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Clamp-on solution for full plant availability

Non-invasive flow measurement of process gases
Clamp-on solution for full plant availability

In the chemical industry, where aggressive and hazardous media are commonplace, experts are happy to resort to clamp-on technology for flow measurements. Long established as a standard method for measuring liquid flow, the non-invasive, acoustic technique is meanwhile being used more and more to measure the flow of gases.

The author: Jörg Sacher Press and Public Relations, Flexim

Among the various possible techniques, clamp-on ultrasonic flow measurement has been an outsider technology for quite some time. Nowadays, it is an “outsider” only in the technical sense, since the transducers are simply clamped onto the outside of the pipe and therefore measure from the safe side, as it were. As a result, they are not subject to wear and tear from the medium flowing through the pipe; they cause no pressure loss whatsoever and there is no risk of leakage.
Due to its practical benefits, ultrasonic technology has long been a widely used, standard measurement method. Its suitability for the gaseous state of aggregation is relatively recent. A typical application in Poland illustrates the method’s characteristic features.
Advantage over vortex
“The crucial advantage is the fact that the system can be installed without interrupting operation and without any pipework”, says Maksym Cichón, Flexim Product Manager at Introl sp. z o.o. The Katowice-based company offers solutions for process control and automation and has been Flexim’s sales partner in the Polish market since 2010.
Cichón reports on his first experiences with non-invasive measurement technology using the clamp-on, ultrasonic method and comments on enthusiastic users: “We received a request from a refinery to measure the hydrogen flow. Initially, we offered a vortex meter because we have had them in our product portfolio for many years. However, this solution has a serious drawback: the measuring instrument is installed in the pipeline, resulting in pipework and inevitable disruptions to operation. Our cooperation with Flexim began during the first phase of the project. The Berlin flow experts not only provide clamp-on solutions for liquids but are also leaders in the area of non-invasive, ultrasonic flow measurement of gases. We then proposed a clamp-on, ultrasonic flow meter instead of the vortex meter. This suggestion met with great interest from the refinery’s engineers.”
Demanding acoustics
Hydrogen is generally considered a complex fluid. It is highly flammable, highly volatile and forms highly explosive mixtures in contact with air. It is no wonder that experts are keen to avoid pipework involving extensive planning and safety measures. The low density of non-invasive flow measurement technology represents something of a challenge. As a general rule, the higher the density, the easier and more accurate the measurement. Since the density of the gas increases with rising pressure, the non-invasive measurement method works extremely well at high to very high operating pressures, which in many other processes are associated with substantial costs and a corresponding amount of effort.
“The pressure at the measuring point is approximately 29 bar”, Cichón reports. “One cubic metre of hydrogen only weighs about 2.5 kg at 29 bar”. This results in a low radiation level of the ultrasonic signals into the medium. A large proportion of the signals produced by the sound transducers are propagated to the pipe wall as ambient noise. For this reason, Cichón decided to use so-called Lamb wave transducers, as their excitation frequency causes the pipe wall resonance to add to the signal insertion. As a result, the measuring signal is irradiated optimally into the medium flowing in the pipe. Practical devices that damp the ambient noise and sophisticated algorithms that filter useful and pipe wall signals in the measurement transmitter take care of the rest.
Conclusive test measurement
Of course, before making any final decision to choose the acoustic measurement method, the refinery’s process engineers insisted on reliable proof of its suitability, reliability and accuracy. “A successful test measurement was naturally a precondition”, Cichón continues. He was convincing from the word “go” with his portable, ultrasonic Fluxus G601 flow meter. “We only needed half an hour to set up the measurement point without any impairment whatsoever to plant operation. After we attached the clamp-on transducers to the pipe, the measurement transmitter produced a strong and stable signal. We left the measurement running for a few hours, during which time the measured values were stored in the internal memory. We read them out afterwards and handed them over to the customer for comparison with the data in the process control system.”
The measured values were equally impressive. Introl was given the task of setting up the measurement point with a permanently installed Fluxus G800. These transmitters are explosion proof and certified for use in Zone 1 hazardous areas (Atex), as are the transducers. The non-invasive solution avoided not only downtime but also extensive planning and bureaucracy. The installation of a wetted measuring system would have necessitated the time-consuming production of dimensional drawings and what are often lengthy approval negotiations with the national technical supervisory board.
Cichón sums up: “It’s always best when the user comes to his own conclusion that the non-invasive solution is optimal for his purpose.” Following this successful project, more measurement points are to be designed with Flexim’s clamp-on ultrasonic technology, amongst other things for non-invasive flow measurement of heavy crude oil.
Flexibility in hazardous areas
Cichón admits that his test measurement was not completely paperless. Like all other portable ultrasonic measuring systems, the portable Fluxus G601 has no Atex certification. Since the revision of the explosion protection directives, the relevant Atex standards are no longer met by any portable system for ultrasonic flow measurement. As there is no alternative in practice to non-invasive ultrasonic technology if temporary flow measurement points need to be set up quickly, plant operators resort to special permits and gas warning devices.
Cichón thus has all the more reason to welcome Flexim’s latest product development. Thanks to the new Fluxus F/G608, unsatisfactory solutions with high administrative and personnel costs are now a thing of the past. In terms of technology, the Fluxus F/G608 is based on the same platform as the well-proven and versatile Fluxus F/G601. It is similarly flexible and easy to handle as well as certified for use in hazardous areas.
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