Positive pressure enclosure
Encyclopedia
A positive pressure enclosure, also known as an isolation chamber, is a chamber used to provide a safe work environment for performing hot work
in the presence of explosive gas
es 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 gases prevents the ingress of explosive gases or vapours which are often present in such work locations. This constant outflow of gases 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.
must be present. The flammability limit
s of gases are expressed in proportions to the other gases 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 gases. In Europe, these regulations are found in the ATEX directive
s, and in the United States, in the National Electrical Code.
) 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.
Hot work
Hot work is any process that can be a source of ignition when flammable material is present or can be a fire hazard regardless of the presence of flammable material in the workplace. Common hot work processes are welding, soldering, cutting and brazing...
in the presence of explosive gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es or vapours. They are used most often in conjunction with the need for underwater welding, and are often associated with the offshore oil industry
Offshore drilling
Offshore drilling refers to a mechanical process where a wellbore is drilled through the seabed. It is typically carried out in order to explore for and subsequently produce hydrocarbons which lie in rock formations beneath the seabed...
.
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 gases prevents the ingress of explosive gases or vapours which are often present in such work locations. This constant outflow of gases 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.
Flammability limits
Flammable gases are not generally explosive under all conditions, and indeed, must be present within specific ranges in order to be explosive, or flammable. Additionally, oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
must be present. The flammability limit
Flammability limit
Flammability limits, also called flammable limits, give the proportion of combustible gases in a mixture, between which limits this mixture is flammable. Gas mixtures consisting of combustible, oxidizing, and inert gases are only flammable under certain conditions. The lower flammable limit ...
s of gases are expressed in proportions to the other gases present. For example, for methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, 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
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
and in North America define the safety requirements for workers working in the presence of flammable gases. In Europe, these regulations are found in the ATEX directive
ATEX directive
The ATEX directive consists of two EU directives describing what equipment and work environment is allowed in an environment with an explosive atmosphere...
s, 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 inchPounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
) 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.