A variety of examples of building occupancies are classified in Sec. A.5.2 of Appendix NFPA 13. The following are designated as light hazard occupations: offices, churches, schools, museums, auditoriums, library seating areas, unused restaurant and attic seating. The maximum sprinkler spacing (area of protection) for these is listed in Table 8.6.2.2.1 (a) if standard sprinklers are used. Generally, the maximum limit is 225 square feet for minor hazards in a hydraulically calculated system. However, when the exposed construction is combustible, with structural members spaced less than 3 feet apart. Additionally, the maximum coverage limit is reduced to 130 square feet.
Group 1 ordinary risk occupations include laundries, restaurant service areas, and car parks. Group 2 ordinary hazard occupations include the aforementioned dry cleaners, auto repair and service areas, auditorium stages, woodworking plants, post offices, and library stacking areas. Standard sprinklers protecting all ordinary hazard occupations must not cover in excess of 130 square feet per head (Table 8.6.2.2.1 (b).
Examples of additional risk occupancy include printing plants, paint and varnish dipping operations, plywood fabrication, solvent cleaning, and plastics processing. The maximum sprinkler space for these occupations is limited to 100 square feet. However, when the required design density is less than 0.25 gpm / sf (and this also applies to high-stack storage), a protection area of up to 130 square feet per sprinkler is allowed (Table 8.6.2.2. 1 C). It should be noted that commercial insurance companies and consultants develop their own literature that contains more extensive listings of examples and occupancy classifications than NFPA 13, data that is often helpful when making an occupancy classification determination.
Design density criteria
The density / area curves for NFPA 13 are found in Fig. 11.2.3.1.1. When calculating a light hazard sprinkler system hydraulically, the design density used is typically 0.10 gpm / sf over a 1500 square foot operating area (the most hydraulically demanding). To begin a calculation, the designer starts with the end sprinkler and works “backwards” to the water supply source. Assume the sprinklers are spaced 14 feet apart. apart on 12 foot branches. Besides. So our square foot coverage is (12 x 14) 168 square feet.
Q (in gpm) is determined by multiplying density times coverage in square feet (.10 x 168), so we know that we will need 16.8 gallons per minute (Q) to discharge from the final sprayer.
The square root of the final head pressure required is determined by “Q” divided by “K”. If the design density is 0.10 and the sprinkler head K-factor is 5.5, we can determine our head pressure by dividing 16.8 by 5.5 and squaring the sum to get a figure of 9.33 psi. 9.33 psi is the final head pressure required. To check, we can simply plug in the numbers while doing the following equations to make sure they match: Q = K multiplied by the square root of the pressure, K = Q divided by the square root of the pressure, and the design density equals Q divided by the square footage coverage. If our area of operation remains 1500 square feet, our design density will change to 0.15 for Group 1 ordinary risk destinations and 0.20 for Group 2 ordinary risk destinations.
Everything changes when you use extended coverage sprinklers. Suppose we decide to extend our coverage to 324 square feet in a light hazard office, spacing the sprinklers 18 ‘x 18’. Now we need to refer to the sprinkler manufacturer’s data sheets for the address. If we choose to install Tyco EC-11 pendent sprinklers, the data sheets dictate that our final sprinkler must discharge a minimum of 33 gpm at 8.7 psi. This means that our design density (Q divided by square foot coverage) is still 0.10 gpm / sf. The K factor for this particular sprinkler is 11.2, which we can validate using the equation K = Q divided by the square root of the pressure.
Extended coverage sprinklers for ordinary hazard destinations work the same way. For example, we could use the Tyco EC-14 Extended Coverage Pendent Sprinkler (K = 14.0) in a restaurant service area (Ordinary Hazard Group 1) to protect an 18 ‘x 18’ area, but here the parameters of the datasheet require a minimum discharge of 49 gpm at 12.3 psi for the final sprayer. In other words, Q = 49, K = 14.0, the square root of the pressure is 3.51, and the coverage is 324 square feet. All equations match, including the required design density (0.15) which is obtained by dividing Q by 324 sq. feet. Of course, the local water supply must still be able to meet the overall demand resulting from the sprinkler system. To achieve this, larger system pipes are installed to supply the additional gpm required by the extended coverage heads.
The sprinkler discharge characteristics are convincingly described in Table 6.2.3.1, which describes the different K-factors for sprinkler identification. Another useful reference table for sprinklers in NFPA 13 is Table 6.2.5.1, which deals with temperature ratings and ratings.
To be absolutely certain of code compliance regarding sprinkler elevations, we refer to Sec. 8.6.4.1 in NFPA 13. Allowable distances noted under ceilings, rafters, or ceilings are always measured to the sprinkler deflector. It is acceptable for designers to refer to the data sheets for the proper clearances under ceilings for specific types of sprinklers, although the safe bet is to ask for a clearance between 1 “and 12” below the bottom of the roof deck. The closer the sprinklers are to the ceiling, the faster they will work. But caution must be exercised as severe interference to lateral water distribution can often occur due to sprinkler placement too close to the ceiling. In all cases, the 1-inch minimum (in code) is to allow for the installation and removal of vertical sprinklers. When sprinklers are installed below sloped ceilings, the highest sprinkler deflector (Sec. 8.6.4.1.3.1) may extend 3 feet. from the highest peak.
