Looking for a source for spring loaded check valves with a cracking pressure less than 5" WC (0.18 psi) for nitrogen service. 316SS/PTFE construction will suffice.API Cast Steel Valves Temperature is 0 - 40C.Forged Steel Valves Sizes could range from 1/4" tubing to NPS 2 Cl 150 flanged.

So far the lowest CP I've found is 1/3 psi.Needle Valves

Rubber "duck-bill" and diaphragm check valves have very low cracking pressure. Spring loaded valves generally do not have very low cracking pressure because they also have to seal reliably when upside-down or dirty. FilterI've used free-floating ball checks mounted so the weight of the plastic ball sealed the valve.It was a 2" check-valve in a 1/2" line.  

And try Generant also.  They have a wide range of valves that meet your specs.  I've found they're also less expensive and you'll get better service than from Circle Seal.

In additionBellow Seal Valves, a gravity based ball check-valve is slow, and might under circumstances give higher loss under opening or part opening and/or high pressure peaks when closing.

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