David W. Spitzer, P.E.

As I noted last month, seemingly minor details can sometimes make a large difference in flowmeter performance. Here’s another example … I recently inspected a large horizontal Venturi flowmeter installation that was used to measure relatively clean water flow from a lake into a water treatment plant. This flowmeter was used to perform a water balance on the entire water distribution system that consisted of over 100 downstream flowmeters.

Both the high and low range differential pressure transmitters had flanged diaphragm seals that were located in nozzles in the upstream section of the Venturi and in its throat. The diaphragm seals and nozzles were designed such that the diaphragm would be located flush with the inside walls of the pipe and throat. As could reasonably be expected, internal observation revealed that the diaphragm seals were close to flush, but not exactly flush. This installation produced measurements that resulted in a water balance of nearly 5 percent, that is, the difference between this flowmeter and the sum of the other flowmeters was almost 5 percent.

The plant then installed an additional pair of differential pressure transmitters that were carefully calibrated to the same calibrations as the original transmitters. The transmitter ports were connected to the thin annular space between the outside diameter of the diaphragm and the inside wall of the nozzle. Measurements with this configuration resulted in a water balance of approximately 1 percent with the downstream flowmeters. The water balance did improve, but the more important point is that the flow measurement shifted by 3-4 percent even though the taps were at the same location in the Venturi.

Think about this for a minute. The Venturi did not change. The transmitter was carefully calibrated. The 100-plus downstream flowmeters did not change. Yet the water balance shifted 3-4 percent on the day that the annular tap was installed. Remember that Venturi flowmeters can exhibit an accuracy of 0.5 to 1 percent of actual flow error. Yet a seemingly small tap change resulted in a 3-4 percent measurement shift.

By this time, the plant did not have much use for the diaphragm seals, so they connected the new transmitters to holes drilled into one set of the diaphragm seals. Despite destroying the seals, the plant water balance with this installation closed within approximately 0.2 percent. Again, the more important point is that this seemingly minor tap change resulted in an almost 1 percent change on a flowmeter that should perform within 0.5 to 1 percent of actual flow.
What does this mean? The details are important and seemingly minor details can sometimes make a large difference in flowmeter performance.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached 845 623-1830.