Assessing the performance of a number of boiler steam flowmeters that were allegedly not accurate because they had not been calibrated for the previous seven years was discussed in a previous column. Removal, inspection and calibration were not possible because the flowmeters could not be taken out of service and the plant refused to calibrate these transmitters containing mercury.
The approach taken to assess performance was to compare these flow measurements with other plant instruments that were regularly calibrated. In particular, the steam flow leaving a particular boiler can be estimated by taking the boiler feedwater flowmeter measurement and subtracting the blowdown.
The boiler feedwater flowmeters were routinely maintained and calibrated to provide accurate measurements. For legal reasons, my investigation was limited to the steam flowmeters. However, the boiler feedwater flowmeter charts were conveniently located alongside the steam flowmeter charts so it was easy for my eyes to wander and record both the instantaneous feedwater flow and the instantaneous steam flow measurements for later analysis.
Legal restrictions made estimating the blowdown flow a bit more complicated because blowdown flow is generally not measured. However, the ratio of the blowdown flow to the boiler feedwater flow (hence, the ratio of the steam flow to boiler feedwater flow) is dependent upon the nature of the water treatment system and its chemistry. The steam producer would likely raise objections and deny access to certain information if detailed information about the water treatment system was requested. As such, we had to carefully request drawings and judiciously request access to certain parts of the plant. Even so, the steam producer suspected that something was awry due to the peripheral nature of our requests.
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.
