Basic Industrial Instruments Used for Flow measurnment. Working , Construction and diagrams with detailed explanations. Major type of Instruments are listed.

1. FLOW MEASURNMENT
Prepared by :Guided by :
Subject : Industrial Instrumentation

2. CONTENT
• Introduction
• Common types of flowmeters –
o Obstruction type Flowmeter (differential pressure or variable area)
o Turbine type Flowmeter
o Electromagnetic Flowmeter
o Ultrasonic Flowmeter
o Hot Wire Anemometer

3. INTRODUCTION
• Accurate measurement of flow rate of liquids and gases is an essential
requirement for maintaining the quality of industrial processes.
• Most of the industrial control loops control the flow rates of incoming liquids
or gases in order to achieve the control objective. As a result, accurate
measurement of flow rate is very important.
• Needless to say that there could be diverse requirements of flow
measurement, depending upon the situation.
• It could be volumetric or mass flow rate, the medium could be gas or liquid,
the measurement could be intrusive or nonintrusive, and so on. As a result
there are different types of flow measuring techniques that are used in
industries

4. OBSTRUCTION TYPE FLOWMETERS
Differential Pressure
Type
Variable Area
Type
• Orifice meter
• Venturimeter
• Flow Nozzle
• Rotameter

5. BASIC PRINCIPLE (DIFFERENTIAL PRESSURE TYPE
FLOWMETERS)
• In a differential pressure drop device the flow is calculated by measuring the pressure drop over an
obstructions inserted in the flow.
• The differential pressure flow meter is based on the Bernoulli Equation where the pressure drop and
the further measured signal is a function of the square flow speed.

6. • We consider the fluid flow through a closed channel of variable cross section, as shown in fig.1
• The channel is of varying cross section and we consider two cross sections of the channel, 1 and 2.
• Let the pressure, velocity, cross sectional area and height above the datum be expressed as p1, v1,
A1 and z1 for section 1 and the corresponding values for section 2 be p2, v2, A2 and z2 respectively.
We also assume that the fluid flowing is incompressible.
• Now from Bernloulli’s equation:
If the fluid is incompressible, then v1 A1 = v2 A2
Considering circular cross section, we define β as the ratio of the
two diameters, i.e

7. • Therefore, the volumetric flow rate through the channel can be expressed as:
• From the above expression, we can infer that if there is an obstruction in the flow path that causes the
variation of the cross sectional area inside the closed flow channel, there would be difference in static
pressures at two points and by measuring the pressure difference, one can obtain the flow rate using eqn.
• However, this expression is valid for incompressible fluids (i.e. liquids) only and the relationship between
the volumetric flow rate and pressure difference is nonlinear.
• A special signal conditioning circuit, called square rooting circuit is to be used for getting a linear
relationship.

8. ORIFICE METER
• Construction :
• An orificemeter provides a simpler and cheaper arrangement for the measurement of fow
through a pipe. An orificemeter is essentially a thin circular plate with a sharp edged
concentric circular hole in it.
• Working :
• The orifice plate, being fixed at a section of the pipe, (Fig. 15.3) creates an obstruction to
the flow by providing an opening in the form of an orifice to the flow passage.

9. • The area A0 of the orifice is much smaller than the cross-sectional area of the pipe. The flow from an
upstream section, where it is uniform, adjusts itself in such a way that it contracts until a section downstream
the orifice plate is reached, where the vena contracta is formed, and then expands to fill the passage of the
pipe.
• One of the pressure tapings is usually provided at a distance of one diameter upstream the orifice plate
where the flow is almost uniform (Sec. 1-1) and the other at a distance of half a diameter downstream the
orifice plate.
• Considering the fluid to be ideal and the downstream pressure taping to be at the vena contracta we can
write, by applying Bernoulli’s theorem
• The main job in measuring the flow rate with the help of an orificemeter, is to find out accurately the value of
C at the operating condition.
• The downstream manometer connection should strictly be made to the section where the vena contracta
occurs, but this is not feasible as the vena contracta is somewhat variable in position and is difficult to
realize.
• In practice, various positions are used for the manometer connections and C is thereby
affected. Determination of accurate values of C of an orificemeter at different operating conditions is known
as calibration of the orifice meter.

10. VENTURIMETER
• Construction:
• A venturimeter is essentially a short pipe consisting of two conical parts with a short
portion of uniform cross-section in between. This short portion has the minimum area
and is known as the throat.
• The two conical portions have the same base diameter, but one is having a shorter
length with a larger cone angle while the other is having a larger length with a smaller
cone angle.
• Working:
• The venturimeter is always used in a way that the
upstream part of the flow takes place through the
short conical portion while the downstream part of
the flow through the long one.

11. • This ensures a rapid converging passage and a gradual diverging passage in the direction of flow to avoid
the loss of energy due to separation. In course of a flow through the converging part, the velocity increases
in the direction of flow according to the principle of continuity, while the pressure decreases according to
Bernoulli’s theorem.
• The velocity reaches its maximum value and pressure reaches its minimum value at the throat.
Subsequently, a decrease in the velocity and an increase in the pressure takes place in course of flow
through the divergent part. This typical variation of fluid velocity and pressure by allowing it to flow through
such a constricted convergent-divergent passage was first demonstrated by an Italian scientist Giovanni
Battista Venturi in 1797.
• Figure shows that a venturimeter is inserted in an inclined pipe line in a vertical plane to measure the flow
rate through the pipe. Let us consider a steady, ideal and one dimensional (along the axis of the venturi
meter) flow of fluid. Under this situation, the velocity and pressure at any section will be uniform.
• In general, few guidelines are to be followed for installation of obstruction type flowmeters. Most important
among them is that, no other obstruction or bending of the pipe line is not allowed near the meter. Though
this type of flowmeters are most popular in industries, their accuracy is low for low flow rates. As a result,
they are not recommended for low flow rate measurement.

12. FLOW NOZZLE
• The flow nozzle as shown in Fig.15.4 is essentially a venturi meter with the divergent part
omitted. Therefore the basic equations for calculation of flow rate are the same as those for
a venturimeter.
• The dissipation of energy downstream of the throat due to flow separation is greater than
that for a venturimeter. But this disadvantage is often offset by the lower cost of the nozzle.
• The downstream connection of the manometer may not necessarily be at the throat of the
nozzle or at a point sufficiently far from the nozzle.
• The deviations are taken care of in the values of Cd, The coefficient Cd depends on the
shape of the nozzle, the ratio of pipe to nozzle diameter and the Reynolds number of flow.

13. ROTAMETER
• Construction :
• The basic construction of a rotameter is shown in fig. 7. It consists of a vertical pipe, tapered
downward. The flow passes from the bottom to the top. There is cylindrical type metallic float
inside the tube. The fluid flows upward through the gap between the tube and the float.
• As the float moves up or down there is a
change in the gap, as a result changing the
area of the orifice. In fact, the float settles
down at a position.
• where the pressure drop across the orifice will
create an upward thrust that will balance the
downward force due to the gravity. The
position of the float is calibrated with the flow
rate.

14. • Working :
• Rotameter works as a constant pressure drop variable area meter. It can be only be used in a
vertical pipeline. Its accuracy is also less (2%) compared to other types of flow meters.
• But the major advantages of rotameter are, it is simple in construction, ready to install and the flow
rate can be directly seen on a calibrated scale, without the help of any other device, e.g. differential
pressure sensor etc. Moreover, it is useful for a wide range of variation of flow rates (10:1).
• If the tube is made in such a way that Area Of float varies linearly with the displacement, one have a
linear relationship in the form
• The scale of the tube can be graduated linearly in terms of flow rate. Otherwise, the displacement of
the float can be converted to electrical signal by using a LVDT or similar type of displacement
sensor.
• The major source of error in rotameter is due to the variation of density of the fluid. Besides, the
presence of viscous force may also provide an additional force to the float.

15. TURBINE TYPE FLOWMETER
• Turbine type flowmeter is a simple way for measuring flow velocity. A
rotating shaft with turbine type angular blades is placed inside the flow
pipe.
• The fluid flowing through the pipeline will cause rotation of the turbine
whose speed of rotation can be a measure of the flowrate. Referring
figure, let blades make an angle α with the body. Then
• From the above expression, the volumetric flow rate can be
related with the angular speed, as
Where,

16. ELECTROMAGNETIC FLOWMETER
• Electromagnetic flowmeter is different from all other flowmeters due to its
uniqueness on several accounts. The advantages of this type of flowmeter can
be summarized as:
1. It causes no obstruction to flow path.
2. It gives complete linear output in form of voltage.
3. The output is unaffected by changes in pressure, temperature and
viscosity of the fluid.
4. Reverse flow can also be measured.
5. Flow velocity as low as 10-6m/sec can be measured.

17. • Working
• he basic principle of operation can be understood from fig. 10. It works on the principle of basic
electromagnetic induction; i.e. when a conductor moves along a magnetic field perpendicular to the
direction of flow, a voltage would be induced perpendicular to the direction of movement as also to
the magnetic filed.
• The flowing liquid acts like a conductor. External magnetic field is applied perpendicular to the
direction of the flow and two electrodes are flushed on the wall of the pipeline as shown. The
expression for the voltage induced is given by:
• where l is the length of the conductor (diameter d in this case) and v is the velocity of the liquid. The
above expression shows the complete relationship between the voltage induced and the velocity.
• However, the magnetic field applied is not d.c. if the liquid medium is water or any other polarizable
liquid. This is because, if the magnetic field is d.d. the voltage induced will also be d.c. and a small
amount of d.c. current will flow if a measuring circuit is connected to the terminals.
• This small d.c. current will cause electrolysis; oxygen and hydrogen bubbles will be formed and they
will stick to the electrodes surfaces for some time. This will provide an insulating layer on the
electrodes surfaces that will disrupt the voltage generation process.
• As a result, the magnetic field applied for these cases is a.c., or pulsed d.c. excitation. The meter can
only be used for liquids having moderate conductivities. As a result, it is not suitable for gases or
liquid hydrocarbons.

18. ULTRASONIC FLOWMETER
• An ultrasonic flow meter is a type of flow meter that measures the velocity of
a fluid with ultrasound to calculate volume flow. Using ultrasonic transducers,
the flow meter can measure the average velocity along the path of an emitted
beam of ultrasound, by averaging the difference in measured transit time
between the pulses of ultrasound propagating into and against the direction of
the flow or by measuring the frequency shift from the Doppler effect.
• Working :
• Ultrasonic flow meters measure the difference of the transit time of ultrasonic
pulses propagating in and against flow direction. This time difference is a
measure for the average velocity of the fluid along the path of the ultrasonic
beam. By using the absolute transit times both the averaged fluid velocity and the
speed of sound can be calculated.
• Using the two transit times and the distance between receiving and transmitting
transducers L and the inclination angle one can write the equations:

19. HOT WIRE ANEMOMETER
• The hot wire anemometer is used to measure fluid velocities by measuring heat
loss by convection from a very fine wire which is exposed to the fluid stream. The
wire is electrically heated by passing an electrical current through it.
• When the heated wire is cooled by a fluid stream its electrical resistance
decreases, because the resistance of metal wire varies linearly with its
temperature.
• Advantages of HWA
• Compared to pneumometric methods, HWA has fast response. Manometers
have transmission lag to a more or less extent depending on the measurement
system employed. Instantaneous measurement of velocity is important for time
dependent fluid phenomena such as turbulence.
• In pneumometric methods, sensitivity decreases as the velocity decreases. In
the case of HWA, sensitivity is more at lower velocities.
• Small probes can be made.

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