In 2017, the NBIC was restructured into four (4) parts, adding a new Part 4, Pressure Relief Devices. This purpose of this restructuring was to provide one distinct integrated part for pressure relief devices compiled from all PRD information referenced in Part 1, Installation; Part 2, Inspection; and Part 3, Repairs and Alterations. And in this new Part 4, Pressure Relief Devices, the National Board had this to say about “intervening stop valves” either between the pressure vessel and the valve or between the valve and the discharge location. Since this is a “new” part to the NBIC, I thought it might be helpful to remind everyone about the value in having valves in their relief systems as well as the DANGERS of doing so without proper administrative controls in place to manage these catastrophic risks. Here are the NBIC requirements: (emphasis by me)
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2.6.6 INLET AND DISCHARGE PIPING REQUIREMENTS a) The opening through ALL pipes and fittings between a piping system and its pressure relief device shall have at least the area of the pressure relief device inlet. The characteristics of this upstream system shall be such that the pressure drop will not reduce the relieving capacity below that required or adversely affect the operation of the pressure relief device. b) A non-reclosing device (SAFTENG NOTE: a rupture disc below the RV) installed between a piping system and a pressure relief valve shall meet the requirements of 2.6.6 a). c) The opening in the pipe shall be designed to provide unobstructed flow between the pipe and its pressure relief device. d) When two or more required pressure relief devices are placed on the connection, the inlet cross-sectional area of this connection shall be sized either to avoid restricting flow to the pressure relief devices or made at least equal to the combined inlet areas of the pressure relief devices connected to it. The flow characteristics of the upstream system shall satisfy the requirements of 2.6.6 a). e) There shall be no intervening stop valves between the piping system and its pressure relief device(s), or between the pressure relief device(s) and the point of discharge EXCEPT UNDER THE FOLLOWING CONDITIONS: 1) These stop valves shall be so constructed or positively controlled that the closing of the maximum number of block valves at one time will not reduce the pressure relieving capacity below the required relieving capacity; 2) Upon specific acceptance of the Jurisdiction, when necessary for the continuous operation of processing equipment of such a complex nature that shutdown of any part is not feasible, a full area stop valve between a piping system and its pressure relief device may be provided for INSPECTION AND REPAIR PURPOSES ONLY. This stop valve shall be arranged so that it can be LOCKED OR SEALED OPEN and it shall not be closed except by an authorized person who shall REMAIN STATIONED THERE during that period of operation while the valve remains closed. The valve shall be locked or sealed in the OPEN POSITION BEFORE THE AUTHORIZED PERSON LEAVES THE STATION; 3) A full area stop valve may be placed on the discharge side of a pressure relief device when its discharge is connected to a common header for pressure relief devices to prevent discharges from these other devices from FLOWING BACK TO THE FIRST DEVICE DURING INSPECTION AND REPAIR. This stop valve shall be arranged so that it can be locked or sealed OPEN and it shall NOT be closed except by an authorized person who shall REMAIN STATIONED there during that period of operation while the valve remains closed. The valve shall be locked or sealed in the OPEN position BEFORE the authorized person leaves the station. This valve shall ONLY be used when a stop valve on the inlet side of the pressure relief device is FIRST CLOSED; or 4) A piping system where the pressure originates from an outside source may have a stop valve between the system and the pressure relief device, and this valve need not be sealed open, provided it also closes off that vessel from the source of pressure. f) Pressure relief device discharges shall be arranged such that they are NOT a hazard to personnel or other equipment and, when necessary, lead to a safe location for disposal of fluids being relieved. g) Discharge lines from pressure relief devices shall be designed to facilitate drainage or be fitted with drains to prevent liquid from collecting in the discharge side of a pressure relief device. The size of discharge lines shall be such that any pressure that may exist or develop will NOT REDUCE the relieving capacity of the pressure relief device or adversely affect the operation of the pressure relief device. It shall be as SHORT AND STRAIGHT as possible and arranged to avoid undue stress on the pressure relief device. h) The reaction forces due to the discharge of pressure relief devices shall be considered in the design of the inlet and discharge piping. i) Pressure relief devices shall be installed, so they are accessible for inspection, repair, or replacement. |
And the code also mentions this in…
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3.2.4.3 PRESSURE VESSELS AND PIPING Standard practice for overpressure protection devices is to not permit any type of isolation valve either before or after the device. However, some pressure vessel standards permit isolation valves under certain controlled conditions when shutting down the vessel to repair a damaged or leaking valve. If isolation block valves are employed, their use should be carefully controlled by written procedures. Block valves should have provisions to be either car-sealed or locked in an open position when not being used. For ASME Section VIII, Div. 1 pressure vessels, see UG-135, Appendix M, and jurisdictional rules for more information. |