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Title: Orifice Metering Uncertainties And Their Impact On Unaccounted-For Gas At PG&E
Author: John W. Stuart
Source: American Gas Association 1990
Year Published: 1990
Abstract: In a recent study, Pacific Gas and Electric (PG&E) attributed 38 percent of its unaccounted-for gas to orifice metering uncertainty biasing caused by straightening vanes. How this was determined and applied to PG&Es family of orifice meters is briefly described. In 1989, Pacific Gas and Electric (PG&E) completed a comprehensive study of its lost and unaccounted-for (UAF) gas. The study, which was co-funded by GRI, investigated and quantified many causes of UAF for the test year 1987. Almost all (97 percent) of PG&Es UAF was found to be attributed to identifiable accounting procedures, measurement problems, theft, and leakage (see Figures 1 and 2). In 1987, PG&Es UAF was calculated to be 1.61 percent. Of that 1.61 percent, 63 percent was attributed to measurement problems such as temperature and atmospheric pressure assumptions at residential meters, mechanical volume integrators used on power plant flow recorders, and orifice meter coefficient shifts caused by straightening vanes. The orifice meter coefficient shifts were estimated to account for 38 percent of the total UAF. This paper will describe the nature of these shifts, how they affect only certain meters, and their impact on UAF at PG&E.

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