Flare back pressure definitions
Constant back pressure (= constant portion of superimposed backpressure): This back pressure is due to fixed components in the flare header (valves, fittings, pipe etc).
Superimposed back pressure - variable : This is the pressure in the flare header due to the flow from other PSVs into the flare header. For an atmospheric discharge it would be 0 psig.
API definition for superimposed backpressure
The static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate.
Superimposed backpressure is the result of pressure in the discharge system coming from other sources and may be
constant or variable.
Superimposed backpressure (constant + variable) is that backpressure that may exist before opening of the device.
Variable back pressure (built-up back pressure): This is the back pressure that is caused by the flow of liquid to be relieved. This is related to the pressure drop due to the release of the particular PSV.
API definition for built-up backpressure
The increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device opens.
(Total) Back pressure
The pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system.
Backpressure is the sum of the superimposed and built-up backpressures.
Or it is the sum of total variable backpressure (superimposed variable + built up) and constant backpressure.