Measurement and Control--FLOW MEASUREMENT

Overview

Flow measurement is a key parameter used by plants for reading values (including accounting needs) and for controlling processes. Of the typical process measurements-flow, level, temperature, and pressure-flow tends to be the most difficult and, therefore, the one for which incorrect devices are most likely to be selected.

The technology of flow measurement has evolved to the point where highly accurate and reliable devices are now on the market. Moreover, new measurement principles are being introduced, and existing principles are continuously improved upon. As a starting point, it should be mentioned that no single flowmeter can cover all flow measurement applications. For that reason, flowmeter selection has been referred to as both a science and an art. This section provides some of the basic knowledge plant personnel need in order to select the correct flowmeter. It is essential that the person selecting the instrument considers the actual users' experiences.

Classification

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 deter mine the meter's 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, tar get, 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 can also be classified into four types:

1. Volumetric, such as positive displacement meters. They measure volume directly.

2. Velocity, such as magnetic, turbine, and ultrasonic meters. These meters determine total flow by multiplying the velocity by the area through which the fluid flows.

3. Inferential, such as differential pressure (dp), target, and variable-area meters. These meters infer the flow by some other physical property such as differential pressure and then experimentally correlate it to flow.

4. Mass, such as Coriolis mass flowmeters. These devices measure mass directly.

Measurement

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 = A x V

where A is the cross-sectional area of the pipe, and V is the average fluid velocity. Therefore, the mass flow may then be defined as volumetric flow × density.

Typically, measurements rely on empirical formulas and on test results. Therefore, the plant considering the specific application of any flowmeter should consider the limitations and test conditions under which certain meters are sold. For example, as temperature changes, the density of a fluid will change as well. That, in turn, may affect the accuracy of the reading unless compensation is implemented.

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 59°F (or 15°C) 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.

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.

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".

General Application Notes:

Depending on which type of flowmeter is selected, many parameters need to be considered when applying flowmeters. Ignoring such parameters will result in a measurement with a high error or one with a short life span. In addition to the requirements common to most measurements-such as process conditions, measuring range, and accuracy-flow measurement also requires a closer look at the following:

• The type of fluid and whether it is dirty or clean

• The velocity profile

• The piping considerations

• The line size

Type of Fluid

The type of fluid may limit the type of flowmeter device available for the application. For example:

• On magnetic meters, severe service for conductive fluids can be measured, where orifice plates or vortex shedders are not suitable.

• On most turbine meters, steam cannot be measured.

• On vortex meters and differential-pressure devices, liquid, gas, and steam can be measured.

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-pres sure 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.

Velocity Profile

The velocity profile has a major effect on the accuracy and performance of most flowmeters.

The shape of the velocity profile inside a pipe depends on the following:

• The momentum or inertial forces of the fluid, which tend to move the fluid through the pipe

• The viscous forces of the fluid, which tend to slow the fluid down as it passes near the pipe walls

Therefore, bends in the pipe, restriction in the lines, and roughness of the pipe walls affect the shape of the flow profile and the speed of recovery from flow disturbances. Flow profiles can be classified into three types: laminar, turbulent, and transitional (see figures 4-1 and 4-2). In laminar flow, the viscous forces cause the fluid to slow down as it passes near the pipe walls. The flow profile is close to parabolic, with more flow traveling at the center of the pipe than at the pipe walls where the flow is slowed. In turbulent flow, the effect of inertial forces is much larger than the effect of the viscous forces, so the effect of pipe wall is reduced. The flow profile is therefore more uniform than laminar flow. However, the fluid layer next to the pipe wall remains laminar. The transitional flow profile is between the laminar and the turbulent flow profiles. Its behavior tends to be difficult to predict and may oscillate between the laminar and turbulent flow profiles.

The flow profile is affected by four factors whose relationship with each other is called the Reynolds number. The Reynolds number (Rd), a dimensionless quantity that indicates the conditions of flow in a given pipe, considers the combined effect of velocity, density, and viscosity. However, the Rd does not take into account the roughness of the pipe wall, which may affect the velocity distribution and applies only to Newtonian fluids. In such fluids, the viscosity is independent from the rate of shear. The Reynolds number is given as follows:

Rd = diameter of pipe average flow velocity x density of fluid / absolute viscosity of fluid

The flow is considered laminar if Rd < 2000. If Rd reaches 4000, it starts becoming turbulent, and by 10,000 the flow profile should by well established as turbulent. The range between 2000 and 10,000 is an unstable and complex condition that is affected by many parameters (such as whether the velocity is increasing or decreasing).

Piping Considerations

Flowmeter performance is normally stated in terms of ideal reference conditions. Variances in the inside diameter of piping and in the upstream and downstream runs-including restrictions, valves, solid buildup, and misaligned gaskets-affect flowmeter performance. There fore, on flow-measuring loops the control valve is typically located downstream of the measuring element. This is to avoid the disturbances to the flow stream caused by the throttling action of the valve, which affects the accuracy of the measuring element.

===

FIG. 1 Flow profiles. TURBULENT FLOW; FLOW; LAMINAR FLOW; FLOW; TRANSITION FLOW; FLOW

===

FIG. 2 Velocity profiles. LAMINAR FLOW; TURBULENT FLOW

===

In addition to the resistance of the line itself, many flow-measuring devices drop some of the line pressure as well. In some cases, this is not desirable, and the amount of pressure drop by the flowmeter is an important consideration when selecting meters. For example, the pressure drop for differential-pressure devices varies from low to moderate. In pitot tubes, pressure drop is low in comparison to other types. Elbow taps have no mentionable pressure loss, and on magnetic flowmeters there is no pressure loss.

The configuration of the piping must take into account the fact that most liquid-measuring flowmeters should remain filled with liquid in order to provide accurate measurement, since gas or vapors will adversely affect performance. Care should be taken if the pipe is drained when the pump is turned off, since restarting the pump may produce sufficient momentum to damage the flowmeter and sometimes the pipe itself.

Most applications require a method for pipe pressure testing called hydrotesting. When this is the case, the flowmeter components must be rated for very high pressures. If they are not, they should either be isolated or removed from the line to avoid damage.

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 flow meters, 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.

Line Size

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).

Measuring 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 guide since in many cases they fall under the responsibility of mechanical engineering activities. Radioactive measuring devices are covered in section 5 (on level measurement).

Comparison Table

TBL. 1 summarizes the main types of flow measurement with respect to a set of common parameters. This comparison table can be used as a guide for selecting flowmeters. The information presented in TBL. 1 indicates typical values; some vendors may have equipment that exceeds the limits shown. For environmental reasons, flowmeters that contain mercury are generally avoided.

Differential Pressure: General Information

The four most common types of differential pressure (dp) flowmeters are the following:

• Orifice plate, segmental orifice plate, and integral orifice plate

• Venturi tube and flow nozzle

• Elbow

• Pitot tube

Typically, a dp flowmeter consists of a primary element (e.g., an orifice plate) and a secondary element (e.g., a dp transmitter). The secondary element measures the differential head produced by the primary element.

Flow rate = constant x __/(differential pressure / density)

Therefore, a square root extracting function is typically required.

Differential-pressure flowmeters have many advantages. They are simple to use, offer low cost (especially orifice plates), have no moving parts, are sturdy, and are available in a wide selection of ranges and models. However, they tend to have low accuracy (which is easily affected by wear on the primary element), some have a high permanent pressure loss, their flow range is generally limited to 4:1 at best, and the impulse lines may block or freeze. Where orifice plates erode, a more expensive solution is required, such as venturis or flow nozzles. Differential pressure flow measurement should occur under turbulent flow-that is, where Rd is > 10,000.

Where possible, the secondary element should be mounted above the primary element for gas measurement (to ensure that condensables do not influence the dp) and below the primary element for liquid, condensables, and steam (to ensure that vapors and gas bubbles flow back to the process). The impulse lines are typically sloped 1:10. Where condensation occurs in the measuring element on a steam line (or wet gas or vapor), condensate chambers are fitted to the impulse points, with both chambers at the same level. On condensables and steam, 1-1/2 in. tees generally provide sufficient capacity as condensate pots. Impulse lines in such cases may need to be insulated and/or heat traced.

For gases, tap connections are generally installed vertically (i.e., from the top of the pipe) or horizontally (i.e., from the side of the pipe). Tap locations are generally installed horizontally (i.e., from the side of the pipe) for steam and liquids to prevent the settling of dirt and sediments in the impulse lines. This approach minimizes the erroneous effects of liquid droplets in gas lines and of gas bubbles in liquid lines. Bottom connections are generally avoided.

Differential-pressure transmitters are typically equipped with three valve manifolds, which are sometimes integral to the transmitters. The integral manifolds are of unitized construction and when compared to part-assembled units, they provide fewer leak points, reduced material and labor costs (especially when supplied with the transmitter), and require less physical space. On toxic and hazardous fluids, a five-valve manifold with drain or vent legs to a safe location is frequently provided, and the impulse lines are flanged or welded, instead of threaded. Refer to section 5 for further information on valve manifolds.

Differential Pressure: Orifice Plate

Principle of Measurement:

This primary element (see FIG. 3), often called a square-edged orifice plate, consists of a flat piece of metal in which a sized hole has been bored (concentric or eccentric). Fluid flow creates a differential pressure across the plate. The square root of the dp is proportional to flow.

A common value used in orifice plate measurement is the beta ratio. This ratio is equal to the inner diameter of the orifice divided by the inner diameter of the pipe. Typically, the beta ratio should be within 0.3 to 0.7 and the dp between 25 and 200" WC (600 and 5000 mmWC). How ever, preferably the beta ratio will be between 0.4 to 0.6 with a dp between 70 and 170"WC (1800 and 4300 mmWC). Ideally, a designer will work around a beta ratio of 0.5 and a dp of 100"WC (2500 mmWC).

The most common pressure taps are flange taps and vena contracta taps. Flange taps are located 1" (25mm) upstream and 1" (25mm) downstream of the orifice plate. They are the most commonly used type of pressure taps in North America, particularly on lines 8" (200 mm) and smaller. They are compact and have been researched extensively, so application data is well documented. Flange taps introduce no disturbance to the piping, have symmetrical locations (and thus can accommodate reverse flow), and offer performance comparable to vena contracta taps.

Vena contracta taps are located 1 diameter upstream and at the vena contracta (point of mini mum pressure) downstream of the orifice plate. They provide the best measurement for lines 10" (250mm) and larger, are commonly used for steam service, and provide the best dp. How ever, it should be kept in mind that the position of the vena contracta is not fixed but varies with flow rate.

Other less commonly used tap locations are radius taps (Up = D, Down = 1/2 D), corner taps (Up at plate, Down at plate), and pipe taps, also known as pressure taps (Up = 2 1/2 D, Down = 8 D).

===

TBL. 1 Flow measurement comparison

Notes:

1. Liquid with vapor or gas.

2. Reynolds number (Rd) is a dimensionless quantity that indicates the conditions of flow in a given pipe (see Flow Profiles in the Introduction section). This number has been developed for Newtonian fluids. A Newtonian fluid has a constant ratio of: shear stress/shear rate. If this ratio is not constant it is considered a non-Newtonian fluid. In most cases, non-Newtonian fluids are fluids in the laminar flow region. Flow measurement data for non-Newtonian fluids is almost non-existent, therefore in such cases, flow measuring devices not dependent on Rd corrections should be used, such as magnetic meters (since the output of a mag meter is basically the average of the flow profile).

3. Where viscosity varies with the rate of shear.

4. Upstream and downstream pipe diameters.

5. Accuracy is measured in% of flow rate or in% of full scale;% of flow rate, measures low flow with the same accuracy as high flow.% of full scale has different measurement accuracies, e.g., a ± 1% FS error = a ± 5% error at 20% flow rate.

6. See section 1 for a definition of repeatability and accuracy.

7. S = sometimes, i.e., it is not a clear yes or no, and is suitable only under certain conditions. Refer to vendors.

8. For diameters less than or equal to 1" (25 mm), use integral orifice plate.

9. This Rd can sometimes reach up to 500,000. However, for orifices with conical entrance, the minimum Reynolds number may be less than 5000.

10. Depends on the capabilities of the secondary element - but generally not recommended.

11. Depending on pressure losses.

12. OK to use on low concentration of the gas/vapor phase

13. Velocity range should be about 0.3 to 10 m/sec (more typically around 2 to 4 m/sec), for abrasive fluids velocity, the velocity should be less than 3 m/sec to minimize damage to the liner.

14. For a higher accuracy a 10 up, 5 down may be required.

15. Some units can reach a 100:1 rangeability

===

FIG. 3 Orifice plate.

===

Application Notes:

Orifice plates have many advantages. They are easy to install, one dp transmitter will apply for any pipe size, and many materials are available to meet process requirements. Type 316 stainless steel is the most common material used in orifice plates unless the process conditions require material of higher quality. Orifice plates have no moving parts, have been researched extensively, and their application data has been well documented.

However, orifice plates also have disadvantages. The process fluid is in the impulse line, meaning there is the potential for freezing and plugging (unless chemical seals are used). Their accuracy is affected by changes in density, viscosity, and temperature, and they require frequent calibration.

The orifice plate typically has a drain hole located at the bottom for steam and gas applications (to drain condensables) and a vent hole on the top for liquid applications (to let gas bubbles through).

Differential Pressure: Segmental Orifice Plate

Principle of Measurement:

The segmental orifice plate (see FIG. 4) is the same as a square-edged orifice plate except that the hole is bored tangentially to a concentric circle whose diameter is equal to 98 percent that of the pipe's inside diameter.

===

FIG. 4 Segmental Orifice Plate

ORIFICE PLATE INFORMATION ENGRAVED ON HANDLE SEGMENTAL ORIFICE PLATE ECCENTRIC ORIFICE PLATE

===

Application Notes:

The segmental orifice plate is less subject to wear than the square-edged orifice plate. How ever, it is good for low flows only. For slurry applications where dp devices are required, segmental orifice plates provide satisfactory measurement. During installation, care must be taken that no portion of the gasket or flange covers the hole.

Differential Pressure: Integral Orifice Plate

Principle of Measurement:

The integral orifice plate is identical to a square-edged orifice plate installation except that the plate, flanges, and dp transmitter are supplied as one unit.

Application Notes:

The integral orifice plate is used for small lines (typically under 2" [50mm]) and is relatively inexpensive to install since it is part of the transmitter.

Differential Pressure: Venturi Tube

Principle of Measurement:

The venturi tube (see FIG. 5) consists of a section of pipe with a conical entrance (typically 20 degrees), a short straight throat, and a conical outlet (typically, a 5- to 6-degree recovery cone). The velocity increases and the pressure drops at the throat. The dp is measured between the input (upstream of the conical entrance) and the throat.

Application Notes:

The venturi tube will handle low-pressure applications and will measure 25 to 50 percent more flow than a comparable orifice plate. It is less affected by wear and corrosion than the orifice plate and is generally suited for measurement in very large water pipes and very large air/gas ducts. Venturi tubes provide better performance than the orifice plate when there are solids in suspension. However, it is the most expensive of dp meters, it is big and heavy in its larger sizes, and its length is considerable.

FIG. 5 Venturi tube.

Differential Pressure: Flow Nozzle

Principle of Measurement:

The flow nozzle (see FIG. 6) is similar to the venturi tube except that there is no recovery cone.

Application Notes:

Flow nozzles are commonly used for steam. They are economical, will handle high flow measurement with low dp loss, and permit approximately 60 percent greater capacity than comparable plates. Pressure taps for flow nozzles are located one pipe diameter upstream and one half pipe diameter downstream from the inlet face of the nozzle.

===

FIG. 6 Flow nozzle. PRESSURE CURVE % PRESSURE

===

Differential Pressure: Elbow

Principle of Measurement:

When liquid travels in an elbow, a centrifugal force is exerted on the outer edge (relative to fluid velocity). Pressure taps (to measure the dp) are located on the outside and inside of the elbow at 45 degrees. Flow can be expressed as follows:

Flow = constant ×

Where:

R = elbow's center-line radius, H = dp, D = elbow/pipe diameter.

RHD3 Density

===

FIG. 7 Elbow.

===

Application Notes:

The advantages of the elbow dp meter (sometimes called a "centrifugal meter") are its low cost, its ease of installation (it can sometimes be mounted in an existing 90-degree elbow), its suitability for measurement in very large water pipes, and its ability to measure flow bi-directionally. Such meters can be used where a rough indication of flow is required. Otherwise, they should be individually calibrated with the fluid to be measured. The disadvantages of the elbow dp meter are that it must be calibrated with the working fluid for accuracy, and it is not recommended for low-velocity streams since it will not generate sufficient dp (e.g., water at 5 ft/sec will generate only 10"WC dp).

Differential Pressure: Pitot Tube

Principle of Measurement:

In a pitot tube (see FIG. 8), which is also called an insertion dp meter, a probe consisting of two parts senses two pressures: impact (dynamic) and static. The impact pressure is sensed by one impact tube bent toward the flow (dynamic head). The averaging-type pitot tube has four or more pressure taps located at mathematically defined locations to measure the dynamic pressure. The static pressure is sensed through a small hole on the side (static head).

The non-averaging type of pitot tube is extremely sensitive to abnormal velocity distribution profiles because it does not sample the full stream. The averaging type corrects this.

Application Notes:

Pitot tubes are easy and quick to install, especially in existing facilities. They can be inserted and removed from the process without shutting down (by using hot taps). They are also simple in design and construction, and they produce energy savings when compared to equivalent orifice plates (due to low permanent pressure loss).

===

FIG. 8 Averaging pitot tube. HI LO DIFFERENTIAL PRESSURE TRANSMITTER FLOW

===

Generally, pitot tubes are suited for making measurements in large water pipes and large air/ gas ducts (6" [150mm] and larger). The disadvantages of pitot tubes are their low differential pressure for a given flow rate and their tendency to plug unless provision is made for purging or flushing.

Magnetic

Principle of Measurement:

The magnetic flowmeter is a volumetric device used for electrically conductive liquids and slurries.

The magnetic flowmeter's design (see FIG. 9) is based on Faraday's law of magnetic induction. Faraday's law states that the voltage induced across a conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor. That is, if a wire is moving perpendicular to its length through a magnetic field, it will generate an electrical potential between its two ends. Based on this principle, the magnetic flowmeter generates a magnetic field that is perpendicular to the flow stream and measures the voltage produced by the fluid passing through the meter as detected by the electrodes. The voltage produced by the magnetic flowmeter is proportional to the average velocity of the volumetric flow rate of the conductive fluid.

The magnetic flowmeter's tube is constructed of non-magnetic material (to allow magnetic field penetration) such as stainless steel and is lined with a suitable material to prevent short circuiting the voltage generated between the electrodes. The tube is also used to support the coils and transmitter assembly. Generally, the electrodes are made of stainless steel, but other materials are also available (choose with care to avoid corrosion). Dirty liquids may foul the electrodes, and cleaning methods such as ultrasonic may be required.

Application Notes:

The magnetic flowmeter has many advantages. Theoretically, it can measure flow down to zero, but in reality its operating velocity should not be less than 3 ft/s (1 m/s). A velocity of 6 to 9 ft/s (2 to 3 m/s) is preferred to minimize coating. It should be noted that accelerated liner wear can result at velocities greater than 15 ft/s (5 m/s).

===

FIG. 9 Magnetic flowmeter.

METERING Section ELECTRODES MAGNETIC COIL MAGNETIC COIL

===

The magnetic flowmeter has no moving parts. It can measure severe service for conductive flu ids, and it is unaffected by changes in fluid density, viscosity, and pressure. It also is bidirectional, has no flow obstruction, is easy to respan, and is available with DC or AC power. The magnetic flowmeter will also measure pulsating and corrosive flow. It will measure multiphase fluid; however, all components should be moving at the same speed since the meter measures the speed of the most conductive component.

The magnetic meter is good for startup and shutdown operations. It can be installed vertically or horizontally (however, the line must be full). Changes in conductivity value do not affect the instrument's performance.

The disadvantages of a magnetic flowmeter are its above average cost, its large size and weight, and its need for a minimum electrical conductivity of 5 to 20 micromhos/cm (5 to 20 mS/cm). However, special low-conductivity units will operate at > 0.1 micromhos/cm. The magnetic flowmeter's accuracy is affected by slurries that contain magnetic solids (for these situations, some meters can be provided with compensated outputs), and the plant may have to provide appropriate mechanical protection for the electrodes.

Other disadvantages include the fact that electrical coating may cause calibration shifts. Also, the line must be full and have no air bubbles since air and gas bubbles entrained in the liquid will be metered as liquid, causing a measurement error. In some applications, vacuum breakers may be required to prevent the collapse of the liner under certain process conditions.

When installing a magnetic meter, the plant must ensure that proper grounding is in place. It must also consider the following points:

• Upstream and downstream pipe requirements are necessary since the meter is sensitive to nonsymmetrical flow profiles. Some meters compensate for such profiles.

• Possible failure of the seal between the electrode and the liner and the consequence of such a failure.

• Insertion meters may be sensitive to piping effects and also create an obstruction in the line.

Magnetic meters are available in DC and AC versions. DC types are commonly used. How ever, AC types are implemented for:

• Pulsating flow applications

• Flow with large amounts of entrained air

• Applications with spurious signals that may be generated from small electrochemical reactions.

• Slurries with non-uniform particle size (they may clamp together).

• Slurries with solids that are not well mixed into the liquid.

• Quick response. The time required to reach 63 percent of the final value of a step input is six times greater for a DC meter than for an AC. For example, a DC meter may require 6 seconds compared to 1 second for AC types.

Mass: Coriolis

Principle of Measurement:

In the Coriolis effect design (see FIG. 10), one or two tubes are forced to oscillate at their natural frequencies perpendicular to the flow direction. The resulting Coriolis forces induce a twist movement of the tubes. This movement is sensed by pickups and is related to mass flow.

There are two common Coriolis effect tube types: straight and curved.

The straight tube is used mainly for multiphase fluids and for fluids that can coat or clog since the straight type can be easily cleaned. In addition, the straight tube requires less room, can be drained, has a low pressure loss, and reduces the probability of air and gas entrapment, which would affect meter performance. However, the straight tube must be perfectly aligned with the pipe (more so than the curved tube).

Compared to the straight tube, the curved tube has a wider operating range, measures low flow more accurately, is available in larger sizes, tends to be lower in cost (due to lower-cost materials), and has a higher operating temperature range. However, it is more sensitive to plant vibrations than is the straight type.

Application Notes:

The Coriolis flowmeter has many advantages. It directly measures mass flow and density, and some also measure temperature. It handles difficult applications, is applicable to most fluids, has no Reynolds number limitation, and is not affected by minor changes in specific gravity and viscosity. In addition, the Coriolis flowmeter device requires low maintenance, is insensitive to velocity profiles, is bidirectional, will handle abrasive fluids, and is non-intrusive.

===

FIG. 10 Mass flowmeter (Coriolis). VIBRATING FLOW TUBE FLOW; FLOW FLUID FORCE FLUID FORCE FLUID FORCES REACTING TO VIBRATION OF FLOW TUBE END VIEW OF FLOW TUBE SHOWING TWIST; TWIST ANGLE TWIST ANGLE

===

On the other hand, the purchase cost of Coriolis flowmeters is high, and inaccuracies are introduced from air and gas pockets in the liquid as well as by slug flow. The pipe must be full and must remain full to avoid trapping air or gases inside the tube. A high-pressure loss is generated due to the small tube diameters. Coating the tube affects the density measurement (since it will affect the measured frequency) but not the flow measurement (since the degree of tube twist is independent of tube coating).

Mass: Thermal

Principle of Measurement:

The operation of a thermal flowmeter (see FIG. 11) is based on the cooling effect that a passing fluid has on a heated resistance temperature device. The flow is measured by either the change in heating power that is required to keep the device's resistance constant or by the change in temperature reading. The thermal flowmeter measurement is an indirect method for measuring mass flow (i.e., it is not a true mass flowmeter). Mass flow rate is inferred from the mass portion in the energy balance equation of the measured fluid.

FIG. 11 Thermal mass flowmeter (in-line type).

There are two types of thermal flowmeters: the insertion type and the in-line type. The insertion type consists of a probe that is inserted in the stream, complete with heating and measuring elements. The in-line type consists of a sensor typically installed on a bypass around a restriction in the main line (see FIG. 11). The in-line element may be supplied with two temperature elements on both sides of a separate heating element. Alternatively, it may be sup plied with only two externally wound self-heating RTDs that heat the tube and measure the temperature.

Application Notes:

The thermal mass flowmeter has no moving parts and is unaffected by viscosity changes.

However, this meter is affected by coating, and some designs are fragile. The thermal mass flowmeter depends on the thermal properties of the fluid (specific heat and heat transfer). For it to produce accurate measurement, properties must remain constant.

Turbine

Principle of Measurement:

A turbine flowmeter (see FIG. 12) consists of a rotor (similar to a propeller) that has a diameter almost equal to the pipe's internal diameter, which is supported by two bearings to allow the rotor to rotate freely. A magnetic pickup, mounted on the pipe, detects the passing of the rotor blades, generating a frequency output. Each pulse represents the passage of a calibrated amount of fluid. The angular velocity (i.e., the rate of rotation) is proportional to the volumetric rate of flow.

Application Notes:

The turbine meter is easy to install and maintain. It is bidirectional, has a fast response, and is compact and lightweight. The device is not sensitive to changes in fluid density (though at very low specific gravities, rangeability may be affected), and it can generate a pulse output signal to directly operate digital meters.

However, turbine meters do have drawbacks. They are not recommended for measuring steam since condensate does lubricate the bearings well, though some designs will handle steam measurement. Also, they are sensitive to dirt and cannot be used for highly viscous fluids or for flu ids with varying viscosity. Flashing, slugs of vapor, or gas in the liquid produce blade wear and excessive bearing friction, which results in poor performance and possible turbine damage.

Turbine meters are sensitive to the velocity profile and to the presence of swirls at the inlet.

Therefore, they require a uniform velocity profile (i.e., they need a straight upstream run and/ or the use of pipe straighteners).

===

FIG. 12 Turbine flowmeter. STRAIGHTENING VANES TURBINE MOTOR BEARING FLOW CONNECTOR MAGNET PIPE FLANGE

===

In addition, turbine meters are affected by air and gas entrained in the liquid (in amounts exceeding 2 percent by volume; therefore, the pipe must be full). Strainers may be required up stream to minimize particle contamination of the bearings (unless special bearings are used).

However, finely divided solid particles generally pass through the meter without causing dam age. Turbine meters also have moving parts that are sensitive to wear and can be damaged by over speeding. They may be destroyed by lines that fill rapidly during commissioning and startup. Thus, to prevent sudden hydraulic impact, the flow should increase gradually into the line.

When turbine meters are installed, the plant may need to use bypass piping for maintenance.

The transmission cable between the magnetic pickup and the transducer must be well protected to avoid the effect of electrical noise. Finally, on flanged meters, gaskets must not protrude into the flow stream.

Additional information on turbine flowmeters is available from ISA-RP31.1-1977, Specification, Installation, and Calibration of Turbine Flowmeters.

Positive Displacement

Principle of Measurement:

The positive-displacement meter separates the incoming fluid into a series of known discrete volumes and then totalizes the number of volumes in a known length of time. It is analogous to pumps operating in reverse. The common types of positive-displacement flowmeters include the following:

• • rotary piston
• • rotary vane
• • reciprocating piston
• • nutating disc (see FIG. 13)
• • oval gear

FIG. 13 Nutating-disk positive-displacement flowmeter.

Application Notes:

Positive-displacement meters have many advantages. Their simple design means electrical power is not required. They are unaffected by upstream pipe conditions, and direct local read out in volumetric units is available. The highly engineered versions are very accurate, and the low-cost mass-produced versions are commonly used as domestic water meters.

On the other hand, positive-displacement meters have many moving parts, clearances are small (and dirt in the fluid is destructive to the meter), and depending on the application, their seals may have to be replaced regularly since they are subject to mechanical wear, corrosion, and abrasion. In addition, they require periodic calibration and maintenance, and they are sensitive to dirt and thus may require upstream filters.

Moreover, positive-displacement meters cannot be used for reverse flow or for steam since condensate does not lubricate well, and viscosity variations have a detrimental effect on their performance. Finally, these meters can block the flow in the line when they fail mechanically.

Positive-displacement meters are selected mainly according to the type of fluid and the rate of flow that the plant wants to measure, and they are normally used for clean liquids where turbines cannot be used. After a plant installs a positive-displacement meter, it should avoid the following because they cause damage:

• over speeding

• backflow

• steam or high press cleaning

Vortex Shedding

Principle of Measurement:

In a vortex flowmeter (see FIG. 14), an obstruction, or "bluff body," is placed across the pipe bore perpendicular to the fluid flow. Vortices are produced from the alternate edges of the bluff body at a frequency proportional to the fluid velocity. That is, the rate at which the vortices are created is proportional to the volumetric flow rate. Vibrations are sensed by strain gages, capacitance sensors, magnetic pickups, and so forth, and are converted into a flow value.

===

FIG. 14 Vortex flowmeter. DELTA BLUFF BODY FLOW VORTICES METER BODY

===

Application Notes:

The vortex meter has no moving parts. It can be installed vertically, horizontally, or in any position (for liquids, the line should be kept full and gas bubbles avoided). The vortex meter does not experience zero drift like a dp device and requires minimal maintenance. It is suitable for many types of fluids, has an excellent price-to-performance ratio, and its frequency output is linearly proportional to the volumetric flow.

However, the vortex meter's bluff body obstructs the pipe's center, and if the bluff body wears to critical shapes, calibration shift may occur. In addition, the meter should not be used where fluid viscosity may vary so much that unacceptable errors occur. It should also not be used where viscosity is greater than 30 cp, where the application produces an on-off flow, where the Rd is less than 20,000 (since as the Rd drops so does the accuracy), or where solids particles are more than 2 percent of the total flow.

Variable Area (Rotameter)

Principle of Measurement:

The variable-area flowmeter, commonly known as rotameter (see FIG. 15), suspends a free-moving float in a tapered tube (sometimes the float is spring loaded). Its movement up and down inside the tube is related to flow and produces a linear signal with flow. Some rotameters are equipped with transmitters that have an output that is proportional to the measured flow.

The term rotameter was derived from the fact that the float used to have grooves that generated a float rotation for the purpose of centering the meter. Today's floats are guided and do not rotate, but the name has stuck.

The rotameter uses the same basic Principle of Measurement: as an orifice plate. The orifice plate has a fixed orifice with a varying pressure drop, whereas the rotameter has a variable orifice (the annular gap between the float and the tube walls) with a relatively constant differential pressure (due to the weight of the float and the density of the fluid). The annular passage increases as the flow increases (i.e., the tube enlarges as the flow rate increases), and the volume of flow is relative to the annular area.

Application Notes:

The rotameter is the simplest form of flowmeter. It will handle low flow rates, is inexpensive and self-cleaning, provides direct indication, needs no power to operate, and is simple to install. However, it can only be mounted vertically (spring-loaded models can be horizontal), and it cannot be used on erosive, crystallizing, or opaque fluids because dirt and sediments make reading difficult. Optional accessories are needed to enable rotameters to transmit data transmission, and costs rise considerably as such options are added.

===

FIG. 15 Variable area flowmeter. FLOAT STOP %, FLOW AREA OF FLUID PASSAGE FLOAT TAPERED GLASS TUBE

===

The rotameter is affected by fluid density and will not handle high-viscosity fluids. However, it has good immunity to viscosity changes (except in small meters) and, where necessary, viscosity-compensating floats can be used. Float bounce is a limitation in gas applications (i.e., damping and/or a minimum back pressure may be required).

The preferred material for rotameter tubes is borosilicate glass (clearly legible scale gradations are engraved directly on the glass). However, the glass type cannot be used with opaque fluids or where the glass may break, causing a hazardous condition. For this reason, the tube may need a protective shield. For safety reasons, the metering tube should be statistically tested at 150 percent of its maximum working pressure. If the process is hazardous, the meter should be of the metal type. Glass and plastic meters should be confined to safe process fluids.

A rotameter should be:

• installed with sufficient clearance to enable read-out and maintenance.

• mounted vertically with horizontal connections, where possible, to allow for a drain plug and/or clean-out openings.

• piped such that no strain is imposed on the meter.

Ultrasonic: Transit Time, Time of Travel, Time of Flight

Principle of Measurement:

In a transit-time ultrasonic flowmeter, also known as "time-of-travel" or "time-of-flight" ultra sonic flowmeter (see FIG. 16), two transducers are mounted diametrically opposite each other, one upstream of the other (at a 45 degree angle). Each transducer sends an ultrasonic beam at approximately 1 MHz (generated by a piezoelectric crystal). The difference in transit time between the two beams is used to determine the average liquid velocity in that the beam that travels in the direction of the flow travels faster than the opposite one.

Each transducer acts as both transmitter and receiver. The two transducers cancel the effect of temperature and density changes on the fluid's sound transmission properties. The speed of sound is not a factor since the meter looks at differential values. The crystals that produce the ultrasonic beam can be in contact with the fluid or be mounted outside the piping (clamp-on transducers).

FIG. 16 Ultrasonic (transit-time) flowmeter.

Application Notes:

Transit-time ultrasonic flowmeters do not obstruct flow, are bidirectional, are unaffected by changes in temperature, and will handle corrosive fluids and pulsating flow. In addition, they can be installed by simply clamping them on the pipe. They are generally suited for measurements in very large water pipes.

However, transit-time ultrasonic flowmeters are highly dependent on the Reynolds number (i.e., the velocity profile), they must be used with pipe made of non-porous pipe material (i.e., not cast iron, cement, and fiberglass), and they require periodic recalibration.

Ultrasonic: Doppler

Principle of Measurement:

In a Doppler flowmeter (see FIG. 17), a piezoelectric crystal generates a sound wave. The receiver measures the velocity of small particles present in the fluid. The frequency of sound reflected from a moving object-solids and entrained gases-is proportional to the speed of the object. The system then averages the reflected velocity signals.

The fluid to be measured must be a liquid that has entrained gas (greater than 30 microns) or suspended solids (depending on particle size, but typically greater than 25 ppm). The crystal can be in contact with the fluid or mounted outside the piping (clamp-on transducers).

Application Notes:

The Doppler flowmeter has many advantages. The common clamp-on versions are easily installed outside the pipe without shutting down the process. This flowmeter is also bidirectional and is unaffected by changes in viscosity. Moreover, the Doppler flowmeter is generally suited for measurements in very large water pipes, it does not obstruct flow, and its cost is independent of line size.

FIG. 17 Ultrasonic (Doppler) flowmeter. FLOW

However, users should consider the following when selecting the Doppler flowmeter. More than 30 ft. (10 m) must be allowed between installations to prevent the meters from interacting.

Some sound energy will travel from the environment through the pipe wall and into the sensor.

This can cause interference, and poor sound penetration produces reading errors. Similarly, the Doppler flowmeter must be used with non-porous pipe material (i.e., avoid cast iron, cement, or fiberglass). Its accuracy depends on the difference in velocity between the particles and the fluid as well as on the particle size, concentration, and distribution. It must be recalibrated periodically.

Weir and Flume

Principle of Measurement:

A weir (see FIG. 18) is a plate with a trapezoidal, rectangular, or V-shaped notch in it. The trapezoidal weir is also known as the Cipolletti weir. The rectangular notch is easy to construct and can handle larger flows, whereas the V-notch has a relatively wide turndown capability.

A flume (see FIG. 19) is a free-flow open channel with a restriction (similar to an open venturi). The entrance section (on the up stream) converges to a straight section that has parallel sides, then the sides diverge. The Parshall flume is the most common type. Level is measured in the entrance section, and its value is converted into rate of flow.

For both weirs and flumes, measuring the height of the water's surface from a datum is a direct indication of flow. Measurement of the liquid head is performed by float, ultrasonic, and other methods (see section 5 on level measurement). A stilling well is sometimes used to eliminate and reduce turbulence.

FIG. 18 Weir with a V-notch. CAPACITANCE LEVEL SENSOR STILL WELL FLOW BAFFLE PLATE WEIR BOX V-NOTCH WEIR; (A) WEIR WITH A V-NOTCH

FIG. 19 Parshall flume. THROAT FLOW CREST STILL WELL; (B) TYPICAL PARSHALL FLUME

Application Notes:

The main advantages of flumes over weirs include their ease of construction and their sturdiness. Their ability to handle flows at higher velocities makes it possible to measure liquids with entrained solids, and flumes' self-cleaning capabilities enable them to handle wastes that have suspended solids. Weirs, which can handle large volumes of liquid, are more accurate than flumes but require cleaning.

Weirs and flumes produce low head loss, are relatively low in cost, and are the only flowmeter that will handle semi-filled pipes. Viscosity changes have little or no effect on weirs or flumes.

Both are used in liquid (clean and dirty) applications in open channels-mainly, water and wastewater applications.

Target

Principle of Measurement:

In a target flowmeter (see FIG. 20), the flow exerts a force on a solid disk that lies in the pipe at right angles to the flow. The force is related to the flow. The target flowmeter can be described as the opposite of an orifice plate.

FIG. 20 Target flowmeter. TRANSMITTER FORCE BAR FLOW; DISC (TARGET)

Application Notes:

The target meter is inexpensive and has no moving parts. It is mainly used for viscous fluids, and is a good method for applications such as hot, tarry fluids and sediment-bearing fluids.

Prev. ------- Next

top of page  Article Index  Home