Gaseous Fire Extinguishing Systems - IG 55 Extinguishing Systems
2004 Edition

Per IS 15506 Clause 29.1, the minimum design concentration for Class A fires (combustible solids) is the extinguishing concentration plus a 20% safety margin; for instance, if extinguishing concentration is 29.1%, the design concentration is 35% by volume. For Class B fires (flammable liquids and gases), a 30% safety factor is applied to the extinguishing concentration. Additionally, a minimum of 40% IG 55 concentration is recommended to maintain residual oxygen at 14%, ensuring effective suppression. Altitude-based atmospheric correction factors should be applied as needed.

To calculate the total IG 55 required, first compute the theoretical agent volume by multiplying the maximum net enclosure volume by the design concentration. Then, divide this by the volume per container to find the number of cylinders needed. Multiply the number of cylinders by container volume for the actual agent quantity, and calculate the actual injected concentration by dividing agent quantity by enclosure volume. Adjust the number of containers using atmospheric correction factors based on altitude. Use formulae and tables provided in IS 15506 to finalize the required quantity.

IS 15506 mandates that IG 55 system concentrations should not exceed 52% in areas occupied by personnel. For concentrations above this threshold, safe evacuation mechanisms must be implemented prior to discharge. The system must include warning alarms, detection devices, and procedures to ensure occupant evacuation before gas release. Oxygen reduction effects are controlled to prevent asphyxiation, and emergency response protocols should be in place to enhance safety.

Piping and fittings may be constructed from carbon steel or stainless steel. Carbon steel components require galvanization on both internal and external surfaces or an equivalent corrosion protection treatment. Stainless steel components do not need additional corrosion protection. All materials must be rated to withstand the maximum pressure and temperature conditions prescribed by IS 15493 and IS 15506 to ensure system integrity and durability.

Nozzles should be installed with a maximum height of 3.5 meters above the floor for a single row, with additional rows added if ceilings exceed this height. The minimum height above the hazard floor is 0.2 meters. Placement should be within 0.5 to 5 meters from walls on ceilings without false ceilings; if false ceilings exist, deflector shields are required to prevent discharge damage. Nozzles must also be provided in concealed areas such as floor and ceiling voids. Spacing between nozzles should not exceed 6 meters, and distances to walls or partitions should be within 3 meters, ensuring uniform distribution and proper pressure.

Vents must be located as high as possible in the protected space to allow pressure relief during gas discharge. Venting area is calculated using formulas incorporating maximum IG 55 flow, allowable overpressure (typically 500 Pa), and specific vapor volumes of the IG 55/air mixture. Free venting is essential to prevent structural damage due to overpressure. The design must comply with safety concentration limits and maintain enclosure integrity as specified in IS 15506.

According to IS 15506 Clause 13.1, full-scale discharge tests are generally not mandatory unless explicitly required by the relevant authorities. Where mandated, tests must follow procedures outlined in Clause 14. Otherwise, acceptance can be achieved via alternative commissioning methods per IS 15493. The system should demonstrate the ability to maintain at least 80% of the design concentration for ten minutes after discharge to confirm effectiveness.