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EssEntials of Flow MeasureMent

This Control Essentials guide made possible by Brooks Instrument. See page 8 for more

information on Brooks’ full range of flow measurement and control solutions.

About the control essentials seriesthe mission of the control essentials series is to provide process industry professionals with an up-to-date, top-level understanding of a range of key process automation topics. Our intent is to present essential engineering concepts in a practical, non-commercial fashion, together with a review of the latest technology and marketplace drivers—all in a form factor well suited for onscreen consumption. check in at controlGlobal.com/essentials for other installments in the series.—The Control Editorial Team

June 2013

A control essentials Guide, by the editors of control

Flowmeter Usage trends

throughout the global process and energy industries, flow rate is among the most often measured and controlled process variables. As such, a broad range of flow measurement technologies has developed over the decades to exploit seemingly every measurable physical property and dynamic characteristic of

a fluid in motion that can be correlated with flow rate. Indeed, flow rate is nearly always an inferred or calculated measurement. Some “flow” measurement devices

actually measure level (as of water through an open weir of known geometry) or loss-in-weight over time (as of a solid from a gravimetric hopper or along a weighing conveyor) to calculate a rate of material movement.

But the workhorse application of industrial flow instrumentation is in determining the flow rate of liquids and gases within closed pipes and ducts. In these applications, a variety of sensor technologies measure a range of secondary variables from which flow is inferred. For example, the pressure drop generated by a fixed flow obstruction as with an orifice meter--or the lift imparted to an unfixed one as with a rotameter. The frequency of von Karman vortex shedding as with a vortex meter, or of a paddle wheel’s rotations as with a turbine meter. The relative speed of sound in a moving fluid as with an ultrasonic flowmeter or the magnetic field created by one with a magmeter. The dispersion rate of thermal energy as with a thermal mass flowmeter or the Coriolis forces exerted on vibrating tubes as with a Coriolis mass meter.

Each of these major flow measurement approaches has its particular advantages and limitations, and the ap-plication of each typically is driven first by compatibility with fluid characteristics and process conditions, sec-ond by overall performance requirements, and third by cost. Supported by the development of more capable digital signal processing algorithms, the overriding usage trend is toward electronic devices such as mass, ul-trasonic and magnetic flowmeters at the expense of traditional “mechanical” alternatives such as differential pressure, turbine and positive displacement devices. Differential pressure devices, however, continue to be commonly used because of deep user familiarity. Multi-variable transmitters that proffer built-in temperature compensation and pre-assembled “integral” flowmeter assemblies also have made this technology easier to specify and install.

“It’s not surprising, nor earth shattering news, to see growth for mechanical flowmeters slowing and an ongo-ing shift to Coriolis and ultrasonic products,” notes Paul Everett, research direct for IMS Research and author of a recent study on market trends in industrial flow measurement. Everett forecasts Coriolis flowmeters to over-take differential pressure flowmeters as the largest flowmeter market as soon as 2014, having replaced magnetic flowmeters as the second largest market in 2012.  “End-users are looking beyond the upfront cost of purchas-ing new technologies, and have begun to assess the overall return on investment that they offer,” Everett says.

the basis of good f lowmeter selection is a clear understanding of the requirements of the particular applica-tion. Therefore, time should be invested in fully evaluating the nature of the process f luid and of the overall installation. The development of specifications that state the application requirements should be a systematic,

step-by-step process.Process conditions and f luid characteristics are of primary importance (table), but one should also consider such in-

tangible factors as technology familiarity of plant personnel, their experience with calibration and maintenance, and spare parts availability at the particular plant site. It is also recommended that the cost of the installation be computed only after taking these steps. One of the most common f low measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly, an attempt is made to justify the use of a device because it is less expensive. Those “inexpensive” purchases can be the most costly installations.

In applications where products are sold or purchased on the basis of a meter reading, absolute accuracy is critical. In other applications, repeatability may be more important than absolute accuracy. Therefore, it is advisable to estab-lish separately the accuracy and repeatability requirements of each application and to state both in the specifications.

If acceptable metering performance can be obtained from two different flowmeter categories and one has no moving parts, select the one without moving parts. Furthermore, if one can obtain the same performance from both a full pipe flowmeter and a point sensor, it is generally advisable to use the flowmeter. The insensitivity of thermal and Coriolis mass flowmeters to density, pressure and viscosity variations also speak to their utility and flexibility.

Specification conSiderationS

Key application paraMeters

• What fluid is to be measured?

• What are normal, minimum and maximum values of pressure, temperature, density, conductivity and viscosity?

• What is the vapor pressure at maximum operating temperature?

• What is the allowable pressure drop?

• Is mixed phase flow expected? Gas with liquids, or liquid with solids?

• Does the fluid have a tendency to coat?

• Can flow reverse?

• Does the fluid always fill the pipe?

Flow From Pressure DroP

the calculation of fluid flow rate from the pressure loss across a pipe re-striction is among the most well established approaches to industrial flow measurement. The pressure drops generated by a wide variety of geometri-

cal restrictions have been well characterized over the decades, and flow elements come in a wide variety of configurations, each with specific application strengths and weaknesses. By far the most common example is the orifice plate (Figure 1), in which the fluid passes through a precisely machined opening concentric with the inside pipe diameter. The loss in pressure across the orifice allows the volumet-ric flow rate to be calculated. And, if fluid density is known (or can be calculated from pressure and temperature measurements), a mass flow rate can be inferred.

Factors affecting the overall accuracy of these devices include proper instal-lation such as with prescribed lengths of straight pipe (or flow straighteners) up-stream and downstream of the device. Erosion of the orifice plate edge over time also can affect accuracy, and the various ancillary components and multiple pro-cess penetrations make installation and maintenance of differential pressure de-vices both expensive and time-consuming.

Rotameters, or variable area meters, exploit essentially the same physics as dif-ferential pressure meters. But instead of measuring the pressure drop across a fixed obstruction, the force of the flowing fluid moves a float/weight upward in a ta-pered, vertical tube (Figure 2). The higher the flow rate, the higher the float’s equilibrium point.

Advantages of the rotameter include its relatively low cost, simplicity, low pres-sure drop, relatively wide range and linear output. If only local display is needed, no power is required: the position of the float can be seen through a glass or other transparent tube on which readings are etched. Alternatively, the float can be mag-netically coupled to a dial indicator, or an electronic transmitter used to send flow rate information to a remote display or supervisory controller.

Figure 1. Orifice plate flowmeters infer volumetric flow rate in a pipe by measuring the difference in static pressure before and after the fluid passes through a concentric circular restriction.

Figure 2. Variable area flowmeters, or rotameters, balance the weight of an in-stream “float” against an upward flow. Knowledge of the tube and float weight and geometry, together with the fluid characteristics, allow calculation of the flow rate.

By their very nature, differential pressure and variable area flowmeters both rely on partially obstructing the flow to be measured in order to produce a measur-able effect. Magnetic meters and ultrasonic meters, on the other hand, perform

their tasks non-intrusively: they calculate the fluid’s average velocity by the magnetic field it generates or by its effect on the relative speed of sound, respectively.

The magnetic flowmeter, or magmeter, works only with conductive fluids such as aqueous solutions, which explains its widespread use in water and wastewater treatment applications. The magmeter itself consists of a non-magnetic pipe lined with an insulat-ing material. A pair of magnetic coils is situated as shown in Figure 3, and a pair of elec-trodes penetrates the pipe and its lining. If a conductive fluid flows through the pipe, a voltage proportional to the average fluid velocity is generated across the electrodes.

Magmeter electrodes and coils also can packaged in an insertable probe for point detection of fluid velocity. This configuration has the advantage that it can be removed without shutting down the process. Overall accuracy, however, is highly depending on how closely the point velocity measured reflects average flow velocity within the pipe. When a magmeter is provided with a level sensor embedded in the liner, it can even measure flow in partially full pipes. In this design, the magmeter electrodes are located at the bottom of the tube (at approximately 1/10 the pipe diameter) to ensure they re-main covered by the fluid. The complementary level sensor allows calculation of the flow area and, in turn, the overall flow rate.

Ultrasonic flowmeters measure fluid velocity by measuring the effect a moving fluid has on the speed of sound. Transit-time, or time-of-flight, meters measure the phase shift between upstream and downstream transducers (Figure 4) and are used primarily with clean liquids and gases. Doppler-effect meters, on the other hand, rely on a single transceiver and measure the frequency shift of sound waves reflected from suspended particles or bubbles within the flowing stream.

More powerful signal processing algorithms, together with multi-path configura-tions, have been developed to better characterize the flow profile and improve overall accuracy. In fact, due to their relatively low cost and high accuracy they now represent the fastest growing industrial flowmeter market segment. Custody transfer applications in the hydrocarbon industry are among the technology’s strongest growth segments.

Flow From Fluid VelocityFigure 3. the magmeter measures the average velocity of conductive liquids: fluid movement through an applied magnetic field creates a measurable voltage across a pair of electrodes.

Figure 4. Ultrasonic flowmeters measure the effect of a moving fluid on the relative speed of sound wave propagation.

two workhorses of industrial flow measurement rely on the measure-ment of frequency to infer flow rate, the vortex meter and the tur-bine meter. The vortex meter is based on the von Karman principle,

which predicts the regular shedding of vortices from alternate sides of a bluff body imposed in a flow (Figure 5). It’s the von Karman effect that makes a flag ripple in a breeze; the flagpole acts as a bluff body, and vortices shed from alternate sides make the flag ripple in a predictable fashion.

In the case of a vortex flowmeter, a piezoelectric sensor detects the pres-sure oscillation around the bluff body caused by the shedding vortices. Alter-natively, a piezoelectric strain gauge detects the forces exert on the shedder bar itself. Vortex shedding frequency is directly proportional to the velocity of the fluid in the pipe, and therefore to volumetric flow rate. The shedding frequency is independent of fluid properties such as density, viscosity and conductivity, but the flow must be turbulent for vortex shedding to occur.

This also means that a zero reading on a vortex meter doesn’t necessarily mean zero flow. For this reason, vortex meters are ill-suited to batch or other intermittent flow operations where the flow rate often moves through low but non-zero values. Relatively to an orifice meter, however, the vortex meter causes only about one-half of the drop in pressure, and is lower in mainte-nance requirements as well.

The turbine flowmeter consists of a multi-bladed rotor mounted at right angles to the flow and suspended in the fluid stream on a free-running bear-ing. The diameter of the rotor is very slightly less than the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate (Figure 6). Turbine rotation is detected by solid state devices (reluctance, inductance, capacitive and Hall-effect pick-ups) or by mechanical sensors (gear or magnetic drives). Turbine meters also come in insertion designs for measuring velocity at a particular point within the pipe. But as with other point sensor technologies, insertion turbine meter accu-racy suffers if at the point chosen the velocity doesn’t represent the average throughout the pipe.

Flow From Frequency

Figure 5. Vortex meters use piezo electric sensors to gauge the frequency of vortices shed from turbulent flow around an in-stream shredder bar.

Figure 6. turbine meters often employ reluctance or inductance sensors to count rotations of the rotor. Frequency is proportional to volumetric flow rate.

the final two flowmeters in our tour of the major flow measure-ment technologies are the mass meters, so named primarily because of their relative insensitivity to variations in tempera-

ture, pressure and other process conditions that can significantly af-fect the mass flow rate accuracy calculated from a volumetric device.

The Coriolis meter takes advantage of the phenomena that Gustav Coriolis first observed as the tendency of bodies moving on the sur-face of the earth to drift sideways because of the eastward rotation of the planet -- to the right of the motion in the Northern Hemisphere and to the left of the motion in the Southern Hemisphere. The first commercial Coriolis flowmeters, which first emerged in the 1970s, work by introducing an artificial Coriolis acceleration on a flowing stream. From a practical perspective, when mass flows through a vi-brating tube, Coriolis forces will tend to twist the tubing. By measur-ing the magnitude of displacement cause by this twisting force, the Coriolis meter can directly calculate the mass flow rate through the tube (Figure 7).

Thermal mass flowmeters also measure the mass flow rate of gases and liquids directly, by introducing a known amount of heat into the flowing stream and measuring an associated temperature change, or by maintaining a probe at a constant temperature and measuring the energy required to do so (Figure 8). Thermal mass flowmeters often are used in monitoring or controlling mass-related processes such as chemical reactions that depend on the relative masses of unreacted ingredients. One of the key capabilities of thermal mass flowmeters is the accurate measurement of low gas flow rates and low gas velocities. For pilot plant or other low flow applications, thermal mass flowme-ters often are packaged as integrated mass flow controllers; a thermal mass flowmeter together with controller and valve are used to main-tain a constant mass flow rate.

Mass Flow MeasureMent

Figure 7. the coriolis effect predicts that the magnitude of twist induced on vibrating tubes is proportional to the mass flow through them.

Figure 8. While many various configurations exist, the thermal mass flowmeter fundamentally relies on the capacity of a moving fluid to transfer heat more heat as mass flow rate increases.

Made possiBle By

this Control Essentials guide on Flow instrumentation was made possible by Brooks instrument, a multi-technology instrumentation company serving a range of demanding markets. Brooks has a proven history of innovation that today includes glass and metal tube variable area meters (rotameters), thermal mass flow controllers and meters from ultra-high purity to industrial grade, and Coriolis mass flow controllers.

To learn more about Brooks’ full range of flow measurement and control solutions, visit Brooksinstrument.com