Relief valves, processing facilities

Process Facilities 15 Area electrical classification 16 Accessibility for mechanical integrity (sampling, maintenance, repairs) 17 Protection of piping and vessels from vehicles and forklifts 18 Protection of small-bore lines, fittings from external impact, personnel 19 Routing of process piping, critical controls cable trays, critical utilities 20 Vent, drain, and relief valve discharge locations... 

All waste hydrocarbon gases (vents, relief valves, and blowdowns) should be routed to a flare or returned to the process through a closed header system. Release of vapors to atmosphere may produce a vapor cloud, and even through the release may be remote from the facility it may drift or the effects of ignition (i.e., blast overpressure) of the cloud will be felt at the facility. 

Woolfolk became concerned over the adequacy of the SRV test facilities, which were commonly used by many commercial SRV test and repair shops and most chemical plants. The typical test stand consisted of a clamp-down table connected to a compressed air cylinder by a 1/4-inch (0.6 cm) tubing. Woolfolk experimented with surge accumulators for testing relief valves, using a substantial volume of air to mimic actual process conditions. 

The function of a relief valve is to protect a vessel, piping system, or heat exchanger from exceeding its maximum allowable working pressure (MAWP). The MAWP is shown on the code stamp (a metal plate) attached to each process vessel, and by design is determined by the size of the impeller on the pump upstream of the vessel (see Sec. 34.4.1 in Chap. 34). Normally, the relief valve is set to open at this MAWP pressure. The relief valve setting is adjustable and is done by a machinist at a shop pressure-testing facility. 

Pressure relief valves are a critical safety item in almost all process facilities. Pressure relief valves simply must work. This means that they must never be blocked in from the equipment item(s) that they are protecting. Yet their very criticality means that relief valves will have to be routinely isolated and/or removed from the system that they are protecting so that they can be maintained and tested— both for the pressure at which they open and, more rarely, for their flow capacity. 

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