a long simple pumping main which allowed time for the pressure surge to

develop.

During this incident, pressure drops within the distribution system were very much less due to

the ameliorating effect of reflections from pipe junctions within this dense network. Other

evidence was found of rapid pressure transients being beneficially modified within the

network.

A catastrophic burst within the system did not trigger the pressure loggers. There are a large

number of pipe junctions between the site of the burst and the loggers. These would severely

reduce the potential pressure drop caused by the burst.

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© WRc plc 2011 30

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© WRc plc 2011 31

5. Surge Site B


Site B was not a typical surge site as the focus was not upon potential failure or issues arising

from a pumping station. Instead, discussions with the water company identified this site as

one in which a large commercial user draws large amounts of water several times a day by

rapidly opening a valve. This was considered to have the potential to cause pressure

problems elsewhere in the distribution system. Since the event of interest was, unlike other

surge sites, frequent and regular, it was agreed that an extended period of pressure logging

was not necessary. Instead, pressure loggers were installed for a period of 14 days to

observe any pressure impacts of the large flow to the commercial user. As with Site A, the

locations of the pressure loggers were selected to represent an increasing level of complexity

across the distribution system. As there was no pumping station related to this site, there was

no requirement to locate a logger at such a location. Therefore, four loggers were considered

sufficient to observe pressures at this site.

The site chosen for the pressure logging has one large commercial user. Several times on

each working day, the commercial demand is large compared with the rest of the site. Figure

5.1 shows the flow for the whole site (DMA, in green) superimposed on the flow excluding the

large user (red).

Figure 5.1 DMA and large customer flows

-2

0

2

4

6

8

10

12

14

16

18

20

17/08/2010 00:00 21/08/2010 00:00 25/08/2010 00:00 29/08/2010 00:00 02/09/2010 00:00

Flo

w (

l/s)

Date_Time

DMA DMA minus major user

DEFRA


© WRc plc 2011 32

This shows a diurnal pattern to the rest of the site which is similar on each day and a number

of very large short-term additional flows. This system was chosen for investigation because of

these “exceptional demands”.

The commercial flow can be as high as 13 l/s expressed as a 15-minute average and as low

as zero. This compares with an average flow to the rest of the site of 3.4 l/s. Therefore, the

commercial flow often imposes a load on the system greater than the combined demand of

other customers within the site.

Pressure was logged at four sites for a period of 14 days. The location of the loggers is shown

in Figure 5.2.

Figure 5.2 Location of the loggers within the network, Site B

Location 1 was chosen because it was the nearest hydrant to the offtake of the major user

(marked as “U” in Figure 5.2). The others were chosen such that the pipework between

offtake and loggers was of different complexity. Location 2 was separated from the offtake by

just one „T‟ junction and 100 m of pipe. Location 3 was at the distant end of a residential area.

12

5

13

0

130

1

2

80

120

115

110

105

100

95

95

100

105

110

120

12

5

125130

110

95

100

105

110

115 120

130

100

105

115

120

130

130

90

4

3

5m contours

Water mains

Hydrants

Logger locations

U

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© WRc plc 2011 33

5.2 Logging results

Pressure loggers and kiosks were installed at the four locations on 17th August 2010. The

data were downloaded daily until the loggers were removed on 1st September 2010.

At each location the logger was set to record when the pressure fell below a “trigger

pressure”. These trigger pressures were set at an initial level and were altered over the first

few days to record an acceptable level of data and prevent real low pressure events being

overwritten. The trigger pressures and number of events logged at each site are given in

Table 5.1.

Table 5.1 Summary of low pressures in Site B

Location

(see

Figure

5.2)

Typical

Normal

Pressure

(bar)

Date Number

of

events

Trigger

level

(bar)

Notes

1 3.3

17/08/2010 0 2.1 Check

telecommunications link

18/08/2010 3 2.1

19/08/2010 0 2.1

20/08/2010 3 2.1

23/08/2010 13 2.1

24/08/2010 6 2.1

25/08/2010 2 2.1

26/08/2010 1 2.1 Trigger dropped to 1.9

bar – no events

subsequently triggered

2 3.2

17/08/2010 0 1.8 Check


18/08/2010 0 1.8 Trigger raised to 2.0 bar

19/08/2010 5 2.0

20/08/2010 11 2.0

23/08/2010 32 2.0

24/08/2010 7 2.0

25/08/2010 6 2.0


bar

DEFRA


© WRc plc 2011 34

Location

(see

Figure

5.2)

Typical

Normal

Pressure

(bar)

Date Number

of

events

Trigger

level

(bar)

Notes

3 3.5

17/08/2010 0 2.4 Check


18/08/2010 22 2.4 Trig dropped to 2.25 bar

19/08/2010 1 2.25

20/08/2010 3 2.25

23/08/2010 17 2.25

24/08/2010 2 2.25

25/08/2010 2 2.25

26/08/2010 2 2.25 Trig dropped to 2.0 bar –

no events subsequently

triggered

4 3.2

17/08/2010 0 1.9 Check


18/08/2010 0 1.9 Trigger raised to 2.1 bar


bar

20/08/2010 8 2.0

23/08/2010 31 2.0

24/08/2010 9 2.0

25/08/2010 6 2.0


bar – no events

subsequently triggered

As can be seen from Table 5.1, the loggers at all four locations were triggered on numerous

occasions over the 14-day logging period. This provided details of the pressure changes in

the distribution system as a result of the exceptional demand of the large commercial user.

However, at no point was a sub-zero pressure recorded at any of the locations. The minimum

observed pressure was 1.691 bars (recorded at location 2).

The pipe pressure at which ingress would be possible depends on the depth of the pipe. It is

therefore unlikely to be greater than 0.1 bars (see Section 2.1). The minimum observed

pressure was far above a value at which ingress would be likely. This is in spite of the fact

that there were many, frequent changes in flow at this site.

DEFRA


© WRc plc 2011 35

5.3 Mechanism of pressure surge in Site B

5.3.1 Potential pressure change

As an example of the flow variation to the commercial premises, consider the period between

10 am and 11 am on August 23rd

(Figure 5.3).

Figure 5.3 Flow to large user on August 23rd

2010

There is an increase of 5.77 l/s between two successive flow readings. However, this is

probably an underestimate of the change because 15-minute averages are used, and these

will smooth out any sudden changes in flow. This customer is supplied from a 5” cast-iron

main in such a location that the increased flow can reasonably be assumed to be supplied

equally from both directions. If the supply had been through a single main, the velocity change

would have been double and the pressure surge more severe.

The velocity change in each main is given by:

--------------------- A

where ΔV velocity change in m/s

ΔQ flow change in l/s

0

2

4

6

8

10

12

14

00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00

Flo

w (

l/s)

Time

Major User Events_Site 1 Events_Site 2 Events_Site 3 Events_Site 4

DEFRA


© WRc plc 2011 36

In this case:

ΔV = 0.235 m/s

The potential pressure change in a 5-inch cast-iron main is given by:

ΔP = 13 x ΔV ---------------------- B

where ΔP pressure change in bars

As the flow is increasing, the pressure upstream of the inlet valve to the major user will fall.

The drop, in this case, is potentially 3.06 bars. If this were to be realised, sub-atmospheric

pressures could be achieved at all four sites. Such drops do not occur because of the

complexity of the system (see Section 2.1).

5.3.2 Speed of pressure change

Because pressure was logged at 0.1-second intervals, it is possible to calculate accurate

short-term rates of change. The rate of change of pressure close to the large user in the

period from 10:00 to 11:00 (considered in Section 5.3) peaked at 0.67 bars per second.

Equation B can be used to calculate that this is caused by a velocity increase of 0.046 m/s in

one second. Equation A gives the increase in flow to the customer over one second as

1.11 l/s, a total change in average velocity of 0.045 m/s over 15 minutes (i.e. considerably

less than the changes that are possible).

If this rate of pressure change had persisted for six seconds or longer, then sub-atmospheric

pressure would have been produced. This does not occur because of reflections of the

pressure wave from pipe junctions which moderate the change. Large diameter pipe junctions

are some 100 m away from the customer off-take in each direction. Since the wavespeed in

these pipes is 1300 m/s the return waves will arrive back at the off-take about 0.15 seconds

after initiation. Well within the first second, the surge is beginning to be alleviated. Further

moderating effects come successively from more distant junctions and changes in diameter.

5.4 Example events

5.4.1 Demand Event on August 23rd 2010

Some of the features of pressure surge in this system are demonstrated by an event

observed on August 23rd

. Pressures at three of the locations are shown in Figure 5.4. The

event was not recorded at location 3. It is assumed that the lowest pressure at that site was

just above the trigger and was, therefore, insufficient to cause the logger to start recording.

DEFRA


© WRc plc 2011 37

Figure 5.4 Surge event on August 23rd

There are a number of interesting features:

1. There is an underlying oscillation with a period of 40 seconds. Using pressure results

from a single day of constant logging from location 1, it is noted that this is observed

throughout the day. The period is remarkably constant and the oscillation is persistent.

This strongly suggests a variation which is imposed on the system from outside the

distribution network. As this oscillation does not seem capable of producing very low

pressures, this is not of any direct concern to this project.

2. The pressure drop at around 100 seconds in Figure 5.4 is almost 2 bars and would

certainly be noticed by customers. However, as with all of the events recorded in this

site, it is much less severe than would be predicted by the flow change (see Section

5.3).

3. The pressure variations from the different locations were very similar both in shape and

magnitude. They vary in absolute value because of ground-level differences and friction

losses between logger locations.

It might be assumed that, as the pressure wave travels away from the source of the event, the

magnitude would decrease due to interactions at junctions and fluid friction. Indeed, this is the

case in a simple main with widely spaced offtakes. However, in a distribution system, where

fittings, changes of material and changes in diameter are close together, the moderating

influences travel rapidly to all locations, including the one closest to the initiating event. Flow

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 50 100 150 200 250 300 350 400 450 500

Bars

Time (s)

1 2 4

DEFRA


© WRc plc 2011 38

changes need to be extremely rapid to produce pressure changes similar to the maximum

potential change (see Section 5.3.1).

4. There is a small difference in the timing of the corresponding peaks and troughs at the

different locations. For example, the timing of the low pressure on Figure 5.4 varies by

less than one second across the three locations which are similar to the travelling time

between locations. It is indicative of the rapid communication through the pipes

between locations.

5.4.2 Demand Event on August 18th 2010

Figure 5.5 shows pressures observed at two locations on August 18th. This includes location

3, which was absent from the previous example.

Figure 5.5 Surge event on August 18th

This shows the four features noted for the other event:

1. The underlying oscillation.

2. The significant drop at both loggers which is less than the surge potential.

3. The similarity of the oscillation at the two locations.

4. The small difference in timing between locations.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 50 100 150 200 250 300 350 400 450 500

Bars

Time (s)

1 3

DEFRA


© WRc plc 2011 39

In addition:

5. The general pattern is (1) an oscillation about a high pressure, (2) a sharp drop, (3) an

oscillation about a lower value, (4) a sharp rise. This is consistent with an increase in

demand (at 100 seconds) which is fast enough to cause a pressure surge followed by

the sudden fall in demand (at 270 seconds) (i.e. the opening and closing of the valve).

6. The average pressures at the two locations in period (1) (oscillation about a high

pressure) are similar. In period (3) (oscillation about a lower pressure), location 3 has a

markedly higher pressure. This is consistent with the large user being the source of the

demand. The pressure drop en-route from DMA entry to location 1 has increased

because of the increased flow. Location 4 shares only a short stretch of this route and,

therefore, suffers only part of the enhanced pressure drop.

5.5 Summary results

1. The flow to one commercial user is large compared to the flow to the rest of the DMA.

2. These large flows occurred frequently on dozens of occasions during the logging

period.

3. Surge pressures developed at the inlet to the commercial user were transmitted to the

logging locations.

4. On no occasion was the pressure close to gauge zero. The minimum pressure

observed at any location was 1.7 bars.

5.6 Discussion and conclusion

The potential pressure drop caused by the exceptional demand can be calculated from the

flow change. In theory, the demand by this large user is frequently large enough to produce

sub-atmospheric pressures in the main. This does not happen. The reason is that, when the

pressure wave reaches a pipe junction, a reflection is returned to the initiation point which

reduces the pressure change. There are junctions with substantial mains within 100 m of the

inlet to the large user. Therefore, the flow change would have to occur in less than

0.2 seconds for the whole of the potential drop to be realised (see Section 2.1). If the flow

change is rapid enough, the potential pressure will be developed before the reflection arrives.

It can be seen from the pressures logged at the nearest point (location 1) that the flow change

is indeed rapid. However, it is not fast enough to reduce pressures to a value near to gauge

zero.

In this (typical) distribution system with cast iron mains, the flow change must be very rapid

(i.e. of the order of 0.1 seconds) to produce very low pressures.

DEFRA


© WRc plc 2011 40

DEFRA


© WRc plc 2011 41

6. Surge Site C


Site C was identified through discussions with water company representatives. This site

consists of a DMA which is fed from a pumping station where the pumps are switched daily to

reduce energy use. The length of the main between the pumping station and the DMA is

considered to be sufficient for surge conditions to develop.

One of the pressure loggers was installed at the pumping station, to observe any low

pressures that may originate here (Figure 6.1, location 1). The direction of flow from the

pumping station to the DMA is shown by the blue arrows.

Figure 6.1 Location of loggers in relation to pumping station, Site C

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5m contours

Water mains

Bulk Meters

Hydrants

Pumps

Logger locations

1

500mm

500mm

500mm

18"

150mm

DEFRA


© WRc plc 2011 42

The other four loggers were located in one of the DMAs fed from the pumping main (Figure

6.2, locations 2 to 5). As with the other surge sites, the locations of these loggers were

chosen to reflect an increasing level of complexity in the system.

There are two junctions part-way along the trunk main which would be expected to modify the

pressure event as it travels towards the distribution system.

Figure 6.2 Location of loggers within the DMA, Site C

6.2 Logging Results

Pressure loggers and kiosks were installed in the network (locations 2 to 5) on 16th November

2010 and at the pumping station a day later, on 17th November 2010. The loggers were

removed on 8th April 2011.

Table 6.1 gives a summary of the low pressures recorded in Site C.

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5m contours

Water mains

Bulk Meters

Hydrants

Pumps

Logger locations

DEFRA


© WRc plc 2011 43

Table 6.1 Summary of low pressures in Site C

Location Trigger

(bar)

Pressures

< 1 bar

Pressures

< 0.1 bar

Minimum

pressure

(bars)

Date of

< 0.1 bar

event

1 1.0 43 0 0.146 5th Feb

2 3.5 0 0 3.132 -

3 1.5 0 0 1.301 -

4 1.5 1 0 0.552 29th Mar

5 1.1 7 0 0.846 5th Dec

In the five-month logging period, the pressure never dropped below 0.1 bars. Indeed, at the

locations within distribution, there were only eight occasions when the pressure dropped

below 1 bar. Seven of these were at the same location.

6.3 Example events

6.3.1 Pump switching event on 8th February 2011

A pump switching event was recorded a number of times during the logging period. It is

probable that this is a frequent event which only triggers the loggers on some occasions

depending on the flow and pressure conditions in the network at the time. However, the ones

that were recorded are all essentially the same.

The pressure results, shown in Figure 6.3, are for one typical example of this pump switching

event.

DEFRA


© WRc plc 2011 44

Figure 6.3 Surge event on 8th

February 2011

The trace for location 1 (the pumping station) shows a dramatic drop in pressure of some nine

bars. This is transmitted along the cast iron pumping main and is reflected from the

distribution networks. Subsequent reflections from the pumping station and network produce

the decayed oscillation which is typical of simple mains. A number of DMAs are supplied by

this station. They are fed from the main at a range of distances from the station. Taking 5 km

as typical and noting that the ferrous main has a wavespeed of around 1100 m/sec, the pipe

period is between nine and ten seconds (See Section 2.1). This is the time taken for the

reflection to return to the station. The time taken for the pressure at the pumps to fall to the

trough is about eight seconds (Figure 6.3). The pipe period is sufficient for the considerable

surge to develop.

If this low pressure had been transmitted to the distribution network, severe negative

pressures would have been developed because of the difference in ground levels. However,

reflections from the large number of pipe junctions modify the pressure change and at no

logging point does the pressure fall below one bar. The pipe lengths are of the order of 10

metres rather than kilometres. The timescales are of the order of 0.01 seconds leaving little

time for surge pressures to develop. Figure 6.3 shows a clear delay in the drop between

location 1 and the others but none between locations 2 to 5. The shape of these four pressure

traces is remarkably similar because of their close proximity to each other (see Section 5.3 for

a discussion of the impact of distribution system features on surge pressures).

0

2

4

6

8

10

12

0 100 200 300 400 500 600

Pre

ssu

re (

bar

s)

Time (seconds)

1 2 3 4 5

DEFRA


© WRc plc 2011 45

The initiating event would have to be completed in the order of 1/100 of a second for the full

effect to be seen at the loggers in the distribution system.

6.3.2 Hydrant testing on 29th March 2011

The Fire Brigade carried out hydrant flow tests in this area. This was apparent because they

temporarily disconnected the pressure logger at location 2. During the 20 minutes that the

logger was disconnected, the Brigade carried out two tests at location 2 which triggered the

logger at location 4. The pressure trace which resulted from the second hydrant flow event is

shown in Figure 6.4.

Figure 6.4 Effect of hydrant flushing at location 4 on 29th

March 2011

The figure shows (at 100 seconds) the pressure drop as a result of opening the hydrant

followed by a minor oscillation as the event is reflected within the system. At 170 seconds, the

pressure rise resulting from closing the hydrant is seen. This is followed by a more dramatic

oscillation. In fact, the minimum pressure is lower than that for the opening transient.

No other logger was triggered by this event. It is clear that the major part of the valve closure

took place in 1 second. In spite of this rapid closure, there was no risk of ingress as a result of

the hydrant testing.

0

1

2

3

4

5

0 100 200 300 400 500 600

Pre

ss

ure

(b

ars

)

Time (seconds)

DEFRA


© WRc plc 2011 46

There had also been hydrant testing in this area on 17th and 18

th March. However, none of the

loggers were triggered by this exercise.

6.4 Summary results

1. In the five-month logging period at five locations, the pressure never dropped below 0.1

bars.

2. Significant drops in pressure were observed frequently at the pumping station. These

were the result of pump-switching, which is undertaken in order to reduce power usage

during winter (see Section 6.3.1).

3. These pressure drops were considerably modified in the distribution system such that

pressures of less than 1 bar were only observed eight times, and all but one of these

were at a single location. These pressures never fell low enough to result in any risk of

ingress.

4. Hydrant tests in the area caused a low pressure to be registered on one occasion

(March 29th) at one of the logging locations (see Section 6.3.2). There was no risk of

ingress.

6.5 Discussion and conclusions

During five month‟s logging, pressure never fell below 0.1 bars.

One type of event caused pressures to fall below one bar at the pumping station and at one of

the distribution sites. This was a routine pump switching operation which caused a pressure

drop at the station of about nine bars. The pumping main is long enough to allow this pressure

surge to develop. If this had been transmitted to the distribution system, severe sub-

atmospheric pressures would have resulted. Reflections within the system prevented this. It is

the relationship between pump switching time and density of the distribution system that

determines the impact of the pressure surge on the distribution pressures. If the pump stop is

rapid compared with surge transit times in the system, a large impact is to be expected.

In some systems it will be advisable to implement pump controls such that pumps stop more

slowly (a “soft stop”).

Hydrant testing was carried out on several days and caused a low pressure at one location on

one occasion. This pressure was not low enough to cause ingress. The lowest pressure was

caused by closing the valve very quickly. Rapid opening and closing should be avoided.

DEFRA


© WRc plc 2011 47

7. Exceptional Demand Site D


The water company provided a number of potential sites where their pressure models had

identified areas which were expected to be at risk of lower pressures. WRc discussed these

sites with the water company and selected Site D as an area which could potentially

experience very low pressures driven by exceptional demands.

Site D is composed of four DMAs fed through a booster station to the east of the area and

also from the north (see Figure 7.1). The booster station and northern feed are located at

lower altitudes than much of the site. The highest areas are to be found to the south, west and

north of the site, with a more isolated hill in the vicinity of locations 10, 11, 11A and 12.

Location 11, for example, is some 50 m higher than the booster station. Pressure loggers

were placed at sixteen locations throughout these DMAs. These locations were selected to

provide a good spatial representation of pressure changes in the DMAs, with particular

attention paid to the areas where it was decided that very low or negative pressures were

most likely to occur.

One location (location 11) was abandoned during the logging period when the hydrant began

to leak when the valve was opened. An alternative location was identified (location 11A), and

a logger was installed at this location the following month.

7.2 Logging results

Fourteen pressure loggers were installed on 23rd

/24th February 2010, with a further two

installed during the first download on 12th/13

th April. Location 11A was identified as an

alternative to location 11, and a pressure logger was installed at the new location during the

scheduled data download on 25th/26

th August 2010. All the loggers were removed on 14

th

February 2011, although a number of loggers stopped working due to frost damage, caused

by the extreme cold during December 2010.

Table 7.1 provides a summary of the low pressure events recorded at all sixteen locations.

The loggers were in situ for almost one year at these locations. Table 7.1 shows that during

that period pressure fell below one bar gauge on only 11 occasions. These were only

observed at six of the sixteen locations (11A is a replacement location close to 11, which it

replaced). At three of these locations only one such low pressure event was observed, where

the pressure was low but positive. For each of these events the minimum recorded pressure

was above 0.1 bar (0.21, 0.26 and 0.20 bar) and, ingress is very unlikely. The pipe would

have to be some 2 or 3 m below water for ingress to occur.

DEFRA


© WRc plc 2011 48

Figure 7.1 Location of loggers at Site D

6

1

25

4

3

10

11A

11

12

16

1514

13

9

8

7

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235

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265

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275170

245

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0

285

DMA 1

DMA 2

DMA 3

DMA 4

Other mains

5m contours

Hydrants

Bulk meters

Pumps

Logger locations

Booster

DEFRA


© WRc plc 2011 49

As can be seen from Table 7.1, pressure fell to zero on one occasion at location 6. This was

due to a brief isolated incident symptomatic of local valve closure to isolate the main in the

vicinity of the logger (i.e. a rapid fall to zero or near zero at one logger, with no measurable

effect or minor effect at other loggers). All the negative pressures (three in total, two events)

were recorded at locations 4 and 11/11A. On the 6th/7

th January 2011 a system-wide event

(failure at the works) caused negative pressures at these points (see Section 7.3.2). Loggers

4 and 11/11A were situated at the highest points in the site, and it is unsurprising that they are

the most vulnerable (see Section 3.3). The third instance of negative pressure (recorded at

location 11 on 28th May) was again symptomatic of local valve closure.

Table 7.1 Summary of low pressures in Site D

Location No of events

< 1 bar

No of events

< 0.1 bar


Pressure

(bars) Date

Duration

below

0.1 bar

1 0 0 2.09 - -

2 1 0 0.21 - -

3 1 0 0.26 - -

4 3 1 -0.30 6th Jan 58 mins

5 0 0 1.92 - -

6 1 1 0.00 22nd

Oct 4 mins

7 0 0 1.49 - -

8 1 0 0.20 - -

9 0 0 2.07 - -

10 0 0 1.25 - -

11 3 1 -0.05 28th May 3 mins

11A 1 1† -0.72 6

th Jan 1 hr 5 min

12 0 0 1.68 - -

13 0 0 1.44 - -

14 0 0 2.28 - -

15 0 0 1.22 - -

16 0 0 1.93 - -

†Pressure fell below zero twice during this one event.

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© WRc plc 2011 50

7.3 Example events

7.3.1 Pump failure event on 29th November 2010

On 29th November, a pressure drop was recorded throughout Site D. The effect was most

marked in DMA 1, and the results for this DMA are shown in Figure 7.2.

Figure 7.2 Pressure trace on 29th

November 2010 in DMA1, Site D

The behaviour of the pressure at each of these locations is essentially the same and that is

consistent with a pump failure at the treatment works (i.e. a pressure change imposed on the

whole system). Each logger records a large pressure transient (a drop of between 2 and

3 bars) followed by a new steady state (approximately 1 bar below normal pumping pressure).

This new pressure is maintained until the problem is resolved about 17 minutes later,

probably as a result of the pump being restored. The lowest pressure (1.1 bars) is recorded at

location 11 (the highest logger in the DMA).

It must be remembered that logging in this case was at 10-second intervals. This time

discrimination is usually inadequate to capture the detail of surge events in distribution (see

Section 2.2). It is possible that there was a brief pressure excursion lower than those logged.

The large surge drop is transferred to DMA 2 but is rather less at location 1 which is the most

remote from the works and where pressures are enhanced by a local booster pump (see

Figure 7.3). The lowest pressure (0.70 bars) is at location 4 (the highest logger in this DMA

0

1

2

3

4

5

6

7

14:45:00 15:00:00 15:15:00 15:30:00 15:45:00 16:00:00 16:15:00 16:30:00 16:45:00

Bars

Date_Time

10 11 12 13 14 15 16

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© WRc plc 2011 51

without a local booster pump). The phenomenon is recorded in DMAs 3 and 4 but is much

reduced because these loggers are hydraulically more remote from the initiating event

(multiple pipe junctions cause reflections which ameliorate the surge).

The temporary steady pressure is significantly lower than normal in DMA 2 except at logger 1

where pressure is maintained by a local booster pump (marked „BP‟ on Figure 7.3). There is

very little impact on steady state pressure in DMA 3 (because supply is solely through the

booster to the east of the DMA) or in DMA 4 (because pressure is controlled by a Pressure

Reducing Valve (PRV)). It is worth noting that a significant surge pressure is recorded in both

of these DMAs (see previous paragraph). The PRV and booster do not react quickly enough

to entirely suppress the surge.

Figure 7.3 Location of local booster pumps for location 1, Site D

7.3.2 Power failure event on 6th and 7th January 2011

On the night of 6th January 2011, the major water treatment works that supplies Site D

experienced a power failure with a subsequent failure of all pumps. This produced pressure

drops across all four monitored DMAs, with the absolute drop in pressure being similar across

the locations. This drop was large enough to produce low pressures at some locations and

negative pressures at locations 4 (DMA 2) and 11A (DMA 1) (see Figure 7.4 and Figure 7.5).

5

6

2

1

4

10

7

3

225

235

240

250

255

260

265

19

0

18

5200195

245

270

275

280

175

18

0

DMA 1

DMA 2

DMA 3

DMA 4

Other mains

5m contours

Hydrants

Bulk meters

Pumps

Logger locations

BP

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© WRc plc 2011 52


January 2011 in DMA 1, Site D


January 2011 in DMA 2, Site D

-2

-1

0

1

2

3

4

5

6

7

22:00:00 22:30:00 23:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00

Bars

Time

10 11A 12 13 14 15 16

-1

0

1

2

3

4

5

6

22:00:00 22:30:00 23:00:00 23:30:00 00:00:00 00:30:00 01:00:00 01:30:00

Bars

Time

01 04

DEFRA


© WRc plc 2011 53

Figure 7.4 shows that the pressure drops in DMA 1 at locations 10, 11A, 12 and 15. The

loggers at locations 13 and 16 were damaged at this time, as a result of freezing during the

cold weather in December 2010 and January 2011. Therefore, there are no data for these

locations. The logger at location 14 does not record the drop in pressure, the only working

logger across all four DMAs not to do so. There is no clear reason why this location should

behave differently. The four locations exhibiting a drop in pressure in DMA 1 all show a similar

pattern and magnitude to the drop and subsequent recovery. Pressures are already lower at

location 11A than at other locations, due to this location being at a higher ground level than

the others (see Figure 7.6). This results in a negative pressure being recorded at this location.

The network at this location is subject to negative pressure for roughly one hour, before

pressures recover briefly. A second pressure drop results in negative pressures for about

30 minutes. Negative pressures are also recorded for the first pressure drop at location 4 in

DMA 2 (Figure 7.5), for a period of roughly one hour. Location 1 shows a lesser effect on

pressures, which is likely due to a separate booster pump serving this location that dampens

the pressure drop (see Figure 7.3).

Figure 7.6 Location of loggers in DMA 1, Site D

A summary of the minimum pressures witnessed at each location during this event is shown

in Table 7.2.

16

11

10

11A9

1213

15

14

19

0

18

5

180

195

20

0

210

220

205

215

225

235

230

DMA 1

DMA 2

DMA 3

DMA 4

Other mains

5m contours

Hydrants

Bulk meters

Pumps

Logger locations

DEFRA


© WRc plc 2011 54

Table 7.2 Minimum pressures recorded at each location during low pressure event

on 6th

January 2011, Site D

Location Minimum pressure (bars)

1 3.388

3 0.255

4 -0.297

6 2.912

8 0.198

10 1.247

11A -0.718

12 1.682

14 3.687

15 1.221

2 No data

5 No data

7 No data

9 No data

13 No data

16 No data

Although the cause of this low pressure event was identified by the contributing water

company as a power failure at a water treatment works, rather than an exceptional demand

event, it is clear that ingress could have occurred at locations 4 and 11A, where pressures

were negative for at least an hour.

7.4 Summary results

1. 16 loggers were deployed for one year.

2. On only 11 occasions did any pressure fall below 1 bar.

3. On only four occasions did pressure fall below 0.1 bars (negative on three occasions).

4. One failure at the works on January 6th caused negative pressures at two high points

(see Section 7.3.2).

DEFRA


© WRc plc 2011 55

5. There were two occasions (October 22nd

and May 28th) where zero or negative

pressure results were symptomatic of mains isolation by local valve closure to isolate

the main in the vicinity of the logger.

6. A PRV and local booster pump each provide some protection against low steady state

pressures in their vicinity.


There were only three occasions when pressure fell to or below zero. It is believed that none

of these were due to exceptional demand events. The system is robust in that it has large

diameter mains which provide alternative supply routes to many areas. Those lower-lying

areas where a large flow would be possible are linked to the higher ground by large diameter

mains. Even a large outflow would have a modest effect on headloss in these large mains and

hence is unlikely to lower pressures to very low values.

Some additional protection against low pressures is provided at some locations by PRVs and

local booster pumps.

Although no negative or very low pressures were observed as the result of exceptional

demand events (e.g. bursts) in this site, a failure at the treatment works did produce negative

pressures at two locations for over an hour.

DEFRA


© WRc plc 2011 56

DEFRA


© WRc plc 2011 57

8. Exceptional Demand Site E


Discussions with the water company identified areas which had characteristics that might

result in low pressures occurring. Some of the areas at risk were considered too small to be

worth investigating, as the low-lying areas where any exceptional demand event leading to

low pressures at the higher areas would have to occur were relatively small. This reduces the

probability of any events being observed during the study period. WRc discussed all the

potential sites with the water company and selected Site E as an area worth investigating

because it had the desirable variation in ground level.

Site E is composed of five DMAs fed from the same supply. Pressure loggers were placed at

sixteen locations throughout these DMAs (Figure 8.1). These locations were selected to


attention paid to the highest areas within the site, where it was decided that very low or

negative pressures were most likely to occur. The blue arrows in Figure 8.1 indicate the

direction of flow through the trunk mains that feed the site.

One location (location 8) was abandoned during the logging period when the hydrant began to

leak when the valve was opened. An alternative location was identified (location 8A), and a

logger was installed at this location the following month.

8.2 Logging results

Fifteen pressure loggers were installed on 15th/16

th March 2010, with one more logger

installed during the first download on 22nd

/23rd

May 2010. Location 8A was identified as an

alternative to location 8, and a pressure logger was installed at the new location during the

scheduled data download on 23rd

/24th August 2010. All the loggers were removed on 21

st

February 2011, although a number of loggers had stopped working during December 2010,

due to frost damage arising from the extreme cold weather.

Table 8.1 provides a summary of the low pressure events recorded at the sixteen locations.

Loggers were in situ for approximately 11 months. Table 8.1 shows that pressures below one

bar were recorded on just 17 occasions. Pressures below 0.1 bars were rarer – ten events

observed at seven locations. Nine of these events were symptomatic of local valve closure to

isolate the main to which the logger was connected (i.e. a rapid fall to zero or near zero at one

logger with no measureable effect or minor effect at other local loggers). Just one low

pressure event is believed to be the result of an exceptional demand event. In this case, the

water was taken from a hydrant with a pressure logger which would need to be removed for

the purpose. The zero value is therefore not good data. The impact on other local loggers was

minor.

DEFRA


© WRc plc 2011 58

Figure 8.1 Location of loggers at Site E

1

163

13

1412

7

9

8

8A

6

1011

2

4

5

15

162

165144

168

147

138

11

1

105

141

132

120

135

150

108

114

12

6

123

171 174

177

15

3

117

159

129

99 102

96

DMA 1

DMA 2

DMA 3

DMA 4

DMA 5

All other mains

Trunk mains

3m contours

Hydrants

Meters

Logger locations

DEFRA


© WRc plc 2011 59

Table 8.1 Summary of low pressures in Site E

Location No of events

< 1 bar

No of events

< 0.1 bar


Pressure

(bars) Date

Duration

below

0.1 bar

1 2 1 -0.05 26th Apr 4 hrs

1

2 0 0 1.71 - -

3 3 1 -0.40 15th Sept 8 min

4 1 1 0.00 24th Aug 7 min

5 0 0 2.76 - -

6 1 1 0.05 14th Sept 3 min

7 1 1 0.00 4th Oct 25 min

8 0 0 2.90 - -

8A 0 0 2.89 - -

9 1 0 0.46 - -

10 0 0 4.02 - -

11 0 0 1.12 - -

12 0 0 1.37 - -

13 0 0 2.64 - -

14 5 4

-0.26 4th May 12 min

0.00 5th May 1 hr 8 min

2

0.00 24th Nov 18 min

0.00 25th Nov 12 min

15 2 0 0.32 - -

16 1 1 0.00 2nd

Feb 11 12 min

1. Pressure went negative for a few seconds. Pressure recorded at zero for the remainder of the event.

2. Pressure went to zero for a few minutes.

8.3 Example events

8.3.1 Event on 19th July 2010

The pressure was seen to fall on a number of the loggers at Site E just after 04:40 am on 19th

July 2010. The pressure remains at this lower level for about 1 hour and 21 minutes before

recovering to normal levels.

DEFRA


© WRc plc 2011 60

The magnitude of the impact of this demand event on pressures varies across the three

affected DMAs. The greatest impact is observed at location 16, in DMA 3, where the pressure

drops by 3.48 bars (Figure 8.2). However, this location exhibits a higher starting pressure, so

the large drop can be accommodated without any significant probability of sub-atmospheric

pressure. All three loggers in this DMA (DMA 3) show a significant pressure drop, suggesting

the causal demand event is likely to have been in this DMA. A burst in the vicinity of location

16 would increase the pressure drop in the smaller diameter mains feeding that location. The

routes from source to locations 1 and 3 have some small diameter mains in common with the

route to 16. There was a major effect at these points but less than that at 16.

The lowest pressure observed during this event was at location 1, where the minimum was

0.788 bars, following a drop of 3.04 bars from the starting pressure. Although the minimum

pressure is not very low or negative, it is noted that there are points at higher altitudes within

this DMA that might be expected to exhibit lower pressures than the minimum observed. The

hydrants at the higher locations are located within a busy main road, and the installation of

loggers at these hydrants would have caused significant planning and traffic disruption on a

monthly basis. Therefore, no loggers were located here. As the difference in the altitude

between location 1 and the highest point in the DMA is about 7.5 m, it is possible that very low

pressures were observed at and around the highest point, but only over a very small area.


July 2010 in DMA 3, Site E

The loggers located in DMAs 1 and 5 were unaffected by this event. Both of these DMAs are

fed from large diameter mains. The flow in these mains would have been affected by the

suspected burst, but the resultant pressure drop would have been small. There was some

impact on loggers in DMA 4 (Figure 8.3) because of shared pipework. This DMA is fed from

0

1

2

3

4

5

6

7

04:00:00 04:30:00 05:00:00 05:30:00 06:00:00 06:30:00 07:00:00

Bars

Time

01 03 16

DEFRA


© WRc plc 2011 61

two large mains, originating from the same source. The lack of any impact at location 5 is

because this location is fed by a large diameter main which is not impacted by flows in DMA 3

(where the burst occurred).



The impact of the demand event varies across DMA 2, with only a small impact observed at

two of the locations (13 and 14), whilst location 12 observed a large pressure drop, although

still remaining above one bar (Figure 8.4). This is also a multi-feed DMA with the flow to the

three logging locations being via different district meters. The flow route to location 12 shares

more common pipework with DMA 3 than 13 or 14.

0

1

2

3

4

5

6

04:00:00 04:30:00 05:00:00 05:30:00 06:00:00 06:30:00 07:00:00

Bars

Time

05 06 07 08 09 10

DEFRA


© WRc plc 2011 62



8.3.2 Hydrant flushing event on 24th August 2010

For some 7 minutes just after 8 o‟clock in the morning, water was drawn from a hydrant in the

study area. It is almost certain that this involved disconnecting and then reconnecting the

logger at location 4. This resulted in the string of zero values seen in Figure 8.5. (It is not

possible for these to be genuine pressure values when the coincident results at the nearby

logger 2 are around 3.5 bars.) Loggers 2 and 11 are in the same DMA (See Figure 8.6). The

increased friction head induced by the imposed high flow is modest (2 to 5 m) and is of no

consequence at these points where the normal pressure is in the 3 to 4 bar range.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

04:00:00 04:30:00 05:00:00 05:30:00 06:00:00 06:30:00 07:00:00

Bars

Time

12 13 14

DEFRA


© WRc plc 2011 63


August 2010 in DMA 5, Site E

Figure 8.6 Location of loggers in DMA 5, Site E

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

07:00:00 07:15:00 07:30:00 07:45:00 08:00:00 08:15:00 08:30:00 08:45:00

Ba

rs

Date_Time

02 04 11

4

2

6

11

5

123

126

120

114

111

105

108

DMA 1DMA 2DMA 3DMA 4DMA 5All other mains

Trunk mains

3m contours

Hydrants

Meters

Logger locations

DEFRA


© WRc plc 2011 64

DMA 5 is fed from DMA 4 and so the hydrant flow affects the pressures in this latter DMA

(Figure 8.7). The effect is barely detectable because of the large diameter mains crossing

DMA 4. The effect is not detectable at location 5 which is fed from the largest (10”) main. The

other DMAs are not affected because they are fed directly from a large diameter main.


August in DMA 4 in Site E

8.3.3 Burst event on 19th September 2010

A significant mains burst occurred on a short length of 8” main which itself is fed from a 10”

main. The 10” main feeds DMAs 2, 3 and 4. However, the impact on pressures in these DMAs

was minor. The results for the loggers in DMA 3 are shown in Figure 8.8. A small fall was

registered at each location and remained until the burst was repaired. A similar result was

observed by the loggers in DMAs 2 and 4.

The pressure drop is the result of increased headloss in the 10” main from supply source to

the burst. Even a large burst has only a small effect on a main of this size.

DEFRA


© WRc plc 2011 65


September 2010 in DMA 3 in Site E

8.4 Summary results

1. Sixteen loggers were deployed for eleven months.

2. Pressure fell below one bar on seventeen occasions.

3. Seven locations observed pressures below 0.1 bars. These values were recorded on

ten occasions. Three of these were negative pressures. The effect at other locations

was usually negligible and sometimes minor.

4. Nine low pressure events were probably due to local valve closure; the remaining low

pressure event (see Figure 8.5) was in all probability a spurious result where the logger

was removed from the hydrant.

5. A possible burst on July 19th had a major impact locally without pressures falling to very

low values (see Section 8.3.1). Impact on other DMAs was small because of the large

diameter spine mains which feed the DMAs.

6. Flow from a hydrant on August 24th had a small effect on logged pressures (see

Section 8.3.2).

0

1

2

3

4

5

6

7

21:00:00 22:00:00 23:00:00 00:00:00 01:00:00 02:00:00 03:00:00 04:00:00

Ba

rs

Date_Time

01 03 16

DEFRA


© WRc plc 2011 66

7. A burst which was reported as “large” had a very small effect on logged pressures

because it was close to a large diameter main which feeds 3 of the 5 DMAs (see

Section 8.3.3).


Sixteen loggers were installed for eleven months. Although ten very low pressures were

recorded in that time, none were caused by large flow events. Most were probably caused by

the valving off of sections of main containing the pressure loggers.

Three high flow events were examined. A reported burst registered on several loggers but the

pressure change was very small. An event which was probably a burst had a marked effect

on local pressures but not, by any means, enough to cause a low pressure problem at any

logger site. It is possible that a small area at higher ground level suffered very low pressure.

Flow from a hydrant had a minor effect at local loggers. In this system, relatively large

diameter mains can supply an exceptional demand with only a small increase in headloss

and, therefore, a small fall in pressure at downstream locations. In particular, the DMAs in this

group are supplied by large mains from the source, so a demand on one DMA has little

impact in some of the others.

A burst is a major demand event and is relatively common in older cast iron systems. This

monitoring has shown that this type of event appears to have very limited impact on the

pressures in this site. Pressure does not fall to very low values and pressure falls do not

extend across the whole system.

DEFRA


© WRc plc 2011 67

9. Exceptional Demand Site F


Discussions with the water company identified areas which had characteristics that might

result in low pressures occurring. WRc discussed these sites with the water company and

selected Site F. This is an area in which pressure problems were known to have previously

occurred, and where the main feeding the site is lower-lying than the site.

Site F is composed of two DMAs fed from the same supply. Pressure loggers were placed at

sixteen locations throughout these DMAs (Figure 9.1). These locations were selected to


attention paid to the high areas where it was decided that very low or negative pressures were

most likely to occur. The blue arrows in Figure 9.1 indicate the main direction of flow.

DEFRA


© WRc plc 2011 68

Figure 9.1 Location of loggers at Site F

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DMA 1DMA 2All Other Mains

Spot heights

5m contours

Hydrants

Logger locations

DEFRA


© WRc plc 2011 69

One location (location 10) was abandoned soon after installation when it became apparent

the hydrant had a leak. An alternative location was identified (location 10A), and a logger was

installed at this location the following month.

9.2 Logging results

Nine pressure loggers were installed on 29th June 2010, with the remaining seven loggers

installed the next week on 6th July 2010. Location 10A was identified as an alternative to

location 10, and a pressure logger was installed at the new location during the first data

download on 10th/11

th August 2010. All the loggers were removed on 28

th February 2011,

although two of the loggers had stopped working during December 2010 due to frost damage

caused by the extreme cold weather.

On 17th August 2010, the Pressure Reducing Valve (PRV) controlling the pressure of water

reaching locations 10, 10A, 11, 12, 13 and 16 (DMA 2) underwent maintenance. This

maintenance, along with the alteration of this PRV from a flow-modulation to a two-point

controller on 4th September 2010, resulted in frequent, regular drops in pressure observed at

locations 10A, 11, 12, 13 and 16. As these drops were the result of the new type of controller

used for the PRV and, as they did not reach below 0.1 bar, they are not of interest to this

study. On 12th October 2010, the PRV was changed to a flow-modulation controller once

again. This resulted in the pressure pattern stabilising, and the regular low pressures were no

longer observed.

Table 9.1 provides a summary of the low pressure events observed at the logger locations.

Loggers were deployed at sixteen locations for eight months. Table 9.1 shows that pressures

fell below 1 bar on a large number of occasions. However most of these were in a pressure-

reduced area as a result of problems, modifications and maintenance on the PRV. Excluding

these, 27 low pressures (below 1 bars) were recorded. Very low pressures (below 0.1 bars)

occurred at 5 locations on a total of 6 occasions. On 4 of these occasions, either the water

company reported that a mains repair had taken place or the effect was symptomatic of local

valving to affect a repair. On the other two occasions, hydrant use was either reported or

suspected by the water company. There was a smaller impact on pressure at one other local

logger.

DEFRA


© WRc plc 2011 70

Table 9.1 Summary of low pressures in Site F

Location

No. of

events

<1bar, inc.

PRV

events

No. of

events

<1bar, exc.

PRV events

No. of

events

< 0.1 bar,

exc. PRV

events

Minimum Observed Pressure

Pressure

(bar) Date

Duration

below

0.1 bar

1 3 3 1 -0.05 26th Aug 23 min

2 0 0 0 2.86 - -

3 0 0 0 1.60 - -

4 0 0 0 3.06 - -

5 0 0 0 1.04 - -

6 0 0 0 1.78 - -

7 4 4 2 0.00 19

th Jul 1 sec

0.05 22nd

Oct 1 sec

8 0 0 0 1.90 - -

9 4 4 1 -0.05 8th Dec 17 min

10 1 1 1 -0.05 29th Jul 2 min

10A 57 5 0 0.57 - -

11 61 5 1 0.05 15th Aug 2 min

12 1 0 0 0.99 - -

13 0 0 0 2.37 - -

14 0 0 0 1.15 - -

15 2 2 0 0.53 - -

16 59 3 0 0.31 - -

9.3 Example events

9.3.1 Hydrant flushing event on 2nd August 2010

Flushing took place in the early afternoon at one of the dead ends downstream of location 9

(Figure 9.2). Although it is not known exactly at which hydrant flushing occurred, it is known to

be one of the hydrants circled and marked “F” in Figure 9.2.

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Figure 9.2 Location of flushing on 2nd

August, Site F

This had an effect on the loggers at three locations, as shown in Figure 9.3.

60

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55555555

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Spot heights

5m contours

Hydrants

Logger locations

F

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Figure 9.3 Flushing on 2nd

August 2010, Site F

The largest effect was at location 9 where the pressure fell to 0.4 bars. Since the downstream

ends of the sections being flushed are higher than this point, the pressure could not fall

further. The pressure drop was caused by the increased headloss in the 3” main which feeds

this area which, in turn, is caused by the exceptional flow from the flushing.

Location 3 can be fed from two directions. One of the routes from the supply source was

directly affected by the flushing flow. The extra flexibility in the system at this point resulted in

a considerably lower pressure drop. In addition, this location is some 5 m lower than location

9, so the observed pressure was roughly 0.5 bars higher before the hydrant flushing.

Location 5 is more distant. The flushing had a small effect on pressure at this point because

there are other flow routes to this location. No other logger registered a pressure fall at this

time. A location will only be affected if the supply to that location has a pipe in common with

the supply to the exceptional demand. If those common pipes are large, the effect may be

negligible. This is the case for many logger locations for this event on this site.

9.3.2 PRV maintenance

There were a number of instances where pressures were at low levels for a short period

during Pressure Reducing Valve (PRV) maintenance. An example from 17th August 2010 is

shown in Figure 9.4.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

11:57:00 12:12:00 12:27:00 12:42:00 12:57:00 13:12:00 13:27:00 13:42:00

Ba

rs

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Figure 9.4 PRV event on 17th

August 2010, Site F

Pressure fell at two locations (11 and 16) to 0.3 bars. These were the highest of the loggers. It

is possible that lower pressures were suffered at other points in the mains which had a higher

ground level, though this area of higher ground is very small. A logger could not be installed in

this area because a water company logger was already present upon the hydrant.

The clue that this is not a demand driven event is provided by the fact that all five loggers in

this pressure controlled area recorded the same pressure fall with the same shape. No low

pressure event was registered outside the PRV area as a result of this maintenance.

9.3.3 Burst event on 3rd December 2010

A burst occurred in the evening of 3rd

December 2010 near logger location 9. It reduced

pressure at that location by approximately 1 bar. A small effect was seen at locations 3 and 5.

(Figure 9.5)

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6

7

10:15:00 10:30:00

Bars

Date_Time

10A 11 12 13 16

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Figure 9.5 Burst on 3rd

December 2010, Site F

This makes an interesting comparison with the results of a flushing exercise which took place

in the same road (Section 9.3.1). Clearly, the burst was a much smaller “exceptional demand”

than the flushing.

The lower pressures produced by the burst persisted for 5 days until the main was isolated for

repair. This also isolated the logger at location 9 which recorded zero or near-zero pressure

for over half an hour (Figure 9.6). In fact, pressure falls just below zero on a number of

occasions during this event. It is likely that this is as a result of attempts by customers situated

at points lower than the logger, to draw water.

The pressures at other local loggers (locations 2, 3 and 5) rose slightly when the burst was

isolated.

0

0.5

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3

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4

4.5

20:30 21:30 22:30 23:30

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© WRc plc 2011 75

Figure 9.6 Mains repair on 8th

December 2010, Site F

A further feature of Figure 9.6 is the fall in pressure at these locations as the main is flushed

prior to reinstatement. This is a potential problem if the main being reinstated had been lower

than the surrounding network.

9.3.4 Demand event on 19th July 2010

A significant fall in pressure was observed at two loggers on this date. Pressure fell to zero at

location 7 and to 0.84 bars at location 15 (see Figure 9.7). There was no noticeable effect at

any other logger.

-0.5

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0.5

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Figure 9.7 Event on 19th

July 2010, Site F

The water company reports that there was an increase in flow to this DMA at the time of the

event with the possibility that this was illegal usage.

Figure 9.8 shows the pipework and the contours in the vicinity of locations 7 and 15, with the

blue arrows indicating the direction of flow.

0

0.5

1

1.5

2

2.5

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3.5

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4.5

16:19:12 16:48:00 17:16:48 17:45:36 18:14:24 18:43:12 19:12:00 19:40:48

Ba

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08 09 10 14 07 15

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Figure 9.8 Location of affected loggers on 19th

July event, Site F

The site of the exceptional demand must have been close to the junctions where the flow to

these two loggers diverged (marked „J‟ on Figure 9.8) since:

1. They each measure the same drop in pressure;

2. There are no loggers downstream of 7 and 15; and,

3. There is no effect at other loggers suggesting that the enhanced headloss is only in the

feed common to 7 and 15.

It is worth noting that location 7 is almost 10 m higher than location 15, and this explains the

difference in minimum pressure that was observed. The point which has been suggested as

the demand location is almost 15 m below location 7. Therefore, during the event, pressure at

that point would have been more than 1 bar which is sufficient to drive a large flow through an

open hydrant.

A similar pressure event took place in this area on 22nd

October 2010. In that case, the

pressure at location 7 fell to 0.1 bars and at location 15 to 0.6 bars suggesting that the

demand was closer to location 15 on this occasion.

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Spot heights

5m contours

Hydrants

Logger locations

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9.4 Summary results

1. Sixteen loggers were deployed for eight months.

2. Pressure fell below 1 bar on 27 occasions.

3. Five locations suffered very low pressures (less than 0.1 bars). Six such values were

recorded. Three were negative.

4. Most of these low pressure events were because the local main had been isolated. Two

hydrant uses had a localised effect.

5. Work on a PRV caused a large number of low pressures in the study area (see Section

9.3.2).

6. A mains flushing event caused a large drop in pressure locally without the possibility of

ingress (see Section 9.3.1). The effect on pressure at other locations was much smaller

because of the robustness of the system.

7. On another occasion (December 3rd

), a burst close to the locations of the flushing

discussed in point 6 (above) had a much smaller effect (see Section 9.3.3).

8. An exceptional demand on July 19th caused zero pressure at one local logger and a low

pressure at another (see Section 9.3.4). It is believed that the demand was at a much

lower ground level than the loggers and caused enhanced headloss only in the main

feeding this small area.


On only two occasions were very low pressures thought to be caused by an exceptional

demand. Some effect was recorded on two other loggers. Other lesser events also had only a

localised effect because of the robustness of the system.

On one occasion, an exceptional demand was thought to be at a much lower ground level

than two of the logger points but such that an enhanced headloss was induced in the main

feeding those two points. These two conditions are needed to cause a very low pressure.

However, upstream of this main, the exceptional flow was carried by a larger main. The effect

on other sites was therefore negligible.

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10. Discussion

10.1 Surge

10.1.1 The mechanism

When flow in a pipe changes, there is a resulting change in pressure. This change can be

very much larger than the normal operating pressure resulting in very high or very low (surge)

pressures. If a rapid increase in demand takes place, pressure at that point will fall. This low

pressure will travel through the system at high speed (the “wavespeed”). Wavespeed in metal

pipes is more than 1000 m/s and in plastic pipes typically between 300 and 600 m/s.

In a simple pumping main, the event will travel to the end of the main where it will be

reflected. Multiple reflections lead to the familiar pressure oscillation. In a distribution network,

the event will travel to the nearest junctions where some of the change will be transmitted and

some will be reflected. Because the distance between junctions are short, the reflection

returns to the demand point whilst the flow is still increasing and the pressure still decreasing.

The reflection will have a beneficial effect on the pressure change, limiting this change to less

than it would otherwise be.

Networks have a large number of closely-spaced junctions providing multiple reflections which

restrict the pressure change until fluid friction causes pressures to return to their steady-state

values.

It is possible to develop very low pressures in distribution networks by this mechanism but the

demand change would need to be very rapid. As an example, consider an iron main where a

junction is 100m away from the sudden change in demand. The total time taken for the

pressure event to travel to the junction and for the reflection to return is approximately

0.2 seconds. Only if the demand change is complete within 0.2 seconds will the full pressure

change develop. In addition there is a small reflection and a small reduction in the transmitted

pressure change when the wave reaches a service pipe. Although this effect is small, it can

be shown that the combined effect of all the service connections in an urban street is

substantial.

The pressure logging carried out during this project provided some striking examples of the

difference between pressure surge in a pumping main and in a distribution network. Large

pressure changes developed in the long mains were clearly sufficient to cause low pressures

in the network. However, the interactions within the network ensured that the changes there

were very modest.

The most important effect is the one described above which limits the fall in pressure at the

initiating point. In addition, the pressure change reduces as it travels away from this point. For

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© WRc plc 2011 80

a particularly fast transient, the change can be seen to reduce rapidly with distance. An

example of this was seen during the logging for this project.

10.1.2 The probability of low pressure

Five loggers were deployed at each of two sites for nine and five months respectively. In

addition four 4 loggers were installed for two weeks at a third site where it was known that a

large commercial demand occurred frequently. There were just five occasions when pressure

fell below 0.1 bars and on some of these negative pressures were observed. These were all

in the same system (site A).

On just one of these occasions was the pressure the direct result of high flow. This was

recorded at the source pumps rather than in distribution but remains a possible source of

ingress which could contaminate the water supplied to all the customers. It is not known

whether pressures at the other locations fell to very low values because these records (if any)

would have been overwritten by the subsequent isolation of the distribution system.

Catastrophic bursts occurred on a trunk main and within a distribution system. Severe

pressure drop occurred at a pumping station. None of these caused very low pressures within

distribution.

Based on the results of this study, there is a very low probability of very low pressure as a

result of surge flows. Valving operations to isolate mains can cause low pressures and

sometimes negative pressures.

10.2 Exceptional demand

10.2.1 The mechanism

An unusually large flow in a main will increase the headloss in that main. While this flow is

maintained, the pressure throughout the part of the system fed through this main will be

lowered. However, if this is a large diameter main, it will accommodate a large additional flow

with only a small increase in headloss. (Since distribution networks are usually designed to

provide 15 m available pressure throughout the system at normal maximum demand, this

increase in headloss must be at least 15 m in order to cause a problem.)

Pressure at the location of the exceptional demand (customer demand, hydrant flow or burst)

must remain positive to drive the outflow of water. Indeed, it may need to be significantly

positive depending on the size of the orifice or service pipe. There is a balance between

pressure in the pipe and outflow. If the pressure is low, the outflow will be low. The ground

level of areas with a higher likelihood of negative pressures due to this mechanism will have

to be higher than the exceptional demand location.

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These two aspects together lead to the following conclusions about an “at risk” system. It

must contain an area of low-lying land and an area of high ground. If an exceptional demand

event occurs in the low-lying area, the pressure may remain high enough to supply this

demand. If the demand is high with respect to the capacity of the pipes feeding it, it will induce

a high headloss in those mains. This will reduce pressure in these mains and in mains

downstream of the demand. The only points where there is a likelihood of very low pressure

are those which are markedly higher than the demand. Although it is feasible for the “at risk”

area to be upstream of the demand, it is unlikely because only part of the enhanced headloss

will apply to upstream points. As the supply source is approached, the mains diameters

typically increase and the increase in headloss will be smaller.

A typical “at-risk” system therefore has a fall in ground level from the source followed by a

substantial rise. The probability is lower if a small part of the network is at low level because

there are now fewer potential locations for a high flow event. The area at risk is also smaller if

the high level area is small. It can be seen that the conditions necessary for a demand to

produce widespread very low pressures by this mechanism are very restrictive.

There are additional distribution features which reduce the probability of low pressure.

Systems are rarely designed as tree structures. Most of the system will have loops providing

alternative routes to most of the customers. This will limit the increase in headloss even to

those areas which would normally be supplied by the pipe with the extra demand. PRVs and

pressure-controlled booster pumps will also limit the effect on controlled areas.

10.2.2 The probability of low pressure

Sixteen loggers were deployed in each of three systems for one year, eleven months and

eight months respectively. There were only 20 instances of pressure falling below 0.1 bars. Of

these, only two were caused by exceptional demands (hydrant flows). The very low pressure

in these two cases was confined to the mains in a few residential streets. As they were dead

ends, the risk (if any) of receiving contaminated water was confined to these few customers.

This suggests that the probability of low pressure caused by exceptional demand is very

small. It should also be noted that the three systems were chosen because they had

substantial variation in ground level which is a pre-requisite for achieving low pressures by

this mechanism.

Many more instances of lower pressure were recorded. These suggested that other issues

are of more concern than exceptional demand. The most common was the result of valve

closure to isolate a main for repair. This accentuates the need for good disinfection practice

when returning the main to service. Other problems are: mains draining down during valving

operations, pump failure and, PRV failure or maintenance

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10.3 Health risk

It should also be noted that other features are needed before the risk of low pressure

becomes a health risk. These are:

1. There must be a source of contamination at points where low pressure occurs,

2. There must be a pathway between the contaminant and the pipe flow.

10.4 Operational practice

Although the probability of very low pressure as a result of either mechanism is low, it can be

lowered further by opening and closing valves and hydrants slowly, and running hydrants at

the lowest necessary rate. There is clearly no action that can be taken to prevent sudden

and/or large bursts, though mains replacement programmes and good operational practice

will minimise risks.

Very low pressures were observed due to operational practices and other events, i.e. valving

off mains for repair, pump start and stop, pump failure, maintenance or failure of PRVs. Action

can be taken to limit low pressures because of operational practice. Pump control (“soft start

and stop”) to avoid rapid start-up and run-down is beneficial as is maintenance of PRVs. Care

should be taken to maintain pressures whilst PRVs are being maintained.

Following good disinfection practice when returning mains to service and paying similar

attention to related mains which may have drained as a consequence of the valving will

minimise potential health effects resulting from low pressures in these instances. The ability to

recognise that local mains have been drained requires a thorough understanding of the mains

layout.

10.5 Practical aspects of pressure monitoring

This study had a number of technical objectives which dictated the choice of equipment

needed to monitor pressure. The main constraints on the choice of equipment were the time

interval of data recording and power requirements. The Primayer loggers used in the

exceptional demand sites (Sites D, E & F) allowed pressure to be recorded at 10 sec intervals

and had a battery life of approximately 40 days. Consequently the loggers were compact

enough to be readily installed within a hydrant chamber. The compromise for the use of such

compact equipment was the necessity for manual retrieval of the data on a monthly basis.

All the installations and monthly data retrievals were carried out by WRc personnel, but

permission was obtained from each water company to allow the connection of the Primayer

loggers to their networks. To safeguard the quality of the water in these sites, the WRc

personnel that undertook the monitoring exercise were fully trained in the operation of the

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© WRc plc 2011 83

loggers and hydrants. Procedures for disinfection of the equipment were agreed and all WRc

personnel were accredited under the National Water Hygiene Scheme.

As discussed in Sections 2.3.2 and 3.3.2, it was important to ensure that alternative hydrants

could be identified in each site, should it prove difficult to install the pressure loggers. These

practical issues included difficult access to the hydrant (parked cars, busy roads), leaking

hydrants or shallow/ buried hydrants.

The Campbell loggers used in the surge sites (A, B & C) allowed pressure to be recorded at

0.1 sec intervals and required significant storage capacity for this data. Data retrieval was

carried out remotely requiring radio communication equipment. The resulting monitoring

assembly was too large to be installed within a hydrant chamber and therefore had to be

placed within a purpose-built kiosk. The installation of the surge equipment therefore required

much greater planning and resources to implement.

There were a number of practical issues that had to be taken into consideration during the

selection of the locations for the surge equipment, in addition to those that relate to the

condition and position of the hydrant. The ideal location had suitable space to house the

kiosk, without causing an obstruction or hazard to the general public. It was also necessary to

identify the owner of the potential site and obtain their permission to install the kiosk. In the

majority of cases the kiosks were installed on adopted highway and permissions were granted

by the local highways authority. Other land owners included a commercial business, a church,

a sheltered accommodation organisation and a district authority. Each of these organisations

responded differently to requests to install the kiosks, ranging from verbal agreement to

requesting the signing of legal agreements.

To ensure that the installations were carried out at efficiently as possible a thorough survey

was undertaken of each potential location. This included measurement of the available space,

testing the hydrant and also obtaining contact details of the possible land owners. A number

of alternative locations were also surveyed in each of the three sites.

The main obstacle that had to be overcome for the installation of the surge loggers was

dealing with the data cable connecting the pressure sensor on the hydrant to the logging

equipment. These cables needed to be buried to avoid causing a trip hazard. As a result, the

cables were classed as “apparatus” under the New Roads and Streetworks Act (NRSWA) and

were therefore subject to the requirements of this act. In practice this meant that licences had

to be obtained from the local highways authority to install them and the appropriate notices

had to be served to allow the highway to be excavated and reinstated. This installation had to

be carried out by personnel qualified to work in the highway, as stipulated in the Specification

for Reinstatement of Openings in Highways.

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11. Conclusions and Recommendations

Detailed pressure monitoring was carried out in 6 separate distribution networks for a combined period

of 56 months. Some low and negative pressures were observed during the study. The following

conclusions can be drawn from this study.

1. The probability of very low (surge) pressures as a result of a sudden demand is very low.

2. The probability of very low pressures as a result of exceptional high demands is very low.

3. A system is most at risk from low surge pressures if:

a. Pipe junctions are widely spaced

b. Property density is low (low number of service pipe connections)

c. There are very rapid increases in demand.

4. A system is most at risk from exceptional demands if:

a. There is a fall in ground level from the source followed by a substantial rise

b. There is a significant area at low level

c. There is a significant area at high level

d. Normal pressures in the high level area are low.

5. Pressures low enough to cause ingress are more likely as a result of the following than they are

from a demand-driven event:

a. Isolating mains for repair

b. Mains draining down during valving operations

c. Pump failure or rapid pump switching

d. PRV failure or maintenance.

6. To pose a health risk a source of contaminant around the low pressure point and a pathway to the

pipe flow are also necessary.

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© WRc plc 2011 86

7. Thorough planning is required to ensure the successful completion of such an extensive monitoring

exercise.

The recommendations from this study are as follows:

1. Additional proactive measures are not required to minimise an already low probability of very low

pressures occurring.

2. The practice of designing distribution systems with alternative routes to most customers (i.e. with

loops) should continue.

3. Good practice should be followed with respect to:

a. Opening and closing valves and hydrants slowly

b. Running hydrants at the lowest necessary flow

c. Returning mains to service (disinfection)

d. Disinfecting local mains which have drained as a consequence of work on other mains

e. Implementing soft start and stop for pumps

f. Maintaining PRVs and maintaining pressures during maintenance.

4. When carrying out research of this nature:

a. A thorough survey should be carried out of all potential locations for any monitoring equipment

b. Sufficient alternative locations should be identified.

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© WRc plc 2011 87

References

WRc, 2008, A review of research on pressure fluctuations in drinking water distribution systems and

consideration and identification of potential risks of such events occurring in UK distribution systems

(WT1205/DWI 70/2/220), Report No: DEFRA7555.01

Wylie, E.B. and Streeter, V.L. Fluid Transients, McGraw-Hill Inc., p3

investigation of instances of low or negative pressures in...

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