Intrinsically safe? 08 June 2012
Hazardous areas and electrical/electronic circuits have been treated with caution ever since the Senghenydd colliery disaster of 1913. Brian Tinham provides a technology update
Electrical equipment destined for installation in designated hazardous areas has to conform to tightly defined specifications, according to the gases (propane, ethylene or hydrogen for surface industries) or dust hazard concerned, likelihood of the risk occurring and the relevant auto-ignition temperature. So much is, or should be, common knowledge.
That there are generally choices available for any protection is less well known, with plant engineers tending simply to opt for their preferred approaches, based on personal experience (See www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm for details). And that some of the technologies – certainly those behind IS (intrinsic safety) – are once again evolving is probably known only to the privileged few involved with their development.
So it's time for an update – and who better to kick us off than Graeme Philp, chief executive of GAMBICA (the association for the instrumentation, control, automation and laboratory technology industries) and formerly chief executive of IS technologies specialist MTL? For him, the greatest changes are those arising out of growing fieldbus (digital plant communications) adoption, under which multiple instruments' power and signals are carried on a single line, rather than each being separately hard-wired, with its own analogue power and signalling.
"It used to be so simple," recalls Philp. "When equipment was wired point-to-point, IS was the preferred protection for low-voltage, low-current instrumentation [24V and 4—20mA, and its predecessors], because of its sheer ease of installation, commissioning and maintenance. If you needed a bit more energy, you might turn to non incendive [Type 'n'] techniques, while EExd flameproof – or in the US, explosion proof – was the default choice for higher voltage and higher current equipment."
Although little has changed with the latter two cases, when it comes to instrumentation linking into hazardous areas on fieldbuses of whatever hue, the issue has been rising electrical energy demand – which makes conventional IS struggle. Why? Because conventional Zener barriers and galvanic isolators – each limiting energy available for release (from capacitors, inductors etc) on any circuit to safe levels, even in the event of two coincident faults – were designed for single instrument loops under the IEC 60079 standard.
"So until recently, the high-tech equivalent designed to limit power on multi-instrument fieldbuses, has come in only two flavours," explains Philp. "One is the FISCO [fieldbus intrinsically safe concept IEC 60079-27] standard, an extension of traditional IS that safely drives eight, but up to 12, instruments per fieldbus segment. The other involves reverting to non-arcing protection concepts that provide a high energy trunk which can power more devices, with individual barriers to protect the individual devices. It means you can power more devices, but you lose the live maintenance capability, which is such a useful feature of intrinsic safety and FISCO."
But that is about to change, with devices building on fast-switching electronics finding their way to the market very shortly. "New devices will constantly monitor energy being supplied by a fieldbus trunk and, if it gets to a level approaching the limit of safety, crowbar themselves," says Philp. "They take advantage of the fact that electronics can shut down faster than the time it takes to develop an arc. We're talking microseconds," he adds.
Installation and commissioning
So the fieldbus spur cuts off before its energy rises to the point where it might cause a problem. Not only that, you also get a high energy trunk capable of powering many devices while retaining the capability to live maintain. Whether plant engineers adopt such devices will depend on how much they trust active electronics to protect a plant. Either way, Philp is confident they're just around the corner, with all the usual suspects in the frame.
Meanwhile, he also confirms that installation and maintenance are bound to remain more or less in line with current practice for FISCO devices – which is not dissimilar to that on earlier analogue IS devices, save for new rules on numbers of devices that can be connected. That's despite the change to high-frequency electronics capable of providing safe power without distorting instrument data waveforms.
That said, testing and commissioning of fieldbuses is more of a challenge than on 4—20mA wiring. "Traditional multimeters aren't much use on a fieldbus carrying data, and complex data analysis equipment requires significant re-training of plant staff. For this reason, most of the companies producing fieldbus power supplies and hazardous area protection equipment add signal and bus diagnostics into their devices to give simple to understand information on the state of health of the bus and its devices."
Indeed, many have gone further. Siemens, for example, has the barriers built into the box on its latest remote I/O for Zone 0 and Zone 20 (dust) hazardous areas. ET200 ISP – typically for Profibus but also, in the future, ProfiNet – was designed not only to keep installation simple, but also to improve flexibility. And it's a similar story with the automation giant's AFD IS active field distributor for Profibus PA-based instruments. "It's all about reducing the infrastructure and keeping it simple, while allowing plants to benefit from intelligent instrument in IS zones," states Steve Leech, who looks after Siemens process automation portfolio.
But there is another issue, also arising out of newer network technology – in this case not fieldbus, but wireless. The point here is that wireless instrumentation generally depends on batteries and/or supercapacitors for power – traditionally awkward to certify as IS, because of the clear and present danger of shorts leading to sudden and serious energy discharges.
The solution, however, has been all about the packaging and technology of the batteries themselves. "These batteries need to deliver a little energy over a long period, but be incapable of releasing potentially incendive levels of energy, if shorted," explains Philp. "So designers have constructed chemistry to achieve exactly that, along with 'smart' battery technology and exterior protection. You can even drive a nail through some of them and nothing untoward happens."
That matters not simply because it enables wireless sensors to be installed in hazardous areas, but also for its power potentially to enable a revolution in gas hazard sensing. "In the future, we might see 'clouds' of sensors everywhere, monitoring for anything from pollution to the presence of explosive gases," comments Philp. "So, in principle, we'll be able to detect when and where explosive gas is building up and then shut down energy to protect certain devices, as and when required. We haven't been able to do that to date, because we didn't know where the gas was."
It seems that developments in wireless sensor technology itself – already used in enclosed hazardous areas – may yet turn the ways in which we protect hazardous plants, at the sensor level, on their head.
Mechanical equipment update
Static and rotating equipment that could act as a source of ignition, either in normal operation or fault conditions, must be assessed, protected and classified for hazardous areas. That's among the legal obligations on plant operators since the enactment of DSEAR (Dangerous Substances and Explosive Atmospheres Regulations 2002), the transitional period for which expired back in 2006.
Yet, according to Rob McSweeney, principal consultant at ABB, there's still non-compliance out there – especially where the perception of risk is low compared to that, say, in petrochemicals. "Some plant operators have traditionally not seen flammability combined with hot surfaces as potentially high hazard," he says. "Risks should have been exposed through HAZOP [hazard and operability] studies – but only if they recognised the need."
And the same goes for DSEAR, with its requirement for ATEX (from the French ATmosphère EXplosif') certification. Failure to comply could be down to ignorance. For the record then, DSEAR changed nothing in terms of electrical and electronic protection. However, McSweeney makes the point that since 2003 all newly installed mechanical equipment should have been ATEX certified, in line with recognised guidance. That includes equipment operating in areas of plant not previously classified.
Why the change? Because some mechanical equipment can spark or, just as risky, attain a temperature that exceeds the auto-ignition point of some gases – particularly in fault conditions. Consider bearing failures.
"The only exception is older mechanical equipment where there was previously no tradition of certification," explains McSweeney. "And that should be subjected to a retrospective process of risk assessment – a structured analysis of the equipment, and the operational and maintenance processes, looking for conditions in which incendive situations might arise."
Normally, if it's Zone 2 or 22 (for dust), the outcome is not too onerous. "It may mean nothing more than good inspection and maintenance practice to minimise the risk of faults, under ALARP [as low as reasonably practicable] principles," advises McSweeney. However, if you're looking at Zone 1 or, even tougher, Zone 0, then retrospective measures can be more difficult. "Then the requirement may be very stringent stipulations around maintenance, condition monitoring, pressurised and purged containment etc."
IS barrier technologies
Zener barriers combine a diode with resistors and a fuse, and were designed to ensure that neither voltage nor current flowing in a circuit can exceed critical values. Most important, they were cheap and are still the most cost-effective way of achieving intrinsic safety on plant.
Also, while some have criticised their requirement of a good earth, the reality is that on most plants, that's not too difficult. Consider the star point of a transformer binging three phase into a plant, for example.
Meanwhile, galvanic isolators are built to transfer dc into ac across a transformer and then back to dc on the other side. Their introduction removed some of the perceived issues around Zeners, but at a price. Typically, they are three to four times more expensive.
FISCO essentially builds on isolator technology, allowing fieldbus spurs in hazardous areas to be installed relatively simply. Similarly, additions to the bus do not require a re-evaluation of the system, as long as the maximum number of devices is not exceeded. Gas group of the system is simply determined by the power supply and the temperature classification of each instrument.
GAMBICA Association Ltd
Siemens Industry Ltd
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