We have a Sour Water drum (Operating Pressure = 46 kg/cm2(g)). We have installed an angle control valve to kill the pressure from 46 kg/cm2(g) to 6 kg/cm2(g) and because of some slurry particles.
System upstream of the Angle valve is designed for 50 kg/cm2(g) and downstream of the valve is designed for 20.5 kg/cm2(g).
In case there is an auto control failure of this Angle control valve, what is pressure can be seen by the system downstream of the valve?
Is it recommended to increase the Design pressure of the downstream system or provide any protection ( safety valve) downstream of this Angle control valve?
Answers
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10/02/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Assuming it is a letdown valve on the liquid from the drum, downstream vessels would normally be protected by their own relief valves set at the lower pressure. Any heat exchangers that could easily be protected by these lower pressure relief valves could be designed for the lower pressure, if they cannot be blocked in at the higher pressure. Pumps and pipe and valves are usually designed for the higher pressure. We really need a P&ID for a simple HAZOP analysis to be specific, after which we would include a disclaimer statement.
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10/02/2010
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Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
You will need to do a hydraulic calculation of the downstream system for the case when the valve fails open. The downstream vessel will need a relief valve sized to relief the excess material, and the piping will need to be designed to handle the pressure profile that occurs. If the flow chokes in the downstream line, there will be a discontinuity in the pressure profile that one should consider in setting design pressures. Having an engineer experienced with such a design is recommended.
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