Hazardous areas, in which potentially explosive atmospheres exist, are encountered in a wide variety of industries. Thankfully the science of how to operate safely in such areas is now well understood, though knowing how best to comply with the requirements is something that still causes widespread confusion. Particular care has to be taken with electrical apparatus because of its potential for creating sparks and hotspots that could ignite a gas, vapour, mist or dust-laden atmosphere.
Such environments are encountered in everyday life, with petrol station forecourts being an obvious example, but industry sectors that are prone to hazardous areas are mining, chemical processing, petrochemicals, and oil and gas (Fig. 1). In addition, pharmaceutical production facilities often have areas where solvents are used, and specialty chemical plants often have hazardous areas. Flour mills, bakeries, sugar processors, timber processors, coal handling plant, paper mills and processors of metals such as aluminium and magnesium, for example, can all have areas where dust-laden atmospheres are potentially explosive, so it can be seen that the full spectrum of production and process plants that can contain hazardous areas is extremely broad.
Regulations, gas groups and zones
Here in the European Economic Area (EEA), hazardous gases are classified in EN 50014: 1997 (Electrical apparatus for potentially explosive atmospheres - General requirements), which is a harmonised standard under Directive 94/9/EC (Equipment and protective systems intended for use in potentially explosive atmospheres) or, as it is commonly known, the ATEX Directive. This standard is about to be superseded by EN 60079-0:2004, which is based on the IEC standard IEC60079-0:2004 (Electrical apparatus for explosive gas atmospheres Part 0: General requirements). EN 50014 divides potentially explosive gases into two groups: Group I relates to mines susceptible to fire damp (methane) and Group II relates to other places. Because of the specialist nature of mining, this present article considers Group II areas only.
Group II is further sub-divided into three to reflect the different flammability of gases (and vapours and mists). Group IIA is for the least flammable gases (such as propane), while Group IIB is for medium-flammability gases (such as ethylene) and Group IIC is for the most flammable (such as hydrogen). However, it should be remembered that, in general, gases on their own are not flammable; they also need oxygen or another oxidant with which they can react in combustion. (The exception is acetylene, which can decompose explosively in the absence of oxygen into carbon and hydrogen.)
Furthermore, specifiers of apparatus for hazardous areas need to know the likelihood of the explosive gas-air mixture being present, so three zones of risk are defined within the hazardous area. In Zone 0 the risk is greatest, with the hazard continuously present - usually due to a continuous source of release. In Zone 1, the risk is lower but the hazard is likely to be present under normal operating conditions – normally due to a primary source of release. In Zone 2, however, the hazard is unlikely to be present and, if so, it will be present only for short periods or due to a fault condition (normally stemming from a secondary source of release). As a general rule, in Zone 0 the hazard will be present for more than 1000 hours per year and in Zone 2 the hazard will be present for less than 10 hours per year – though these figures are not laid down in any standard.
Protection to suit Zones
Having established the nature of the hazard and the level of risk associated with it, a protection method needs to be selected to suit (Fig. 2).
For Zone 0 the preferred method is intrinsic safety type ‘ia’ (two-fault tolerant), though special protection can be employed if specifically certified for this use, and it is possible to use encapsulation in some limited circumstances. Nonetheless, intrinsic safety is almost always the only practical option, especially for sophisticated apparatus such as instrumentation.
There is a much wider choice for applications in Zone 1 areas. Intrinsic safety (type ia, which is two fault - or type ib - which is single-fault tolerant) is often the preferred method, though flameproof protection, increased safety and purge/pressurisation protection are also commonly used. Less frequently encountered are sand/powder filling, oil immersion, encapsulation and special protection.
Within a Zone 2 area any of the above methods may be used, or Type-n (non-incendive) protection - in which the apparatus is not capable of causing ignition through the creation of sparks or hot surfaces during normal operation (though fault conditions could potentially cause ignition).
In all cases the requirement is to provide the necessary level of protection at a reasonable cost, though specifiers should be aware that a lower purchase cost will almost always lead to a higher cost-of-ownership. For Zone 0 we have seen that the only practical option is intrinsic safety, and for Zone 2 the considerably lower purchase cost of Type-n protected apparatus will often make replacement more cost-effective than repair, so cost-of-ownership is less of an issue. However, the choice is not so straightforward for Zone 1.
Intrinsically safe apparatus is designed and constructed such that, even under fault conditions, the electrical energy within the circuits is less than the minimum ignition energy of the flammable atmosphere in which it is to operate. One of the main benefits of intrinsically safe equipment is that live maintenance within the hazardous area is permitted, which greatly reduces the cost-of-ownership. If maintenance is required, the equipment can be left in place, which saves time and minimises plant downtime. Furthermore, cables can be installed without additional mechanical protection, and this helps to reduce installation costs compared with using other protection methods. The, literally, intrinsic safety of this method of protection, means many users view this as the safest option.
European specifiers have favoured intrinsically safe equipment for some time, yet this mature market is still growing at an estimated 3.4 per cent per year. In contrast, the North American market is starting to swing from using flameproof to intrinsically safe apparatus, hence an annual growth rate of 6.5 percent is estimated and elsewhere, growth rates in the Middle East and Asia are estimated at 7.3 per cent and 11.2 per cent respectively.
Issues to be aware of with intrinsic safety are the fact that some associated apparatus will always be required to connect the apparatus to the non-hazardous area (typically by means of a safety interface unit – such as a zener safety barrier or galvanic isolator – located in the non-hazardous area), and the purchase cost is often higher than for other protection methods due to the greater design effort, the need for low-power electronic components, and the higher degree of fault-tolerance that is built-in.
Flameproof protection essentially refers to the placement of all electrical apparatus within a special enclosure that is capable of containing an explosion that initiates inside. In some cases the enclosure has a complete and perfect metal-to-metal seal at all openings and the enclosure is capable of withstanding an internal explosion. Other designs use special wide flanges that enable any flame escaping through a joint gap to be quenched before it reaches the potentially flammable atmosphere outside the enclosure.
Except for some component enclosures, certification is generally required for each specific application. Furthermore, some flameproof enclosures are suitable for all gas groups, whereas others are only certified for gas group IIA or IIB, for example.
Installation and maintenance clearly requires great care in order to ensure that the flameproof characteristics of the enclosure are not compromised. Similarly, cable glands and conduit must be correctly specified and installed to provide adequate physical and flameproof protection. Because of the nature of the components within the enclosure, live maintenance is forbidden, which can add significantly to the cost of ownership. Moreover, the concept allows for the ingress of flammable gases into the enclosure from outside, and accepts that combustion will occur. Where flammable gas is continually supplied into the enclosure this creates the risk of the enclosure itself heating to the point that its external temperature may present an ignition risk even if the combustion is contained inside. For this reason, it is understandable why many users prefer the philosophy appertaining to intrinsic safety.
Another alternative protection method for Zone 1 areas is increased safety. This relies on safeguards applied during design and construction that ensure the apparatus contains no normally arcing or sparking devices or hot surfaces that could cause ignition. Measures that are taken include the use of high-integrity insulation, the temperature de-rating of insulation materials, enhanced creepage and clearance distance, careful attention to terminal design, protection against the ingress of solids and liquids, high impact strength for the enclosure and the control of maximum temperatures.
Increased safety is generally considered to be suitable for medium-power apparatus, with typical applications being small motors, luminaries and junction boxes.
The last of the popular protection methods for Zone 1 hazardous areas is over-pressurisation of the apparatus enclosure using clean air or an inert purge gas. Depending on the exact procedure employed, the region inside the cabinet becomes either a non-hazardous or a Zone 2 area, with resultant implications for the design and construction of the electrical equipment contained therein.
Purge/pressurisation is often used where the function of the electrical equipment makes it difficult to redesign it so that it is intrinsically safe. It is also an easy concept to understand, and it is human nature to trust what is readily understood. Another advantage of using the purge/pressurisation method is that the cost of the core apparatus is, relatively speaking, lower than the intrinsically safe equivalent.
However, the complexity and cost associated with a purge/pressurisation system should not be underestimated. If the purge gas is to be clean air, this may have to be piped from some considerable distance away, and there is a need to install the pumping system outside the hazardous area, plus protected pipework must be run through the hazardous area to the apparatus. Correct connection of the pipework to the apparatus enclosure is also essential. All of this hardware needs to be maintained and suitable fail-safe monitoring and alarm systems need to be in place to detect any failure of the pressurisation system and shut down the apparatus.
If, on the other hand, an inert purge gas is used, this can reduce the installation cost, but there is an additional ongoing cost associated with the consumable gas. Furthermore, whichever purge/pressurisation method is used, live maintenance of the apparatus itself is prohibited, which can add to maintenance costs and lead to significantly longer downtime when the apparatus needs to be maintained.
Other methods that can be used, such as oil immersion, encapsulation and filling with sand or powder, are only suited to a limited range of applications - in particular, those where maintenance is not likely to be required.
Furthermore, having outlined the alternative methods for protecting electrical apparatus in hazardous areas, it has to be pointed out that it is not always a case of using one or another in isolation; in some circumstances it is beneficial to combine methods to optimise the protection of an overall system. Take, for example, instruments such as the Servomex 2200 or 1900 oxygen analysers (Fig. 3). These use a combination of intrinsic safety and a flameproof enclosure together to give what is thought by many to be the ideal combination of safety, reliability and cost-of-ownership.
With this system of protection, the equipment is suitable for use in demanding environments (Zone 1 hazardous areas and high-flammability group IIC gases), yet the principle of intrinsic safety is only applied to the elements of the apparatus that require it. For instance, in a gas analysis system there may be a requirement to process a flammable sample gas, which means that the sensors must, by definition, be suitable for use in a Zone 0 hazardous area. But to power the instrument, the mains supply must be fed into a flameproof enclosure – which will typically be sited in a Zone 1 area.
With the Servomex 2200 transmitter, there is an intrinsically safe compartment and a flameproof compartment, both housed within a common instrument casing (Fig. 4). The customer terminates high voltage mains electrical connections only within the flameproof enclosure and intrinsically safe circuits in the intrinsically safe compartment.
In comparison, other manufacturers of hazardous area oxygen analysers tend to use purge/pressurisation of the sensor and power supply, or house everything in a flameproof enclosure. However, because of the possible need to introduce a flammable sample gas, extreme measures need to be taken to ensure that a leak will not escape from the sample path into the flameproof enclosure.
An expert opinion
Nigel Picket, a Design Engineer at Servomex and an expert on intrinsic safety, comments: “Here in Europe we have the ATEX Directive, which includes specific requirements for apparatus sited in hazardous areas. For explosion protection, the equipment itself should not generate an explosive atmosphere; if that is not possible, then the equipment must prevent ignition of the explosive atmosphere.
“When handling flammable gas samples, we recognise that ‘prevention is better than cure’ and therefore adopt intrinsically safe methods as standard as it ensures that an ignition cannot take place.
“While purge/pressurisation may be a much simpler concept to understand than flameproof protection or intrinsic safety, the greater cost-of-ownership can far outweigh the higher purchase cost of the alternatives.”
In addition to supplying gas analysers, Servomex also designs and builds complete sample systems that incorporate analysers and associated equipment. Stephen Firth, Manager of the Servomex systems business, highlights other issues to consider: “Flameproof enclosures, which are required to contain an explosion, are generally constructed from cast metal and therefore heavy and costly; a flameproof enclosure would typically be an order of magnitude more expensive than a non-flameproof equivalent.
“Every application has to be considered on its own merits. We always encourage customers to compare the long-term costs of alternative approaches, and often they decide to pay more for an intrinsically safe instrument or sample system because they know maintenance will be so much easier. Nevertheless, it is sometimes prohibitively expensive to produce customised equipment with intrinsic safety protection, in which case an alternative will be specified.
“Ten years ago the market was very different, and it is definitely changing now. Even specifiers in the USA are now switching from flameproof to intrinsically safe because they appreciate the benefits - even if the concept is harder to understand. In all global markets, and across all industries, there is a movement towards intrinsic safety. Nowhere is there a rush, and intrinsic safety will seldom achieve 100 per cent market share, but there is an undeniable trend in that direction.”
More information about Servomex gas analysers and sampling systems, all of which are available and fully supported worldwide, can be obtained from Servomex. www.servomex.com