Pressure Relief Valve Inlet Line Stresses

The discharge flow from a pressure relief valve generates forces that can result in substantial stresses on the inlet nozzle and its connection to the protected vessel. The magnitude of these forces depends on the size of the valve, the configuration of the outlet piping, and the temperature change due to the discharge. API Recommended Practice 520 Part II presents an equation for quantifying the magnitude of this force for open discharge systems. In addition to this reaction force stress, temperature gradients across a pressure relief valve discharging an auto-refrigerating fluid can induce additional stress on the inlet line due to contraction of the piping. An experienced piping designer should generally be consulted to specify support members for pressure relief valves discharging directly to atmosphere. For large closed systems, a pipe stress analysis is usually performed prior to installation.

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