Effect of Check Valve on Water Hammer Shock-Energy Dissipation

Assuming flow is from left to right in both cases, the check valve is there to prevent flow from right to left in the event of a lower pressure on the left than in the reservoir on the right.

To prevent sudden changes in flow causing a reaction movement in the pipework, a.k.a. "water hammer", change the position of the shut off valve in a non-abrupt manner.

Dear all,

1. The flow direction is from right to left, so it is the flow allowed by the check valve.

When a valve shuts off suddenly, specially at the end of a long pipe with high speed flow, the complete pipe is pushed by the hammering in the direction of the flow.

The check valve it is used normally to protect the pump (when existing) against the back-blow, or to avoid backflow, if the layout of the installation allows it.

2. Nonetheless, with the diagram sent, I absolutely agree with the slower operation of the closing valve as better approach. The check valve as it is showed, it is a "brute force" stopper for the back-blow.

3. I do not understand why dissipation by friction would be possible in the first case.

Friction would exist if the fluid was allowed to move, and in the first case this movement it is prevented by the check valve.

¿Could somebody correct/ellaborate about this point?

4. If the pipe lenght, the fluid speed and the process requirements, make absolutely necessary a fast closing operation, I would consider the use of an accumulator as damper just before the valve to reduce the hammer effect.

Salu2 cordiales,

Abel

<...3...not understand why dissipation by friction would be possible in the first case. Friction would exist if the fluid was allowed to move, and in the first case this movement it is prevented by the check valve. Could somebody correct/ellaborate about this point?...>

The fluid moves as determined by the shut-off valve. The check valve is there to prevent reverse flow.

I don't see that the check valve would have any effect on the water hammering phenomena, which is usually caused by the momentum of water flow being abruptly halted, resulting in a lurching motion of unsecured pipe...The cure would be to provide an expansion area that the force can dissipate into....

http://www.plumbingmart.com/water-hammer-information.html

Very nice link!

The "echoes" it refers to, they are the successive rebounds of the water hammer in the system.

I have seen pipes shifted backwards due to the back-blow, because supports and fixations considered only the hammer in the flow direction.

In any case, it is better to avoid it completely. (Except in a water hammer pump!!! ;)

Sorry, there is a correction in the above post.

The intention to convey was, Initial steady flow is from Reservoir towards shutoff valve and without check valve in circuit, reverse flow to the reservoir is possible during the propagation of the water hammer shock wave cycle, and so friction losses are possible. But with Check valve, reverse flow of liquid is not possible and so how the shock energy will dissipate. So

Read this "How the shock energy will dissipate in the second case" as "How the shock energy will dissipate in the First case"

Read this "I understood that in first case the energy dissipation by friction is possible" as "I understood that in Second case the energy dissipation by friction is possible"

Thank you

I'd like to add. after check gets closed waves travels between ends a longer time if it was an engine cilinder i would say that fenomena is similar to "tumble" in engines, then to make a more or less sharped frequence response ,designers could change in several forms the head cilinder.Back to the case i wonder if differents pipes length with closed ends at the middle in between the check and shut valve could have similar effect than a damper.-

Sir,

My doubt is in "The mass keeps traveling against the now closed valve,and sets up compression waves, that travels back and forth, until friction or a buffer (with friction) absorbs it"

Once the valve closed, compression wave travels towards reservoir. Since the check valve is there (downstream of check valve high pressure & upstream low pressure), it will get closed and so compression wave can't reach reservoir. So wave can't travel back. Because as compression wave reaches reservoir, only then wave will travel back towards valve as liquid flows into the reservoir(reverse direction)

Please correct me.

Thank you

Liquids are very stiff for compression. Hence a compression wave is a high peak pressure and very low amplitude phenomenon traveling at the speed of sound. Comparably, checkvalves I encountered are very large amplitude, slow devices. A small amplitude wave will not do much to it, in the time it takes the wave to decay.

The buffer eases the situation by allowing the wave to expand into a small volume. It cuts the peak pressure, and provides added friction for absorption.

Other than that, I have no strong feeling about either solutions. In particular, since 0msec control valve does not exist. Hence you have to decide, which of the models predominate: the one with abrupt no flow, or the other with gradually stopped flow.

When the shut off valve is in the closed position the pressure on both sides of the check valve are equal.

In a hammer situation an oscillation will develop between the two valves and the energy will dissipate via friction inside the check valve as it chatters to its steady state position.

If the check valve is closer to the reservoir then the pipe friction will have more of an effect.

A hammer arrestor would be a prudent addition here if the shutoff valve is closed abruptly and frequently.

Alternatively place the check valve downstream of the shut off valve then the reservoir can absorb the energy too.

Who's on first? What's on second? I am confused somewhat to say the least

Gravity flow? Or pumped? Definitely makes a difference.

The check valve will induce/create a mechanical shock wave in the system when/if the fluid flow reverses and the check assembly closes.

The only way this would not happen is if the control valve were to close first and the distance between the control valve and check valve is very minimal.

The severity and magnitude of the reaction is dependent upon flow magnitude, pressure drop across the check valve, check valve closing time, control valve closing time, and distance of fluid travel (length of pipe).

If you are attempting to lessen or control the affect of the hammer then installing an accumulator or lengthing the control valve closing time interval will reduce the affect.

Maximum affect would be attained by installing an accumulator and adopting/applying the proper timing curve for closing the control valve. (Initial values can be calculated but fine tuning requires the "Trial and Error" method)

You did not give the critical information as to what type of valve is being used which is paramount to reaching a valid conclusion.

Example:

If this is a "real" existing application and the control valve is a 90 degree operating ball valve, then; the closing operation is extremely fast, the flow interruption is very non-linear and adopting a slower time curve for closing the valve would have a small affect on the induced hammer.

If this were the case, using a "V" ball type unit will affect the flow in a very linear fashion and provide much better control of the hammer though it will not totally eliminate it.

Use of a plug or globe valve provides a much more linear flow control range that is significantly more conducive to eliminating hammer.

Many of these types of valves have the option to specify the proper size of internal flow cavitation ring (cage) which will even further help reduce and/or eliminate or at least minimize the vibration and hammer.

Hope this helps,

Jim