FLOW MEASUREMENT

ORIFICE PLATE

VENTURE METER

When a venture meter is placed in a pipe carrying the 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 and when 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 size of the pipe through which fluid flows. This enables the venture to be fitted to the pipe.

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

3) Following the converging section, there is a cylindrical section with 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 and 2 in the diagram) of the venture meter for attaching a differential pressure 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 on reducing and attains a minimum value P2 when it enters the 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 the pressure difference(P1-P2) which becomes an indication of 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 the recovery.

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 greater than 20KiloHertz using specialized techniques.

Diagram of Ultrasonic Flowmeter

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 are mounted conveniently, so that the ultrasonic signals are transmitted between them as well as through the liquid.

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

The detector is designed to measure the transit time from upstream 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 and p2.

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 direction perpendicular to the direction of the fluid motion and the magnetic field, according to Fleming's Right-hand Rule.

 An electromagnetic flowmeter can be used to measure 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 to 4-20 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 the measuring 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.