Part III: Open-Channel Flowmeter Problems
For the past two months, I asked what can be wrong with a flowmeter and referred to an article where an open-channel measurement (used to measure the wastewater effluent prior to the pumping station) was suspected to be “way off” (Flow Control, May 2012, page 14; June 2012, page 6). A number of end-users and consultants provided responses—and their suggestions varied all over the map, including:
Checking that the relation between flow and level is linear does not seem logical given that the relationship between flow and level for this type of flowmeter is not linear. However, a logarithmic graph between flow and level yields a straight line, so let’s assume that this is what was meant. Continuing, part of the explanation suggests that if the flowrate and corresponding height does not match what you expect, you should use a few known data points to generate your own linearly extrapolated calibration data. This solution appears to address a transmitter calibration problem and discards the possibility of having other hydraulic, installation, design, electronic, or other problem(s) that could clearly occur. For example, what if the improper upstream piping causes the flowmeter to measure high at some flowrates and low at others? What if jetting is present? What if the level sensor is not properly located? What if the level sensor is plugged? What if…?
Determining the flowrate using dilution techniques is a reasonable way to verify whether the open-channel flowmeter is accurate (at the tested flowrate). However, it does not solve the open-channel flow measurement problem.
Making sure that the depth sensor is clean and that an ultrasonic sensor is not reading foam is a reasonable and easy first step that can typically be performed by visual inspection.
So, the lesson learned here when asking what can be wrong with a flowmeter is … Just about anything. The flowmeter can be designed wrong, installed wrong, calibrated wrong, operated wrong, and/or maintained wrong. To find the problem (or problems), one should examine the entire flowmeter system and its installation, operation and maintenance from the start of its upstream straight run to the end of its downstream straight run. Subtleties and nuances can make a world of difference.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.
Mr. Spitzer can be reached at 845 623-1830 or www.spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.
