Multiple PR valve

Number of Valves Required - Normally a manufacturer s standard PR valve with Drifdce area equal to or larger than the calculated requirement is specified. In some cases, e.g., large reheving rates or to prevent chattering, two or more valves are necessary. Likewise, if there is an appreciable difference between the calculated orifice size and the available size, multiple PR valves are desirable to more nearly match the available area to the required orifice area. The column for "spares" indicates the requirement, if any, for spare PR valves installed on the equipment. Normally, this applies only in the case of refinery preference or local regulations, but is required in many European countries. 

Multiple PR Valve Discharges to a Single Atmosphere Riser - Where two or more PR valves are manifolded into a single riser discharging to the atmosphere, the following additional requirements apply ... 

The entire inlet line must be at least the size of the PR valve inlet. When multiple safety valves are manifolded on the inlet side, the cross-sectional area of the maitifold piping must be equal to or greater than the sum of all the inlet areas of valves open to the manifold. 

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