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Orifice meter is a type of head type flow measurement. Head type flow measurement measures volumetric flow rate rather than mass flow rate, but mass flow rate can be calculated mathematically.
The principle of operation of head type device is that a restriction in the line of a flowing fluid produces a differential pressure across the element (restriction) which is proportional to the flow rate. The proportionality is not linear but has a square root relationship. The relations or flow elements are made in a predetermined manner and there are several types.
The orifice meter consists of an orifice plate placed in between the two flanges which obstructs the passage of the flowing fluid. Due to this, there is a pressure drop in the fluid which is measured by a differential pressure measuring device, and the flow is measured using the barnauli equation.
Orifice meter principle
Orifice meter works on Barnauli Theorem. An orifice flowmeter measures differential pressure that is developed by the restriction element (Orifice Plate). The orifice plate is used to pass the liquid or gas whose flow rate needs to be determined. The orifice plate experiences a pressure loss that changes with flow rate, resulting in a pressure differential between the outlet and inlet portions. The flow rate of a fluid or gas is calculated by measuring the pressure decrease.
According to Barnauli Theorem–
“The total mechanical energy of a moving fluid, which includes the gravitational potential energy of elevation, the energy associated with the pressure of the fluid, and the kinetic energy of fluid motion, remains constant.”
Bernoulli’s equation formula is a relation between pressure, kinetic energy, and gravitational potential energy of a fluid in a container.
The formula for Bernoulli’s principle is given as:
p + ρ v2 + ρgh =constant
- p is the pressure exerted by the fluid
- v is the velocity of the fluid
- ρ is the density of the fluid
- h is the height of the container
Bernoulli’s equation gives great insight into the balance between pressure, velocity, and elevation.
Construction of an orifice meter
It is generally made of carbon steel, and the main components of the device are:
Inlet Section – A linear section emerging from the device and acting as an end connection for the fluid flowing inside.
Outlet section – also a linear section similar to the inlet section where the gas or fluid pressure is determined.
Orifice plate – present between the outlet and inlet sections and used to generate a pressure drop that enables the flow rate to be measured.
Flow Conditioner – Installed in the inlet section of the meter tube and used to increase the linear flow to the inlet section of the meter tube.
Orifice meter working principle
The fluid flow in the pipeline can be monitored with an orifice meter, which is a thin plate with a hole in the middle that is put between two pipe flanges. It will be mounted perpendicularly to process the flow. This device is simple to build, install, and replace in the event of damage. When the liquid runs through the pipe, it reaches the orifice plate, where the pressure is dropped and the velocity is increased as it enters the vena contracta. The velocity and pressure will change again if the flow continues and reaches beyond the vena contracta. This pressure drop can be measured using differential pressure measurement devices.
Watch this animation video From E&h to understand better:-
Type of orifice meter
There are four types of orifice meters according to their design and measuring conditions.
Concentric Orifice meter:-
This type of orifice meter is mostly used to measure the flow of clean liquids, gases, and low-velocity vapor flows. It has a sharp square-edged hole drilled into a flat thin plate. This type is used to restrict flow and reduce pressure. High accuracy can be achieved if this type of meter is installed properly.
segmental orifice meter
This type is mostly used where there is enough water or air and also where there is a suspension in liquids. This will avoid the build-up in front of the orifice plate. This type is used for slurry and extremely dirty gases
eccentric orifice meter
In an eccentric orifice, there is no opening of flow in the center of the plate. This type is mostly used to measure liquids that contain small amounts of non-abrasive solids. This type of orifice meter is mostly used in many chemical industries.
Quadrant Edge orifice meter
This type of orifice is mostly used for liquids that have high viscosity. The inlet is one-quarter of a circle and the thickness of the plate shall be the least radius of the inlet.
Orifice meter Specifications:
- 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:
- Orifice meter can be used in Natural Gas
- This flow meter can be used in Water Treatment Plants
- This flow meter can be used in Oil Filtration Plants
- it can be used in Petrochemicals and Refineries
Orifice meter Installation
- Meter manifold piping should always be installed to enable calibration as well as to protect the differential element from being over the limit.
- The meter should be installed as close as possible to the orifice fitting.
- Always gently slope the manifold lines up to a meter from the orifice fitting to eliminate any high or low points in the manifold lines.
- Use condensate chambers or air traps to remove either the liquid from the gas system or to remove the gas from the liquid system if a low or high in the manifold piping cannot be avoided.
- It is important when applying pressure or pressure to differential measuring instruments to apply or release pressure equal to or from the high and low meter chambers, so as not to exceed excessive limits.