Relief piping

Dust explosibility characteristics Length and shape of relief pipe if existent... 

Without relief pipe, up to 2 har With relief pipe, up to 4 har... 

Particularly reasonably priced explosion isolation of systems involves the use of a relief pipe with which the flow direction can be diverted by 180°. It prevents flame jet ignition with precompression in constructionally protected equipment. If suction is present, explosion propagation can occur. To prevent this, the use of an additional extin-gui ing barrier or a rapid-action gate valve is necessary. If a diverter is installed where positive pressure feed is given, then the diverter is, in general, safe against an explosion propagation. 

Water flowed from the relief pipe (20 ft elevation) to tank E610. It first flowed through the jumper to the process pipe to the normally closed blow-down valve which was open (possibly inadvertently left open or failed to seat in the attempt to pressurize the tank). About 119 gallons of water flowed through this open blow-down valve, through another, normally open, isolation valve to enter tank E610, where it reacted with MIC. 

For gases with specific heat ratios of approximately 1.4, the critical pressure ratio is approximately 0.5. For hydrocarbon service, this means that if the back-pressure on the relief valve is greater than 50% of the set pressure, then the capacity of the valve will be reduced. In other words, if the pressure in the relief piping at the valve outlet is greater than half (he set pressure, then a larger relief valve will be required to handle the same amount of fluid. 

The pressure in the relief piping is usually equal to atmospheric pressure as long as no relief valve is relieving. There is a common miscon-... 

The relief piping should be segregated into pressure relief and atmospheric relief systems. If there is a wide range of set pressures on relief valves, the pressure relief piping is sometimes divided into high pressure and low pre.ssure systems. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

A-8 SPECIFIC INDUSTRIAL PIPING SYSTEMS -STOP VALVES IN PRESSURE RELIEF PIPING (SEE PARA. IP-7.2.1)... 

If stop valves are located in pressure relief piping, and if any of these stop valves are to be closed while the equipment is in operation, an authorized person should be present. The authorized person should remain in attendance at a location where the operating pressure can be observed and should have access to means for... 

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. 

The chlorine absorber is enclosed by a jacket through which a hot or cold fluid can be pumped to maintain the absorber contents at any desired temperature. A pressure relief pipe emerging from a point near the top of the absorber is equipped with a rupture disk, a thin metal membrane designed to break if the absorber pressure rises above a certain value. If this happens, gas flows from the absorber through the relief pipe to a surge tank, lowering the pressure in the absorber while containing the released gas. 

Pa = Accumulated relieving pressure, psia P = Pressure inside relief pipe near the vessel, psia... 

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