Sometimes What the Ultrasonic Flow Meter Doesn’t Tell You is Important

A customer needed to determine the flow rate on a pair of large pipes draining a sump. There were issues with the sump filling and the rate out flow did not appear to be sufficient. A pair of GE Panameterics PT878 Ultrasonic Flow Meters were rented from TechRentals for the task.

When the meters were installed one of the meters worked correctly, the second meter would only show an error indicating problems with the liquid and indicated a low signal strength (in this case TechRentals were asked to fix the meters).

The two meters were swapped to ensure that there was not a problem with the meter. The same pipe still indicated a problem. It was assumed that the flow in that troublesome pipe was either too chaotic or that there was a lot of air in the pipe.

Another Ultrasonic Flow Meter technology was obtained, a Polysonics Hydra SX30.The Hydra SX30 is a Doppler meter and Doppler meters tend to be more accepting of high levels of particles in the flow. Often if you examine the frequency spread of the returned signal, the FFT display on the meter, and it does not show a clean ‘bell curve’, it is a good indication of problems with chaotic flow.

The Doppler meter also would not give a reading on that pipe, but worked correctly on the other pipe! The Doppler meter also indicated a very low signal strength. Typical if there is too much air in the pipe, these results are reported.

Subsequently, the pumps that delivered water to these pipes were examined. The ‘good’ pipe had a single pump which was operating correctly, whilst the pump delivering liquid to the second pipe could not be measured because it had two pumps delivering water to it.

Because of the mechanical arrangement one of those pumps was cavitating badly and basically pumping air. This was not obvious because of where and how that pump was installed. So, even though the ultrasonic flow meters in this case were not able to tell us the flow rate, the result was still positive because it directly indicated where the problem actually was!