Sometimes, the obvious is not quite evident. Have you specified a flowmeter for natural gas service in your career? I suspect that a few individuals routinely specify natural gas flowmeters, while many others have specified a handful of such flowmeters in their careers, and the remainder has not specified any flowmeters for natural gas service.
Regardless of where you fall in this spectrum, consider how you would describe the process fluid in your specification. The obvious choice would be "natural gas," and you would be in good company if this was your response.
However, natural gas is not a pure substance but rather a naturally occurring gas that largely contains methane but can also contain significant amounts of other combustible and inert gases. The North American Energy Standards Board (NAESB) suggests that a typical natural gas composition consists of 94.9 percent methane, 2.5 percent ethane, 1.6 percent nitrogen, 0.7 percent carbon dioxide, 0.2 percent propane, 0.03 percent each of iso-butane and normal-butane, 0.1 percent oxygen, 0.01 percent each of iso-pentane, normal-pentane and hexanes, and a trace of hydrogen. However, NAESB suggests that these percentages can vary considerably, such as 87 to 96 percent methane, 1.8 to 5.1 percent ethane or 1.3 to 5.6 percent nitrogen.
In short, the chemical composition of natural gas at a given location is determined by the nature of its source and treatment. Further, the composition of the natural gas can be different than typical NAESB natural gas and can vary over time as its source ages and/or changes. Therefore, its physical properties such as its density, heat content and thermal properties can be different and vary over time. This reality begs one to question whether these (potentially large) differences from a typical NAESB composition can affect the measurement of the flow of natural gas.
David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.