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Custody Transfer of Oil & Gas

Flow measurement accuracy with money on the line

November 24, 2012
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Flowmeters for oil and gas measurement have many applications. These include allocation metering, check metering, process measurement, flare and stack gas measurement, district heating, LNG measurement, and shale gas measurement. Different flow technologies are used for these applications, but custody transfer is one of the most important applications.

Custody transfer occurs when the possession of a fluid, such as oil, gas, water, or steam, changes hands from one owner to another. The owner could, for example, be an oil or gas production company, a pipeline company, or a utility company. What typically happens in a custody-transfer flow measurement situation is that one or two custody-transfer flowmeters measure the volume or mass of fluid before the transfer is made, and then another set of flowmeters measures the flow after the transfer. What makes custody transfer unique among flowmeter applications is that money changes hands and that accuracy requirements are higher than they are for most other applications.

AGA Approvals
Custody-transfer applications have become increasingly important in oil and gas flow measurement. The American Gas Association (AGA, www.aga.org) began studying custody transfer for natural gas applications in the late 1920s. Its first report, called AGA-1, was issued in 1930. AGA-1 dealt with the use of differential-pressure flowmeters with orifice plates for custody-transfer applications. This report was the predecessor of AGA-3, which was first issued in 1955 and reissued in 1992. In 1981, the AGA issued a report on the use of turbine flowmeters for custody-transfer applications. This report applied to gas applications. It was called AGA-7, and it was reissued in 2006. Prior to that, the AGA published AGA-11 in 2003, a report on the use of Coriolis flowmeters for custody-transfer applications.

The move to standardize the use of ultrasonic flowmeters for custody transfer began in Europe in the mid-1990s. At that time, Groupe Europeen de Recherches Gaziers (GERG) published Technical Monograph 8, which laid out the criteria for using ultrasonic flowmeters for custody-transfer applications. Following this, the AGA published AGA-9 in 1998, which also specified custody-transfer applications for ultrasonic flowmeters. Though it took some time for this standard to be widely accepted, it has created a surge in the use of ultrasonic flowmeters for custody transfer, especially for natural gas pipeline applications.

API Approvals
While the AGA and the American Petroleum Institute (API) work together on many standards projects, the AGA is more focused on industrial and natural gas, while the API focuses more on petroleum liquids. It should not be surprising, then, that the API has issued its own reports on the use of flowmeters involving custody transfer of liquids. These include API MPMS 5.2 (positive-displacement meters), API MPMS 5.3 (turbine meters), API MPMS 5.6 (Coriolis flowmeters) and API MPMS 5.11 (ultrasonic flowmeters). Other API reports address the use of vortex, magnetic, thermal dispersion, and variable-area flowmeters.

Coriolis Flowmeters
Coriolis flowmeters are used for custody-transfer of both liquids and gases, but they traditionally have had a more difficult time measuring gas than liquid. This is because gas is less dense than liquid, and Coriolis meters operate by measuring the impact of a flowing fluid on a vibrating tube. The main limitation on custody transfer for natural gas is line size, since in the past nearly all Coriolis meters were sold for line sizes of six inches or less. Because many of the line sizes for upstream oil & gas applications are above six inches, most Coriolis meters will not work for these applications. However, they are widely used for both compressed natural gas (CNG) and liquefied natural gas (LNG) applications.

Coriolis flowmeters are widely used for custody transfer of petroleum liquids, especially for downstream applications. They are used to measure the transfer of fuel and other petroleum liquids from trucks to planes, trains, ships, and also to buildings. The line sizes for these applications are much smaller than for upstream applications, and they are well-suited to Coriolis meters. Here Coriolis meters mainly compete with positive-displacement meters. They are replacing positive-displacement meters for some applications because of their high accuracy and reliability, and because, unlike positive-displacement meters, they have no moving parts.

One important development for Coriolis meters is the introduction of large flowmeters for line sizes of 8–16 inches. While most of these meters are designed for petroleum liquids, some are also being developed for gas flow measurement. These meters are still not large enough to compete with turbine and ultrasonic meters for large natural gas pipeline measurement, many of which are 20 inches and above. However, they are likely to make an impact on flow measurement for some upstream oil and gas applications. While not all the large Coriolis meters are for custody transfer, some are designed specifically for custody-transfer applications. Companies involved in developing these meters include KROHNE, GE Measurement & Control, Micro Motion, and Endress+Hauser.

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