Flow Sensors
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Transcript of Flow Sensors
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Flow Sensors
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• 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.
Reasons for Flow Metering
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1. Inferential type flow meters2. Quantity flow meters
a. Positive displacement metersb. Metering pumps
3. Mass flow meters
Types of Flow Meters
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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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1. Variable head or differential meters2. Variable area meters3. Magnetic meters4. Turbine Meters
5. Target meters
6. Thermal flow meters
7. Vortex meters
8. Ultrasonic flow meters
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Inferential Meters
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• Orifice Plate• Dall Tube• Venturi Tube• Pitot Tube• Rota meter• Target mater• Averaging Pitot• Nozzle• Spring Loaded• Intake Meter• Elbow Meter• Bypass Meter
Differential Pressure Meters
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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.
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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.
Parts of differential flow meters
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Obstruction Meters
• Orifice Meters
• Venturi Meters
• Flow Nozzles
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P1 P2
P
P1
d D
Flow Through an Orifice Meter
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-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
CM
A
AC
2
1
21
1
0.6
0.85
=d/D0.1 0.8
Re
100k
5000
10k
Flow Through an Orifice Meter
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In a venturi, 0.95 < C < 0.98
Advantage:
Pressure recovery
Uses little power
Flow Through an Venturi Meter
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P1 P2
P
P1
P2
Flow Through a Nozzle
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Shorter and cheaper than venturi
But larger pressure drop.
Thus, more power lost in operating.
C
0.86
0.98
103
105
Re
Flow Through a Nozzle
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2211
21
222111
21
vAvA
ibleincompress
vAvA
Avm
mm
1
1
1
1
v
A
m
2
2
2
2
v
A
m
Basic Equations:
a.) Continuity:
mass in = mass out
b.) Bernoulli’s Eqn.
Total pressure is constant throughout
Flow Through a Nozzle
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pressuredynamicv2
1
pressuretotalP
pressurestaticP
PPv2
1Pv
2
1
Pv2
1
.constessurePrTotalP
Bernoulli
2
0
02
2
221
2
11
2
0
Flow Through a Nozzle
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P2
A
A1
1YCAQ
FlowalReFor
IdealP2
A
A1
1AvAQ
RateFlow
P2
A
A1
1v
2
1
2
2
2
1
2
222
2
1
2
2
Flow Through a Nozzle
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P21
2
1
222
22
2
1
21
222
211
22221
12
1
2
1
2
1
2
1
2
1
when
A
Av
vA
Av
vvPPP
Flow Through a Nozzle
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Y = Compressibility Factor
=1 for incompressible flow or when P<< Pabs
C= Discharge Coefficient
=f(Re) and
nature of specific flow meter
P
P
Flow Through a Nozzle
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Elbow Flow Meters
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Pitot Tube
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• Force balance– Drag Force– Gravity – Buoyancy
• (usually negligible)
Derived on next slide
Rotameter, variable-area-flowmeter
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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.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
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Turbine Flow MetersElectrical pulses can be counted and totalized and it gives the total flow rate.
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-- 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 rateK = a known coefficientF = force
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Target Flow Meters
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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.
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For high corrosive applications
Induced voltage is given by• E=CBLv• v=E/CBLEquation of continuity:• Q=vAso• Q=EA/CBL• Q = KEWHERE• K= A/CBL=CONSTANT• So induced voltage is directly and linear proportional to the
volumetric flow rate.
Magnetic Flow Meters (Cont‘d)
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Based on specific heat equation which is given as
Q=WCP(T2 – T1)
W= Q/CP(T2 – T1)
WhereQ= heat transferW=mass flow rate of fluidCp=specific heat of fluidT1=initial temperature of the fluid after
heat has been transferredT2= final temperature after heating the
fluid
Thermal Flow Meters
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Swirl Meter
Vortex Flow Meters
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.
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Vortex Shedding Meter
Vortex Flow Meters (Cont‘d)
• 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.
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Time difference Type
Ultrasonic Flow Meters
• TAB-TBA=2LVcosθ/C• 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
• V= ΔfC/2fo cosθ• 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
Doppler Type
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AdvantagesVery 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
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High pressure drop that increases drastically with viscosity
Relatively high cost
Indirect measurement
Disadvantages
Inferential Meters
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Assignment
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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.
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• 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 meters2.Metering pumps
Quantity Flow Meters
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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)
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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)
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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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Principle of Operation
PD Rotary Meters ( Displacement Meters)
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• 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)
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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)
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Oval GearTwo 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)
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Oval GearThe flow rate can be calculated by measuring the rotation speed.
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PD Rotary Meters ( Displacement Meters)
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Roots MeterThe 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)
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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)
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Rotating Impeller
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PD Rotary Meters ( Displacement Meters)
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Rotary Meters ( Displacement Meters)
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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
PD Rotary Meters ( Displacement Meters)
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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)
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DisadvantagesIncreased 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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PD Rotary Meters ( Displacement Meters)
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1. It is a positive displacement pump which is used to provide a predictable and accurate rate of process fluid flow.
2. Reciprocating Piston Pumps3. Peristaltic Pumps4. Diaphragm pumps
Metering Pumps
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• 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
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Reciprocating Piston Pumps
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• 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
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Diaphraghm Pumps
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• 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
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Peristaltic Pumps