Implementing Thermal Dispersion Technology for Mass Flow Measurement: Common Questions and Answers

During the past few months, the thermal mass flow measurement portal for Flow Control magazine has featured Magnetrol® Product Manager Tom Kemme as its resident expert on thermal dispersion technology. Tom has received many excellent questions about the proper use of thermal dispersion in mass flow measurement, and we will be sharing answers to some of these inquiries on this blog during the next two weeks. Consider subscribing to the MAGNETROL blog to receive the complete discussion.

Question: For wet gases/vapors (e.g., compressed air), what is the maximum allowable moisture content that will allow the thermal mass flow sensor to work without incurring significant error due to heat loss from surface condensation? Also, can temperature sensor pins be coated with PFA or ETFE for use in corrosive acid vapor service?

Answer: Thermal flow meters are used in applications that have vapors present as well as gases that are considered “wet,” such as digester gas. If actual condensation is present and comes into contact with the probe tips, this can cause a spike in the reading due to the additional cooling of the liquid.

Liquid drops themselves do not damage the sensor, but corrosion is a different issue. Most manufacturers offer a standard stainless steel probe option, but many also have a Hastelloy option for more corrosive gases. Other sensor materials may also be available, depending on the manufacturer.

Thermal dispersion flow control technology

One option for an application where condensation could be present is to install the probe at an angle to prevent the liquid from dripping down the probe and coming into contact with the pins. Because thermal flow meters do not have to be installed in the top center of a pipe or duct, they can go into horizontal or vertical lines. An example of this is shown here.

Coating the sensor is not typically recommended because it would change the thermal characteristics of the sensor. This could affect sensitivity and achievable flow rates.

As important as accuracy is, many customers are looking for a relative indication of the amount of leakage in compressed air applications. Knowing this measurement assists in calculating the approximate savings by eliminating such leaks.

Every flow meter has advantages and disadvantages. For a thermal flow meter, some of the main advantages include:

1. Direct mass flow measurement
2. Strong signal at low flows or low pressures
3. High turndown

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

Answer: Thermal flow meters are at the forefront in flow measurement for emissions reporting and energy management projects in many markets, including power and oil and gas. Popular applications include monitoring gas fuel flow to a combustion source to report sulfur dioxide (SO2) emissions and stack (flue) gas flow in power plants as part of a continuous emissions monitoring (CEM) system of nitrous oxide. These applications prove difficult for many flow meter technologies.

For example, in a flare application, the gas may not be flared off. However, the user will still want to monitor the low flow of pilot gas keeping the flare lit in case of an actual event. This requires a flow meter with a very high turndown and good low flow sensitivity, which is a limitation of some technologies, such as differential pressure flow meters.

The most popular gas in the emissions conversation is carbon dioxide (CO2). However, we often find applications that need methane measurement. Methane is a greenhouse gas that has more than 20 times the global warming potential of CO2, so coal mines and landfills cannot emit it directly into the atmosphere. If owners do not flare the gas off, they capture and treat it to produce usable natural gas.

Some facilities that emit landfill gas – or produce biogas – are involved in carbon credit programs or clean development mechanisms. Similar applications are 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 has become well accepted in all of these markets.

Flow Instrumentation For Energy Management Bulletin



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