The Fisher Universal Gas Sizing Equation FUGSE

The angle, 0, will be small when the valve pressure ratio, P2/P1, is high, allowing application of the approximation sin 0 0, which transforms equations (9.44) and (9.45) essentially into equation (9.1). At very high-pressure drops, on the 

The valve conductance ratio, C, is related to the friction coefficient at high valve pressure ratios, as follows  

Substituting for the limiting gas conductance, C, from equation (9.14) and for the conductance as far as the throat, C,., from equation (9.9) transforms equation (9.48) into 

From equation (9.47), the valve conductance ratio, C, may be written as  

The onset of choking is predicted from equation (9.45) when 

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A3.5 Gas flow at intermediate pressure drops the Fisher Universal Gas Sizing Equation (FUGSE)... 

The historical dominance of US valve manufacturers has led to valve characteristics often being given in standard US units (US gallons, standard cubic feet per hour, etc.). A particularly important influence because of its extensive research into and testing of control valves was the Fisher Controls Company of Marshalltown, Iowa (now part of Fisher-Rosemount Ltd.) This appendix converts the basic liquid-flow equation from US to SI units. The appendix then goes on to derive Fisher s Universal Gas-Sizing Equation ( FUGSE ) from the basic liquid-flow equation and shows how this may be converted into SI units. 

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