water hammer

[130684]

dears,
does anyone know if pressure waves caused by suddenly closing of a valve are mitigate by the elbows situated upstream of it?

if i have a 2500 m pipe length and from the last elbow to the CV are 50 m should i calculate the "critical time" with the total length, or only for the last 50 m?

thank you,

 

[130684]

Which is a reasonable value of required time to close the valve as %of critical time (in water hammer issues)?

 

[gustorf]

I may misinterpret your question, but you are facing water hammer by check valve closure, right?

If yes, ask check valve manufacturers for reverse velocity figure below 0,3 m/s, when the closing element hits the seat. This may only be achievable using non-slam nozzle check valves like made by Noreva.
These valves close in 0,2 to 1,0 seconds slam-free due to their short stroke.
Hope this helps.

 

[130684]

Dear Gustorf,
Thank you for your answer.
I read that water hammer is produced by the suddenly closure of a valve. The fluid hits the valve, and then it creates a backward pressure wave (through the hole pipe system (bend, valves, etc), hitting at the end the check valve) leading to mechanical damages.
To avoid this issue (suddenly closing of the control valve and pressure waves), a critical closing time should be calculated for the control valve.
Is there any recommandation for closing time as %of the calculated critical closing time?

 

[ione]

The presence of an elbow reduce the length of straight pipe, and consequently mitigate the effect of pressure waves.
Please don’t forget to take into account the type of valve you’re dealing with. Butterfly valves allow a considerable flow even when they’re partly closed, and so the time necessary to go from say 80% closed position to complete closure should be considered (check Cv vs open %).

 

[gustorf]

130684,
I am not into control valves, you may want to consult waterhammer analysis experts like at

http://waterhammer.hopout.com.au

 

[130684]

Thank you for support.

 

[gerhardl]

Further tip: use this site's search engine (top of the page) using for instance 'water hammer calculation' and both threads and sites for calculations will show up.

As others have commented: in practical engineering the clue is to select components (valvetypes), piping layout and closing sequences to avoid waterhammer (non-slam nozzle valves or soft closing if checkvalves, slower last 1/3 closing if motorized, good solutions to empty pipes of air, etc. etc) all adapted to your special conditions, normal use and failure analysis

 

[CMA010]

Elbows do not mitigate water hammer. In order to estimate closing times follow gustorf's suggestion, contact a consultant or an engineering firm.

 

[stanier]

Suggest you get a copy of Fluid Transients in Pipeline Systems by ARD Thorley or Pressure Tarnsients in Water Engineering by Ellis.

Waterhammer is like backgammon, you can learn to use waterhammer analysis software (ie play the game) in a matter of days but it takes a lifetime to understand it.

Find someone who knows what they are doing.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/