Where,
• Cv = Valve flow coefficient, US GPM with P = 1 psi
• Qa =
Air or gas flow, standard cubic feet per hour (SCFH) at 14.7 psig and FT Flowing air or gas temperature (F)
• P = Pressure drop (PP) at maximum flow, psi
• P = Outlet
pressure at maximum flow, psia (abs)
• P = Inlet pressure at maximum flow, psia (abs) The relationship between the inlet and
outlet pressure is important, as seen above, in determining which equation to use for gaseous flow. When the outlet pressure is less than ½
the inlet pressure, this is said to be ab criticalb pressure drop. The gas will behave differently when the pressure drop is critical therefore it is necessary to use the correct equation depending on the extent of pressure drop. Once you have computed the desired value of
Cv, you can choose the valve. The chosen valve must have a valve coefficient greater than or equal to the computed value.
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