Required Information for Sizing the Correct Thermal Mass Flow Meter for Your Application

Thermal mass flow meters offer accurate, repeatable, and dependable measurements of liquids and gases for both industrial and environmental processes.  They are used in a multitude of applications including landfills, GHG emissions, flare gas, natural gas measurement, pollution control, petrochemical, chemical processing, measuring hydrogen gas, food and beverage, pharmaceutical, semi-conductor manufacturing, sub-metering of clean process gases, and power / utility.

When sizing a thermal mass flow meter, the following information is required in determining the correct flow meter for your application:

1. Fluid
2. Insertion or inline meter
3. Minimum and maximum line pressure
4. Minimum and maximum flow rate
5. Minimum and maximum fluid temperature
6. Pipe diameter, schedule and material
7. Standard, sanitary or ultra-high purity finish
8. Desired process connections
9. Flow direction
10. Available power for transmitter
11. Transmitter output

Once these parameters are determined, a mass flow meter may be properly specified for your application. To be safe the instrument will perform safely and without problem, it is highly recommended you consult with a process instrumentation expert for assistance.

M.S. Jacobs and Associates
https://msjacobs.com
800-348-0089

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 Measurement of Tank Blanketing Gas

Insertion style thermal mass flow meter can measure
low flow rates of gas for tank blanketing.
Courtesy Magnetrol

Closed liquid tanks and other vessels contain two substances, liquid and not liquid. The liquid, in this case, is the subject material of a process. The "not liquid" is whatever fills the space not filled by the subject liquid. There are many cases where the process, the subject liquid, and safety are best served by filling the space with a known gas. There may be concerns about ignition of the vapor from the liquid, reactivity of the liquid with oxygen, or a wide range of other issues that call for filling the tank space with a known gas.

Nitrogen is a commonly employed gas for tank blanketing. It is comparatively inexpensive and widely available. It can inhibit combustion by displacing atmospheric oxygen and is not reactive with most industrial process chemicals.

Vessels with rapidly changing levels, or those of very large size, will require larger available flow capacity of blanketing gas to maintain the desired conditions within the tank. There are regulating valves designed specifically for tank blanketing operations. Vents intended for use in the same application are also commercially available.

Monitoring tank liquid level and gas flow are part of best practices for a tank blanketing operation. Confirming that gas flow rate is commensurate with the requirements for current tank level confirms proper operation. Too high a flow rate could indicate a leak or malfunction of a blanketing system component. It may also be useful to totalize gas flow for use in operational planning.

Thermal insertion flow meters are suitable for measuring the wide range of gas flow rates employed in tank blanketing applications. The instruments are available for insertion installation, as shown in the image near the top of this article, or as inline units. Either configuration delivers accurate measurement with no moving parts, a high turndown ratio, and minimal maintenance requirement.

Share your tank blanketing requirements and challenges with process measurement and control specialists, combining your own process knowledge and experience with their product appliction expertise to develop effective solutions.

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.

Thermal Mass Flow Meters - Mature Technology With Modern Application

Configurations of Thermal Mass Flow Meters
Courtesy Thermal Instrument Co.

Thermal mass flow measurement technology has been used in industrial process measurement and control applications for many years. The technology measures the amount of heat required to maintain a sensor at a constant reference temperature, offsetting the cooling effect of the fluid flow over the sensor. The technology is well suited for installations requiring the measurement of low pressure gases where the fluid components are known and remain constant. Thermal mass flow measurement is also employed effectively in a number of other scenarios.


Potential advantages of thermal mass flow technology for industrial process measurement and control:

• Provides true mass flow reading using a single instrument
• Not significantly affected by fluid pressure or temperature
• Moderate comparative cost
• No moving parts
• Minimal restriction of flow introduced by sensor

Typical applications for thermal mass flow meters:

• Landfill gas outflow measurement
• Compressed air systems
• Gas distribution in semi-conductor manufacturing
• Beverage carbonation
• Tablet coating and compression in pharmaceutical industry
• Flare gas measurement in oil and gas industry
• Natural gas flow to boilers, furnaces, and other consuming equipment

Thermal mass flow sensors have attributes making them very suitable for a family of applications. Like all measurement technologies, there are also areas of caution in their application. Any measurement technology must be properly applied in order to obtain reliable results. Talk to a sales engineer about your flow measurement ideas and applications. Combining your process expertise with the knowledge of a product application specialist will produce good results.