the air main

The KAESER Compressed Air Seminar Part 9 1

9. The Air Main

9.1 Air Losses

9.2 Pressure Drop

9.3 Mains Sizing and Selection of Pipework

9.4 Air Mains with or without Air Dryers

9.5 Outdoor Air Main

9.6 Selection of Pipe Materials

9.7 Identification of Pipelines

9.8 Save Energy and Lower Costs


9.1 Air Losses

Air losses increase operating costs !!!

Example: a 3 mm diameter holeAir loss: 0,5 m/min (6 bar)0,5 m/min x 60 min/h = 30 m/h30 m/h x 8,000 h/year = 240,000 m/year240,000 m/year x 0.02 P = 4,800 \HDULeakagesCompressed air must be routed with the minimum of air volume reduction (through leaks)And thus at minimum cost from the compressed air installation to the consumers.

Correspondingdiametre of hole

Size mm

Air lossat 6 barm/min

LosskW

1 0.065 0.3 240,--

2 0.240 1.7 1,360,--

4 0. 980 6.5 5,200.--

6 2.120 12.0 9,600,--

Example:0.1 kWh is needed to compress 1 m of air at 7,5 bar. At 4,000 service hours per year and power costs of0.10 N:KWKHIROORZLQJVXPIRUDWRWDOOHDNDJHLQDQDLUPDLQRIP/min is calculated:5 m/min x (4,000 h x 60 min/h) x 0,1 kWh/m x 0,10 N:K - \HDULeakage losses raise the costs of producing compressed air or lower the performance of pneumatictool or machine.

9. The Air Main

* Electricy price: 0.10 N:Kworking period: 8,000 sh/year


Measuring leakages by emptying the air receiver

9. The Air Main

Supply line shut off

Amount of leakage

VR x (pI pF)VL =

t

VL = volume of leakVR = receiver volumepl = initial receiver pressurepF = final receiver pressuret = measuring time

This method of measurement is suited to airsystems in which the pipework volume is lessthan 10 % of the air receiver volume. If not,the accuracy of the measurement cannot beguaranteed.

Beispiel:VB = 500 lpl = 9 bar (g)pE = 7 bar (g)t = 3 minVL = 500 l x (9 bar 7 bar)/3 min

= 333 l/min

Leakage losses in the air main:333 l/min

Tools not in use


Measuring leakages by measuring the cut-in periodof the compressor with pneumatic equipment out of use

VL = volume of leak in m/minVC = volume flow of compressor in m/mint = sum of time units during which the compressor ran under loadT = total time for measuring procedure

Example:Volume flow VC of the compressor = 3 m/minLoad time t of the compressor t = t1 + t2 + t3 + t4 + t5 = 120 sTotal time T for the measuring procedure = 600 s

9. The Air Main

1

2

3

4

5

6

7

8

Workingpressure

bar(

g)

time

VK x tVL =

T

3 x 120VL = = 0,6 m/min = 20%

600


Measuring consumer leakages

9.2 Pressure Drop

Air tools only work at their best if sufficient airvolume at the correct pressure is available.The following illustration shows howinsufficient pressure affects the performance ofa tool:

Pressure drop is caused by:

Internal friction (molekules)

Friction on the pipe walls

Turbulence

high flow velocities (small pipe bore sizes)

9. The Air Main

In plants in which a lot of air tools, machinesand equipment are used, hose coupling andvalves cause significant air leakages.

Using the previously described methods, twomeasurements will be carried out:

a.) tools, machines and equipment areconnected for normal use (total leakage)

b.) the shut-off valves upstream of thecouplings for the air consumers are closed (airmain leakage)

The difference between a) and b) is the sum ofthe losses at the tools and their fittings.

%

kW

Pressure in bar (g)

Perform

ance

The normal pressure required by an air tool is 6 bar (g). Increasing the pressure in thecompressor system to compensate for the pressure drop (pressure loss) costs money.Example: V = 30 m/min requires 160 kW of power at 7 bar (g). At 8 bar (g), 6% more poweris required, i.e. approximately 9.4 kW more.Costs: 9.4 kW x 0.10 N:K[K\HDU -- \HDU


The right fittings are also a pressure loss factor

A = valve (ball valve recommended)B = filter (to separate water and rust)C = pressure regulator (for constant

working pressure)D = lubricator (mostly oil mist lubricators)E = quick release coupling (flexibility)F = hose (3-5 m in length)G = spring balancer (working help)

Pressure drop in the air main

On a well designed air main a pressure drop of 0.1 bar is to be expected.

9. The Air Main

A

B

C E

F

G

1. Main supply line 0.03 bar2. Distribution line (ring)0.03 bar3. Branch line 0.04 bar4. Refrigeration dryer 0.2 bar5. Filter, regulator,

lubricator and hose 0.5 barmax. 0.8 bar

Switching difference 0.2 bar1.0 bar

Max. pressure at the compressor 7.0 bar (g)Consumer pressure 6.0 bar (g)Pressure difference 1.0 bar

0.1 bar

1

23

4

5

Pressure drop 0.8 bar

3

5

At low working pressures, e.g. 3 bar (g), apressure drop of 0.1 bar means, however, arelatively higher loss of energy than in 7 bar (g)air main.

The max. pressure drop in the air main 1.5 %

of the working pressure

Air supply using a ring main Air supply using a dead end split-flow main

D


9.3 Mains Sizing and Selection of Pipework

The following points must be observed when sizing and installing an air main:

Flow impedance of fittings, converted into equivalent pipe length

Pipe flow calculations

Equivalent length of pipe in mFittings

25 40 50 80 100 125 150 200 250 300 400

shut-off valve opened half closed

0,35

0,58

0,610

1,016

1,320

1,625

1,930

2,640

3,250

3,960

5,280

diaphragm valve 1,5 2,5 3,0 4,5 6 8 10 - - - -

elbow valve 4 6 7 12 15 18 22 30 36 - -

seat valve 3-6 5-10 7-15 10-25 15-30 20-50 25-60 30-75 - - -

check plate 2,0 3,2 4,0 6,4 8,0 10 12 16 20 24 32

bendR = 2d

0,3 0,5 0,6 1,0 1,2 1,5 1,8 2,4 3,0 3,6 4,8

bendR = d

0,4 0,6 0,8 1,3 1,6 2,0 2,4 3,2 4,0 4,8 6,4

knee bend 1,5 2,4 3,0 4,8 6,0 7,5 9 12 15 18 24

T-piece in direction of flow 0,5 0,8 1,0 1,6 2,0 2,5 3 4 5 6 8

T-piece in direction ofbranche flow

1,5 2,4 3,0 4,8 6,0 7,5 9 12 15 18 24

adaptor piece 0,5 0,7 1,0 2,0 2,5 3,1 3,6 4,8 6,0 7,2 9,6

9. The Air Main

Bore size of the pipeline:- air consumption- length of the air line- working pressure- pressure drop- flow impedance

Fittings and connectors:- type of outlets- shut-off valves- condensate separators- tool lubricators- dust filters- oil filters- pressure regulating valves- hoses- couplings

Selection of pipe materials:- ambient conditions (humidity,

temperature, chemical pollution of theair)

- quality of the air (humidity content, oilcarry-over, temperature)

- costs- expected service life

Installation:- ring mains- interconnecting lines- dead end lines- pipe connections- fittings


Example:- pipe length (air main) 100m- bore size of pipe 100 mm- required fittings

Fittings No. off Equivalent pipe length in mmper fittimg total

seat valve 4 30 120

bend r = d 12 1,6 19

T-piece 2 6 12

adapter piece 4 2,5 10

Total 161

Total pipe length (technical flow length):

Ltotal = Lstraight + Lcomparable

or, estimated

Ltotal = 1,6 x Lstraight

Minimum bore size of pipelines

Air deliverym/min

Working pressure 7,5 bartotal length:

up to 50 m to 100 m to 200 m over 200 m

Working pressure 10 bartotal length:

to 50 m to 100 m to 200 m over 200 m

Working pressure 13 bartotal length:

to 50 m to 100 m to 200 m over 200 m

up to 0,5 1 1 1 1 1 1 up to 1,0 1 1 1 1 1 1 1 1 1 up to 1,5 1 1 1 1 1 1 1 1 1 up to 2,0 1 1 2 1 1 2 1 1 2up to 3,0 1 1 2 1 1 2 1 1 2up to 5,0 1 2 2 1 2 2 1 1 2up to 7,5 2 2 2 2 2 2 1 2 2up to 10 2 2 2 2 2 2 2 2 2

up to 12,5 2 2 3 2 2 3 2 2 2 up to 15,0 2 2 3 2 2 3 2 2 3up to 17,5 2 3 DN100 2 3 DN100 2 3 DN100up to 20,0 3 3 DN100 3 3 DN100 2 3 DN100up to 25,0 3 DN100 DN100 3 DN100 DN100 3 DN100 DN100

up to 30,0 3 DN100 DN100 3 DN100 DN100 3 DN100 DN100

up to 40,0 DN100 DN100 DN125 DN100 DN100 DN125 DN100 DN100 DN125

Nominal bore size of pipelines - comparison

mm (DN: Diameter Nominal) Nominal bore size (inches)

DN 6 R 1/8DN 8 R 1/4DN 10 R 3/8DN 15 R 1/2DN 20 R 3/4DN 25 R 1DN 32 R 1 1/4DN 40 R 1 1/2DN 50 R 2DN 65 R 2 1/2DN 80 R 3DN 100 R 4DN 125 R 5DN 150 R 6

9. The Air Main

seestraightlinegraphforairlines




Determination of pipeline bore size with astraight line diagram

How to use the straight line graph:Mark axes A and B according to the total pipe length and the air delivery. Connect both points with a straight linethat cuts axis C.Now mark axes E and G with the system pressure and the required pressure loss. Connect both points with astraight line that cuts axis F.Now connect the two points of intersection on axes C and F with a straight line and read off the required pipelinebore size on axis D.

9. The Air Main

pipe lengthl[m]

FAD[m

3/h] [m

3/min]

bore sized[mm]

pressure loss

'p[bar]

gaugepressurep[bar]


No falling pipelinevertical branch lines possible

9.4 Air Mains with or without Air Dryers

Air main without air dryer (pipe-laying principles in wet areas)

Air main with air dryer

9.5 Outdoor Air Main

9. The Air Main

Compressor Air receiver withcondensate drain

Filter, waterseparator,regulator,lubricator

Filter,regulator Condensate

(water) drain

a) Installation in ducting or shaftsAdvantage: no spatial restrictionDisadvantage: complicated, expensive andpoor accessibility; separators must be locatedin frost-free space

b) Underground installationAdvantage: low costsDisadvantage: repair and maintenanceexceptionally difficult, pipes in stainless steelbecause of corrosionc) Overground installation with supportedor suspended pipesAdvantage: relatively low costDisadvantage: spatial restriction, risk offreezing, not aesthetic

Winter operation is possible with shortoutdoor pipelines, and with a pressure dewpoint of + 3 C (refrigeration dryer), if:

Larger bore sizes (no freezing) are used

The compressed air is subsequently re-warmed and a condensate drain/filter isinstalled at the point of re-entry into aheated area

The pipeline is vented during non-operational periods

The corresponding section of the pipelineis heated

CompressorAir receiver withcondensate drain

Dryer, incl.condensate drain

Regulator,lubricator

Regulator

Falling pipelineapprox. 2


9.6 Selection of Pipe Materials

9. The Air Main

Concrete duct

Condensate line

water separator

Inspection pit

Cover

Drain valve

Nominalpressure(bar())

Seamless steelpipe

DIN 2448

Max. permissible working pressure in (bar())

up to 120 C up to 200 C

2,5

6

10

16

25

40

64

100

St 35

St 35

St 35

St 35

St 35

St 35.8

St 35.8

St 35.8

2,5

6

10

16

25

40

64

100

2

5

8

13

20

36

50

80

Influence of temperature on themaximum permissible presssureIf the temperature increases, the yield pointof all materials reduces. DIN sheet 2401shows the interrelationship betweennominal pressure and the max. permissibleworking pressure for various materials.

Example: An air main that is designed for 6bar (g) and is subjected to a test pressure of6 bar (g) may only be operated at apressure of 5 bar (g) at temperatures above120 C.

Air line

Example of an outdoorseparator not subject tofreezing temperature


Steel pipelines

Seamlesssteel pipe

Threaded pipe Stainless steel pipe Copper pipe

Type

Black or galvanized to DIN2448

Medium weight to DIN 2440Heavyweight to DIN 2441

black or galvanized

Seamless to DIN 2462Welded to DIN 2463

Soft, in coils to DIN 1786Hard, in straight lengths

to DIN 1754/1786

Material e.g. St 35to DIN 1629

Seamless St 00 to DIN 1629Welded St 33 to DIN 17100

e.g. material No. 4301,4541, 4571

Copper

Dimensions 10,2 - 558,8 mm 1/8 - 6 6 - 273 mm6 - 22 mm soft6 - 54 mm hard

54 - 131 mm hard

Permissibleworkingpressure

12,5 - 25 bar 10 bar Up to 80 bar and, in part,higher

16 - 140 baraccording to type

Pipe ends Plain Tapered, plain or threaded Plain Plain

Pipe connection Welded Screw, welded Welded(inert gas arc welding)

Srew, brazed (fittings),welded

Advantages Air light pipe connection Many fittings(for srew connections)

Airtight connection, nocorrosion

No corrosion smooth innerwalls

Disadvantages

Corrosion on black steelpipe

Installation by experiencedtechnicians only

Corrosion (sometimes ongalvanized piping too)High flow and friction

Impedance can leak afterlong service life

Time consuming installationbecause of thread cutting or

weldingInstallation by experienced

technicians only

Installation by experiencedtechnicians only

Limited number of fittings

Special installation skillsrequired

Possibility of coppersulphate formation

Plastic pipelines

Material GIRAIR (PVC) Polyamide Polyethylene netted Polyethylene

Dimensions 16 - 110 mm 2 - 40 mm 10 - 160 mm 10 - 160 mm

DIN DN 8061/62 DN 16982 DN 8074 DN 16893

Permissibleworking pressure

at 20 C

12,5 bar Up to 100 bar 10 bar Up to 20 bar

Pipe ends Plain Plain Plain Plain

Pipe connection Cold solvent welding Fittings Welding Fittings/press fit

Advantages No leaksFittings of the same

materialHardly inflammable

Higher pressuresHigh chemical resistance

No leaksFittings of the same

materialNormal inflammability

High temperatureresistance

High chemical resistanceHardly inflammableMax. resistance to

aggressive compressed air

Simple to lay, low weight, no corrosion

Disadvantages Limited size range in partHigher coefficient of expansionOnly normal inflammability in part check in each individual casePossibility of static charge in partLegislative warranty only in partSome fittings of metal

9. The Air Main


Material comparison

The following selective criteria should be taken into account when planning an air main.The costs of material and installation are disregarded in this table.

Steel pipe to DIN 2440, 2441, 2448Criteria

Individualrequirements

Black,threaded

Black,welded

Galvanized,threaded

Galvanized,welded

Copper toDIN 1786,

17545

StainlessSteel toDIN 2462,

2463

Plastics

Size rangeup to 50 mmup to 100 mmover 100 mm

*XX(X)

XXX

XX(X)

XXX

XX(X)

XXX

XXX

Pressure rangeup to 10 barup to 12,5 barover 12,5 bar

*X (DIN2440/41)

X1)

X1)

X (DIN2440/41)X

1)

X1)

X (DIN2440/41)X

1)

X1)

X (DIN2440/41)X

1)

X1)

XXX

XXX

X(X)(X)

CorrosionAir quality

* 3 3 2 2 2 1 1

Temperature rangeup to 20 Cup to 50 Cup to 80 Cover 80 C

*XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

X(X) up to 8 bar

--

Flow characteristik * 2 2 2 2 1 1 1

Toxicologicalcharacteristic

3 3 3 3 3 1 1

Anti-static 1 1 1 1 1 1 3

Installation effort,specialists and

mates

3X-

2X-

3X-

2X-

2X-

2X-

1-X

Weight 3 3 3 3 3 3 1

Maintenance 3 2 3 2 1 1 1

Air tightness * 3 1 3 1 1 1 1

SUM of* criteria

8 6 7 5 4 3 3Technically advantageous

Example:High demands on the air quality (no corrosion), minimum energy loss(airtight and hydraulically smooth inner walls), simple installation,normal working pressure, e.g. 7 bar (g).Used in the following areas: air and space travel precisionmechanics/optics/watch and clock making wood processing electrics textiles printing food industries office machines/DP machine tools chemicals.

1 = very good, 2 = acceptable, 3 = with limitations1) DIN 2448 according to quality specification to DIN 1629

9. The Air Main

* = identification of thecriteria relevant to theindividual selection


9.7 Identification of pipelines

Medium Group Colour and number

Air 3 grey RAL 7001

Water 1 green RAL 6018

Inflammable liquids 8 brown RAL 8001

Gas 4/5 yellow RAL 1012

Steam 2 red RAL 3003

Acid 6 orange RAL 2000

Solvent 7 violet RAL 4001

Oxygen 0 blue RAL 5015

9. The Air Main

DIN 2403 and BS 1710 states that in theinterests of safety, correct maintenance andeffective fire-fighting methods, it is imperativethat pipelines are clearly identified according tothe flow medium.This identification must indicate dangers in orderto prevent accidents and personal injury.The colour code identification to DIN 2403provides immediate information on the mediumflowing at the point of identification

The colour code must be clearly marked:

at the start of the pipeline

at the end of the pipeline

at junctions

at wall breakthroughs

at fittings

using coloured rings along thecomplete length

Compressd Air8 bar

SignsUsing either text or codenumbers

3.1 8 bar

No. of the sub-group

Group number

Grey = Colour of Group 3 Air RAL 7001

Classification for compressed air


9.8 Save Energy and lower your CostsPlanning, Installation, Redevelopment of an Air Mainby Erwin Ruppelt

9. The Air Main

Compressed air may be a versatilecarrier of energy, but contrary toopinions that are still very widespread, itis a very expensive one. It only becomesa really economical option when all thecomponents of the air system airproduction, treatment and distributionare optimally matched to each other.Correct sizing of the system, correctmaintenance and, when necessary,appropriate refurbishing of the air mainare all requirements maintaining systemefficiency. The following articledescribes points that should beobserved in the trade.

If one takes all expenditure for energy,cooling, maintenance and depreciation ofthe compressor into account then a cubicmetre of compressed air, depending on thesize, loading, state of repair and type ofcompressor, costs between DQG0.03. In many plants, great value is placedon really efficient production of compressedair: older compressors are being replacedby efficient fluid-cooled screw compressors.Because of the more effective cooling andlower maintenance requirements of thesecompressors, up to 20% of former costs forthe production of compressed air can besaved. Naturally, it is precisely in the tradewhere many uses for reciprocatingcompressors are still to be found. Theseinclude transportable and mobile workshopcompressors that are often used to powerair tools on construction sites as well. Inthese cases, where easy transport andhandling are paramount, oil lubricated,single-stage reciprocating compressors givethe craftsman the possibility of usingcompressed air economically and flexibly.The lower delivery limit of mobilecompressors lies at around 20 l/min, theupper limit at around 450 l/min; their workingpressure is normally between 4 and 10 bar.Pressure up to 20 bar are possible forspecial applications such as in glassworking (fig. 1).

Also, single-stage reciprocatingcompressors with gauge pressure up to 10bar are well suited as reasonably pricedstationary supplies in smaller workshopshaving a low air demand.

In the displacement range between 120and 2000 l/min they are the ideal solutionif there is no continuous compressed airrequirement. In contrast, in workshopsthat need working pressures up to 15 bar,

Fig. 1: Air at a maximum gauge pressure of 10 or20 bar, needed for inserting putty in windows forexample, is generated by transportablereciprocating compressors of this series. Thesesmall, powerful compressors can be takenpractically anywhere and can be used for manyof the different craftsmans tasks

a two-stage reciprocating compressor isrecommended they are ideal ifcompressed air is not required constantly.In contrast, workshops needing a workingpressure up to 15 bar, a two-stagereciprocating compressor isrecommended. The duty of both thesingle and the two-stage compressorshould not exceed 70% however,otherwise under higher load conditions,the maintenance requirement would bedisproportional. The same applies if theair delivery exceeds 2000 l/min orcompressed air is demandedcontinuously. In such situations, it is farbetter to use a suitably sized screwcompressor because a reciprocatingcompressor is distinctly inferior to a screwcompressor when maintenance costs andefficiency are compared.


9. The Air Main

In most workshops, much attention is paidto reasonably priced air production.Correct treatment of the application isusually ignored. This is to be regretted,because only properly treated air reducesmaintenance costs for consumer equipmentand the air distribution system.In 80% of all applications refrigerationdryers are more than sufficient for such airtreatment.

Fig. 2: A compact, ready to use systemcomprising a fluid-cooled screw compressor,refrigeration dryer and 300 l air receiver provideseconomical compressed air production, dryingand storage for workshops with an air demand ofaround 1100 l/min

If one accepts average production costs of0.02 SHUP3 of compressed air then theannual loss amounts to 5,256.- 7KHOHDN-rate in a well maintained compressed airnetwork should not be more than 10% of totalair consumption. The average value through,is between 20 and 25%. This means that aloss equating to that given in the example isnot the exception but rather the rule. Theleak is not confined to one isolated point butnormally comprises countless small leaks inthe hundredth or tenth of millimetre rangeand amount to quite large losses whenadded up together.High losses of energy are also caused if thecompressor installation is enlarged to combatincreasing air demand, but air main bores arenot increased to match the new conditions. Ina modern compressed air system thepressure loss in a fixed air main should notexceed 0.1 bar. In fact, the average value inmost plants is much higher at around 0.7 to0.8 bar. In other words: if a pressure loss of0.8 bar in a fixed air main was reduced to 0.1bar through refurbishing then 4.2% of thepower costs could be saved. If a powerrequirement of 25 kW is taken as a basis,this saving corresponds to 1.05 kW everyservice hour. If the total annual service hoursadd up to 8,000 then the annual powerrequirement would reduce by 8,400 kWh, orfor a kWh price of 0.11 FRVWVDYLQJVRI924.- ZRXOGEHPDGHThey often save on filters (with theirpressure losses) and consume only 3% of

the overall power used by the compressorto produce the corresponding volume ofcompressed air. Added to this are thecost savings brought about by lowermaintenance and repair effort on pipelinesand pneumatic equipment that are up toten times the investment needed fordrying. In workshops with an air demandup to 1100 l/min there are reallyeconomical, compact systems availablethat are ready to plug in and use,comprising a fluid-cooled screwcompressor, a refrigeration dryer and anair receiver (fig. 2).

A defective air main is direct cause ofwasted energy

Leakages in the air main causeconsiderable losses of energy andincrease power costs. At 6 bar air mainpressure, a leak of 3 mm diameter causesa pressure loss of 0.5 m

3/min! In an hour,

that is 30 m3. On the assumption that an

air main is pressurized the whole yearthrough, the loss amounts to 262,800 m

3.

AS 31

Fig. 3: The pressure drop between thecompressor and the pneumatic equipment ismaximum 0,8 bar in a well designed air main

Planning a new air main

The first decision in the design of a newair main is whether air should be suppliedon central or decentral basis. Inworkshops in the trade a central system isgenerally installed. In such installations,none of the problems arise that occur incentral systems supplying large plants,namely high installation costs, freezing ofinsufficiently insulated outdoor pipelines inwinter or increased pressure dropscaused by very long pipelines.

Pressure drop: 0,8 bar


9. The Air Main

Right sizing

Pipe size should always be calculatedfirst. The basis of this calculation is amaximum pressure drop of 1 bar betweenthe air consumers, including normaltreatment of the air (refrigeration drying),(see fig. 3).

The following pressure losses can beassumed:

main supply line 0.03 bardistribution line 0.03 barconnecting line 0.04 bardryer 0.3 barfilter, regulator,lubricator and hose 0.6 bar

total (maximum) 1.0 bar

Looking at this list you can see howimportant it is that the pressure losses inthe individual pipeline sections arecalculated together with all fittings andshut-off valves, etc. Just inserting thestraight length of pipe into the formula ortables is not sufficient. More important,the equivalent pipe length of the pipelinemust be calculated. Generally, at the startof planning, it is impossible to know theoverall composition of the air main with allits fittings and shut-off valves, etc. Tocompensate for this, the equivalent pipelength is calculated by multiplying thestraight length of pipe in metres by thefactor 1.6. The bore size of the pipelinecan then be evaluated using a straightline graph as shown on page 10.

Laying pipelines with energy saving inmind

Pipelines should be laid as straight aspossible in order to save energy. Bendscircumventing pillars, etc. can be avoidedby laying the pipeline in a straight linenext to the hindrance. High pressurelosses caused by 90corners can bereduced with 90large radius bends.Instead of water traps, which are stilloften encountered, ball valves or butterflyvalves with full bores should be used. Inthe wet pipe section, i.e. in moderninstallations in the compressor space, theinlets and outlets to the main pipe mustemerge in the form of an upward loop orat least to the side. The main pipelinemust be installed with a fall of 2% and a

condensate drain must be fitted at the lowestpoint of this line. In the dry area the pipelinescan be installed horizontally and the outletscan point directly downwards.

Steel or plastic pipe materials?

In view of material characteristics, no generalrecommendations can be made with regardto the material selection. Prices are no guideeither to making a positive decision.Galvanized, copper and polymer pipelinescost about the same when installation andmaterial costs are considered. Theinstallation of stainless steel pipelines cancause a cost increase of about 20%,although, in the meantime, better installationmethods have brought about lower prices.Many manufacturers these days providetables from which optimum conditions forevery pipeline material can be taken. Beforethe final decision is made however, thesetables should be studied in detail, the in-service loading taken into account and aspecification catalogue for the pipelineswritten. This is the right way to make a reallygood choice of materials. The individualpipelines should be connected using eitherwelding or solvent welding or screw andadhesive techniques. Even though removal ismade more difficult, these types ofconnections are very secure and leakagesare kept to a minimum.

Refurbishing an existing air main?

Although many faults and subsequentproblems can be avoided when a new airmain is designed, refurbishing of an existingpipework is often thwart with difficulty. Thiscan become a really helpless task if the airmain is subsequently fed with moist air again.Before any refurbishing is attempted, acentral drying unit must be installed.If, after installation of an air treatmentsystem, the pressure drop within the air mainis still quite large even though sizing issufficient, then deposits within the pipescaused by contaminants drawn along by theair are the problem. These contaminantshave reduced the available cross-section offlow to a minimum. If these deposits havealready hardened then in most cases onlyreplacement of the section can solve theproblem. Often though, if reduction is not toograve, the flow cross-section can beincreased by blowing through and drying outthe pipeline.


9. The Air Main

The amount of leakage is then calculated onthis basis using the following formula:

VC x tVL =

T

VL = leakage volume in m3/min

VC= compressor volume flowin m

3/min

t = time the compressor ran on loadin min

T = total time in min

To determine the leakage caused by theconsumers, all tools, machines and the sumtotal of all leaks are measured (see page 5,fig. a). Then the shut-off valves immediatelyupstream of the consumers are closed andair main leakage is measured (see formulaand page 5, fig. b). The difference betweenthe sum total of all leaks and air mainleakage itself equals the leakage of theconsumers and their fittings. To locate theleaks more precisely, the measurementshould be carried out several times.

Experience shows that about 70% of theleaks are to be found in the last few metresof the network, i.e. at the final take-off points.These leaks can be located with the help ofsoapy water or special sprays. An air mainoften suffers from numerous large leaks if itwas originally a wet main fitted with old jointssealed with sisal and then supplied with dryair. These joints dry out after a while, causingleaks. Ultrasonic equipment is recommendedfor precise location of air main leaks.

When all these leaks have been located andremoved and the cross-section of thepipelines matched to the current airconsumption, the resulting air main is (again)an efficient compressed air distributionsystem.

Narrowed branch lines can be improvedby laying a new line in parallel with theold one (fig. 4a, overleaf). In the sameway, a second ring solves the problemscaused by a narrow ring main (fig. 4b,overleaf).

If such a dual branch or ring main iscorrectly designed, then increasedreliability is gained as well as anoticeable reduction in pressure drop.Another method of reducing pressuredrop is the integration of so-calledintermediate networks (fig. 4c, overleaf).

All these refurbishing measures only leadto success if the leaks in the existing airmain are reduced to a minimum.However, before searching for leaks acheck of the overall leakage volume mustbe made.

With the help of a compressor relativelysimple methods are used to measure thelosses involved. All consumers are shutdown and then the cut-in times of thecompressor over a defined period aremeasured (see page 4).

Fig. 4: Refurbishing or enlargement of an existing airmain using a parallel pipeline (4 a), by installing asecond ring (4 b) or installation of so-calledintermediate lines (4 c); 1: screw compressor; 2:hose; 3: ball valve; 4: pressure switch

Old air mainExtended air main



the air main

Documents

air consumers

air dryers9

air losses9

hduleakagescompressed

air receiver9

air receiver volume

compressed air installation

air lossesair losses