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Title: Paper Summarizing API Investigative Testing For Existing Square Edge Flange Tap Orifice Expansion Factor Equations
Author: Casey Hodges
Source: 2006 International Symposium on Fluid Flow Measurement
Year Published: 2006
Abstract: When measuring compressible fluid flow through a differential producing flowmeter, an expansion factor or coefficient must be used to correct for the changes in density and pressure as the fluid is accelerated through the flowmeter. In the 1930s, Buckingham and Bean developed the expansion correction that is currently being used in API 14.3 and ANSI 2530, which is applied in the United States and other countries. In the mid 1980s, work by Kinghorn showed that the Buckingham equation was in error. Further work by Seidl in the mid 1990s confirmed the errors seen by Kinghorn. In the late 1990s, Reader-Harris collected data from several facilities, and formed a new equation for the expansion factor. This equation has been implemented into ISO 5167-2. This paper describes the results of testing to determine the validity of existing expansion factor equations for flange-tapped sharp-edged orifices. Tests were performed at two independent facilities and on four different line sizes 2, 4, 6, and 10 inch, with diameter ratios ranging from 0.2 to 0.66. After initial testing, it was decided to also test a 3 inch line. At the Colorado Engineering Experiment Station Inc. (CEESI) facility dry air was used as the compressible flowing fluid, while at the Southwest Research Institute (SwRI) facility natural gas was used. The ratio of differential pressure across the orifice to the static inlet pressure was varied from near zero, to approximately 0.25 at a constant Reynolds number. The expansion factor was determined from the change in discharge coefficient with relation to the change in the differential pressure to inlet static pressure ratio at a constant Reynolds number. This paper shows that new equations fit the API expansion factor data set more effectively than the existing Buckingham and ISO 5167.2 equations.

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