 ORIFICE PLATE
 Orifice plates, as shown in fig., are the simplest and cheapest form of primary
element and are used more frequently than all other types.
 An orifice plate is inserted in the line and the differential pressure across it is
measured.
 Sometimes, orifice plates are provided with an additional small hole for the
passage of gases, as shown in fig.
 This hole is located at the bottom when gases are measured, to allow the
condensate to pass in order to prevent its building up at the orifice plate.
 When the fluid is a liquid, this hole is located at the top so that the gases can
pass and gas pockets cannot build up.
 The use of such a drain hole may produce errors in measurements and thus is
considered a disadvantage.
 There are four types of orifice plates which are listed below.
1)
2)
3)
4)
Concentric orifice plate
Eccentric orifice plate
Segmental orifice plate
Quadrant edge orifice plate
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1) CONCENTRIC ORIFICE PLATE
 It is most widely used.
 It is usually made of stainless steel and its thickness varies from 3.175 to
12.70mm, depending on pipe line size and flow velocity.
 It has a circular hole (orifice) in the middle, and is installed in the pipe line
with the hole concentric to the pipe.
 It is also made from other materials such as nickel, phosphor bronze, etc. to
withstand the corrosive effect of the fluid.
 The plate thickness at the orifice edge should not exceed any of the
following:
D/50
d/8
(D-d)-8
Where, D=the pipe internal diameter
d=the orifice diameter or bore
2) ECCENTRIC ORIFICE PLATE
 It is similar to the concentric plate except for the offset hole which is bored
tangential to a circle, concentric with the pipe and of a diameter equal to
98% of that of the pipe.
 Bore is located such a way that location of the bore prevents damming of
solid materials or foreign particles and makes it useful for measuring fluids
containing solids ,dirty particles ,slurries etc.
3) SEGMENTAL ORIFICE PLATE
 This orifice plate is used for the same type of services as the eccentric
orifice plate.
 It has a hole which is a segment of a circle, the diameter of which is
customarily 98% of the pipe diameter.
 It is installed horizontal and with the lower surface of the pipe.
4) QUADRANT EDGE ORIFICE PLATE
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 This type of orifice plate is used for flows such as heavy crudes, syrups and
slurries, and high viscous flows.
 It is constructed in such a way that the edge is rounded to form a quartercircle.
 The plate has a concentric opening with a rounded upstream edge rather
than the sharp, square edge normally used.
 It may be used when the Reynolds number range from 100,00 or above,
down to 3000 to 5000 with accuracy of approximately 0.5%.
 ADVANTAGES OF ORIFICE PLATE

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Its cost is low
They can be used in a wide range of pipe size (3.175 to 18211.8mm)
They can be used with differential pressure devices.
They are well-known and have predictable characteristics.
They are available in many materials.
 DISADVANTAGES OF ORIFICE PLATE
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They cause relatively high permanent pressure loss.
They tend to block slurry services.
They have square root characteristics.
Their accuracy is dependent on care during installation.
They have changing characteristics because of erosion, corrosion and
scaling.
 FLOW NOZZLE
 The flow nozzles are used for flow measurement at high fluid velocities
 They are rugged and more resistant to erosion than the sharp edge orifice
plate.
 Basically, there are two types of flow nozzles,
(1) The long-radius flow nozzle and
(2) The I.S.A. (international federation of the national standardizing
associations) flow nozzle.
 A flow nozzle consists of a cylindrical throat, as shown in fig.
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 One differential pressure tap is located on the pipe upstream and other is
located on other end of flow nozzle at downstream having one-half diameter
as shown in figure 2.
Figure 1.
Figure 2.
 For a given diameter and a given differential pressure, it allows measurement
of flow rates almost 65% more than of the orifice plate.
 Flow nozzles are manufacture commonly from materials such as stainless
steel or chrome-moly steel.
 Flow nozzle should be used at Reynolds numbers of 50,000 or above.
 However, data is available for Reynolds number down to 6,000; so it Is
possible to use nozzles with more viscous fluids.
ADVANTAGES

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Its permanent pressure loss is lower than that for an orifice plate.
It is available in various materials.
It is useful for fluids containing solids.
It is widely accepts for high-pressure and temperature steam flow.
DISADVANTAGES
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 Its cost is higher than orifice plate.
 it is limited to moderate pipe sizes.
 It requires more maintenance (it is necessary to remove a section of pipe to
inspect or install it.)
 VENTURI TUBE
 As we know that orifice plates can make permanent pressure loss of the
flowing fluid
 Venturi tube is used where permanent pressure loss is of prime important
and where maximum accuracy is desired in the measurement of high viscous
fluids.
 It consists of
1) A straight inlet section of the same diameter as the pipe in with the
high pressure tap is located,
2) A converging conical inlet section I which the cross sectional of the
stream decreases and decrease of pressure head.
3) A cylindrical throat which provides for the low pressure tap location of
the decrease pressure in an area where flow velocity is neither
increasing nor decreasing and,
4) A diverging recovery cone in which velocity decreases.
5) The pressure tap are located one-quarter to one-half pipe diameter
up-stream of the inlet cone and at the middle of the throat section.
⇜FIG. 1. LONG-FORM OR CLASSIC VENTIRI TUBE⇝
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⇜FIG.2. ECCENTRIC VENTURI TUBE⇝
⇜FIG.3. RECTANGULAR VANTURI TUBE⇝
 So by measuring the pressure difference at the two ends of the venturi tube
it is possible to measure flow of the liquid.
 The venturi tube can be used to handle a fluid which is handled by an orifice
plate and fluids that contains some solids, because this venture tube contains
no sharp corners and not project into the fluid stream.
 It can be also used to handle slurries and dirty liquids that build up around
primary element.
 The venturi tubes are usually made of cast iron or steel, and are built in
several forms.
 Venturi tubes are available in sizes from 100mm to 813mm with a flow
coefficient value of 0.984.
 Its overall accuracy range ±1/4 to ±3% .
 Venturi coefficient is less affected by a decreasing Reynolds number.
 ADVANTAGES
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It causes low permanent pressure loss.
It is widely used for high flow rates.
It is available in very large pipe sizes.
It has well known characteristics.
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 It is more accurate over wide flow ranges than orifice plates or nozzles.
 DISADVANTAGES
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Its cost is high.
It is generally not useful below 76.2 mm pipe size.
It is more difficult to inspect due to its construction.
it has the limitation of a lower Reynolds number of 150,000.
 PITOT TUBE
 Pitot tube is mainly used for the measurement of fluid velocity.
 The operation principle of a pitot tube is based on the fact that when a solid
body is kept centrally and stationary in a pipe line with a fluid stream, the
velocity of the fluid starts decreasing due to the presence of the body till it is
reduce to zero directly in front of the body.
 This point is known as the stagnation point.
 Here kinetic head pressure is lost by the liquid, it gains a static head pressure.
 Thus, by measuring the differential between pressure at normal flow line and
that at stagnation point, the fluid velocity is determined.
 A pitot tube consists of a tube having diameter of 3.125 to 6.35 mm which is
placed directly in the line of flow, and a static opening at 90° from the
streamline.
 The differential pressure across these taps is proportional to the velocity of
the fluid.
 For an accurate measurement, the Pitot tube is moved across the entire
diameter of the pipe to measure the velocity at several points and then the
true average velocity is calculated.
 The accuracy of a pitot tube may range from ±1/2 to ±5%.
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 Pitot tubes are rarely used in process stream but are used occasionally in
utility stream where high accuracy is not necessary:
 ADVANTAGES
 These tubes have no process loss.
 They are economical to install.
 DISADVANTAGES
 These tubes have poor accuracy.
 They are unsuitable for dirty or sticky fluids.
 They are sensitive to upstream disturbance.
 VARIABLE AREA METERS OR ROTAMETER
(GLASS TUBE AND METAL TUBE TYPE)
 The rotameter is the most widely used form of the variable area flow meter.
 It consists of a vertical tapered tube with a float which is free to move up or
down within the tube, as shown in fig.
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 The tube is made tapered so that there is a linear relationship between the
flow rate and flow area of the fluid in the tube.
 The tube is mounted vertically with the small end at the bottom.
 The fluid to be measured enters the tube from the bottom and passes upward
around the float.
 When there is no flow through the rotameter, the float resets at the bottom
of the metering tube.
 At the bottom the diameter of the tube is kept approximately equal to that of
tube.
 When fluid enters the metering tube, the float moves up, and the flow area of
the metering tube increases.
 The pressure differential across the metering tube is proportional to the
square of its flow area and to the square of the flow rate.
 The float is pushed upward until the lifting force produced by the pressure
differential across its upper and lower surface is equal to the weight of the
float.
 If the flow rate rises, the pressure differential also rises and hence the lifting
force increases , and the float then floats upwards.
 Any decrease in flow rate causes the float to drop to a lower position.
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 A calibration scale printed on the tube or near it, provides a direct indication
of flow rate.
 The tube materials of rotameter may be of glass or metal.
 The glass metering tubes are commonly used for relatively low pressure and
temperature fluids such as water and air.
 The glass tubes are generally not used for the measurement of high pressure
because there may be possibility of broken of glass.
 Metal metering tubes are used in application where glass is not acceptable.
 Since the float cannot be seen inside the metal tube, some different
technique for indication is necessary.
 Since the majority of fluids application of rotameter is for flow viscosity fluids,
corrections must be provided for the changes in fluids density or specific
weight.
 In addition to flow rate indication, Rotameter can be equipped with additional
functions such as alarm, pneumatic or electric transmission recording,
controlling etc.
 The accuracy of Rotameter is from ±0.5% of rate to ±10% of full scale
depending upon size, type and calibration.
 Rotameter can directly measure flows as high as 4000gpm (920 liter/hr.).
 ADVANTAGES
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Its cost is relatively low.
Rotameter have good rangeability.
It is easily equipped with alarm switches, or transmitting devices.
It handles wide variety of corrosive materials.
 DISADVANTAGES
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The glass tube is easily subject to breakage.
It must be mounted vertically.
It is not good in pulsating services.
Rotameter are generally limited to small pipe sizes.
Rotameter are limited to relatively low temperature.
The accuracy of Rotameter is fair (about ±1/2 to ±10%).
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 MAGNETIC FLOWMETERS
 Magnetic flow meter is the first type of flow meter that can be used for
measurement of high corrosive and erosive application.
 This meter utilizes the principle of faraday’s law of electromagnetic induction
for making flow measurement.
 It states that whenever a conductor moves through a magnetic field it cuts
the magnetic lines of flux, and a voltage is induced in the conductor which is
proportional to the relative velocity between the conductor and the magnetic
field.
 In the case of magnetic flow meter conductive flowing fluids works as the
conductor.
 The induced voltage is given by the equation,
E=CBVL
Or
𝑽=
𝑬
𝑪𝑩𝑳
Where, E=induced voltage in volts
C=dimensional constant
B=magnetic field in Weber/m2
L=length of conductor (fluid) m
V=velocity of the conductor (fluid) in m/s
 the equation of continuity to convert a velocity measurement to volumetric
flow rate is given as,
Q=VA
where, Q=volumetric flow rate
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V=fluid velocity
A=cross-sectional area of the flow meter
 now, putting the value of V from equation.1 the volumetric flow rate can be
written as,
𝑬𝑨
𝑸=
𝑪𝑩𝑳
 since, for given size of flowmeter, A,C,B and L become constant, the Q can be
written as,
Q=KE
Where, 𝐾 =
𝐴
𝐶𝐵𝐿
 Therefore, the induced voltage is directly proportional and linear with
volumetric flow rate.
 CONSTRUCTION AND WORKING
 The magnetic flow meter consists of a non-conductive pipe such as fiber glass,
with a pair of electrodes mounted opposite to each other.
 As shown in figure magnetic coil mounted around the pipe So that a magnetic
field is generated in a plane of the electrodes.
 Fig. illustrates the basic operation principle of a magnetic flowmeter in which
the flowing liquid acts as the conductor, the length L of which is the distance
between the electrodes.
 As the liquid passes through the pipe section, it also passes through the
magnetic field set up by the magnet coils, thus inducing the voltage in the
liquid which is detected by the pair of electrodes mounted in the pipe wall.
 The amplitude of the induced voltage is proportional to the velocity of the
flowing liquid.
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 Thus by measuring induced emf, velocity can be measured which helps in
measuring volumetric flow rate.
 Magnetic meters are available in sizes from 2.54 to 2540 mm in diameter,
with an accuracy range ±1/2 to ±2%.
 The measurements of these meters are independent of velocity, density,
temperature and pressure.
 ADVANTAGES
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It can handle slurries and greasy materials.
It can handle corrosive fluids.
It has very low pressure drop.
It is totally obstruction less.
It is available in several construction materials.
It is available in large pipe sizes and capacities.
It is capable of handling extremely low flows.
It can be used as bi-directional meter.
Measurement are unaffected by viscosity, density, temperature and
pressure.
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 DISADVANTAGES
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It is relatively expensive.
It works only with fluids which are electrical conductors.
It is relatively heavy, especially in larger sizes.
It must be full at all times.
 ULTRASONIC FLOW METER
 In ultrasonic flow meter, the measurement of flow rate is determined by
the variation in parameters of ultrasonic oscillation.
 There are two types of ultrasonic flow meter.
i.
ii.
i.
TIME DIFFERENTIAL TYPE
DOPPLER FLOWMETER
TIME DIFFERENTIAL TYPE
 These devices measure flow by measuring the time takes for ultrasonic wave
to transverse a pipe section, both with and against the flow of liquid within
the pipe.
 It consists of two transducers, A and B, inserted into a pipe line, and working
both as transmitter and receiver, as shown in fig.
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 The ultrasonic waves are transmitted from transducer A to transducer B and
vise versa.
 An electronic oscillator is connected to supply ultrasonic waves alternately
to A or B which is works as transmitter through a changeover switch, when
the detector is connected simultaneously to B or A which is working as
receiver.
 The detector measures the transit time from upstream to downstream
transducer and vice versa.
 The time TAB for ultrasonic wave to travel from transducer A to transducer B
is given by the expression:
𝑳
TAB =
(𝑪+𝑽 𝑪𝑶𝑺𝜽)
And, the time (TAB) to travel from B to A is given as,
𝑳
TBA =
(𝑪−𝑽 𝑪𝑶𝑺𝜽)
Where, L=the acoustic path length between A and B
C=velocity of sound in the fluid
Θ=angle of path with respect to the pipe axis.
V=velocity of fluid in pipe
 The time difference between TAB and TBA can be calculated as,
ΔT=TAB-TBA =
𝟐 𝐋𝐕 𝑪𝑶𝑺𝜽
𝑪
or
V=
𝚫𝐓 𝐂
(𝟐𝑳 𝑪𝑶𝑺𝜽)
 So by measuring the velocity it is possible to measure volumetric flow rate.
 Since, the type of flow meter uses an ultrasonic signal traversing across the
pipe, the liquid must be relatively free of solids and air bubbles.
ii.
DOPPLER FLOWMETERS
 In Doppler flow meter, an ultrasonic wave is projected at an angle through
the pipe wall into the liquid by a transmitting crystal in a transducer
mounted outside the pipe, as shown in fig.
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 Part of the ultrasonic wave is reflected by bubbles or particles in the liquid
and is returned through the pipe wall to a receiving crystal.
 Since the reflectors (bubbles) are travelling at the fluid velocity, the
frequency of the reflected wave is shifted according to the Doppler principle.
 The velocity of the fluid is given by the equation,
𝛥𝑓 𝐶𝑇
𝑉=
= ∆𝑓𝐾
2𝑓0 COSθ
Where, Δf= difference between transmitted and revived frequency
Ct=velocity of sound in the transducer
fo=fequncy of transmission
θ=angle of transmitted and receiver crystal with respect to the pipe
axis.
K=constant
ADVANTAGES OF ULTRASONIC FLOWMETER
 It does not impose additional resistance to the flow or disturb the flow
pattern as the transducers are inserted in the wall of pipe.
 Its velocity/ output relationship is linear.
 It has no moving parts.
 Its repeatability is in the order of 0.01%.
 WEIRS
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 For open channel flow measurement, a change in depth of flow at some
point is typically measured and correlated with water flow rate. The most
common methods of measuring open channel flow rate are with a weir or
a flume.
 Weirs are used to measure flow rate primarily in open channels such as
waste and sewage systems, and in pipes and conduits that are generally not
completely filled with liquid.
 A weir is basically an obstruction in the flow path in an open channel.
 The weir will cause an increase in the water depth as the water flows over
the weir.
 In general, the greater the flow rate, the greater will be the increase in
depth of flow, The depth of water is generally used to correlate with flow
rate.
 The two major types of weir are sharp crested weir and broad crested
weir.
 The crest is the term used for the top of the weir, where the water flows
over it. The two diagrams here show a sharp crested weir and a broad
crested weir.
 The height of the water above the crest of the weir is called the head over
the weir and is shown as H in both of the diagrams. It is the parameter
that is measured and used to determine the flowrate.
 With the use of weir, flow rates can be measured from a few gallons per
minute to millions of gallons per day.
 ADVANTAGES
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 Its cost is low.
 It can be constructed on location where being used.
 It is not easily damaged.
 DISADVANTAGES
 It is applicable only to open-channel measurement.
 It’s field calibration is required.
 Its accuracy is poor.(generally not over 2 to 3%)
 FLUMES
 Instead of putting in a vertical obstruction, as with a weir, a flume consists
of a constriction in the channel width.
 This constriction causes the water level to change, and that water level in
the constriction can be correlated with flow rate.
 The diagram at the right shows a top view of a venturi flume and a top
view and side view of a Parshall Flume.
 A venturi flume has no bumps or dips in the floor of the channel. The
Parshall Flume has the specific shape shown for its floor.
 ADVANTAGES
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It handles greater flow than weir.
It can be constructed on location.
It is easy to construct as all its section are plane surface.
Its dimensions are not easily altered to cause incorrect measurement.
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 DISADVANTAGES
 It is more expensive than weir.
 It requires calibration on location.
 Its accuracy is not over 2 to 3%.
 TURBINE FLOWMETERS
 The turbine flowmeter is used for the measurement of liquid, gas and very
low flow rates.
 It works on the basic principle of turbine.
 The basic construction of the turbine flow meter incorporates a bladed
turbine rotor installed in a flow tube.
 The rotor is suspended axially in the direction of flow through the tube.
 The turbine flowmeter is a transducer, which senses the momentum of
the flowing stream.
 The bladed rotor rotates on its axis in proportion to the rate of the liquid
flow through the tube.
 As the liquid product strikes the front edge of the rotor blades, a
differential-pressure area is produced between the upstream cone and
the rotor.
The blades of the turbine rotor will tend to travel toward this lowpressure area as a result of this pressure differential across the blades.
 Fluid flowing through the meter impacts an angular velocity to the
turbine rotor blades, which is directly proportional to the linear velocity
of the liquid.
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 Electrical output is generated using the principle of reluctance.
 A pickup coil, wound around a permanent magnet, is installed on the
exterior of the flow tube.
 The magnet is the source of the magnetic flux field that cuts through the
blade of the turbine rotor passing in close proximity to the pickup coil
causes a deflection in the existing magnetic field.
 This change in the reluctance of the magnetic circuit generates a voltage
pulse within the pickup coil.
 The turbine flow meters provide very accurate flow measurement over wide
flow range.
 The accuracy range is from ±1/4 to ±1/2%, and the repeatability is excellent,
ranging from ±0.25% to as good as ±0.02%.
 The turbine meters are available in sizes ranging from 6.35 to 60 mm and
liquid flow ranges from 0.1 to over 50,000gallons per minute.
 The turbine meters are widely used for military application.
 ADANTAGES
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Its accuracy is good.
It provides excellent repeatability and rangeability.
It allows fairly low pressure drop.
It is easy to install and maintain.
It gives good temperature and pressure ratings.
 DISADVANTAGES
 Its cost is high.
 Its use is limited for slurry application.
 It faces problems caused by non-lubricating fluids.
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