Ch. 3 flow measurement
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Ch. 3 flow measurement






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Ch. 3 flow measurement Ch. 3 flow measurement Presentation Transcript

  • 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 is f h i forcing the fluid through the pipe. G i h fl id h h h i Greater the h d h 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. ) y p y g 4)Fluid viscosity: its physical resistance to flow. Higher the viscosity the fluid, the slower fluid flow.
  • 5) The specific gravity of the fluid: At any given operating condition, higher the fl id' hi h th fluid's specific gravity, lower th fluid's flow rate. ifi it l the fl id' fl t 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 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 flow: which is b t T iti l fl hi h i between 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 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 Measurement Flow meters operate according to many different principles of measurement although thi could be i i l f t lth h this ld b classified roughly as follow: 1. Differential pressure flowmeters 2. Variable area flowmeters 3. Mechanical flowmeters 4. 4 Electronic flowmeters 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 METERS VENTURI •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, and the velocity (V), th volumetric fl d d th l it (V) the l t i flow (Q) and the d th mass flow (W) can all be calculated using the following generalized formulas: k is the discharge coefficient of the element ( g (which also reflects the units of measurement), A i th cross-sectional area of th pipe's opening, and is the ti l f the i ' i 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 the inside diameter of th pipe. th i id di t f the 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 is the i i th viscosity i centipoises. it in 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. B i Because of the f 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 orifice plates are typically used f di i ifi l i ll d 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 p preference to eccentric bore plates, because it allows p , 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, but also the most differential 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 pp p diameters downstream from the orifice. They detect the smallest pressure diff ll t difference. With pipe t i taps measurement t 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 • Wh When the diff th differential pressure i obtained experimentally and ti l is bt i d i t ll 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. Furthermore, the total unrecovered head loss rarely exceeds F th th t t l dh dl l 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 to wear and corrosion compared to orifice p p plates • It is suitable for measurement in very large water pipes and very l large air/Gas d t i /G ducts. • 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 • I has considerable length Its h id bl 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 g type. p plug yp
  • Float Type (Rotameter) The float is inside a tapered tube. The fluid tube 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 p upwards. 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 meter 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 id through a series of gears or fluid th h i f c a be s (pos t e displacement ete s) chambers (positive d sp ace e t meters) OR by means of a spinning turbine or rotor (Turbine Flowmeters)
  • 3.2) TURBINE FLOWMETERS The Th turbine flowmeter is an bi fl i 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 and suspended in the fluid 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 devices ( d d (inductance pickk ups).
  • Volumetric Flow Rate Equation o The outputs of reluctance and inductive pick-up coils are continuous pick up 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 of 20 mV peak-to-peak. It is not advisable to transport such a d f V k t k i t d i bl t t t h 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 = kQ p 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 displacing a k di l i known volume of fluid through the meter and summing the l f fl id th h th t d i 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 to the velocity profile to the presence of swirls at the ii h l i fil h f i l 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 S i Strainers may be required upstream to minimise particle b i d i i i i l 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 FLOWMETERS • Electronic flowmeters represent a logical grouping of flow measurement technologies. All have no moving parts, are relatively non-intrusive, and are p y y p made possible by today's sophisticated electronics technology. 3 types of flowmeters: 1. Magnetic flowmeters, 2. Vortex flowmeters, 2 V t 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, induction which states that: "The voltage induced across a The 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 ( g y) v = velocity of the conductor (average flow 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 of each fl t b f h flowtube. Th K value th The l thus obtained i valid f bt i d is lid for 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. • Ch Changes i conductivity value d not, affect the i in d i i l do ff h instrument performance. f Disadvantages • It's above average cost • It' l It's large size i • 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 coating may cause calibration shifts Electrical ti lib ti 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. 2 Transit time
  • 4.3.1) The Doppler Shift o D Doppler-effect flow meters use a transmitter that projects a l ff t fl t t itt th t j t 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 p particles or air bubbles, it is said to be sonically opaque. More , y p q opaque the liquid, greater the number of reflections that originate near the pipe wall, a situation exemplified by heavy wall 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 gp ) pipe having a certain inside diameter (ID in inches) can b obtained by: be bt i d 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 shutdown. sh tdo n • It can be installed bi-directional • Flow 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. Disadvantage Di d t • 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 re-calibration.
  • 4.3.2) Transit Time Measurement o In this design, the time of flight of the ultrasonic signal is measured g , g g 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 the following equation: y g 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 y g g the change in sonic velocity can change the refraction angle "a", which 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 • It can be installed bi-directional 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 for measurements in very large water pipes. Disadvantages • This type of meters are highly dependent on the Reynolds number (the velocity profile) • It requires nonporous pipe material (cast iron, cement and fibreglass should be avoided) • It requires periodic re calibration re-calibration • 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 this information, factor also can be entered into a microprocessor to obtain mass flow readout. However, when density changes may occur with some frequency, and frequency 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 diff differential (h) developed b th flow element is ti l d l d by the fl l ti 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-sectional area of the pipe's opening, and cross sectional pipe s D is the density of the flowing fluid.
  • 5.3) THERMAL MASS FLOWMETERS o Th power supply directs h t t th midpoint of a sensor tube The l di t heat to the id i t f 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, some distance away, there is no difference ), e, so e d sta ce a ay, t e e s o d e e ce between wall and fluid temperature. o Therefore the temperature of the unheated fluid (Tf) can be Therefore, detected by measuring the wall temperature at this location further away from the heater. This heat transfer process is nonnon linear, 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 by b an electric h t l t i heater. A th process fl id fl As the fluid flows th through th h the pipe, resistance temperature detectors (RTDs) measure the temperature rise while the amount of electric heat introduced is rise, 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: