Sound Wave Principles

Be wary of Doppler installations located near radio antennas because their frequency of operation is approximately the same as that of AM radio stations. In one such installation, it was possible to listen to a local radio station on the test leads. That installation was a failure because there was no practical way to shield the instrument.

Doppler ultrasonic flowmeter applications include fluids with a solids concentrations between 100 mg/l and 30%. Solids size should be greater than approximately 75 microns, and the solids’ specific gravity should be at least three times greater than the carrier’s specific gravity. That is, the specific gravity of the solids alone (not the bulk density of the slurry) should be over three when flowing in water. Avoid stainless-steel tubing and high-density polyethylene pipe.

Velocity Matters

Regardless of the flowmeter technology, you should ensure that there is a good velocity profile in the flowmeter. Distorted velocity profiles can significantly increase measurement errors. Multi-path transit-time ultrasonic flowmeters are available to measure flows that exhibit non-uniform velocity profiles. Increasing the number of paths generally increases the accuracy with which the flow can be measured. It is advisable to use straight runs or flow conditioners to develop a good velocity profile for the flowmeter rather than increase the number of paths (and the cost). That said, in critical applications, such as for custody transfer, it is not uncommon to install a multi-path spool-piece ultrasonic flowmeter with lots of straight run in conjunction with a flow conditioner to achieve superior accuracy.

Figure 4.  Clamp-on Doppler ultrasonic flowmeter (Courtesy Siemens)

Small pipe sizes can present a measurement constraint on transit-time ultrasonic flowmeters. Because the transit time is small in small pipes, the difference between the upstream and downstream transit times is smaller yet. This can be remedied by using multiple traverses that direct the ultrasonic energy to bounce off the pipe wall one or more times as it travels between the transducers. Increasing the transit time increases the difference between the upstream and downstream transit times, making the difference easier to measure.
Transit-time ultrasonic flowmeter installations that exhibit internal pipe coating and/or reflectivity issues can adversely affect the effective path length and, hence, the transit-time and flow measurements. For example, the path length of a transit-time ultrasonic flowmeter with four traverses (three reflections) in a 25-mm pipe with 5 mm of coating will be reduced by over 25 mm.

Be sure that the fluid itself is uniform in the flowmeter. Some fluids can separate when flowing in horizontal piping, so the top and bottom phases may travel at different velocities and cause measurement problems. Relocating the sensors in a vertical pipe section often resolves this problem.
Life would be simpler if there were fewer complications. Wouldn’t it be great if you could clamp a sensor onto the pipe and measure the flow through the pipe? Ultrasonic flowmeters solve this problem in some, but alas, not all cases.

David W. Spitzer is a principal in Spitzer and Boyes,  LLC, and can be reached at 845.623.1830 or www.spitzerandboyes.com.

For more information on flow technology go to www.controlglobal.com/flowmeter.html.