Built-up backpressure at the outlet flange of a pressure relief valve

摘要

Even for pressure relief valves (PRVs) discharging directly to the atmosphere through short tailpipes, API 520 Part 1 warns that choked flow may develop at the outlet flange, resulting in high backpressure. API 520 Part 2 provides the formula for the reaction force on the pipe supports as a function of the backpressure, both for gas flow and two-phase flow. No procedure is given, however, to estimate this pressure at the outlet flange when the valve opens.