Can you tell me about the application of pilot operated PSVs? Where or why are these type of PSVs used?
Answers
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08/03/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
For a given flow rate and valve size, it allows a higher backpressure for a given set pressure. That is because the pressure to open is supplied by the inlet pressure rather than by the outlet pressure. Common applications include low pressure accummulators/receivers, such as the atmospheric column overhead accummulator where the backpressure during a plantwide power failure calculates to be up to 50% of the set pressure.
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08/03/2010
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keith bowers, B and B Consulting, kebowers47@gmail.com
Pilot operated safety relief valves utilize an auxiliary, much smaller, adjustable spring loaded relief valve to assist the main valve opening and CLOSING. The main valve may also be spring loaded, but process fluid pressure, through use of a piston arrangement forcing the main valve closed, is the main source of sealing pressure (the higher the process pressure, the higher the sealing force!!).
When the process pressure reached the 'set point' the pilot valve opens and dumps (releases) the pressure on top of the main valve, which then immediately fully opens. The 'supply' of process fluid to the pilot and closing piston is metered through a carefully sized restriction orifice so there is no possibility of the pilot being unable to release the main valve closing pressure.
The pilot valve assembly is very similar to a hydraulic 'spool valve' assembly so the on/off characteristic is very sharp and not subject to 'chatter' or built-up back pressure, thus stabilizing operation of the main relieving valve.
Pilot operated relief valves are useful the maximum allowable (relief) pressure is not very much higher than normal operating pressure, and both a high. A conventional spring operated valve is very susceptible to , instability , chatter when relieving, leaking, simmering, and failure to re-seat under those conditions. However, the precision pilot assembly, which has a very small, but essential to operation small leak rate past the pilot spool, is not suitable for dirty fluids containing any solids or materials which will plug up the pilot valve assembly.
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