If you want a spirited process safety debate, simply mention the practice of having “intervening valve(s)” before or after a Pressure-Relieving Device (PRD). There are some who swear that it is not allowed, but in fact, it is. This is how I learned about the “Car-Seal” program early in my career. ASME B31.12 is one of those RAGAGEPs that allows this practice WHEN it meets certain criteria, such as a Car-Seal Program.
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IP-7.2.1 Stop Valves in Pressure-Relief Piping When installing stop valves between piping and protective devices or between protective devices and points of discharge, the stop valves shall meet the requirements of (a) or (b) and either (c) or (d) below. (a) Full-area stop valves may be installed on the inlet and/or discharge side of pressure relieving devices. (b) Stop valves of less than full area installed on the inlet and/or discharge side of pressure relieving devices shall not cause pressure drops that reduce the relieving capacity below that required, nor adversely affect the proper operation of the pressure relieving devices. (c) Stop valves installed in pressure relief piping shall be so constructed or positively controlled that the closing of the maximum number of block valves possible at one time will not reduce the pressure relieving capacity provided by the unaffected relieving devices below the required relieving capacity. (d) As an alternative to (c) above, stop valves shall be so constructed and arranged that they can be locked or sealed in either the open or closed position. |