Gaseous Fire Extinguishing Systems - HCFC Blend A Extinguishing Systems
2004 Edition

HCFC Blend A, as defined in IS 15505, is a mixture of several HCFC gases with specific proportions detailed in Clause 4.3, Table 1. It commonly includes HCFC-22, HCFC-123, and HCFC-124. Physically, it is stored as a liquefied gas under pressure, appearing colourless with a citrus scent and exhibiting non-conductive electrical properties. Its density is approximately three times that of air, and it requires pressurized containment for proper storage.

According to IS 15505, the maximum fill density for HCFC Blend A cylinders is 0.9 kg per litre (900 kg/m³) for both 2.5 MPa and 4.2 MPa pressure systems. The maximum working pressure at 55°C is 3.5 MPa for 2.5 MPa systems and 5.3 MPa for 4.2 MPa systems. Cylinders are superpressurized with dry nitrogen to maintain 2.5 MPa ±5% or 4.2 MPa ±5% at around 21°C, ensuring moisture content remains below 0.005% by volume. Adhering to these limits prevents liquid overfilling and excessive pressure buildup.

The minimum design concentration for HCFC Blend A fire suppression is based on experimentally established extinguishing concentrations, with typical values like 7.2% volume for Class B fires. Safety factors are applied—20% for general hazards and 30% for flammable liquid/gas fires—to ensure effectiveness. The final design concentration is the extinguishing concentration multiplied by (1 + safety factor), with a minimum threshold of 8.6% volume. Altitude adjustments are made using atmospheric correction factors to account for pressure differences.

IS 15505 requires carbon steel piping and fittings to be galvanized internally and externally or provided with equivalent corrosion protection. Stainless steel pipes and fittings may be used without additional protection, provided they meet design strength criteria outlined in IS 15493. All piping must withstand the maximum system pressure developed at 55°C. Compliance with IS 15493 and local regulations is essential, and hydraulic considerations must address two-phase flow for accurate pipe sizing.

Discharge time calculation involves measuring the interval from valve activation or initial liquid discharge at the nozzle until the predominantly gaseous agent is expelled. The system must discharge 90% of the HCFC Blend A mass at 21°C sufficient to reach the minimum design concentration plus a 20% safety margin. The time to reach 95% of the design concentration should not surpass 10 seconds. Verification requires hydraulic flow calculations or approved pre-engineered system methods, with accuracy tolerances of -5% to +10% for agent weight discharged.

IS 15505 establishes toxicological thresholds: NOAEL at 10% volume concentration, LOAEL above 10%, and a 4-hour LC50 at 64%. If the design concentration exceeds LOAEL, the system should only be employed as a total flooding solution in unoccupied spaces. Safety protocols per IS 15493 include ventilation, leak detection, alarms, evacuation procedures, and personal protective equipment. Monitoring and emergency readiness are critical whenever exposure levels approach or exceed the LOAEL.

Nitrogen superpressurization enhances system performance by dissolving partially into the HCFC Blend A liquid and maintaining a vapor phase pressure above the agent’s natural vapor pressure. This combined pressure drives the agent through the distribution piping effectively. Typical superpressurization levels are 2.5 MPa ±5% or 4.2 MPa ±5% at around 21°C with moisture-controlled nitrogen. This improves flow consistency, delivery pressure, and overall system reliability while ensuring minimal moisture contamination.