With today’s increased emphasis on strategic energy management, many throughout the chemical process industries and elsewhere are attempting to obtain better information on the natural gas consumption in their facilities. While custody-transfer flowmeters are typically in place at the property line (to track total gas consumption throughout the facility), the flow to individual combustion sources, such as heaters, furnaces, boilers and so on, generally remains unknown. When armed with better information on actual natural gas utilization, users can optimize the combustion performance by operating combustion processes at peak efficiency.
Similarly, when users measure actual natural gas flow, they are able to determine which of their units is the most efficient. The operating efficiency of furnaces or dryers will vary. Knowing which process unit is the most efficient can result in significant cost savings. For instance, if more than one furnace, dryer or other type of gas-consuming unit is available, the user will be able to choose the combustion unit that provides the highest efficiency.
The first step to better energy management and improved energy efficiency is to obtain good measurements of the flow rates of each individual combustion source. In addition to providing tools for improving energy management, the measurement of the natural gas utilized by individual combustion sources may also permit users to meet the regulatory requirements for determining emissions by reporting actual (rather than estimated) natural gas usage for each individual combustion source within the facility.
Concerns When Installing Flowmeters
There are many different ways to measure the flow of gases. The difficulty in obtaining good gas flow measurements is the simple fact that gases are compressible, and thus the volume of the gas is dependent upon the pressure and temperature at the point of measurement.
Chemical engineers will recall the basic concepts of the Ideal Gas Law, whereby gas volume is proportional to the temperature and inversely proportional to the pressure. This complicates gas flow measurement because, with the exception of thermal mass and Coriolis flowmeters, many gas flow measurement technologies measure the flow at the actual operating pressure and temperature.
Thermal Mass Flowmeters
Thermal mass flowmeters provide an inferred measurement of the mass flow of the gases passing through them. Specifically, thermal mass flowmeters measure heat transfer that is caused as the molecules (hence, the mass) of gas flow past a heated surface. The relationship between heat transfer and mass flow is obtained during the calibration of the instrument.
In addition to providing a mass flow measurement without the need for additional devices to correct for pressure and temperature (as is required with many other flow measurement devices), thermal flowmeters also provide the following advantages:
- Lower flow sensitivity. A thermal mass flowmeter will easily measure flow rates that are much lower than those that can be measured using orifice plates or vortex flowmeters. This permits a thermal flowmeter to be retrofitted into existing natural gas pipes using a simple NPT (national pipe thread) thread or flange connection on the pipe. This simplifies installation compared to other flowmeters, which may require a reduction in the pipe size in order to obtain the desired rangeability.
- Higher turndown capabilities. Some combustion systems may have a high natural gas firing rate during initial warm-up operation and then, once the desired temperature has been obtained, the flowrate of the gas is typically reduced to maintain the desired operating temperature. A thermal mass flowmeter can easily handle this range, which may be difficult to obtain with other technologies.
- Simplified installation. An insertion device permits simplicity of installing the flowmeter using NPT connection, flange, compression fitting or even a complete retractable probe assembly. Using a “hot tap” permits the user to install the flowmeter without having to shut down the operation. The insertion design also permits the use of the same instrument in different pipe sizes. Some use the insertion probe as a semi-portable instrument and reconfigure the transmitter for the different pipe sizes.
- Lower pressure drop. There is virtually no pressure drop when using a thermal mass flowmeter. This is advantageous in low-pressure applications where other technologies would consume operating pressure.
Magnetrol® produces a thermal mass flowmeter, the Thermatel® TA2, which can be used for natural gas applications. To learn more about the THERMATEL TA2 and other advantages of flowmeters, please visit flow.magnetrol.com.