Accurate measurement of natural gas consumption: the economic impact on power generation operations

Deregulation of electricity under FERC Order 888 will result in open access for consumers and competition among suppliers. The method of measuring natural gas transfers is normally specified in the contract between the buyer and the seller. Monthly billings are based on a custody transfer flow meter, which is usually owned and maintained by the transporter. An inaccurate custody transfer meter can be a detriment to either the buyer or the transporter of the gas, depending on whether the meter over- or under-registers the volume of gas involved in the transaction. For this reason, most commercial gas buyers monitor the gas exchange with an in-plant check meter or compare to inplant meters used on process equipment to help ensure that both the buyer and seller are treated fairly and equitably. This paper presents a case study in which the buyer, Nevada Cogeneration Associates (NCA), and the transporter, Southwest Gas Corporation (SWG), agreed to a recalibration of a custody transfer turbine meter and the attached pressure/temperature corrector. The meter and corrector were sent to the Gas Research Institute (GRI) Metering Research Facility (MRF) located at Southwest Research Institute (SwRI) in San Antonio, Texas. The custody transfer meter was calibrated (proven) against critical flow venturi reference flowmeters that are accurate to a nominal 10.2%. Following calibration of the meter and evaluation of the corrector, it was found that the combination had overregistered the volume of gas by an average of about 1.088% of reading.