CO₂ is not a clean agent in the NFPA 2001 sense; it is designed to a separate standard, NFPA 12. Total flooding design concentrations start at 34 per cent, and that level, or indeed much lower levels, is lethal to people. We therefore install CO₂ systems in normally unoccupied spaces and treat time delays, lock-off valves and warnings as mandatory parts of the design.
ATypes and components
Total flooding systems
The whole of an enclosed volume is brought to extinguishing concentration. Used in transformer rooms, generator and turbine enclosures, cable galleries and similar enclosed, normally unoccupied spaces.
Local application systems
CO₂ is aimed directly at the hazard, such as the ink unit of a printing press or the surface of a dip tank. The space does not need to be enclosed; nozzle layout and discharge time are calculated from the geometry of the hazard.
High pressure systems
CO₂ is stored in standard steel cylinders at ambient temperature (around 58 bar at 21 °C). Suitable for small and medium hazards; as cylinder numbers grow, so do space requirements and weighing work.
Low pressure systems
CO₂ is stored in an insulated, refrigerated tank at about -18 °C and roughly 20 bar. Economical on large sites that need tonnes of agent, and where several areas are protected through selector valves.
Pneumatic time delay and lock-off valve
A pneumatic delay unit in the discharge line holds the gas back for a set time even if power is lost. The lock-off valve physically prevents discharge while maintenance or other work is going on inside.
Odoriser and warning devices
Because CO₂ is colourless and odourless, an odoriser can be added to the discharge. Sounders and beacons inside the space and at entrances, and warning signs on the doors, are required by the standard.
Detection and control
The extinguishing control panel processes signals from detectors and from manual release and abort buttons. For local application, flame detectors or pneumatic detection tubing can also be used.
BHow it works
- Detectors sense the fire, or someone presses the manual release button.
- The panel operates the warning devices and starts the delay; ventilation is shut down and the space is evacuated.
- At the end of the delay the cylinder valves open and, after the pneumatic delay, CO₂ discharges through the nozzles.
- Oxygen falls below the level that supports combustion and the fire goes out; nobody enters until the space has been thoroughly ventilated.
CAdvantages
- Non-conductive, safe on live equipment
- No residue and no clean-up
- Fast knockdown on flammable liquid and surface fires
- Cheap, widely available agent
- Local application can protect open machinery
DApplications
- Transformer and switchgear rooms
- Generator and turbine enclosures
- Printing presses
- Paint booths and solvent rooms
- Cable galleries and tunnels
- Industrial fryers and dip tanks
EFrequently asked questions
Is a CO₂ system dangerous to people?
Yes. The concentrations needed to extinguish a fire are lethal, and far lower concentrations can cause unconsciousness within a short time. CO₂ systems are therefore used in normally unoccupied spaces and must never be commissioned without time delays, warnings and a lock-off valve. Where people are present, clean agent or water mist systems are the better choice.
How often should CO₂ cylinders be checked?
Under NFPA 12, high pressure CO₂ cylinders are weighed at least every six months, and a cylinder that has lost more than 10 per cent of its contents is refilled or replaced. On low pressure tanks the level gauge is monitored regularly. Periodic hydrostatic testing of cylinders follows the pressure vessel regulations.
What is the difference between CO₂ and a clean agent system?
Clean agents extinguish at concentrations people can survive; CO₂ is lethal at its extinguishing concentration. On the other hand, CO₂ allows local application, the agent is inexpensive and it has a long history on industrial machinery risks. We make the choice based on whether the space can be occupied and on the type of hazard.