I have built a low pressure fluid system for an R&D application. It is made with 1/4" pvc tubing and push connect fittings. It operates at about 10 mm Hg (about 0.20 psi) and uses a paristaltic pump rated at 24 ml/min. I have installed a reservoir on the pump inlet for filling the system, a low point drain, and a high point vent. I have also installed a pump discharge needle valve and a pump bypass line with a needle valve. Now I would like to install a pump inlet check valve, but I can't find one with a low enough 'cracking pressure'. I need it to open at less than 0.20 psi. Most of what I have found has a cracking pressure of 1 psi. Can anyone give me any leads on a check valve for this application? Schematic attached.

Smart Products check valve was what I needed, but they have a 25 piece minimum order. The cost was $200 plus shipping. They can build a check valve to your specs by making a custom wound spring.
I found a check valve with a 1/3 psi cracking pressure at US Plastic Corp. About $6.50.

You can design a check valve, it has to operate in the vertical position, using the proper density ball without a spring.  I have the the spring out of several ball checks to lower the cracking pressure. Just remember that  a check valve isn't the most reliable component in the system.

I would start with a square edge seat if the process fluid if clean. I would also use some guides to keep the ball on centers.

Their are also flexible rubber type check valves out there that also have essentially 0 cracking pressure and operate in any position. These are the same type of valve used toys and whoopee cushions. Size maybe a problem.

I would give Smart a call and ask for distributor of their valves where you could get a one off two off prices.

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