What Is an Ultrasonic Flow Meter?
An Ultrasonic Flow Meter is a flow meter device that utilizes the acoustic vibration caused by ultrasonic waves. It has the feature of non-contact measurement with the object to be measured.
There are two types of ultrasonic flow meters: time-difference and doppler.
Ultrasonic flow meters have a simple structure with no pressure loss because there is no obstruction in the pipe. They have features such as low failure rate, wide measurable range, they are unaffected by density and viscosity of fluid, and have high accuracy.
In addition, ultrasonic flow meters are a clamp-on device and can be mounted outside the pipe to measure corrosive fluids without corroding the sensor.
Uses of Ultrasonic Flow Meters
Ultrasonic flow meters are used to measure the flow rate of gases, liquids, and solids moving through piping. Specifically, they are often used to control the flow of fluids in water treatment facilities and factories.
Among ultrasonic flow meters, the propagation time difference method cannot ensure measurement accuracy if solids or bubbles are mixed in. Therefore, it is used to measure fluids with high purity, for example, to manage the flow rate of chemicals in semiconductor factories.
On the other hand, the doppler method uses solid substances and bubbles in the fluid to measure the flow rate, so it is used to measure the flow rate of suspended fluids, for example, sludge and wastewater.
Ultrasonic flow meters are a clamp-on device that attaches to and detaches from the outside of the piping for both the time-difference-of-propagation and doppler methods, and transmits and receives ultrasonic waves from the outside of the piping. This means that there are no obstructions inside the piping, and the pressure loss is zero.
The simple structure makes them difficult to break down, and they’re not affected by the density or viscosity of the fluid. In addition, since there is no need to place sensors or other components inside the piping, corrosive fluids can be measured without causing corrosion of the components.
Principle of Ultrasonic Flow Meters
There are two types of ultrasonic flow meters: time-differential and doppler, each of which have a different configuration.
1. Ultrasonic Flow Meters with Time Difference Propagation
Time differential ultrasonic flow meter is the most common type of ultrasonic flow meter. They are designed to transmit and receive ultrasonic waves diagonally across the fluid from upstream to downstream, and vice versa.
If the fluid is not moving, the propagation speed of ultrasonic waves transmitted in the forward direction is the same as the speed of propagation of ultrasonic waves transmitted in the reverse direction. However, if the fluid is moving, the propagation velocity of ultrasonic waves transmitted in the forward direction will be the velocity of the fluid plus the velocity of the ultrasonic waves transmitted in the reverse direction.
On the other hand, the propagation velocity of ultrasonic waves transmitted in the reverse direction is the velocity of the fluid minus the velocity of the fluid. The ultrasonic flow meter calculates the velocity of the fluid from this difference in velocity, and then calculates the flow rate.
Because of this structure and principle, ultrasonic flow meters have high accuracy in measuring flow rate. However, if there are solids or bubbles in the liquid, measurement accuracy cannot be maintained and measurement cannot be performed.
2. Doppler Ultrasonic Flow Meter
Doppler ultrasonic flow meters use the Doppler effect. The Doppler effect is typified by the phenomenon where the sound of an ambulance siren is heard differently when approaching and when moving away. This phenomenon occurs because the apparent wavelength of the sound wave changes when the source of the sound approaches the observer and when it moves away from the observer.
They use a transducer to irradiate ultrasonic waves to a fluid flowing inside a pipe, and take advantage of the phenomenon that the ultrasonic waves are reflected by grains and bubbles in the fluid. The transducer then receives ultrasonic waves in which the wavelength is shifted by the reflections from the grains and bubbles.
Since there is a linear relationship between the frequency change of the ultrasonic wave and the flow velocity, the flow rate can be calculated from the flow velocity.
Other Information About Ultrasonic Flow Meters
1. Issues Related to Bubbles in Ultrasonic Flow Meters With Time-Differential Propagation
When measuring gas or liquid flow with a time-differential ultrasonic flow meter, the measured value is affected by the velocity distribution in the system, so the upstream side of the flow meter must be rectified.
Especially when the flow velocity of the fluid is high, cavitation is likely to occur upstream of the measurement device and in the system. When cavitation occurs, it is easy for air bubbles to be generated or mixed in the fluid, and the measurement accuracy of the time differential ultrasonic flow meter cannot be ensured.
A countermeasure against these bubbles is to install a gas-liquid separation tank upstream of the device. However, this requires modification of existing equipment, and the cost is a disadvantage.
In recent years, therefore, ultrasonic flow meters with a function to measure flow rate by canceling the data when bubbles pass through has been developed.
2. Advantages of Clamp-on Ultrasonic Flow Meters
There are volumetric flow meters, turbine flow meters, flow cell flow meters, etc., but all of them require the installation of a sensing component in the piping to measure the flow rate of fluid in the piping. For newly constructed facilities, the location of the flow meter can be determined at the design stage and its specifications can be finalized.
However, in the case of existing facilities, there are many cases where piping modifications are required and the equipment must be shut down. For example, when installing a differential pressure flow meter or turbine flow meter in an existing facility, cutting the piping is required.
However, a clamp-on ultrasonic flow meter does not require any piping work and can be easily installed in an existing facility.