Flow Sensors
Reasons for Flow Metering
• Plant control, for product quality and safety
reasons.
• Custody transfer, both interplant and selling to
outside customers.
• Filling of containers, stock tanks and transporters.
• Energy, mass balancing for costing purpose and
health monitoring of heat exchangers.
• Health monitoring of pipelines and on-line
analysis equipment, Government and company
legislation may dictate the use here of such
equipment.
Types of Flow Meters
1. Inferential type flow meters
2. Quantity flow meters
a. Positive displacement meters
b. Metering pumps
3. Mass flow meters
Inferential Meters
The inferential type meters are so-called
because rather than measuring the actual
volume of fluid passing through them, they
“infer” the volume by measuring some other
aspect of the fluid flow and calculating the
volume based on the measurements
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Inferential Meters
1.
2.
3.
4.
5.
6.
7.
8.
Variable head or differential meters
Variable area meters
Magnetic meters
Turbine Meters
Target meters
Thermal flow meters
Vortex meters
Ultrasonic flow meters
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Differential Pressure Meters
•
•
•
•
•
•
•
•
•
•
•
•
Orifice Plate
Dall Tube
Venturi Tube
Pitot Tube
Rota meter
Target mater
Averaging Pitot
Nozzle
Spring Loaded
Intake Meter
Elbow Meter
Bypass Meter
Parts of differential flow meters
1. Primary element
(Part of meter used to restrict the fluid flow in
pipe line to produce differential pressure)
They include
• Orifice plate
• Venturi tubes
• Flow nozzles
• Pitot tube etc.
Parts of differential flow meters
1. Secondary element
(measure the differential pressure produced by
primary elements and convert them to
usable forces or signals )
Secondary elements;
• Manometers
• Bellow meters
• Force balance meters etc.
Obstruction Meters
• Orifice Meters
• Venturi Meters
• Flow Nozzles
Flow Through an Orifice Meter
P1
P2
d
P1
P
D
Flow Through an Orifice Meter
-Cheapest and Simplest
-But biggest pressure drop and power lost
(C~0.6 - 0.7)
-Side Note:
Pressure drop caused by friction and
turbulence of shear layer downstream of
vena contracta



1
C
2

 A2 
 1   
 A1 

Re
0.85



  CM



10k
5000
100k
0.6
0.1
b=d/D
0.8
Flow Through an Venturi Meter
In a venturi, 0.95 < C < 0.98
Advantage:
Pressure recovery
Uses little power
Flow Through a Nozzle
P1
P2
P1
P
P2
Flow Through a Nozzle
Shorter and cheaper than venturi
But larger pressure drop.
Thus, more power lost in operating.
0.98
C
0.86
105
103
Re
Flow Through a Nozzle
1 m
2
m
  Av
m
Basic Equations:
a.) Continuity:
mass in = mass out
b.) Bernoulli’s Eqn.
Total pressure is
constant throughout
1A1 v1   2 A 2 v 2
incompress ible
1   2
A 1 v1  A 2 v 2
1
m
2
m
1
2
A1
A2
v1
v2
Flow Through a Nozzle
Bernoulli
P0  Total Pr essure  const .
1 2
 v  P
2
1
1
2
2
1 v1  P1   2 v 2  P2  P0
2
2
P  static pressure
P0  total pressure
1 2
v  dynamic pressure
2
Flow Through a Nozzle
v2 
2 P

1
 A2 
1   
 A1 
Flow Rate
Q  A2v2  A2
2
1
 A2 
1   
 A1 
2
2 P

For Re al Flow
Q  YCA 2
1
 A2 
1   
 A1 
2
2 P

Ideal
Flow Through a Nozzle
P  P1  P2 
1
1
 2 v22  1v12
2
2
2
1
1  A2  2
2
  2 v2  1   v2
2
2  A1 
2

 A2  
1 2
 v2 1    
2
  A1  
when
1   2
P
Flow Through a Nozzle
Y=
Compressibility Factor
=1 for incompressible flow
or when P<< Pabs
C=
Discharge Coefficient
=f(Re) and
nature of specific flow meter
P
P
Elbow Flow Meters
Pitot Tube
Rotameter, variable-area-flowmeter
• Force balance
– Drag Force
– Gravity
– Buoyancy
• (usually negligible)
Derived on next slide
Turbine Flow Meters
It consists of a multi-bladed rotor
mounted at right angles to the flow and
suspended in the fluid stream on a freerunning bearing.
Used for measurements of liquid, gas
and very low flow rates.
It basically works on the principle of
turbine.
The diameter of the rotor is very slightly
less than the inside diameter of the
metering chamber, and its speed of
rotation is proportional to the volumetric
flow rate.
The rotational speed is a direct function
of flow rate and can be sensed by
magnetic pick-up coil.
As each rotor blade passes the magnetic
pick-up coil, it generates a voltage pulse
which is a measure of flow rate.
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Turbine Flow Meters
Electrical pulses can be counted and totalized and it
gives the total flow rate.
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Target Flow Meters
-- It measures flow by measuring the amount of force exerted by the
.
flowing fluid on a target suspended in the flow stream
-- The fluid flow develops a force on target which is proportional to the
.
square of the flow
--measure the flow of liquids and gases, such as water, air, industrial gases,
and chemicals.
Q=K(F)1/2
Where
Q= flow rate
K = a known coefficient
F = force
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Magnetic Flow Meters
The physical principle at work
is Faraday's
law of electromagnetic induction,
it states that whenever a conductor
moves through a magnetic field of
given strength , a voltage is induced
in a conductor which is proportional
to the relative velocity between the
conductor and the magnetic field.
Magnetic Flow Meters (Cont‘d)
For high corrosive applications
Induced voltage is given by
• E=CBLv
• v=E/CBL
Equation of continuity:
• Q=vA
so
• Q=EA/CBL
• Q = KE
WHERE
• K= A/CBL=CONSTANT
• So induced voltage is directly and linear proportional to the
volumetric flow rate.
Thermal Flow Meters
Based on specific heat equation which is
given as
Q=WCP(T2 – T1)
W= Q/CP(T2 – T1)
Where
Q= heat transfer
W=mass flow rate of fluid
Cp=specific heat of fluid
T1=initial temperature of the fluid after
heat has been transferred
T2= final temperature after heating the
fluid
Vortex Flow Meters
Swirl Meter
Operates on principle of vortex
precession.
It gives an output in the form of pulses
whose frequency is proportional to the
fluid flow rate.
In the area where expansion occurs , the
swirling flow precceds or oscillates at a
frequency proportional to the fluid flow
rate.
Each high velocity vortex passed the
thermistor, changes the resistance and
since a constant current is applied, the
resistance changes is converted into
voltage pulses which are amplified ,
filtered and transformed into constant
amplitude high level pulses of square
waveform.
Vortex Flow Meters (Cont‘d)
Vortex Shedding Meter
• Based on the phenomenon of
Vortex shedding.
• The frequency at which the
vortices are formed is directly
proportional to the fluid velocity.
• The velocity and pressure
distribution in the fluid around the
sluff body change at the same
frequency as the vortex shedding
frequency.
Ultrasonic Flow Meters
Time difference Type
Doppler Type
• TAB-TBA=2LVcosθ/C
• V= ΔfC/2fo cosθ
•
•
•
•
•
•
•
•
Where
L=acoustic path length between A
and B
C=velocity of sound in fluid
Θ=angle of path wrt to pipe axis
V=velocity of fluid in pipe
•
•
Where
C=velocity of sound in fluid
Θ=angle of transmitter and receiver
wrt to pipe axis
fo = frequency of transmission
Δf = difference between transmitted
and received frequency
Inferential Meters
Advantages
Very good repeatability
• Reduced susceptibility to fouling and deposits
• Less sensitive to viscosity changes
Available in large sizes, good value for high flow
rates
• Low maintenance
Registers near zero flow rate
Disadvantages
High pressure drop that increases drastically with viscosity
Relatively high cost
Indirect measurement
32
Assignment
Coriolis Mass Flowmeter
In the Coriolis meter the fluid is passed through a tube. The tubes are available in
different design like tubes of U-shape or horseshoe-shaped. The tubes can either be
curved or straight. When two tubes are used the flow is divided when entering the
meter and then recombined. The flow when enters the tube encounters oscillating
excitation force that causes the tubes to vibrate at a fixed frequency. The vibration is
induced in the direction that is perpendicular to flow of fluid. This creates the rotation
frame of reference. Consider the tube during oscillation moving up and downward,
when the tube is moving upward the fluid flowing in it tends to resist this and forces it
downward. When the tube moves in the opposite direction, so does the fluid and a
twist in introduced in the tube. All this might not be visible by directly observing. The
twist at inlet of fluid and outlet of fluid results in phase difference or time lag and that
is dependent on the fluid mass passing through the tube.
Quantity Flow Meters
• Used for the measurement of small percentage
of industrial flow rates.
• These meters operate by passing the fluid to be
measured through the meter in separate and
distinct increments of alternately feeling and
emptying containers of known fixed capacity.
• The number of times the container is filled and
emptied gives the quantity of flow.
• Types are:
1. Positive displacement meters
2. Metering pumps
PD Rotary Meters ( Displacement Meters)
Positive displacement
flow meters, also
know as PD meters,
measure volumes of
fluid flowing through
by counting repeatedly
the
filling
and
discharging of known
fixed volumes.
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PD Rotary Meters ( Displacement Meters)
Principle of Operation
POSITION 1. As the bottom impeller rotates in a
counterclockwise direction towards a horizontal position,
fluid enters the space between the impeller and cylinder.
POSITION 2. At the horizontal position, a definite volume
of fluid is contained in the bottom compartment.
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PD Rotary Meters ( Displacement Meters)
Principle of Operation
POSITION 3. As the impeller continues to turn, the
volume of fluid is discharged out the other side.
POSITION 4. The top impeller, rotating in opposite
direction, has closed to its horizontal position confining
another known and equal volume of fluid.
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PD Rotary Meters ( Displacement Meters)
•
•
•
•
•
•
•
•
•
Oval Gear
Nutating Disk
Oscillating Piston
Multi Piston
Rotating Impellers
Rotating Valve
Birotor
Roots Meter
Helix Meters
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PD Rotary Meters ( Displacement Meters)
Nutating Disk
A nutating disc meter has
a round disc mounted on
a spindle in a cylindrical
chamber.
By
tracking
the
movements
of
the
spindle, the flowmeter
determines the number of
times the chamber traps
and empties fluid.
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PD Rotary Meters ( Displacement Meters)
Oval Gear
Two identical oval rotors
mesh together by means
of slots around the gear
perimeter.
The oval shaped gears are
used to sweep out an
exact volume of the liquid
passing
through
the
measurement
chamber
during each rotation.
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PD Rotary Meters ( Displacement Meters)
Oval Gear
The flow rate can be calculated by measuring
the rotation speed.
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44
PD Rotary Meters ( Displacement Meters)
Roots Meter
The roots meter is
similar in many
respects to the oval
gear meter.
Two-lobed impellers
rotate in opposite
directions to each
other within the body
housing.
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PD Rotary Meters ( Displacement Meters)
Roots Meter
• These peanut-shaped gears sweep out an
exact volume of liquid passing through the
measurement chamber during each rotation.
• The flow rate can be calculated by
measuring the rotation speed.
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PD Rotary Meters ( Displacement Meters)
Rotating Impeller
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Rotary Meters ( Displacement Meters)
PD Rotary Meters ( Displacement Meters)
Advantages
 High accuracy over a wide range of viscosities
and flow rates up to 2000 cP with proper
clearances.
 Extremely good repeatability on high viscosity
fluids, very low slippage, long life if little or
no abrasive material in the fluid
Low pressure drop
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PD Rotary Meters ( Displacement Meters)
Advantages
Special construction available for high
viscosities and temperatures
Can register near zero flow rate
Measures directly, not an inferential
device, for more consistent results
Easy to repair and economical.
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PD Rotary Meters ( Displacement Meters)
Disadvantages
Increased maintenance compared to other
meters, more moving parts
• May become damaged by flow surges and
gas slugs
Chance of corrosion and erosion from
abrasive materials
Relatively high cost for large sizes
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Metering Pumps
1. It is a positive displacement pump which is
used to provide a predictable and accurate
rate of process fluid flow.
2. Reciprocating Piston Pumps
3. Peristaltic Pumps
4. Diaphragm pumps
Reciprocating Piston Pumps
• Used in heavy chemical and manufacturing
industry.
• It contains a piston or plunger with the inlet
and outlet check valves and the piston moves
with a reciprocating motion within a chamber.
• As the piston retracts from its cylinder, the
inlet check valve opens and the cylinder is
filled.
• When the piston re-enter the cylinder , the
inlet check valve closes and liquid is forced
throughout the outlet check valves and enters
into the discharging pipe.
Reciprocating Piston Pumps
Diaphraghm Pumps
• Same as reciprocating piston pump except
that the process fluid is separated by a flexible
diaphragm.
• Consists of a diaphragm which is directly
flexed by a piston
Diaphraghm Pumps
Peristaltic Pumps
• Fluid is moved forward by progressively
squeezing a flexible container from the
entrance to the discharge.
• This container is usually a tube that can be
made out of any material that possesses a
property to recover its original shape
immediately after compression.
• The flow rate is adjusted by changing the
speed of squeezing mechanism.
Peristaltic Pumps