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Theoretically, what minimum flow requirement shuld be specified for selection of Recycle Valve with respect to compensate for drop in suction pressure, provided my maximum flow through the RC valve is known?
 
Answers
23/11/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Not sure I answered the question, because not sure what is given and what is to be solved. To decide the minimum amount to recycle or spillback with good control depends on the slope of the pump curve at the operating point under consideration and the size and type of control valve.
23/11/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I am assuming that this is a pump service that, during normal operation, is maintaining a set suction pressure, such as a crude “Desalter Pump” service, where the desalter pressure is controlled by this pump. For a minimum flow situation, even an equal percentage control valve on the spillback should be operated at 15% or more open for steady control.
By the way, I don’t think my example is the best way to control the desalter pressure, even though it is often done that way. But that is another subject.