My process engineer requested a low pressure check valve with a differential pressure of 0 kg/cm2 on a line that goes from a tank to a wet flare header. The question is: a check valve with Dp=0 kg/cm2 is always closed or open i suppose, so what is its real use?You need to have a differential pressure to open the check valve. Anyhow There are available check valves that ask for low differential pressure to work (i.e. disk check valve with no spring).
To go further into this matter take a look to this thread
Slightly diverging: the conservation vents offered by Protectoseal, Groth, Varec, etc are really check valves.  They start with a very light aluminum pallet (disc) and only the weight determines the cracking pressure. There are configurations offered with flanged connections on both ends, and the minimum cracking pressure is about 1/2 oz/in2 

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