Within the three situations that should be carefully considered: operation under vacuum, measuring downward liquid flow and gravity flow, we continue the discussion on measuring downward flow.
One of the most insidious problems encountered with flowing downward is maintaining the pipe full of liquid. Often, an initial analysis of a given downward flowing piping geometry would indicate that the pipe remains full of liquid. However, measurement errors on the order of 10% to 50% during operation can quickly paint another picture.
Consider an installation where a liquid was pumped from a tank on the first floor to a drum located at grade. The piping was routed upward to the first-floor ceiling before running horizontally to where it dropped to a manual filling valve located above the drum. A flowmeter was installed above the manual valve so the flowmeter should have been completely full of liquid. Flow measurement errors of 10% to 50% — as compared to a weigh scale — prompted an investigation that concluded that the flowmeter was not full of liquid, even though it was installed in a pumped system upstream of a throttling valve.
In another installation, the continuous overflow from a tank was measured by a flowmeter located in the (upstream) downward flowing leg of a U-shaped trap. From a hydraulic perspective, the flowmeter was located in a submerged leg — below the inverted U — and should have been full of liquid. However, erratic flow measurements led to an investigation that revealed that the falling overflow liquid caused turbulence and bubbles that reached the flowmeter and affected its measurement. The definitive solution to this problem would have been to relocate the flowmeter to the upward flowing part of the U-shaped trap. However, the pragmatic, no-cost solution was to throttle the flowmeter discharge to raise the liquid level in the overflow pipe above the flowmeter to isolate the flowmeter from the effects of the falling liquid.
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.