In this article, you’ll learn what is a venturimeter? Its diagram, definition, parts, working, formula, types, and uses are explained with pictures.
You can download the PDF file of this article at the end of it.
What is Venturimeter?
Fluid flow measurement means measuring the rate of flow of a fluid flowing through a pipe or through an open channel. The rate of fluid flow through a pipe is measured by a venturimeter.
It is a device in which pressure energy is converted into kinetic energy and is used to measure the rate of flow through a pipe. Or in other words, a venturimeter is a tube with a constricted throat that increases velocity and decreases pressure.
They are used for measuring the flow rate of compressible and incompressible flow in pipelines. The venturimeter basically works on the principle of Bernoulli’s equation. It consists of a converging portion, throat, and diverging portion.
Parts of a Venturimeter
- Converging part
- Diverging part
#1 Converging Part
A converging cone or part is a short pipe that converges from a diameter d1 to a small diameter d2. The slope of the converging sides is between 1 in 4 and 1 in 5.
The throat is a small portion of a circular pipe in which the diameter d2 is kept constant.
#3 Diverging Part
Diverging cone or part is an outlet of the venturimeter, which diverges from a diameter d2 to a large diameter d1. The length of the divergent cone is about 3 to 4 times that of the convergent cone.
Working of Venturimeter
The liquid, while flowing through the venturimeter, is accelerated while flowing through the convergent cone. As a result of the acceleration, the velocity of liquid in the throat becomes higher than at section convergent.
This increase in velocity results in considerably decreasing the pressure on the throat. If the pressure head at the throat falls below the separation head (which is 2.5 meters of water), then there will be a tendency of separation of the liquid flow. This is called the venturi vacuum.
In order to avoid the tendency of separation at the throat, there is always a fixed ratio of the diameter of the throat and the pipe (i.e., d2/d1). This ratio should be 1/3 to 1/2.
The purpose of the diffuser in a venturi meter is to assure gradual and steady deceleration after the throat. It is designed to ensure that the pressure rises again to close to the original value before the venturimeter.
If the pressure is rapidly recovered in the divergent part, then there is every possibility for the stream of liquid to break away from the walls. In order to avoid this, the divergent cone is made longer.
Another reason for making the diverging cone longer is, that it reduces frictional losses. So it is 3 to 4 times longer than the convergent cone.
Discharge Though Venturimeter
According to Nptel, the function of the converging portion is to increase the velocity of the fluid and temporarily lower its static pressure. The pressure difference between the inlet and throat is developed. This pressure difference is correlated to the rate of low.
The expression for the theoretical flow rate is obtained by applying the continuity equation and energy equation at the inlet and throat section, and assuming the fluid to be ideal is given by,
- p1 = Pressure at section 1,
- v1 = Velocity of water at section 1,
- z1 = Datum head at section 1,
- a1 = Area of venturi meter at section 1, and
- p2, v2, z2, a2 = are corresponding values at section 2
- C = Venturi meter
- 13.6 = Specific gravity of mercury, and
- ω = Specific weight of the oil
By applying Bernoulli’s equation at sections (1) and (2), we can get
As the pipe is horizontal, hence z1 = z2
But p1-p2/ρg is the difference of pressure heads at sections 1 and 2 and it is equal to h or p1-p2/ρg = h
Substituting this value of p1-p2/ρg in the above equation, we get
Now, we need to apply the continuity equation in sections 1 and 2
Substituting this value of v1 in the equation (8.1)
Discharge, Q = a2v2
Equation (8.2) gives the discharge under ideal conditions and is called theoretical discharge. Therefore, the actual discharge will be less than the theoretical discharge.
Where Cd = is the co-efficient of venturimeter and its value is less than 1.
Types of Venturimeter
- Horizontal venturimeter
- Vertical venturimeter
- Inclined venturimeter
1. Horizontal Venturimeter
The horizontal venturimeter has the greatest kinetic energy and lowest potential energy. A horizontal venturimeter with an inlet diameter of 200 mm and a throat diameter of 100 mm is used to measure water flow.
2. Veritcal Venturimeter
The vertical venturimeter has the greatest potential energy and lowest kinetic energy. These are normally fitted with a circular pipe of the diameter of 30 cm and the throat diameter is 15 cm. The difference in the readings of the two parts of the manometer is 30 cm. Using this type, you can easily determine the amount of water flowing through the pipe.
3. Inclined Venturimeter
In this type, both the kinetic and potential energy lies in between the two types mentioned. An inclined venturimeter is usually inserted into an inclined pipe in a vertical plane to measure the flow rate through the pipe.
How To Install Venturimeter?
Installing a proper venturi meter is the solution to correct operation. Therefore, venturimeters should be installed following the manufacturer’s guidelines. Generally, the following guidelines should be followed when installing venturi meters:
- The direction of flow in the venturi meter should be checked and set up to agree the flow direction.
- When aligning the flanges on the venturimeter, it should be properly checked whether the piping flanges are aligned with the venturimeter ends.
- Keep in mind that pipe supports should not be installed on venturimeters.
- When fitting the venturimeter with a bolt, it should not be over-tightened.
- Applying tolerances should be as per industry standards.
- Pressure taps for liquid service applications must be oriented horizontally.
- The power loss is considered low compared to other types of fluid measuring instruments.
- These are employed where a small head is available.
- Accuracy will achieve higher over a wide flow range.
- In venturimeter, the co-efficient of discharge is very high.
- These devices are easy to handle and are also used for compressive and incompressible fluids.
- The Venturimeters are commonly used for high flow rates or discharge.
- Venturimeters are quite expensive to install.
- These devices require maintenance.
- This system requires more space than the orifice meter.
- Venturimeter is costlier and a bit bulky.
- These are not used where the pipe diameter is 76.2 mm.
- By using the venturimeter you can easily find out the flow rate of the fluid.
- It is used in the industrial sector to determine the pressure of a volume of gas and liquid inside a pipe.
- These are very helpful to measure airflow in engine carburetors.
- It is used to measure and control process flow in process industries.
- In addition, they are also used for arterial blood flow, which is measured by a venturimeter in the medical industry.
- These devices are used in the wastewater treatment process.
- It is also quite useful where high-pressure recovery is needed.
Wrapping It Up
As I discussed above, It is a device used to determine the flow rate of fluid flow through a pipe. There are many varieties of venturi meters available, and choosing the right venturimeter can solve your problems.
That’s it. If you still have any questions about “Venturimeter” you can ask in the comments I’ll respond to you. If you like this article then share it with your friends.
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