My previous article described the existing installation and operation of a fume flowmeter for an incinerator. The new installation is more complex and uses fumes to replace up to 80 percent of the combustion air, but the fume flow can drop to near zero flow in approximately five seconds after the unit shuts down or trips.
The existing Pitot tube flowmeter measured reliably over the years, but it had its negatives. In particular, the Pitot tube is an inferential flowmeter; its low-range, differential pressure transmitter was difficult to calibrate, it drifted (especially when subjected to sunlight), and its squared output degraded accuracy at the low flow rates that needed to be measured in the new process.
The new process diverted most of the fumes away from the fume nozzles to the burner, so the new combustion control strategy required measurement of its flow to achieve stable combustion. In addition, the measurement of fume flow to the fume nozzles was not feasible, so measurement of the fume flow to the burner was also utilized to calculate the fume flow to the fume nozzles (which was monitored and controlled). In other words, this measurement was important because it played a role in keeping the fume nozzles from overheating as well as account for the equivalent of up to 80 percent of the combustion air to the burner.
An insertion thermal flowmeter was selected for this application because it was linear and able to measure mass over a wide flow range of flows. The new flowmeter worked well for a week before it started to measure incorrectly. The sensor was removed, cleaned and returned to service. The same symptom returned a few days later, so the new flowmeter was replaced with an insertion Pitot tube similar to the existing fume flowmeter that had been in service for years.
More next month.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control.
Spitzer may be reached at 845-623-1830 or via spitzerandboyes.com. Click on the "Products" tab to find his Consumer Guides to various flow and level measurement technologies.
