Understanding Water Hammer 8

[rgvrider]

I've been doing some reading on water hammer and trying to 'really' understand it.

I've been asked to look at a relatively simple problem of a pump station pump water up a relatively constant slope. Initially this was to be done by others, but I was keen (stupid) to learn.
Details:
Flow: 50 MLD
Head: ~250 m (~200m static)
Length: 10,000m
Diameter: ~1000mm
Pipe: Steel, ~10mm thick

I've assumed the most likely cause of water hammer will be due to pump trip as; pump isolation valves are manual, there is a control valve near discharge, but this is relatively 'slow' (over 60s open/close). At this stage I've assumed pump will take less than 5s before it's head drops below static head, hence flow 'stops'.

I've assumed time for 'valve closure' (in this case pump stop) is 5s, therefore given celerity is in the order of 1100 m/s, this disturbance is rapid (5s < 2L/a < 20). Therefore elastic theory should used.

Elastic theory indicates that I'll get a pressure rise of about 100m.

I have two question:

1. Does a pressure rise of 100m seem to be accurate based on my simple analysis? When the pump stops, the water column continues to move, causing a low pressure at the pump discharge. This low pressure wave travels up the water column. The low pressure cause the water column to stop, then it will 'pull' the water back, at which point it will collide with the check valve and cause a pressure spike. Should I assume that if the spike is 100m, the low pressure will be -100m (if static head 200m, then 100m)?
2. It has been suggested that a fast acting non return valve can reduce the pressure spike. Is this true and if so how? Or does it just protect the pump?

 

[rneill]

I'm not sure that your analogy of the pump stopping as being equivalent to a slow closing valve is necessarily correct. In the water hammer case where we look at a closing valve, the upstream pressure remains constant and we have a certain amount of momentum in the fluid that wants to keep moving but can not. In the case of a pump trip, there is nothing to stop the fluid momentum from carrying it forward but we gradually lose the upstream pressure and so the "high" downstream pressure starts to want to drive fluid back the other direction. If the fluid is allowed to reverse direction and then is subsequently stopped (perhaps by a slow closing check valve), we can have water hammer. If on the other hand we are able to stop the fluid from reversing direction, or we can catch it before it builds appreciable reverse momentum, we will avoid water hammer.

A fast acting check valve will minimize the chance for reverse flow to occur and will minimize the potential for water hammer. Consequently, the potential for water hammer in a pump trip scenario should I think be dependent on the characteristics of the check valve and I'm not sure that looking at the pump as a "valve closure" fits the situation.

An old valve catalogue I have says the following:

"forward flow continues for a while after the pump is switched off, but the downstream pressure decelerates the flow more rapidly and then reverses its direction. Without a check valve, the reverse flow would increase and stabilize at some value, unless the downstream pressure declined. An "ideal" check valve would allow no reverse flow and would close exactly at the time the velocity curve passes through zero; there would be no water hammer. A "real" check valve starts closing while the flow is still forward, but it lags the velocity curve. Still, with fast response, it closes before a high reverse velocity develops and thus minimizes the water hammer surge."

This particular catalogue recommended tilting disc check valves in pump discharge applications because of their rapid response relative to other designs but I don't think they are all that commonly used anymore. I know one end user with a strong preference for Durabla check valves in pump discharge applications.

Anyway, I'm sure many of the reputable check valve manufacturers can provide all sorts of information on this.

 

[stanier]

Suggest you download a demo copy of AFT's Impulse.

http://www.aft.com

you get the manual with it and you can see some of the issues.

Also go to

http://www.pipingdesign.com

and look at papers on surge.

Check out

http://www.engineersaustralia.org.a...7-0528-B401-36CA-F0E6D07C1841&siteName=ieaust

Also find a copy of Fluid Transients in Pipelines by ARD Thorley.

For check valves you cannot go past

http://www.noreva.de

for non slam valves.

 

[rgvrider]

hmmmm. I think I may have discovered my mistake.

The pump is 'driving' the water up the hill. Due to pump inertia, the pump continues to drive the flow and until the check valve closes. If it is a perfect check valve there will be no pressure surge. Is this correct?

So with a pump, when it trips there is a complication as the system can still draw water through the pump. However, if the pump had no inertia, it would stop instantly. The water column has momentum so it takes a discrete time to stop. As the discharge side of the pump has higher pressure than the suction, no water would flow through the pump (it may even reverse flow). The check valve will remain open until the flow stops (assume perfect check valve). When the check valve closes, the local pressure around the check valve should be less than the static pressure (due to moment of water). The water column must reverse flow and then there would be a surge. Is this correct?

I realise I'm mixing equipment that ignores some rules of physics and that this may make my examples invalid.

 

[BigInch]

"If it is a perfect check valve there will be no pressure surge. Is this correct?"

Not correct. The check valve (if it survives) will stop the surge from continuing back to the pump. There may be sufficient volume between the check and pump impeller to attempt to reverse spin the pump for a fraction of a second, or at least slow it down considerably. Its common petroleum pipeline design practice to consider the cases where the pump trips and also where a check or other control valve, or an automatic shutdown valve fails.

"When the check valve closes, the local pressure around the check valve should be less than the static pressure (due to moment of water)."

When the check valve closes, the flow has stopped AT THE CHECK VALVE, but the flow farther downstream is still moving along so you will not yet have anything like a static pressure situation at the check valve. When the check valve closes all you can say is that the pressure upstream is less than the pressure downstream at that instant in time. When the moving fluid column impacts a closed valve, say at the end of the pipeline, (or just runs out on an uphill section) the algebraic sum of the velocity head converted to pressure plus original operating pressure there will then attempt to accelarate the column from that end back to the pump station. That reversed pressure wave will not reach the check valve until a time equal to the pipeline length/sonic_velocity has expired.

Caution: Should lowest pressure go below the vapour pressure of water, the fluid column will part at that point creating a vapour space filling with water vapour as the downstream portion of the column continues to move along. The reverse pressure wave will probably be sufficient to collapse that vapour space on its return and may increase pressures well above that predicted by elastic theory, as the two segments of the fluid column impact each other.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[MortenA]

If avoiding water hammer were as easy as installing a check valve - then I think some of us would be out of jobs.

Anyway - indicating that you are pumping uphill is important - not downhill or over a hill.

If the"elestic theory" i Joukowsky -then its usually pretty accurate - although conservative when disregarding risk of column separation.

Best regards

Morten

 

[MortenA]

As Stainer suggests:

If your problem is sufficient complicated then you should consider simulation - although i use different software (Flowmaster or pipeline studio (formerly TGNET/TLNET)).

Best regards

Morten

 

[4Pipes]

The normal flow velocity is relatively modest that you still have a decent static head compared with friction. I would agree with earlier posts that you will still need a properly engineered check valve. I would add that you also need a very heavy anchor support at the check valve and a spare pair of trousers. Take absolutely no notice of the smart asses who tell you that the anchor support is over designed. Stand them next to the check valve when you trip test the pump - have the trousers ready just in case. (or pants if you are in the US).

 

[rgvrider]

Thanks for the feedback.

I played around with some modelling programs and they confirmed the 'simple' Joukowsky result.

I think that my original reasoning of check valves was correct. That they can't reduce water hammer, but they can increase the problem due to rapid closure at reverse velocities. Hence increasing the rate of deceleration.

Cheers.

 

[stanier]

You may be interested in this presentation given to ASME/Engineers Australia and IMechE.

I reiterate the suggestion to get a copy of Prof Thorley's book Fluid Transients in Pipelines. He did a lot of research into the behaviour of check valves including some with Delft laboratories Also check out the Delft website as there are some good technical papers to be found.

 

[rgvrider]

Stainer, it's on the shopping list.

 

[LSThill]

Technical Reference

ASME How to Predict Thermal-Hydraulic Load on Pressure Vessel and Piping PD382 & ASME Introduction to Unsteady Thermofluid Mechanical PD382
Frederick J. Moody
General electrical Company
and
Mechanical Engineering Department
San Jose State University
San Jose, California

 

[BigInch]

Thermal-hydraulic load on vessels, sounds like a bit OTT for "trying to understanding waterhammer".

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[LSThill]

Mr. BigInch (Petroleum)

How to Predict Thermal-Hydraulic Loads on Pressure Vessels and Piping PD382

Frederic J. Mooody

Mr. Moody has worked on thermal-hydraulic problems at the General Electric Company in the design and analysis of boiling water nuclear reactors, containment, components, and fluid flow systems for over 41 years. His specialties have include the application of fluid mechanics, thermodynamics, eat transfer, and fluid-structure interaction theory to postulated accidents, transients, and safety analyses. He have been an adjunct professor in the Mechanical Department at San Joes University for 27 years, and an instructor for company sponsored advanced engineering course for 25 years

He received his Ph.D. in Mechanical Engineering from Stanford in 1971. Honors included the George Westinghouse Fold Medal Award in 1980 for his contributions to two-phase flow and reactor accident analysis, the ASME Pressure Vessel and Piping Award in 1999 for contributions to unsteady flow technology, and induction into the Silicon Valley Engineers Hall of Fame in 2000 for technical contributions to the energy industry and mentoring numerous engineers. He was also elected to the National Academy of Engineering in 2001. He has been an ASME Fellow since 1982

Not able to upload 15 pages

Predicting preoperative flow loads

Waterhammer loads

Please advise if required

Leonard Stephen Thill

 

[BigInch]

Oh yes, I'm sure its a good book, but the OP is simply trying to "understand" waterhammer. That may be good reading later, but perhaps slightly advanced right now. He was only talking about pumping water up a hill. Nothing about pressure vessels, nothing about structural interactions, nothing about thermal loads. Since the book title was so off topic, I just wondered why you suggested it. There must be pleanty of books that you know of that address only the basics of waterhammer, so why that one?

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[LSThill]

TECHNICAL NOTE (2) OPTIONS:

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Water Hammer

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Setting an Appliances Parameters


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Interact has been funded by the Teaching and Learning Technology Programme, jointly funded by the four UK higher education funding bodies, HEFCE, HEFCW, SHEFC and DENI.
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Dr Ruth Thomas
CEE, Heriot Watt University
Riccarton, Edinburgh, EH14 4AS
Email: rct@eng.cam.ac.uk
Tel: (0131) 449 5111
Fax: (0131) 451 3327

University of Cambridge > Department of Engineering
Forbidden Access
The Interact project is complete. Products are only being maintained for local use. Contact Tim Love (tpl@eng.cam.ac.uk) for details.

TEXT FILE SIZE = 2,189 KB VERY GOOD REFERENCE AND GOOD READING

SECOND OPTION (2)
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L S THILL

 

[BigInch]

Here's a legal download for you,
Corps of Engineer's WHAMO program,
Have fun.

http://www.cecer.army.mil/usmt/whamo/whamo.htm

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[IRstuff]

Dynaflow's downloads can be found here:

http://www.dynaflow.com/research/papers/

TTFN

FAQ731-376

 

[DSB123]

LSThill,
From your first post can you clarify a few items:-

1. Is the guy's name Moody or Mooody?
2. What is "eat" transfer - something to do with calorific exchange??