API 520-2 pdf free download
API 520-2 pdf free download.Sizing, Selection, and Installation of Pressure-relieving Devices Part lI–Installation.
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 superimposed and built-up backpressure on the operating characteristics of the PRDs. The discharge piping system should be designed so that the backpressure does not exceed an acceptable value for any PRD in the system. See API 520, Part I for limitations on backpressure. The rated capacity corrected for the actual overpressure of a conventional spring-loaded, balanced spring- loaded, or pop-action pilot-operated PRV should typically be used to size the atmospheric vent piping or the discharge line from the PRV to the relief header. Note that the rated capacity corrected for the actual overpressure can vary depending on the overpressure scenario. Common relief header piping in closed discharge 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 capacity of the system that the valve is protecting. Whenever the atmospheric vent, discharge piping, or common relief header piping is sized using the system’s required relieving capacity instead of the rated capacity of the valve corrected for the actual overpressure, the backpressure should be re-checked whenever changes are made to the process that affect the required relieving capacity of the system the valve is protecting. For pressure drop calculations, when discharging incompressible or subsonic compressible flow to either a closed reservoir or the atmosphere, the static pressure within the exit of the pipe is numerically equal to the reservoir or atmospheric pressure, respectively. When discharging sonic compressible flow, the pressure within the exit of the pipe is the calculated choking pressure.!8] Additional information on sizing of discharge piping systems for vapor or gas service is covered in API 521)7.2.4 Acoustic Interaction PRV instability due to acoustic interaction is complex and is an area of active research. A simple acoustic interaction model for a direct spring-loaded PRV is described below. When the PRV opens rapidly, the pressure just upstream of the valve disc drops and a rarefaction pressure wave travels upstream at the speed of sound in the fluid. The pressure reduction at the PRV inlet will tend to return the valve disc to its closed position. When the pressure reduction wave reaches a large reservoir (a hydraulic boundary), a pressure wave reflection occurs. If the pressure wave returns quickly, the PRV will stay open and should flow in a stable manner or may flutter. If, on the other hand, the PRV closes before the pressure wave returns, the PRV may cycle or chatter. The acoustic pressure waves are recoverable, so the PRV inlet pressure would rapidly build back up and the process would repeat. This phenomenon may contribute to instability in all fluid regimes; however, the effects of acoustic interaction are more pronounced with liquid reliefs as described in 7.1.3 [19] [24] [25]. See Annex C. There is a separate acoustic phenomenon that may lead to PRV chatter in which the PRV inlet pipe acoustic quarter wave couples with the PRV disk motion. This is supported by recent test data and one-dimensional fluid dynamics modeling for gases and liquids [14] [15] [20] [30] [33]. PRV chatter is most severe where the PRV natural frequency matches or approaches the inlet piping acoustic quarter wave frequency.API 520-2 pdf download.
