Custody transfer is one of the most important applications for flow measurement. Many flow measurement technologies are used for custody transfer applications; these include differential pressure DP flow meters, turbine flow meters, positive displacement flow meters, Coriolis flow meters and ultrasonic flow meters.
Differential Pressure Flowmeters
Differential pressure (DP) flow meters are used for the custody transfer of natural gas to measure the flow of liquid, gas, and steam. The DP flow meter consists of a differential pressure transmitter and a primary element. The primary element places a constriction in a flow stream, while the DP transmitter measures the difference in pressure upstream and downstream of the constriction.
In many cases, pressure transmitters and primary elements are bought by the end-users from different suppliers. However, several vendors have integrated the pressure transmitter with the primary element to form a complete flow meter. The advantage of this is that they can be calibrated with the primary element and DP transmitter already in place.
Standards and criteria for the use of DP flow meters for custody transfer applications are specified by the American Gas Association (AGA) and the American Petroleum Institute (API).
An advantage of using a DP flow meters is that they are the most studied and best understood type of flow meter. A disadvantage of using a DP flow meters is that they introduce a pressure drop into the flow meter line. This is a necessary result of the constriction in the line that is required to make the DP flow measurement.
One important development in the use of DP flow meters for custody transfer applications has been the development of single and dual chamber orifice fittings.
The first turbine flow meter was invented by Reinhard Woltman, a German engineer in 1790. Turbine flow meters consist of a rotor with propeller-like blades that spins as water or some other fluid passes over it. The rotor spins in proportion to flow rate (see turbine meters). There are many types of turbine meters, but many of those used for gas flow are called axial meters.
The turbine flow meter is most useful when measuring clean, steady, high-speed flow of low-viscosity fluids. In comparison to other flow meters, the turbine flow meter has a significant cost advantage over ultrasonic flow meters, especially in the larger line sizes, and it also has a favorable price compared to the prices of DP flow meters, especially in cases where one turbine meter can replace several DP meters.
The disadvantage of turbine flow meters is that they have moving parts that are subject to wear. To prevent wear and inaccuracy, durable materials are used, including ceramic ball bearings.
Positive Displacement Flowmeters
Positive displacement (PD) flow meters are highly accurate meters that are widely used for custody transfer of commercial and industrial water, as well as for custody transfer of natural gas and many other liquids. PD flow meters have the advantage that they have been approved by a number of regulatory bodies for this purpose, and they have not yet been displaced by other applications.
PD meters excel at measuring low flows, and also at measuring highly viscous flows, because PD meters captures the flow in a container of known volume. Speed of flow doesn’t matter when using a PD meter.
Coriolis flow meters have been around for more than 20 years and are preferred in process industries such as chemical and food and beverage. Coriolis technology offers accuracy and reliability in measuring material flow, and is often hailed as among the best flow measurement technologies, however, significant limitations exist in conventional Coriolis meters for custody transfer. This is because Coriolis meters have not performed well in measuring two-phase flow conditions, which involve a combination of gas and liquid.
Flow is measured using Coriolis meters by analyzing the changes in the Coriolis force of a flowing substance. The force is generated in a mass moving within a rotating frame of reference. An angular, outward acceleration, which is factored with linear velocity is produced due to the rotation. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid.
A Coriolis meter has two main components: an oscillating flow tube equipped with sensors and drivers, and an electronic transmitter that controls the oscillations, analyzes the results, and transmits the information. The Coriolis principle for flow measurement requires the oscillating section of a rotating pipe to be exploited. Oscillation produces the Coriolis force, which can be sensed and analyzed to determine the rate of flow.
Ultrasonic flow meters were first introduced into industrial markets in 1963 by Tokyo Keiki (now Tokimec) in Japan. Custody transfer measurements have been around for a long time, and over the past ten years, Coriolis and ultrasonic meters have become the flow meters of choice for custody transfer in the oil and gas industry.
Ultrasonic meters provide volumetric flow rate. They typically use the transit-time method, where sounds waves transmitted in the direction of fluid flow travel faster than those travelling upstream. The transit time difference is proportional to fluid velocity. Ultrasonic flow meters have negligible pressure drop, have high turndown capability, and can handle a wide range of applications. Crude oil production, transportation, and processing are typical applications for this technology.
The use of ultrasonic flow meters is continuing to grow for custody transfer. Unlike PD and turbine meters, ultrasonic flow meters do not have moving parts. Pressure drop is much reduced with an ultrasonic meter when compared to PD, turbine, and DP meters. Installation of ultrasonic meters is relatively straightforward, and maintenance requirements are low.