Flow

FLOW INSTRUMENTATIONS

IntroductionFlow of a fluid can be expressed in terms of its velocity, its instantaneous volumetric or mass flow rate in terms of the total quantity passed (either volume or mass) in a given time.It should be noted that liquid flow measurement is affected by Temperature, Turbulence, Transitional and Laminar Flow profiles. These areas of study are quite detailed and should be understood for a full understanding of flow measurement.

Velocity

Flow inferred from velocityFluids do not move at the same velocity at all points across a pipe - generally they move faster in the centre and slower near the walls. Thus the velocity of the fluid can be either its velocity at a given point (e.g. at the centre of the pipe etc.) or its average velocity. The SI unit for both point and average flow velocity measurement is meters/second. (m/s)

INFERRED MEASUREMENTIn an inferred measurement, flow rate is not measured directly. Instead, some other variable, in this instance, velocity is measured and then translated into a flow rate based on the cross sectional area of the pipeline.

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, i.e. 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:Volumetric Flow Rate = Average Velocity x Cross sectional Area of PipeThus Q= AXVWhere:Q=Volumetric flow rate (m/s) v = Average velocity (m/s) A = Cross sectional area of pipe (m) Non metric units such as Imperial and US gallons, barrels, cubic feet etc. may still be encountered

LIMITATIONS OF VOLUMETRIC MEASUREMENTThere 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 measurements.

Mass Flow Rate

When very precise flow rate measurements are required, mass flow meters 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.Mass Flow Rate = Volumetric Flow Rate x Density of the FluidThus W = Q X PWhere:W= Mass flow rate (kg/s) Q = Volumetric flow rate (m/s) P = Density of fluid (kg/m)

Total Quantity (Volumetric)

The total quantity expressed in terms of volume passed in a given time.Total Quantity = Mass Flow Rate x TimeThus V = Q X tWhere:V = Total quantity (m) Q = Volumetric flow rate (m/s) t = Time (s)

Total Quantity (Mass)

The total quantity expressed in terms of mass passed in a given time.Total Quantity = Mass Flow Rate x TimeThus Q = W X t Where:Q = Total quantity(m) W = Mass volumetric flow rate (m/s) t = Time (s)

Why Measure Flow?CUSTODY TRANSFERCustody transferthe measurement of fluid passing from a supplier to a customeris one of the most important flow measurement applications. In custody transfer applications, flow meters are essentially the cash register of the system. For example, a flow meter at gas station measures how much gas pump into your vehicle and bills you accordingly. Given the economic implications, custody transfer applications require high measurement accuracy.

PRODUCT CONSISTENCYAccurate flow measurement ensures product consistency.Flow is used as an input to process control systems so that the product produced is the same. As a consumer, you expect the processed food you eat or gasoline you used in your car to be the same each and every time you purchase these products.

EFFICIENCYPrecise 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 burner's ability to burn fuel. The amount of unburned fuel and excess air in the exhaust are used to assess a burner's 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 CONTROLFlow 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.

SAFETYRegulation 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.

Fluid PropertiesThe 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:DensityViscosityFluid typeFlow profile

DENSITYDensity (), one of the most commonly used measure, 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 / volumeThe density of the process fluid is important to flowmeter selection and performance.

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. The higher a fluids viscosity, the greater the force required to shear the fluid and the slower the resultant 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 centipoises (cp).

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.

FLUID TYPEA 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:1.Clean fluidA fluid that is free from solid particles (e.g., water)2.Dirty fluidA fluid containing solid particles (e.g., muddy water)3.SlurryA liquid with a suspension of fine solids that can flow freely through a pipe (e.g., pulp and paper, oatmeal)4.SteamThe type of fluids to be measured can give an indication of the type of flowmeter that may work best for that particular application.

FLOW PROFILEchange profiles several times point in time, a fluid will have one of the following three flow profiles:1.LaminarTurbulentTransitionLaminar:In laminar flow, fluid flows in smooth, uniformed layers. As a result, there is a 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.

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.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 densityv = Fluid velocityD = Pipe inside diameter = Fluid viscosityBecause the Reynolds number expresses the characteristics of a flow stream, it is useful when determining whether a particular flowmeter is appropriate for an application. TheReynolds number is especially helpful in predicting the flow profile:LaminarRD 4,000Some flowmeters have Reynolds number restrictions on the accuracy of measurement.

Pipe geometry (design) and conditionsPipe geometry (design) and conditions are the third key component in flow equations. Pipe geometry can cause changes in 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.

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 (constant).

FLOW PROFILE DISTURBANCESA 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:In a symmetrical flow profile disturbance, the velocity profile of the fluid remains symmetrical about the process pipe axis.Asymmetrical profile disturbance:In an asymmetrical profile, the velocity profile is not symmetrical about the process pipe axis.Swirl:Swirl occurs when the velocity profile of a fluid moves in a circular motion as it flows forward.

Asymmetrical Swirl Caused by Two 90 Elbows in Different PlanesFlow ConditionersEliminating the Effects of Flow Profile Disturbances:Some flowmeters are more sensitive to flow irregularities than other flowmetersmost 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.

Classes of Flow metersFlowmeters are grouped into four classes:DP flowmetersVelocity flowmetersMass flowmetersPositive displacement flowmeters (also called volumetric flowmeters)

DP FlowmetersDP 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 a primary element 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 rate, a decrease in pipe diameter (A) results in an increase in fluid velocity (v):v1A1=v2A2

Bernoullis equation says 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 Bernoulli's 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 (a) 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:

1.Orifice plate2.Pitot tube3.Flow nozzle4.Venturi tube5.Wedge6.Rotameter

Energy and Flow Equation of FluidsThe total energy of fluid in a flow system is comprised of three components: potential energy, kinetic energy and pressure energy. When described in terms of meters head of the flowing fluid, we must consider: Total Energy = Potential + Kinetic + PressureWhich includes? Z = Elevation of the center line of the pipe (m) V = Velocity of the fluid (m/sec) g = Acceleration due to gravity (9.8 m/sec2) P = Static Pressure (N/M2) = Weight density of fluid (N/M3)Flow quantity inside a pipe is given as the product of the velocity of the fluid and the cross-sectional area of the pipe, that is: Q = V . A Where Q = Flow rate (m3/sec) V = Velocity (m/sec) A = Cross-sectional area (m2)

Now consider the flow in a pipe with a restriction as shown in Figure 1.Flow in A Pipe With a RestrictionIf the flow is steady, then the same quantity of fluid must pass through the two different sections of the pipe work in a given time. Section 2 has a smaller cross-sectional area than Section 1, therefore, the fluid must travel faster in Section 2than in Section 1. Relating the flows for these two cross sectional areas: Q = A1V1 = A2V2 And when A1 > A2, then V1 < V2

Because of the Principle of Conservation of Energy, an increase in velocity in Section 2, which causes an increase in kinetic energy, must be compensated for by a corresponding decrease in potential or pressure energy.We can write an equalization equation to approximate the change in the potential energy (mgh) and the change in kinetic energy (1/2 mv2) resulting from this velocity change in the flowing fluid.

Change in Potential Energy = Change in Kinetic EnergyMgh = (1/2) MvCanceling M from both sides and solving for V;.we get;V = KhSo the flowing velocity will be proportional to the square root of the differential pressure sensed across the flow restriction.Q = AV = AKhWhere:Q = flow quantityA= flow restriction areaK = flow constanth= differential pressure measured across the restriction

The flow rate is proportional to the square root of the differential pressure developed across a flow restriction.This is the principle behind flow metering the flow can be calculated if we measure the differential pressure across a defined flow restrictionIn order to obtain a linear flow signal, we must always take the square root of the measured differential pressure

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.The pressure drop is typically measured with a DP or multivariable 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.

AccuracyBecause 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 multivariable measurement, the variations in discharge coefficient are compensated for and an accuracy of 1% of rate can be achieved over a much wider range (6:1 to 12:1 depending on the application).CompatibilityOrifice 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.

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.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 flow stream. 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 TUBEAveraging pitot tubes are also available, with designs that include several measurement ports over the entire diameter of the pipeline. The Annubar port design yields a much more accurate flow measurement than the regular Pitot tube.

BENEFITS AND LIMITATIONSAccuracyAveraging 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 range ability or turndown. In addition, pitot tubes have a very low permanent pressure loss.CompatibilityAveraging pitot tubes are an insertion-type DP flow primary elements that can be used in pipe sizes from 2 - 72 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 lagging is a concern. When impulse lines are used instead of remote seals, lagging is required on the impulse lines to prevent solidification of process fluids such as slurries and other viscous fluids.

BENEFITS AND LIMITATIONSBecause 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.

Venturi Tube:A venturi tube is composed of three main sections.Converging inlet conethe 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.Throatthe 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 conethe 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 LIMITATIONSVenturi 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 locations having 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:

Elliptical inletthe 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.Throatthe inlet tapers to a cylindrical throat section, where the low pressure tap is located.

BENEFITS AND LIMITATIONSAccuracyFlow nozzles retain long-term precise calibration even under hostile 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.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 flow rate 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 LIMITATIONSRotameters 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 re-setting the balance. Unlike with most flowmeters, pressure loss through a Rotameter is constant throughout the flow range.Rotameters are inexpensive and have a simple design, although they do have moving parts that require some maintenance.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.

Velocity FlowmetersVortex flowmeterTurbine flowmeterOnly above mentioned velocity flow meters are used in MCR

Vortex Flowmeter:A vortex flowmeter is a bluff body, or shedder, placed in the fluid flow stream that causes vortices or small eddies to form. 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.

Turbine Flowmeter:Turbine flowmeters consist of a section of pipe that contains a multi blade rotor and a magnetic pickup coil.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.

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