A positive pressure enclosure, also known as welding habitats or hot work habitats, is a chamber used to provide a safe work environment for performing hot work in the presence of explosive gases or vapours. They are used most often in conjunction with the need for 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. The qualities of good Indoor Air Quality should include comfortable temperature and humidity, adequate supply of fresh outdoor air, and control of pollutants from inside and outside of the Habitat. Indoor air quality describes how inside air can affect a person's health, comfort, and ability to work. It can include temperature, humidity, lack of outside air, mold from water damage, or exposure to other chemicals. Currently, OSHA has no indoor air quality standards but it does provide guidelines about the most common IAQ workplace complaints, the habitat technician need to provide daily practical solutions to ensure the worker inside habitat is safety and healthy.
Air Quality Assurance
The air exchange rate inside the Habitat shall be maintained above 20 cycles per hour in compliance with Factories and Machineries Act Regulation 1970, under Regulation 25 - Ventilation. Air flow inside the Habitat shall be arranged in one-direction airflow with the inlet and outlet at the furthest and opposite side. The ventilation flow rate of habitat shall be 2000 cfm from a clean source, per welder to dilute the polluted air inside habitat.
Positive Pressure Assurance
A minimum positive pressure of 0.1 inch of water or 0.00025 bar shall be maintained inside the Habitat enclosure to prevent ingestion of hydrocarbon in the event of leak occurred outside.
Heat Stress (Working Temperature)
Maximum allowable temperature inside the Habitat enclosure is 40 Deg C. Work must stop when the temperature inside has reached above this allowable threshold. This is to prevent heat stress exposure of those working inside.
Technician Training and Competency
The technicians must have attended the habitat manufacturer or operator in-house training on the Habitat set-up, operations and basic maintenance. The technician must have attended training on Local Exhaust Ventilation and able to calculate air exchange flow rate to ensure fumes generated from welding activity in the enclosure is maintained to acceptable limit.
Standards & Certification
IEC Standard
Almost all developed countries are members of the International Electrotechnical Commission. Positive pressure enclosures, or “welding habitats” are working on the principle of overpressure. This protection principle is regulated by Equipment protection by pressurized room "p". The IEC60079-13 standard forms the bases of welding habitat certification to the ATEX directive 2014/34/EU, and the IECEx equipment certification scheme. The IEC60079-13 standard is adopted into national certification standards by changing the prefix to the applicable national prefix i.e. IEC60079-13 becomes BS EN 60079-13, or CAN/CSA C22.2 No 60079-13, ANSI/UL 60079-13 etc. There may be some national foreword added to the IEC60079 series standard, but the main content of these national version of the IEC standards are identical to the IEC standard.
ATEX Certification
ATEX certification is the national certification standard of the European Union, and mandatory to operate equipment in explosive atmosphere in Europe. ATEX certification is not based on the IECEx certification scheme, certification is bases on the ATEX directive 2014/34/EU – see main article. Under the ATEX 2014/34/EU Directive, all equipment requires a proper manufacturer's EU Declaration of Conformity. For Zone 1 welding habitats this EU Declaration of Conformity must be based on a Notified Body issued EU–Type Examination Certificate for pressurized habitats EN 60079-13. Warning - there are habitat companies on the market that are using ATEX or IECEx component certification to convince customers that the habitat is ATEX compliant, but these habitats lack the required system certification to IEC60079-13. Practically the same as trying to convince someone about the roadworthiness of a car, by providing the certification for the tyres, lights, engine, etc., instead of the certificate for the car. Welding habitat certification is not about the individual component compliance – that’s a given – it is about what the habitat as a system does: completely separating potential explosive atmosphere from hot work using a controlled overpressure system combined with an automatic shutdown unit certified to IEC60079-13.
IECEx Certification
The IECEx certification scheme is regulated by the IECEx 02 IECEx certification is more transparent than ATEX, as all certification is web-based and can be checked & downloaded on the . IECEx certification is compulsory to operate electrical equipment in explosive atmosphere in most Countries outside Europe. Australia, New Zealand, UAE, Malaysia, and the Philippines accept IECEx certification directly. In Canada and the USA, all electrical equipment must bear the recognized Certification Mark of an accredited Certification Body, i.e., cQPS for Canada, QPSus for the USA and cQPSus for both, the USA and Canada. This QPS mark is only an extension of the IECEx certification - meaning IECEx + the national certificate. Brazil has the INMETRO regulations as their national extension of the IECEx certification scheme. IECEx Certificate may be accepted in other jurisdictions where no National electrical safety system exists.
Operating principles
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, oxygen must be present. The flammability limits of gases are expressed in proportions to the other gases present. For example, for methane, the lower explosive limit is 4.4% and the upper explosive limit is 17%. A positive pressure enclosure works by making sure that the methane present in the work area never approaches the 4.4% LEL.
Positive pressure
The operating pressure inside a typical isolation chamber is set only marginally above local pressure; typically only 0.05 kilopascals above local atmospheric pressure. This is sufficiently low to be undetectable 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.
