Energy Demand Propels Custody Transfer Flow Measurement

The world’s drive for energy has put oil and gas flow measurement in the spotlight. Rising oil prices, along with increasing prices for natural gas, have made it more important to oil and gas producers, distributors and end-users to have accurate and reliable measurements of flow. As many of the world’s economies recover from a severe recession, demands for energy are increasing.

Custody transfer is one of the most important applications for flow measurement. In custody transfer applications, the ownership of a fluid is transferred from one person or company to another. For these applications, it is important that both parties agree on the type of flowmeter being used and on the conditions under which the flowmeter is being used. Having this agreement reduces the likelihood that a dispute will arise at a later time over the amounts being measured.

The main types of flowmeters used for custody transfer of natural gas are differential pressure (DP), turbine and ultrasonic. DP and turbine flowmeters have been around for many years, while ultrasonic flowmeters are a more recent participant in this market. Both DP and turbine flowmeters have the advantage of having been studied for many years, so their performance in gas flow is well understood. These two flowmeter types were first to receive industry approvals for use in custody transfer applications. Industry approvals have played a key role in determining which types of flowmeters are used for custody transfer.

Differential Pressure Flowmeters
DP flowmeters are made up of a DP transmitter and a primary element. The primary element places a constriction in the line, resulting in reduced pressure downstream from the element. The DP transmitter detects the difference in upstream and downstream pressure and uses this value to compute the flowrate. There are many different types of primary elements, including: orifice plates, venturi tubes, pitot tubes, flow nozzles, wedge elements and laminar flow elements.

While each of these primary elements has its advantages and disadvantages, orifice plates are the most widely used type of primary element. And the standards that have been written for DP flowmeters have been written for DP flowmeters with orifice plates. Both the American Gas Association (AGA) and the American Petroleum Institute (API) have played an important role in laying out standards and criteria for the use of DP flowmeters for custody transfer applications. The AGA began in 1927 by publishing a preliminary report on the use of differential pressure flowmeters using orifice plates. In 1930, this report became AGA-1, the first industry standard published on the use of orifice meters in gas flow measurement. This was followed up in 1935 with the publication of AGA-2, which incorporated the results of additional tests.

Shipments of Primary Elements Worldwide for Flow by Type in 2009 (percents of dollars)
Source: Volume X: The World Market for Flowmeters, 2nd Edition (Flow Research)