Chapter 3 FLOW MEASUREMENT
o There are many types of instruments for measuring liquid and/or gas flow. 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 process control. o It is one of the most frequently measured process variables. variables o Flow tends to be the most difficult variable to measure. o No single flow meter can cover all flow measurement applications. applications
Physical Properties Affecting the Fluids' Flow 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 direction of flow. Fluid velocity depends on the head pressure that h is i fforcing i the h fluid fl id through h h the h pipe. i G Greater the h h head d 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. ) viscosity: y its p physical y resistance to flow. Higher g the 4)Fluid viscosity the fluid, the slower fluid flow.
5) The specific gravity of the fluid: At any given operating condition, hi h th higher the fluid's fl id' specific ifi gravity, it lower l th the fluid's fl id' flow fl rate. t 6) Fluid Condition: The condition of the fluid (clean or dirty) also limitations in flow measurement, some measuring devices become blocked/plugged /p gg or eroded if dirty y 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 T iti l flow: fl which hi h is i b between t th the laminar and turbulent flow profiles. Its behaviour is difficult to predict and it may oscillate between the laminar and turbulent flow profiles.
Flow-straightening devices • These devices are used to improve p the flow-pattern p 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 i i l off measurementt although lth h thi this could ld be b classified roughly as follow: 1. Differential pressure flowmeters 2. Variable area flowmeters 3. Mechanical flowmeters 4 Electronic flowmeters 4. 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: •ORIFICE METERS. •VENTURI 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 difference ∆P is created. It can be shown for all these meters that the 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 measured, d and d the th velocity l it (V), (V) th the volumetric l t i fl flow (Q) and d the th mass flow (W) can all be calculated using the following generalized formulas:
k is the discharge g coefficient of the element ((which also reflects the units of measurement), A is i th the cross-sectional ti l area off th the pipe's i ' opening, i and d 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 th iinside the id di diameter t off th the pipe. i 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/minute, gallons/minute SG is the fluid specific gravity at 60°F, and i th is the viscosity i it iin centipoises. ti i
ORIFICE FLOWMETERS The components of a typical orifice flowmeter installation are: • Orifice plate and holder • Orifice taps • Differential pressure transmitter • Flow indicator / recorder
ORIFICE PLATES 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 opening. i B Because off the th smaller area the fluid velocity increases, causing a corresponding decrease in pressure.
• 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. •E Eccentric i orifice ifi plates l are typically i ll used d ffor di 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 p p plates,, because it allows more drainage around the circumference of the pipe.
Orifice Holders 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. inspection In contrast, Senior orifice fitting allows the orifice to be removed from the process without depressurizing the line and shutting down flow.
Orifice taps 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 in diameter. 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, 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 p taps p are located 2.5 p pipe p diameters upstream p and 8
diameters downstream from the orifice. They detect the smallest ll t pressure diff difference. With pipe i ttaps measurementt 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 scaled indicator.
Relationship between Differential pressure and flow • When Wh the th diff differential ti l pressure iis obtained bt i d experimentally i t ll and 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 transmitter.
DP Flowmeter Installations
Advantages and Disadvantages of Orifice flowmeters
Advantages • 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 elements). Disadvantages • 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, and temperature. • They require frequent calibration
VENTURI TUBES 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 Furthermore, the th total t t l unrecovered d head h d loss l rarely l exceeds d 10% of measured d/p.
Advantages and Disadvantages of VENTURI TUBES Advantage g • It can handle low-pressure applications • It can measure 25 to 50% more flow than a comparable orifice plate • It is less susceptible p to wear and corrosion compared p to orifice plates • It is suitable for measurement in very large water pipes and very large l air/Gas i /G d ducts. t • Provides better performance than the orifice plate when there are solids in Suspension. Suspension Disadvantage • It is the most expensive among the differential pressure meters • It is big and heavy for large sizes • Its I has h considerable id bl length l h
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 p plug g type. yp
Float Type (Rotameter) The float is inside a tapered tube. 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 p pressure forces the float upwards. Because the tube is tapered, p , 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 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, 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 the outside.
3) ) MECHANICAL FLOWMETERS • Mechanical flow meters that measure flow using an arrangement of moving parts, either by passing isolated known volumes of a fl fluid id through th h a series i off gears or chambers c a be s (positive (pos t e d displacement sp ace e t meters) ete s) OR by means of a spinning turbine or rotor (Turbine Flowmeters)
3.2) TURBINE FLOWMETERS Th turbine The bi flowmeter fl is i an accurate and reliable flowmeter for both liquids and gases. It consists of a multi-bladed multi bladed rotor mounted at right angles to the flow p in the fluid and suspended 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 d devices ((inductance d pickk ups).
Volumetric Flow Rate Equation o The outputs of reluctance and inductive pick pick-up 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 d off 20 mV V peak-to-peak. k t k It is i nott advisable d i bl to t transport t t such ha 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 p in the form of an equation: q f = kQ 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 displacing ak known volume l off fluid fl id through th h the th meter t and d summing i the th number of pulses generated by the meter.
Advantages and Disadvantages of the turbine meters Advantages g The turbine meter is easy to install and maintain. They: • Are bi-directional bi directional • Have fast response • Are compact and light weights Disadvantages • 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. • Th They are sensitive ii to the h velocity l i profile fil to the h presence off swirls i l at the h 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 Strainers S i may be b required i d upstream to minimise i i i particle i l contamination of the bearings. o Turbine meters have moving parts that are sensitive to wear and can be damaged g by y over speeding. p g To prevent p sudden hydraulic 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 p 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 possible by y today's y sophisticated p electronics made p technology. 3 types of flowmeters: 1. Magnetic flowmeters, 2 Vortex 2. V t flowmeters, fl t 3. Ultrasonic flowmeters
MAGNETIC FLOWMETERS Base principle of magnetic flowmeter The magnetic flow meter design is based on Faraday’s law of magnetic induction which states that: "The induction, 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=BxLxv Where: B = the strength of the magnetic field (induction) L = the length of the conductor (distance of electrodes) y of the conductor (average ( g flow velocity) y) v = 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 off each h flowtube. fl t b Th K value The l th thus obtained bt i d is i valid lid for f any other th conductive liquid and is linear over the entire flowmeter range.
Advantages and Disadvantages of Magmeter Advantages • 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 component. • 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 Ch i conductivity in d i i value l d not, affect do ff the h instrument i performance. f Disadvantages • It's above average cost • It's It' large l size i • Its need for a minimum electrical conductivity of 5 to 20 µmhos / cm • Its accuracy is affected by slurries containing magnetic solids. • Electrical El t i l coating ti may cause calibration lib ti shifts hift • 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 error). • In some applications, appropriate mechanical protection for the electrodes must be provided.
4.3) ULTRASONIC FLOWMETERS Base Principle: 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 flowing fluid. There are 2 types of ultrasonic flowmeters: 1. Doppler shift, and 2 Transit time 2.
4.3.1) The Doppler Shift o Doppler-effect D l ff t flow fl meters t use a transmitter t itt that th t projects j t 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. 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 p y opaque. p q More opaque the liquid, greater the number of reflections that originate near the pipe wall, 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 g in frequency. q y The flow (Q in gp gpm)) in a pipe having a certain inside diameter (ID in inches) can be b obtained bt i d by: b
• The presence of acoustical discontinuities is essential for the proper operation of the Doppler flowmeter.
Advantages and Disadvantages of Doppler Meter Advantage •
The common clamps-on versions are easily installed without process sh tdo n shutdown.
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It can be installed bi-directional
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Flow measurement is not affected due to change in the viscosity of the process.
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Generally suitable for measurements in large water pipes
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The meter produces no flow obstruction
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Its cost is independent of line size.
Di d Disadvantage t •
The sensor may detect some sound energy travelling in the causing interference reading errors.
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Its accuracy depends on the difference in velocity between the particles, the fluid, the particle size, concentration, and distribution.
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The instrument requires periodic re re-calibration. calibration
4.3.2) Transit Time Measurement o In this design, g , the time of flight g of the ultrasonic signal g is measured between two transducers; one upstream and one downstream. The difference in elapsed time going with or against the flow determines the fluid velocity. 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 y the following g equation: q
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, g in sonic velocity y can change g the refraction angle g "a", which the change 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 Advantages •
It does not cause any flow obstruction
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It can be installed bi bi-directional directional
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It is unaffected by changes in the process temperature
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It is suitable to handle corrosive fluids and pulsating flows.
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It can be installed by clamping on the pipe and is generally suited for measurements in very large water pipes.
Disadvantages •
This type of meters are highly dependent on the Reynolds number (the velocity profile)
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It requires nonporous pipe material (cast iron, cement and fibreglass should be avoided)
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It requires periodic re-calibration re calibration
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It is generally used where other metering methods are not practical or applicable.
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 directly. 3. The use of thermal mass flow meters that infer mass flow by way of measuring heat dissipation between two points in the pipeline. pipeline
5.1) MICROPROCESSOR-BASED VOLUMETRIC FLOW METERS
o with microprocessors it is relatively simple to compensate a volumetric flow meter for temperature and pressure. o With reliable composition (density) information, information this factor also can be entered into a microprocessor to obtain mass flow readout. However, when density changes may occur with some frequency, frequency and particularly where the flowing fluid is of high monetary value (for example, in custody transfer), 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 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 pressure differential diff ti l (h) developed d l db by th the flow fl element l t is i measured, and the mass flow (W) can all be calculated using the following generalized formulas: Where: k is the discharge coefficient of the element (which also reflects the units of measurement), A is the cross cross-sectional sectional area of the pipe's pipe s opening, and D is the density of the flowing fluid.
5.3) THERMAL MASS FLOWMETERS o The Th power supply l directs di t h heatt tto th the midpoint id i t off a sensor tube t b 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) 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 (detected as Tw), ), while, e, so some e d distance sta ce a away, ay, tthere e e is s no o d difference e e ce between wall and fluid temperature. o Therefore, 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 non nonlinear, 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 by l t i heater. h t A the As th process fluid fl id flows fl th through h the th pipe, resistance temperature detectors (RTDs) measure the temperature rise, rise while the amount of electric heat introduced is held constant. 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 follows:
