Ch. no. 4 flow measurement

Unit No. 4 Flow Measurements
Babasaheb Phadtare Polytechnic (DME) Prof. Kokare A.Y.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Subject- Mechanical Engineering
Measurement

Unit No. 4
Flow Measurements
Course Outcome (CO):
To use relevant Instruments
for measurement of flow

Flow Measurement aspects:
1. Measurement of the velocity of flow
It may be necessary to determine local velocities, mean
and fluctuating component of velocities or velocity
distribution profile in a cross section of fluid flow.
2. Flow rate measurement
It involves the determination of volume, mass or weight
of fluid flow per unit time (m3/s). Obstruction meters
are commonly used for the purpose.
3. Total Flow measurement
It involves the determination of total quantity of
volume, mass or weight of fluid flow in a given time.
e.g. total volume of water consumed in a month.

Various Type of Flow
1. Uniform Non-uniform flow
2. Steady unsteady flow
3. Laminar/Turbulent flow
4. Compressible/ Incompressible flow
5. Viscous / Non viscous flow
Non-dimensional Numbers in fluid flow
1. Reynold's number: It is a non-dimensional
number and is the ratio of inertia forces to viscous
forces. If Re> 2000 laminar & Re<4000 turbulant.
2. Mach number: It is a non-dimensional number
and is square root of the ratio of inertia forces to elastic
forces. Also, it is equal to M = V/C that is, velocity of
fluid to the acoustic velocity in that fluid flow.

Classification of Flow measurement method
(A) Primary or quantity methods
(a) Weight or volume tank.
(b) Positive displacement meter.
(i) Coriolis flow meter. (ii) Oscillating piston
flow meter. iii) Rotary vane flow meter.
(B) Secondary or rate meters.
(a) Obstruction meters.
(i) Orifice (ii) Flow nozzle.
(iii) Venturi meter (iv) Variable-area meter
(b) Velocity probes.
(i) Static pressure probe (ii) Total pressure probe
(iii) Direction of flow sensing probe
(c) Special methods.
(i) Turbine-type meter
(ii) Thermal or hot wire/film anemometer
(iii) Magnetic flow meter

Variable Head Flow meters
Variable head meter is also called restriction type flow
meter. This results in the acceleration of the fluid,
causes change in the pressure.
1. Orifice plate
Orifice plates are simplest and cheapest form of primary
elements and are used more frequently than other
element for measurement of flow.

i) Concentric type:
It is installed in pipeline with hole concentric to pipe.
ii) Eccentric orifice plate:
Location of offset hole prevent damaging of solid
material or foreign particles and makes it useful for
measuring fluids containing solids.
iii) Segmental plate: The segmental plate has a hole
that is only partly circular heated below its center.
Advantages Orifice plate
1. It is less expensive. 2. Space required is less.
3. They can be used for different sizes of pipes.
Disadvantage
1. Accuracy is very poor, pressure loss is more.
2. Coefficient of discharge is very low as compared to
venturi and nozzle. 3. It can't be used for dirty fluids.

Venturimeter
The converging takes place at an angle of 21° +2° the velocity of
fluid increases as it passes through the converging section and
correspondingly pressure falls.
To accomplish a maximum recovery of kinetic energy the diffuser
section is made with an included angle 5° to 7o.
Advantages:
1. High pressure recovery is attainable.
2. Because of smooth surface, the meter is not much affected by
wear and abrasion. 3. Well established characteristics.
4. Due to low value of losses the co-efficient of discharge is high.

Flow Nozzle
A compromise between orifice plate and venturi tube is
flow nozzle. The flow nozzle consists of a smooth, gradual
contraction to throat followed by a free, uncontrolled
expansion back to the original pipe flow area.
Advantages:
1. Coefficient of discharge is high as compared to orifice.
2. Permanent pressure loss is less than orifice.
3. It has high flow velocity.
4. It used for high pressure and stem.
Disadvantages
1. it is expensive and also difficult
to install. 2. They are limited to
moderate pipe sizes, not available
above 120 cm.

The segmental wedge
A wedge acts as a restrictor in the pipe reduces the
available flow area, a narrowing the flow. The
construction increases the fluid velocity, and decreases in
static pressure. When this happens the fluid upstream of
wedge will have higher static pressure than the fluid
down stream.
The pressure taps on either side of the wedge gives the
differential pressure which can then be used to directly
calculate the flow rate.

Pitot tube
A right angled glass tube placed in the pipe one end of
the tube faces the flow while other end is open to
atmosphere hollow tube is mounted on the wall of the
pipe. Which measures only static pressure at the pipe
These two tubes senses the pressure at different place
within the pipe.
V = C√2gh

Dall Flow Tube
The two conicals are separated by a slot or gap between
the two cones. The throat is formed by a slit located
between the inlet and outlet cones.
The lower pressure is measured at circular slit area and
higher pressure is measured at upstream edge at the
inlet. The differential pressure produced by dall tube is
much higher than venturimeter.
V=K √(P1-P2)

Rotameters
The float adjusts its position in relation to the discharge through the
passes the float rides higher or lower depending on the flow rate.
With an increase in the flow rate, the float rises in the tube and there
occurs an increase in the annular area between the float and the
tube.
Advantages
1. Low cost, direct indicating, minimum piping required.
2. Pressure loss in rotameter is nearly constant and small.
3. It can handle any corrosive fluid. 4. It has quite good accuracy
especially at low flow rates. 5. It provides
linear scale. 6. It can be compensated
for changes in fluid density and viscosity.
Limitations
1. It must be installed in vertical position only.
2. For high pressure and temp., it is expensive.
3. When opaque fluid is used, float may not
be visible.

Anemometer
The anemometer works on the basis of convective heat
transfer from a heated sensor to the surrounding fluid, the
heat transfer being primarily related to the fluid velocity.
A sensor of 5 micron diameter platinum tungsten wire
welded between two prongs of the probe and heated
electrically to form part of Wheatstone circuit.
the probe is introduced into the flowing fluid, it tends to be
cooled by the velocity & there is decrease in its resistance.
The rate of cooling of wire depends
upon the : (i) Dimension and physical
properties of the wire. ii) Difference of
the temperature between the wire.
iii) Physical properties of the fluid.
(iv) Stream velocity under
measurement.

Constant-current mode (Anemometer)
Constant-current mode hot wire anemometer in which the
voltage across the bridge is kept constant.
When the fluid flows the hot wire cools the resistance changes
and the galvanometer deflects.
Constant-temperature mode
Constant- temperature mode hot wire anemometer in which
resistance of the wire and its temperature is maintained
constant.
Advantages:
1. For measurement of steady velocities the constant
temperature type is often used.
2. Majority of application is in gas flow meters.
Disadvantages:
1. Fine wire has limited physical strength
2. Due to dirt accumulation calibration of instrument changes.

Coriolis flow meter:
The coriolis flowmeter is primarily used to measure the
mass flow rate of liquids, and in some cases gas flo
measurements.
A coriolis flowmeter requires force acting on a tube
carrying a flowing fluid. This force actually deforms tubes
through which the fluid flows.
Positive displacement flow meters are the only flow meters
which directly measure the volume of fluid that passes
through the flow meter.

Oscillating piston flow meter:
Liquid enters in a cylinder containing an oscillating piston. Liquid
pressure drives the piston to oscillate and rotate on its center
hub.
As it does so, a specific volume of fluid is moved through the
meter. A sensing system typically magnetic or optical senses the
pulse each time a part of oscillation occurs.
Each revolution of piston hub is equivalent to a fixed volume of
fluid, which is indicated as flow by a indicator.
Advantages:
1. At low flow rates, it gives good
accuracy.
2. It is easy for installation and maintenance.
Disadvantages:
1. It can be used for liquids only
that too clean liquids as the meter
may block due to suspended large particles in liquid

Rotary Vane Flow meter:
The passage between each set of vanes delivers a specific
volume of fluid to the outlet side of the meter, allowing the
flow rate to be determined which is based on revolutions
of a piston.
At any instance, there is one vane in the stream at any one
time, in which case the rate of rotation of the vane is
proportional to the mean flow velocity.
Advantages:
1. It gives precise measurement
over a wide range of flow.
2. It has a smaller pressure losses
3. It has a precise and high pressure
and temperature ratings.
Disadvantages:
1. It is suitable for clean liquids.

Doppler Ultrasonic Flow Meter:
The sensor is mounted on the outside of the pipe, and en
ultrasonic beam from a piezoelectric crystal is transmitted
through the pipe wall into the fluid at an angle to the flow.
Signals reflected off flow disturbances are detected by a second
piezoelectric crystal located in the same pipe at distance L.
Transmitted and reflected signals are compared in an electrical
circuit, and the corresponding frequency shift is proportional to
the flow velocity. Advantages:
1. They show minor obstruction to
the flow of fluid.
2. No moving parts, no wear.
3. Ultrasonic flow meters are available
in sizes to 2 m and larger.
Limitation:
1. It needs expert to repair and
maintain the flow meters.

Ch. no. 4 flow measurement

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