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 orificemeter  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 flange.

There is a vena -contacta considered as minimum area -cross section.

A differential pressure is developed across the Orifice Plate which is directly  proportion to the flow-rate of the liquid or gas.

Principle:
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 the Bernoulli’s Equation hence the differential pressure instrument can be configured to display flow-rate instead of showing differential pressure.

Working:

The working principle of Orifice Meter is the same, as 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 describes the Orifice Meter Specifications as per the following:

1. Concentric Orifice Plate
2. Eccentric Orifice Plate
3. Segment 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 enter 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.): Upto 800 degC
• Operating Pressure (Max.): Upto 400 bar

Applications of Orifice meter:

• Natural Gas
• Water Treatment Plants
• Oil Filtration Plants
• Petrochemicals and Refineries

• 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
• 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.
• Discharge Co-efficient obtained is low.