Industry and Academia go with the flow
29 November 2015
The benefits of collaboration between industry and academia are once again being proven, this time in the area of ultrasonic flow meters. Yifei Zhang examines the results of recent tests demonstrating the suitability of new generation materials for key applications in utility and industrial flow measurement.
The ultrasonic flow meter configuration showing the reflector blocks in position
Accurate flow measurement is crucial in utility applications not just to enable accurate tracking of usage and therefore billing, but for reasons of safety. Similarly, in industrial applications, close monitoring of consumption is vital to identify inefficiency and allow remedial action to be taken, while once again safety and the service life of other components can be compromised if flow is either too high or too low.
In utilities, the concept of ‘smart’ metering is now firmly entrenched and has brought with it a growing demand for accurate, reliable and durable ultrasonic flow meters. A flow meter is now fitted on the supply to each dwelling to track water usage.
Compared with traditional mechanical flow meters, ultrasonic meters provide greater sensitivity, accuracy and longevity. This is largely due to their use of solid state technology, which means they have no internal moving parts and so suffer no internal wear. That, in turn, means ultrasonic meters retain their accuracy over the long term.
In a fully assembled ultrasonic flow meter, a pair of sensors is mounted in parallel to a flow tube, along with other electronic devices required. The meter calculates the flow rate of a medium, generally a liquid or gas, by assessing the delay in response between the two sensors, which are driven electronically to create ultrasonic waves.
It is very important that the ultrasonic wave created by one sensor be transmitted to the other sensor with minimal loss. Blocks with an angled reflective surface are used to achieve this. The blocks must retain their reflective surface throughout service, since any loss or degradation of reflection will impair the transmission of the signal between sensors.
Traditionally, stainless steel has been used to make such blocks, as a material which is proven, easy to polish and cost-effective. However, questions have been raised over its ability to maintain its performance and accuracy over many years, particularly in relation to its corrosion and surface degradation resistance. Concerns have also been raised in terms of its proneness to sustaining scratches from abrasive materials (grit and sand) in the media passing through the meter. These scratches have the potential to impact significantly on the integrity of the sensors and therefore on the accuracy of readings, with a commensurate detrimental effect on the performance of the entire system.
These concerns, along with the increased use of the latest generation of ceramic materials across an ever broader spectrum of applications, led the team at Morgan Advanced Materials to examine whether a ceramic material would be capable of delivering the requisite performance attributes in these applications.
The materials deemed most suitable for this, following extensive research by teams within Morgan’s Technical Ceramics business, was its Hilox grades of alumina (Al2O3). As well as a high degree of hardness, it also boasts strong corrosion resistance properties and excellent acoustic performance which does not deteriorate noticeably over time when compared with many alternative materials.
However, despite these theoretically beneficial properties, the alumina ceramics – all of which have been approved by the Water Regulations Advisory Scheme (WRAS), which approves the materials commonly used in water management applications - needed to be tested to see exactly how well they would perform in the conditions.
Morgan decided to rekindle a long-term and highly successful partnership with the Advanced Ceramics Research Group at Loughborough University – one which has successfully overseen the evaluation and eventual commercial verification of a plethora of new materials as well as new applications for existing materials. The Loughborough group, led by Professor Bala Vaidhyanathan, is globally respected and widely acknowledged as one of the leading teams concerned with the understanding, characterisation and processing of advanced materials.
Together, the two organisations evaluated the degradation of stainless steel and alumina ceramics, through a test that simulates 20 years of service within a flow tube. This length of service is a key factor, as some essential maintenance to the wider system is inevitable at that interval.
The research found that under normal drinking water conditions, the development of scale on stainless steel reflectors resulted in a loss of signal of the sensors in the region of 15 percent, creating in an inaccuracy of the same level. Further testing showed that this increased to 25 percent over 40 years, and 50 percent over 60 years. This compared with an accuracy loss of just 10 percent for the alumina sensors after 20 years.
What is more, testing of the alumina products to 40 and 60 years revealed that there was virtually no further deterioration after the 20-year point, meaning that these systems can outlast the current, estimated 20 years in service life of these devices drastically increasing maintenance intervals and ensuring long-term, uninterrupted and accurately measured supply. Even after 10 years, the loss of acoustic response was less than 10 percent for alumina compared with nearly 13 percent for stainless steel.
These figures need to be understood in the context that while a discrepancy of up to 10 percent in accuracy in flow meters can be adjusted by tuning the external electrical circuits, anything above that is not considered to be satisfactory. Meanwhile, the clear inference from the research programme was that the effect of scale on the performance of stainless steel sensors would be accelerated and further exacerbated in some applications with greater mineral content.
The results also suggest that, as well as utility applications, alumina sensors are, for the same reasons, likely to be more frequently specified in industrial applications - where their ability to withstand the effects of coarse, abrasive media over extended periods without significant loss of accuracy, as well as their impressive service life, will make them attractive to specifiers seeking high-performance, low-maintenance solutions for applications such as the pumping of corrosive fluids in the chemical and pharmaceutical sectors.
Dr Yifei Zhang is a production engineer at Morgan Advanced Materials
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