Flow Measurement Equipments and Techniques - Fluid Mechanics - Chemo Concept

In every chemical industry that deals with fluid, It is very important to measure the flow of fluid that is being transported by the piping network.

Flow measurement is key to understand the status of the process.

What is Flow Measurement?

Flow measurement is a process of measuring the flow of fluid.

In liquid flow can be directly measured or can be measured by calculating mean velocity and multiplying it with a cross-sectional area of flow area.

In gas flow generally flow is not directly measured but the more important parameter which is pressure is calculated.

Types of Flow Measurement Devices

There are major two types of flow measurement devices.

Full bore type: 

• Full bore flow measurement devices are fitted between pipes in which fluid flow is to be measured. 
• Full bore flow measurement devices have the same diameter as the pipe. 
• To insert full bore meters in the working process it is needed to cut the whole pipe, join meters between them which may lead to more downtime but where accuracy in flow measurement is required, full bore flow measurement devices are more preferred than insertion meters. 
• Generally, a full bore meter is used to measure the flow in small size pipes.
• Some examples of full bore meters are 

1. Rotameter
2. Venturimeter
3. Orifice Meter
4. Electromagnetic devices

Insertion type: 

• Insertion type of flow meters are meters having one part inside the fluid which is flowing. 
• The part inserted in the fluid can be Impeller, Ultrasonic Wave Generator, Turbine, Vortex Generator, Pitot tube or Electromagnetic Device, etc. 
• To insert an insertion meter into the running process require less downtime because it takes only a few hours to drill a hole of the desired size and add a measuring element into it. 
• Different types of insertion meters are used for a specific type of fluid that is compatible with the fluid that is flowing. 
• Generally for flow measurement in bigger pipe sizes insertion meters are used. 

So these are the major type of flow measurement device let's study them one by one.

1. Rotameter: 

• Rotameter is one of the simplest flow measurement devices. 
• Rotameter is generally used for flow measurement when the fluid is flowing from down to up. 

• Rotameter works on the principle of buoyancy. 
• Rotameter is consists of a diffuser, made up of transparent glass or plastic with a very slight increase of flow area from bottom to top. 
• In Rotameter there is a cone size element with a cylindrical top also called a probe which floats when fluid flows. 
• The resultant force of gravity and buoyancy force decides the position of the probe. There is marking on transparent diffuser according to predefined experimental data. 
• The reading coincides with the Rotameter reading is the flow rate of the fluid. 
• Pressure drop in Rotameter is very less due to its pressure recovery section(Increasing diameter pipe helps in reducing pressure drop). 

2. Venturimeter: 

• Venturimeter is a flow measurement device based on pressure drop calculation and then convert into flow by the Bernoulli and Continuity equation. 

• Venturimeter is consists of three sections

1. Nozzle
2. Throat
3. Diffuser

• The pressure drop from the flowing pipe to the throat of the venturimeter is calculated and then converted into velocity by the two equations. 
• In nozzle the area decreases and velocity increases. 
• The lowest flow area is called the throat.
• The diffuser is generally longer than the nozzle to recover the pressure drop that taken place in the nozzle. 

3. Orifice Meter: 

• The orifice meter is consists of one orifice plate which is fitted in between the flow path. 

• The flowing fluid forms the Vena contract (Minimum flow area zone) where pressure drop is calculated concerning general pipe pressure. 
• Pressure drop in Orificemeter is higher than venturi meter but low cost, less space, and easy maintenance are advantages of Orifice meter. 

4. Vortex Shading Meter:

• In vortex shading meter a bluff body of trapezoidal shape is put in the path of fluid flow. 
• The vortex generated is observed and fluid flow is predicted based on turbulence generated. 

5. Turbine Meter: 

• The turbine meter has a turbine that obstructs the path of flowing fluid. 
• The speed of turbine rotation reveals the fluid flow. 

The ultrasonic meter uses ultrasonic sounds waves.
The electromagnetic meter uses Electromagnetic waves to calculate fluid flow generally used for conductive fluid.
Coriolis meters measure the fluid flow based on vibration generated at U bent in pipe sue to fluid flow.
There are also thermal meters that measure the fluid flow by the temperature rise in the fluid by the heating element.
The pitot tube is a special type of flow measuring device which is used to find the local velocity of the fluid at any place inside the pipe by comparing its pressure with atmospheric pressure.