Fire Water (Part 1)
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 re-evaluation.
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 evaluations.
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 intervention).
Occupancy and Commodity Classes
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 some hazards.
Detector, Nozzle and Pipe Variables
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).
Special fire-protection 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