Installation of an isolation (i.e., block) valve in the inlet line to a pressure relief valve enables the valve to be removed for repair or for routine inspection and maintenance without isolating or venting the protected equipment. Most applicable codes allow the installation of such a valve, provided it meets certain minimum requirements designed to ensure that the relief device is not inadvertently isolated from the equipment it is intended to protect. Furthermore, as the installation of an isolation valve implies the intention to remove the pressure relief valve while continuing to operate the process equipment, an alternative protection scheme should be planned. This alternative can be a spare relief device that would be installed immediately upon removal of the “primary” pressure relief valve, a normally isolated relief device that would be connected to the equipment when the primary valve is isolated, specialized operating procedures used while the primary valve is removed, or a combination of these approaches.
Some specific guidelines concerning installation of isolation valves in a pressure relief valve’s inlet line are:
a. Such valves are to be full bore.
b. They are to be capable of being car-sealed or locked open.
c. If a gate valve is used as such an isolation valve, it is to be installed with its stem oriented horizontally so that an internal failure will not cause the gate to fall closed.
If two full-capacity pressure relief valves are installed via a tee on the same vessel nozzle, with one valve intended as a spare for the other, it is important that only one inlet isolation valve be open at a time. The excess flow capacity afforded by both pressure relief valves could produce excessive inlet pressure drop. Consideration should be given to the use of a three-way isolation valve capable of creating an open pathway to only one of the two pressure relief valves at a time.