In a continuing series on natural gas flow meter types, here we discuss the ultrasonic meter. This meter is used to measure natural gas volume and excels in custody transfer. While this flow meter is used to measure both liquid and gases, this post highlights information pertaining to natural gas.
Natural Gas Flow Measurement
Our blog typically discusses applications for thermal mass flowmeters because that is our business. While our thermal mass flow meters serve a variety of applications, there are additional devices that excel in other gas flow measurement areas.
Transit-Time Ultrasonic Flow Meter
An ultrasonic flow meter is a device commonly used to calculate volumetric flow of fluid. There are two leading types, the transit-time and Doppler style meter. In the transit-time ultrasonic flow meter, the transducers are upstream and downstream of each other and each act as a transmitter and receiver. One transducer, of course, emits the ultrasound signal with the flow, while the other emits it against the flow. The meter measures the difference in transit time between the two transducers, and the velocity difference is used to calculate flow.
Over the years there have been technological advances that have helped improve the limitations of the meters. For example, improvements in electronic processing have permitted the meter to measure some fluids that aren’t clean. Additionally, the introduction of a multi-path transit-time meter allows the meter to measure flow at more than one location which increases the accuracy of the meter and permits measurement in demanding flows (swirls, irregularities and cross-flow).
The ultrasonic flow meter is extremely accurate, usually within 1%, and is approved for use in custody transfer (fiscal metering). The meter has good repeatability, high turndown and rangeability, and can measure at both high and low pressures. The device tolerates high and low temperatures, and the clamp-on style can be fastened on the outside of the pipe without penetrating it. It is self-diagnosing, and once calibrated the meter requires less frequent calibrations. Since the meter has no moving parts, it has lower maintenance because it does not have to be lubricated, or parts do not have to be replaced. Additionally, the meter has long-term reliability.
Ultrasonic meters can also measure flows in large pipe sizes, unlike other popular choices (like Coriolis meters). As a result, ultrasonic meters are used more in pipes 6” and larger.
Additionally, the ultrasonic meter works better than magnetic meters for oil and gas, since gases and hydrocarbons are nonconductive.
On the onset, ultrasonic meters are extremely expensive. Additionally, the ultrasonic flow meter is disturbed by noise and can be impacted by dirt and fluids. The pipe walls can interfere with signals for the clamp-on ultrasonic meters, and buildup on the pipe walls can affect the accuracy. Additionally, if one needs to determine mass flow with an ultrasonic meter, pressure and temperature measurement is required because ultrasonic meters calculate volumetric flow.
Ultrasonic Meter Manufacturers
Some of the ultrasonic flow meter manufacturers are Daniel (Emerson), Endress+Hauser, FMC Technologies, GE Measurement & Control Solutions, Greyline, Honeywell (RMG), Instrumart, Krohne and Sierra.
If you are interested in natural gas flow measurement, you may want to review the following meter styles:
Photo: Wikipedia public domain