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University of Engineering and Technology Taxila

Mechanical Engineering Department

Lecture No: 07

Course Title: Measurement and Instrumentation

(ME-314)

Tutor: Khalid Masood Khan

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Flow Measurement

Flow rate is expressed as either volume flow

per unit time or mass flow per unit time

Size, accuracy, cost, pressure drop, pressure

losses, and compatibility with the fluid are

important design considerations for flowmetering devices

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Velocity of a fluid at a point is

In Cartesian coordinates:

In cylindrical coordinates:

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Categories of Flow Metering Instruments/Devices

Pressure differential meters

Insertion volume flow meters

Mass flow meters

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Pressure differential meters manipulate/

obstruct the flow path. These obstructionmeters are

Orifice meter

Venturi meter

Flow nozzles

Usually inserted in-line with a pipe

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Orifice Meter

It consists of a circular plate, containing a hole

(orifice)

The plate is inserted into a pipe such that theorifice is concentric with the pipe inside

diameter

Among the variety of designs, the square-

edged orifice is common

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Orifice plate represents a relatively

inexpensive flow meter that provides an easily

measurable pressure drop

However, it introduces a large permanent

pressure loss into the flow system

Because loss effects vary along the length of

the flow meter, flow rate estimate based onpressure drop is very sensitive to the pressure

tap location requiring judicious tap placement

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Venturi Meter A venturi meter consists of a smooth

converging contraction to a narrow throat

followed by a shallow diverging section

The divergent section angle is 7or 15. The

meter is inserted between two flangesintended for this purpose

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Pressure is sensed between a location upstream ofthe throat and a location at the throat and

calculated using the equation

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The product is often represented by what iscalled the flow coefficient,

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Venturi meter is more expensive than anorifice meter in terms of initial cost

However, the system operating cost is muchsmaller because of low permanent pressure

loss for a given installation

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Flow Nozzles

ASME Long-Radius Nozzle

Sonic Nozzle

A flow nozzle consists of a gradual contractionto a narrow throat. It needs less installation

space and is cheaper than a venturi meter

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ASME Long-Radius Nozzle

For this nozzle, the contraction is that of the

quadrant of an ellipse with major axis aligned

with the flow axis

Pressure tapes are usually located at one pipe

diameter upstream of the nozzle inlet and at thenozzle throat by using either wall or throat taps

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The flow rate is determined from the sameequation as for the venturi with and based

on the throat diameter while and are

obtained graphically

Because it lacks the gradual divergent section

of a venturi, the permanent loss associated with

the nozzle is larger for the same pressure drop

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Sonic Nozzles

Sonic nozzles are used to meter and to control

the flow rate of a compressible gas. For flow

reaching speed of sound (sonic flow) at the

throat , the mass flow through the nozzle

reaches a maximum for the given inlet

conditions. For choked flow, the critical

pressure ratio is

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For an ideal gas undergoing an isentropic

process between and

As with all obstruction meters this ideal

mass flow rate must be modified using adischarge coefficient to account for losses

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Insertion Volume Flow Meters

Electromagnetic Flow Meters

Vortex Shedding Meters

Rotameters

Turbine Meters

Positive-Displacement Meters

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Electromagnetic Flow Meter

The meter employs the fundamental principle

that an emf is induced in a conductor which

moves through a magnetic field, i.e.,

Or,

where is the angle between and ,

usually at 90.

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The volume flow rate is found by

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It has very low pressure loss. It issuitable for metering corrosive and

dirty fluids

Use is limited to metering fluids with

electrical conductivity above a

certain value

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In general, a fluid particle passing through the

meter tube that is rotating (as a result of the

oscillating tube) relative to the fixed pipeexperiences an acceleration

The meter principle is unaffected by changing

fluid properties but temperature changes will

affect the overall meter stiffness, an effect that

can be compensated for electronically

The meter is also used as an effective

densitometer