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Before calculating the size of your safety-relief valve, make sure you understand the limitations of the methods available
(ProQuest Information and Learning: ... denotes formulae omitted.)
Safety-relief valves (SRVs) and rupture disks are typically used to protect equipment from excessive overpressure. Typical scenarios that can result in such overpressure in excess of the vessel's MAWP (maximum-allowableworking pressure) include external fire, blocked outlet line, power failure, loss of cooling water or steam, thermal expansion, excess inlet flow, accumulation of noncondensables, failure of check or control valve, exchanger tube rupture, runaway reaction, and human error (for example, opening or closing the wrong valve). These and other scenarios are discussed in more detail elsewhere [I].
Relief devices should be installed on all pressure vessels, including reactors, storage tanks, towers, and drums. Other locations where relief devices are required are blocked in sections of liquid-filled lines that are exposed to external heating, the discharge from positive-displacement pumps, compressors and turbines, and vessel steam jackets. Storage vessels containing volatile liquids and a vapor space should be protected not only from excessive pressures from external heat or flow input but also from the possibility of a vacuum due to condensation of the vapor.
Relief valves are designed to open at a preset pressure, and are sized to allow mass flow out of the vessel at a rate sufficient to remove excess energy from the vessel at least as fast as it is inputted to the vessel contents (from either external or internal sources, such as external heating or a runaway reaction) to prevent further pressure buildup. The valve will close when the pressure drops to a safe level, thus containing and protecting the bulk of the vessel contents. Since the capacity of a valve is limited, it cannot accommodate the extreme flowrate that might be required to protect against an extremely high-energy-input rate such as might result from a very energetic runaway reaction, a deflagration or an explosion. Rupture disks are a less expensive alternative to safety valves, especially for very large-capacity requirements, but of course do not reclose to contain the vessel contents.
Proper design of a relief system requires not only determining the correct size for the valve or rupture disk, but also the proper size and selection...