Application Corner: Measuring downward flow, part 3

Under no flow conditions in downward flow, the pressure actually increases through the flowmeter as a result of static pressure due to decreasing elevation.


We continue the discussion of three important situations to carefully consider: operation under vacuum, measuring downward liquid flow and gravity flow. Operating under vacuum was discussed in the previous two columns, so let’s move on to measuring downward flow. 

When selecting a flowmeter for liquid service, it is common to consider its upstream pressure, downstream pressure and the pressure drop to ensure that the flowmeter will pass the design flow in the process piping system. Interestingly, under no flow conditions in downward flow, the pressure actually increases through the flowmeter as a result of static pressure due to decreasing elevation. This pressure increase gets smaller as flow is increased and may eventually become a pressure drop at higher flow rates when the pressure drop across the flowmeter becomes larger than the static pressure increase due to elevation. 

It is not clear how this pressure rise might affect flowmeters — if at all. However, some years ago, I was charged with fixing a particular vortex shedding flowmeter installed in downward flow that consistently differed from process calculations by approximately 10% since its installation approximately a decade prior. This particular flowmeter design had exhibited problems in other plant locations subject to high vibration, but vibration was not an issue in this location, and similar vortex shedding flowmeters of the same design not subject to vibration performed accurately. Being pragmatic, we purchased a new vortex shedding flowmeter of a different design, but installed it with upward flow, and it performed accurately. Given the potential vibration problem, it is not clear whether this measurement issue can definitively be assigned to downward flow. Reinstalling the existing flowmeter in upward flow would have reduced this doubt but might have compromised plant operation. 

Read more on this topic next month. 

David W. Spitzer

 

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.

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