Recently, NFPA released an “Alert” regarding the hazards of antifreeze solutions in fire sprinkler systems. Antifreeze has become a popular alternative to compressed air for protecting sprinkler pipes from freezing in cold climates. In its undiluted form, however, antifreeze will exacerbate a fire instead of extinguishing it.
Alerts such as these can serve to motivate businesses to
re-evaluate their existing fire-protection solutions. This two-part column
provides readers with some of the fundamentals about fire sprinklers and
related fire-protection systems should they decide to undertake a
Every fire-protection system
design should begin with clear definitions of the top-level objectives for the
system. Life safety (protection of the structure to permit safe egress of
occupants) should be an obvious motivation factor, but protection of a
building’s contents and equipment should also be considered. At a minimum,
applicable fire code requirements should be met.
Tradeoffs are frequently
important. One to consider is extinguishing efficacy versus negative
consequences of agent in the protected space. Another tradeoff involves
up-front system cost versus downstream costs to replace fire-damaged goods and
infrastructure. Insurance loss-control experts can assist with these
The inter-relationship of the
various motivation factors will influence the basic “performance objective” of
the automatic fire-protection system.
The three categories of performance are: fire control (fire
growth is stopped and heat damage is contained, but manual firefighter
intervention is required to complete extinguishment); fire suppression (fire
intensity is reduced by the automatic system, but manual extinguishment is
still required on a smaller scale); and fire extinguishment (complete automatic
suppression of a fire until burning is eliminated, without firefighter
Occupancy and Commodity
A fire-protection system design must also take into account the nature of
the facility being protected (“occupancy”) as well as the nature of the
combustible materials present (“commodity”).
For the purpose of sprinkler design, occupancies can be “light hazard”
(e.g., churches, kennels and nursing homes), “ordinary hazard” (e.g.,
restaurants, machine shops and plastics molding) or “extra hazard” (e.g., die
casting, solvent cleaning and oil quenching) spaces.
Fuel and Ventilation
The quantity of combustibles in a space and their location (e.g., high
elevations, stacked against walls, etc.) also play roles in determining
preferred fire-protection designs. For rack storage or other spaces with
obstructions that could impede the effective application of water from
ceiling-mounted sprinklers, spray nozzles at different elevations may be
necessary. The supply of fresh air into a space by natural and mechanical means
also impacts design choices.
Different extinguishing agents
are well suited for different occupancies and commodities. Liquid water is the
most common extinguishing agent, but steam, carbon dioxide, dry chemicals
(e.g., bicarbonate of soda), wet chemicals (e.g., aqueous solution of potassium
carbonate and/or potassium acetate) and other materials may be preferred for
Detector, Nozzle and Pipe
When water is selected as the
extinguishing agent, a number of nozzle styles are available (sprinkler, water
spray, water mist and steam). Choices also exist for piping and valve design
(e.g., wet pipe, dry pipe, deluge, circulating, preaction and antifreeze).
Detectors can be mechanical (fusible links) or electrical (temperature switch
or fused contacts).
solutions are often implemented for special hazards. The interior of processing
equipment may be incompatible with conventional sprinklers due to chemicals or
extreme temperatures and pressures. Such locations could be best served by
inert gas or other extinguishing agents. Ducts that convey combustible
particulate matter may require spark-detection and rapid extinguishing-agent
discharge to prevent a deflagration and explosion. Non-metal, product-conveying
ducts and plenums should be manufactured with fire-retardant material or
protected by sprinklers.
Obviously, the design of an effective fire-suppression system
involves numerous subsystems and many choices for the plant owner. In Part 2,
some specific fire-protection examples will be discussed along with reasons to
select or avoid many of these design choices.IH
Fire Water (Part 1)
August 31, 2011