Flexible conduit systems can provide a proven safe and cost effective protection solution for cabling installed in explosive atmospheres. Better safe than sorry is a sensible approach, but make sure that you are not fooled into unnecessarily expensive options warns Tim Creedon, Sales and Marketing Director for Flexicon.
Explosive areas exist where a flammable mixture of gas and air or dust and air exist in large enough quantities and for long enough. If an ignition source exists then there is a real danger of an explosion.
Naturally we all think of the oil and gas industry in such situations, but there are a surprising number of other industries where explosive atmospheres could exist such as in building and construction, transport, marine and defence, food processing, water treatment and power generation to name but a few.
Wherever possible it is important to minimise the risk of explosive mixtures forming and/or prevent the risk of ignition. Where this is impossible or impractical then you need to consider providing protection.
Any electrical installation in such an environment is a potential source of ignition. The degree of protection any equipment requires depends to a large extent on the risk of an explosion occurring in a given area.
To understand the level or protection required, you must understand the nature of flammable mixtures and ignition sources and also how different zones are classified depending on the level of risk.
Gases are classified into 3 groups with group A being the least explosive and group C being the most. Equipment classification is from T1 to T6 according to the maximum allowed temperature resulting from the ignition temperatures of the gas/air mix. It is important to remember that certain fine dusts dispersed in the air can also be explosive.
|Gases, Vapours and Mist||Continuous or long term frequent||0|
|Gases, Vapours and Mist||Occasional||1|
|Gases, Vapours and Mist||Occasional, then only briefly||2|
|Dusts||Continuous or long term or frequent||20|
|Dusts||Occasional, then only briefly||22|
The above table shows the zone designations, which are divided first into the hazardous areas for gases, vapours and mists and secondly into the hazardous areas for dusts. It also shows their risk categories, i.e according to the probability of a risk being present.
Assuming that the electrical equipment is correctly specified, it is important not to overlook any cabling that connects into it.
Until recently those specifying cables for such areas had to select from the products offered by various cable manufacturers. In addition each individual cable needed a flameproof gland, which added to both the cost and the time needed for installation.
Such cables could also be difficult to terminate and, if a number need terminating in an enclosure, could necessitate the specification of a larger enclosure.
The development of flameproof ATEX and IECEx approved barrier glands for flexible conduit means that you can use liquid tight conduit systems in hazardous areas without compromising safety.
There are a number of ways of classifying protection techniques used to address hazardous zones. Most glands are classified as Ex d or Ex e.
An Ex d classified gland forms a flameproof or explosion proof barrier – they are strong enough to contain any explosion or fire that may occur. An Ex e classification is defined as ‘increased safety’.
If something is classified as Ex d then it can also be used for Ex e applications in Zone 1 and Zone 2 areas for gases and Zones 21 and 22 where explosive dust may be present. In most cases such glands do not add to the temperature of the enclosure into which it terminates, so it can be used with all temperature classes.
By using Ex d glands with liquid tight conduit, you can group several cables together into one system. The conduit provides protection for all of these cables and, if correctly selected, offers all the necessary mechanical protection for a given application.
So, for example, by using steel cored armoured metallic conduit you might be able to use standard cables instead of more expensive SWA specialist cables. In this example one braided conduit effectively takes the place of several more expensive braided cables. It also means that only one flameproof barrier gland is needed, rather than several.
This single termination, instead of the multiple terminations that would be needed with individual cables, limits the risk of the enclosure integrity being compromised since there is only one point of entry. Using a flexible conduit system also provides additional mechanical protection for the cables.
It is worth pointing out at this stage that it is the flameproof gland that is rated at Ex d so that if there were an explosion within the electrical equipment enclosure it would be contained. Some conduit manufacturers have in the past muddied the waters and inferred that the conduit is also classified as Ex d and been able to charge a premium.
The role of flexible conduit in any application is to protect cabling so you should take care in its specification. This is even more important in hazardous areas since you want the installation to remain safe throughout its lifetime, not just once the system is installed.
Fortunately when you specify the correct barrier gland, you can effectively use a liquid tight flexible conduit for the vast majority of applications.
There are a number of different types of liquid tight flexible conduit that are suitable for both indoor and outdoor applications.
With this in mind the specifier needs to conduct a thorough risk assessment of all of the hazards potentially faced by the conduit in addition to the explosive atmosphere.
From this risk assessment they can then accurately specify the most appropriate liquid tight conduit for the project. As an illustration Flexicon has eight different grades of liquid tight conduit which, between them, would suit most applications.
Other environmental factors
If conduit is left in an exposed situation, it could be crushed or there is a chance that something could be dropped onto it. For such applications you should specify conduit with an adequate compression and/or impact strength.
Other factors that you may need to consider when selecting the correct conduit might include: extremes of temperature, EMC screening requirements, moving equipment, abrasion, resistance to chemicals or corrosion and UV resistance for external installations.
Note this list is not comprehensive, hence the need for a full risk assessment. If you have any concerns then most manufacturers should be able to advise.
A great deal of attention has been paid to hazardous area equipment. Do not forget cable protection and electrical lugs.
For more information on our range of Flexicon flexible conduits, please contact our Sales Team.
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