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# Switch Flow Rate - Check valve

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## Switch Flow Rate - Check valve

(OP)
I am looking for help on this forum since the fluid dynamics is not my strength. I have a check valve (air) that I would like to compute the air flow (SCFM) at which the check valve is activated. Check valve is used to maintain a safe level of vacuum in a system. It closes when there is differential pressure (atmospheric/vacuum).
The force on the spring due to displacement and preload is known and I calculated the equivalent pressure on the plunger based on spring force. The pressure acting on the plunger is the air pressure as it flows through the valve.
What air flow through the valve (switching flow rate) can generate this pressure (in terms of SCFM)?
This switching flow rate would be unique to the valve and depended offspring 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. I have no instruments available to establish the flow rate or pressure.
Any reference, examples will be greatly appreciated.

### RE: Switch Flow Rate - Check valve

If you want to protect the vacuum chamber from collapse, that's normally done with structure, since the maximum differential pressure that can be applied is less than 15 psi. Failure is normally governed by buckling, so you have to use the equations for externally pressurized shells like submarines, not the equations for internally pressurized shells like pipe.

Check valves do a terrible job of regulating pressure. They are normally rated for 'cracking pressure', i.e. the pressure needed to just start flow, which is governed by the spring force at seated position and the area of the seat. The ports have no influence. ... except that they are involved in computation of a Cv vs lift or delta pressure rating.

If you want to have precise control of the vacuum level within the chamber, you would just buy a vacuum regulator, which can do a much better job of controlling pressure. It also has a Cv rating, when you are interested in the flow rate through the regulator, but most people 'size' regulators and check valves by matching the port size to what they have, and that's usually sufficient.

Better, or more precise, recommendations will probably arrive after you reveal such things as the size of the vessel, the size and HP of the pump, how far and how fast you want to pull the vacuum down, how many times in a day, stuff like that.

OR, rent a local engineer who has more experience with the 'plumbing' side of what you are trying to do.

Mike Halloran
Pembroke Pines, FL, USA

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