Introduction to orifice meter
An Orifice Meter is basically a type of flow meter which is used to measure the rate of flow fluids (mainly Liquids or Gases), using the Differential Pressure Measurement principle.
There is basically an Orifice Plate installed in the orifice meter which provides obstruction to the fluid flow. Here, the streamline contracts because of the area contraction due to orifice which is placed between the pipe by the flange.
There is a vena -contacta considered as a minimum area -cross-section.
Differential pressure is developed across the Orifice Plate which is directly proportional to the flow-rate of the liquid or gas.
Principle of Orifice meter :
When a liquid/gas, whose flow-rate is to be determined, is passed through an Orifice Meter, there is a drop in the pressure between the Inlet section and Outlet Section of Orifice Meter. This drop in pressure can be measured using a differential pressure measuring instrument.
Since this differential pressure is in direct proportion to the flow-rate as per Bernoulli’s Equation hence the differential pressure instrument can be configured to display flow-rate instead of showing differential pressure.
Working of Orifice Meter :
The working principle of Orifice Meter is the same, like that of Venturi meter
The orifice plate is being fixed at a section of the pipe, creates an obstruction to the flow by providing an opening in the form of an orifice to the flow passage.
Orifice meters are built in different forms depending upon the application-specific requirement, The shape, size, and location of holes on the Orifice Plate describe the Orifice Meter Specifications as per the following:
- Concentric Orifice Plate
- Eccentric Orifice Plate
- Segment Orifice Plate
- Quadrant Edge Orifice Plate
Operation of Orifice meter:
- The fluid flows inside the Inlet section of the Orifice meter having a pressure P1.
- As the fluid proceeds further into the Converging section, its pressure reduces gradually and it finally reaches a value of P2 at the end of the Converging section and enters the Cylindrical section.
- The differential pressure sensor connected between the Inlet and the and the Cylindrical Throat section of the Orifice meter displays the difference in pressure (P1-P2). This difference in pressure is in direct proportion to the flow rate of the liquid flowing through the Orifice meter.
- Further, the fluid passed through the Diverging recovery cone section and the velocity reduces thereby it regains its pressures. Designing a lesser angle of the Diverging recovery section helps more in regaining the kinetic energy of the liquid.
Specifications of Orifice meter:
- Line Size: 6 mm to 800 mm
- Accuracy: +/-0.5% to +/-3.0%.
- Operating Temperature (Max.): Up to 800 degC
- Operating Pressure (Max.): Up to 400 bar
Applications of Orifice meter:
- Natural Gas
- Water Treatment Plants
- Oil Filtration Plants
- Petrochemicals and Refineries
Advantages of Orifice meter:
- The Orifice meter is very cheap as compared to other types of flow meters.
- Less space is required to Install and hence ideal for space-constrained applications
- The operational response can be designed with perfection.
- Installation direction possibilities: Vertical / Horizontal / Inclined.
Limitations of Orifice meter:
- Easily gets clogged due to impurities in gas or in unclear liquids.
- The minimum pressure that can be achieved for reading the flow is sometimes difficult to achieve due to limitations in the vena-contracta length for an Orifice Plate.
- Unlike Venturi meter, downstream pressure cannot be recovered in Orifice Meters. Overall head loss is around 40% to 90% of the differential pressure.
- Flow straighteners are required at the inlet and the outlet to attain streamline flow thereby increasing the cost and space for installation.
- Orifice Plate can get easily corroded with time thereby entails an error.
- The discharge Co-efficient obtained is low.
Some Questions and Answers :
- What is an Orifice Plate?
An orifice plate: It is a thin plate with a hole in it, which is usually placed in a pipe. When a fluid (whether liquid or gaseous) passes through the orifice, its pressure builds up slightly upstream of the orifice but as the fluid is forced to converge to pass through the hole, the velocity increases, and the fluid pressure decreases. A little downstream of the orifice the flow reaches its point of maximum convergence, the vena contracta where the velocity reaches its maximum and the pressure reaches its minimum. Beyond that, the flow expands, the velocity falls and the pressure increases.
2. Explain the term vena-contracta with neat sketch
The figure shows a sharp-edged orifice in one side of a reservoir containing water. The water will emerge from the orifice as a free jet, that is, a jet discharged in the atmosphere and will, therefore, be under the influence of gravity only. The section C-C of the jet, at which the streamlines are straight and parallel to each other and perpendicular to the plane of the orifice, and the jet has the minimum cross-sectional area, is known as vena contracta. The pressure at section C-C is uniform and it is equal to the pressure of surrounding the jet. The velocity of flow of water at this section will be maximum by the principle of continuity. Beyond the section C-C the jet may, however, diverge again and it undergoes a downward deflection due to gravity. The area of jet i.e. at vena contracta may be related to the area of the orifice by following expression
Cc=Coefficient of contraction
3. Define all hydraulic coefficients.
There are four hydraulic coefficients-
1. Coefficient of contraction (Cc): It is the ratio of the area of the jet at vena contracta to the area of Orifice is known as Coefficient of contraction.
2. Coefficient of velocity(Cv): It is the ratio of actual velocity of jet at vena contracta to the theoretical velocity of the jet is known as the Coefficient of velocity
3. Coefficient of discharge (Cd): It is the ratio of actual discharge through an orifice to the theoretical discharge is known as the Coefficient of discharge.
4. Coefficient of Resistance (Cr): It is the ratio of loss of head in the orifice to the head of water available at the exit of the orifice is known as the Coefficient of resistance.
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