Flow

Reader feedback: September 2019

Reader offers insights on Doppler flow measurement applied to slurries

I enjoyed the article on slurry flow measurement. Very well written and an easy read.

I would think that the suggestion to rotate a meter one bolt hole (along with other recommendations) has been well received. In my 30 years of working in the flow measurement industry, I found that meter orientation issues are routinely overlooked. With more focus on signal advanced diagnostics, signal processing, communication protocols, etc., installation and operation of the primary element is an afterthought.

Lately, I've been immersed in clamp-on Doppler flow measurement technology applied to slurries (frac flowback water, HDD mud, tar sands crude, etc.) This isn't custody transfer measurement (clamp-on ultrasonic flowmeters would not be acceptable), but a challenge nonetheless. I’ve been involved in some testing on HDD mud (oil and gas and municipal) and frac flowback water. The results substantiate what you point out in your article.

As you probably know, a clamp-on Doppler meter emits an ultrasonic frequency into the pipe through the pipe wall through a transducer. The emitted signal seeks out reflectors (solids, air bubbles, etc.) moving in the flow stream. The returned signal to the transducer is an altered frequency and the difference is proportional to velocity.

For the initial test, we pumped dirty water from a vessel through a closed return loop. The Doppler meter tracked the pump output (measured with a magmeter) within 2-3% consistently from roughly 2 FPs to 18 FPS. We had the choke open on the line, which introduced aeration—favorite target of a Doppler meter. Bags of sand were added to the water progressively in 50 lb increments. While the meter tracked the output up and down, the scale, the accuracy diminished to 5-7% (lower).

By moving the transducer around the circumference of the pipe to find the best location, we brought the accuracy back to an acceptable range. In viewing the waveform captures, it was fairly apparent that the Doppler signal was reflecting off the slower moving sand at certain locations in the pipe (mostly at the bottom). Ideally, the Doppler signal should take an average of the flowing solids, air bubbles, etc. but, as you pointed out, the conditions along the pipe wall (sediment, build-up, etc.) can greatly affect the operation of the meter and its accuracy.

John Good
Regional sales manager, Greyline Instruments
jgood@greyline.com

It looks like you're doing interesting work with ultrasonic doppler meters. Though not a doppler meter per se, we had success in the oil sands with the Cidra Sonartrac device for multiphase slurries. Sonartrac works on recognizing sound wave patterns, so perhaps if you can add a feature along those lines to your signal analysis it will help you improve accuracy and/or additional information such as impending sanding or bridging along part of the pipe. Have you also tried installing multiple transducers along the same line at slightly different orientations, then using the combined signals? For some customers, dual transducers for reliable measurements may be a feasible tradeoff.

One thing that is certain is that you always need to install the meter in the correct orientation for the service in which it's being installed.

—Ian Verhappen