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Overhead Control System

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#1 sudheer


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Posted 23 August 2007 - 02:03 AM

Our crude column overhead pressure control valve which is letting down to flare is 'air fail to open', whereas Visbreaker fractionator overhead pressure control valve which is letting down gases to flare is 'air fail to close'. Both systems are similar with heater - fractionator(& soaker in visbreaker) -overhead drum arrangement. Can forum throw light on philosophy of selection of 'air fail to open' and 'air fail to close' type of control valves
Thanks in advance
Sudheer Pai

#2 djack77494


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Posted 23 August 2007 - 08:30 AM

Your question has no simple answer other than a valve should always be made to fail in a position least likely to result in an unsafe condition. That determination is often made (or at least reviewed) by a knowledgable group of engineers during a HAZOP. Other reviews are also common. So when you tell me that one valve fails open and one closed, all I am left thinking is that those were determined to be the safest fail positions. I would assume that both columns also have PSV's in their overhead systems.

The PCV in the Crude Column overhead system that I most recently worked on failed Open. I have not worked on a Visbreaker unit. BTW, if your valves have solenoid valves in the instrument air supply that pressurizes the valve operators, then those solenoids should fail (i.e. if de-energized) to a position that would vent the air pressure.

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