Flow Meter Calibration

With the effective calibration of flow meters at the GRI Metering Research Facility, the natural gas industry can cut consumer costs and increase profits.

By Edgar B. Bowles Jr.      image of PDF button

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Edgar B. Bowles Jr. is manager of flow measurement in the SwRI Mechanical and Fluids Engineering Department, which oversees operation of the GRI Metering Research Facility. Bowles is a specialist in fluid dynamics and heat transfer. His most recent research efforts have been primarily for the oil and gas industry.

tt18.jpg (108532 bytes)Deregulation and open access in the U.S. natural gas industry in the 1990s has built a competitive market, motivating more efficient company operations. Gas companies face constant pressure to earn greater profits and cut costs. With many more participants buying and selling natural gas in the U.S. market, accurate measurement of gas at custody transfer points is paramount.

A custody transfer point is the location where gas changes ownership, such as from gas supplier to gas consumer. If a bias in the custody transfer meter favors the seller, the buyer cannot pass on those cost inefficiencies and effectively compete in the open marketplace. The seller wants to be paid correctly for the volume of gas delivered, and the buyer wants to receive the volume of gas purchased. It's strictly a business proposition that's fair to both parties.

The GRI Metering Research Facility

For more than eight years, Southwest Research Institute (SwRI) has operated, on behalf of the Gas Research Institute (GRI), the Metering Research Facility (MRF). Located on the grounds of SwRI, the MRF is a world-class, natural gas flow measurement test facility.

It is one of only two facilities in North America that can perform commercial meter calibration on natural gas. The MRF is a closed-loop, recirculating gas flow calibration facility that provides maximum control of test conditions, resulting in excellent repeatability and high accuracy calibrations. The facility consists of a high-pressure loop for large-diameter, high-pressure, volume metering applications and a low-pressure loop for lower pressure, volume metering applications. A key feature of the MRF is its primary calibration system, based on gyroscopically balanced weigh tanks, the most accurate means known for measuring mass flow rate of gas.

Clients find great operational and economic advantage in having their meters calibrated at the MRF before installation. All meter calibrations conducted at the MRF are traceable to the U.S. National Institute of Standards and Technology. The following example demonstrates the commercial value of the MRF to the U.S. natural gas industry.

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Flow calibrations are performed at the MRF on gas flow meters, such as this 12-inch ultrasonic meter, prior to installation to ensure measurement accuracy meets acceptable standards set by the U.S. National Institute of Standards and Technology and the U.S. natural gas industry.

Flow meter calibration precision

A flow calibration on a 20-inch diameter ultrasonic gas flow meter was recently performed at the MRF for a major gas transmission pipeline operator. The meter was scheduled to be installed at a point of sale (custody transfer point) on a large gas transmission pipeline.

Typically, ultrasonic meter fabrication tolerances and performance limitations on meter electronics can result in metering inaccuracies. Most of these "built-in" inaccuracies can be corrected by flow calibrating a meter prior to installation. In a flow calibration, the test meter output is compared to a known reference value. In this case, the MRF is the flow reference. The test meter output is then "corrected" to give the same value as the flow reference.

The measurement accuracy customarily accepted by the U.S. natural gas industry for ultrasonic meters is ±0.7 percent. A "positive" measurement error means that the meter is over-registering or that less gas is passing through the meter than is being indicated by the meter output. Conversely, a "negative" measurement means that more gas is passing through the meter than is being registered.

In the example case, the 20-inch ultrasonic meter was found to have a measurement bias error of approximately +0.3 percent over its normal operating range. That is, the output of the meter consistently read +0.3 percent high compared to the MRF reference flow rate.

Assume that this meter will, on average, be operated at its mid-range, which means the gas velocity at the meter will be nominally 50 feet per second at a line pressure of 850 pounds per square inch. Also assume that the value of the gas passing through the meter will be $2 per thousand standard cubic feet, which is the typical current market value. In this scenario, the cost of the meter error, in terms of incorrectly measured gas volume, over one-year's time, would be almost $1.2 million in favor of the seller.

The cost of the meter calibration at the MRF was $5,000. Thus, the cost for the calibration was paid back in approximately 1.5 days. Furthermore, the seller avoided possible litigation from the buyer because of the substantial overcharge that would have resulted from the meter not being flow calibrated before installation. Because ultrasonic meters are a relatively new technology, there is no established calibration interval; however, some companies suggest recalibration every three to five years.

In 1999, the MRF will calibrate approximately 50 ultrasonic flow meters. The average bias error correction provided by flow calibration of the meters will be between 0.25 percent and 0.5 percent. Assuming that each meter calibration saved $1.2 million per year in costs because of measurement error, the value to the industry would be $60 million. Assuming each meter calibration at the MRF costs $5,000, the cumulative payback period for industry would only be 75 days.

2000 and beyond

As the MRF concludes its first decade of service, its mission remains unchanged. The facility continues to help advance natural gas flow measurement technology worldwide, assist with industry standardization, address specific metering problems faced by gas companies, and expedite the transfer of research results to field applications. With the current business environment, in which natural gas company consolidation and downsizing is the norm and in-house technical expertise continues to diminish, the MRF will play a more prominent role in the coming years in developing and assessing gas measurement technology.

Comments about this article? Contact Bowles at (210) 522-2086 or ebowles@swri.org.

Published in the Fall 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.

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