PSV Constant Back Pressure Correction Factor (Kb) for Vapor & Gas
Constant Back Pressure Correction Factor (Kb)
For Conventional Pressure Relief Valves (Vapors and Gases Only)
Note: This chart is typical and suitable for use only when the make of the valve or the actual critical flow pressure point for the vapor or gas is unknown; otherwise, the valve manufacturer should be consulted for specific data. This correction factor should be used only in the sizing of conventional (nonbalanced) pressure relief valves that have their spring setting adjusted to compensate for the superimposed back pressure. It should not be used to size balanced-type valves.
For Balanced-Bellows Pressure Relief Valve (Vapors and Gases Only)
The curves above represent a compromise of the values recommended by a number of relief valve manufacturers and may be used when the make of the valve or the critical flow pressure point for the vapor or gas is unknown. When the make of the valve is known, the manufacturer should be consulted for the correction factor. These curves are for set pressures of 50 psig and above. They are limited to back pressure below critical flow pressure for a given set pressure. For set pressures below 50 psig or for subcritical flow, the manufacturer must be consulted for values of Kb.
See paragraph 3.3.3. in API Rp 520 Part 1
For 21% overpressure, Kb equals 1.0 up to PB/PS = 50%.