Thermal Flow Meters

Magnetrol TA2 thermal flow meter configured
for insertion mounting.
Image courtesy Magnetrol International 

There are numerous methods and technologies adapted for the measurement of fluid flow, each with its own set of positive attributes and limitations. Understanding the basic operating principles behind each is useful for effective selection of a technology to be applied on a specific application. One technology long employed for continuous fluid flow measurement is thermal dispersion. The operation of a thermal flow meter is as simple as placing a heated object into a moving stream. The amount of heat drawn away, or dispersed, from the heated object is a measurable quantity that is directly proportional to mass flow rate. This is similar in concept to a principle commonly observed in wind chill where someone perceives the temperature to be colder than it actually is at the moment of measurement.

One example of a thermal mass flow meter is a hot wire anemometer, with which air speed is measured via a metal wire charged with an electric current. The faster the air moves around the wire, the more the temperature of the wire will correspondingly drop. The electrical power required to keep the temperature of the wire constant is directly proportional to the flow rate of the air moving past the wire. However, thermal flow meters are inherently used to measure mass air flow and not volumetric air flow.

A common application of thermal flow meters is mass air flow measurement for combustion control, such as in engines and boilers. Maintaining fuel to air ratios in a range resulting in efficient combustion is essential to controlling fuel costs and the level of regulated emissions. Keeping combustion efficient relies on a controller’s ability to modify the combustion air mass flow rate to match the demand for fuel under changing load and input conditions. Thermal dispersion technology can be applied to gases or liquids, making the range of applications very broad.

Typically, thermal mass flow meters used in processing industries will have a flow tube or insertion probe with two temperature sensors, one which is heated and the other which is not. The heated sensor serves as the mass flow sensor, meaning it will cool at a rate directly dependent on mass flow. The purpose of the second temperature sensor is to deliver an accurate measure of fluid temperature. Various processing methods are employed to determine the degree of thermal dispersion, but all are related to the same basic principal.

One of the best applications for thermal mass flow meters is at a particular point of a flow stream, where the flow meter can be inserted or installed to measure a specific amount of fluid being used in the process, such as the amount of gas being sent to a flare. Their design simplicity and ease of production allows thermal flow meters to be very compact. Some are even coupled with built-in throttling mechanisms and other control devices, incorporating measurement and control functions into a single integrated device.

Share your flow measurement requirements and challenges with an instrumentation specialist, combining your own process knowledge and experience with their product application expertise to develop an effective solution.

Thermal Mass Flow Meter Questions Answered by Experts

Thermatel, insertion style thermal mass flow meter
Image courtesy of Magnetrol

Knowledgeable individuals that share expertise and experience with others in their field are a valuable resource, worthy of our attention.

Tom Kemme, from Magnetrol®, expertly fielded some questions about thermal mass flow meters in a recent blog post. Mr. Kemme's responses were so useful and clear that I decided, with all the credit flowing his way, to share them here for those of you that may not closely follow the Magnetrol® Blog.

Will thermal mass flow meters be affected by changes in the composition of gas (i.e. will they require recalibration every time the composition changes)?

Thermal mass flow meters measure a flow rate based on convective heat transfer. Fluid properties are some of the many factors that influence convection. Each gas has unique properties, which is why these flow meters are calibrated for a specific application. You would not want a meter calibrated for an air application placed into a natural gas application without recalibration or some type of field adjustment if applicable.

All gas mixes are not created equal. If you had a gas mix with high hydrogen content, a variation in hydrogen would have a much greater effect than typical variation in natural gas content. Hydrogen has a tendency to create more heat transfer than most gases. For natural gas, it is common to have some slight variation in composition between the calibration of the device and the application itself. However, the effect is minimal for slight changes in methane or ethane at different times of the year. Natural gas fuel flow is one of the most prevalent applications for thermal mass.

Based on our experience, the biggest cause of malfunction in flow meters is improper installation. If you do not install a flow meter per the manufacturer’s recommendation this will greatly influence the performance of the meter. For thermal mass, this includes proper straight run, depth into the pipe (insertion probes) and flow arrow alignment.

Each application presents unique difficulties for every flow meter technology, and each end user has unique needs. There is no exact answer as to when a recalibration would be needed for thermal mass flow, as it is application dependent. You do not always need recalibrations for variation in gas composition.

What role do thermal flow meters play in emissions monitoring applications?

Thermal flow meters are at the forefront in flow measurement for emissions reporting and energy management projects. The energy management arena spans many markets, including some of the largest in the oil & gas and power industries. Some popular applications include monitoring gas fuel flow to a combustion source to report SO2 (sulfur dioxide) emissions, stack (flue) gas flow in power plants as part of a continuous emissions monitoring (CEM) system of NOX (nitrous oxide) and SO2, and flares in a gas field that need to be reported to environmental authorities. These applications prove difficult for many flow meter technologies.

For example, in a flare application most of the time gas is not being flared off, but it needs to be measured in case of an event. The user will want to monitor the low flow of pilot gas keeping the flare lit. This requires a flow meter with a very high turndown with good low flow sensitivity, which is a limitation of some technologies, such as differential pressure flow meters.

Many operators are most concerned with measuring CO2 (carbon dioxide) emissions. However, with thermal flow meters we are increasingly finding applications with the need for methane measurement. Methane is a greenhouse gas that has more than 20 times the global warming potential as CO2. No longer can coalmines or landfills emit this directly to the atmosphere. If not flaring the gas off, the owners are beginning to capture it, treat it, and produce usable natural gas from it. Some facilities that emit landfill gas, or facilities that produce biogas, are involved in carbon credit programs or clean development mechanisms. Similar applications can be found in wastewater treatment plants where customers are reporting digester gas emissions and even capturing this gas to produce electricity and reduce energy costs. Thermal dispersion flow meter technology, such as the MAGNETROL Thermatel® TA2, has become well accepted in all of these markets.

You can easily tap into Magnetrol® expertise to solve your flow measurement challenges. Reach out to a product specialist and combine your process knowledge with their flow measurement expertise to develop effective solutions.

Corrosion Resistant Flow Meter

There are numerous flow measurement technologies available for application in process measurement. Each technology is represented by a broad array of product variants, each with a set of attributes making it suitable for certain applications.

ICON Process Controls specializes in corrosion resistant industrial fluid handling and process control equipment, offering the most complete line of all plastic instrumentation products supported by the largest inventory in North America. Applications for their corrosion resistant instruments include Municipal and Industrial Water & Wastewater Treatment, Bulk Chemicals, Steel Processing, Metal Finishing, Chemical Dosing Skids, Food & Beverage.

Share your process measurement and control requirements with instrumentation specialists. Combine your own knowledge and experience with their product application expertise for effective solutions.

Ultrasonic Clamp-On Flowmeter with SIL 2 Rating

FLUXUS F/G70X and F/G80X series meters
Courtesy Flexim

Measuring the flow quantity of gases and liquids is a common industrial processing task. There are numerous technologies available for measuring fluid flow, each with its own set of advantages and drawbacks for any particular application. Some of the technologies and methods have been in use for a very long time, with recent enhancements provided by electronics or smart sensor designs.

Ultrasonic flow measurement devices employ a comparatively recent technology to measure gaseous or liquid flow. Whether the transit time differential or Doppler method is utilized, ultrasonic flow meters have a distinctive characteristic in that they can be deployed in a form factor that does not require insertion into the fluid. A common installation method is to clamp the ultrasonic transducer assembly onto the exterior of a pipe. This makes the technology attractive for applications that involve adding a flow measurement point to an existing piping system.

Flexim, a globally recognized leader in ultrasonic flow measurement, offers a number of permanent and portable units for measuring liquid and gaseous flow rates. Some of their instruments have been certified as SIL 2 capable, along with a host of other third party certifications. The product range includes simple and accurate instruments designed for general industrial use, and extends to multi-beam units intended for applications, such as custody transfer of fluids, that require the highest accuracy and overall performance levels.

Share your flow measurement challenges and requirements with instrumentation specialists, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Non-Invasive Gas Flow Measurement from MSJacobs-Cali

Niagara Meters: Overview of Flow Measurement Devices

Cutaway and exterior views of target flowmeter

Niagara Meters, based in Spartanburg, South Carolina, is a well recognized brand in the flow measurement field. For over a hundred and fifty years, their products have been used in a myriad of industries, including agriculture, petrochemical processing, and even aboard United States Navy ships. The company’s flow meter products target three basic functions: flow measurement of liquid, gas flowmeters, and flowmeters for steam. The three product application groups are anchored by Niagara’s operating technology thesis statement, “specializing in innovative, reliable products.”

The largest array of Niagara meters are those utilized for liquid flow measurement. For measuring liquid in an open channel, such as a weir or flume, Niagara offers an application specific ultrasonic open channel flowmeter, which comes equipped with one or more sensors, along with a level monitor. The device is flexibly configurable for different arrays, and calculates liquid level, open channel flow and differential level measurement.

Cutaway view of turbine flowmeter

Measuring potable water flow through a pipe branch or system can be accomplished easily with the reliable mechanical MTX/WPX model series. In this turbine technology flowmeter, water pressure and resulting flow drives the internal turbine rotor. Magnetic coupling of the rotor to a flow indicator makes this instrument simple, reliable, and rugged for totalizing, rate of flow, and batch control applications.

Flow measurement in applications involving liquids with viscosity similar to oil are candidates for Niagara’s positive displacement oscillating piston flowmeter. This positive displacement device employs a piston which rotates in a flow chamber inside the meter. Liquid flow forces the piston to rotate, with the rotations recorded by a totalizer or pulse transmitter. Mechanical and smart electronic versions are available in a number of variants to meet the range of register and interface requirements.

Another mechanical positive displacement technology from the company uses a nutating disc and gear train to measure flow and provide a totalized flow register. The nutating disc is precisely fitted into the flow chamber and wobbles in a predictable manner that can be counted and used to measure volumetric flow. As with the oscillating piston products, a wide array of variants, including a smart meter version, are available to accommodate any register requirement.

Niagara also offers a fully electronic magnetic flowmeter, or Magmeter. The 6600 Series Magmeter can only be applied with conductive fluids, and converts the voltage produced when the fluid flows through a magnetic field to a volumetric flow signal for a high accuracy solution with no moving parts.

The company offers one additional category of flowmeter technology; applicable to liquids, gas, or steam. The target meter has a solid disk (the target) located in the flow path. The dynamic force of fluid movement acting on the target is converted to an electrical output signal that is proportional to flow rate.

All of Niagara’s flow measurement devices are time proven through many applications. For assistance in selecting and configuring a flowmeter for a particular application, share your requirements and process measurement challenges with a product specialist. The combination of your process knowledge and their product application expertise will produce effective solutions.

Installing a Clamp-on Ultrasonic Flow Meter

Ultrasonic flow meter with clamp on sensor
Courtesy Flexim

Ultrasonic flow meters are utilized throughout the fluid processing industries, as well as for compressed air and energy monitoring. The non-invasive nature of the sensor installation couples with sufficient accuracy and low maintenance requirements to give this technology a competitive edge for many applications.

Producing consistently accurate results with an ultrasonic flow meter depends heavily on a proper installation. Flexim, a globally recognized leader in the manufacture of ultrasonic flow meters, provides us with a video that steps through the installation process, with recommendations and guidance along the way.

Flexim manufactures a full range of ultrasonic flow measurement equipment and instruments for industrial and commercial applications. Share your flow measurement challenges with process measurement experts, combining your process knowledge with their product application expertise to develop effective solutions.

Applying Precision Turbine Flow Meters

High precision turbine flow meters
Courtesy Cameron Measurement Systems

Precision turbine flow meters are specially designed to accommodate a broad range of precise fluid measurement applications. They accommodate greater flow rates with lower pressure drops than other meters in their class. Some have a self-flushing design for longer sustained accuracy. The turbine's high-frequency digital output is suitable for interfacing with an assortment of readout and recording equipment. Some turbine flow meters have a symmetrical bi-directional design that supports reverse flow applications without a reduction in accuracy or capacity.

Operating Principle

(The following is excerpted from Model 700 Series Turbine Flowmeter User Manual, from Cameron Measurement Systems....with some editing)

Fluid flows over a diffuser section and is accelerated onto a multi-blade hydro-dynamically balanced turbine rotor. The rotor speed is proportional to the volumetric flow rate. As the rotor turns, a reluctance type pickup coil (mounted on the meter) senses the passage of each blade tip and generates a sine wave output with a frequency that is directly proportional to the flow rate.

The rotor is the only moving part of the turbine flow meter. The small lightweight rotor hubs ensure fast response to process flow changes. The rotor is hydro-dynamically balanced during operation, eliminating the need for mechanical thrust leveling. This low-friction design improves metering linearity and reduces wear and maintenance.

A variable reluctance generating pickup coil contains a permanent magnet and a wire winding. In some cases, the rotor blade of the turbine meter is made of a ferritic stainless steel such as grade 430. The movement of the rotor blade in proximity to the magnetic field of the coil tip produces an AC type voltage pulse within the coil winding. An alternate arrangement finds the ferritic bars embedded in the rotor shroud, where they can interact with the pickup coil. Increasing the quantity of bars on the shroud to outnumber the rotor blades provides more pulses per unit volume (resolution). This feature can be valuable when proving large-capacity meters with a small-volume prover. Shielded wire cable conveys the output of the pickup coil to compatible electronic instruments to indicate flow rate, record, and/or totalize the volumetric flow. The coil itself does not require electrical power to operate.

The meter may be factory-fitted with multiple coils for redundancy, indication of flow direction or pulse train verification. The pickup coil type and magnetic strength vary with application requirements.

The turbine flow meters are calibrated in a horizontal position. Therefore, the best correlation of calibration occurs when the meter is operated in this plane. However, the meter will operate satisfactorily in any position.

System Pressure

The maximum and minimum system pressures must be considered when applying the turbine meter. To obtain proper response, a back pressure should be applied to the meter. This back pressure should be at least twice the pressure drop of the meter at maximum flow. For liquid meters, the back pressure should be twice the pressure drop of the meter at maximum flow, plus twice the fluid vapor pressure.

Summary

Turbine flow meters, with their simple, durable construction and wide operating range, may be the right choice for a number of applications. As with all instrumentation, there are a number of factors to consider when making a selection. Share your flow measurement challenges and requirements with instrumentation specialists, combining your process knowledge with their product application expertise to develop the most effective solutions.

Turbine Liquid Flow Meters from MSJacobs-Cali

Don't Ignore the Simple Solution for Flow Measurement

Flow indicator also permits visual
inspection of fluid
Courtesy ERDCO Engineering Corp.

For process control and commercial or industrial applications, there are numerous methods of flow measurement from which to choose. Technologies range from very simple applications of physical principles to deployment of very specialized electronics and sensors. The available range of accuracy, response, and cost is quite broad, with a general expectation that higher cost will deliver better performance and accuracy.

Making the best instrument selection for a flow measurement application should include an assessment of what the operators really need in order to safely and effectively run the process or perform the task related to the measurement of fluid flow. Installing instrumentation with capabilities far beyond what is required is almost certainly a waste of financial resources, but may also have an unexpected impact on operators. Through the generation of data that, while accurate, does not provide any actionable information about process condition, operators can be misled, similar to the occurrence of a false or nuisance alarm. Some applications call for high accuracy, some do not. Define your informational needs and select instruments that will meet those needs.

There is a large array of applications that can be satisfied with simpler, less costly measurement technology. These devices often employ turbines or vanes to produce an indication of flow rate. Incorporated into some of the instruments is a means to visually observe the flowing liquid to verify color and clarity. Simple devices sometimes are intended only to indicate the presence of fluid flow, and whether the flow rate is high or low. Configurations are available that allow insertion into lines under pressure (hot tap) through a full port ball valve. Other variants with combinations of features and capabilities abound.

The selection range is enormous, so define your minimum needs first, then search for a compatible product. Your search can be enhanced by contacting an instrumentation specialist. Combining your process expertise with their broad product knowledge will produce effective solutions.

New Product From Cameron - CamCor Coriolis Flow Meters

CamCor Pro - Industrial Coriolis Flow Meter
Courtesy Cameron

Whether it's high viscosity crude oils, ultra-low flow conditions or applications at the extremes of temperature, Cameron's new CamCor™ range of Coriolis flow meters deliver value beyond the whole-life operational benefits associated with non-mechanical architectures.

CamCor comes in two architectures, CamCor CT for custody transfer, where the emphasis particularly is on high accuracy, and the Cameron CamCor PRO for process operations.

CamCor CT offers flow rate accuracy of ± 0.1% for liquids, as a result of its deep “U” shaped dual sensors, with outstanding zero stability performance (0.071 lb/min for 2” model). The range includes models purpose-designed for application extremes, covering temperatures from -200°C (Cryogenic/ LNG) up to 350°C. Nominal sensor sizes go from less than 2mm to 250mm, with 1/4” to 10” end connections, offered in ANSI class 150 to 900, other flange types, threaded, or Tri-Clover.

The unit is manufactured of 316/316L stainless steel and Hastelloy Alloy C22.

The CamCor PRO Series possesses a flow rate turndown ration up to 50:1, flow rate accuracy of
+/- 0.2% and density accuracy of +/- 0.003 g/ml. It comes with the same transmitter, output, communications, diagnostics and configuration as the CamCor CT and is available in four nominal sensor sizes, from 6mm to 50mm, with 1/2” to 2” end connections in ANSI flanges and Tri-Clover.

Review the product brochure below, or contact a product specialist for more details or a discussion of your application.

Coriolis Flow Meter - Cameron Camcor Brand from MSJacobs-Cali

Multivariable Vortex Flowmeter

Multivariable Vortex Flowmeter
Courtesy Azbil North America

A new multivariable vortex flowmeter has recently been added to the product line at MS Jacobs & Associates. It is intended for industrial process measurement and control. The AX Series, from Azbil North America, utilizes a vortex shedding velocity sensor, solid state pressure transducer, and resistance temperature detector in a single unit to allow accurate measurement of mass flow in gases, liquids and steam. The combination of multiple variable measurement in a single package simplifies installation and setup cost, while reducing potential leakage paths of an arrangement employing separate instruments for each variable. The AX Series is available for in-line or insertion installation. Your product sales engineer can provide a sizing guide and other assistance in selecting and configuring the best product for your application.

Azbil Mulitvariable Vortex Flowmeter from MSJacobs-Cali