Differences and determination of several back pressures in safety valve calculations

Thread Content

Recently, when performing calculations and selecting safety valves, I have often been confused by the various back pressures associated with them. Generally, the total back pressure of a safety valve = additional back pressure + discharge back pressure. Furthermore, the additional backpressure is divided into additional constant backpressure and additional variable backpressure. The parts highlighted in yellow below are an explanation copied from a knowledgeable user on a forum: ASME PTC-25 -1994 specifies as follows: a) Superimposed Back pressure: the static pressure existing at the outlet of a pressure relief device at the time when the device is required to operate. It results from pressure in the discharge system coming from other sources, and it can be either constant or variable. Additional back pressure: the static pressure present at the outlet of a safety valve just before it operates. It is caused by other pressure sources in the exhaust system, and it is divided into constant and variable types. 1) Variable back pressure is the additional back pressure that changes over time. Variable back pressure refers to the supplementary back pressure which varies with time. 2) Constant back pressure is the back pressure that remains constant over time. It is an additional back pressure that does not change as time passes. b) built-up back pressure: pressure existing at the outlet of a pressure relief device due to the flow through that device into the discharge system. Discharge back pressure: the pressure that forms at the valve outlet as a result of the flow of fluid through the safety valve into the discharge system. c) Back pressure: the static pressure existing at the outlet of a pressure relief device due to the pressure in the discharge system. Back pressure refers to the static pressure at the outlet of a safety valve resulting from the pressure in the discharge system. We can summarize the relationship among all types of backpressures as follows: We can summarize the relationships between various types of backpressure as follows: 1. Total backpressure = Built-up Backpressure + Superimposed Backpressure. The total backpressure equals the backpressure resulting from emissions plus the additional backpressure. 2. The Superimposed Backpressure can be either CONSTANT or VARIABLE; for example, if a relief device releases pressure to the atmosphere, then it is CONSTANT, while if the relief device releases pressure into a flare header where several other relief devices are also present, then the superimposed backpressure is VARIABLE. The additional backpressure can be either constant or variable. For example, if the safety valve vents to the atmosphere, its additional back pressure remains constant ； If this safety valve vents to the flare header, and several other safety valves are also venting to the flare header at that time, the additional backpressure for this safety valve becomes variable. In point 2 above, if that safety valve vents to the flare header, then is the additional constant backpressure of that safety valve what we usually refer to as the backpressure of the flare header (which is generally around 50 kPaA)? So how is the additional variable back pressure determined in this case? Additionally, how is the discharge backpressure of a safety valve usually determined?