What can be wrong with a flowmeter? I recently dug up an old magazine article in which an open-channel measurement was used to measure the wastewater effluent prior to the pumping station. Accurate flow measurement was required for environmental compliance purposes. It was suspected that the flowmeter was “way off” because the readings did not make sense, were consistently low, and did not match consumption rates. The user wanted to end the calibration nightmare and verify the actual flowrate.
Where should you start on this problem? It depends. You can start where the person with the problem suggests that you start—note that the last sentence in the preceding paragraph states that the problem is related to calibration. That may be, but there is a lot more to a flowmeter installation than its calibration. What about that orifice-plate flowmeter located two diameters downstream of an elbow? On one occasion, I ran into an orifice flowmeter installation where the bypass on the differential-pressure transmitter was cracked open so that the impulse lines would not freeze during cold weather. Needless to say, the flowmeter was not measuring correctly when its hydraulic signal was partially “shorted.” Calibration would not have resolved this issue. I suggest that one should consider all aspects of the flowmeter design, installation and operation before coming to any conclusions.
A number of end-users and consultants provided responses … and their suggestions varied all over the map to include decreasing water velocity, installing an ultrasonic level transmitter, verifying the type of flow through the flowmeter, measuring the velocity with a Pitot tube, checking to ensure the relation between flow and level is linear, determining the flowrate using dilution techniques, and making sure the depth sensor is clean and not measuring foam. Taken individually, they are quite interesting, so let’s examine them one at a time.
The first suggestion was to decrease the water velocity suggesting putting “artificial barriers” in the pipe, or rebuilding the entrance box so that the flow enters the entrance box at a right angle to the flowmeter. Because the responder did not suggest increasing the flowmeter size to decrease velocity, I suggest that this response relates to the possibility that the water flow was jetting as it entered the flowmeter. This is certainly possible and could be determined by examination of the upstream piping and/or the water flow as it enters the flowmeter.
More next month …
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.
David can be reached at 845 623-1830 or spitzerandboyes.com. Click on the “Products” tab to find his “Consumer Guides” to various flow and level measurement technologies.
