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# Check Valve - Switch Flow rate

## Check Valve - Switch Flow rate

(OP)
Hello,
I have a check valve (air) that I would like to compute the air flow (SCFM) at which the check valve is activated. This would be used to maintain a safe level of vacuum in a system. There are no descriptions on the valve and I have no instruments to measure the flow rate so I would like to solve it mathematically.
I have calculated the spring force counting for the displacement and preload and therefore the equivalent pressure on the base of the plunger is known. The pressure acting on the plunger is the air pressure past the intake orifice as it as it flows through the valve.
The problem that I have is that I am not sure how to calculate the air flow through the valve (switching flow rate) that can generate this pressure (in terms of SCFM)?
This switching flow rate would be unique to the valve and depends on the spring and I assume on the diameter (areas) of the in/out ports. Knowing the “switch flow rate” of the valve I can select the right number of them to connect to a pump of know flow rate.
Any reference, examples will be greatly appreciated.
Thank you,
PS. I posted this question before on a different forum and i apologize for multiple posting. I think with will be the appropriate forum to post it.

### RE: Check Valve - Switch Flow rate

You say this valve is " to maintain a safe level of vacuum in a system". However what you've drawn means, as far as I can see that once the level of vacuum is more than 0.306 psi below atmospheric pressure then the valve closes and doesn't open any more. Hence the vacuum level can go on decreasing.??

The problem you have is that the moving air will also impose a force and create pressures which are not easy to calculate. That internal plug will only move so far, but as the flow increases the drag on the plug increases until it slams shut, then pops off then slams shut then .... ~The closer the internal pressure is to 0.306 psi differential the worse it would get..

There are a few different things going on with this valve in terms of pressure exerted by a moving flow, flow through an annulus between plug and valve, flow through he spring, flow through the entry and exit.

Can you describe what this is a bit better, how it is supposed to work and why you're doing this and not just buying an off the shelf pressure relief valve?

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

### RE: Check Valve - Switch Flow rate

Oh and if you post in a different forum ( frowned upon) please provide a link back to that post so we can see what was already written.

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

### RE: Check Valve - Switch Flow rate

First you can directly measure the actuation pressure with a simple differential pressure gauge.
How much air flow you get will depend then on the delta P, so these valves have a flow coefficient attached to them.
It would likely take less time to go to a catalog (I would start with Swagelok) and spec out check valves with known cracking pressure and flow curves, and replace what you have rather than try to calculate theoretical values that may not be accurate.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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