Some years ago, I was assigned to upgrade the instrumentation on a hazardous waste incinerator. For those of you not familiar with hazardous waste incineration, the fundamental process involves generating a large flame into which wastes are injected and subsequently thermally reduced to their products of combustion, which do not pose a hazard to the environment.
In this particular process, one of the waste-fume streams was relatively large and contained a significant amount of oxygen. The concept of using this stream to replace a portion of the combustion air had been partially attempted in a rudimentary manner a few years prior. This attempt was doomed to failure for a number of reasons. Nonetheless, when resurrected, substituting the waste-fume stream for combustion air placed an enormous design burden on the combustion-air flowmeter.
Assuming the incinerator operates from 20 to 100 percent of its firing rate, the combustion-air flowmeter would be reasonably expected to operate with at least a five-to-one turndown. However, as fume substitution is increased, the amount of combustion air required decreased, so the turndown requirement of the meter had to increase to operate over a larger range of flowrates.
Compounding the situation, the waste-fume stream was large enough that it could supply the entire combustion-air requirement. Theoretically, no combustion airflow would be required at this operating condition. However, if the combustion airflow were allowed to reach zero, the waste-fume pressure would overpower the combustion-air fan and cause waste fumes to be released into the atmosphere without being incinerated. Therefore, a minimum combustion-air flowrate had to be maintained at approximately 2 percent of the full-scale flowrate. This necessitated a combustion-air flowmeter turndown of approximately 50-to-one.
To avoid potential control issues, the combustion-air flowmeter was required to measure zero flow under reverse flow conditions. It is important to note that many flowmeters measure flow and do not distinguish between forward and reverse flow directions.
The aforementioned requirements placed an onerous burden on the combustion-air flowmeter that eliminated many options. After all was said and done, a turbine flowmeter that measured zero flow under reverse flow conditions was selected. Its problems were primarily related to maintenance because its rotor had to be replaced annually due to bearing wear. As for its overall performance — it did what it was supposed to do.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME, MFC, and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
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