Fundamentos de flujo

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Flow is the study of fluids in motiona fluid is any substance that can flow, and thus the term applies both to liquids and to gases. Precise measurement and control of fluid flow through pipes can be difficult, but is extremely important in almost all process industries. ACCURACY Accuracy is typically specified either as “% of flow rate” or as “% of full scale”. The user should be careful when defining accuracy since “% of flow rate” and “% of full scale” are not the same. In “% of flow rate”, the accuracy is the same for low flows as it is for high flows. For example, a device with 0-100 L/m range and ±1% flow rate accuracy, will have, at 100 L/m, an error of ±1 L/m and at a flow of 20 L/m, the error will be ±0.2 L/m (i.e., 1% of measurement in both cases). On the other hand, a “% of full scale” device has different measuring accuracies at different flow rates. For example, a device with 0-100 L/m range and ±1% full scale accuracy will have, at 100 L/m, an error of ±1 L/m and at a flow of 20 L/m, the error will still be ±1 L/m (i.e., 5% of measurement). This is a much larger error than the flow of 20 L/m under “% of flow rate”. Different process industries measure flow for different reasons, and one flowmeter may be used for more than one application even within one industry. REPEATABILITY Repeatability is the ability of a flowmeter to produce the same measurement each time it measures a flow. High repeatability does not ensure accuracy. Depending on the application, repeatability of a flowmeter may be more important than accuracy. For example, in a flow control loop, if a flowmeter gives a stable, repetitive reading, the true accuracy of the measurement is not necessarily important.

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Buenos fundamentos de flujo

Transcript of Fundamentos de flujo

  • Flow is the study of fluids in motiona fluid is any substance that can flow, and thus the term applies both to liquids and to gases. Precise measurement and control of fluid flow through pipes can be difficult, but is extremely important in almost all process industries.

    ACCURACY Accuracy is typically specified either as % of flow rate or as % of full scale. The user should be careful when defining accuracy since % of flow rate and % of full scale are not the same. In % of flow rate, the accuracy is the same for low flows as it is for high flows. For example, a device with 0-100 L/m range and 1% flow rate accuracy, will have, at 100 L/m, an error of 1 L/m and at a flow of 20 L/m, the error will be 0.2 L/m (i.e., 1% of measurement in both cases). On the other hand, a % of full scale device has different measuring accuracies at different flow rates. For example, a device with 0-100 L/m range and 1% full scale accuracy will have, at 100 L/m, an error of 1 L/m and at a flow of 20 L/m, the error will still be 1 L/m (i.e., 5% of measurement). This is a much larger error than the flow of 20 L/m under % of flow rate. Different process industries measure flow for different reasons, and one flowmeter may be used for more than one application even within one industry. REPEATABILITY Repeatability is the ability of a flowmeter to produce the same measurement each time it measures a flow. High repeatability does not ensure accuracy. Depending on the application, repeatability of a flowmeter may be more important than accuracy. For example, in a flow control loop, if a flowmeter gives a stable, repetitive reading, the true accuracy of the measurement is not necessarily important.

  • Why Measure Flow?

    CUSTODY TRANSFER The measurement of fluid passing from a supplier to a customeris one of the most important flow measurement applications. In custody transfer applications, flowmeters are essentially the cash register of the system. For example, a flowmeter at your local gas station measures how much gas you pump into your vehicle and bills you accordingly. Given the economic implications, custody transfer applications require high measurement accuracy. PRODUCT CONSISTENCY Accurate flow measurement ensures product integrity. Flow is used as an input to process control systems so that the product produced is the same. As a consumer, you expect processed food you eat or the gasoline you use in your car to be the same each and every time you purchase these products. EFFICIENCY Precise flow measurement can also provide indications of process efficiency based on the amount of inputs used and the amount of product produced. For example, in a boiler, combustion efficiency is an indication of the burners ability to burn fuel. The amount of unburned fuel and excess air in the exhaust are used to assess a burners combustion efficiency. Burners resulting in low levels of unburned fuel while operating at low excess air levels are considered efficient. Well-designed burners firing gaseous and liquid fuels operate at excess air levels of 15% and result in negligible unburned fuel. By operating at only 15% excess air, less heat from the combustion process is being used to heat excess air, which increases the available heat for the load. Combustion efficiency is not the same for all fuels and, generally, gaseous and liquid fuels burn more efficiently than solid fuels. PROCESS VARIABLE CONTROL Flow rate is measured and controlled during applications. For example, during heat exchange, fluid temperature can be controlled by changing the flow rate of steam through the heat exchanger. Other process applications use flow rate control to manipulate such variables as pressure, level in a vessel, chemical composition, and weight. SAFETY Regulation of flow is often essential for safety reasons. Flow rates outside the desired range can be an indication that something else in the process is in an upset condition, such as a compressor or a pump or even a valve.

  • Terminology FLOW RATE Flow rate refers to the velocity of the fluid being measured. Velocity is generally measured in feet or meters per second. Flow rate answers the question: How fast is the fluid moving? Flow can be defined as a volume of fluid in a pipe passing a given point per unit of time. This can be expressed by

    Q = v x A Where A is the cross-sectional area of the pipe, and v is the average fluid velocity. VOLUMETRIC FLOW RATE More often the question is: How much fluid is passing through the pipeline or system? One way to describe a quantity of fluid is by giving the volumetric flow rate; the volume of fluid that is transported over some period of time such as gallons per minute, liters per hour, and so forth. Volumetric flow rate can be determined from the velocity of the fluid if the area of the pipeline is known. The equation that describes the relationship between velocity and volumetric flow rate is: Q = v x A Where:

    Q = the flow rate in units of volume per unit of time v = the velocity of the fluid A = the cross-sectional area of the pipe

    Flow rate is not measured directly. Instead, some other variable (in this instance, velocity) is measured and translated into a flow rate based on the cross-sectional area of the pipeline.

  • Limitations of volumetric measurement There are a few limitations inherent to volumetric flow. For example, volumetric flow measurement devices usually do not account for changes in fluid density, which is especially important when measuring gases or vapors. As the temperature of a gas increases, the molecules move further apart. This means there is a smaller amount by weight of the measured fluid in a given volume than there would be at some lower temperature. Similarly, increases in pressure will cause the molecules to move closer together, resulting in more of the measured fluid by weight in a given volume. One solution to this problem is to use devices that provide temperature and pressure compensation. Another solution is to use mass flow measurement. These concepts will be discussed later.

    MASS FLOW RATE When very precise flow rate measurements must be made, mass flowmeters are often preferred. Mass flow measurements give the actual weight of the fluid that is being transported per unit of time, such as pounds per hour, kilograms per second, and so forth. The mass flow may then be defined as

    volumetric flow density

    TOTALIZED FLOW Totalized flow gives an ongoing measurement of the total amount of fluid passing by the point of measurement. Everyday examples of totalized flow measurement are the measurements made by the gas and water meters attached to homes and businesses.

  • Fluid Properties The following fluid properties are often used in process industries both as variables in flow equations and separately to evaluate and predict process efficiency and safety:

    Density

    Viscosity

    Fluid type

    Flow profile DENSITY Density (), one of the most commonly used measures, is the mass per unit volume of a fluid, typically given at a reference temperature and pressure. Table 3.1 shows how density is affected by temperature and pressure both for liquids and for gases. In general, density is proportional to pressure and inversely proportional to temperature. Density = Mass / Volume

    The density of the process fluid is important to flowmeter selection and performance. Gravedad especfica en gases SG = MW de gas MW de aire SG (Oxgeno) = 32 / 28.8 = 1.11

  • FLUID TYPE A wide variety of process fluid types can be measured. Often, the fluids contain suspended solids or other particulate matter that may affect flowmeter function or measurement accuracy:

    Clean fluidA fluid that is free from solid particles (e.g., water) Dirty fluidA fluid containing solid particles (e.g., muddy water) SlurryA liquid with a suspension of fine solids that can flow freely through

    a pipe (e.g., pulp and paper, oatmeal)

    Steam The condition of the fluid (i.e., clean or dirty) also presents limitations. Some measuring devices may become plugged or eroded if dirty fluids are used. For example, differential-pressure devices would normally not be applied where dirty or corrosive fluids are used (though flow nozzles may handle such applications under certain conditions). On the other hand, magnetic meters are capable of accurately measuring dirty, viscous, corrosive, abrasive, and fibrous liquids. Flow of Solids The flow measurement of solids typically involves using a weighing device or a radioactive (radiation) device. For example, a batch in a hopper could be measured with load cells and then discharged. For a continuous process, isolated weighing conveyors provide the weight measurement. Such measurements are not provided in this handbook since in many cases they fall under the responsibility of mechanical engineering activities. VISCOSITY Viscosity can be thought of as fluid thickness. Viscosity is a measure of a fluids tendency to resist a shearing force or to resist flow (Figure 3.3). The higher a fluids viscosity, the greater the force required to shear the fluid and the slower the fluids flow rate. For example, honey has a higher viscosity than water, so water flows faster and more easily around obstructions in its flow path than honey. Typical units used to represent viscosity are poise (cm/g/sec) and centipoise (cP).

  • In simple terms. Viscosity is:

    The resistance of fluid to flow

    The internal friction generated by one layer of liquid flowing over another o In a Newtonian Fluid the friction (viscosity) is constant o In a Non-Newtonian Fluid the friction (viscosity) is a function of the

    shear rate - hence viscosity is not constant Generally, fluid viscosity is inversely proportional to temperatureas temperature increases, fluid viscosity decreases. Gas viscosity is an exception. Gas viscosity is proportional to temperatureas temperature increases, gas viscosity increases.

    Viscosity Values of Selected Substances @ 20C (64F): Substance Dynamic Viscosity (cP) Gases 0.01 to 0.02 Liquid Hydrogen 0.01 Air 0.018 Acetone 0.32 Water 1.0 Mercury 1.55 Blood plasma 1.77 Full blood 5 to 120 Sugar solution 57 Engine oil 100 to 500 Castor oil 1000 Honey 10,000 Molten plastic 10e4 to 10e8 Tar 1-e5 to 10e8 Earth Crust 10e24

  • Environmental Conditions Although pressure is an absolute quantity, everyday pressure measurements, such as pipeline pressure, are usually made relative to ambient air pressure. In some cases, measurements are made relative to absolute pressure or vacuum.

  • FLOW PROFILE Flow profile characterizes the behavior of a fluid as it flows through a pipe (e.g., smooth or turbulent, symmetrical or asymmetrical). A fluid may change profiles several times before it reaches its destination point. Generally, at a given point in time, a fluid will have one of the following three flow profiles:

    Laminar

    Turbulent

    Transition Laminar In laminar flow, fluid flows in smooth, ordered layers. As a result, there is very little mixing of fluid across the pipe cross section. The layers in the center of the pipe have the highest velocity, while friction between the fluid and the pipe wall causes a lower velocity near the pipe wall (Figure 3.5). Laminar flow profiles occur when viscous (restraining) forces have more influence in the flow stream than do inertial (driving) forces. Laminar flow streams may be symmetrical or non-symmetrical.

    Turbulent Turbulent flow profiles often occur with low-viscosity fluids, when inertial forces have more influence in the flow stream than do viscous forces. The low viscosity enables turbulent eddies (whirlpools) to form, which occur randomly in the fluid stream (Figure 3.6). In turbulent flow, the fluid velocity is nearly constant across the pipe cross section (uniform flow), with significantly lower velocity occurring only very near the pipe wall. Because of the turbulence, considerable mixing takes place across the pipe cross section.

  • Transition Transition flow profiles mark the change from laminar to turbulent flows. Transition flow varies depending on the pipe radius and may have characteristics of laminar flow, turbulent flow, or both.

    REYNOLDS NUMBER The effects of the most important factors affecting fluid flow can be combined and expressed with a dimensionless, numerical value called the Reynolds number (RD). The Reynolds number can be thought of as the ratio of the inertial force to the viscous force in the flow stream. The basic equation for the Reynolds number is:

    Where:

    = Fluid density v = Fluid velocity D = Pipe inside diameter = Fluid viscosity

  • Because the Reynolds number expresses the characteristics of a flow stream, it is useful when determining whether a particular flowmeter is appropriate for an application. The Reynolds number is especially helpful in predicting the flow profile:

    LaminarRD 4,000

    STANDARD VS. ACTUAL VOLUMETRIC FLOW RATE For gases, pressure and temperature must be compensated for, if the measured values differ from the ones used for calculations. Unlike gases, liquids are incompressible but they may require temperature compensation since their density may vary significantly after a large change in temperature. To standardize expressions of gas flow, process measurement professionals often refer to the gas flow at operating conditions to standard pressure and temperature conditions. Standard conditions are presumed to be 14.696 psia (101.325 kPa absolute) for pressure and 59F (or 15C) for temperature. However, such standard conditions may vary from industry to industry, so it is good practice to define these conditions to avoid errors. Gas flow expressed in standard units is the amount of gas at standard conditions that is required to effect the same mass flow. The reasoning behind this approach is to relate the volumetric flow to mass flow at given operating conditions, since the mass flow at 100 psig is quite different from the mass flow at 5000 psig due to density change. Standard volumetric flow rate is the volumetric flow rate that would occur if the process pipe conditions were set at a reference temperature and pressure. The standard volumetric flow rate is multiplied by the density of the fluid to determine the mass flow rate. Actual volumetric flow rate is the volumetric flow rate at the actual pressure and temperature conditions within the process pipe.

  • Figure 3.7 shows the difference between actual and standard volumetric flow rates. The pipe flow conditions of Pipe 1 are at the standard reference conditions, so one actual cubic meter (Am3) equals one normal cubic meter (Nm3). Most process pipes do not operate at standard conditions.

    More often, the conditions are like those in Pipe 2, where the pressure in the pipe is higher than the standard reference pressure. The same mass of gas that was in 5 m3 in Pipe 1 is compressed into 1 m3 in Pipe 2, where the pressure is five times higher. In Pipe 2, one Am3 is equal to five Nm3 of the process gas. Ley de Avogadro: El nmero de molculas integrales en cualquier gas; a iguales condiciones de presin y temperatura es siempre el mismo en volmenes iguales. La masa de un gas es directamente proporcional a su peso molecular. Amadeo Avogadro 1776-1856

    1 mol contiene 6.02 x 1023 tomos (No. De Avogadro)

  • The base reference conditions used to describe a standard cubic foot (SCF) or Nm3 vary. There are actually several different base pressures used to define an SCF (Table 3.2):

  • Name of GasChemical

    Formula

    Approx

    Mol.

    Weight

    Specific

    Gravity

    Acetylene (ethyne) C2H2 26.0 0.0682 1.0930 0.907

    Air ~ 29.0 0.0752 1.2052 1.000

    Ammonia NH3 17.0 0.0448 0.7180 0.596

    Argon Ar 39.9 0.1037 1.6619 1.379

    Butane C4H10 58.1 0.1554 2.4904 2.067

    Carbon dioxide CO2 44.0 0.1150 1.8430 1.529

    Carbon monoxide CO 28.0 0.0727 1.1651 0.967

    Chlorine Cl2 70.9 0.1869 2.9953 2.486

    Ethane C2H6 30.0 0.0789 1.2645 1.049

    Ethylene C2H4 28.0 0.0733 1.1747 0.975

    Helium He 4.0 0.0104 0.1665 0.138

    Hydrogen chloride HCl 36.5 0.0954 1.5289 1.268

    Hydrogen H2 2.0 0.0052 0.0838 0.0695

    Hydrogen sulphide H2S 34.1 0.0895 1.4343 1.190

    Methane CH4 16.0 0.0417 0.6683 0.554

    Methyl chloride CH3Cl 50.5 0.1342 2.1507 1.785

    Natural Gas ~ 19.5 0.0502 0.8045 0.667

    Nitric oxide NO 30.0 0.0780 1.2500 1.037

    Nitrogen N2 28.0 0.0727 1.1651 0.967

    Nitrous oxide N2O 44.0 0.1151 1.8446 1.530

    Oxygen O2 32.0 0.0831 1.3318 1.105

    Propane C3H8 44.1 0.1175 1.8831 1.562

    Propene (propylene) C3H6 42.1 0.1091 1.7484 1.451

    Sulphur dioxide SO2 64.1 0.1703 2.7292 2.264

    Standard Density

    lb/ft3 kg/m3

    Table 3.3: Standard densities

    STEAM PRESSURE AND TEMPERATURE Process pressure and temperature of the fluid inside the pipe are also key elements in flow equations. Changes in pressure and temperature are especially important when measuring steam, which is one of the most commonly measured fluids in the process industry. As water transits into steam, transition from the liquid phase to the gaseous state occurs.

    RELACION CALOR-TEMPERATURA La temperatura no es indicador de la cantidad de calor: El calor es la causa y la temperatura es la consecuencia. La relacin entre el calor y la temperatura para las sustancias se enuncian en las Leyes de cambio de Estado:

    1. A Presin constante una sustancia cambia de estado a una temperatura definida llamada Punto de Transformacin.

  • 2. Mientras ocurre el cambio de estado, el Punto de Transformacin de la

    sustancia permanece constante.

    3. El Punto de Transformacin no cambia, cualquiera que sea el sentido de la transformacin, por ejemplo, a presin atmosfrica a nivel del mar, el agua se congela a 0C y se funde a 0C y tambin hierve a 100 C y se condensa a 100 C.

    Esto puede explicarse mejor en con ayuda del grfico siguiente:

    Lquido Saturado:

    Es una condicin de un fluido, a una Presin dada en la cual su calidad de vapor es 0, es decir, es completamente lquido y si recibe una cantidad infinitesimal de energa, empieza su cambio de fase hacia vapor.

    Vapor Saturado:

    Es vapor a la temperatura de ebullicin del lquido. Es la condicin de un fluido a la misma Presin y Temperatura que su Lquido Saturado, en la cual su calidad de vapor es 100% y si pierde infinitesimalmente una cantidad de energa, empezar a cambiar su fase hacia lquido.

    Vapor Sobrecalentado:

    Es vapor a una temperatura mayor a la de ebullicin del lquido. Siempre su calidad de vapor es 100% y slo si pierde grandes cantidades de energa, se aproximar a la condicin de saturacin.

    Calidad de Vapor o vapor hmedo:

    Es la masa en fase vapor que existe en cada unidad de masa de un fluido. Se expresa en forma adimensional o bien como porcentaje:

    x = masa en fase vapor

    unidad de masa

    Agua a

    760

    mmHg-

    abs

  • Saturation Point Saturation point is the point under a specific set of conditions at which a liquid turns to vapor. Saturation pressure and temperature vary according to a well-defined relationship called the vapor-pressure curve. Water at its saturation point may be saturated liquid (all liquid), saturated steam (all vapor), or a mixture of water liquid and vapor.

    Diagrama de Mollier:

  • STATES OF STEAM Pressure and temperature in the pipeline determine the state of the steam. Saturated steam is steam in the process pipe that is exactly at its saturation point. If the pressure drops or the temperature rises, superheated steam (steam heated beyond its saturation point) results. For example, at 350 psia (pounds per square inch absolute), the saturation temperature for water is 432 F (222 C). Thus, steam at 350 psia and 532 F (278 C) includes 100 F (56 C) of superheat. If the pressure rises or the temperature drops, the steam will start to condense and become saturated (the steam wont condense until the superheat is removed at 432 F or 222 C). Most flowmeters will measure steam well. However, almost all flowmeters have some additional error when measuring saturated steam. The error usually becomes greater as steam quality decreases because of the difficulty in determining the density of the liquid/vapor mixture.

    Pipe Geometry and Conditions Pipe geometry (design) and conditions are the third key component in flow equations. Pipe geometry can cause changes in the flow profile. Process pipe conditions, such as roughness of the inner wall, can also affect the flow. For example, the texture of the inner pipe wall can cause a slight increase (smooth wall) or decrease (rough wall) in fluid velocity. Not all measuring devices cover all line sizes. For example, the maximum size of most vortex meters is eight inches. Therefore, the question is whether the selected flow device can handle the line size (and required flow). PIPE INSIDE DIAMETER In most industries, the inside diameter of a process pipe does not remain constant throughout the entire process. Fluctuations in pipe inside diameter affect several factors (e.g., Reynolds number). For example, doubling the diameter of a process pipe can increase the flow rate by as much as four times if the velocity remains unchanged. (See size-sch.xls file) CAVITACIN Ocurre cuando la presin de operacin se reduce por debajo de la presin de vapor del fluido. Esto causa que el vapor cavite o burbujee. Si la presin se incrementa rpidamente por encima del punto de vapor, las burbujas colapsan con gran fuerza.

  • FLOW PROFILE DISTURBANCES A uniform, symmetrical, turbulent flow profile is desirable for most flowmeters. Factors that cause the flow profile to change are called flow profile disturbances. Most flow profile disturbances are caused by pipe geometry. Flow profile disturbances can affect flowmeter accuracy, although to what degree depends on the sensitivity of the flowmeter. There are three types of flow profile disturbances:

    Symmetrical profile disturbance

    Asymmetrical profile disturbance

    Swirl

    Symmetrical Profile Disturbance In a symmetrical flow profile disturbance, the velocity profile of the fluid remains symmetrical about the process pipe axis, but it is no longer uniform. Symmetrical profile with a higher core velocity may be caused by either a reducer (pipe section inserted to decrease the cross-sectional area) or an expander (pipe section inserted to increase the cross-sectional area) (Figure 3.9). Reducers and expanders may also cause a symmetrical profile velocity to become asymmetrical.

    Asymmetrical Profile Disturbance In an asymmetrical flow profile disturbance, the velocity profile of a fluid is not symmetrical about the process pipe axis. Asymmetrical profile can be caused by anything that tends to push the flow to one side of the pipe. Elbows (Figure 3.10), valves, and tees are common sources of this disturbance.

  • Swirl Swirl occurs when the velocity profile of a fluid moves in a circular motion as it flows forward. Swirl can be caused by pumps, compressors, or two pipe elbows in different planes (Figure 3.10). Swirl causes fluids to flow across the diameter of the process pipe rather than parallel to the pipe. Such flow characteristics can take more or less time to pass by the point of measurement, which can produce erroneous readings in flowmeters that measure velocity.

    Eliminating the Effects of Flow Profile Disturbances Some flowmeters are more sensitive to flow irregularities than othersmost flowmeters require a specific length of straight piping between disturbances to ensure a uniform flow profile at the flowmeter. For each flowmeter, industry or manufacturers standards specify the required length of straight pipe. The standard is referred to as an upstream or downstream straight piping requirement. Straight piping length is usually specified in pipe diameters. Knowing the cause of the disturbance can also help determine how much straight piping is required. For example, swirl may take 100 or more pipe diameters to dissipate, and a reducer requires significantly less straight piping to dissipate than a double elbow. Flow conditioners can be disks with circular holes in them or bundled tubes that are inserted in the process pipe to eliminate swirl (Figure 3.11). Flow conditioners create a turbulent, uniform (constant across the pipe cross section), flow profile and can be used to decrease the length of the straight piping requirement. Many types of flowmeters use a minimum number of upstream and downstream straight pipe runs because irregular velocity profiles affect the accuracy of the measurement. This requirement has a direct effect on the piping and may sometimes be a problem (especially on existing installations). For example, for

  • orifice plates, typically a straight run of 10 to 20 upstream pipe diameters is required, with five pipe diameters for the downstream side. On the other hand, a Pitot tube requires 40 upstream and 10 downstream pipe runs respectively, depending on the fluid dynamic disturbance. Major vendors offer tables to guide the user in determining the recommended upstream and downstream straight pipe runs. For Coriolis and variable area flowmeters, no upstream and downstream pipe runs are required. There are many applications where appropriate upstream and downstream pipe lengths are not available to provide accurate measurement. In these applications, straightening vanes or flow conditioners (consisting, for example, of tube bundles) can be used. The length of these tubes should be more than ten times the diameter of the tubes, with the inside diameter of the tubes less than 1/4 the inside pipe diameter.

    Flowmeter Selection There are several devices and instruments available for measuring fluid flow. Each is designed to measure accurately and efficiently in a variety of applications, although some flowmeter designs perform better with certain applications. Of the many flowmeters available for measuring fluid flow, the type of flowmeter used often depends on the nature of the fluid and the process conditions under which the fluid is measured. Each type of flowmeter has benefits and limitations that depend partly on the flowmeter design and partly on the application. You will need to be familiar with flowmeter specifications in order to select the best flowmeter for a particular application.

  • Classes of Flowmeters Flowmeters operate according to many different principles of measurement. They can be broadly classified into four categories:

    1. Flowmeters that have wetted moving parts (such as positive displacement, turbine, and variable area). These meters utilize high-tolerance machined moving parts, which determine the meters performance. These parts are subject to mechanical wear and thus are practical for clean fluids only.

    2. Flowmeters that have wetted non-moving parts (such as vortex, differential pressure, target, and thermal). The lack of moving parts gives these meters an advantage. However, excessive wear, plugged impulse tubing, and excessively dirty fluids may cause problems for these meters.

    3. Obstructionless flowmeters (such as coriolis and magnetic). These meters

    allow the fluid to pass undisturbed and thus maintain their performance when handling dirty and abrasive fluids.

    4. Flowmeters with sensors mounted externally (such as clamp-on ultrasonic

    and weir flow measurements). These meters offer no obstruction to the fluid and have no wetted parts. However, their limitations prevent them from being used in all applications.

  • Flowmeters are grouped into four classes:

    DP flowmeters

    Velocity flowmeters

    Mass flowmeters

    Positive displacement flowmeters (also called volumetric flowmeters) Which class a flowmeter belongs to is important when deciding which flowmeter to use for a specific application. The classes are delineated based upon how each measures fluid flow. With most classes of flowmeters, flow is first measured indirectly by measuring differential pressure or a quantity proportional to fluid velocity. Volumetric flow rate is then determined electronically from this first measurement. HOW DP FLOWMETERS WORK DP flowmeters, also called differential producers, are the most common type of flowmeter used and account for just over half of all industrial flow measurements. Flowmeters in this class measure the differential pressure (P) caused by an obstruction in the flow stream. The differential pressure is the difference in pressure between a point before the obstruction and a point after the obstruction. DP flowmeters work because of the equation of continuity and Bernoullis equation. The equation of continuity shows that for a steady, uniform flow, a decrease in pipe diameter (A) results in an increase in fluid velocity (V):

    Bernoullis equation states that the total of kinetic, potential, and pressure energy within a fluid stream remains constant. If velocity increases, there must be a corresponding decrease in either pressure energy or potential energy. If we assume a horizontal pipeline, we can ignore the potential energy consideration. Therefore, according to Bernoullis equation, an increase in fluid velocity at the restriction will produce a corresponding decrease in pressure. The flow equation used for DP flowmeters is based on Bernoullis equation. Volumetric flow rate (Q) is proportional () to the square root of differential pressure:

    DP flowmeters consist of two parts: a primary device and a secondary device. The primary device is placed in the process pipe to restrict the flow and create a pressure drop. The secondary device measures the differential pressure and transmits the result to a control system.

  • Some of the most common DP flowmeters are:

    Orifice plate

    Pitot tube

    Flow nozzle

    Venturi tube

    Wedge

    V-cone

    Rotameter HOW VELOCITY FLOWMETERS WORK Velocity flowmeters work by producing an output based upon fluid velocity that is proportional to the volumetric flow rate. Velocity is the speed of a fluid flowing past a stationary point ina process pipe. Typical units used to represent velocity are ft/ s and m/s. Some of the most common velocity flowmeters are:

    Magnetic flowmeter

    Vortex flowmeter

    Turbine meter

    Ultrasonic flowmeter Velocity flowmeters also have a primary device and a secondary device. The primary device generates a signal proportional to fluid velocity, while the secondary device interprets and transmits this signal to a control system. HOW MASS FLOWMETERS WORK Mass flow rate is the mass (actual weight) of a fluid that is transported through a process pipe per unit of time. Typical units used to represent mass flow rate are lb/hr and kg/sec. The two most common mass flowmeters are: Coriolis mass flowmeter and Thermal mass flowmeter. True mass flowmeters measure mass flow rate directly, without an intermediate calculation from volume or density. HOW POSITIVE DISPLACEMENT FLOWMETERS WORK Volumetric flow rate is the volume of fluid that is transported through a process pipe per unit of time. Typical units used to represent volumetric flow rate are gallons per minute (gpm) and liters per hour (L/hr). Volumetric flow rate can be determined from the velocity of the fluid if the cross-sectional area of the process pipe is known. Positive displacement flowmeters measure the volumetric flow rate directly by repeatedly trapping and measuring a sample of the process fluid. The

  • total volume of the fluid passing through the flowmeter in a given period of time is the product of the volume of each sample and the number of samples taken. FLOWMETER MARKET Just over half of all industrial flow measurements are made by DP flowmeters. Velocity flowmeters hold the second largest market share, at about 28%. Mass and positive displacement meters are least used for industrial flow measurements. Figure 3.12 shows more specifically the percentages of industrial flow measurements made by each type of flowmeter.

    DP Flowmeters

  • Orifice Plate An orifice plate is a thin disk placed in the path of fluid flow with a sharp-edged opening (orifice) in it. The orifice plate acts as the primary element of a DP flowmeter. Fluid velocity increases and pressure decreases as a fluid passes through the orifice, which creates a pressure drop. The value of the pressure drop is determined by measuring the pressure before the plate at a high pressure tap and after the plate at a low pressure tap (Figure 3.13). The pressure drop is typically measured with a DP or multi-variable transmitter.

  • BENEFITS AND LIMITATIONS Reliability Industry standards, such as AGA Report No. 3, ISO 5167, and ASME MFC 3M, ensure industry-accepted measurement performance without the need for flow lab calibration. In addition, extensive research and data are available concerning the performance of orifice plates with various process fluids and in various industries. Accuracy Because the discharge coefficient varies over the flow range, the accuracy of an orifice plate varies with the type of measurement device used. Discharge coefficient is a laboratory-determined factor for a DP flow primary element. If only differential pressure is measured, an accurate measurement can be expected over a 3:1 to 5:1 range. With multi-variable measurement, an accuracy of 1% of rate can be achieved over a much wider range (6:1 to 12:1 depending on the application). Compatibility Orifice plates can accommodate virtually all clean fluids, although abrasive or sticky fluids may reduce accuracy and increase maintenance costs because of clogged pressure taps or particulate matter buildup near the orifice plate. Orifice plates are compatible with most pipe sizes. Cost Initially, orifice plates are inexpensive (instrument cost only), but because they require impulse lines, a three-valve manifold, and a pipe stand, the installation cost is high. While orifice plates are relatively easy to maintain, maintenance costs can increase as well because the orifice edge must be clean and sharp for optimal meter performance. In addition, orifice plates may cause a high relative pressure loss that consumes energy and may result in higher costs. In comparison to some velocity flowmeters, orifice plates will provide an output over a very wide operating range.

    Pitot Tube A common pitot tube design for flow measurement consists of a cylindrical probe inserted into the process pipe. The probe is bent at a 90 angle so that it points toward the source of fluid flow, parallel to the pipe wall (Figure 3.14). The velocity of the moving fluid creates a high-impact pressure inside the probe. Using a differential pressure transducer, this impact pressure is measured and compared

  • with the static pressure measured through a port on a surface parallel to the pipe wall (usually on the probe). The differential pressure measured is proportional to the square of the velocity of the fluid. In some pitot tube designs, both impact and static pressure are measured by the same device installed in one pipeline tap.

    Because of its one-point velocity measurement, the accuracy of the pitot tube is easily affected by changes in velocity profile. In order to attain an average measurement, the tube must be moved back and forth in the flowstream. For this reason, pitot tubes are most often used as a simple means for obtaining a rough measurement (e.g., for low- to medium-flow gas applications where high accuracy is not required).

    AVERAGING PITOT TUBE Averaging pitot tubes are also available, with designs that include several measurement ports over the entire diameter of the pipeline (Figure 3.15). The Annubar port design yields a much more accurate flow measurement than the regular pitot tube.

  • BENEFITS AND LIMITATIONS Accuracy Averaging pitot tubes have good long-term accuracy (13%) partially because they have no leading edge to wear. However, dirty fluids can clog the measurement ports and reduce accuracy. Compared to other DP flow primary elements, pitot tubes create a relatively low differential pressure, which can make measurement of the pressure drop difficult and may limit rangeability or turndown. In addition, pitot tubes have a very low permanent pressure loss. Compatibility Averaging pitot tubes are an insertion-type DP flow primary element that can be used in pipe sizes from 272 inches. In larger lines especially, pitot tube installation is convenient and inexpensive. Some averaging pitot tubes can be used for the measurement of fluids flowing in either direction (bidirectional capability), and can be installed in the process pipe without shutting the process down (hot tap).

    Wedge Flow Element Wedge flow elements are inserted in the process pipe to create a wedged obstruction on the inner wall of the pipe. A differential pressure is created as the fluid flows past the obstruction. Wedge flowmeters are usually used with remote seals in applications where plugging of inpulse lines is a concern. When impulse lines are used, heat tracing may be required on the impulse lines to prevent solidification of process fluids such as slurries and other viscous fluids (Figure 3.16).

    BENEFITS AND LIMITATIONS Because the wedge flow element presents no sudden changes in contour and no sharp corners, it can be used for measuring dirty fluids, slurries, and fluids at high viscosities (low Reynolds numbers) that tend to build up on or clog orifice plates and impulse lines.

  • V-Cone The V-cone is a differential pressure type flowmeter with a unique design that conditions the flow prior to measurement. Differential pressure is created by a cone placed in the center of the pipe. The cone is shaped so that it flattens the fluid velocity profile in the pipe, creating a more stable signal across wide flow downturns. Flow rate is calculated by measuring the difference between the pressure upstream of the cone at the meter wall and the pressure downstream of the cone through its center. A V-cone is normally lab-calibrated flowmeter (Figure 3.17). For users who have limited room for straight piping requirements, the v-cone is useful because it works equally well both with short and with long straight pipe requirements. In addition, the V-cone can be used with some dirty processes.

    Venturi Tube A venturi tube is composed of three main sections (Figure 3.18):

    Converging inlet cone: The converging inlet cone gradually decreases the pipe diameter and creates a pressure drop. A high pressure tap is located at the start of the inlet cone.

    Throat: The inlet cone ends at the throat, where the low pressure tap is found. Fluid velocity is neither increasing nor decreasing in the throat.

    Diverging outlet cone: The outlet cone increases in cross-sectional area, which enables the fluid to return to very near its original pressure. The outlet cone also eliminates air pockets and minimizes frictional losses.

  • BENEFITS AND LIMITATIONS Venturi tubes are usually used in applications that require a low pressure drop and high accuracy. Venturi tubes provide very low permanent pressure loss when compared to other DP flowmeters, although they are also larger and more expensive. Venturi tubes work well with short straight piping requirements and, therefore, are useful for users who have limited space for straight piping. Because they present no sudden changes in contour, they can be used for measuring dirty fluids and slurries that tend to build up on or clog orifice plates.

    Flow Nozzle Flow nozzles consist of two main sections (Figure 3.19):

    Elliptical inlet: The flow nozzle is mounted in the pipeline so that the elliptical entrance of the nozzle is facing the source of the fluid flow. Fluid velocity increases as it enters the inlet and pressure decreases.

    Throat: The inlet tapers to a cylindrical throat section, where the low pressure tap is located.

  • BENEFITS AND LIMITATIONS Accuracy Flow nozzles retain long-term precise calibration even under severe conditions because the exact contour of the flow nozzle is not particularly critical for accurate measurement. For this reason, flow nozzles are often used for measurement of steam flow and other high-temperature or high-velocity fluid flows where erosion may be a problem.

    Rotameter Rotameters, also known as variable-area flowmeters, are tapered glass, plastic, or metal tubes that must be mounted vertically (Figure 3.20). A float inside the tube rises in response to the fluid flow rate. Because the tube is tapered, pressure is higher at the bottom, or narrow end, of the tube than at the top. The float rests where the differential pressure between the upper and lower surfaces of the float balances the weight of the float. Depending on the meter design, the flowrate may be read directly from a scale inscribed on the transparent tube or sensed electronically. Rotameters are commonly used for indication onlythat is, they provide only a local indication of flow and do not transmit the measurement readings to another location.

    BENEFITS AND LIMITATIONS Accuracy Rotameters are not as accurate as other flowmeters, although they are highly repeatable. Rotameters must be removed and disassembled in order to change their flow range, by resetting the balance. Unlike with most flowmeters, pressure loss through a rotameter is constant throughout the flow range.

  • Compatibility Rotameters are available in process pipe sizes from 1/46 inches. They can be tailored to specific applications by selection of various flowmeter components. For example, stainless steel rotameters are better for measurement of high-pressure flows than are the glass tube rotameters. Rotameters have a Reynolds number constraint for liquid measurement and cannot be used with abrasive fluids. They have no upstream or downstream straight piping requirements. Maintenance and Cost Rotameters are inexpensive and have a simple design, although they do have moving parts that require some maintenance.

    Velocity Flowmeters Magnetic Flowmeter Magnetic flowmeters, also called magmeters, provide obstructionless flow measurement that is ideal for metering any conductive process fluid. Magmeters consist of two main components:

    Sensor: The sensor generates an electronic signal.

    Flow transmitter: The flow transmitter conditions the signal and sends it to a process control system or computer.

    The operating principle of magmeters is based on Faradays law of induction, which states that a voltage will be induced in a conductor moving through a magnetic field (Figure 3.21):

    E=kBDV The magnitude of the induced voltage (E) is directly proportional to the conductor velocity (V). Magnetic field coils placed on opposite sides of the pipe wall generate a magnetic field (B). As the conductive process fluid moves through the magnetic field, electrodes sense the induced voltage. The distance between electrodes represents the conductor width (D). An insulating liner prevents the signal from shorting to the pipe wall. The output voltage from the magnetic flowmeter sensor is amplified and sent to a magnetic flowmeter transmitter where the signal can be conditioned and sent to a control system.

  • BENEFITS AND LIMITATIONS Accuracy Magnetic flowmeters are not constrained by Reynolds number or flow profiles. They provide high accuracy and rangeability and do not contribute to pressure loss. Voltages measured at the electrodes represent the average fluid velocity in laminar and in turbulent flows.

    Compatibility Magnetic flowmeters provide flow measurement with a signal inherently linear to the average volumetric flow rate regardless of fluid temperature, pressure, density, viscosity, or direction of flow. The only limitation is that the fluid must be electrically conductive and non-magnetic. They are commonly used in applications that contain large particles, including highly corrosive chemicals or fibrous slurries. A variety of construction materials provides compatibility with virtually all process fluids. Magnetic flowmeters do not measure gases or non-conductive fluids, such as hydrocarbons, oils, or gases. Conductivity of a process fluid is expressed in microseimens. Typical magmeters require that process fluid conductivity measure greater than five microseimens/cm. Maintenance and Cost

  • Magnetic flowmeters have no moving parts, and thus require little, if any, maintenance. Occasionally, residues may deposit in the flowtube and coat the electrode, but maintaining a higher fluid velocity would solve this problem. Older, AC-powered magnetic flowmeters require power to be supplied at the flowtube, which consumes more energy than todays pulsed DC-powered flowtubes. The initial price of a magnetic flowmeter can be higher than other flow devices. Typically, an additional set of wires is required for the device to work properlyone set to transmit the signal and one set to power the flowtube coils. The additional wires add to normal installation costs.

    Vortex Flowmeter A vortex flowmeter is a bluff body, or shedder, placed in the fluid flow stream that causes vortices to form (Figure 3.22). The shedder acts as the primary device. As the fluid flows around the shedder, velocity increases and pressure decreases on one side, while velocity decreases and pressure increases on the other side. The alternating forces cause vortices to form that are picked up by the sensing mechanism. The fluid flow rate is obtained from the frequency (detected by the sensor), which is directly proportional to the velocity of the fluid.

    BENEFITS AND LIMITATIONS Accuracy In general, vortex flowmeters are highly accurate, and rangeability is commonly 20:1 to 30:1. If properly sized, you may need to add a reducer to increase velocity at the part where the vortex meter is operational. Vortex meters produce only a small pressure loss in the pipeline. Compatibility

  • Vortex flowmeters are often thought of as good all-purpose flowmeters because they can be used with liquid, gas, or steam without recalibration. Their versatility can help simplify spare parts inventory and maintenance training. Vortex flowmeters have minimum requirements for velocity that vary with the viscosity and density of the measured fluid. In general, they are recommended for measurement of fluids with Reynolds numbers over 10,000 (well-developed turbulent flow). They are not useful at very low flow rates or with fluids with very low Reynolds numbers because of the absence of vortex formation. In addition, vortex meters are not recommended for the measurement of abrasive fluids or fibrous slurries because of the lack of vortex formation and the need to keep the shedder bar free of erosion. Maintenance and Cost Vortex flowmeters are fairly simple and have no moving parts and few connections where leaks may occur. Their installation cost is low, and there is no need for the meter body to be winterized.

    Turbine Flowmeter Turbine flowmeters consist of a section of pipe that contains a multi-blade rotor and a magnetic pickup coil (Figure 3.23). The entire fluid to be measured enters the flowmeter and passes through the rotor, which then turns at a velocity that is proportional to the fluid velocity. The magnetic pickup probe converts the rotor velocity to an output signal that has a frequency proportional to volumetric flow rate. The turbine flowmeter is based on the principle that the speed of a turbine that is driven by a flowing fluid is proportional to the velocity of the fluid.

    BENEFITS AND LIMITATIONS

  • Accuracy Turbine meters are high-accuracy (0.150.5% of rate), highrangeability (up to 20:1) meters, although sudden flow surges could cause large calibration shifts or damage the flowmeter. Compatibility Turbine meters have several applications: clean liquid and gas, custody transfer, and lubricating fluids operating at Reynolds numbers in excess of 4,00020,000. They are compatible with pipe sizes >1/4 inch with flow ranges of 0.0650,000 gpm on liquids. The fluid momentum must be sufficient to operate the rotor. Maintenance and Cost Turbine meters have moving parts and thus require high maintenance and have a high ownership cost. One meter cannot be used for both liquid and gas applications. Turbine meters create a high pressure loss.

    Ultrasonic Flowmeter Ultrasonic flowmeters determine flow by measuring the velocity of sound as it passes through a fluid flowing through a pipe. Pulses from a piezoelectric transducer travel through a moving fluid at the speed of sound and provide an indication of fluid velocity. Two different methods are currently used to make this velocity measurement:

    Time-of-flight

    Doppler effect

    TIME-OF-FLIGHT ULTRASONIC FLOWMETER Time-of-flight ultrasonic flowmeters operate on the principle that the speed of an ultrasonic sound wave (sound at a frequency too high to be heard by the human ear) will increase when directed with flow and decrease when directed against flow. A simple analogy is that an airplane can travel faster when moving in the direction of the prevailing current than it can when moving against the current. Two or more transducers, acting as primary elements, are used in transit time flowmeters. The transducers are positioned so that a signal sent between them will travel at an angle to the flowstream (Figure 3.24). An ultrasonic signal is sent from the upstream transducer to the downstream transducer and then back again. As the signal crosses the flowstream to the downstream transducer, its velocity increases. As the signal travels back, its velocity decreases. The difference in time taken for signals to move upstream and downstream is a direct measure of fluid velocity and the basis for a volumetric flow rate measurement.

  • DOPPLER EFFECT ULTRASONIC FLOWMETER The Doppler effect refers to the change in the frequency of sound waves. The frequency increases or decreases based on the velocity of the fluid. A common example of the Doppler effect is the change in the pitch of a train whistle as the train passes at high speed. Doppler effect flowmeters direct an ultrasonic beam of known frequency into the pipeline, usually at an angle (Figure 3.25). Moving solids, bubbles, or particles in the flow stream reflect the ultrasonic beam back to a receiver. Because these particles are moving, the frequency of the reflected beam is shifted away from the original frequency of the transmitted beam. Flowmeter electronics detect the shift in frequency, calculate fluid velocity, and use the velocity measurement as the basis for a volumetric flow rate calculation.

  • BENEFITS AND LIMITATIONS Design Options Ultrasonic flowmeters are available in a variety of styles and configurations. In wetted designs, the transducers are exposed to the process fluid. Wetted designs may be more sensitive than noninvasive meters because the ultrasonic signal does not have to pass through a pipe wall. Noninvasive flowmeters use strap-on or bolt-on transducers that mount on existing piping.

    Accuracy Newer ultrasonic flowmeters with a multipath, microprocessor-based, spool design are achieving good results in gas measurements and are gaining wider acceptance. The clamp-on design meters have had mixed success. Proper installation of ultrasonic flowmeters is essential for optimal performance. Accuracy depends on the design of the flowmeter, and is typically as follows:

    Multipath0.5% Wetted1 to 2% Clamp on3 to 10%

    Ultrasonic flowmeters have a rangeability of 20:150:1, and because they present no obstruction to flow, there is no pressure loss in the pipeline. Compatibility Ultrasonic flowmeters can measure liquids or gases, depending on the flowmeter design. Fluids measured must be above a certain Reynolds number as well. The noninvasive designs can be used for the measurement of sterile, corrosive,

  • erosive, or otherwise difficult or hostile fluids. Ultrasonic flowmeters are bidirectional and can be used in applications where flow direction is reversed. Maintenance and Cost The noninvasive flowmeter design provides low maintenance, easy replacement, and cost economy for large pipelines, although it may be more expensive than other flowmeters in smaller pipes. Because it is mounted on the outside of the pipe, the noninvasive design requires no piping modifications (e.g., holes cut into the pipe). However, there are long upstream and downstream straight piping requirements. The ultrasonic flowmeter has a four-wire operation, which can also affect cost and maintenance.

    Mass Flowmeters

    Coriolis Mass Flowmeter Coriolis mass flowmeters use a curved tube as a sensor and apply Newtons Second Law of Motion to determine flow rate. An electromagnetic drive coil is located in the center of the bend of the tube that causes the tube to vibrate like a tuning fork (Figure 3.26).

    When fluid moves through the sensors tubes, it is forced to take on the vertical momentum of the vibrating tube. When the tube moves upward during the first half of its vibration cycle, the fluid flowing into the sensor tube resists moving upward and pushes down on the tube. The fluid has the tubes upward momentum as it travels around the bend and out of the tube the fluid resists having its vertical motion decreased by pushing up on the tube. The opposing forces of the fluid and the vibrating tube causes the tube to twist. The twisting is referred to as the Coriolis effect (Figure 3.27). According to Newtons Second Law of Motion, the angle of the

  • twist is directly proportional to the mass flow rate of the fluid flowing through the tube.

    BENEFITS AND LIMITATIONS Accuracy Coriolis mass flowmeters provide extremely accurate mass measurements that are independent of such fluid properties as temperature, pressure, viscosity, and solid content, which eliminates the need for pressure and temperature compensation. Their high accuracy makes Coriolis meters popular in many custody transfer applications and other applications that require tight control, such as chemical processes and management of precious or expensive fluids. Once Coriolis meters are factory calibrated, they can be used in a variety of services without recalibration. One of the chief limitations of Coriolis meters is an inherent susceptibility to system noiseboth hydraulic and mechanical. Because the sensors measure vibration of the tubes, any other influences that cause the tubes to vibrate may introduce error. Installation according to manufacturers guidelines, tranquilizers in the process pipe, and electronic filtering can help minimize these problems. However, conditions such as water hammer and other large disturbances must be avoided. Compatibility Coriolis meters are available in pipe sizes up to six inches. Coriolis meters are noninvasive and can be used with sterile or difficult fluids and virtually any liquid or gas flowing with sufficient mass flow rate to operate the meter. The density of low-pressure gases is usually too low to accurately operate the flowmeter. Coriolis meters have no Reynolds number constraints. One device provides multiple process variables, such as density, temperature, mass, and volumetric flow rate. Maintenance and Cost

  • Coriolis meters require four-wire operation and have a very high initial cost. They are a low-maintenance meter and have no straight piping requirements. Coriolis meters introduce a high pressure drop.

    Thermal Mass Flowmeter In thermal mass flowmeters, part of the fluid flows through a bypass, or shunt, sensor tube. The process fluid is heated at the midpoint of the sensor tube (Figure 3.28). The flowmeter measures the temperature of the fluid at resistance temperature detectors (RTDs) located upstream and downstream of the meter. The temperature variation (change in temperature between the two points upstream and downstream of the meter) is inversely proportional to mass flow rate.

    BENEFITS AND LIMITATIONS Accuracy In general, thermal mass flowmeters provide good measurement accuracy: 0.5% of full scale for liquids and 1.0% of full scale for gases. However, accuracy varies with changes in fluid specific heat. With a constant specific heat, thermal mass meters provide mass flow measurement without pressure or temperature correction. They also have a widerangeability (50:1100:1) and high repeatability (0.2% of rate). Thermal meters cause a significant pressure drop.

    Compatibility

  • Thermal mass flowmeters are widely available as pressure and flow controllers. They can measure fluids with pressures up to 5,800 psi (liquids) and 1,500 psi (gases). However, thermal meters are generally limited to fluids with viscosities under 200 cp. In addition, thermal meters have a temperature maximum of 150 F (66 C).

    Positive Displacement Flowmeters An oval gear meter is an example of a positive displacement meter. Oval gear meters consist of two touching oval gears that rotate as fluid flows through them. The gears trap a known quantity of fluid as they rotate (Figure 3.29). Each complete revolution of both gears represents the passage of four times (4) the amount of fluid that fills the space between the gear and the meter body. Therefore, the volumetric flow rate is directly proportional to the rotational velocity of the gears.

    BENEFITS AND LIMITATIONS Accuracy Positive displacement flowmeters can provide high accuracy (0.250.5% of rate) and repeatability in many applications, although errors can be introduced by slip (leakage) around the gears. Compatibility Positive displacement flowmeters are mechanical devices with many moving parts that are prone to wear and cannot be used with dirty or gritty fluids. To eliminate wear from excessive friction, the process fluid should have good lubricating properties. Positive displacement flowmeters are recommended for use with high-viscosity fluids, which tend to seal small clearances and reduce slip. However, if a fluid residue coats the inner chambers of the meter, the fluid volume is reduced, thus producing an error.

  • Maintenance and Cost Positive displacement flowmeters have a moderate amount of maintenance because of their moving parts, but have no upstream or downstream straight piping requirements. Because positive displacement meters are self-powered, they extract some energy from the fluid stream, which may result in a high pressure loss.