FLOWMETER WORKING.

What are flow meters?

Flowmeter is the devices that measure the flow rate of the fluid. They are installed in any process vessels in order to measure the process flow data like volumetric flow rates, velocities etc.

They are designed in such a way that it can measure and calibrated to get parameters at an exact specific time at a specific location.

Today we are going to discuss various flow meters and their classification and also there working principals.

Flowmeter Classification.

Flowmeter is classified as constant area meter or variable head meter and variable area flow meter.

In constant area flow meter, area of devices are constant I.e area of construction of such devices are constant. The pressure drop across the device changes and the area of flow remains constant.

Whereas in the variable area, the area of devices is design in such a way that can be used to measure the parameter. They are either increasing area or decreasing area meter. Pressure drop across throat is constant and the area of flow is changing.

Variable head or constant area meter includes Venturi meter and orifice meter. And Variable area meter includes Rotameter.

Another classification is that devices that neither fall in above classifications. They are called a Pitot tube that measures point velocity and local velocity.

In Pitot tube, velocity is directly converted to Pressure head.

VENTURI METER :

The construction of these devices includes a cylindrical converging section followed by a cylindrical throat followed by diverging section. This simple construction is responsible for creating different pressure drops in a different section as fluid flows.
Here pressure drop across throat is constant and flow area are changing.


In the converging section, the cross-sectional area decreases that increase the fluid velocity and decrease the pressure along with that location.
While in the diverging section, the cross-sectional area increases that decreases the fluid velocity and increases the pressure along with that location.


One thing should be noted here that pressure along the converging section is larger than pressure along the diverging section.
It follows continuity equation that states that when area decreases velocity along that area increase.

Use of Venturi meter to find volumetric flow rate.

These difference in pressure can be measured by a simple manometer and reading can be correlated to flow rate by the application of Bernoulli's equation.
By applying Bernoulli equation at location 1 & 2

∆h = Difference in kinetic head.
Kinetic head can be directly obtain by application of manometers..

By applying manometric balance on manometers, a general equation for volumetric flow rate can be achieved that would be referred as theoretical discharge.

As per equation of continuity with constant density fluids.

Here final equation can be written as,

Above equation is used for theoretical discharge.

And to achieve real discharge a termed called coefficient of discharge is used.

Usually it values is take around 0.90 to 0.98
This value shows that venturi meter is efficient and economical.

Application of venturi meter can be found in many process industry were it is important to measure the volumetric or mass flow rates.

It can be directly installed in between joints of two pipes.Venturi meter are very costly on construction point of view but this gives it a great advantage of having less kinetic head loss.
Venturi meter with large coefficient of discharge are favourable.