What is Capacity Correction Factor (CCF) due to Back pressure for PSV ?

What is capacity correction factor, Kb in orifice area calculation in PSV. How do we calculate this value ?

* PSV - Pressure Relief Valve

Back pressure will tend to produce a closing force on the unbalanced portion of the disc. This force may result in a reduction in lift and an associated reduction in flow capacity. Capacity correction factors, called back pressure correction factors, are provided by manufacturers to account for this reduction in flow.

For preliminary PSV sizing, figures in API RP 520 Part 1 can be referred.

Reference :
i) API RP 520 Part 1

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