API 520-2-2020 pdf download Sizing, Selection, and lnstallation of Pressure-relieving Devices Part ll-Installation
6.2 Safe Disposal of Relieving Fluids
For a comprehensive source of information about the safe disposal of various releving fluids, see AP) 521
6.3 Backpressure Limitations and Sizing of Pipe
6.3.1 General
When discharge piping is designed, consideration should be given to the combined effect of superimposedand built-up backpressure on the operating characteristics of the PRDs. The discharge piping system shouldbe designed so that the backpressure does not exceed an acceptable value for any PRD in the system. SeeAPI 520, Part I for limitations on backpressure.
The rated capacity corrected for the actual overpressure of a conventional spring-loaded, balanced springloaded, or pop-action pilot-operated PRV should typically be used to size the atmospheric vent piping or thedischarge line from the PRV to the relief header. Note that the rated capacity corrected for the actualoverpressure can vary depending on the overpressure scenario. Common relief header piping in closeddischarge systems can be sized using the protected system’s required relieving capacity (see API 521)
For a modulating pilot-operated PRV, the discharge piping can be sized using the required relieving capacityof the system that the valve is protecting.
Whenever the atmospheric vent, discharge piping, or common relief header piping is sized using thesvstem’s reauired relieving capacity instead of the rated capacity of the valve corrected for the actuaoverpressure, the backpressure should be re-checked whenever changes are made to the process thataffect the required relieving capacity of the system the valve is protecting.
For pressure drop calculations, when discharging incompressible or subsonic compressible flow to either aclosed reservoir or the atmosphere, the static pressure within the exit of the pipe is numerically equal to thereservoir or atmospheric press,re, respectively. When discharging sonic compressible flow, the pressurewithin the exit of the pipe is the calculated choking pressurels Additional information on sizing of dischargepiping systems for vapor or gas service is covered in API 5213

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