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FLOW MEASUREMENT



ORIFICE PLATE







VENTURE METER

When a venture meter is placed in a pipe carryingthe fluid whose flow rate is to be measured, a

pressure drop occurs between the entrance and

throat of the venturimeter. This pressure drop is

measured using a differential pressure sensor andwhen calibrated this pressure drop becomes a

measure of flow rate.



The following are the main parts and areas of venture meter:

1) The entry of the venture is cylindrical in shape to match the sizeof the pipe through which fluid flows. This enables the venture tobe fitted to the pipe.

2) After the entry, there is a converging conical section with anincluded angle of 19’ to 23’.

3) Following the converging section, there is a cylindrical sectionwith minimum area called as the throat.

4) After the throat, there is a diverging conical section with an

included angle of 5’ to 15’.

5) Openings are provided at the entry and throat (at sections 1 and2 in the diagram) of the venture meter for attaching a differentialpressure sensor (u-tube manometer, differential pressure gauge,etc) as shown in diagram.

CONSTRUCTION OF VENTURI METER



OPERATION

1) The fluid whose flow rate is to be measured enters the

entry section of the venture meter with a pressure P1.2) As the fluid from the entry section of venture meter flows

into the converging section, its pressure keeps onreducing and attains a minimum value P2 when it entersthe throat. That is, in the throat, the fluid pressure P2 will

be minimum.3) The differential pressure sensor attached between the

entry and throat section of the venture meter records thepressure difference(P1-P2) which becomes an indicationof the flow rate of the fluid through the pipe when

calibrated.4) The diverging section has been provided to enable the

fluid to regain its pressure and hence its kinetic energy.Lesser the angle of the diverging section, greater is therecovery.



ULTRASONIC FLOWMETER

When pressure waves are released into the flowing

fluid, their velocity and amplitude are affected by

the fluid velocity. Ultrasonic flowmeters help in

measuring these pressure wave changes,

especially in the ones having frequencies greaterthan 20KiloHertz using specialized techniques.



Diagram of Ultrasonic Flowmeter

http://www.instrumentationtoday.com/ultrasonic-flowmeter/2013/01/

http://www.instrumentationtoday.com/ultrasonic-flowmeter/2013/01/





WORKING

The working of Ultrasonic flow measurement system by

measuring phase difference is shown in the figure below. The two peizo-crystals p1 and p2 working both as

transmitter and receiver of signals alternatively aremounted conveniently, so that the ultrasonic signals aretransmitted between them as well as through the liquid.

Switch ‘sw’ is utilized to supply p1 and p2 alternately froman oscillator simultaneously connecting the detector to p2and p1 respectively.

The detector is designed to measure the transit time fromupstream to downstream and vice versa via phase shift

measurement. If C is the velocity of the ultrasonic wave and v is the fluid

velocity, then for a distance b between the crystals pa andp2.



ELECTROMAGNETIC FLOWMETER

when conductive fluids pass through a magnetic field they generate

electromotive force in proportion to flow velocity.

The operation of a magnetic flowmeter is based on Faraday's Law.

The electromotive force is

generated in a directionperpendicular to the direction of

the fluid motion and the

magnetic field, according to

Fleming's Right-hand Rule.



An electromagnetic flowmeter can be used tomeasure the flow of fluids like corrosive acids, acid

slurries, paper pulp, detergents, beer, and so on.



The magnetic flowmeter detects this electromotive

force by using a pair of electrodes installed inside the

measuring tube, calculates the flow rate, converts it to4-20 mA and pulse signals, and outputs them.

It is one of the most accurate industrial flowmeter

types.

It has no movable parts or other obstructions inside themeasuring tube, can be used with corrosive fluids, and

is maintenance-free.

Magnetic flowmeters are therefore extensively used in

such industries as chemicals, food, iron & steel, pulp &

paper, and water supply.



ANEMOMETER

Basic Principle

When an electrically heated wire is placed in a flowing

gas stream, heat is transferred from the wire to the gas and

hence the temperature of the wire reduces, and due to this,

the resistance of the wire also changes.This change in resistance of the wire becomes a measure of

flow rate.

Types of anemometer.

1. Hot wire anemometer.

2. Hot film anemometer.





The bridge arrangement along with the anemometer has been shown

in diagram. The anemometer is kept in the flowing gas stream to

measure flow rate.

A constant current is passed through the sensing wire.

That is, the voltage across the bridge circuit is kept constant, that is,

not varied.

Due to the gas flow, heat transfer takes place from the sensing wire to

the flowing gas and hence the temperature of the sensing wire

reduces causing a change in the resistance of the sensing wire. (this

change in resistance becomes a measure of flow rate).

Due to this, the galvanometer which was initially at zero position

deflects and this deflection of the galvanometer becomes a measure

of flow rate of the gas when calibrated.



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Flow measurement.



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