Ultrasonic Flow Meters In The Energy Measurement Spotlight

By Jesse Yoder, PhD | July 2009 Vol. 236 No. 7

Ultrasonic flow meters are among the most popular of the flow meter types. They are used for a wide range of applications, including natural gas and petroleum liquids custody transfer, check metering and flare gas measurement.

While they are most widely used in the oil and gas industry, ultrasonic flow meters can also be found in the refining, power, chemical, water and wastewater and other process industries. What accounts for the popularity of ultrasonic flow meters in these industries?

Ultrasonic flow meters were first introduced into commercial markets in Japan by Tokimec in 1963. Controlotron brought out the first clamp-on ultrasonic flow meter in the United States in 1972. Unfortunately, ultrasonic flow meters were not well understood during the 1970s and 1980s when they were first used. As a result, they were often misapplied, and many users got a negative impression of them. They did not come to be widely used in industrial markets until the 1990s.

In many ways, the ultrasonic flow meter market is a market of dichotomies. The fit of ultrasonic flow meters for a given application depends on the ultrasonic technology, the mounting type, the number of paths, and other considerations. For example, transit time flow meters are best suited for clean liquids, while Doppler meters are better suited to dirty liquids. End-users who understand these different aspects of ultrasonic technology will be in a better position to select the right type of meter for their application.

Two Main Technologies

The two main ultrasonic flow meter technologies are transit time and Doppler. Transit time meters have both a sender and a receiver. A transducer sends an ultrasonic signal at an angle from one side of the pipe to the other and back. The signal travels faster when it travels with the flow than when it travels against the flow. The flow meter determines how long it takes for the signal to cross the pipe in one direction, and how long it takes the signal to cross the pipe in the reverse direction. The difference between these two times is proportional to flow rate.

Doppler ultrasonic flow meters also send a signal across the pipe. However, with Doppler technology, the signal bounces off particles in the flow stream instead of the other side of the pipe. The flow particles are traveling at the same speed as the flow. As the signal from the transducer travels through the flow stream, its frequency shifts in proportion to the mean velocity of the fluid. The reflected signal is detected by a receiver, which measures its frequency. The meter calculates flow by comparing the transmitted and reflected frequencies.

Mounting Types

Ultrasonic flow meters come in the form of either clamp-on, spoolpiece or insertion meters. The transducers of clamp-on ultrasonic flow meters are mounted outside the pipe. The ultrasonic signal passes through the pipe wall and then through the fluid inside the pipe. For accurate measurement, it is important to know the thickness and material of the pipe so that the inside diameter of the pipe can be accurately determined. Clamp-on flow meters have the advantage that they are completely non-intrusive, and can easily be moved from one location to another.