ORIFICE PLATE
The simplest pipeline restriction for flow metering is the orifice plate, which is a thin, circular metal plate with a hole in it. Its held in the pipeline between two flanges called orifice flanges. The pressure below the orifice less than above the orifice, this difference in pressure is a measure of the rate of flow.
There are four types of orifice plate.
(1) Concentric orifice plate
(2) Eccentric orifice plate
(3) Segmental orifice plate
(4) Quadrant orifice plate
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CONCENTRIC ORIFICE PLATE
it is usually made of stainless steel and it’s thickness varies from 3.175 to 12.7mm (⅛” to ½”) depending on pipeline size. It has a circular hole in the middle. It is also made from other material such as nickel, phosphor bronze. Use for liquid, gas and steam service
ECCENTRIC ORIFICE PLATE
The eccentric orifice plate has a circular hole located below its center. Used for viscous and slurry flow measurement.
SEGMENTAL ORIFICE PLATE
The segmental orifice plate has a hole that is only partly circularly located below its center. Used for viscous and slurry flow measurement.
QUADRANT ORIFICE PLATE
This type of orifice plate is used for flows such as heavy crudes and slurries and viscous flows.
ADVANTAGES OF ORIFICE PLATE
Ø It’s cost low.
Ø They can be used in a wide range of pipe size (3.175 to 1828.8mm).
Ø They can be used with differential pressure device.
Ø They are well known and have good characteristics.
Ø They are available in many materials
The orifice plate is provided
1. An indication of the orifice plate in the line.
2. Orifice diameter marked on it.
3. The material of orifice plate.
4. The tag number of the orifice plate.
5. The marked the inlet of the orifice plate.
Guidelines for the installation of the orifice plate
1. Confirm the location of the orifice carrier (orifice flange) from P & I.
2. Confirm the tag number to service using P & ID.
3. Confirm the flow direction.
4. Ensure HP tapping of DP transmitter is on the upstream (inlet) of orifice flange.
Don’t assume transmitter is installed correctly.
5. Checked the orifice plate for any signs of damage.
6. Installed the orifice plate between two new spiral wound (metal flex) gasket.
For orifice plate in nonvertical (Horizontal) line
1. Tab to point down for liquid service.
2. Tab to point up for gas service.
3. Text on the tab to face upstream (inlet).
Orifice plate in vertical line
1. Have no drain hole.
2. orifice plate is horizontal line must have a drain hole.
3. A hole is at the bottom in a stem and wet gas line.
4. A hole is at the top for liquid service.
VENTURI TUBE
The venturi tube is used for large pipelines. It is more accurate than the orifice plate but it is more expensive and more difficult to install. It is used in the measurement of high viscous fluid (liquid).
Venturi tube
(a) Long form or classic venturitube.
(b) Eccentric venturitube.
PITOT TUBE
It is the simplest form an averaging pitot tube senses the natural average differential pressure created by the velocity of the fluid flowing in a pipe.
When the fluid passes through the sensor it creates a high-pressure zone greater (more) then the static pressure in front of the sensor. As the fluid moves past the sensor low-pressure zone are created to the side of the sensor so that differential pressure is produced. It is not suitable for dirty and sticky fluids.
ANNUBAR
An annular tube consists of two probes inserted into the line in the same manner as a pitot tube, with one probe facing the flow to sense the velocity pressure and the second probe behind the fast with its opening facing downstream to sense the static pressure. The difference between the two pressures gives the average flow. The annual is available in pipe size from 3.175 to 3750mm.
FLOW NOZZLE
The flow nozzle is almost as accurate as the venturi tube and is not so expensive to buy or as difficult to install. Differential pressure measurements taps are normally located 1 pipe diameter upstream and 1½ diameter downstream from the nozzle. It is used for more viscous fluid (liquid).
TURBINE
The turbine meter consists of three basic components.
(1) The housing. (2) The rotor assembly. (3) The magnetic pickup coil.
The turbine meter is a flow rate device. The velocity of a flowing fluid force to the blades’ or rotor which rotates at a speed proportional to a flow rate. The output is usually a pulse of AC voltage. The frequency of generated pulses is a measure of flow rate and a total number of pulses a measure of total flow. The turbine meter is applied to the measurement of clean, noncorrosive material. Turbine meter range in size from ¼ to 24 inches and 0.1 to over 50,000 GPM.
ROTAMETER
The rotameter has mostly used the variable area flow meter. It consists of a vertical taper tube with a float which is free to move up or down within the tube. In the rotameter, the flow rate varies directly as the float rises and falls in the tapered tube.
MAGNETIC FLOWMETER
The meter consist of an electrically insured or pipe with a pair of electrodes mounted opposite each other and magnetic coil are mounted around the tube.
A magnetic flow meter is based on faradays law of electromagnetic induction, which states that the voltage is developed in a conductor proportional to the rate of flow of liquid which moves in a magnetic field. In a magnetic flow meter, the conductor is a moving electrically conductive liquid.
The magnetic flow meter is a suitable to measurement of slurries, greasy and dirty fluids.
ADVANTAGES
(1) There are no obstructions to fluid flow.
(2) Piping constructions are not critical.
(3) Available in large pipe size and capacities.
PRECAUTION
(1) The meter must be full at all the time.
(2) Care must be taken in handling the low voltage generated by the meter.
(3) Cleaning method must be used to keep the electrodes clean and conductive because electrodes are covered by some coating in some liquid or fluid.
POSITIVE DISPLACEMENT METER
(LOBED IMPELLER METER)
P.D. METER:- Liquid flow passes through the meter it moves the measuring element (lobed impeller) holding a definite volume. As the measuring element moves the compartment are filled and empty. Each cycle the fixed quality of liquid is passed from the inlet to the outlet. The number of cycle of the measuring element indicated by means of a pointer moving over the dial and totalized the total volume past through the meter.
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