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Title: Real-Gas Effects In The Flow Of Methane And Natural Gas Through Critical-Flow Nozzles
Author: Robert C. Johnson
Source: American Gas Association 1970
Year Published: 1970
Abstract: When critical-flow nozzles are used for metering the mass flow rate of natural gas, the isentropic flow equations for a perfect gas do not apply. (A critical- flow nozzle is one in which the throat velocity equals the local speed of sound. It also has been called a sonic-flow nozzle or a choked nozzle.) In this paper a perfect gas is defined as one having an invariant specific heat and a compressibility factor of unity. A perfect gas is to be distinguished from an ideal gas, which has a temperature-dependent specific heat and unity compressibility factor. A nonperfect gas is a real gas. In the absence of dissociation, all real gases approach the ideal-gas condition as the pressure is reduced. The assumption that the gas is perfect is sufficiently accurate for computing the flow of such gases as air and nitrogen at atmospheric pressure and room temperature. However, natural gas cannot be considered perfect even at pressures less than atmospheric, since natural gas has an appreciable specificheat variation with temperature. That is, under this condition, natural gas can be considered ideal but not perfect. At higher pressures the compressibility factor variation also becomes important.

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