The purpose of hazardous area classification is to avoid ignition of those releases, which inevitably occur from time to time in the operation of the facilities handling flammable liquids and gas. The approach is to reduce to an acceptable level the probability of coincidence of a flammable atmosphere and an electrical or another source of ignition.
The definition of a hazardous area is: –
“A hazardous area is defined as a three-dimensional space in which a flammable atmosphere may be expected to be present at frequencies as to require special precautions for the construction and use of electrical apparatus.”
The main reasons for classifying an installation into hazardous areas are:
- To perform the correct selection of equipment according to the degree of hazardous area classification where they are situated;
- To ensure that sources of ignition are segregated from sources of flammable gas or vapour;
- To assist in the appropriate location of ventilation and combustion air inlets and exhausts for buildings and combustion equipment;
- To define the extent of hazardous area around degassing vents;
- To assist in the appropriate location of flammable gas detectors;
- To determine the maximum allowable surface temperatures for particular areas.
The classification of hazardous areas shall only take into consideration events for which the probability of occurrence is significant enough to be described as “liable to occur during normal or abnormal operation conditions”.
The classification of the hazardous areas shall be carried out according to the frequency of the existence of a flammable gas/air mixture. Three main subdivisions exist:
- Zone 0 is that part of hazardous area in which a flammable atmosphere is
continuously present or present for long periods;
- Zone 1 is that part of a hazardous area in which flammable atmosphere is likely to occur in normal operation; and
- Zone 2 is that part of a hazardous area in which flammable atmosphere is not likely to occur in normal operation, and if it occurs, will exist only for short periods.
In the absence of client specification or code requirement for hazardous area classification, the extent and definition of hazardous areas shall be in accordance with IP Model Code of Safe Practice Part 15. The methodology given in this code is Risk Based and the hazardous radius given are specific to the pressure contained within the process equipment, probable leak sizes, type and categories of fluids, fluid composition, number of ignition sources present in the vicinity of a potential leak source, probability of presence of personnel in an area and probability of personnel exposure to a hazard. This classification code is preferred to other more generic codes such as API 505, which gives a hazard radius regardless of system pressure, potential leak size, etc.
The overall design philosophy shall be able to make the site as safe as possible by minimising the potential sources of release of flammable gas and ensuring efficient ventilation to disperse any gas that is released. Process facilities shall be designed to minimise areas classified as Zone 0 and Zone 1.
Hazardous area classification drawings
Hazardous area drawings will be prepared during FEED Engineering. They shall
identify the governing potential and continuous gas sources. Hazardous area drawings
shall include the following: –
- All major items of equipment;
- All ventilation inlets and outlets;
- Type and extent of hazardous areas;
- Intakes and exhausts of all fuelled equipment;
- Location of all fuelled units (e.g. diesel power generator);
- Tank or process vents; and
- Type of ventilation present / provided.
Hazardous area drawings shall be updated during Detailed Engineering taking into consideration the exact location of equipment and vents. Hazardous Equipment Tables have been prepared during FEED Engineering and shall be updated during Detailed Engineering.
Equipment and protective systems for all places in which explosive atmospheres may
occur shall be selected on the basis of the categories set out in accordance with European ATEX Directive 94/9/EC.
The following categories of equipment shall be used in the zones indicated, provided they are suitable for gases, vapours or mists as appropriate: –
- In zone 0: Category 1 equipment;
- In zone 1: Category 2 equipment;
- In zone 2: Category 3 equipment.
All electrical equipment located in the restricted area and that may be required to operate in an emergency situation shall be of Cat. 2 unless otherwise specified. Such equipment includes, but not limited to: –
- Field fire and gas detection (FGS) equipment;
- Field safety systems (PSS, SSS) equipment;
- Field emergency lighting and escape signs.
All electrical equipment shall be rated for the hazardous area in which it is located.
Electrical equipment in the Battery Room shall be suitable for Gas Group II C.
Primary muster and embarkation areas, LQ, helideck, control rooms, switch rooms, fire pump rooms, main and emergency generator rooms, workshops, offices shall be located in safe areas.
Suppression of ignition sources
Diesel power generators and the crane diesel engine are generally installed outside any hazardous area. However, they shall be regarded as potential ignition sources.
The diesel power generators shall be automatically shutdown and inhibited to start following confirmed gas detection.
For crane diesel engines, in the case of confirmed gas detection, automatic shutdown of the engine is not advisable as a load may be left hanging on the crane boom during the lifting operation. An alarm (visual and audible) shall be provided in the crane cabin to inform the crane operator of confirmed gas detection event and to stop the crane diesel engine manually.
Their ventilation air inlets shall be located at 2 m minimum from any hazardous area limit. Their exhausts shall be kept 2 m away from the border of any Zone 2 hazardous area and 9 m away from the border of any Zone 1 hazardous areas.
Hot surfaces temperature limitations
Because combustible materials are present and the possibility of leakage or spillage exists, a suitable low surface temperature of vessels/equipment/piping is very important in preventing the ignition of these materials. The maximum allowable surface temperature depends on the auto-ignition condition of the combustible material present. When the condition exists, all hot surfaces, such as valves, flanges, exhausts and any other hot metal projections shall be insulated so as to maintain all hot exposed surfaces below the maximum allowable surface temperature.
Escaped flammable gases shall not be allowed to come into contact with a surface whose temperature can exceed their auto-ignition temperature. As the ignition point of the combustible gas is not known, the conservative value of 250°C shall be considered as the maximum acceptable surface temperature.
Spilled liquid hydrocarbon should not be allowed to come into contact with a surface whose temperature exceeds its flash point. 200°C shall be taken as default value, but it is highlighted that process hydrocarbons (condensate, lube oil and diesel) have significantly lower flash points. This requirement is not applicable for components that are, by design, connected onto the hot equipment itself.
Insulating materials shall be non-asbestos, and to prevent fire spreading, the insulation material must be non-combustible and non-absorbent, such that it does not contribute fuel to the fire.