While NFPA 13 is a prescriptive standard, it still requires the design team to make some critical decisions. This includes selecting the appropriate system type for the hazard present.
Many people believe NFPA 13 dictates what type of system is required for the various occupancies and hazards that may appear in a building. The reality is NFPA 13 uses a “menu-style” by giving the designer several options that he or she can choose from. The designer needs to select the one best fit for not only the hazard, but also the needs of the owner or operator of the facility.
Whether the decision is the design professional’s (specifying engineer) or the sprinkler contractor’s engineering technician, selecting the type of system or systems that will be protecting the building is a critical step. NFPA 13 allows the designer to select from any of the systems outlined in Chapter 7:
In order for the designer to figure out which of these systems is the best fit, they need to first understand what the different types of sprinkler systems are and how they function.
The most common sprinkler system installation is a wet system. In a wet system all the piping is filled with water. This allows for the immediate delivery of water to the fire once the sprinkler activates. The only major drawback to wet systems is they have the potential to freeze when subjected to temperatures below 32° F.
To combat the potential for freezing, NFPA 13 allows dry-pipe sprinkler systems to be installed. A dry sprinkler system consists of automatic sprinklers that are attached to a piping network containing air or nitrogen under pressure. When a sprinkler in a dry system activates, the air or nitrogen is expelled from the system. When the gas is expelled from the system, a dry valve (see Figure 1), located in a heated portion of the building, recognizes the change in pressure and allows water to flow to the open sprinkler(s). This type of system will keep the piping from freezing up but there can be a considerable delay in getting water to the fire.
In situations where it is important to get large volumes of water to a fire or the fuel packages adjacent to a fire very quickly, a deluge system may be needed. The sprinklers in a deluge system are “open,” meaning they do not have bulbs or fusible links. This means when a fire occurs and the system activates, all the sprinklers in the system will discharge water, not just the ones that reached their activation temperature.
The deluge system is supervised by a detection system that upon activation releases water to all the open sprinklers in the area. This can result in hundreds if not thousands of gallons of water being discharged every minute. These systems, such as the one shown in Figure 2, typically are found in industrial applications or other areas where the discharge of large volumes of water is an acceptable condition.
The final type of system addressed by NFPA 13 is the preaction system, which is used where there is a significant concern surrounding accidentally filling pipes with water and/or accidentally discharging water in a room or space. Preaction systems require not only a sprinkler to activate to discharge water, but also a detection system. If a sprinkler were to be accidentally damaged by being hit with a ladder or forklift, no water would be discharged unless the detection system signaled the preaction valve to let water into the piping network. This redundancy adds a level of safety against accidentally discharging water on valuable building contents.
Once the designer has established an understanding of the different types of systems, they must consider the specifics of the project that they are working on to determine what system makes the most sense. The factors that the designer should consider when determining the most appropriate system type are:
Type of fire expected;
Contents being protected; and
While it is impossible to exactly know what type of fire will occur in any building, the designer should be aware of the various fuel packages present in the building and how the building will be used. NFPA 13 requires the owner to provide this information to the designer in the form of an “owner’s certificate” (see info box below) before they begin their design. This requirement is established in section 4.3 of NFPA 13 and is a critical first step for the designer:
The critical part of the owner’s certificate is the requirement for the owner to identify the “intended use of the building including the materials.” Many designers feel as if they can ascertain this information from design drawings, however these drawings may not tell the whole story. Assuming an understanding of how a building functions or worse yet what fuel packages might be in it can result in selecting a sub-optimal system that could expose the designer to liability down the road.
For most building uses and fuel sources, wet systems will be the starting point for the system designer. There are some types of fuel, however, that create severe hazards such as flammable liquids storage or chemicals used in industrial process where fire originating in these spaces may necessitate immediate water delivery over a large area. In other cases the types of fuel present may require a system that contemplates pre-wetting certain fuel sources to provide a form of exposure protection (tank farms, aircraft hangars, etc.). In these instances a wet system might be a permitted option by NFPA 13 but wouldn’t be the best choice for the owner.
|4.3* Owner’s Certificate|
|The owner(s) of a building or structure where the fire sprinkler system is going to be installed or their authorized agent shall provide the sprinkler system installer with the following information prior to the layout and detailing of the fire sprinkler system [see Figure A.23.1(b)]:|
|(1) Intended use of the building including the materials within the building and the maximum height of any storage;|
|(2) A preliminary plan of the building or structure along with the design concepts necessary to perform the layout and detail for the fire sprinkler system;|
|(3) *Any special knowledge of the water supply, including known environmental conditions that might be responsible for corrosion, including microbiologically influenced corrosion (MIC).|
Freeze protection is another factor to consider when selecting the system type. In spaces where the temperature cannot be maintained above 40°, a wet system would not be permitted by NFPA 13. In these instances a dry or preaction system would be the best approach. Areas such as loading docks and spaces exterior to the building that require sprinkler protection will typically be protected by a dry system. Spaces that are intentionally maintained below 40°, such as cold storage freezer spaces, often will be protected by a double-interlock preaction system. This is done so the piping in the freezer is not filled with water until a sprinkler and the detection system activates. It is difficult to drain water from these systems and the piping will freeze up fairly quickly in these environments. In addition to the concern of accidental discharge, accidentally filling the pipes with water also is a major concern that can be avoided with the double-interlock preaction system.
The contents of a building also may help dictate which system is the best fit. Some occupancies or some building contents are very sensitive to water, so eliminating the chance of accidental water damage is a high priority. These spaces include server rooms, museums, document/archival storage, storage freezers and historic structures. If these spaces are identified on the owner’s certificate, this should prompt a discussion between the designer and the owner (and possibly the insurance representative) over the needs for the system.
Finally, system cost is a major factor when looking at which system to install. In addition to wet pipe systems being the most reliable and cost-effective system to install, they also are the most cost-effective to maintain.
Dry systems are more expensive than wet to install and much more expensive to maintain. Preaction and deluge systems are the most expensive primarily due to the added cost of installation of a detection system. When dry and preaction system piping becomes filled with water, either due to a fire or accidental activation, it is imperative all the water is eliminated from the piping.
If all the water is not removed, the chances for corrosion dramatically increase and the need to replace piping or other system components quickly can become expensive. When thinking about the cost of the system, the designer should consider not only the upfront costs but also the long-term cost of maintaining the system.
NFPA 13 provides the designer with several options when it comes to protecting a building and its contents. It is important for the designer to understand what the options are, how the different systems function and which one would be the best fit based on the specific information about the building and the owner’s needs.
Keep in mind that many buildings require multiple system types to be employed to achieve the necessary level of protection.