Pressure relief devices discharge piping

Where, of necessity, the vent pipe rises to a considerable height, excessive internal pressure on the tank may result, due to the pressure head of oil should an overflow occur. To prevent any possible tank failure due to such an occurrence a vent pipe pressure-relief device must be provided. It should be self-draining to reduce the risk of blockage, particularly when using heavy oil fuel. Codes require that these devices should not place any restriction on oil flow and must discharge within the bund area. 

This guideline covers only nonroutine or accidental events. Many hazardous events start with the discharge or loss of containment of a flammable and/or toxic material from a vessel or pipe. These discharges, which may take the form of vapor, liquid, solid, or multiphase vapor-liquid-solid mixtures, may be released into a confined area, such as a dike, building, or an equipment array, or into an open, unconfined area. The sources of these releases could be holes in vessels or pipelines, open pressure-relief devices, pipe ruptures, flange and seal leaks, or catastrophic vessel mptures. The range of releases is illustrated in Figure 2.1. 

Design of Pressure-relief Systems. Discharge rates are calculated by charts, tables, or equations supplied by manufacturers. In general, the method is to establish the discharge-pressure rates by code requirements and use fluid-flow calculation. for the pressure-relief devices, which behave as nozzles or orifice.s, and the associated piping. Discharge coefficients for pressure-relief devices can he approximated as follows ... 

Pressure relief equipment includes relief valves, safety valves, rupture discs, piping, drums, vent stacks, pressure indicators, pressure alarms, pressure control loops, and flare systems. Pressure relief devices can be placed on pumps, compressors, tanks, piping, reactors, distillation columns, refrigeration systems, and many other kinds of equipment. Materials that cannot be released to the atmosphere are recycled back to the system, or sent to a scrubber or flare system. The discharge from pressure relief equipment is collected in a closed piping system and sent to a flare stack. Harmless gases are discharged at a safe distance from plant operations areas. 

Discharge piping from emergency pressure relief devices unrestricted by 90 degree ells. 

Pressure relief devices on converters located in confined areas where the carbon dioxide discharged cannot be dissipated must be vented outdoors away from personnel and building air intakes. Such piping must not be capped at the outlet end or equipped with valves or other means of stopping or restricting the flow of the gas. 

The pressure of the environment downstream of a relief pipe or relief device, into which the pipe or device discharges., Back pressure can be "constant" or "built-up". A constant back pressure always exists, irrespective or the relief process, e.g. atmospheric. pressure. A built-up back pressure, exists due to the frictional pressure drop caused by flow through the relief system. 

Safety devices should be piped with a minimum of inlet pipe runs because excess pressure drop before a safety valve will affect its operation. For a minimum of discharge piping, relief valves are located high on a tower in open relief systems. In closed systems relief valves are located just above the relief header. 

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