Carbon dioxide (CO2) has successfully been used as a fire extinguishing agent for many years. The demise of Halon 1301 as a gaseous fire extinguishing agent, due to its ozone depleting characteristics, has renewed interest in CO2. CO2 is a colorless, odorless, generally non-reactive and electrically nonconductive gas that is slightly heavier than air. It is self-propelling, leaves no residue and is inexpensive.
If it weren’t for one major drawback, CO2 would be the perfect solution to just about all fire protection needs. The drawback is that CO2displaces oxygen, which is its main fi re extinguishing mechanism. The minimum theoretical CO2 concentration to extinguish fire with most fuels is 28 percent. But the maximum concentration at which harmful effects become noticeable in humans is about 6 percent. It is obvious that any CO concentration suitable for fire
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protection is also lethal to humans. Although the safety record of
CO as a fire extinguishing agent is relatively good, several fatalities
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have resulted from unintended CO system discharges. One of the
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more recent incidents occurred in 1998 at the Test Reactor Area of
Idaho National Engineering and Environmental Laboratory, which
resulted in one fatality and several life-threatening injuries.
The industry design and installation standard, National Fire
Protection Association Standard 12 (NFPA 12), Carbon Dioxide
Extinguishing Systems, has been periodically revised since its
inception in 1929. Starting with the 2005 edition of NFPA 12,
new requirements for personnel safety were added to the standard
which required that all existing CO fire suppression systems be
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retrofitted with warning signs, lock-out valves, pneumatic time
delays and pneumatic predischarge alarms. These requirements
affect most existing CO2 systems in the power generation industry.
The 2008 edition of NFPA 12 was partially revised to add language
concerning occupiable vs. unoccupiable spaces.
CO can migrate or collect in the event of a discharge from a storage 2
container’s safety device.
The sign’s required text differs according to its location (for example, within the protected space as opposed to at the entrance to a protected space as opposed to outside the entrance to CO2 storage rooms) and whether or not the CO2 is odorized. There are, therefore, six different sign options, although typically only two or three will apply for a CO2 system at a power plant.
WARNING
Carbon dioxide gas can cause injury or death. When alarm operates, vacate immediately.
While most CO2 systems come with some type of warning sign, the signs need to be upgraded in accordance with the 2008 NFPA 12 edition to meet the new (2002) revision of American National Standards Institute (ANSI) Z535 format—the only format now permissible. The only time signage can differ from ANSI Z535 format is when a formal signage training program exists. This means that all personnel with access to the protected space must either be trained or accompanied by personnel trained on the signage program. Because this exception can be cumbersome, most facilities find it easier to simply change to or install the ANSI Z535 signs. These signs follow a three-panel pictogram format (Figure 1). Warning signs are required in every protected space; at every entrance to protected spaces; in spaces near the protected spaces where it is determined that CO could migrate, creating a hazard to
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personnel; and at each entrance to CO storage rooms and where
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Manual lockout valves were provided on many CO systems
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in the past and became a requirement for all systems beginning
with the 2005 edition of NFPA 12. The only exception is where
dimensional constraints prevent personnel from entering the
protected space. Even then, a lockout valve is required if CO2 could
migrate and create a personnel hazard. Essentially, the lockout
valve requirement applies to every CO2 system typically found in a
power plant.
Because all components in a fire protection system are required to be listed for fire protection service, the valve must have a visual indication of its position and a provision that allows it to be locked in the closed position. In addition, the valve must be supervised. This means that the valve must have a normally closed electromechanical switch. This switch should be electrically connected to the CO
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system’s control panel so that when the valve is in the closed position, a supervisory signal is initiated, and an open circuit, ground fault or loss of integrity results in a trouble signal, as required by NFPA 72, National Fire Alarm Code. A service disconnect switch is not a permissible substitute for a lockout valve.
NFPA 12 permits the “authority having jurisdiction” (federal, state or local fire prevention bureau or insurance carrier) to waive the valve supervision requirement. A typical example would be a situation in which an older existing system is provided with a lockout valve that does not include electronic supervision. In such
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