As the debate rages on whether an intervening valve can be placed between a relief valve and the vessel it is protecting, I offer up NFPA 59A as another source for the answer YES. In fact, NFPA 59A (2016) requires either a stop valve or RV tree so that the RV’s can be changed without emptying and degassing the storage vessel.
7.3.6.4 Each pressure and vacuum safety relief valve for LNG tank systems shall be able to be isolated from the tank systems for maintenance or other purposes by means of a manual full-opening stop valve.
(A) The stop valve(s) shall be lockable or sealable in the fully open position.
(B) Pressure and vacuum relief valves shall be installed on the LNG tank system to allow each relief valve to be isolated individually while maintaining the required relieving capacity.
(C) Where only one relief device is required, either a full-port opening three-way valve connecting the relief valve and its spare to the container or two relief valves separately connected to the container, each with a valve, shall be installed.
(D) No more than one stop valve shall be closed at one time.
(E) Safety relief valve discharge stacks or vents shall be designed and installed to prevent an accumulation of water, ice, snow, or other foreign matter and shall discharge vertically upward.