Downstream protective relief valves

As long as pressure, level, and temperature control devices are operating correctly, the safety system is not needed. If the control system malfunctions, then pressure, level, and temperature safety switches sense the problem so the inflow can be shut off. If the control system fails and the safety switches don t work, then relief valves are needed to protect against overpressure. Relief valves are essential because safety switches do fail or can be bypassed for operational reasons. Also, even when safety switches operate correctly, shutdown valves take time to operate, and there may be pressure stored in upstream vessels that can overpressure downstream equipment while the system is shutting down. Relief valves are an essential element in the facility safety system. 

Inflow exceeding outflow is sensed by a high-level sensor (LSH). Back-up protection is furnished by the PSH (to keep the relief valve from operating) or an LSH in a downstream vent scrubber if the vessel gas outlet goes to atmosphere. That is, a vent scrubber must be installed downstream of any vessel that discharges directly to atmosphere. 

When a spec break is taken from a higher to a lower MAWP, there must be a relief valve on the lower pressure side to protect the piping from overpressure. The relief valve can be either on the piping or. more commonly, on a downstream vessel. Spec break problems most commonly occur where a block valve exists on a vessel inlet, or where a bypass is installed from a high pressure system, around the pressure vessel which has a relief valve, to a lower pressure system. 

To protect the HPRT outlet casing integrity and mechanical seals from possible downstream back pressure transients a relief valve installed in the HPRT outlet piping circuit should be considered (see Figure C.1). 

The high-pressure pumps are normally supplied with a pressure relief valve and associated product piping to protect the scraped-surface heat exchanger equipment downstream and the pump itself, should a blockage of the production line occur. 

We recommend that you install a pressure-relief valve that releases pressures above 2000 psig (140 bar) on each main gas line, to protect downstream equipment from high-pressure failures. The best location for the device is after the regulator shutoff valve (Figure 10.17). Pressure-relief valves on nonflammable gas lines need not be vented to a hood, but be sure to direct the vents downward (away from operator). 

The system consists of two blowdown trains, one for each steam generator. The blowdown water is extracted just above the tube sheet of a steam generator. Flow control valves adjust the blowdown flow rate from each steam generator and depressurise it. The blowdown flow is cooled by means of a regenerative heat exchanger, which uses the heat to warm the condensate. It then enters an ion-exchange purification unit, which removes impurities. Downstream of the purification unit, both trains combine into a common header, which contains a relief valve for providing overpressure protection for the low-pressure portion of the system. 

It was standard practice to protect refinery delayed coker, coke drums, from overpressure by locating a relief valve on the overhead vapor line. However, the main reason the coke drums would overpressure was coke accumulation in the inlet of the vapor line, immediately downstream of the coke drum itself and typically just upstream of the relief valve. Thus, it was a common occurrence (for example, at the... 

Pressure safety relief valves (PSRVs), the symbol for which is shown in Figure 13.1, frequently cross systems boundaries, i.e., they protect more than one process section. The scenarios to consider for isolation here are when the equipment the PSV is protecting needs to be worked on or the PSV itself needs to be removed and worked on. Each PSV will have an isolation valve both upstream and downstream of the PSV. 

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

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