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A positive pressure enclosure, also known as an isolation chamber, is a chamber used to provide a safe work environment in the presences of explosive gasses or vapours. They are used most often in conjunction with the need for underwater welding, and are often associated with the offshore oil industry.
A positive pressure enclosure works by providing a constant inflow of breathable atmosphere, which in turn causes gas to continuously leak out of the chamber. This outflow of gasses prevents the ingress of explosive gasses or vapours which are often present in such work locations. This constant outflow of gasses from the chamber also serves to cleanse the air within of the undesirable gaseous by-products of the welding process. Most commercial versions of positive pressure enclosures are referred to by their manufacturers as habitats.
Operating principles
Flammability limits
Flammable gasses are not generally explosive under all conditions, and indeed, must be present within specific ranges in order to be explosive, or flammable. Additionally, oxygen must be present. The flammability limits of gasses are expressed in proportions to the other gasses present. For example, for methane, the lower explosive limit (LEL) is 4.4% and the upper explosive limit (UEL) is 17%. A positive pressure enclosure works by making sure that the methane present in the work area never approaches the 4.4% LEL.
Government regulations in the European Union and in North America define the safety requirements for workers working in the presence of flammable gasses. In Europe, these regulations are found in the ATEX directives, and in the United States, in the National Electrical Code.
Positive pressure
The operating pressure inside a typical isolation chamber 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 (a person sitting in a bathtub full of water is exposed to greater pressures) 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 into the enclosure by fan units.
References
- The Basics of Explosion Protection, www.r-stahl.com/fileadmin/Dateien/explosionsschutz/pdf/grundlagen_en.pdf retrieved 20th Oct 2008
- www.hse.gov.uk/fireandexplosion/zoning.pdf - retrieved 20th October, 2008