I have written about this topic numerous times, and each time it causes controversy among the SAFTENG members. Some members are adamant that we should NEVER allow any “intervening valve(s)” in our pressure relief systems; others have seen it done many times. ASME VIII does indeed allow “intervening valves” before and/or after a Pressure Relieving Device (PRD), and in my previous articles, I mention the reasons why and discuss the need for such a design (e.g., PRDs connected to a treatment system or a header). Now we see that ASME B31.3, 2018 mentions using these “Stop Valves” while the equipment operates. WTH?!?!? I have never heard of anyone closing a valve in a PRD system “while the equipment is in operation,” I would STRONGLY RECOMMEND this, not even enter into a discussion. It is risky enough to have the valve in the PRD system, and just closing it while the equipment is out of service is risky enough (because these valves get forgotten and stay closed after the equipment is returned to service). But for the sake of openness, here is what ASME B31.3 says…
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322.6.1 Stop Valves in Pressure Relief Piping. If one or more stop valves are installed between the piping being protected and its protective device or devices, or between the protective device or devices and the point of discharge, they shall meet the requirements of (a) and either (b) or (c), below. (a) A full-area stop valve may be installed on the inlet side of a pressure-relieving device. A full area stop valve may be placed on the discharge side of a pressure-relieving device when its discharge is connected to a common header with other discharge lines from other pressure-relieving devices. Stop valves of less than full area may be used on both the inlet side and discharge side of pressure-relieving devices as outlined herein if the stop valves are of such type and size that the increase in pressure drop will not reduce the relieving capacity below that required, nor adversely affect the proper operation of the pressure-relieving device. (b) Stop valves to be used 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. (c) As an alternative to (b) above, stop valves shall be so constructed and arranged that they can be locked or sealed in either the open or closed position. See Appendix F, para. F322.6. |
PLEASE NOTE that we MUST meet (a) AND EITHER (b) or (c)!!!
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APPENDIX F – GUIDANCE AND PRECAUTIONARY CONSIDERATIONS … F322 DESIGN CONSIDERATIONS FOR SPECIFIC SYSTEMS F322.6 Pressure Relief Piping Stop Valves in Pressure Relief Piping. If stop valves are located in pressure relief piping in accordance with para. 322.6.1(a), and if any of these stop valves are to be closed while the equipment is in operation, an authorized person should be present. The authorized person should remain in attendance at a location where the operating pressure can be observed and should have access to means for relieving the system pressure in the event of overpressure. Before leaving the station the authorized person should lock or seal the stop valves in the open position. |