Zones (Vapours & Gases)
|If you look at the size of a refinery or chemical factory and the amount of liquids and gases that circulate the various processes in that plant there must be a certain amount of risk of leaks and other hazards. In some cases the gas, vapour or dust is present all the time or for long periods.
|Refineries and chemical complexes should thus be divided into areas of risk of release of gas, vapour or dust known as zones. The type and size of these hazardous areas is determined using area classification.
Typical gas hazards are from hydrocarbon compounds.
A domestic domain such as a house would be classed as safe area where the only risk of a release of explosive or flammable gas would be the propellant in an aerosol spray. The only explosive or flammable liquid would be paint and brush cleaner. These are classed as very low risk of causing an explosion and are more of a fire risk (although on rare occasions gas explosions in domestic property are known to occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 0.25% of its lower explosive concentration limit.
Zone 2 area
This is a step up from the safe area. In this case it has been decided that in this zone the gas, vapour or dust would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time. Explosion safety compliant equipment should be used.
Zone 1 area
These areas are where special or classified electrical equipment must be used. It is expected that the gas, vapour or dust will be present or expected for long periods of time under normal running. As a guide this can be defined as 10–1000 hours/year or 0.1–10% of the time. Explosion safety equipment that has a higher safety level than Zone 2 equipment must be used.
Zone 0 area
This is the worst scenario as gas or vapour is present all of the time (over 1000 hours/year or >10% of the time). Although this is the worst case it is very rare that a zone 0 area will be in the open. Usually this would be the vapour space above the liquid in the top of a tank or drum.
In the case of dusts there is still a chance of explosion. An old system of area classification to a British standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002
The boundaries and extent of these three dimensional zones should be decided by a competent person. There must be a site plan drawn up of the factory with the zones marked on.
The zone definitions are:
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods, or frequently.
A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.
A place in which an explosive atmosphere in the form of a cloud of combustible dust is not likely to occur in normal operation, but if it does occur will persist for a short period only.
Hazardous area equipment is specified in terms of the types of gases present, being:
Group I : For mining applications
Group II : For general industry
Equipment must be categorized as being suitable for use with gases in Group II industries.
Group I equipment has a representative gas of Methane and all equipment used in underground mining applications fall into this category Any other equipment will be Group II classified.
Group II is again segmented into three different representative gas groups, Propane, Ethylene and Hydrogen. Groups IIA, IIB and IIC respectively.
|As stated these are only representative gases where in fact there are many different types of gases one can be presented with when classifying equipment.
Temperature classification takes into account the ignition temperature of the flammable gases present in the particular environment.
Ambient temperatures are of course considered in this appraisal, however circumstances which may increase this surface temperature must also be taken into account.
For example, an enclosure shall over a period of time, collect a layer of dust on its surfaces. This shall increase the units overall operating and surface temperatures, to a point where if it rises to a point nearing the flash or combustion point of surrounding gases, liquids or dusts, then an explosive situation would occur.
For Group I classified electrical equipment, where coal dust can form a layer, the maximum surface temperature shall not exceed 150 °C. If this layer can be avoided through sealing or protection from the ingress of dust, the internal surfaces must not exceed 450 °C.
In respect to Group II electrical equipment, temperatures are classified as per the following table.
Temperature classes T1 to T6 (AS 1280.1) are used for all protection techniques, and correspond to the electrical devices maximum operational temperature.
The better the equipment, the lower the rated surface temperature. In other words a rating of T4 is better than that of T2.
Therefore, those items certified as being T6, can be used in T1, T2, T3, T4, T5 and T6 gases. Whereas a T2 certification only allows a piece of equipment to be used in T1 and T2 gases.
Protecting Electrical Apparatus to make it suitable for use in hazardous areas
Electrical apparatus for use in hazardous areas needs to be designed and constructed in such a way that it will not provide a source of ignition. There are ten recognized types of protection for hazardous area electrical apparatus. Each type of protection achieves its safety from ignition in different ways and not all are equally safe. In addition to the equipment being suitable for the Gas Group and the Temperature Class required, the type of protection must be suitable for the zone in which it is to be installed. The different types of protection and the zones for which they are suitable are as follows: