Diaphragm Valve hydraulic lock 1

[Frank Elliott]

I am wondering if closing the diaphragm valves in a system can cause hydraulic lock of fluid trapped inside and whether that can damage the valve
If this is common, does it have a name other than hydraulic lock?
Thanks

 

[LittleInch]

I don't believe diaphragm valves have a sealed off fluid section so no.

Valves like ball valves can see high pressure in trapped in fluid due to thermal expansion but not diaphragm valves.

If you have a specific valve in mind please post details.

Of course shutting off fluid in a system which then sees higher temperature can result in high pressures which need to be relieved using thermal relief valves.

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[Frank Elliott]

Sorry I will expand on my original question. Imagine 3 Diaphragm valves in series. The outside 2 are closed, then the center valve is closed. This would put pressure on the other two valves as the water tries to compress but does not.
Is this how it works? Is there a name for this "fault"

Thank you for your answer

 

[LittleInch]

Well given that the diaphragm is flexible I'm not sure that closing the middle valve will result in excess pressure or that the outer two won't flex a bit also to relieve the build up of pressure.

Is this a real scenario or a hypothetical one?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Many valves have the issue described by the OP. Gate valves, globe valves, and needle valves have parts that move into the valve and displace some liquid volume. This is rarely an issue in real life, but can be. Many dispensing valves take this into consideration and are designed for the opposite effect where, when the valve closes the volume in the valve body increases. These are called snuff-back valves and are used to prevent dripping after the valve closes.

 

[Frank Elliott]

Thanks for the responses. Yes this is a real scenario. Lately I seen an employee get water squirting out of the diaphragm from an near hose because of not following the "recommended" way for closing the system. I heard from people here when I started to push the water along to the vent or back to tank when closing a bunch of valves. And usually people do it and there are no incidents. This time that method was not followed and the person closed in on another valve and she seen water coming out of the diaphragm of another close valve. The procedures we have for shut down to not take this problem into account.

 

[bimr]

I believe that it is called a "trapped" liquid. Closing the valve quickly will create what is known as a water hammer. Diaphragm valves will close bubble tight with zero leakage.

This valve closing incident can happen with any type of valve and is generally not a problem with normal flow rates. Valves are also typically supplied with features that moderate the valve closing time to minimize the water hammer in the pipe segment.

If the trapped fluid in the pipe segment is subject to a thermal source such as the sun, the resulting thermal expansion may over pressurize the pipe segment.

 

[LittleInch]

It's not really water hammer which is a transient surge effect caused by sudden cessation of flow. It all depends the pressure you're seeing and what the rating of the valves and systems are.

You could have a situation where the locked in pressure is close to the valve design pressure when the action of closing an intermediate valve causes the pressure to rise, but I still don't see where the water is coming form unless the valve wasn't fully sealed.

But getting back to the OP, yes, it may result in pressures higher than the design pressure of your valve.

But as bimr says, the real pressure rise comes from thermal expansion of the liquid between closed valves. This is something that is normally evaluated during design and relief valves installed where required. Depending on the volume involved, size of pipe etc.

But anything which involves a procedure to prevent leakage is open to failure of the individual to follow the procedure. This is part of your safety evaluation as to whether a procedure is adequate or you need to install thermal relief valves to cope with this small volume of water.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

An accumulator would accommodate this displacement, whether thermal or physical valve displacement.

Ted