Chapter 3MEASUREMENTFLOW MEASUREMENT

o There are many types of instruments for measuring liquid

and/or gas flow 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. application to application.

o Measuring flow is one of the most important aspects of

process control.

o It is one of the most frequently measured process variables 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

applicationsapplications.

Physical Properties Affecting the Fluids' FlowThe major factors affecting the flow of fluids through pipes The major factors affecting the flow of fluids through pipes are:

1)The velocity of the fluid: is defined as the fluid speed in the 1)The velocity of the fluid: is defined as the fluid speed in the

direction of flow. Fluid velocity depends on the head pressure

h i f i h fl id h h h i G h h dthat is forcing the fluid through the pipe. Greater the head

pressures, faster the fluid flow rate.

2)Pipe size: The larger the pipe, the greater the potential flow

rate

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 ) y p y g

viscosity the fluid, the slower fluid flow.

5) The specific gravity of the fluid: At any given operating condition,

hi h th fl id' ifi it l th fl id' fl thigher the fluid's specific gravity, lower the fluid's flow rate.

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./p gg y

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 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 of flow pattern found in pipes. Turbulent flow is the flow pattern which has a transverse velocity (swirls, eddy current).

T iti l fl hi h i b t th Transitional flow: which is between the laminar and turbulent flow profiles. Its behaviour is difficult to predict and it may oscillate between the laminar and turbulent

oscillate between the laminar and turbulent flow profiles.

Flow-straightening devices• These devices are used to improve the flow-pattern from p p

turbulent to transitional or even to laminar.

• There are three common elements; tubular element, radial Vane element and aerodynamic straightening vanes.

Fluids' Flow MeasurementFlow meters operate according to many different

i i l f t lth h thi ld b principles of measurement although this could be

classified roughly as follow:

1. Differential pressure flowmeters

2. Variable area flowmeters

3. Mechanical flowmeters

4 Electronic flowmeters4. Electronic flowmeters

5. Mass flowmeters

1. DIFFERENTIAL PRESSURE FLOWMETERSDifferential 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:common types of differential pressure flow meters are:

•ORIFICE METERS.

VENTURI METERS•VENTURI METERS

•NOZZLE METERS

•PITOT TUBES.

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 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 volumetric flowrate Q is related to ∆p by the following

basic formula.

Q = K (∆p)0.5

K is the meter constant.

The pressure differential (∆p = h) developed by the flow element is

d d th l it (V) th l t i fl (Q) d th measured, and the velocity (V), the volumetric flow (Q) 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 g (

the units of measurement),

A i th ti l f th i ' i d A is the cross-sectional area of the pipe's opening, and

D is the density of the flowing fluid.

The discharge coefficient k is influenced by the Reynolds number and by theThe 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

th i id di t f th ithe inside diameter of the pipe.

the Reynolds number (Re), which for liquid flows can be calculated using the

relationship:

ID is the inside diameter of the pipe in inches,

Q is the volumetric liquid flow in gallons/minuteQ is the volumetric liquid flow in gallons/minute,

SG is the fluid specific gravity at 60°F, and

i th i it i ti i is the viscosity in centipoises.

ORIFICE FLOWMETERSORIFICE FLOWMETERS

The components of a typical orifice flowmeter The components of a typical orifice flowmeter

installation are:

• Orifice plate and holder

• Orifice taps

• Differential pressure transmitter

• Flow indicator / recorder

ORIFICE PLATESo 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 o The typical orifice plate has a

concentric, sharp edged

i B f th opening. Because of the

smaller area the fluid velocity

increases, causing a

corresponding decrease in

pressure.

• The concentric orifice plate has a sharp (square-edged) concentric bore that provides an almost pure edged) 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 (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 minimum static pressure occurs at some 0.35 to 0.85 pipe diameters downstream from the orifice plate.

E i ifi l i ll d f di • Eccentric orifice plates are typically used 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 p p ,more drainage around the circumference of the pipe.

Orifice HoldersThe 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 maintenance or inspection.

In contrast, Senior orifice fitting allows the orifice to be removed from the process without depressurizing the line and shutting from the process without depressurizing the line and shutting down flow.

Orifice tapsThere 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 They are not recommended for use on pipelines under 2 inches in diameter.

2. Vena contracta taps are located one pipe diameter upstream 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 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.

4. Pipe taps are located 2.5 pipe diameters upstream and 8 p p p p p

diameters downstream from the orifice. They detect the

ll t diff With i t tsmallest pressure difference. With pipe 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 of the transmitter is not

linear. To solve this

problem some form of problem some form of

signal conditioning is

needed to condition the needed to condition the

signal for use with a linear

scaled indicator.

Relationship between Differential pressure and flow

Wh th diff ti l i bt i d i t ll d • When the differential pressure is obtained experimentally and 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 Root Extractor is fitted to the output of a differential pressure transmitter.

DP Flowmeter Installations

Advantages and Disadvantages of Orifice flowmetersAdvantages

• They are easy to install.

• One differential pressure transmitter applies for any pipe size.

• Many DP sensing materials are available to meet process requirements.

• Orifice plates have no moving parts and have been researched extensively; therefore, application data well documented (compared to other primary differential pressure documented (compared to other primary differential pressure elements).

DisadvantagesDisadvantages

• The process fluid is in the impulse lines to the differential transmitter may freeze or block.transmitter may freeze or block.

• Their accuracy is affected by changes in density, viscosity, and temperature.

• They require frequent calibration

VENTURI TUBESo Venturi tube consists of a section of pipe with a conical entrance, 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. F th th t t l d h d l l d Furthermore, the total unrecovered head loss rarely exceeds 10% of measured d/p.

Advantages and Disadvantages of VENTURI TUBESAdvantageg

• It can handle low-pressure applications

• It can measure 25 to 50% more flow than a comparable orifice • It can measure 25 to 50% more flow than a comparable orifice plate

• It is less susceptible to wear and corrosion compared to orifice p pplates

• It is suitable for measurement in very large water pipes and l i /G d tvery large air/Gas ducts.

• Provides better performance than the orifice plate when there are solids in Suspensionare solids in Suspension.

Disadvantage

• It is the most expensive among the differential pressure meters• It is the most expensive among the differential pressure meters

• It is big and heavy for large sizes

I h id bl l h• Its 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, 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 plug type.p p g yp

Float Type (Rotameter)The float is inside a tapered tube The fluid 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 pupwards.

Because the tube is tapered, the restriction p ,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 float float.

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 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 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 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 of the plug is transmitted with a magnet to an indicator on the outside.

3) MECHANICAL FLOWMETERS)

• Mechanical flow meters that measure flow Mechanical flow meters that measure flow

using an arrangement of moving parts,

either by passing isolated known volumes of

fl id th h i f a fluid through a series of gears or

chambers (positive displacement meters) c a be s (pos t e d sp ace e t ete s)

OR by means of a spinning turbine or rotor

(Turbine Flowmeters)

3.2) TURBINE FLOWMETERSTh bi fl i The turbine flowmeter is an accurate and reliable flowmeter for both liquids flowmeter for both liquids and gases. It consists of a multi-bladed rotor mounted multi bladed rotor mounted at right angles to the flow and suspended in the fluid pstream 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 d ( d kdevices (inductance pick-ups).

Volumetric Flow Rate Equationo The outputs of reluctance and inductive pick-up coils are continuous 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 d f 20 V k t k It i t d i bl t t t h order of 20 mV peak-to-peak. It is not advisable to transport such a

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 = kQp q Q

Where,

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)

k Factor

• A turbine flowmeter’s K factor is determined by the manufacturer by di l i k l f fl id th h th t d i th displacing a known volume of fluid through the meter and summing the number of pulses generated by the meter.

Advantages and Disadvantages of the turbine metersAdvantagesg

The turbine meter is easy to install and maintain. They:

• Are bi directional• Are bi-directional

• Have fast response

• Are compact and light weights

Disadvantages

• They generally are not available for steam measurement (sincecondensate 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 andexcessive bearing friction that can result in poor performance andpossible turbine damage.

Th i i h l i fil h f i l h• They are sensitive to the velocity profile to the presence of swirls at theinlet; they require a uniform velocity profile (i.e. pipe straightness mayhave to be used).

o Air and gas entrained in the liquid affect turbine meters.

S i b i d i i i i lo Strainers may be required upstream to minimise particle

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 g y p g p y

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 p

effect of electrical noise.

4) ELECTRONIC FLOWMETERS4) ELECTRONIC FLOWMETERS• Electronic flowmeters represent a logical grouping 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 p y y p

technology. 3 types of flowmeters:

1. Magnetic flowmeters,

2 V t fl t2. Vortex flowmeters,

3. Ultrasonic flowmeters3. Ultrasonic flowmeters

MAGNETIC FLOWMETERS

Base principle of magnetic flowmeterBase 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 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 (E)(E)

E = B x L x v

Where:

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)y ( g y)

Magmeter Flow Equation

o If a conductive fluid flows through a pipe of diameter (D) through ao If a conductive fluid flows through a pipe of diameter (D) through amagnetic field density (B) generated by the coils, the amount ofvoltage (E) developed across the electrodes will be proportional to thevelocity (V) of the liquid. Because the magnetic field density and thevelocity (V) of the liquid. Because the magnetic field density and thepipe diameter are fixed values, they can be combined into a calibrationfactor (K) and the equation reduces to:

Manufacturers determine each magmeter's K factor by water calibrationf h fl t b Th K l th bt i d i lid f thof each flowtube. The K value thus obtained is valid for any other

conductive liquid and is linear over the entire flowmeter range.

Advantages and Disadvantages of MagmeterAdvantages• 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 conductivethe same speed; the meter can measure the speed of the most conductivecomponent.

• It can install vertically or horizontally (the line must be full, however) andcan be used with fluids with conductivity greater than 200 umhos/cm.Ch i d i i l d ff h i f• Changes in conductivity value do not, affect the instrument performance.

Disadvantages• It's above average cost

It' l i• 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 ti lib ti hift• Electrical 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 measurementerror).

• In some applications, appropriate mechanical protection for the electrodesmust be provided.

4.3) ULTRASONIC FLOWMETERS

Base Principle:

The speed at which sound propagates in a fluid is 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 passage (or reflection) to determine the velocity of a

flowing fluid.

There are 2 types of ultrasonic flowmeters:

1. Doppler shift, and

2 Transit time2. Transit time

4.3.1) The Doppler Shift

D l ff t fl t t itt th t j t o Doppler-effect flow meters use a transmitter that projects a

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. 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 p , y p q

opaque the liquid, greater the number of reflections that

originate near the pipe wall a situation exemplified by heavy originate near the pipe wall, a situation exemplified by heavy

slurries.

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 g q y (Q gp )

pipe having a certain inside diameter (ID in inches)

b bt i d bcan be obtained by:

• The presence of acoustical discontinuities is essential

for the proper operation of the Doppler flowmeter.

Advantages and Disadvantages of Doppler MeterAdvantage• The common clamps-on versions are easily installed without process

sh tdo nshutdown.

• It can be installed bi-directional

• Flow measurement is not affected due to change in the viscosity of theFlow measurement is not affected due to change in the viscosity of the process.

• Generally suitable for measurements in large water pipes

• The meter produces no flow obstruction

• Its cost is independent of line size.

Di d tDisadvantage• 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• The instrument requires periodic re-calibration.

4.3.2) Transit Time Measuremento In this design, the time of flight of the ultrasonic signal is measuredg , g g

between two transducers; one upstream and one downstream. Thedifference in elapsed time going with or against the flow determines thefluid velocity.fluid velocity.

o When the flow is zero, the time for the signal T1 to get to T2 is thesame as that required to get from T2 to T1. When there is flow, theeffect 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:y g q

o where K is a calibration factor for the volume and time units used, dt iso where K is a calibration factor for the volume and time units used, dt isthe 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 andtemperature. Therefore, both have to be compensated for. In addition,the change in sonic velocity can change the refraction angle "a", whichg y g gin 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 MeterAdvantagesAdvantages• It does not cause any flow obstruction

• It can be installed bi-directionalIt 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 formeasurements in very large water pipes.

DisadvantagesDisadvantages• This type of meters are highly dependent on the Reynolds number (the

velocity profile)

• It requires nonporous pipe material (cast iron, cement and fibreglass shouldbe avoided)

• It requires periodic re calibration• It requires periodic re-calibration

• It is generally used where other metering methods are not practical orapplicable.

5) MASS FLOWMETERSTraditionally fluid flow measurement has been made interms of the volume of the moving fluid even though themeter user may be more interested in the weight (mass) ofmeter user may be more interested in the weight (mass) ofthe fluid. Volumetric flow meters also are subject to ambientand process changes, such as density, which changes withtemperature 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 directly.

3. The use of thermal mass flow meters that infer mass flow by way of measuring heat dissipation between two points in the pipelinein the pipeline.

5.1) MICROPROCESSOR-BASED VOLUMETRIC FLOW METERS

with microprocessors it is relatively simple to o with microprocessors it is relatively simple to

compensate a volumetric flow meter for temperature

and pressure.

With reliable composition (density) information this 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 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 precise density compensation (to achieve mass) can

be expensive.

o For the precise measurement of gas flow (steam) at varying pressures and temperatures, it is necessary to determine the 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 or temperature.

o This unit will automatically correct for variations in pressure, temperature, specific gravity, and super-compressibility. The

diff ti l (h) d l d b th fl l t i pressure differential (h) developed by the flow element is measured, and the mass flow (W) can all be calculated using the following generalized formulas:the following generalized formulas:

Where:

k is the discharge coefficient of the element (which also reflects 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 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

Th l di t h t t th id i t f t b o The power supply directs heat to the midpoint of a sensor tube that carries a constant percentage of the flow. On the same tube at equidistant two temperature elements (RTD) are installed 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) is equal.

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 (detectedas Tw), while, some distance away, there is no differenceas ), e, so e d sta ce a ay, t e e s o d e e cebetween wall and fluid temperature.

o Therefore the temperature of the unheated fluid (Tf) can beo Therefore, the temperature of the unheated fluid (Tf) can bedetected by measuring the wall temperature at this locationfurther away from the heater. This heat transfer process is non-further away from the heater. This heat transfer process is nonlinear, and the corresponding equation differs from the oneabove as follows:

o In the direct-heat version, a fixed amount of heat (q) is addedb l t i h t A th fl id fl th h thby an electric heater. As the process fluid flows through thepipe, resistance temperature detectors (RTDs) measure thetemperature rise while the amount of electric heat introduced istemperature rise, while the amount of electric heat introduced isheld constant.

o The mass flow (m) is calculated on the basis of the measuredo The mass flow (m) is calculated on the basis of the measuredtemperature difference (T2 - T1), the meter coefficient (K), theelectric heat rate (q), and the specific heat of the fluid (Cp), asfollows: