2 Check Valves in Series

[althebear]

If you have 2 50psi check valves in series, does that then make the cracking pressure 100psi? Whereas if they were in parallel the cracking pressure would be 50 psi down each line?

Thanks in advance!

 

[Compositepro]

Essentially, yes.

 

[pennpiper]

althebear,
You wrote:
If you have 2 50psi check valves in series, does that then make the cracking pressure 100psi? Whereas if they were in parallel the cracking pressure would be 50 psi down each line?

Issue #1 - If you have 2 50psi check valves in series? Why would you have two check valves in series?

Issue #2 - does that then make the cracking pressure 100psi? If you have a check Valve that has a Cracking Pressure of 50 psi how does it get to be 100 psi?

Issue #3 - Whereas if they were in parallel? Why would you design the system with parallel flows?

Issue #4 - Why do you think the Cracking Pressure would be different with two parallel flows?

Issue #5 - What type of Check Valve are you using?

Issue #6 - What is the Fluid?

Issue #7 - What is the Fluid Operating Temperature?

Issue #8 - What kind of Pump are you using?

Issue # 9 - What is the size of the Pump discharge piping?

Sometimes its possible to do all the right things and still get bad results

 

[gerhardl]

Hmmm!!!??

You have to look at the hydraulic conditions.

If two checkvalves are mounted in series in such a way that the first valve reaches the cracking pressure, opens and let fluid pass to the second valve, the second will also open at the same (normal) cracking pressure.

(Two springs in series will not double the force needed to strech or compress)

If you put two checkvalves in parallell, either in a common chamber, or in parallell pipelines fed from the same fluid source, both will open at the same (normal) cracking pressure, under the condition that the flow is large enough to maintain the cracking pressure if one opens a bit before the other.

(Two springs in parallel will double the force if pressed by a common plate/area. In this hydraulics valve-case you will have a common fluid pressure, maintained from outside, giving equal pressure working at two equal valve areas)

 

[LittleInch]

Hmm. If the gap between the two valves sits at 50 psi if this acts as a backorder on the first valve plus the spring force will that make it 100 psi?

Not sure.

Once flow goes then it will be clear but right at the initial stage?

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

 

[althebear]

I am away so I can't get on to my work systems to get in the detail but I got a call today saying that the check valves are not cracking until in excess of 100psi, I expected them to crack at over 50psi so I wanted to check whether I just understood it wrong, it's not our design as the compensation system is third party so not my speciality so to speak. So my initial thought was maybe a 100psi check was installed incorrectly instead of a 50psi one, so I just wanted to make sure I understood it correctly.

We have 2 checks, firstly for redundancy if one fails and also to prevent backflow so no ingress of any debris occurs to keep the the lines clean. The setup is on a compensation system for valve operations subsea, the idea being it vents to sea if a valve leaks into the compensation system if a seal goes etc and doesn't damage the compensation system. The compensation system is rated to 290psi, valve operating pressures being much higher.

Reality is its fine as the weak point (the bladder) is rated to 290psi but I just wanted to make sure I understood it correctly for my own learning to be honest, arrangement is per the following image.

 

[3DDave]

It should be a combination. Once the first one cracks it will allow flow at some pressure drop lower than its cracking pressure. The second won't crack until the down stream pressure from the first is at its cracking pressure. When it cracks, it will also allow flow at below the cracking pressure. Aside from some ugly dynamic interactions operating near the cracking pressure and assuming the total pressure/ drops due to flow is high enough to force both open and keep them open, the drop will be dependent on the flow-dependent characteristics.

 

[itsmoked]

With a pressure source in the region of 50psi I can envision a nasty oscillation where the first cracks and when it does the second cracks reducing the pressure enough to cause the first to close causing the second to close followed by the first cracking and around and around.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[gerhardl]

Hello itsmoked,

I agree! As in my first post: hydraulic conditions! If we do not have a steady flow supply, below the capacity checkvalves are dimensioned for, nasty things usually happens.

 

[LittleInch]

I know everyone keeps calling them check valves but really these are simple pressure relief valves once you stick a spring on them.

As such they work on the basis of differential pressure.

No once CV1 has passed a little bit the pressure between it and the second one is anywhere from zero to 50 psi. NOTE - this assume the pressure D/s CV2 is zero psi - Is it??

If its close to 50 then the require pressure does become 50 psi PLUS the downstream pressure.

Your second valve in series should be rated at only a few psi then it would work.

If you think these valves will fail in service then you really need better valves. Especially subsea.

How far subsea? As said your valves are differential pressure relief so if you have pressure eon the D/S side then the pressure as measured versus atmosphere will increase.

This design is not good.

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

 

[dgallup]

I think it will take close to 100 PSI to open both valves and keep them open. If the pressure differential across either valve drops below 50 PSI it will close again.

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