m&i notes-7
TRANSCRIPT
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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