Formula source of flow capacity of pressure relief deives for gas cylinders in CGA S 1.1 -2019

[yanxingqing]

Hi everyone, I am doing some pressure relief device calculations to gas cylinders according to CGA S 1.1-2019. There are two equations for flow capacity of PRD, Qa=0.154Wc for other PRD except PRV, and Qa=0.00154PWc for PRV. Qa is the flow capacity in ft3/min of free air, Wc is water capacity of the cylinder in pounds, and P is flow rating pressure in psia. I am confused about the deviation of these two equation. Why Qa is irrelevant to P for other PRD except PRV but is relevant to P for PRV? Could anyone tell me the devation of the euqations? or tell me where I can find the information on this. The information like documents, standards that require payment can also be accepted.
Thanks in advance for the help.