Safety code for working in compressed air
1977 Edition

IS 4138 prescribes that decompression should not exceed a rate of 0.4 bar per minute, both when reducing pressure to the initial stage and between subsequent pressure stages. The time intervals between stages are excluded from this decompression rate calculation. For pressure values near the maximum working limit, the longer decompression method must be followed. Transfer of personnel to decompression chambers should be completed within 5 minutes, and recompression within 3 minutes, with decompression chamber pressures matching the original working chamber pressure. Decompression tables for exposure durations between 1 and 2 hours illustrate staged pressure reductions with corresponding total times to ensure safe pressure transitions.

According to IS 4138, air-locks and medical-locks must meet specific design and equipment requirements. Medical-locks should have a minimum clear headroom of 1.8 meters and consist of two compartments to maintain pressure integrity. These locks must be properly heated, ventilated, and illuminated. Doors are required to have viewports (bull’s eyes) and air valves operable from both inside and outside. Essential equipment includes a couch at least 2 meters long, blankets, dry woollen clothing, a food lock, and communication systems such as intercoms or telephones alongside non-verbal signaling between compartments and external observers. Air supplied must be free from oil and carbon monoxide, capable of quickly increasing pressure to 5.27 kg/cm² within 5 minutes. Man-locks should provide at least 1.8 meters of clear headroom and a minimum of 0.85 cubic meters of air per occupant, equipped with pressure gauges, clocks, signaling devices, and remote control for air supply.

IS 4138 mandates that workers engaged in compressed air environments undergo medical evaluations by qualified doctors to certify fitness before commencing work. Initial certification must occur within three days prior to first exposure or be valid within the previous three months without any significant illness. For workers exposed to pressures exceeding 1.25 kg/cm², a recent medical fitness certificate issued within four weeks before employment is required. Employers are responsible for continuous medical supervision and must maintain records in the compressed air health register. Workers must also carry identification cards indicating their status and emergency protocols.

IS 4138 emphasizes several fire safety precautions for compressed air workplaces. The air supply must be filtered and free from contaminants, maintaining a continuous fresh air flow of 0.3 cubic meters per minute per person at chamber pressure to prevent oxygen deficiency and reduce fire risk. A qualified supervisor should continuously monitor air quality and equipment. Alcohol consumption and entry under its influence are strictly prohibited to avoid impaired judgment and increased fire hazards. Ignition sources such as sparks and open flames must be eliminated, and fire-resistant materials used. Proper ventilation and emergency preparedness, including accessible fire extinguishers and evacuation plans, are essential to ensure safety.

Per IS 4138 Clause 14.2, gas detection must be carried out at least once every eight hours, with air samples collected from both the bottom and top of the well to monitor for methane, other hazardous gases, and oxygen deficiency. Only certified personnel equipped with approved methanometers should conduct these tests. Lighting used during inspection must be approved electric lamps or torches designed to prevent ignition. Upon detection of hazardous gases, immediate reporting is required and all compressed air operations must cease until safe conditions are restored.