As we discussed in last week’s blog article, thermal dispersion mass flow technology is rapidly becoming a preferred method to measure the flow rate of gas for industrial process operations. Thermal mass flow meters offer an attractive alternative to older measurement technologies, because they provide a direct, accurate measurement of mass flow rate, without having to correct for temperature and pressure conditions. Additionally, because of its properties of low flow sensitivity, wide turndown and low pressure drop, thermal dispersion mass flow measurement can improve process control in a wide variety of typical and more challenging applications.
- Compressed air/gas. Measurement and totalization of compressed air or gas flow is utilized for internal plant allocation and measurement of overall consumption. Can be used for detecting and identifying general location of leaks. Government estimates show that leakage can account for 20-30% of compressed air generation. Eliminating a small ¼” leak can result in cost savings up to $4000 in a year.
- Combustion air flow. Measurement of the mass flow rate of combustion air is desirable when determining fuel-to-air mixtures for proper combustion control. Thermal mass measurement is very appropriate due to its combination of direct mass measurement, excellent low flow sensitivity, wide turndown and low pressure drops.
- Natural gas. In-plant measurement of natural gas flows to boiler, furnaces, dryers and heaters is an ideal application for thermal mass flow measurement. Knowing the natural gas usage of individual combustion sources can help identify efficiency, leading to reduced fuel usage. The composition of natural gas may vary slightly during the year. These changes in heat transfer characteristics of the gas are minor and will not have any noticeable effect on a thermal mass flow meter’s performance.
- Greenhouse gas emissions. The EPA requires many facilities to annually report their greenhouse gas emissions from each combustion source. A thermal mass flow meter is an ideal method of determining the natural gas usage, in order to follow the EPA guidelines to calculate greenhouse gas emissions.
- Flare lines. Thermal dispersion mass flow offers many benefits for flare lines – wide turndown, low flow detection and low pressure drop. Thermal flow measurement has successfully been used in this application. However, consideration must be given to changes in gas composition.Different gases have different thermal properties that affect convective heat transfer. Changes in gas composition will change heat transfer rates, resulting in inaccuracies in flow measurement.If used in a flare line with a consistent gas composition such as natural gas, there is no difficulty. However, if used in an application with wide variations in gas composition, especially major changes in concentration of hydrogen, the user must be aware of the considerable potential for inaccurate flow measurement. Hydrogen cools the sensor much greater than other gases; a small flow of hydrogen will appear like a much larger flow of hydrocarbon gases. In those applications with varying gas compositions, a thermal mass flow meter, such as the Thermatel® Model TA2, will provide a relative flow measurement. It can be used to provide an indication of changes and magnitude in flow rate, as well as duration of a release to the flare. Often a Model TA2 mass flow meter will be used to monitor the flow to the flare from individual production units, with a different technology flow meter measuring the main flare flow for environmental reporting or obtaining a mass balance. Provided that the flow meter is used for flow monitoring rather than flow measurement, consideration should be given to a simple calibration rather than trying to calibrate for an exact gas mixture.
- Stacks. While thermal mass flow measurement has successfully been used for measurement of stack flow, generally multiple point array systems are utilized for large diameter stacks. Another option is to use four or more single point probes inserted from opposite sides of the stack. An external device is needed to average the flow rate.
For a complete overview of thermal mass flow technology, methodology and applications, Magnetrol® invites you to download our THERMATEL Thermal Dispersion Mass Flow Measurement Handbook.