This paper presents the new information on orifice flow metering in ASME Performance Test Code (PTC) 19.5, “Flow Measurement” [1], and discusses many of the clarifications that have been made based on experience and commentary to the review drafts of the Code. In particular, this paper expounds upon details regarding piping installation requirements for accurate measurement. A major advancement incorporated into ASME PTC 19.5 is the development of a coefficient of discharge equation that is based on fluid dynamic theory. The theoretical concept and a summary of the derivation of the new discharge coefficient equation for orifices are presented in this paper. It is shown that the calibration interpretation methodology introduced in PTC 19.5, which is similar to that developed earlier for nozzle calibrations, reduces the uncertainty of calibrated orifice metering sections, even when used outside the calibration range.

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