o There are many types of instruments for measuring liquid
and/or gas flow
o The accuracy of flow measurement will vary from instrument
to instrument and the desired accuracy will vary from
application to application.
o Measuring flow is one of the most important aspects of
o It is one of the most frequently measured process variables
o Flow tends to be the most difficult variable to measure.
o No single flow meter can cover all flow measurement
Physical Properties Affecting the Fluids' Flow
The major factors affecting the flow of fluids through pipes
1)The velocity of the fluid: is defined as the fluid speed in the
direction of flow. Fluid velocity depends on the head pressure
that is f
h i forcing the fluid through the pipe. G
h fl id h
Greater the h d
pressures, faster the fluid flow rate.
2)Pipe size: The larger the pipe, the greater the potential flow
3)Pipe Friction: reduces the flow rate through the pipe. Flow rate
of the fluid is slower near walls of the pipe than at the centre.
4)Fluid viscosity: its physical resistance to flow. Higher the
viscosity the fluid, the slower fluid flow.
5) The specific gravity of the fluid: At any given operating condition,
higher the fl id'
hi h th fluid's specific gravity, lower th fluid's flow rate.
the fl id' fl
6) Fluid Condition: The condition of the fluid (clean or dirty) also
limitations in flow measurement, some measuring devices become
blocked/plugged or eroded if dirty fluids are used.
7) Velocity Profiles: Velocity profiles have major effect on the
accuracy and performance of most flow meters.
The shape of the velocity profile inside a pipe depends on the
momentum or internal forces of the fluid, that moves the fluid
through the pipe, the viscous forces of the fluid that tend to slow
the fluid as passes near the pipe walls.
There are three types of flow profile:
Laminar or Streamlined: is described as
liquid flowing through a pipeline, divisible
into layers moving parallel to each other.
Turbulent flow: is the most common type
of flow pattern found in pipes. Turbulent flow
is the flow pattern which has a transverse
velocity (swirls, eddy current).
Transitional flow: which is b t
hi h i between th
laminar and turbulent flow profiles. Its
behaviour is difficult to predict and it may
oscillate between the laminar and turbulent
• These devices are used to improve the flow-pattern from
turbulent to transitional or even to laminar.
• There are three common elements; tubular element, radial Vane
element and aerodynamic straightening vanes.
Fluids' Flow Measurement
Flow meters operate according to many different
principles of measurement although thi could be
i i l
classified roughly as follow:
1. Differential pressure flowmeters
2. Variable area flowmeters
3. Mechanical flowmeters
4 Electronic flowmeters
5. Mass flowmeters
1. DIFFERENTIAL PRESSURE FLOWMETERS
Differential pressure type flow meters provide the best results
where the flow conditions are turbulent. Some of the most
common types of differential pressure flow meters are:
The working principle for DP flowmeters is that
something makes the velocity of the fluid change and
this produces a change in the pressure so that a
difference ∆P is created.
It can be shown for all these meters that the
volumetric flowrate Q is related to ∆p by the following
Q = K (∆p)0.5
K is the meter constant.
The pressure differential (∆p = h) developed by the flow element is
measured, and the velocity (V), th volumetric fl
l it (V) the
t i flow (Q) and the
mass flow (W) can all be calculated using the following generalized
k is the discharge coefficient of the element (
(which also reflects
the units of measurement),
A i th cross-sectional area of th pipe's opening, and
f the i '
D is the density of the flowing fluid.
The discharge coefficient k is influenced by the Reynolds number and by the
"beta ratio," the ratio between the bore diameter of the flow restriction and
the inside diameter of th pipe.
th i id di
f the i
the Reynolds number (Re), which for liquid flows can be calculated using the
ID is the inside diameter of the pipe in inches,
Q is the volumetric liquid flow in gallons/minute
SG is the fluid specific gravity at 60°F, and
is the i
i th viscosity i centipoises.
The components of a typical orifice flowmeter
• Orifice plate and holder
• Orifice taps
• Differential pressure transmitter
• Flow indicator / recorder
o Are metal plates have an
equal outer diameter of the
pipeline. These plates have an
opening “orifice bore” smaller
than the pipe inner diameter.
o The typical orifice plate has a
concentric, sharp edged
Because of the
smaller area the fluid velocity
increases, causing a
corresponding decrease in
• The concentric orifice plate has a sharp (squareedged) concentric bore that provides an almost pure
line contact between the plate and the fluid. The beta
(or diameter) ratios of concentric orifice plates range
from 0.25 to 0.75. The maximum velocity and
minimum static pressure occurs at some 0.35 to 0.85
pipe diameters downstream from the orifice plate.
Eccentric orifice plates are typically used f di
d for dirty
liquids/ gases. Liquids containing vapour (bore above
pipeline flow axis). Vapours containing liquid (bore
below pipeline flow axis).
• Segmental orifice plates are used for heavy fluids, in
preference to eccentric bore plates, because it allows
more drainage around the circumference of the pipe.
The orifice is inserted into the pipeline between the two flanges of
an orifice union. This method of installation is cost-effective, but it
calls for a process shutdown whenever the plate is removed for
maintenance or inspection
In contrast, Senior orifice fitting allows the orifice to be removed
from the process without depressurizing the line and shutting
There are 4 common arrangements of pressure taps:
1.Flange taps are located 1 inch from the orifice plate's surfaces.
They are not recommended for use on pipelines under 2 inches
2. Vena contracta taps are located one pipe diameter upstream
from the plate, and downstream at the point of vena contracta.
This location varies from 0.35D to 0.8D. The vena contracta taps
provide the maximum pressure differential, but also the most
noise. Normally are used only in pipe sizes exceeding 6 inches.
3. Corner taps are predominant for pipes under 2 inches.
Pipe taps are located 2.5 pipe diameters upstream and 8
diameters downstream from the orifice. They detect the
smallest pressure diff
difference. With pipe t
i taps measurement
errors are the greatest.
DP Flow Measurement
When a DP cell is used to
transmit a flow
measurement the output
of the transmitter is not
linear. To solve this
problem some form of
signal conditioning is
needed to condition the
signal for use with a linear
Relationship between Differential pressure and flow
When the diff
th differential pressure i obtained experimentally and
is bt i d
plotted against flow, the resulting graph is a square function.
• If the square root of differential pressure is plotted against flow,
a straight line is obtained showing that the rate of flow is in
direct proportion to the square root of differential pressure.
Therefore, in many flow measurement installations a Square
Root Extractor is fitted to the output of a differential pressure
DP Flowmeter Installations
Advantages and Disadvantages of Orifice flowmeters
• They are easy to install.
• One differential pressure transmitter applies for any pipe size.
• Many DP sensing materials are available to meet process
• Orifice plates have no moving parts and have been
researched extensively; therefore, application data well
documented (compared to other primary differential pressure
• The process fluid is in the impulse lines to the differential
transmitter may freeze or block.
• Their accuracy is affected by changes in density, viscosity,
• They require frequent calibration
o Venturi tube consists of a section of pipe with a conical entrance,
a short straight throat, and a conical outlet. The velocity
increases and the pressure drops at the throat. The differential
pressure is measured between the inlet (upstream of the conical
entrance) and the throat.
o Venturi tubes are available in sizes up to 72", and can pass 25
to 50% more flow than an orifice with the same pressure drop.
Furthermore, the total unrecovered head loss rarely exceeds
th t t l
10% of measured d/p.
Advantages and Disadvantages of VENTURI TUBES
• It can handle low-pressure applications
• It can measure 25 to 50% more flow than a comparable orifice
• It is less susceptible to wear and corrosion compared to orifice
• It is suitable for measurement in very large water pipes and
large air/Gas d t
i /G ducts.
• Provides better performance than the orifice plate when there
are solids in Suspension
• It is the most expensive among the differential pressure meters
• It is big and heavy for large sizes
• I has considerable length
2) VARIABLE AREA FLOWMETERS
• Variable area flowmeters are simple and versatile
devices that operate at a relatively constant
pressure drop and measure the flow of liquids,
gases, and steam.
• There are two main types of this meter
1.Float type (Rotameter)
2.Tapered p g type.
Float Type (Rotameter)
The float is inside a tapered tube. The fluid
flows through the annular gap around the
edge of the float.
The restriction causes a pressure drop over
the float and the pressure forces the float
Because the tube is tapered, the restriction
is decreased as the float moves up.
Eventually a level is reached where the
restriction is just right to produce a pressure
force that counteracts the weight of the
The level of the float indicates the flow rate.
If the flow changes the float moves up or
down to find a new balance position.
Tapered Plug Type
In this meter, a tapered plug is aligned inside a hole or
orifice. A spring holds it in place. The flow is restricted as it
passes through the gap and a force is produced which
moves the plug. Because it is tapered the restriction
changes and the plug takes up a position where the
pressure force just balances the spring force. The movement
of the plug is transmitted with a magnet to an indicator on
3) MECHANICAL FLOWMETERS
• Mechanical flow meters that measure flow
using an arrangement of moving parts,
either by passing isolated known volumes of
a fl id through a series of gears or
c a be s (pos t e displacement ete s)
chambers (positive d sp ace e t meters)
OR by means of a spinning turbine or rotor
3.2) TURBINE FLOWMETERS
Th turbine flowmeter is an
accurate and reliable
flowmeter for both liquids
and gases. It consists of a
multi-bladed rotor mounted
at right angles to the flow
and suspended in the fluid
stream on a free-running
bearing. The rotor speed of
rotation is proportional to
the volumetric flow rate.
Turbine rotation can be
detected by solid state
devices ( d
Volumetric Flow Rate Equation
o The outputs of reluctance and inductive pick-up coils are continuous
sine waves with the pulse train's frequency proportional to the flow rate.
o At low flow, the output (the height of the voltage pulse) may be on the
order of 20 mV peak-to-peak. It is not advisable to transport such a
t d i bl t t
weak signal over long distances. Therefore, the distance between the
pickup and associated display electronics or preamplifier must be short.
o In an electronic turbine flowmeter, volumetric flow is directly
proportional to pickup coil output frequency. We may express this
relationship in the form of an equation: f = kQ
f = Frequency of output signal (Hz, equivalent to pulses per second)
Q = Volumetric flow rate (e.g. gallons per second)
k = Turbine meter factor (e.g. pulses per gallon)
• A turbine flowmeter’s K factor is determined by the manufacturer by
displacing a k
di l i
known volume of fluid through the meter and summing the
f fl id th
number of pulses generated by the meter.
Advantages and Disadvantages of the turbine meters
The turbine meter is easy to install and maintain. They:
• Are bi directional
• Have fast response
• Are compact and light weights
• They generally are not available for steam measurement (since
condensate does not lubricate well.
• They are sensitive to dirt and cannot be used for highly viscous fluids.
• Flashing or slugs of vapour or gas in the liquid produce blade wear and
excessive bearing friction that can result in poor performance and
possible turbine damage.
They are sensitive to the velocity profile to the presence of swirls at the
inlet; they require a uniform velocity profile (i.e. pipe straightness may
have to be used).
o Air and gas entrained in the liquid affect turbine meters.
o S i
Strainers may be required upstream to minimise particle
i i i
contamination of the bearings.
o Turbine meters have moving parts that are sensitive to wear and
can be damaged by over speeding. To prevent sudden hydraulic
impact, the flow should increase gradually into the line.
o When installed, bypass piping may be required for maintenance.
o The transmission cable must be well protected to avoid the
effect of electrical noise.
4) ELECTRONIC FLOWMETERS
Electronic flowmeters represent a logical grouping
of flow measurement technologies. All have no
moving parts, are relatively non-intrusive, and are
made possible by today's sophisticated electronics
technology. 3 types of flowmeters:
1. Magnetic flowmeters,
2. Vortex flowmeters,
2 V t
3. Ultrasonic flowmeters
Base principle of magnetic flowmeter
The magnetic flow meter design is based on Faraday’s law of magnetic
induction which states that: "The voltage induced across a
conductor as it moves at right angles through a magnetic field
proportional to the velocity of that conductor.“
That is, if a conductor is moving perpendicular to its length through a
magnetic field, it will generate an electrical potential between its two ends
B = the strength of the magnetic field (induction)
L = the length of the conductor (distance of electrodes)
v = velocity of the conductor (average flow velocity)
Magmeter Flow Equation
o If a conductive fluid flows through a pipe of diameter (D) through a
magnetic field density (B) generated by the coils, the amount of
voltage (E) developed across the electrodes will be proportional to the
velocity (V) of the liquid. Because the magnetic field density and the
pipe diameter are fixed values, they can be combined into a calibration
factor (K) and the equation reduces to:
Manufacturers determine each magmeter's K factor by water calibration
of each fl t b
h flowtube. Th K value th
thus obtained i valid f
bt i d is
lid for any other
conductive liquid and is linear over the entire flowmeter range.
Advantages and Disadvantages of Magmeter
• Are bi-directional
• Have no flow obstruction
• Are easy to re-span
• Are available with DC or AC power
• It can measure pulsating and corrosive flow.
• It can measure multiphase; however, all components should be moving at
the same speed; the meter can measure the speed of the most conductive
• It can install vertically or horizontally (the line must be full, however) and
can be used with fluids with conductivity greater than 200 umhos/cm.
Changes i conductivity value d not, affect the i
h instrument performance.
• It's above average cost
• It' l
It's large size
• Its need for a minimum electrical conductivity of 5 to 20 µmhos / cm
• Its accuracy is affected by slurries containing magnetic solids.
• El t i l coating may cause calibration shifts
• The line must be full and have no air bubbles (air and gas bubbles
entrained in the liquid will be metered as liquid, causing a measurement
• In some applications, appropriate mechanical protection for the electrodes
must be provided.
4.3) ULTRASONIC FLOWMETERS
The speed at which sound propagates in a fluid is
dependent on the fluid's density. If the density is
constant, however, one can use the time of ultrasonic
passage (or reflection) to determine the velocity of a
There are 2 types of ultrasonic flowmeters:
1. Doppler shift, and
2 Transit time
4.3.1) The Doppler Shift
Doppler-effect flow meters use a transmitter that projects a
ff t fl
itt th t
continuous ultrasonic beam at about 0.640 MHz through the
pipe wall into the flowing stream. Particles in the stream reflect
the ultrasonic radiation, which is detected by the receiver.
o The frequency reaching the receiver is shifted in proportion to
the stream velocity.
o The frequency difference is a measure of the flow rate.
o When the measured fluid contains a large concentration of
particles or air bubbles, it is said to be sonically opaque. More
y p q
opaque the liquid, greater the number of reflections that
originate near the pipe wall, a situation exemplified by heavy
The Doppler Flow meter works satisfactorily for only some
applications and is generally used when other metering methods
are not practical or applicable. It should not be treated as a
“universal“ portable meter.
• Thus, flow velocity V (ft/sec) is directly proportional
to the change in frequency. The flow (Q in gpm) in a
pipe having a certain inside diameter (ID in inches)
can b obtained by:
be bt i d b
• The presence of acoustical discontinuities is essential
for the proper operation of the Doppler flowmeter.
Advantages and Disadvantages of Doppler Meter
The common clamps-on versions are easily installed without process
sh tdo n
It can be installed bi-directional
Flow measurement is not affected due to change in the viscosity of the
Generally suitable for measurements in large water pipes
The meter produces no flow obstruction
Its cost is independent of line size.
The sensor may detect some sound energy travelling in the causing
interference reading errors.
Its accuracy depends on the difference in velocity between the particles, the
fluid, the particle size, concentration, and distribution.
The instrument requires periodic re calibration
4.3.2) Transit Time Measurement
o In this design, the time of flight of the ultrasonic signal is measured
between two transducers; one upstream and one downstream. The
difference in elapsed time going with or against the flow determines the
o When the flow is zero, the time for the signal T1 to get to T2 is the
same as that required to get from T2 to T1. When there is flow, the
effect is to boost the speed of the signal in the downstream direction,
while decreasing it in the upstream direction. The flowing velocity (Vf)
can be determined by the following equation:
o where K is a calibration factor for the volume and time units used, dt is
the time differential between upstream and downstream transit times,
and TL is the zero-flow transit time
o The speed of sound in the fluid is a function of both density and
temperature. Therefore, both have to be compensated for. In addition,
the change in sonic velocity can change the refraction angle "a", which
in turn will affect the distance the signal has to travel. In extreme cases,
the signal might completely miss the downstream receiver.
Advantages and Disadvantages of Transit Meter
It does not cause any flow obstruction
It can be installed bi-directional
It is unaffected by changes in the process temperature
It is suitable to handle corrosive fluids and pulsating flows.
It can be installed by clamping on the pipe and is generally suited for
measurements in very large water pipes.
This type of meters are highly dependent on the Reynolds number (the
It requires nonporous pipe material (cast iron, cement and fibreglass should
It requires periodic re calibration
It is generally used where other metering methods are not practical or
5) MASS FLOWMETERS
Traditionally fluid flow measurement has been made in
terms of the volume of the moving fluid even though the
meter user may be more interested in the weight (mass) of
the fluid. Volumetric flow meters also are subject to ambient
and process changes, such as density, which changes with
temperature and pressure.
There are three ways to determine mass flow:
1. The application of microprocessor technology to
conventional volumetric meters.
2. Use of Coriolis flow meters, which measure mass flow
3. The use of thermal mass flow meters that infer mass flow
by way of measuring heat dissipation between two points
in the pipeline
5.1) MICROPROCESSOR-BASED VOLUMETRIC FLOW METERS
o with microprocessors it is relatively simple to
compensate a volumetric flow meter for temperature
o With reliable composition (density) information this
factor also can be entered into a microprocessor to
obtain mass flow readout. However, when density
changes may occur with some frequency, and
particularly where the flowing fluid is of high
monetary value (for example, in custody transfer),
precise density compensation (to achieve mass) can
o For the precise measurement of gas flow (steam) at varying
pressures and temperatures, it is necessary to determine the
density, which is pressure and temperature dependent, and
from this value to calculate the actual flow. The use of a
computer is essential to measure flow with changing pressure
o This unit will automatically correct for variations in pressure,
temperature, specific gravity, and super-compressibility. The
differential (h) developed b th flow element is
d by the fl
measured, and the mass flow (W) can all be calculated using
the following generalized formulas:
k is the discharge coefficient of the element (which also reflects
the units of measurement),
A is the cross-sectional area of the pipe's opening, and
D is the density of the flowing fluid.
5.3) THERMAL MASS FLOWMETERS
o Th power supply directs h t t th midpoint of a sensor tube
t heat to the id i t f
that carries a constant percentage of the flow. On the same tube
at equidistant two temperature elements (RTD) are installed
upstream and downstream of the heat input.
o With no flow, the heat reaching each temperature element (RTD)
o With increasing flow the flow stream carries heat away from the
upstream element T1 and an increasing amount toward the
downstream element T2. An increasing temperature difference
develops between the two elements.
o This temperature difference detected by the temperature
elements is proportional to the amount of gas flowing, or the
mass flow rate.
o The pipe wall temperature is highest near the heater (detected
as Tw), while, some distance away, there is no difference
e, so e d sta ce a ay, t e e s o d e e ce
between wall and fluid temperature.
o Therefore the temperature of the unheated fluid (Tf) can be
detected by measuring the wall temperature at this location
further away from the heater. This heat transfer process is nonnon
linear, and the corresponding equation differs from the one
above as follows:
o In the direct-heat version, a fixed amount of heat (q) is added
b an electric h t
l t i heater. A th process fl id fl
fluid flows th
pipe, resistance temperature detectors (RTDs) measure the
temperature rise while the amount of electric heat introduced is
o The mass flow (m) is calculated on the basis of the measured
temperature difference (T2 - T1), the meter coefficient (K), the
electric heat rate (q), and the specific heat of the fluid (Cp), as