Wired for sense 09 December 2014

Although electrochemical sensors remain the bread and butter for gas detection, alternative technologies have come a long way, with the oil and gas sector leading the charge.

It is two years since research, commissioned by electrochemical sensor manufacturer Analytical Technology (ATI), revealed an alarming lack of confidence in gas detection technologies – with less than a third (32%) of managers "very confident" that systems installed in their workplaces were working. Even more worrying, despite the fact that frequency of testing and calibration are critical to ensuring safety with these sensor types, only three in five managers believed their gas detectors were being checked frequently enough.

Little is likely to have changed in terms of conventional gas detectors' application or management (good or bad) since 2012, but technology – or, more accurately, its industrial acceptance – certainly has moved on. And with its progression have come not only improved gas sensing capabilities, but also solutions to the underlying problems. Latest instruments check their own health, with digital diagnostics, while the newer breed of sensors is also immune to the poisoning issue that has always bedevilled electrochemical sensors.

So what's new? Well, if we take our lead from the oil and gas industry, then recent years have seen infrared, laser and ultrasonic sensors increasingly stealing the limelight. Eliot Sizeland, head of global sales and marketing at Rosemount Measurement (part of Emerson Process Management), points to his own company's novel ultrasonic system, aimed squarely at large-scale leak detection, but also Simtronics' and Senscient's tuneable laser diodes and GasSecure's wireless infrared devices – all of which started life quietly in the high-value, high-risk offshore sector.

"We started manufacturing our ultrasonic leak detectors back in 2006, but we're now on the second generation," explains Sizeland, adding that they are designed to detect any gas (non-specific) leaking from pressurised containers at above 7bar. "They can sense leaks from, for example, compressors, and are capable of covering wide, exposed areas. But they really come into their own where point-type detectors might fail for lack of gas accumulation, while open path devices could lose alignment, due to plant vibration."

Reliable detection
Sizeland makes the point that there's generally not much ultrasound to compete with around oil and gas plants – the exceptions being control valve movements, some compressor noise, etc. However, Rosemount's system sets a threshold above the background and builds on that with a time delay to prevent spurious alarms. "This technology has been increasingly favoured following a study by HSE [Health and Safety Executive] several years ago, which found the effectiveness of existing gas detection technologies at the time was 62%."

Clearly, that meant 38% of offshore gas leaks – particularly smaller leaks – were not being detected. Reasons included device location, inadequate maintenance frequency and the gas just dispersing too quickly to be measured. "So our system plugs the gap," he says.

Interestingly, although Rosemount's ultrasonic systems come with hazardous area ATEX (ATmosphères EXplosibles) approval, there are, as yet, no international standards covering operational performance. In fact, that is a common theme with several of the more novel gas detecting technologies. However, insurers like them for their proven track record, particularly on higher-risk upstream and offshore plant (such as wellheads, receiving terminals and mid-stream compressor stations) where cost is less of an issue – frankly because failure to prevent a catastrophe isn't an option when there's nowhere for people to run.

"Other industries aren't adopting this technology," concedes Sizeland, agreeing that at a price of £11,000 for a detector, 4–20mA failsafe I/O, local displays, etc, they're not cheap. That said, refinery complexes wanting to protect hydrogen compressors and onshore storage tanks from gas leaks could certainly benefit, given the limits of traditional catalytic sensor technology, which is neither failsafe nor maintenance-free.

Moving on to tuneable laser systems, among the latest from Simtronics is its GD 1 near infrared toxic gas detector, which can monitor path lengths up to 60 metres and is claimed to offer the fastest response on the market to, for example, hydrogen sulphide. This offers much the same technical and operational advantages as ultrasonics over conventional alternatives, but is obviously fully molecule-specific. Again, however, although this equipment could be just as at home in boiler rooms and wastewater treatment plants as it is in refineries, fuel loading facilities and compressor stations, it is the oil and gas sector that currently buys it.

Value for money
Why? Price: you're looking at around £14,000 all-up. Against that, you get a much wider range of coverage than is possible with alternatives, such as electrochemical sensors. They're also solid-state and very robust; they fail to safety; and they require minimal installation and commissioning, with virtually zero ongoing maintenance and no calibration.

This latter factor is particularly important in a potentially toxic and/or explosive gas environment: no one likes sending technicians into such plants unless they absolutely have to. Sizeland suggests that, taking all that into account, it's easy to argue a break-even point of two years, compared with cheaper conventional gas detection approaches, which do require regular intervention and calibration – particularly on applications such as sour gas detection on some of the very large plants in the Middle East.

What about Senscient's version? Its technology is dubbed ELDS (enhanced laser diode spectroscopy), which provides for failsafe detection of flammable and toxic gases, ranging from hydrogen sulphide to methane, hydrocarbons generally, ammonia and CO2 – although, again, there are still no international performance standards. Its latest gas detectors (Series 2000) are replacing some infrared gas detection equipment offshore – notably on Suncor's Terra Nova FPSO (floating production storage and offloading) facility off the east coast of Canada.

Shell is among keen advocates of this technology, having been instrumental in its development, testing and eventual uptake at several of its oilfield, refinery and gas plants. Its latest to commit to Senscient is the giant Carmon Creek enhanced-recovery bitumen project, currently under construction in Alberta, Canada. Reasons for the choice: impressive gas leak detection coverage; excellent early warning capability; good elimination of false alarms; as well as reduced maintenance, compared to other methods – collectively leading to improved plant reliability and uptime.

Finally, on infrared gas sensing technology, one other major development concerns GasSecure's point detector wireless devices. It's not so much that they are capable of detecting hydrocarbon gases over a 60-metre range (and smoke potentially up to 200 metres), but that they can do so on battery power alone for two years, without intervention.

GasSecure chief executive Knut Sandven explains that, whereas infrared systems typically consume power at 5–8W, its wireless instruments require one thousandth of that – only 5mW. "We designed this gas detector from the bottom up with proprietary MEMS [micro electrical mechanical system] technology to give the same or better performance as wired infrared systems," he explains. "We use a single customised infrared source and the heart of our gas detector is the configurable MEMS filter, which manages the infrared wavelengths to detect gas composition."

In operation, this micro-miniature system handles all the infrared processing, rapidly switching between wavelengths where hydrocarbon gases absorb energy and two reference spectra. Hence the single infrared source and detector design – and the impressively low power draw. And, since the instrument's launch in 2012, major oil and gas companies have moved rapidly from test installations to significant projects both on- and offshore.

What about cost? "They're expensive," admits Sandven. "Three times the price of wired devices and you need a gateway [router] to receive the signals. But you save on all the cabling and installation work and that can equate to an overall cost reduction of 50–80%. We have also completed several installations where there were no alternative technologies, or the distances were just too great – for example, on tank farms – to make conventional infrared gas detection feasible."

Brian Tinham

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