How do Ultrasonic Flowmeters Work?

Ultrasonic Flowmeters | Flowmetrics

Ultrasonic flowmeters are one of the numerous technologies for monitoring the flow of fluid through a pipe. These flowmeters use ultrasonic transducers and focus on monitoring velocity based on the use of sound waves sent through the material in the pipe. There are three primary versions of ultrasonic flowmeters including:

  • Transmission Ultrasonic Flowmeters
  • Doppler Shift Ultrasonic Flowmeters
  • Open-Channel Ultrasonic Flowmeters

Each version of the ultrasonic flowmeter completes the same task using slightly different methods. To better understand how these flowmeters work, consider these overviews.

Transmission Ultrasonic Flowmeters

Transmission or contra-propagating transit-time flowmeters, operate by measuring the different amount of time required for a pulse sent into the flowing fluid takes to reach a corresponding sensor. The time difference provides a measurement that can calculate the velocity of the fluid moving within that section of the pipe. The point of comparison is based on the time received by the transmission ultrasonic flowmeter and the known speed of sound.

There are two variants of transmission ultrasonic flowmeters. One version is referred to as an in-line. These are built into the pipe, leading to them also being called intrusive or wetted ultrasonic flowmeters.

The other variant is a clamp-on, or non-intrusive, variant. These are mounted on the pipe instead of being integrated into them. These versions must be calibrated to compensate for any resistance related to the pipe material itself, as purely comparing the sensor readings to the speed of sound would produce an inaccurate result without these adjustments.

Doppler Shift Flowmeters

Doppler shift flowmeters are also called reflection or Doppler flowmeters. These use the reflection of an ultrasonic beam off of specified materials. When a fluid is clean, the turbulence of that fluid can be used to measure velocity. However, when fluids are not clean, the beam is reflected off of solid particles or air bubbles that are free-floating in the material.

These flowmeters are commonly used to measure blood flow, as the ultrasonic beam can safely penetrate the skin before bouncing off a reflective plate. The movement of the blood within the vessel affects the return speed of the beam, allowing the volume to be estimated.

Open-Channel Ultrasonic Flowmeters

Open-channel ultrasonic flowmeters are more often used when measuring water levels in an open channel. The ultrasonic element determines the height of the water and compares it to the overall geometry of the channel itself. That allows the height to be correlated to a flow rate.

Often, open-channel ultrasonic flowmeters also have temperature sensors in their construction. This helps compensate for how temperature can affect the speed of sound through the air, allowing for a more accurate calculation.

Pros of Ultrasonic Flowmeters

Ultrasonic flowmeters are highly accurate when used for custody transfer of materials like natural gas and petroleum liquids. They are also suitable for use in high-temperature situations as the calculations can compensate for shifting temperatures. Accuracy levels tend to be consistent and provide a high turndown.

When using a clamp-on variant, accurate readings can be realized without having to compromise the integrity of the pipe. Additionally, they do not obstruct the flow of fluids through the pipe and are ideal for applications where the integrity of the liquid is critical. Since clamp-on ultrasonic flowmeters aren’t in contact with the fluid, this helps when concerns about maintaining a sanitary environment are concerned. They also allow for proper metering when the materials being tracked are corrosive or abrasive in nature, while providing safe solutions for hazardous conditions, such as piping involved in nuclear operations.

Generally, maintenance requirements are low, and clamp-on versions can often be maintained with relative ease due to the accessible location.

Cons of Ultrasonic Flowmeters

Often, ultrasonic flowmeters require a substantial upfront investment. Additionally, they are not ideal for situations where stray process vibrations may be present. If improper maintenance of the pipe occurs, such as issues of buildup or corrosion, the ultrasonic flowmeter may not be properly calibrated to compensate for the change in the diameter of the pipe, leading to inaccurate results.

Further, each form of ultrasonic flowmeter requires certain conditions be present for proper operation. For example, transmission and Doppler ultrasonic flowmeters require the fluids involved to conduct ultrasonic waves properly. Otherwise, the waves are unable to penetrate the fluid to reach the transducers. Often, if a fluid tends to be opaque, penetration issues may be present, creating faulty results.

Similarly, Doppler ultrasonic flowmeters need the fluid to reflect the ultrasonic waves adequately. For example, small bubbles in the flow stream can be sufficient but would need to be naturally occurring under ideal circumstances. Often, generating bubbles artificially is difficult in practice, so results may not be accurate.

When measuring the movement of a slurry, the particles within the fluid may not move at the same rate as the fluid itself. This can lead solid particles to become concentrate in a certain area, which leads to inaccurate results.

Maintenance and Calibration

All flowmeters require regular maintenance and calibration, as well as the pipes in which the fluid is traveling. Failing to properly maintain any components or equipment can lead to inaccurate outputs. Often, proper maintenance and calibration schedules can be obtained from the manufacturer. But, if you observe less than optimal output or suspicious readings, it may be necessary to complete these processes more frequently.

When maintaining a flowmeter, make sure to meet recognized national standards. You also want to calibrate based on higher standards than actually required based on the unit under test. This ensures your flowmeter is calibrating more stringently than you actually have to meet for operations.

Failing to maintain calibration standard creates certain risks in business operations. First, a flowmeter that provides a high reading means your actual output is less than your flowmeter suggests. In cases where flowmeters are used to measure product for customers, this means customers are not getting all for which they have paid. When readings are falsely low, you are giving out more product than has been purchased. Either of these instances can affect a company’s reputation and potential leads to lost profit or customers.

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