| |
| David W. Spitzer, P.E.
|
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.
For More Information: www.spitzerandboyes.com
|
|
