There are many types of meters in general industrial use, many of these are not suitable for custody transfer. Generally, five technologies are used when flow measurement means money.
Differential Pressure (DP) Flowmeter
Differential pressure (DP) flowmeters are the oldest of the technologies and the first to be studied and approved for custody transfer for natural gas. In 1930, the AGA issued Report AGA-1 to cover the use of DP flowmeters with orifice plates for custody transfer applications. DP meters can operate in very harsh environments and also feature a high degree of robustness and reliability.
It wasn’t until 1981 that AGA-7, involving turbine meters, was published. During the 1990s, turbine flowmeters began to displace DP flowmeters, especially for gas applications. The main reasons were the higher accuracy of many turbine flowmeters, along with their greater rangeability. Turbine meters are also used for custody transfer of petroleum liquids.
Positive Displacement (PD) Meters
Positive displacement (PD) meters are common for small line size applications. It is unusual to find PD meters in line sizes above 10”. They are very good at measuring fluid at low flowrates. Downsides of PD meters include pressure drop and mechanical moving parts.
Recent advances in measurement technology have introduced two highly accurate and repeatable flow measurement technologies to custody transfer: Coriolis mass flow meters and multiple-path ultrasonic flow meters.
In the oil and gas industry, as in many others, the transition from traditional-technology to new-technology flowmeters is evident not only in custody transfer applications, but also elsewhere in the flowmeter world. However, traditional meters still have the advantage of a large installed base.
Ultrasonic flowmeters are volumetric devices that measure the velocity of flowing liquid or gas, and use that velocity to calculate flow rate.
Transit-time ultrasonic flowmeters use a pair of transducers, one pointed upstream and one pointed downstream in the meter body. The difference in transit times of the downstream-directed pulses and the upstream directed pulses is used to determine the average velocity of the fluid. Multiplying velocity by the cross-sectional area of the pipe gives volumetric flow rate (Q=VA). Mass flow can be computed using input from a densitometer.
Ultrasonic flowmeters often use multiple sets of transducers mounted in the pipe wall. Multi-path meters allow highly accurate readings of average axial velocity, with diagnostic information about flow disturbances which could impact measurement accuracy.
Difference between Custody Transfer & Non-Custody Transfer
One difference between custody transfer and non-custody transfer applications for ultrasonic flowmeters is that custody transfer applications require three or more measurement paths within the meter. Multi-path ultrasonic meters measure flow velocity at more than one location in the pipe. A path is the route traveled by an ultrasonic signal from one side of a pipe to the other and back.
Since approval by AGA in 1998, ultrasonic flowmeters have become widely used for custody transfer of natural gas. They are typically available from 2”-line sizes and can handle large natural gas pipelines, which often range from 20 to 42 inches-line sizes. Ultrasonic flowmeters are also used for custody transfer of petroleum liquids – from the oil well through the refinery to the ultimate distribution point.
Advantages of Ultrasonic Flowmeter
Advantages of ultrasonic flowmeters include no moving parts, high accuracy and turndown ratio, and virtually no pressure drop. Because there is little or no pressure drop, ultrasonic meters minimize the loss of energy due to friction losses through the meter, and improve the efficiency of pump stations in oil and gas pipelines.
They can be used for measurement of crude, including the heavy crude found in oil shale and oil sands. Advanced models have sophisticated transmitters and flow computers with full diagnostic suites that make calibration easier and reduce measurement uncertainty. This capability simplifies operations.
Coriolis Mass Flowmeters
Measure mass flow
Unlike ultrasonic meters, Coriolis flowmeters are not volumetric flowmeters, but instead measure mass flow directly. As fluid flows through a Coriolis flowmeter, the measuring tubes twist slightly due to the Coriolis force. The natural vibration frequency of the tubes changes with the mass flow of the fluid.
Micro Motion was the first manufacturer of Coriolis mass flow meters, beginning in the 1970s. In 2002, the API approved the use of Coriolis flowmeters in custody transfer and fiscal metering (API Chapter 5.6). Coriolis meters are currently supplied for line sizes 1/14” to 16” (1-400 mm).
Limitations of Coriolis Flowmeter
While Coriolis flowmeters can have pressure drop and are not available for line sizes above 16” (400 mm), these disadvantages are outweighed by a lack of moving parts and significant accuracy improvement over many other flow meters, even those that are temperature and density compensated.
In fiscal metering and custody transfer applications, this accuracy is critical.