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Habitat may be used to refer to a pressurised enclosure used on gas and oil platforms in the North Sea and anywhere where hot work may reequired in the presence of explosive hydrocarbons. The enclosures are designed to leak so the air inside is constantly changing, preventing the ingress of explosive gasses or vapours. This allows operators to perform hot work on metal structures within Zones 1 and 2 as defined by the ATEX directive allowing the Oil platform or Gas platform to operate without incurring an expensive shutdown.
Background
Explosive atmospheres are formed when gas, vapour or even flammable dust, reaches a concentration between two limits - the LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) see Flammability limit) expressed as a percentage. Below the LEL, the mixture of explosive material and air is so rarefied that an ignition source will not cause an explosion; above the upper limit, the mixture is too concentrated to produce an explosion but may cause a deflagration incident.
Atex Zoning
ATEX defines three areas or zones according to the risk of explosion as follows:
Zone 0 (20): A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist (or dust in Zone 20) is present continuously or for long periods or frequently.
Zone 1 (21): A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist (or dust in Zone 21) is likely to occur in normal operation occasionally.
Zone 2 (22): A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist (or dust in Zone 22) is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
These groupings broadly match those of the North American National Electrical Code Article 505-7 and Commission of the European Communities CEC 18-006 specifications.
Operating Principal
The operating pressure inside a typical habitat system is set only marginally above local pressure; typically only 0.05 kilopascals (about 0.007 pounds per square inch) above local atmospheric pressure. This is sufficiently low to be undetectable (we are exposed to more pressure just sitting in a bath of water) to operators working inside the enclosure, but due to the leaky nature of the unit, ensures that the volume of air inside is constantly and rapidly changing. Intake air is piped from a source well inside Zone 2 by fan units. Enclosures produced by StS Norway and Safehouse Ltd share a similar leaky baloon principal. Units manufactured by Hot-Hed also feature an extraction fan and claim a 2:1 ratio of input to output air.
According to the official website, StS claims a patent because its unit has a number of gas sensors, including one located in the intake fan, which monitor for explosive hydrocarbon gases and shuts the system down. Further gas detectors are arranged inside of Zone 1; heat and pressure detectors are located inside the enclosure itself.
Current Instalations
Overpressure enclosures known commercially as Habitats are currently manufactured and deployed by StS Gruppen AS in Norway, Safehouse Ltd. in Scotland for use in the North Sea oil and gas fields. Habitat is a trademark of Hot-Hed in the United States.
References
- www.hse.gov.uk/fireandexplosion/zoning.pdf - retrieved 20th October, 2008
- The Basics of Explosion Protection, www.r-stahl.com/fileadmin/Dateien/explosionsschutz/pdf/grundlagen_en.pdf retrieved 20th Oct 2008
- http://hot-hed.com/services/habitat.html retrieved 28th October 2008
- http://stsadvancedhabitat.com/hans.html retrieved 4th November 2008
External Links
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