October 2018

Special Focus: Plant Safety and Environment

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

Even for pressure relief valves (PRVs) discharging directly to the atmosphere through short tailpipes, API 520 Part 11 warns that choked flow may develop at the outlet flange, resulting in high backpressure.

Cordero, G. O., Techint Engineering & Construction

Even for pressure relief valves (PRVs) discharging directly to the atmosphere through short tailpipes, API 520 Part 11 warns that choked flow may develop at the outlet flange, resulting in high backpressure. API 520 Part 21 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. FIG. 1. A PRV that is either discharging to the atmosphere or to a manifold collecting other streams. The design engineer normally defines the size of the discharge manifold leading to the

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