INSTRUMENTATION IN HAZARDOUS AREAS
When combustible materials are mixed with air an explosive mixture is produced creating a potential danger particularly in handling and storage areas. In the oil and gas industries these areas are commonly referred to as hazardous areas.
More specifically, a hazardous area is an area in which explosive gas-air mixtures are present (or may be present) in quantities such as to require special precautions in the construction and use of electrical apparatus.
With the introduction of electronic and electrical instrumentation in process control, the potential risk of ignition by electrical energy has increased sharply. To protect personnel and installations, special precautions, as laid down by the various standards organisations, are taken to prevent ignition of an explosive mixture.
Conditions within electrical apparatus likely to ignite explosive mixtures are:
-Electrical sparks and arcs produced when circuits are opened or closed such as relay or switch contacts,
-Components, and subsequently their containers, heated by passage of electrical current or by faulty apparatus.
The probability of a hazardous condition being present is established first by classification of hazardous areas into zones according to the degree of hazard involved. Having classified hazardous areas by zones, then appropriate electrical apparatus must be selected according to the zone of risk, ignition energy and the surface temperature produced by the equipment.
All electrical equipment for use in hazardous areas is constructed to conform to standards issued by the British Standards Institute, (BSI), or the International Electro-technical Commission, (IEC), and CENELEC, the European Committee for Electro-technical Standardisation.
In addition, manufacturers submit production samples for testing to the British Approval Service for Electrical Equipment in Flammable Atmospheres (BASEEFA).
The highest level of safety in hazardous areas is attained by the use of intrinsically safe electrical apparatus. This is a protection technique, which avoids exposing an explosive atmosphere to any possible ignition source.
Finally, standards within the UK and Europe have evolved to simplify the selection and use of electrical equipment in hazardous areas. It is mandatory that maintenance of this equipment be such that no modifications are made, which can render the equipment no longer intrinsically safe or flameproof.
Hazardous areas are classified into zones according to the degree of risk.
ZONE 0 Where an explosive atmosphere is continuously present for long periods.
ZONE 1 Where an explosive atmosphere is likely to occur in normal operation.
ZONE 2 Where an explosive atmosphere is not likely to occur in normal operation and if it occurs it is only present for a short period.
Typical example of a hazardous zone side plan for an offshore installation is shown in Figure 1.
Selection of apparatus according to Zones
Electrical installations in hazardous areas should be kept a minimum. However, in areas where electrical equipment has to be installed, a code of practice has been developed to enable specific selection of various electrical equipment suited to the zones and the types of protection allowed, see the table in figure 2.
There are various methods used to prevent electrical apparatus from igniting the surrounding atmosphere when energised.
1) Flameproof Enclosures, type Ex ‘d’
Equipment that could ignite an explosive atmosphere is housed within a substantial enclosure. This is capable of containing an internal explosion without transmitting that explosion to the surrounding explosive atmosphere.
2) Increased Safety, type Ex ‘e’
An increased measure are taken to prevent the generation of arcs, sparks and excessively hot areas in equipment thus preventing the risk of explosion inside or outside of the enclosure.
3) Intrinsic Safety, type Ex 'i'
Power is limited to this equipment in such a way that a spark or a hot surface would not be hot enough to ignite the explosive atmosphere.
4) Pressurised, type Ex 'p'
Here the concept used is to house the equipment in an enclosure, which is pressurised or purged by inert gas, thus preventing an explosive atmosphere from reaching the equipment.
5) Oil Immersion, type Ex ‘O’
All equipment is totally immersed in oil, thus preventing an explosive atmosphere from reaching the equipment.
6) Powder filled, type Ex 'q'
All equipment is totally immersed in powder, thus preventing an explosive atmosphere from reaching the equipment.
7) Non-sparking and Restricted Breathing, type Ex 'n' or ‘N’
This gives a level of protection by housing electrical equipment in substantial enclosures that inhibit mechanical damage and give some degree of ingress protection.
8) Special Protection, type Ex 'S'
This applies to items of equipment not entirety covered by any of the foregoing concepts but one that can clearly be demonstrated to be explosion-proof.
9) Moulded/Encapsulated, type Ex 'm'
The apparatus is totally encapsulated by a non-porous compound. Electrical connection is by flying leads.
Hazardous materials vary in their ease of ignition and this is recognised by grouping either them, or the apparatus, into order of severity. This is done in two separate ways:
1. By their capability of being ignited by sparks or flames (gas group)
2. By their capability of being ignited by hot surfaces (gas temperature class)
Spark or Transmitted Flame Ignition
The possibility of spark ignition exists in electrical apparatus and may occur during either its normal operation or under fault condition.
All electrical apparatus is grouped according to the ignition energy produced, in terms of the gas-air mixture that it will not ignite. There are four groups in Europe:
Gas Groups
Gases are grouped together according to their degree of hazard. This is based on the amount of energy required to light the gas/air mixture.
Gas Group Representative Gas
I : (Mining)
IIA : (Surface)
IIB : (Surface)
IIC : (Surface) Methane
Propane
Ethylene
Hydrogen
Hot Surface Ignition
If hot surfaces, such as hotplates and light fittings, come into contact with a flammable atmosphere whose ignition temperature is lower than their surface temperature, ignition will result. A gas/air mixture can ignite when it comes into close contact with an excessively hot surface. The surface temperature at which equipment operates is therefore of crucial importance. This equipment is temperature classified as follows:
Temp. Class Max. Surface Temp.
T1 450°C
T2 300°C
T3 200°C
T4 135°C
T5 100°C
T6 85°C