For two years, Flow Control’s thermal mass flow measurement technology portal has been an important resource for flow measurement and instrumentation data. Tom Kemme, our thermal dispersion product manager, answers questions about the technology in the portal’s Ask the Expert column. This week’s blog shares some recent Q&As.
Question: Are thermal dispersion flow meters a good fit for natural gas flow measurement applications?
Answer: Natural gas flow measurement is a popular application for thermal flow meters. While thermal flow meters are not an approved custody transfer meter for natural gas, they are often used to measure natural gas flow to individual combustion sources. There are many advantages to thermal over other technologies. For example, thermal flow meters have better low flow sensitivity and a higher turndown than traditional flow meters that utilize differential pressure technology. There is more information available in the Magnetrol® white paper, “Tracking Natural Gas With Flowmeters,” which is available on the flow measurement and instrumentation portal.
Question: How does a thermal dispersion mass flow meter compare to a vortex flow meter for gas measurement applications, in terms of advantages and disadvantages?
Answer: There are advantages and disadvantages to every flow meter technology. The most common applications for vortex flow meters are steam and liquid flow measurement. They are also used in high velocity gas flow applications, but there are limitations in terms of the low flows a vortex flow meter can measure. Gas flow rates must be high enough to create vortices around the blunt element in the line, which is the basis of the measurement. The flow rates measured by vortex flow meters are actual flow rates, or the flow rate at operating conditions. To convert to standard conditions (mass flow) the user must make a conversion based on measurement of the operating temperature and pressure, or have a flow meter that is integrated with a multivariable transmitter.
By contrast, most thermal flow meters are used in gas flow applications. Condensed moisture in the line can cause high measurements as the relative cooling of the sensor increases. Thermal flow meters are often installed at points where condensation is knocked out of the line. Thermal flow meters have high sensitivity at low flow rates and low pressures, which is a difficult measurement for many other technologies. It is also one of the only technologies that outputs a mass flow, taking away the need for external temperature and pressure measurement.