Last month’s column began a review of fire-protection strategies and systems for industry. This month, we conclude that review.
publishes more than a dozen standards addressing automatic fire-protection
systems, including NFPA 10 (portable fire extinguishers), NFPA 12 (carbon
dioxide), NFPA 13 (sprinklers), NFPA 17/17A (dry-chem and wet-chem), NFPA 15
(water spray), NFPA 750 (water mist) and NFPA 25 (water-based system maintenance).
The water-based technologies each have advantages and
disadvantages that users should be aware of. Water-based systems should never
be employed to control fires where water-reactive chemicals are located.
Steam is the least-utilized
option in this grouping, having only non-mandatory design guidance (see NFPA
86). Steam can be useful to extinguish fires inside enclosures by displacing
oxygen. It may have fewer negative side-effects than liquid water. However,
steam has two major drawbacks that limit its usefulness. Escape of steam into
occupied spaces creates a scald hazard. And unlike liquid water, steam’s
ability to cool hot materials (e.g., burning embers or burning liquids) is
Water-mist is a relatively new technology.
The first systems were installed in passenger ferries in the 1940s, and the
first NFPA standard was published in 1994 as Halon systems were being phased
out. Because the agent application rate is considerably lower than sprinklers,
water-mist systems are particularly useful where the water supply is limited.
Since the water is delivered in liquid form, however, the quenching effect is
preserved. Further, the small size and large number of the droplets can block
radiant heating of adjacent combustibles. Negatives for mist include the
requirement for compressed air as an atomizing medium and the limited “reach”
for each nozzle.
systems provide droplets that are intermediate in size to those of sprinklers
and mist systems but do not require an atomizing medium. Water-spray nozzles
are often employed to extinguish combustible liquid fires through a combination
of wider coverage than mist and smaller droplets than sprinklers. Because the
volume of water released is high, means for drainage is required. Examples of
water-spray applications include conveyors that move combustible materials,
fixtures with densely packed combustibles such as cable-trays and the exterior
of enclosures requiring heat-exposure protection.
Sprinkler systems are by far the most commonly implemented
fire-protection technology. The first edition of NFPA 13 was published in 1896,
and the current version incorporates a great deal of engineering know-how on
detection, actuation, piping, water supplies and nozzle coverage. Sprinkler
systems are distinguished by the detection/actuation method and whether the
pipe is wet or dry out to the nozzles. A “deluge” system employs open heads
installed in a dry pipe with an upstream actuation valve. An automatic
sprinkler system contains closed heads with a plug/stopper that ruptures at a
prescribed temperature to permit flow. Automatic sprinklers may be employed
with a wet-pipe or dry-pipe distribution network, depending on the temperature
of the protected space (i.e. ambient spaces, ovens or freezers).
Carbon dioxide and dry- or wet-chemical extinguishment
systems round out the primary technologies for fire protection of commercial
and industrial hazards.
Dry- and wet-chemical systems are designed to
discharge an agent (often sodium or potassium bicarbonate in particle or slurry
form) onto surfaces to smother flames and cool heated materials. The agent is
distributed using an expellant gas through a fixed network of pipes. As the
agent contacts burning materials, the particles or droplets suppress the fire
by both heat removal and chemical interruption of combustion chain reactions.
Dry- and wet-chemical systems are often employed where lubricants, fuels,
hydraulic fluids or cooking oils are at risk of being overheated and ignited.
systems, the agent is stored as a high-pressure liquid that expands to a gas
upon passage through nozzles, thereby providing a rapid cooling effect and
displacement of oxygen. Due to the maturity of this technology, available
design guidance is extensive. Precautions must be taken to prevent asphyxiation
when utilizing CO2 in “occupiable” but “normally unoccupied” spaces
such as transformer rooms, utility tunnels and liquid storage areas. The
discharge of CO2 systems produces ice particles that can lead to the
accumulation of static charge followed by sparking discharge.
Design, installation and maintenance of any
fire-protection system should be carried out by qualified personnel. Local fire
authorities frequently require and participate in annual inspections to ensure
Fire Water (Part 2)
October 3, 2011