Unit 4 - LEVEL - AND - FLOW - MEASUREMENT - AND - CONTROL
Unit 4 - LEVEL - AND - FLOW - MEASUREMENT - AND - CONTROL
Unit 4 - LEVEL - AND - FLOW - MEASUREMENT - AND - CONTROL
Study Guide
School of Engineering
Compiled by: Dr. E.M. Migabo (PhD Computer Science & DEng Electrical Engineering)
May, 2023
INI3601
Study Guide
I. Learning objectives
a. Understand the principles of level measurement and the different types of level sensors
used in industrial instrumentation, such as ultrasonic, radar, capacitive, and differential
pressure sensors.
b. Learn about the methods used for level control, including on-off control, proportional
control, and integral control, and the different types of level control valves.
c. Understand the principles of flow measurement and the different types of flow sensors
used in industrial instrumentation, such as electromagnetic, vortex shedding, and
Coriolis flow meters.
d. Learn about the methods used for flow control, including on-off control, proportional
control, and integral control, and the different types of flow control valves.
e. Understand the principles of mass flow measurement and the different types of mass
flow meters used in industrial instrumentation, such as thermal and Coriolis mass flow
meters.
f. Learn about the methods used for mass flow control, including on-off control,
proportional control, and integral control, and the different types of mass flow control
valves.
g. Understand the principles of viscosity measurement and the different types of viscosity
sensors used in industrial instrumentation, such as rotational and capillary viscometers.
h. Learn about the factors that affect level and flow measurement and control, including
changes in fluid density, viscosity, and temperature, and how to compensate for these
factors.
Summary of contents from chapters “Level Measurement and Control" and “Flow Measurement
and Control” both from the textbook “Industrial Instrumentation and Control” by SK Singh
2
Industrial Instrumentȋ͵ͲͳȌ
UNIT
MEASUREMENT OF FLOW and LEVEL
AIM
To understand the various types of Flow and Level measurement techniques.
PRE-REQUISTIE:
Sensors and Actuators
PRE-MCQ:
1. In _____________ velocity of fluid is constant on every point at a specific time.
a) Steady flow
b) Rotational flow
c) Non steady flow
d) None of the mentioned
2. If all particle of fluid has a path parallel to the wall, it is known as ____________
a) Stream line flow
b) Laminar flow
c) Viscous flow
d) All of the mentioned
3. Which of the following conversions take place in float element?
a) Level to force
b) Level to voltage
c) Level to displacement
d) None of the mentioned
Flow Measurement:
Flow, or volumetric flow rate, is simply the volume of fluid that passes per unit of time.
In water resources, flow is often measured in units of cubic feet per second (cfs), cubic
meters per second (cms), gallons per minute (gpm), millions of gallons per day (MGD),
or other various units.
Introduction:
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There are different types of flow measuring techniques that are used in industries. The
common types of flow meters that find industrial applications can be listed as below:
(c) Electromagnetic
(f) Anemometer
Obstruction or head type flow meters are of two types: differential pressure type and
variable area type. Orifice meter, Venturimeter, Pitot tube fall under the first category,
while rotameter is of the second category. In all the cases, an obstruction is created in
the flow passage and the pressure drop across the obstruction is related with the flow
rate.
Basic Principle:
We consider the fluid flow through a closed channel of variable cross section. 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.
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𝐶𝑑 𝐴2 2𝑔(𝑝1 − 𝑝2)
𝑄= √
√1 − 𝛽 2 𝛾
Cd is defined as the ratio of the actual flow and the ideal flow and is always less than
one.
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 the equation. 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.
Orifice:
An Orifice Meter is basically a type of flow meter used to measure the rate of flow of
Liquid or Gas, especially Steam, using the Differential Pressure Measurement principle.
It is mainly used for robust applications as it is known for its durability and is very
economical. As the name implies, it consists of an Orifice Plate which is the basic
element of the instrument. When this Orifice Plate is placed in a line, a differential
pressure is developed across the Orifice Plate. This pressure drop is linear and is in
direct proportion to the flow-rate of the liquid or gas. Since there is a drop in pressure,
just like Turbine Flow meter, hence it is used where a drop in pressure or head loss is
permissible.
Working
As the fluid approaches the orifice the pressure increases slightly and then drops
suddenly as the orifice is passed. It continues to drop until the “vena contracta” is
reached and then gradually increases until at approximately 5 to 8
diameters downstream a maximum pressure point is reached that will be lower than
the pressure upstream of the orifice. The decrease in pressure as the fluid passes thru
the orifice is a result of the increased velocity of the gas passing thru the reduced area of
the orifice. When the velocity decreases as the fluid leaves the orifice the pressure
increases and tends to return to its original level. All of the pressure loss is not
recovered because of friction and turbulence losses in the stream. The pressure drop
across the orifice increases when the rate of flow increases. When there is no flow there
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Types
Orifice Plates are normally mounted between a set of Orifice Flanges and are
installed in a straight run of smooth pipe to avoid disturbance of flow patterns
from fittings and valves. Orifice plates cover a wide range of applications
including fluid and other operating conditions. They give an acceptable level of
uncertainties at lowest cost and long life without regular maintenance.
Concentric is by far the most common, it’s used on most processes, but not
recommended for use on slurries or highly corrosive processes.
Eccentric Bore – Eccentric bore orifice plates are plates with the orifice off-center,
or eccentric, as opposed to concentric. Location of the bore prevents accumulation
of solid materials or foreign particles and makes it useful for measuring fluids
containing suspended solid particles. Eccentric bore orifice plates are more
uncertain as compared to the concentric orifice.
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Segmental Bore – The segmentally bored orifice plates contain a hole that is a
segment of a concentric circle. Like the eccentric orifice plate design, the segmental
hole should be offset downward in gas flow applications. Segmental bores are
generally used for measuring liquids or gases which carry non-abrasive impurities
such as sewage treatment, steel, chemical, water conditioning, paper and
petrochemical industries.
Venturi Meter
The major disadvantage of using orifice plate is the permanent pressure drop that is
normally experienced in the orifice plate as shown in fig.3. The pressure drops
significantly after the orifice and can be recovered only partially. The magnitude of the
permanent pressure drop is around 40%, which is sometimes objectionable. It requires
more pressure to pump the liquid. This problem can be overcome by improving the
design of the restrictions. Venturimeters and flow nozzles are two such devices. The
construction of a venturimeter is shown in figure. Here it is so designed that the change
in the flow path is gradual. As a result, there is no permanent pressure drop in the flow
path. The discharge coefficient Cd varies between 0.95 and 0.98. The construction also
provides high mechanical strength for the meter. However, the major disadvantage is
the high cost of the meter. Flow nozzle is a compromise between orifice plate and
venturimeter.
Construction
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. After the entry, there
is a converging conical section with an included angle of 19’ to 23’. Following the
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converging section, there is a cylindrical section with minimum area called as the
throat. After the throat, there is a diverging conical section with an included angle of 5’
to 15’. 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.
Working
The fluid whose flow rate is to be measured enters the entry section of the venturi meter
with a pressure P1. As the fluid from the entry section of venturi 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.
The differential pressure sensor attached between the entry and throat section of the
venturi meter records the pressure difference (P1-P2) which becomes an indication of
the flow rate of the fluid through the pipe when calibrated. 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.
They are large in size and hence where space is limited, they cannot be used.
Expensive initial cost, installation and maintenance.
Require long laying length. That is, the veturimeter has ti be proceeded by a
straight pipe which is free from fittings and misalignments to avoid turbulence in
flow, for satisfactory operation. Therefore, straightening vanes are a must.
Low turndown (can be improved with dual range Δp cells)
Greater cost to manufacture
Greater susceptibility to “tapping errors” in high Reynolds number gas flows
owing to the high velocity fluid passing the pressure tapping at the throat.
Less experimental data than orifice plates
Flow Nozzle:
The flow nozzles are a flow tube consisting of a smooth convergent section leading to a
cylindrical throat area. The throat is the smallest section of the nozzle. Pressure taps are
located on the upstream side of the nozzle plate and on the downstream side of the
nozzle outlet. They may be in the form of an annular ring, i.e. equally spaced holes
connected together which open into the pipeline, or in the form of single holes drilled
into the pipeline.
As mentioned above, flow nozzles are primary elements in differential pressure flow
meters. These flow meters use the primary elements as an obstruction to generate a
pressure drop to calculate the flow rate. This is based on Bernoulli’s principle, according
to which any obstruction placed in the path of a flowing fluid will cause the velocity of
the fluid to increase and the pressure to decrease in the area of the obstruction.
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As the fluid passes through the nozzle, the obstruction causes the velocity of the fluid to
increase while its static pressure decreases simultaneously. At the point of maximum
convergence, i.e. at the vena contracta, the velocity is at its maximum and the pressure
is at its minimum. As the fluid exits the nozzles, its flow expands and the velocity
reduces and the pressure rises again. This difference in pressure before and after the
primary element is measured using differential pressure transmitters, also called
secondary elements
Rotameter:
A rotameter is a device that measures the flow rate of liquid or gas in a closed tube. It
belongs to a class of meters called variable area meters, which measure flow rate by
allowing the cross-sectional area the fluid travels through, to vary, causing a
measurable effect. A rotameter consists of a tapered tube, typically made of glass with a
‘float’, made either of anodized aluminum or a ceramic, actually a shaped weight,
inside that is pushed up by the drag force of the flow and pulled down by gravity. The
drag force for a given fluid and float cross section is a function of flow speed squared
only.
A higher volumetric flow rate through a given area increases flow speed and drag
force, so the float will be pushed upwards. However, as the inside of the rotameter is
cone shaped (widens), the area around the float through which the medium flows
increases, the flow speed and drag force decrease until there is mechanical equilibrium
with the float’s weight. Floats are made in many different shapes, with spheres and
ellipsoids being the most common. The float may be diagonally grooved and partially
colored so that it rotates axially as the fluid passes. This shows if the float is stuck since
it will only rotate if it is free. Readings are usually taken at the top of the widest part of
the float; the center for an ellipsoid, or the top for a cylinder. Some manufacturers use a
different standard. The “float” must not float in the fluid: it has to have a higher density
than the fluid; otherwise it will float to the top even if there is no flow. The mechanical
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nature of the measuring principle provides a flow measurement device that does not
require any electrical power.
A rotameter requires no external power or fuel, it uses only the inherent properties
of the fluid, along with gravity, to measure flow rate.
A rotameter is also a relatively simple device that can be mass manufactured out of
cheap materials, allowing for its widespread use.
Since the area of the flow passage increases as the float moves up the tube, the
scale is approximately linear.
Clear glass is used this is highly resistant to thermal shock and chemical action.
Disadvantages:
Due to its use of gravity, a rotameter must always be vertically oriented and right
way up, with the fluid flowing upward.
Due to its reliance on the ability of the fluid or gas to displace the float,
graduations on a given rotameter will only be accurate for a given substance at a
given temperature. The main property of importance is the density of the fluid;
however, viscosity may also be significant. Floats are ideally designed to be
insensitive to viscosity; however, this is seldom verifiable from manufacturers’
specifications. Either separate rotameter for different densities and viscosities may
be used, or multiple scales on the same rotameter can be used.
Due to the direct flow indication the resolution is relatively poor compared to
other measurement principles. Readout uncertainty gets worse near the bottom of
the scale. Oscillations of the float and parallax may further increase the uncertainty
of the measurement.
Since the float must be read through the flowing medium, some fluids may
obscure the reading. A transducer may be required for electronically measuring
the position of the float.
Rotameter are not easily adapted for reading by machine; although magnetic floats
that drive a follower outside the tube are available.
Rotameter are not generally manufactured in sizes greater than 6 inches/150 mm,
but bypass designs are sometimes used on very large pipes.
Pitot Tube:
The Pitot tube is named after Henri Pitot who used a bent glass tube to measure
velocities in a river in France in the 1700s. Pitot tubes can be very simple devices with
no moving parts used to measure flow velocities.
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Pitot tubes are a common type of insertion flowmeter. The below figure shows the
basics for a Pitot tube, where a pressure is generated in a tube facing the flow, by the
velocity of the fluid.
This ‘velocity’ pressure is compared against the reference pressure (or static pressure) in
the pipe, and the velocity can be determined by applying a simple equation.
The tube inserted in the center of the pipe is used to measure Total Pressure and the
next second tube is used to measure the static pressure.
When the flow rate through the pipe changes, the pressures at the total pressure tube
and static pressure tube varies with respect to the flow velocities. The difference
between the total pressure and static pressure is used to measure the proportional flow
rate passing through the pipe.
A DP type transmitter is used to measure the difference between total pressure and
static pressure and it is converted into proportional flow rate.
In practice, two tubes inserted into a pipe would be cumbersome, and a simple Pitot
tube will consist of one unit as shown in Below Figure. Here, the hole measuring the
velocity pressure and the holes measuring the reference or static pressure are
incorporated in the same device.
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Because the simple Pitot tube (Above Figure ) only samples a single point, and, because
the flow profile of the fluid (and hence velocity profile) varies across the pipe, accurate
placement of the nozzle is critical. To avoid this type of problems by using averaging
Pitot tubes.
2∆𝑝
𝑈1 = √
𝜌
Where,
ρ is Density
The averaging Pitot tube was developed with a number of upstream sensing tubes to
overcome the problems associated with correctly siting the simple type of Pitot tube.
These sensing tubes sense various velocity pressures across the pipe, which are then
averaged within the tube assembly to give a representative flowrate of the whole cross
section.
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Turbine Flow Meter is a volumetric measuring turbine type. The flowing fluid engages
the rotor causing it to rotate at an angular velocity proportional to the fluid flow rate.
The angular velocity of the rotor results in the generation of an electrical signal (AC sine
wave type) in the pickup. The summation of the pulsing electrical signal is related
directly to total flow.
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The frequency of the signal relates directly to flow rate. The vaned rotor is the only
moving part of the flow meter.
The Turbine flow meter (axial turbine) was invented by Reinhard Woltman and is an
accurate and reliable flow meter for liquids and gases. It consists of a flow tube with
end connections and a magnetic multi bladed free spinning rotor (impeller) mounted
inside; in line with the flow. The rotor is supported by a shaft that rests on internally
mounted supports.
The Supports in Process Automatics Turbine Flow Meters are designed to also act as
flow straighteners, stabilizing the flow and minimizing negative effects of turbulence.
The Supports also house the unique open bearings; allowing for the measured media to
lubricate the bushes – prolonging the flow meters life span. The Supports are fastened
by locking rings (circlips) on each end.
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The rotor sits on a shaft, which in turn is suspended in the flow by the two supports. As
the media flows, a force is applied on the rotor wings. The angle and shape of the wings
transform the horizontal force to a perpendicular force, creating rotation. Therefore, the
rotation of the rotor is proportional to the applied force of the flow.
Because of this, the rotor will immediately rotate as soon as the media induces a
forward force. As the rotor cannot turn thru the media on its own, it will stop as soon as
the media stops. This ensures an extremely fast response time, making the Turbine Flow
Meter ideal for batching applications.
A pick-up sensor is mounted above the rotor. When the magnetic blades pass by the
pickup sensor, a signal is generated for each passing blade. This provides a pulsed
signal proportional to the speed of the rotor and represents pulses per volumetric unit.;
and as such the flow rate too.
Advantages
1. They do cause some pressure drop where that may be a factor such as gravity
flows.
2. Not reliable for steam
3. Bearings wear out.
Applications
This type of measurement is not affected by the liquid’s viscosity, density or the
turbulence in the pipe. All incompressible fluids will occupy the same volume and there
is no need to correct the meter’s output to compensate for these factors.
Positive Displacement Meter is a type of flow meter that requires fluid to mechanically
displace components in the meter in order for flow measurement. Positive displacement
(PD) flow meters measure the volumetric flow rate of a moving fluid or gas by dividing
the media into fixed, metered volumes (finite increments or volumes of the fluid).
A basic analogy would be holding a bucket below a tap, filling it to a set level, then
quickly replacing it with another bucket and timing the rate at which the buckets are
filled (or the total number of buckets for the “totalized” flow). With appropriate
pressure and temperature compensation, the mass flow rate can be accurately
determined.
These devices consist of a chamber(s) that obstructs the media flow and a rotating or
reciprocating mechanism that allows the passage of fixed-volume amounts. The number
of parcels that pass through the chamber determines the media volume.
The rate of revolution or reciprocation determines the flow rate. There are two basic
types of positive displacement flow meters. Sensor-only systems or transducers are
switch-like devices that provide electronic outputs for processors, controllers, or data
acquisition systems.
2. Gear
Gear flow meters rely on internal gears rotating as fluid passes through them. There are
various types of gear meters named mostly for the shape of the internal components
Oval Gear
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Two rotating oval gears with synchronized teeth “squeeze” a finite amount of fluid
through the meter for each revolution. With oval gear flow meters, two oval gears or
rotors are mounted inside a cylinder.
As the fluid flows through the cylinder, the pressure of the fluid causes the rotors to
rotate. As flow rate increases, so does the rotational speed of the rotors.
Helical Gear
Helical gear flow meters get their name from the shape of their gears or rotors. These
rotors resemble the shape of a helix, which is a spiral-shaped structure.
As the fluid flows through the meter, it enters the compartments in the rotors, causing
the rotors to rotate. Flow rate is calculated from the speed of rotation.
3. Nutating disk
A disk mounted on a sphere is “wobbled” about an axis by the fluid flow and each
rotation represents a finite amount of fluid transferred. A nutating disc flow meter has a
round disc mounted on a spindle in a cylindrical chamber.
By tracking the movements of the spindle, the flow meter determines the number of
times the chamber traps and empties fluid. This information is used to determine flow
rate.
4. Rotary vane
A rotating impeller containing two or more vanes divides the spaces between the vanes
into discrete volumes and each rotation (or vane passing) is counted.
5. Diaphragm
Fluid is drawn into the inlet side of an oscillating diaphragm and then dispelled to the
outlet. The diaphragm oscillating cycles are counted to determine the flow rate.
PD Meters types
PD flow meters are mainly named after the inbuilt mechanical device in the meter unit.
Various types of positive displacement flow meters are available for industrial use. All
these types are based on the common operating principle. Besides, they all are
volumetric flow measuring devices.
Major types of positive displacement flow meters are mentioned below:
Other types available are double acting pistons and rotary pistons. Selection of a
particular type of piston meter depends on the range of flow rates necessary for an
application.
Although piston meters are smaller in size and considered apt for handling only low
flows of viscous liquids, yet they are proficient enough to deal with an extensive range
of liquids. Major application areas of a reciprocating piston meter include viscous fluid
services like oil metering on engine test stands, specifically where turndown ratio is not
considered much crucial.
Also these meters can be employed on residential water service where they tend to pass
partial quantities of dirt and fine sand along with water.
Oval-shaped gears
These types of meters consist of two rotating, oval-shaped gears constructed with
synchronized, close fitting teeth. In an oval gear meter, the rotation of gear shafts causes
a fixed amount of liquid to pass through the meter. By monitoring the number of shaft
rotations, one can calculate liquid flow rate.
These types of meters prove to be very accurate when slippage between the housing
and the gears is set very small. Turndown ratio of an oval gear meter gets influenced by
the lubricating properties of the process fluid.
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They exist in various sizes and capacities and can be constructed from a wide range of
materials. Their typical size range varies from 5/8-in to 2-in sizes. They are ideal for
pressure ranges around 150-psig with an upper limit of 300 psig.
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Based upon the construction material, maximum pressure and maximum temperature
limits of rotary vane meters are 350°F and 1,000 psig respectively. Their Viscosity limit
ranges between 1 and 25,000 centipoises.
Helix Meters
These types of meters are made up of two radically pitched helical rotors which results
in an axial liquid displacement from one side of the chamber to the other side.
Both the rotors are geared together and there is a very small clearance between the
rotors and the casing.
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E=k*B*D*V
where,
The induced voltage (E) is directly proportional to the velocity (V) of the fluid moving
through the magnetic field (B). The induced voltage is carried to the transmitter
through the electrode circuit. The transmitter then converts this voltage into a
quantifiable flow velocity. The volumetric flow rate of the fluid is calculated using this
known velocity along with the area of the pipe.
When a flow meter is installed and activated, its operations begin with a pair of
charged magnetic coils. As energy passes through the coils, they produce a magnetic
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field that remains perpendicular to both the conductive fluid being measured and the
axis of the electrodes taking measurements. The fluid moves along the longitudinal
axis of the flow meter, making any generated induced voltage perpendicular to the
field and the fluid velocity. An increase in the flow rate of the conductive fluid will
create a proportionate increase in the voltage level.
The meter features flanged construction and is available with a choice of liner and
electrode material. All meters consist of a sensor and a converter that may be mounted
integral to the sensor or remotely either with a field mount kit.
(i) The obstruction to the flow is almost nil and therefore this type of meters can be used
for measuring heavy suspensions, including mud, sewage and wood pulp.
ii) There is no pressure head loss in this type of flow meter other than that of the length
of straight pipe which the meter occupies.
(iii) They are not very much affected by upstream flow disturbances.
(iv) They are practically unaffected by variation in density, viscosity, pressure and
temperature.
(v) Electric power requirements can be low (15 or 20 W), particularly with pulsed DC
types.
(vii) The meters are suitable for most acids, bases, water and aqueous solutions because
the lining materials selected are not only good electrical insulators but also are
corrosion resistant.
(viii) The meters are widely used for slurry services not only because they are
obstruction less but also because some of the liners such as polyurethane, neoprene and
rubber have good abrasion or erosion resistance.
(i) These meters can be used only for fluids which have reasonable electrical
conductivity.
(ii) Accuracy is only in the range of ± 1% over a flow rate range of 5%.
(iii) The size and cost of the field coils and circuitry do not increase in proportion to
their size of pipe bore. Consequently small size meters are bulky and expensive.
This electromagnetic flow meter being non intrusive type, can be used in general for
any fluid which is having a reasonable electrical conductivity above 10
microsiemens/cm.
Fluids like sand water slurry, coal powder, slurry, sewage, wood pulp, chemicals, water
other than distilled water in large pipe lines, hot fluids, high viscous fluids specially in
food processing industries, cryogenic fluids can be metered by the electromagnetic flow
meter.
An ultrasonic flow meter utilizes ultrasound to measure the velocity of a fluid and is
used in a variety of fluid applications. Ultrasonic flowmeters are ideal for water and
other liquids. Clamp-on ultrasonic flow meters achieve high accuracy at low and high
flows, save time with no pipe cutting or process shutdown, and are not affected by
external noise.
Ultrasonic flow meters use sound waves at a frequency beyond the range of hearing
(typically 0.5, 1, or 4 MHz). This ultrasound signal is sent into a stream of flowing liquid
by using wetted (insertion) transducers that make direct contact with the liquid or
external (clamp-on) transducers that send the ultrasound through the pipe wall. Clamp-
on ultrasonic flow meters allow users to measure the volumetric flow rate of a fluid in a
pipe without having to penetrate the pipe which decreases installation and maintenance
costs.
A typical transit-time ultrasonic liquid flow meter utilizes two ultrasonic transducers
that function as both ultrasonic transmitter and receiver. The ultrasonic flow meter
operates by alternately transmitting and receiving a burst of ultrasound between the
two transducers by measuring the transit time that it takes for sound to travel between
the two transducers in both directions. The difference in the transit time (∆ time)
measured is directly proportional to the velocity of the liquid in the pipe.
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The same process is then repeated in reverse as the second transducer transmits the
ultrasound. The time difference between the times of flight up and down is the ∆ Time.
When the liquid in the pipe is not moving, the ∆ Time would be zero.
To calculate the velocity of the liquid, you need to convert the raw ∆ Time into the
velocity of the liquid in the pipe. The angle of the ultrasound path is calculated by
knowing the speed of sound of the pipe and the liquid. This angle is used in a common
trigonometry equation to convert the ultrasound path into a straight line in the pipe.
This will be the velocity of the liquid in the pipe.
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Once the velocity is known, it’s just a matter of converting it into a flow rate by
multiplying it times the cross-sectional area of the pipe, as with any velocity based flow
meter.
Advantages:
Low maintenance: There are no blades to wear out or bearings to replace as in turbine
meters, nor are there electrodes that can foul over time as in magnetic flow meters.
The common method of measuring flow through an open channel is to measure the
height or HEAD of the liquid as it passes over an obstruction (a flume or weir) in the
channel. Using ultrasonic level technology, Open channel flow meters include a non-
contacting sensor mounted above the flume or weir. By measuring the time from
transmission of an ultrasonic pulse to receipt of an echo, the water level or “Head” is
accurately measured.
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Flumes and weirs are specially designed channel shapes that characterize the flow of
water.
Common types are
Rectangular Weirs,
V-Notch Weirs,
Parshall flumes and
Palmer Bowlus flumes.
The choice of flume or weir type depends on the application: flow rate, channel shape
and solids content of the water.
There are several different versions of solids flowmeter. In this article we will discuss
and evaluate these, so that you too will have the right tool for your industrial flow
application.
There are a handful of technologies used for metering the flow of solids:
Impact Flow meter – the most popular form of solids flow meter, impact meters, as they
are often called, guide the material through an infeed pipe or chute and create a specific
trajectory for the material to strike a flat sensing plate.
The amount of force the impact creates is measured by means of load cells or an LVDT
(linear variable differential transformer). As the plate is deflected by the force of the
material, the load cell or LVDT deflects and generates a signal, which is converted into a
flow rate by an integrator. Impact Flow meters has two distinct Advantages. First, they
can handle very low to very high flow rates. Second, material buildup on the sensing
plate does not affect their accuracy or repeatability, as only the horizontal force of
impact causes a deflection on the sensor. Any additional weight, e.g. if some material
sticks, does not shift the output of the system.
a motor. The motor is connected to a torque arm, which is mounted to a load cell. As the
amount of material fed into the coriolis meter increases, the torque on the motor
increases. The load cell detects this increase and sends a signal to an integrator, which
translates it into a flow rate.
Microwave Flowmeter (Low Cost but lower Accuracy)– one of the lesser-used
technologies, microwave or radar flowmeters, emits a 24 or 125 GHz microwave into
the material flow in a pipe or chute. Based on the Doppler principle, the change in
microwaves reflected back to the sensor is measured and transmitted as a 4 to 20 mA
signal for scaling in a PLC system to become a flow rate. Microwave-based products can
be used in pneumatically-fed systems, as the extra force of the material flow does not
affect the measurement as is the case with the three technologies discussed above .
Capacitive Flowmeter (Simple Installation, lower Accuracy)– solids flow sensors using
capacitance are based on two independent measurements. One is the change in
capacitance from an empty pipe to a full pipe, which is proportional to the
concentration of the material. The other is a velocity measurement, which uses two
sensors to indicate the time it takes for the material to move from the first sensor to the
second. The signals from these measurements are then fed into an integrator, which
outputs a flow rate. Capacitive measurement can also be used with pneumatic systems.
Industrial Instrumentation
Level Measurement:
1. Direct Method
Sight Glass Type Level Measurement
Float type
2. Indirect Method
Electrical method
Resistive
Capacitive
Ultrasonic
Sight Glass Type Level Measurement
Use of a sight glass is probably the simplest method of measuring liquid level. The
sight glass is attached to the outside of the tank so that the liquid level can be seen
through the glass. The sight glass is marked with graduations to allow the level to be
measured. The main disadvantage of this method is that it only gives level indication
local to the vessel. A visual indication of the level can be obtained when part of the
vessel is constructed from transparent material or the liquid in a vessel is bypassed
through a transparent tube. The advantage of using stop valves with the use of a bypass
pipe, is the ease in removal for cleaning.
Industrial Instrumentation
Float devices use the buoyancy of a float to indicate the liquid level in the tank. One
common approach is to attach the float to a chain. The chain is attached to a
counterweight which indicates the level as the float moves up and down. These types of
device are often found on large atmospheric storage tanks.
Other devices transfer the buoyancy force to a metering device using a torque tube. The
float is connected to the torque tube which twists as the height of the displacement
device changes. The twisting force drives the position of a pointer which then indicates
liquid level. For some applications – where the liquid is corrosive, toxic or in some way
hazardous – a magnetic level gauge is used. The float contains a magnet which changes
the orientation of small indicator wafers as it moves up and down.
Industrial Instrumentation
Other systems use the float method by sensing the position of the float magnetically or
electrically. Float systems can also be used when measuring granular solids as well as
liquids.
Disadvantages
High maintenance
Expensive
In atmospheric conditions, a seal is used to protect the sensing head from the
process fluid. However in pressurized applications, it is better to fill the head
with the sensing fluid, particularly if the fluid is clean and lubricating.
Application Limitations
Industrial Instrumentation
Float and tape systems have a common problem with the tape hanging up. This
often occurs if the long guide pipes are not perfectly vertical, where the tape rubs
against the inside of the pipes.
Another common problem is with corrosion or dirt, where the tape can be held
in place while the float is moving. These problems are more likely to result in a
lower than actual reading.
The force from the float weight is usually great enough to overcome the friction
of the tape against the impurities, whereas the force of the take-up device may
not. Tanks controlled using tape level gauges will often overflow when the tape
is stuck.
This problem can be protected against by the use of a separate high level switch.
More advanced controllers are available that monitor tank capacity and pumping
rates to check the actual level, rate of change and direction of change.
Although the fundamental theory of operation has been outlined, in a very practical
displacer level sensor, the construction is built to attain the specified measuring
objective with sophisticated electronic circuitry.
In these kinds of displacer level sensors, the displacer is connected to a spring that
restricts its movement for every increment of buoyancy (i.e. level change).
A transmitter incorporating a Linear Variable Differential transformer (LVDT) is
employed to trace the increase and fall of the displacer rod as liquid level changes.
Sophisticated electronics is then accustomed process the voltage signal from the LDVT
into a 4-20mA o/p signal.
Industrial Instrumentation
In the bubble type level system, liquid level is determined by measuring the pressure
required to force a gas into a liquid at a point beneath the surface. This method uses a
source of clean air or gas and is connected through a restriction to a bubble tube
immersed at a fixed depth into the vessel. The restriction reduces the airflow to a very
small amount. As the pressure builds, bubbles are released from the end of the bubble
tube. Pressure is maintained as air bubbles escape through the liquid. Changes in the
liquid level cause the air pressure in the bubble tube to vary. At the top of the bubble
tube is where a pressure sensor detects differences in pressure as the level changes.
Most tubes use a small V-notch at the bottom to assist with the release of a constant
stream of bubbles. This is preferable for consistent measurement rather than
intermittent large bubbles.
Industrial Instrumentation
The Bubbler System is an inexpensive but accurate means of measuring the fluid level
in open or vented containers, especially those in harsh environments such as cooling
tower sumps, swimming pools, reservoirs, vented fuel tanks, drain sumps, air washers,
etc. The system consists of a source of compressed air, air flow restrictor, sensing tube
and pressure transmitter. The only component of the Bubbler System that is exposed to
the elements is the sensing tube. All other components can be remotely located in a
protected area.
The sense tube is installed in the tank and connected to the pressure transmitter and the
air supply through the flow restrictor as shown in Figure. Since the pressure required to
displace the liquid is determined by the depth of the fluid, the transmitter output
indicates the fluid depth above the open end of the sense tube.
Working
Bubblers, are all hydrostatic measurement devices. This technology is used in vessels
(tanks) that operate under atmospheric pressure. A dip tube having its open end near
the vessel bottom carries a purge gas (typically air, although an inert gas such as dry
nitrogen may be used when there is danger of contamination of or an oxidative reaction
with the process fluid) into the tank.
As gas flows down to the dip tube’s outlet, the pressure in the tube rises until it
overcomes the hydrostatic pressure produced by the liquid level at the outlet. That
pressure equals the process fluid’s density multiplied by its depth from the end of the
dip tube to the surface and is monitored by a pressure transducer connected to the tube.
Industrial Instrumentation
Advantages
1. Simple assembly
2. Suitable for use with corrosive fluids.
3. Intrinsically safe
4. High temp applications
Disadvantages
1. Requires compressed air and installation of air lines
2. Build-up of material on bubble tube not permissible
3. Not suited to pressurized vessels
4. Mechanical wear
Application Limitations
1. Bubble tube devices are susceptible to density variations, freezing and plugging
or coating by the process fluid or debris. The gas that is used can introduce
unwanted materials into the process as it is purged.
2. Also the device must be capable of withstanding the maximum air pressure
imposed if the pipe should become blocked.
Hydrostatic level transmitters, or pressure level transmitters, use the pressure exerted
by a resting body of fluid in a container to determine how much of it is in the container.
Basically, the more force the liquid exerts on the container, and thus the sensor, the
more liquid there is present in the container.
This is one of the most common types of fill level detection technologies. However, to
ensure accuracy, the density of the liquid being measured must remain consistent. As
the specific gravity of the liquid increases, the pressure it exerts on the transmitter per
cubic inch goes up too.
So, these types of level transmitters are sometimes less useful for applications where a
liquid might have variable density because of impurities or other factors.
Industrial Instrumentation
Hydrostatic head instruments for measuring liquid level in vessels operating above
atmospheric pressure uses the full capability of the differential pressure instruments
with both sides of the measuring element connected to the vessel. The differential
pressure transmitter enables an automatic subtraction of the pressure on the LP side,
from the total pressure appearing at the HP side. This is accomplished as shown in
diagram above, where the LP is connected above the maximum predicted level. With
this arrangement, each increment of pressure above the liquid surface is applied to both
capsule assemblies of the transmitter, and since they are in opposition, the increment is
cancelled. Only the hydrostatic pressure, which is applied to the HP, is effective in
causing any response to the transmitter, which is proportional to the level.
The DP transmitter have inbuilt pressure sensors like Diaphragm, capsules, strain
gauges etc to measure the differential pressure. The pressure sensor converts the
measured pressure into parameters like millivolts, capacitance, resistance etc depending
on the type of pressure sensor we are using inside the DP transmitter. Generally a
Wheatstone bridge will be used to convert resistance; capacitance or inductance type of
pressure sensor outputs into electrical signal like milivolts or volts which is
proportional to the pressure, then transmitter converts the pressure into equivalent
Level Signal accordingly.
The tank bottom tapping point is High pressure (HP) tapping point and Tank top
tapping is Low Pressure (LP) tapping point. The DP Transmitter is connected at these
HP & LP tapping points accordingly.
Industrial Instrumentation
The DP Transmitter calibration parameters will vary depending on installation & seal
system also. Generally we can see three possibilities of installation of a transmitter in
the field.
They are :
The electrical sensing element consist of a precision-wound helix resistor that has about
28 contacts per foot.An outer jacket of flexible protective material also acts as a pressure
receiving diaphragm.
Industrial Instrumentation
The pressure of the liquid in the tank acts upon the jacket-diaphragm and causes the
resistance element to contact the steel base trip. Below the surface of the liquid,the
resistor is shorted. The resistor remains unshorted above the surface,and it is this
portion that is metered to provide the level reading. Two wires from the sensor top
transmit an electrical resistance signal that is related to the distance from the tank top to
the surface of the liquid. Typically 1 ohm is equal to 1 cm.
The sensor strip remains fixed in position has no moving parts and provides a
continuous indication of resistance. The windings these sensors are purely resistive,
they cannot store or release electrical energy. Thus resistance sensors qualify as
intrinsically safe devices and can be used with explosive liquids and dust.
Since capacitance level sensors are electronic devices, phase modulation and the use of
higher frequencies makes the sensor suitable for applications in which dielectric
constants are similar.
Working Principle:
The principle of capacitive level measurement is based on change of capacitance. An
insulated electrode acts as one plate of capacitor and the tank wall (or reference
electrode in a non-metallic vessel) acts as the other plate. The capacitance depends on
the fluid level. An empty tank has a lower capacitance while a filled tank has a higher
capacitance.
Industrial Instrumentation
C = E.(K . A/d)
Where:
Measurement:
Measurement is made by applying an RF signal between the conductive probe and the
vessel wall.
The RF signal results in a very low current flow through the dielectric process material
in the tank from the probe to the vessel wall. When the level in the tank drops, the
dielectric constant drops causing a drop in the capacitance reading and a minute drop
in current flow.
Industrial Instrumentation
This change is detected by the level switch’s internal circuitry and translated into a
change in the relay state of the level switch in case of point level detection. In the case of
continuous level detectors, the output is not a relay state, but a scaled analog signal.
Conducting Material:
In conducting liquids, the probe plates are insulated using thin coating of glass or
plastic to avoid short circuiting. The conductive material acts as the ground plate of the
capacitor.
Proximity level measurement does not produce a linear output and are used when the
level varies by several inches.
1. Liquids
2. Powered and granular solids
Industrial Instrumentation
When ultrasonic pulse signal is targeted towards an object, it is reflected by the object
and echo returns to the sender. The time travelled by the ultrasonic pulse is calculated,
and the distance of the object is found. Bats use well known method to measure the
distance while travelling. Ultrasonic level measurement principle is also used to find
out fish positions in ocean, locate submarines below water level, also the position of a
scuba diver in sea.
An ultrasonic level transmitter is fixed at the top of a tank half filled with liquid. The
reference level for all measurements is the bottom of the tank. Level to be detected is
marked as “C”, and “B” is the distance of the ultrasonic sensor from the liquid level.
Industrial Instrumentation
Ultrasonic pulse signals are transmitted from the transmitter, and it is reflected back to
the sensor. Travel time of the ultrasonic pulse from sensor to target and back is
calculated. Level “C” can be found by multiplying half of this time with the speed of
sound in air. The measuring unit final result can be centimeters, feet, inches etc.
The above principle of measurement looks quite straightforward and true only in
theory. In practice, there are some technical difficulties which are to be taken care to get
correct level reading.
Ultrasonic Sensor is the heart of the ultrasonic level Transmitter instrument. This sensor
will translate electrical energy into ultrasound waves. Piezoelectric crystals are used for
this conversion process. Piezoelectric crystals will oscillate at high frequencies when
electric energy is applied to it. The reverse is also true. These piezoelectric crystals will
generate electrical signals on receipt of ultrasound. These sensors are capable of sending
ultrasound to an object and receive the echo developed by the object. The echo is
converted into electrical energy for onward processing by the control circuit.
Ultrasonic level transmitter has no moving parts, and it can measure level without
making physical contact with the object. This typical characteristic of the transmitter is
useful for measuring levels in tanks with corrosive, boiling and hazardous chemicals.
The accuracy of the reading remains unaffected even after changes in the chemical
composition or the dielectric constant of the materials in the process fluids.
Ultrasonic level transmitters are the best level measuring devices where the received
echo of the ultrasound is of acceptable quality. It is not so convenient if the tank depth
Industrial Instrumentation
is high or the echo is absorbed or dispersed. The object should not be sound absorbing
type. It is also unsuitable for tanks with too much smoke or high density moisture.
POST-MCQ:
4. Which of the following devices are used for a level to force conversion?
a) Load cell
b) Membrane
c) Diaphragm
d) Voltmeter
7. Which of the following represents the correct relation between flow rate and area
of pipe?
a) Direct proportionality
b) Inverse proportionality
c) Equal
d) None of the mentioned
10. Bernoulli’s theorem is applicable for fluid path with moderate frictional force.
a) True
b) False
Reference:
[1] https://www.engineeringclicks.com
[2] http://www.instrumentationtoday.com
[3] D. Patranabis, “Principles of Industrial Instrumentation”, Tata McGraw
Hill, 2ndEdition, New Delhi, Reprint2009.
[4] S. K. Singh, “Industrial Instrumentation & Control” 3rd Edition, Tata
McGraw Hill, Reprint 2009.
[5] K.Krishnaswamy & S.Vijayachitra, “Industrial Instrumentation” New age
International, Reprint 2008.
INI3601
Study Guide
III. Tutorials
1. What is the basic principle of flow measurement? Answer: The basic principle of flow
measurement is to determine the volume or mass of a fluid that passes a certain point
in a given time.
2. What is an orifice plate and how is it used in flow measurement? Answer: An orifice
plate is a thin plate with a hole in the center that is placed in a pipeline. It is used to
create a pressure drop in the fluid, which can be measured to calculate the flow rate.
3. What is the difference between laminar flow and turbulent flow? Answer: Laminar
flow is a smooth, orderly flow of fluid in which the velocity at any point remains
constant. Turbulent flow is characterized by fluctuations in velocity and direction, and
is often seen in high-speed flows.
4. What is a rotameter and how is it used in flow measurement? Answer: A rotameter is
a type of flow meter that uses a tapered tube with a float inside. The float is pushed up
by the flow of fluid, and the position of the float indicates the flow rate.
5. What is a pitot tube and how is it used in flow measurement? Answer: A pitot tube is
a device used to measure fluid velocity. It consists of a tube with an opening facing the
flow, and a pressure sensor that measures the difference in pressure between the
opening and a point perpendicular to the flow.
6. What is the purpose of a flow switch? Answer: A flow switch is used to monitor fluid
flow and trigger an alarm or shut down a system if the flow falls below or exceeds a
certain threshold.
7. What is a venturi meter and how is it used in flow measurement? Answer: A venturi
meter is a type of flow meter that uses a constricted section of pipe to create a pressure
drop in the fluid, which can be measured to calculate the flow rate.
8. What is a magnetic flow meter and how does it work? Answer: A magnetic flow meter
is a type of flow meter that uses a magnetic field to measure the velocity of a fluid. As
the fluid flows through a magnetic field, an electric voltage is induced, which can be
used to calculate the flow rate.
9. What is the difference between open channel flow and closed channel flow? Answer:
Open channel flow refers to fluid flow in which the surface of the fluid is open to the
atmosphere, such as in a river or canal. Closed channel flow refers to fluid flow in a
closed pipe or conduit.
10. What is a level transmitter and how is it used in level measurement? Answer: A level
transmitter is a device used to measure the level of a fluid in a container or tank. It
typically uses a pressure sensor or ultrasonic sensor to determine the height of the fluid.
11. What is a float switch and how is it used in level measurement? Answer: A float switch
is a type of level switch that uses a buoyant float to detect the level of a fluid. As the
fluid level rises or falls, the float moves and triggers a switch.
12. What is a differential pressure transmitter and how is it used in level measurement?
Answer: A differential pressure transmitter is a device used to measure the level of a
fluid in a container or tank. It works by measuring the pressure difference between the
top and bottom of the container, which can be used to calculate the level.
13. What is a hydrostatic level transmitter and how does it work? Answer: A hydrostatic
level transmitter is a device used to measure the level of a fluid in a container or tank.
It works by measuring the pressure exerted by the fluid on a diaphragm or other sensing
element.
14. What is the difference between a contact and non-contact level sensor? Answer: A
contact level sensor
3
INI3601
Study Guide
1. A tank has a diameter of 5 meters and a height of 10 meters. Calculate the volume of liquid
in the tank if the level is at 7 meters. Answer: The volume of liquid in the tank is V = πr^2h
= π(2.5)^2(7) = 137.5 m^3.
2. A flow meter has a range of 0-1000 liters per minute (LPM). If the flow rate is 450 LPM,
what is the percentage of the range? Answer: The percentage of the range is (450/1000) x
100% = 45%.
3. A weir has a rectangular notch of width 0.2 meters and height 0.1 meters. If the measured
head is 0.05 meters, what is the discharge in liters per second (L/s)? Answer: The discharge
is Q = 2/3 x (0.1) x (2g)^0.5 x (0.2)^1.5 x (0.05)^1.5 = 0.0079 L/s.
4. A flow meter has a diameter of 4 inches and a flow rate of 10 gallons per minute (GPM).
Calculate the flow velocity in feet per second (ft/s). Answer: The flow velocity is v =
(4/12)^2 x π/4 x 10/7.48 = 1.21 ft/s.
5. A level transmitter has a range of 0-5 meters and a span of 4 meters. If the measured level
is 3.5 meters, what is the output signal in milliamps (mA) if the transmitter is a two-wire
system? Answer: The output signal is (3.5/4) x 20 mA = 17.5 mA.
6. A tank has a diameter of 3 meters and a height of 6 meters. If the liquid density is 1000
kg/m^3, what is the weight of liquid in the tank when the level is at 4 meters? Answer: The
volume of liquid in the tank is V = πr^2h = π(1.5)^2(4) = 28.3 m^3. The weight of liquid
in the tank is W = V x ρ x g = 28.3 x 1000 x 9.81 = 277,521 N.
7. A differential pressure transmitter has a range of 0-100 kPa and a span of 50 kPa. If the
measured differential pressure is 30 kPa, what is the output signal in volts (V) if the
transmitter is a four-wire system? Answer: The output signal is (30/50) x 10 V = 6 V.
8. A level transmitter has a range of 0-10 meters and a span of 8 meters. If the measured level
is 4 meters, what is the output signal in milliamps (mA) if the transmitter is a three-wire
system? Answer: The output signal is (4/8) x (10-4) mA = 3 mA.
9. A flow meter has a diameter of 2 inches and a flow rate of 20 gallons per minute (GPM).
Calculate the flow velocity in meters per second (m/s). Answer: The flow velocity is v =
(2/39.37)^2 x π/4 x 20/60 = 1.24 m/s.
10. A level transmitter has a range of 0-20 meters and a span of 16 meters. If the output signal
is 4 mA when the level is 2 meters and 20 mA when the level is 18 meters, what is the
level when the output signal is 12 mA?
Answer:
The span of the level transmitter is 16 meters, which means that the output signal changes
by 16 mA over the range of 0-20 meters.
Output signal = ((Level - Lower range) / (Upper range - Lower range)) x Span + Lower
output
Where:
• Output signal = 12 mA
• Lower range = 0 meters
4
INI3601
Study Guide
Level = ((Output signal - Lower output) x (Upper range - Lower range)) / Span + Lower
range
Level = 15 meters