Flow Meters-2

Ultrasonic Flow Meter:

Ultrasonic Flow meter is a type of flow meter, Which uses ultrasonic sound waves to determine the velocity of fluid flowing in a pipe. There are two different types of ultrasonic flow meters which include, Doppler flow meter and another one is Transit time flow meter. 

Doppler Flow Meter:

Doppler ultrasonic flow meter operate on the doppler effect. In Doppler effect, One transducer transmits an ultrasonic beam of 0.5Mhz into the flow stream. Liquid flowing through the pipe must contain sonically reflected materials such as solid particles or air bubbles. the movement of these materials alters the frequency of the wave reflected onto a second receiving transducer. The frequency shift is linearly proportional to the rate of flow of materials flowing in the pipe.

The basic equations defining the doppler flow meter are,

Equation(3) clearly shows that the Flow velocity is equal to the Doppler frequency shift. Now, because the diameter(D) of the pipe is known, Volumetric flow rate(e.g Galloons per minute) can be measured using the following expressions,

Where, K= a constant dependent on the units of Flow velocity and D.

One Doppler meter design mounts both the transmitting and receiving transducers in the same case attached to one side of the pipe. Reflectors in the flowing liquid return the transmitting signal to the receiver with a frequency shift proportional to the flow velocity.

Advantages Of Doppler Flow Meter:

The main advantage of doppler flow meter is its non-intrusive design. An acoustic-coupling compound is used on the surface of the pipe and the sensors are simply held in place to take a measurement or, for a more permanent installation, they are strapped around the pipe. Some manufacturers offer a special clamp-on probe which allows connection to smaller pipe sizes. Other advantages include:
1) Easy installation and removal.
2) No process downtime during installation.
3) No moving parts to wear out.
4) No process contamination.
5) Works well with dirty or corrosive fluids.
6) No leakage potential.
7) Insensitive to liquid temperature, viscosity, density or pressure variations.

Disadvantages Of Doppler Flow Meter:

The main disadvantage of the doppler flow meter is the fact that the liquid stream must have particulates, bubbles, or other types of solids in order to reflect the ultrasonic signal. This means that the doppler flow meter is not a good choice for the water and very clean fluids.
The another disadvantage is that the accuracy can depend on particle-size distribution and concentration and also on any relative velocity that may exist between the particulates and the fluid. if there are not enough particulates available, the repeatability will degrade.
The other potential problem of this technology is that it can have trouble operating at very low flow velocities.

Applications Of Doppler Flow Meter:

Being an non-intrusive, Doppler flow meters have a wide variety of applications. Some of them include:
1) Influent and effluent water flow.
2) Digester feed control.
3) Waste water.
4) Crude oil flow.
5) Mining slurries.
6) Acids.
7) Liquified gases.

Transit Time Flow Meter:

Transmit time flow meter sends and receives the 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 downstream wave. When the fluid moves faster, The difference between the upstream and downstream time increases. The difference in transit times in the upstream and downstream directions(Tu-Td) measured over the same path can be used to calculate the flow passing through a pipe.
Transmit time ultrasonic flow meters are more accurate and economical than the doppler ultrasonic flow meters.

Advantages Of Transit Time Flow Meter:

The main advantage of the transit time flow meter is, that it works non-invasively with clean and ultrapure fluids. This allows the user to maintain the integrity of the fluid while still measuring the flow. Some of the other advantages include:
1) Easy installation-transducer set clamps onto pipe.
2) No moving parts to wear out.
3) Zero pressure drop.
4) Works well with clean and ultrapure fluids.
5) No leakage potential.
6) Insensitive to liquid temperature, viscosity, density or pressure variations.

Disadvantages Of Transit Time Flow Meters:

The transit time flow meter perfomance can suffer from pipe-wall interference, accuracy and repeatability problems can result if there is any Space Gap between the fluid and pipe wall.
The transit time flow meters will not operate on dirty, bubbly, particulate-ladden fluids. Sometimes, the purity of a fluid may fluctuate so as to effect the accuracy of the flow measurement. For such cases, there are Hybrid meters onthe market that will access the fluid conditions within the pipeand automatically choose doppler or transit time meter operations where needed.

Applications Of Transit Time Flow Meter:

Transit time flow meter have wide applicability for flow measurement of clean or ultrapure streams. Some of the applications include:
1) Clean water flowrate in water treatment plants.
2) Hot or cold water in power plants.
3) Pure and ultrapure fluids in pharmaceutical, food and beverage industries.
4) Liquid to medium crude oils in the petroleum refining industry.
5) Water distribution systems used in agriculture and irrigation.

In my next post i will go through Vortex & Electromagnetic flow meter.
If i skipped any main points in this post, Then please let me know through your valuable comments.
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