hammer v8i ss3 si qanda formdocument mar-2012

8/10/2019 HAMMER V8i SS3 SI QandA FormDocument Mar-2012

1/20

HAMMER, Transient Analysis and

Design - Results Tables/Questions/Answers

Version: V8i (SELECTseries 3)Units: Metric

This document has been created so that you

can easily input your answers into the resultstables and answer the questions.


2/20

Mar-12 2 Transients in an Unprotected Pipeline - Q and A

Copyright 2012 Bentley Systems, Incorporated

Transients in an Unprotected

Pipeline - Q and A

Results Tables

Alternatives

Without

Protection

HT-1 Air Valve Surge Tank

with Check

Valve

Surge Tank

no Check

Valve

SRV

Pressure (Maximum Transient) @ PJ2(bars)

Pressure (Minimum Transient) @ PJ2(bars)

To be completed in Workshop:

Pipeline Protection

Vapor Volume (Maximum Transient)@ P2 (L)

Head (Maximum Transient) @ P5 (m)

Alternatives Junction Pressure (Maximum Transient) (bars)

PJ2 J2 J3 J4 J5 J6

Steady State Pressure 9.8 6.5 6.4 7.2 7.6 3.6

Max. Allowable Pressure 14 14 14 14 14 14

Without Protection

Hydropneumatic Tank(HT-1)

Air Valve To be completed in Workshop: Pipeline Protection

Surge Tank with CheckValve


3/20

Mar-12 3 Transients in an Unprotected Pipeline - Q and ACopyright 2012 Bentley Systems, Incorporated

Results Tables

Surge Tank no Check Valve To be completed in Workshop: Pipeline Protection

SRV


PJ2 J2 J3 J4 J5 J6




4/20


Workshop Review

Workshop Review

Now that you have completed this workshop, lets measure what you have

learned.

Questions

1 Where does the largest vapor pocket occur and what is its maximum

volume? Explain the mechanism behind the development of this vapor

pocket.

2 Select the Time History for P1:J1and click on Animate. When does the

vapor cavity at junction J1start to form and when does it close?

3 What happens to the flow at J1 at about 13.5 sec and what effect does

this have on the vapor cavity?

Enter your answer below:



Hint: In the Transient Results Graph you can click on the Data tab for easier viewing of graphvalues.


5/20


Workshop Review

4 Considering your answers to the previous questions and the role of the

vapor cavity in the transient event, what are some of the possible

strategies for reducing the upsurge?

5 Why is the pump (PMP1) not subjected to the transient upsurge?




6/20


Workshop Review

Answers

Alternatives

Without

Protection


with Check

Valve

Surge Tank

no Check

Valve

SRV


31


-1.0 To be completed in Workshop:Pipeline Protection

Vapor Volume (Maximum Transient)@ P2 (L)

446

Head (Maximum Transient) @ P5 (m) 723


PJ2 J2 J3 J4 J5 J6



Without Protection 31 27 27 28 31 26


Air Valve


To be completed in Workshop: Pipeline Protection

Surge Tank no Check Valve

SRV


7/20


Workshop Review

1 Where does the largest vapor pocket occur and what is its maximum

volume? Explain the mechanism behind the development of this vapor

pocket.

The largest vapor pocket occurs at J1 with a maximum volume of 446.4 L.

Water is being pumped up a steep rise to a well-defined vertical bend or

knee at node J1, after which the pipe turns about 90 degrees to levelout with a slight fall. After the pump trips and the vapor pocket forms,

water being pumped up the rise decelerates at a much faster rate than

the water in the more level pipeline after the knee. This causes the vapor

pocket to expand at the knee (J1) as these columns of water separate.

2 Select the Time History for P1:J1and Animate it. When does the vapor

cavity at junction J1start to form and when does it close?

The first vapor cavity opens at about 8.8 sec (when flow reaches zero) and

closes just after about 13.8 sec., when a large pressure rise occurs.

Note: Transient Tip: The first vapor pocket at a well-defined knee (J1) usually opens-up as flow in the upstream segment (P1) reaches zero: the departing liquid

column in the downstream pipe (P2) actually leaves a vacuum behind at J1.

This allows you to correlate the flow and volume graphs to get the

approximate start time of 8.8 seconds.

3 What happens to the flow at J1 at about 13.8 sec and what effect does

this have on the vapor cavity?

Flow returns from Res2 and results in a negative flow, or flow reversal, at

J1. This in turn causes the vapor pocket to collapse and results in a high

pressure upsurge.

4 Considering your answers to the previous questions and the role of the

vapor cavity in the transient event, what are some of the possible

strategies for reducing the upsurge?

Mitigating the formation of the vapor cavity is at the heart of the solution.

This could be achieved by installing a surge tank or gas vessel to ensure

that water is supplied to the area where the cavity would form without

surge protection.

5 Why is the pump (PMP1) not subjected to the transient upsurge?

The time history at the pump shows that the check valve closes before

these pressure waves reach the pump (zero flow), effectively isolating itfrom the system and protecting it against damage.


8/20

Mar-12 8 Pipeline Protection - Q and A


Pipeline Protection - Q and A

Results Tables

Alternatives

Without

Protection


with Check

Valve

Surge Tank

no Check

Valve

SRV


31


-1.0

Vapor Volume (Maximum Transient)@ P2/P-9/P-11/P-13/P-15 (L)

446

Head (Maximum Transient) @ P5 (m) 723


PJ2 J2 J3 J4 J5 J6





Air Valve



9/20

Mar-12 9 Pipeline Protection - Q and ACopyright 2012 Bentley Systems, Incorporated

Results Tables

Surge Tank no Check Valve

SRV


PJ2 J2 J3 J4 J5 J6




10/20


Workshop Review

Workshop Review


learned.

Questions

1 What effect does placing the gas vessel at J1 have on the formation of the

vapor cavity and the transient pressure spike?

2 Based on your investigation of other protection equipment, what might

be the most effective strategy for protecting the pipeline from transient

pressures?

3 How effective is the SRV in reducing the formation of vapor cavities?





11/20


Workshop Review

Workshop Results

Alternatives

Without

Protection


with Check

Valve

Surge Tank

no Check

Valve

SRV


31 15 31 13 10 12


-1.0 6 4 4 8 3

Vapor Volume (Maximum Transient)@ P2/P-9/P-11/P-13/P-15 (L)

446 0 0 0 0 379

Head (Maximum Transient) @ P5 (m) 723 486 603 489 460 530


PJ2 J2 J3 J4 J5 J6




Hydropneumatic Tank(HT-1) 15 11 10 10 9 4

Air Valve 31 22 21 21 22 14


13 10 9 10 11 6

Surge Tank no Check Valve 10 7 6 7 8 4

SRV 12 11 13 14 15 11


12/20


Workshop Review

Answers

1 What effect does placing the gas vessel at J1 have on the formation of the

vapor cavity and the transient pressure spike?

The gas vessel effectively stops the formation of a vapor cavity at J1 and

other locations along the pipeline. The high pressure spike resulting fromthe collapse of the vapor cavity is therefore eliminated.

2 Based on your investigation of other protection equipment, what might

be the most effective strategy for protecting the pipeline from transient

pressures?

The simple surge tank might seem like the most favorable alternative but

the construction of a 60 m tall tower would definitely rule this out as a

practical/economical option. The simple surge tank with check valve

would be a better option provided the system can accommodate a shortperiod of relatively low pressure that occurs just after pump shut down.

Alternatively the hydropneumatic tank is an excellent option in terms of

controlling transients, though, as a pressure vessel, the construction and

maintenance costs are likely to be higher than a simple tank. A cost

benefit analysis on these options is required to come up with the final

decision.

3 How effective is the SRV in reducing the formation of vapor cavities?

The SRV is effective in confining the occurrence of sub-atmosphericpressures to a small portion of the pipeline adjacent to SV-1 and Res2,

however a sizable vapor cavity is allowed to form near SV-1. The reason

for this is that the SRV works to control the symptoms (high pressures)

and not the cause. Other strategies are directed at restricting the

formation of the vapor cavity.


13/20

Mar-12 13 Network Risk Reduction - Q and A


Network Risk Reduction - Q

and A

Workshop Review


learned.

Questions

1 Select Path1 in the viewer and click animate. When does the vapor cavity

open and collapse?

2 What causes the vapor cavity to collapse and what happens to the local

pressure when it does?




14/20

Mar-12 14 Network Risk Reduction - Q and ACopyright 2012 Bentley Systems, Incorporated

Workshop Review

3 Why is the upsurge resulting from the pump restart less than that for the

initial pump shutdown?

4 How long does it take the system to return to near steady state

conditions?

5 Apart from the area adjacent to junction J1 where else in the network do

you find appreciable formation of vapor cavities?





15/20


Workshop Review

6 Given that the subdivision may contain older pipes, what can you say

about the transient pressures that are experienced in the network,

especially along Path3?

7 What effect has the addition of the subdivision had to the transient

pressures in the main transmission line (Path1)? Set up the pump trip as

per Lesson 1 and compare the results to those obtained in that lesson to

confirm your answer.

8 Experiment to learn the sensitivity of this system to an automatic,

emergency shutdown and restart:

Set different shutdown and restart ramp times for the pump. Forexample, try 10 second ramp times for the pump. How fast does the

flow decrease to zero? Why?





16/20


Workshop Review

Select different time delays between the pump shutdown and restart.

What happens if you try to restart the pump when pressure is at its

lowest, rising, or highest?

9 What are the fastest ramp times and shortest time delay which do not

result in unacceptable transient pressures anywhere in the system? Since

the maximum transient envelopes depend on these two variables, several

valid solutions are possible.




17/20


Workshop Review

Answers

1 Select Path1 in the viewer and click animate. When does the vapor cavity

open and collapse?

The vapor cavity begins to open and about 8 seconds and collapses at 16

seconds.

2 What causes the vapor cavity to collapse and what happens to the local

pressure when it does?

The flow returning to the pump station causes the vapor cavity to collapse

and results in a large pressure upsurge at junction PJ2.

3 Why is the upsurge resulting from the pump restart less than that for the

initial pump shutdown?

The flow generated by the pump restart helps to prevent subsequent

formation (and therefore collapse) of vapor cavities. The pump restarts at25 seconds or 20 seconds after the start of the emergency pump

shutdown, just as the high-pressure pulse from the collapse of a vapor

pocket at node J1 is reaching the pump station. This pulse closes the

check valve against the pump for a while, until it reaches its full speed and

power at around 30 seconds.

4 How long does it take the system to return to near steady state

conditions?

The system approaches a new steady state after 50 seconds and it has

essentially stabilized to a new steady state by 90 seconds.

5 Apart from the area adjacent to junction J1 where else in the network do

you find appreciable formation of vapor cavities?

There is appreciable vapor formation at the localized high point in the

network at junction J19.

6 Given that the subdivision may contain older pipes, what can you say

about the transient pressures that are experienced in the network,

especially along Path3?

Maximum transient pressure heads are of the order of 100% above

steady-state pressures along the majority of Path3. This is likely to be

very significant compared to the pipes surge-tolerance limit.


18/20


Workshop Review

7 What effect has the addition of the subdivision had to the transient

pressures in the main transmission line (Path1)? Set up the pump trip as

per Lesson 1 and compare the results to those obtained in that lesson to

confirm your answer.

The subdivision has the effect of dampening the transient pressures.

(Pump Shutdown scenario in Lesson3_Workshop 3_answer.wtg)

8 Experiment to learn the sensitivity of this system to an automatic,

emergency shutdown and restart:

Set different shutdown and restart ramp times for the pump. For

example, try 10 s ramp times for the pump. How fast does the flow

decrease to zero? Why?

Select different time delays between the pump shutdown and restart.

What happens if you try to restart the pump when pressure is at its

lowest, rising, or highest?

With a 10 second ramp time, the flow decreases to zero in 7.3 seconds

after the pump shutdown starts (versus 4.3 seconds for a 5 second ramp

time). With different time delays, there are only minor differences in the

results.

9 What are the fastest ramp times and shortest time delay which do not

result in unacceptable transient pressures anywhere in the system? Since

the maximum transient envelopes depend on these two variables, several

valid solutions are possible.

The pump needs to ramp down slowly in order to avoid negative

pressures at J19, so the ramp down time should be 140 seconds or more.

The resulting gradual ramp down means the delay before ramping up can

be very short (5 seconds or less), since the system doesnt need extra time

to stabilize. The pump can ramp back up to full speed relatively quickly

(approx. 5-10 seconds) without causing significant high pressure

transients.


19/20

Mar-12 19 Multipoint Network Protection - Q and A


Multipoint Network Protection

- Q and A

Workshop Review


learned.

Questions

1 Do vapor cavities or subatmospheric pressures occur in the protected

system?

2 Are high transient pressures experienced in the subdivision network?




20/20

Mar-12 20 Multipoint Network Protection - Q and A

Workshop Review

Answers

1 Do vapor cavities or subatmospheric pressures occur in the protected

system?

No sub-atmospheric pressures occur anywhere in the distribution

network.

2 Are high transient pressures experienced in the subdivision network?

High transient pressures are comparable to the steady-state pressures for

the downstream half of Path3-1. Keeping transient water pressures within

a narrow band reduces complaints and it could be important for certain

industries.

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