Each relief valve should be equipped with inlet piping no smaller than the valve inlet flange size, and inlet piping should be as short as practical. Inlet piping should be designed so that the pressure drop from the source to the relief valve inlet flange will not exceed 3% of the valve set pressure. Pressure drop larger than 3% may cause the relief valve to “chatter” (or rapidly open and close), which can damage the relief valve. The 3% pressure drop should include the losses due to the inlet from the
vessel to the piping.
Relief valves vented to the atmosphere should have “tail pipes” equal to or larger in diameter than the relief valve outlet that extend vertically a minimum of one foot above building eaves, or eight feet above adjacent platforms on operating areas. The tail pipes should be provided with a drain located such that the exhaust through the drain hole does not impinge on vessels, piping, other equipment or personnel.
Discharges from relief valves are sometimes tied together in a common header so that the relief fluids can be routed to a safe location for venting or flaring. Piping should be installed in such a manner that liquid in the piping will drain into the header. Relief valves should be located above the header to prevent liquid from collecting at the outlet of the relief valve. In the event of a release, this liquid can be propelled at high speed, causing damage to the relief piping. If it is not possible to locate relief valves and branches above the main header, then branches should enter the header from the top to prevent liquid from draining out of the header into the branch. Branches below the main header, PSV outlets below the header, and unavoidable low spots in the piping should be equipped with a drain valve piped to a safe location.
Relief valves should be tested on a periodic basis even if not required by regulations. Pilot operating valves can be tested by sending a test signal to the pilot through a test connection in the pilot sensing line. Spring loaded relief valves must (1) be removed from service for testing, (2) be tested by subjecting the equipment being protected to set pressure, or (3) have an upstream block valve, which can isolate the relief valve from the equipment being protected, and a test connection between the block and relief valves installed. There is no industry consensus on which of these three test methods provides the highest level of safety. Therefore, some relief valves are installed with upstream block valves and some without.
If relief valves discharge to a common header, it is sometimes convenient to install downstream block valves so that the relief valve can be removed for repairs without shutdown of all equipment tied into the common header. Where either upstream or downstream block valves are used they should be full bore gate or ball valves with a device that enables them to be locked open and sealed. These are often referred to as “car-seal-open” valves. A lock out/tag out procedure should be in place to ensure that the block valves are not inadvertantly left closed.
Various arrangements employing three-way valves and multiple relief valves are sometimes used to provide the benefits of being able to isolate the relief valve for testing and maintenance without the disadvantage of decreasing safety through inadvertent closing of a block valve. Threeway valve arrangements are much more costly than block valves.
In so far as possible, relief valves should be located so as to be accessible from platforms. Relief valve connections to equipment and all relief piping should be designed to withstand the high impact forces that occur when the valve opens. Discharge piping supports should be arranged to minimize bending moments at the connection to the equipment being protected.