Flow Sensors

Flow Sensors

• 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

1. Inferential type flow meters2. Quantity flow meters

a. Positive displacement metersb. Metering pumps

3. Mass flow meters

Types of 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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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

• 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

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.

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

Obstruction Meters

• Orifice Meters

• Venturi Meters

• Flow Nozzles

P1 P2

P

P1

d 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

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

In a venturi, 0.95 < C < 0.98

Advantage:

Pressure recovery

Uses little power

Flow Through an Venturi Meter

P1 P2

P

P1

P2

Flow Through a Nozzle

Shorter and cheaper than venturi

But larger pressure drop.

Thus, more power lost in operating.

C

0.86

0.98

103

105

Re


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


pressuredynamicv2

1

pressuretotalP

pressurestaticP

PPv2

1Pv

2

1

Pv2

1

.constessurePrTotalP

Bernoulli

2

0

02

2

221

2

11

2

0


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


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


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

• Force balance– Drag Force– Gravity – Buoyancy

• (usually negligible)

Derived on next slide

Rotameter, variable-area-flowmeter

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

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

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.

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)

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

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.

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.

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

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

32

High pressure drop that increases drastically with viscosity

Relatively high cost

Indirect measurement

Disadvantages

Inferential Meters

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.

• 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

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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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


• Oval Gear• Nutating Disk• Oscillating Piston• Multi Piston• Rotating Impellers• Rotating Valve• Birotor• Roots Meter• Helix 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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41

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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Oval GearThe flow rate can be calculated by measuring the rotation speed.

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44

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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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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Rotating Impeller

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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


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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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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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

• 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

Reciprocating Piston 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

Diaphraghm 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

Peristaltic Pumps

Date post:	23-Jan-2016
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Flow Sensors

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