Gas flow at very large pressure drops

Equation (A3.15) possesses the limitation that it does not predict the choking effect that will occur when the flow becomes sonic in the valve throat. For such conditions, US manufacturers predict choked flow, Orrir, in terms of the measured gas sizing coefficient . 

Qcril is the critical flow rate in standard cubic feet per hour, 

C is the gas sizing coefficient (scf/hr/psia) for a given valve opening. 

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