Noninvasive multiphase level measurement with an ultrasonic probe

Q: Is it possible to measure levels in multiphase (water+oil) tanks using an ultrasonic probe? Plus, noninvasively?

With this question in mind, we visited our colleagues at UTFPR’s Multiphase Flow Research Center (NUEM) and performed some experiments. And the answer is: YES!

Prof. Eduardo N. dos Santos kindly granted us some time at their separation tank, where some very interesting research has been conducted on oil-water separation methods. We counted on the help of, among others, former LASSIP member Everton Trento Jr., who currently works at NUEM.

The tank was partially filled with water and oil. An 3.5MHz ultrasonic probe (transducer) was coupled to the bottom of the tank using glycerine as coupling liquid and connected to an OmniScan MX2 inspection system.

The method is quite simple: the MX2 system repeatedly emits ultrasonic pulses to the tank. The pulses cross the steel wall of the tank and start to travel through the water. On the water-oil interface, part of the acoustic energy will be reflected back and captured by the transducer, which acts as both emitter and receiver. Another part of the acoustic energy will travel through the oil and hit the oil-air interface, where it will be reflected back and travel all the way back to the transducer. The Times of Flights (TOF) of these two pulses are proportional to the levels of water and oil.

We tracked the two TOFs on the screen of the MX2 system. Since the level of oil used in the experiment was quite low, the two pulses were close to each other in time. The Figure below shows the screen of the MX2 system. The voltage trace shows the acoustic pressure measured by the transducer after the last pulse was emitted. The lower graph shows the same information, but the voltages are represented in a color scale. A small history of the last traces is also shown, where each line represents a different trace.

The video below shows a small time lapse. Note that the history shows the two pulses shifting to the left as the level of water drops.

When water is poured from the top, the mixture is perturbated and things get a bit more messy. Nevertheless, the history of TOFs is still visible, as can be seen in the Figure below. This suggests that, with some signal processing, a measurement system based on this principle can even be robust to perturbations.

Now some of our students will carry on with the development of this idea. Translating the TOFs into levels is a simple task that involves the multiplication of times by the velocities of sound in both fluids. Some interesting topics for investigation are:
– Developing a stand-alone system that yields the level measurements (instead of voltage traces);
– Integrating the stand-alone system to a SCADA system for level control;
– Assessing the accuracy of the method for transducers of several frequencies;
– Assessing the robustness of the method relative to perturbations;
– Using signal processing to make the system more robust to perturbations;
– Using off-the-shelf clamp-on transducers suitable for industrial environments.

Currently, three undergraduate students from LASSIP (Lucas R. R. I. Ocampos, Guilherme K. T. Kaneko, and Roberto K. Roehrig) are working on these and related topics. All of them are members of the PRH 21 program. The tests shown in this post were conducted by Lucas with professors Thiago A. R. Passarin and Daniel R. Pipa on September 22nd, 2023.

Thanks to Prof. Eduardo Santos and colleagues from NUEM!