You must have studied in your college that venturi meter and orifice meter are used to measure the flow rates of the running fluids in pipes. But, We prefer venturi meter most of the time as it gives more precise results with less pressure drop. Also, both the venturimeter and orifice meter work on the same principle.
So, When do you prefer orifice meter over venturimeter? We prefer orifice meter over venturi meter when space or the money is limited. Venturimeter is longer in length and bigger in size as compare to orifice meter. That’s why sometimes we can not install venturi meter when the space is limited. So, in such cases, we prefer orifice meter as it can be easily installed in the small places, unlike venturi meter.
Venturimeter is costlier and it has a higher maintenance cost than orifice meter. So, when it is not possible to afford the installation and maintenance cost of the venturi meter, then you can use an orifice meter.
Difference between Orifice meter and Venturimeter
Venturimeter and orifice meter work on the same differential pressure measurement principle.
Both the instruments are used to create pressure drops in the running fluid and then that pressure drop is used to calculate the discharge of fluid using Bernoulli’s principle.
The difference between orifice meter and venturimeter is in their construction. Venurimeter consists of three sections that are convergent, throat, and divergent. In the case of an Orifice meter, it doesn’t possess parts like convergent and divergent. The orifice plate is the main part of the orifice meter.
The additional parts in Venturimeter help to recover maximum pressure drop. That’s why energy losses in venturi meter are far lower than other instruments.
Why Cd value is high in Venturi meter than Orifice meter
You may have learned that the coefficient of discharge for venturimeter is about 0.98 whereas the coefficient of discharge of orifice meter is between 0.58 to 0.65. So, as compared to the orifice meter, the venturi meter has a bigger Cd value.
So, why cd value is high in venturi meter than orifice meter? The reason behind this is the energy loss or pressure loss due to eddy formation in the orifice meter. Venturimeter possesses a convergent and divergent section that significantly reduces pressure losses in venturi meter. Whereas in orifice meter there are no convergent and divergent sections to prevent eddy formation.
Eddy formation is defined as the swirling of the fluid when it faces sudden contraction or expansion and it consumes a lot of energy of the running fluid. In the orifice meter, the running fluid faces the sudden contraction that results in the eddy formation. The eddy formation in orifice meter then results in large pressure drop or energy loss. That’s why pressure losses in orifice meter are high.
Whereas in the venturi meter, the converging section and diverging section helps to prevent the eddy formation. That’s why energy losses or pressure losses are far lower in venturi meter.
The coefficient of discharge (Cd) or the discharge coefficient is the ratio of actual discharge to the theoretical discharge. As pressure losses start to increase, the actual discharge of the running fluid starts to decrease. So, you will get a small value of the coefficient of discharge, if the value of actual discharge is less than the value of theoretical discharge.
That means, if the pressure losses are more, then the actual discharge will be less, due to which the value of the coefficient of discharge will be less. Now, let’s discuss the working of the orifice meter.
Orifice meter working principle
The working of orifice meter is based on the differential pressure measurement principle.
Firstly, the fluid whose flow rate we want to know is passed through the orifice meter. When it passes through the orifice meter, the pressure drop is created at the inlet and outlet sections of the orifice meter.
This pressure drop is created due to the orifice plate. The orifice plate is the most important part of the orifice meter. It is inserted between the inlet and outlet sections of the orifice meter.
When the fluid reaches near the inlet section of orifice meter, it faces sudden contraction. This sudden contraction results in eddy formation or swirling of fluid. Eventually, the pressure drop is generated in the running fluid due to eddy formation. This pressure drop is generated at the outlet of the orifice meter. In this way, the orifice meter is used to create a pressure drop.
This pressure drop can be measured with differential pressure measuring instruments like a manometer. Then the calculated pressure drop can be used to calculate the discharge of the fluid with the help of the Bernoulli’s principle. According to the Bernoulli’s principle, when the speed of the fluid increases, it results in a simultaneous decrease in the pressure. So, with the help of pressure drop and Bernoulli’s principle, we can find the discharge of the fluid.
Advantages of Orifice meter
Now let’s discuss some of the advantages of an orifice meter.
1) It is shorter in length as compared to venturi meter
2) It is easy to install the Orifice meter
3) Its construction is simple
4) It is cheaper as compare to venturi meter
5) It is available in different materials
6) The orifice meter is cheaper as compare to venturi meter
Disadvantages of Orifice meter
1) The orifice meter is not as precise as Venturimeter
2) Energy Losses in orifice meter are high as compare to venturi meter
3) Orifice meter doesn’t work properly with high viscosity fluids
The orifice meter is convenient to use than venturi meter when space and money are limited, but other parameters need to be discussed where venturi meter has a better performance than orifice meter.
On what principle does the Venturi meter work
Venturi meter works on the differential pressure measurement principle. It has three main sections that are convergent, throat, and divergent.
- Convergent Section
Convergent is the intake port of the venturi meter. The diameter of the convergent section decreases gradually. The velocity of the incoming fluid increases in the convergent area due to a gradual decrease in the cross-sectional area. One end of the convergent is attached to the pipe while the other end is attached to the throat section of the venturi meter.
- Throat Section
Throat is the middle section of the venturi meter. It’s one end is connected to the convergent while the other end is connected to the divergent. The throat is the section where we get the required pressure drop.
- Divergent Section
Divergent is the last part of the venturi meter. The diameter of the diverging part increases gradually. The divergent part plays a very important role in preventing eddy formation.
This construction of venturimeter helps to achieve desired conditions with minimum loss in energy. Then, the pressure drop is measured with the help of a manometer. And finally, by using Bernoulli’s principle, we can find out the discharge of fluid.
Advantages of venturimeter over orifice meter
1) Venturimeter gives best results as energy losses in venturi meter are lower than orifice meter
2) It is easy to predict the behavior of venturi meter
3) The value of the coefficient of discharge is high
4) It can significantly recover the pressure drop
Disadvantages of Venturi meter
1) It is larger in size that’s why they are hard to use in a limited space
2) Their cost is high
3) Their installation and maintenance cost is high
4) Venturimeter can not be used if the diameter of the pipe is less than 7.5 cm
So, Venturimeter gives precise results with less loss in energy. But it is costly and it is difficult to install in small places. Unlike Venturimeter, the Orifice meter is cheaper and small in size. That’s why orifice meter can be easily installed in small places, unlike venturi meter.