Safety Relief Valve FAQs
Companies that repair, test, maintain or supply valves routinely receive inquiries from end users about safety-relief valves. Here are a few questions frequently asked of one VMA member.
A: Mount SRVs in a vertical, upright position. Installing an SRV in any position other than vertical (within plus or minus one degree) will induce misalignment of moving parts, which will adversely affect its operation and may even reduce the service life. The inlet piping should be fastened directly to the valve inlet, without any interference from other equipment. Piping length should not exceed the face-to-face dimension of a standard tee of the same pressure class of the valve. Inlet piping should never be smaller in diameter than the inlet connection of the valve.
Pressure drop from the vessel to the valve should not exceed 3% of valve set pressure when the valve is at full-flow capacity. Excessive pressure drop at the inlet of the SRV will cause extremely rapid opening and closing of the valve, which is known as “chattering.” This can result in lowered flow capacity and may damage the valve’s seating surfaces SRV inlets should not be located where excessive turbulence is present, near elbows, tees, bends, orifice plates or throttling valves. The outlet piping must not be reduced from the nominal outlet size of the valve; it should be designed to compensate for expansion (if required) and must be fully supported.
Stress distortion of the valve body may cause misalignment of the internal components, causing leakage. When two or more valves are piped to discharge into a common header, the built-up backpressure resulting from the opening of one or more valves may cause a superimposed backpressure in the remaining valves. Valve function, overpressure and flow capacity reduction must be considered when designing common collection systems.
A: The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code requires some valve models to include a drain below the seating surfaces of the valve. When installed on boilers, this drain must be open to prevent the accumulation of condensation in the valve body, which could corrode internal components and prevent the valve from operating properly. Where possible, use properly supported drain piping to channel any discharge or condensation to the desired location.
A: Typically, on a single valve system, the valve must be set at or below the Maximum Allowable Working Pressure (MAWP) of the vessel or system that it is intended to protect. On a system that has multiple valves, one valve must be set at or below the MAWP of the vessel or system. The additional valves may be set to open at a higher pressure but must be within the applicable ASME code standards. The current ASME Boiler and Pressure Vessel Code should be consulted for specific application parameters, regulations and tolerances.
A: There are two types of backpressure: constant and variable. The effects of constant backpressure on the opening pressure of the valve can be eliminated by reducing the opening pressure of the valve by the amount of the constant backpressure. The effects of variable backpressure on the opening pressure of the valve can be negated by installing a bellows in the valve. However, in either case the flow capacity may be reduced. Verification that the calculated flow capacity reduction meets or exceeds the required flow capacity is essential.
A: Is the valve actually leaking? In most cases, the leaking is caused by an outside source affecting the valve. A few troubleshooting questions that may help address this issue are: Is this a new installation or repair? Is the valve installed vertically? Is the installation piping (inlet and outlet) correct? Has the system been recently changed or modified (including the valve manufacture and model)? Is a proper operating gap being maintained? Have pressure-measuring devices been calibrated recently? Where is the pressure measurement being sampled in relationship with the valve and is there a correction required for pressure differences between the two areas? Does the valve exhaust into a single exhaust or a common header? Is the system seeing any vibration? Was the valve opened in an overpressure condition? If so, how many times?
It’s advisable to have an experienced, qualified technician troubleshoot the valve on the application. Also, keep in mind that the prospect of receiving an SRV that fails to meet the compulsory code standards is highly unlikely for those dealing with a reputable firm.