NOTE ON GASEOUS FIRE EXTINGUISHING SYSTEMS

 NOTE ON GASEOUS FIRE EXTINGUISHING SYSTEMS

Gaseous /Clean agent fire extinguishing systems are three-dimensional agents that are used efficiently and effectively to suppress fire through physical or chemical action.  Gaseous/Clean agent fire extinguishing systems are electrically non conducting and leave no residue upon evaporation. Gaseous/ Clean agent fire extinguishing systems are categorized into two types, 

 

1.      Halocarbon agents: These are sold under TWO commercial names 

a.     HDFC BLEND A – It is a mixture of  4 CHEMICALS

CHEMICAL NAME	%
Isoproponyl-1 -methylcyclohexane	3.75
Dichlorotrifluoroethane	4.75
Chlorotetrafluoroethane	9.5
Chlorodifloromethane	82

 

b.     HDFC – 227ea – (Heptaflouro propane- 100%)

2.     Inert gas agents : These are sold under FOUR commercial names 

a.     IG 01 - (Argon-100%)

b.     IG 100 - (Nitrogen-100%)

c.     IG 55 - (Nitrogen-50% and Argon-50%)

d.     IG 541 - (Nitrogen-52%, Argon-42%, Carbon dioxide-8%)  

 

SUITABILITY AND APPLICATION OF GASEOUS FIRE EXTINGUISHING SYSTEMS

Gaseous/clean agent fire extinguishing systems are stored in containers which are designed to hold that specific agent at ambient temperatures which is between -21°C to 55°C, they are stored as per the recommended pressure and concentration as per the manufacturer,  The clean agent contained in the systems should meet the requirements of Department of explosives, Nagpur. 

 

Gaseous / clean agent fire extinguishing systems are suitable to the following like

 

1. Enclosures - such as rooms, vaults, enclosed machines, containers, storage tanks and bins.
2. Enclosed electrical hazards - such as transformers, control cubicles, switchboards, circuit breakers, and rotating equipment.
3. Enclosed storage areas - such as flammable liquid/gas storage and processing areas.
4. Engines - For engines using flammable fuels.
5. Electronic hazards: such as computers, data processing equipment, control room, telecommunication facilities
6. Concealed spaces and assets: Such as Sub-floors and other concealed spaces and Other high-value assets.

LIMITATIONS OF GASEOUS FIRE EXTINGUISHING SYSTEMS

Gaseous fire extinguishing systems cannot be used on fires involving the following materials

 

1. Chemicals such as cellulose nitrate, gun powder, which are capable of rapid oxidation in the absence of air.
2. Metals which are very reactive such as lithium, sodium, potassium, magnesium, titanium, zirconium and plutonium.
3. Some Metal hydrides and metal amides react violently with some gaseous agents.
4. Chemicals capable of undergoing auto-thermal decomposition, such as certain organic peroxides and hydrazine
5. Mixtures containing oxidizing materials, such as sodium chlorate or sodium nitrate.
6. Environments, where significant surface areas exist at temperatures greater than the breakdown temperature of the extinguishing agent and, are heated by means other than the fire.

g.     Electrostatic charging of non-grounded conductors may occur during the discharge of gaseous fire extinguishing systems. These conductors may discharge to other objects causing an electric arc of sufficient charge to initiate an explosion in potentially explosive atmospheres.

 

SAFETY REQUIREMENTS WHILE USING GASEOUS/CLEAN AGENT FIRE EXTINGUISHING SYSTEMS

In the vicinity of discharge, some of the gaseous fire extinguishing/ clean agents creates serious hazards for personnel present near the discharge of extinguishing agent and near the areas to which the gas may migrate. These hazards can reduce visibility, increase suffocation and toxicity, during and after the discharge period. 

All clean agents contain fluorine.  This fluorine in the presence of available hydrogen (from water vapour, or the combustion process itself) can form HF (hydrogen fluoride) which is the main decomposition product. The decomposition products especially HF (Hydrogen fluoride) generated by the clean agent breaking down in the presence of very high amounts of heat can be hazardous. The decomposition products have a sharp, acrid odour which creates a noxious and irritating atmosphere. Longer exposure of the clean agent vapour to temperatures above 482°C would produce greater concentrations of decomposition products.  The two main important safety precautions to be taken by personnel present are 

·       Avoiding unnecessary exposure to all gaseous extinguishants.

·       The rate of discharge should be selected to minimize the exposure time of the vapours to the elevated temperature if the concentration of breakdown products is to be minimized. 

SAFETY PRECAUTIONS IN GASEOUS/CLEAN AGENT TOTAL FLOODING SYSTEMS

In total flooding systems, occupants may get exposed to the agent discharged, that is the agent starts flooding the area even before the occupants evacuate the area making them exposed to hazards of the agent.  To protect occupants from getting exposed, factors such as the time for egress and the risk to the occupants by the fire must be considered when determining the system discharge time delay. The mandatory requirements regarding the safety aspects of these total flooding systems are 

1. A time delay shall be provided for the evacuation of occupants before agent starts discharging. The agent discharge delay time shall be based and should be set on an engineering assessment of egress time for occupants in that area calculated for protection.
2. Safety interlocks and Lock off valve shall be provided based on the NOAEL (No Observed Adverse Effect Level) and LOAEL (Lowest Observed Adverse Effect Level) level which indicates the toxicity concentration of different clean agents.
3. All Exit routes should be kept clear at all times and the provision of emergency lighting and other audio-visual signals along with adequate direction signs to minimize travel distances.
4. Outward swinging self-closing doors should be provided which can be opened from inside including when locked from the outside.
5. There should be continuous visual and audible alarms at entrances and required warning and instruction signs should be arranged at designated exits until the protected area has been evacuated and made safe
6. There should be Means of prompt ventilation like forced draft ventilation for protected areas after any discharge of clean agent. Care should be taken to completely dissipate hazardous atmospheres and not just move them to other locations, as agents are generally heavier than air
7. Mock drills and Instructions of all personnel should be provided properly, including maintenance and construction personnel to make them understand the correct procedure when the system operates
8. If possible, an odour can be added to the agent so that the hazardous atmospheres are recognizable
9. Self-contained breathing apparatus (SCBA) to be provided along with the personnel trained for its use
10. There should be proper Means to detect a hazardous atmosphere in or around the protected area.

Article by PJ Mohan

Sr.Faculty, NIFS