Measurement of hydrocarbon liquids at various points in the distribution chain is a primary requirement for production, gathering, and transportation of crude oil and refined products. In this article, Emerie Dupuis of Daniel Measurement and Control, Emerson Process Systems, explains why flow measurement accuracy is critical whenever money is changing hands at custody transfer. A common application is the Lease Automatic Custody Transfer unit (LACT unit) used for automatic unattended measurement of crude oil and other wellhead liquids when ownership is transferred from lease site to a pipeline, truck/rail loading/unloading terminal, or other points of transfer.
The American Petroleum Institute (API) maintains a comprehensive program for hydrocarbon measurement standards based on technical principles consistent with current technology, accounting standards, engineering practices and stewardship of industry committees.
Measurement, often referred to as Metering, consists of a variety of static and dynamic measurement devices, each having advantages and disadvantages dependent on the metering application, along with chemical and physical conditions of the hydrocarbon liquid.
In production, transportation, and refining applications, there are four widely used measurement devices; positive displacement meter, turbine meter, Coriolis meter, and ultrasonic flowmeters.Click to tweet
Positive Displacement (PD) Meters and Turbine Meters have historically been the preferred measurement device for custody transfer, with Turbine Meters having an advantage for lower viscosity fluids.
In many of today’s custody transfer applications, Coriolis Meters are preferred due to their higher turndown ratio along with their ability to measure density as well as throughput.
Liquid Ultrasonic Flowmeters are now gaining wider use in more critical applications, particularly in downstream refining markets. These are volumetric devices that measure the velocity of flowing liquid and use that velocity to calculate flow rate. Advanced models have sophisticated transmitters, flow computers, and full diagnostic capabilities that make calibration easier and more reliable.
Integrated modular LACT units are ideal for easy transportation to site
A common application of these devices is the Lease Automatic Custody Transfer unit (LACT unit) used for automatic unattended measurement of crude oil and other wellhead liquids when ownership is transferred from lease site to a pipeline, truck/rail loading/unloading terminal, or other points of transfer.
LACT Units can incorporate a wide variety of configurations, but systems designed for custody transfer will incorporate several other key components in addition to the metering device such as temperature and pressure transmitters, flow computers, charge pumps, sampling systems, and other process safety devices.
LACT Units are designed as a measurement system typically constructed in a modular design on a steel structure frame with all process fluid piping and control devices integrated into a unit that is easily accessible for operation and maintenance. Modules are also designed for easy mobility and truck transportation to site.
Meter Provers ensure accuracy of flow data
Metering systems must be checked for accuracy periodically. A Meter Prover is a system that verifies the performance accuracy of the liquid meter at the point of custody transfer, ensuring the metering system is providing accurate flow data and conforms to the API Standard and requirements of regulatory bodies. For detailed considerations when planning ahead for accurate liquid meter proving, click here.
The most common devices used for proving liquid meters are Pipe Provers. They can be designed as Uni-directional or Bi-directional and are manufactured in various sizes according to the application requirements. Piping systems on meter provers must meet strict manufacturing tolerances, and additional measurement standards.