Selecting liquid flowmeters

12/01/1999


Measuring liquid flow rate in a pipe is done for many reasons: product blending, process heating or cooling, monitoring energy costs, machine lubrication or manufacturing, and determining the amount of product transferred for billing purposes, to name a few.

Reliability and accuracy of measurement are two of the selection factors used in choosing a particular type of flowmeter. Inaccurate or lack of flow measurement could result in serious damage to equipment or product. In some instances accuracy is important because it can make the difference between profit and loss when dispensing liquids.

Flowmeter selection is complicated by the fact that there are about two dozen designs to choose from. Many have been around for years and have an established user base. Others, such as thermal mass, Coriolis, and ultrasonic, have benefited from new technology, improved designs, and electronics to find increasing applications. The thermal mass design has moved to second place in just two decades of existence. A recent study by Cahners Research determined there are eight flowmeter designs widely used (Fig. 1).

There are two basic types of flowmeters used with pipes: full-bore inline and insertion. The velocity-type inline flowmeter allows the entire flow to pass through and inferentially derives a flow rate from average velocity. Other inline meters base flow rates on positive displacement or mass flow techniques.

Insertion-type flowmeters protrude into the pipe. They sample a point in the flow stream that is assumed to represent average velocity, or they create a pressure differential dependent on flow.

Popular flowmeters

There are several types of flowmeters available: differential pressure, mass, turbine, magnetic, positive displacement, vortex, and ultrasonic.

Differential pressure

Differential pressure flowmeters rely on changes in flow to create an increase or decrease in pressure across a fixed orifice or restriction in the pipeline (Fig. 2). This pressure difference is used to calculate flow rate.

Orifice plates are the most popular flowmeters in use today. Most orifice plates are concentric, but eccentric and quadrant or segmental designs are available. They produce the best results when measuring turbulent flows of clean liquids. Major advantages of orifice plates are no moving parts and a cost that does not increase significantly with pipe size. Metering accuracy depends on installation, orifice area ratio, and physical properties of the liquid. They are susceptible to erosion, which produces readings on the low side for a given flow rate, and must be installed in straight pipe runs.

Venturi tubes can handle large flows with a low pressure drop and good accuracy. Applications are usually limited to high flow rates because of the relatively high cost. They can be used with most liquids, including those with a high solids content. Venturis are not recommended for highly viscous liquids or those containing large amounts of sticky solids.

Flow nozzles represent a good compromise between an orifice and venturi. At high velocities they can accommodate approximately 60% greater flow than an orifice plate with the same pressure drop. They can handle large solids, high velocities, high turbulence, and very high temperatures. Liquids with suspended solids can be metered.

Variable area flowmeters maintain a relatively constant, rather than varying, pressure differential with varying flow rates by having a moveable restriction in the flow path. The position of the piston in the housing indicates flow rate (Fig. 3). Because the flow rate can be read directly, secondary reading devices are unnecessary.

Mass

There are two types of mass flowmeters: thermal and Coriolis. Thermal mass flowmeters rely on heat loss or heat gain of the flow stream. The amount of loss or gain is proportional to mass flow. In a Coriolis meter, fluid passes through a vibrating tube, causing the tube to twist. The amount of twist is proportional to mass flow.

Thermal mass flowmeters operate independently of pressure and viscosity. They use a heated sensing element isolated from the fluid flow path (Fig. 4). The flow stream conducts heat from the sensing element. The conducted heat is directly proportional to mass flow rate. The amount of heat carried away from the probe depends on the fluid's velocity, density, specific heat, and thermal conductivity. If the probe becomes coated, this interferes with heat transfer and negatively affects accuracy and response time.

Coriolis mass flowmeters (Fig. 5) accurately measure flow rates independent of temperature, pressure, viscosity, and solids content. Two tubes are constantly vibrated. Fluid flow causes the tubes to twist. The amount of twist depends on the flow rate. This design is noninvasive and used with many fluids over a wide range of flows. Because Coriolis flowmeters maintain accuracy and factory calibration, they are widely used in applications that require tight control, management of high-value fluids, and custody transfers.

Turbine

Turbine flowmeters use a rotor, with propeller-like blades that rotate as fluid passes over them (Fig. 6). Flow rate is proportional to rotational speed as sensed by a magnetic pickup, infrared beam, or radio-frequency field in a pickup coil.

Turbine flowmeters provide excellent short-term accuracy, repeatability, and rangeability. They are usually used with clean liquids, but are not effective with swirling or high viscosity fluids. The design is prone to bearing wear, blade coating, and damage. Meters must be calibrated for each application, adding to installation and maintenance costs.

Magnetic

Magnetic flowmeters are constructed with a coil around the flow stream that creates a magnetic field (Fig. 7). An electrically conductive fluid generates a voltage as it moves through the magnetic field. This voltage is proportional to the flow rate.

Magnetic flowmeters have major advantages. They can measure difficult and corrosive liquids and slurries and forward and reverse flow. However, the fluid must be electrically conductive and nonmagnetic. Most water-based fluids can be measured, but petroleum-based fluids cannot. The pipe must be completely filled with fluid or significant measuring errors will result.

These meters do not have obstructions to cause pressure loss and no moving parts to wear out. They are not sensitive to changes in viscosity, density, or pressure and work in both laminar and turbulent flow. Accuracy is affected if the insulating liners and electrodes become coated with a conductive film. A nonconductive film makes the meter inoperable.

Positive displacement

Positive displacement flowmeters measure incremental volumes of flow as line pressure fills and displaces each chamber's volume downstream. Flow rate is determined by counting the number of times this action occurs.

Because these meters have many moving parts, they are not suitable for dirty or gritty fluids. They require energy from the fluid stream, which causes a slight pressure drop. Leakage around gears or vanes can cause inaccurate readings, but using these devices with viscous fluids reduces this effect. However, viscosity variations below a given threshold affect measuring accuracy. Several common designs include reciprocating single or multiple pistons, nutating disk, oval gear (Fig. 8), lobed impeller, and rotary vane.

Vortex

Vortex flowmeters use a bluff body or shedder bar to generate vortices in the flow stream. Flow rate is determined by counting the vortices that form alternately behind the bluff body. Frequency of vortex formation is directly proportional to the fluid velocity.

Vortex flowmeters are rugged devices with no moving parts. They are equally suitable for flow rate and flow totalization measurements. Use with slurries or high viscosity liquids is not recommended. These meters are not useful at very low flow rates because vortex formation is inhibited by a lack of available energy in the fluid.

Multivariable vortex mass flowmeters (Fig. 9) monitor mass flow rate by directly measuring three variables: fluid velocity, temperature, and pressure. A computer then calculates mass flow rate based on these direct measurements. The result is improved accuracy and reliability. Installed cost is low due to fewer penetrations and less hardware.

Ultrasonic

Ultrasonic flowmeters are available in two designs: Doppler and transit time. Doppler measures the frequency shift of a sound wave to determine flow rate. Transit time measures the time it takes a sound wave to travel a specified distance through a flow stream. The variation in time is related to flow rate.

In a Doppler flowmeter, a constant-frequency sound wave is transmitted through the pipe walls and fluid to a receiver. The sound wave is reflected back to the receiver by suspended solids, entrained gases, or flow turbulence moving with the fluid. Because the liquid causing the reflection is moving, frequency of the returned signal is shifted proportionately to the liquid's velocity.

Transit time flowmeters (Fig. 10) have transducers mounted at a 45-deg angle to the flow, either on the same side or opposite sides of a pipe, depending on pipe and liquid characteristics. Speed of the signal or shift of frequency between the transducers increases or decreases with the direction of transmission and velocity of the fluid. A limitation is that the liquid being measured must be relatively free of entrained gases or solids.

Ultrasonic meters operate without physically touching the fluid or obstructing flow, and are not affected by corrosive, abrasive, or highly viscous liquids. They provide bi-directional capability and can be strapped to existing piping, reducing installation costs.

-- Joseph L. Foszcz, Senior Editor, 630-320-7135, jfoszcz@cahners.com

The cover picture was taken with the cooperation of Universal Flow Monitors, Inc., Chem Flow, Inc., Nalco Chemical Co., and Universal Instruments Div. of Dinno Equipment, Inc.

Key concepts

Application determines the proper flowmeter.

There are two basic flowmeter designs: full-bore inline and insertion.

Not all meters can measure bi-directional flow.

Advantages and disadvantages of liquid flowmeters

Type Advantages Disadvantages

Differential Low initial cost Subject to plugging

pressure Familiar technology Pressure drop

(orifice plate) Easy to use Orifice plate wear

Thermal mass Low cost Periodic cleaning

Handles low density fluids Not highly accurate

Turbine Accurate Wear

Accepted technology High flow velocity can

cause damage

Magnetic Accurate Requires conductive fluid

No pressure drop Electrodes subject to

Bi-directional coating from fluid

Adaptable to large pipes

Coriolis mass High accuracy Sensitive to vibration

True mass flow measurement High initial cost

Not suitable for large pipes

Positive Accurate Wear

displacement Wide rangeability Limited use on large pipes

Requires clean fluids

Vortex Accurate Sensitive to vibration

Easy to install Lacks approvals

Ultrasonic Low maintenance High initial cost

Nonintrusive May require clean fluid

Adaptable to large pipes Clamp-on installation

Selection factors

At a minimum, specifiers of flowmeters should consider the following:

- Ability to withstand the process environment: fluid, pressure, temperature, etc.

- Ability to provide the accuracy of measurement required

- Serviceability and maintenance requirements

- Long term stability, durability, and frequency of calibration

- Cost of purchase and installation

- Ease of interfacing with existing equipment

- Pressure loss incurred, level of swirl generated, or pulsation produced

&READERSERVICE> Liquid flowmeter manufacturers

This list of manufacturers was prepared from information appearing in the Plant Engineering Product Supplier Guide and from companies that supplied data for the article. For detailed information on their products and assistance in applications, circle the number on the reader service card or visit their web sites. Not all types of flow meters are presented in this table; only those shown to be the most popular in a study by Cahners Research .

Circle Company, web site Types*

221 Badger Meter, Inc. A-B-C-D-E

www.badgermeter.com

222 Blancett Flow Meters B

www.blancett.com

223 Brooks Instrument B-D-E

www.brooksinstrument.com

224 Cadillac Meter Co. B

www.cadillacmeter.com

225 Cole-Parmer Instrument Co. B-C-D-E-G-H

www.coleparmer.com

226 Digiflow Systems C

227 Dynasonics Div.,

Racine Federated, Inc. D-E

228 Endress+Hauser A-B-C-D-E-F-G-H

www.us.endress.com

229 Flowmetrics, Inc. C

www.flowmetrics.com

230 Gems Sensors, Inc. D

www.gemssensors.com

231 George Fischer, Inc. C-D-H

www.us.piping.georgefischer.com

232 Hedland Flow Meters A-C

www.hedland.com

233 Hoffer Flow Control, Inc. B-C

www.hofferflow.com

234 Intek, Inc. G

www.intekflow.com

235 J-Tec Associates H

236 Kobold Instruments, Inc. A-B-C-D-G-H

www.koboldusa.com

237 Liquid Controls, LLC B-C-D-F

www.lcmeter.com

238 Mesa Laboratories, Inc. E

239 Micro Motion, Inc. F

www.micromotion.com

240 Panametrics, Inc. E

www.panametrics.com

241 Sierra Instruments, Inc. G-H

www.sierrainstruments.com

242 Spirax Sarco, Inc. A-B-C-D-E-G-H

243 Universal Flow Monitors, Inc. A

www.flowmeters.com

244 Yokogawa Industrial Automation H

www.yca.com

* Types of flowmeters:

A-Orifice

B-Positive displacement

C-Turbine

D-Electromagnetic

E-Ultrasonic

F-Coriolis mass

G-Thermal mass

H-Vortex

&/READERSERVICE>

More info

See the "Fluid handling" channel on www. plantengineering.com for more articles and information on this topic.

Additional information on specific flowmeters can be obtained by contacting the manufacturers listed in the box at right.





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