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Application Impossible by Walt Boyes
One of the problems with automated systems is
that they are often an afterthought.
In the process industries, batch processing can sometimes be done in the pilot stages without much control. Then control is "grafted on" to make bigger batch systems. In systems where the production is continuous, sometimes flow was never considered, and now someone has decreed that there should be a measurement of flow from a particular spot.
Usually, as we've seen, that spot is unsuitable for measuring flow. If you can get a decent flow value, say 10 to 15 percent of actual flow, and it is repeatable, in a continuous process, you have a workable system. If you want real accuracy, especially in batching applications, you may be forced to explain to somebody who really doesn't want to hear this, that the cost, and the downtime, to make the change will be much more than simply buying a flowmeter and putting it in.
Remarkably, few process control engineers have much training in this sort of presentation, although they get to do it quite a bit. Sooner or later you'll be faced with the difficult-to-impossible application.
Chemical Composition of Natural Gas: A Game of Alphabet Soup by David W Spitzer
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 at all. 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 considerable, 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.
Effect of Gas Temperature Variation by David W Spitzer
A flowmeter that measures velocity is designed to measure the flow of a gas whose temperature is controlled within 3 degC of its setpoint of 27 degC. What is the approximate flow error resulting from the expected temperature fluctuation?
+/- 1 percent
+/- 3 percent
+/- 5 percent
The output of a flowmeter that measures fluid velocity will increase (or decrease) linearly as the size of the fluid increases (or decreases). Therefore, the effect of a 3 degC temperature change on the flow measurement depends upon the amount by which the gas volume changes.
One might be tempted to select Answer D because the temperature change is almost 10 percent (27 to 30 degC). However, the change in size of the gas is related to the change in its absolute temperature which is relative to the temperature at which all heat is removed --- not relative to the freezing and boiling points of water.
The setpoint of 27 degC was, not coincidentally, chosen for this application because it approximates room temperature and corresponds to an absolute temperature of 300 Kelvin. A 3 degC temperature change represents a 1 percent change in absolute temperature and a 1 percent volume change (Charles' Law), so Answer A is correct.
The answer may seem academic but a deeper understanding reveals that even relatively small temperature variations under nonextreme operating conditions can have a significant effect on flowmeter performance, especially since flowmeter accuracy specifications are often better than 1 percent of rate. The effect under actual conditions can be dramatically larger.
Additional Complicating Factors
Flowmeters that do not measure linearly as a function of velocity will exhibit different relationships. For example, a differential pressure flowmeter will exhibit approximately one-half the error as compared to a linear flowmeter. In contrast, thermal flowmeters should not be affected by temperature variations.
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 450 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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