Seemingly minor details can sometimes make a large difference in flowmeter performance. 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. The diameter of the throat was approximately 60 percent of the pipe size. The high- and low-range differential pressure transmitters were calibrated for (say) 0-200 and 0-20 mm WC, respectively. Flanged diaphragm seals were specified on both transmitters.

The existing installation had two flanged nozzles near the inlet of the Venturi and another two flanged nozzles in its throat. The two upstream nozzles were located slightly above and below the horizontal centerline of the Venturi. The two downstream nozzles were similarly offset above and below the horizontal centerline in the throat.

The flow could not be stopped to check the transmitter zero calibration, so a procedure was used to simulate this condition. The recommended procedure was to extract both diaphragm seals by 200 mm. Then the zero and span were checked using a calibrator.

This calibration procedure seems straightforward, but it is in error because it does not account for the geometry of the Venturi taps. Note first that the taps are not on the horizontal centerline of the pipe. This means that extracting the diaphragm seals from the tap above the centerline will not only remove them horizontally from the pipe, but will also raise them above the centerline of the tap. This would be acceptable if the upstream and downstream taps were located at the same distance above the centerline of the same size pipes.

However, the diameter of the downstream pipe is only 60 percent of that of the upstream pipe, and the upstream and downstream taps were not located at the same elevation. As a result, extracting the diaphragm seals by 200 mm located the diaphragm seals at different elevations, so zeroing the transmitter in this position would result in a zero error.

This zero error may seem small, but it amounted to millions of liters per day in this particular application. Depending upon the geometry, the influent flow could measure higher or lower than the actual influent flow. Higher measurements limit revenue by limiting the amount of water that the plant could process and sell. Lower measurements could cause regulatory compliance issues when daily flow is near regulatory limits.

Aside from creating these measurement issues, diaphragm seals also tend to degrade transmitter performance. Elimination of the diaphragm seals in this relatively clean service would go a long way to reduce complexity and increase flow measurement accuracy.

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.