The Working Principle of Ultrasonic Flowmeters

Ultrasonic Flowmeters measure the velocity of a flowing medium by monitoring the interaction between the flow stream and an ultrasonic sound wave transmitted through it.

The main techniques used are:

•  Doppler
• Time of flight/Transit-time

Doppler Flowmeters

These types of ultrasonic flowmeters use Doppler Effect which states that the frequency of sound changes if its source or reflector moves relative to the listener or monitor. The magnitude of the frequency change is an indication of the speed of the sound source or sound reflector.

Doppler flowmeter comprises a housing in which two piezoelectric crystals are potted, one being the transmitter and the other a receiver. This whole assembly is located on the pipe wall as shown below:

The transmitter transmits ultrasonic waves of frequency F₁ at an angle ϴ to the flow stream. If the flow stream contains particles, entrained gas or other discontinuities, some of the transmitted energy will be reflected back to the receiver. If the fluid is travelling at a velocity V, the frequency of the reflected sound as monitored by the receiver can be shown to be F₂ such that:

Where C is the velocity of sound in the fluid.

Rearranging the equation:

Which show velocity is proportional to the frequency change.

Applications of Doppler flowmeters

The Doppler flowmeter is normally used as an inexpensive clamp on flowmeter. The only operational constraints being that the flows stream must contain discontinuities of some kind, without which the device won’t work. Note the device cannot monitor clear liquids. For the Doppler flowmeter to work, the pipeline must be able to transmit acoustic signals.

Doppler flowmeter is mostly used as a flow switch or for flow indication where the absolute accuracy is not required.

Time of Flight Flowmeters

Ultrasonic flowmeters that use time of flight technique differ from Doppler flowmeters in that they rely on transmission of an ultrasonic pulse through the flow stream and therefore do not depend on the discontinuities or entrained particles in the flow stream for operation.

The principle of operation is based on the transmission of an ultrasonic sound wave between two points, first in the direction of flow, and then in the opposing flow. In each case the time of flight of the sound wave between the two points will have been modified by the velocity of the flowing medium and the difference between the flight times can be shown to be directly proportional to the flow velocity.

The sound waves are not generated in the direction of flow but at an angle across it as shown below:

Pulse transit times downstream T₁ and upstream T₂ along the path length D can be expressed as:

T₁ = D/(C + V) and T₂ = D/(C –V), where C is the velocity of sound in the fluid and V is the fluid velocity.

Now, T = T_{1 –} T₂ = 2DV/ (C² –V²)  (equation 1)

Since V² is very small compared to C² it can ignored. It is convenient to develop the expression in relation to frequency and remove the dependency on the velocity of sound C.

Since F₁ = 1/T₁   and F₂ = 1/T₂ and the average fluid velocity V_av = V/ (cosϴ)

Replacing T₁ and T₂   in equation 1 with respective Frequencies, we get:

F₁  - F₂  = (2V_av cosϴ)/D

The frequency difference is calculated by an electronic converter which gives an analog output proportional to average fluid velocity.

In practice, the piezoelectric ceramic transducers used act as both transmitters and receivers of the ultrasonic signals and thus only one is required on each side or the pipe.

Typically the flowmeter consists of a flow tube containing a pair of externally mounted transducers and a separate electronic converter/transmitter. Transducers may be wetted or non-wetted and consist of a piezoelectric crystal sized to give the desired frequency (typically 1 – 5 MHz for liquids and 0.2 -0.5 MHz for gases.

You can also read: 

• The Ultimate Guide to Industrial Flow Instruments
• Working Principle of Coriolis Flowmeters

Advantages of ultrasonic Flowmeters

The unique advantage of ultrasonic flow measurement is the ability to measure flow through the use of temporary clamp-on sensors rather than a specialized flow tube with built in ultrasonic transducers.

Some modern ultrasonic flowmeters have the ability to switch back and forth between Doppler and transit-time (counter propagation) modes, automatically adapting to the fluid being sensed. This capability enhances the suitability of ultrasonic flowmeters to a wider range of process applications.

Limitations of Time of Flight Ultrasonic Flowmeters

Due to the fact that the flowmeter measures velocity across the center of the pipe, it is susceptible to flow profile effects and care should be taken to ensure there is sufficient length of straight pipe upstream or downstream of the flow tube to minimize this kind of effects.

To overcome this problem, manufacturers use multiple beam techniques where several chordal velocities are measured and the average computed, but note that, since ultrasonic flowmeters are easily affected by swirl and other large scale fluid disturbances, it is advisable to ensure an approximately 10 upstream and downstream diameters of straight pipe of the measurement flow tube, when installing them, this helps to stabilize the flow profile.

Also since this type of flowmeter relies on transmission through the flowing medium, fluids with a high solids or gas-bubble content cannot be measured well using ultrasonic meters.

Applications of Ultrasonic Flowmeters

Ultrasonic flowmeters are used in various industrial process measurement applications, and some of them include:

• Measurement of both conductive and non-conductive liquids
•  Measuring aqueous liquids as well as extreme viscous oils
•  Measuring multiple products e.g. allocation measurements in on/off loading
• The are used in all process industries: make up water, demineralized water, boiler feed water etc.
• They are also considered for custody transfer natural gas

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