Ultrasonic flow meters use sound waves to determine the velocity of a fluid flowing in a pipe. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe and its reflections from the fluid are the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.
Transit time ultrasonic flow meters send and receive ultrasonic waves between transducers in both the upstream and downstream directions in the pipe. At no flow conditions, it takes the same time to travel upstream and downstream between the transducers. Under flowing conditions, the upstream wave will travel slower and take more time than the (faster) downstream wave. When the fluid moves faster, the difference between the upstream and downstream times increases. The transmitter processes upstream and downstream times to determine the flow rate.
Transit time ultrasonic flow meters are usually more accurate than Doppler ultrasonic flow meters. Doppler ultrasonic flow meters are usually more economical.
Ultrasonic flow meters are commonly applied to measure the velocity of liquids that allow ultrasonic waves to pass, such as water, molten sulfur, cryogenic liquids, and chemicals. Transit time designs are also available to measure gas and vapor flow. Be careful because fluids that do not pass ultrasonic energy, such as many types of slurry, limit the penetration of ultrasonic waves into the fluid. In Doppler ultrasonic flow meters, opaque fluids can limit ultrasonic wave penetration to near the pipe wall, which can degrade accuracy and/or cause the flow meter to fail to measure. Transit time ultrasonic flow meters can fail to operate when an opaque fluid weakens the ultrasonic wave to such an extent that the wave does not reach the receiver.
Ultrasonic flow meters
do not obstruct flow so they can be applied to sanitary, corrosive and abrasive liquids. Some ultrasonic flow meters use clamp-on transducers that can be mounted external to the pipe and do not have any wettedparts. Temporary flow measurements can be made using portable ultrasonic flow meters with clamp-on transducers. Clamp-on transducers are especially useful when piping cannot be disturbed, such as in power and nuclear industry applications. In addition, clamp-on transducers can be used to measure flow without regard to materials of construction, corrosion, and abrasion issues. However attractive, the use of clamp-on transducers introduces additional ultrasonic interfaces that can affect the reliability and performance of these flow meters. In particular, if not properly applied and maintained, attenuation of the ultrasonic signal can occur at the interfaces between the clamp-on transducers and the outside pipe walls, and between the inside pipe walls and the fluid.
Ultrasonic flow meters are available in sizes to 72 inches and larger.
For transit time ultrasonic flow meters, be sure that the fluid can adequately conduct ultrasonic waves, because the flow meter will not measure when the ultrasonic waves cannot penetrate the flow stream between the transducers. Similarly, ultrasonic waves must be able to penetrate the fluid for Doppler flow meters to operate accurately. When the fluid is relatively opaque and does not penetrate the fluid, Doppler flow meters tend to measure the velocity of the fluid at or near the pipe wall, which can cause significant measurement error and/or cause the flow meter to fail.
For Doppler ultrasonic flow meters, be sure that the fluid adequately reflects ultrasonic waves, because the flow meter will not operate without a reflected ultrasonic signal. Depending upon design, reflections can occur due to small bubbles of gas in the flow stream or the presence of eddies in the flow stream. If not already present in the flowing stream, generating these sources of reflection can be difficult in practice. Fortunately, some combination of bubbles of gas and/or eddies are present in most applications.
The velocity of the solid particles in slurry can be different than its liquid carrier fluid. Be careful applying ultrasonic technology when the solid particles can become concentrated in one part of the flowing stream, such as in a horizontal pipe flowing at a relatively low velocity. Be careful when applying Doppler ultrasonic flow meters in slurry applications because the solid particles can produce strong signals that can cause the Doppler flow meter to measure the velocity of the solids and not the velocity of the liquid.
Avoid fluids that can coat wetted transducers or coat the pipe wall in front of non-wetted transducers, because the flow meter will not measure when the ultrasonic waves cannot enter the flow stream. Be sure to maintain reliable clamp-on transducer connections to the pipe wall, because the flow meter will not measure when the ultrasonic waves are not able to reach the fluid.
Be sure to understand the process and apply these flow meters properly. For example, a periodic cleaning process upstream may cause the flow meter to stop working because the dirt may not allow ultrasonic energy to pass through the fluid. Further, if the dirt coats wetted transducers, the flow meter may fail to operate until it is cleaned.
