One puff of smoke is all it takes to trigger the sprinklers in the home of “The Incredibles.” Shattering the glass of a fire alarm soaks students in the hallways of North Shore High School in “Mean Girls.” Ben Affleck lights a piece of paper on fire and somehow activates every sprinkler in the building in “Changing Lanes.”

None of these movie examples of how fire sprinkler systems work is accurate, which is why safety professionals need to develop their knowledge of them.

Fire protection engineer Chris Butts, P.E., PMSFPE, AET, SET, CFPS, ARM, property risk control specialist with Sompo International, addressed several of these myths and provided a thorough review of fire sprinkler systems in a webinar for our Fire Protection Practice Specialty.

In that discussion, he highlighted 10 things safety professionals should know about fire sprinkler systems.

1. Use NFPA 13 as a Guide

NFPA’s standard provides information on the design, installation and testing requirements for fire sprinkler systems, but it does not tell you when fire sprinkler systems are required. The organization offers more guidance in NFPA 1: Fire Code, but most requirements are implemented by local jurisdictions and municipalities, so be sure to check your local fire codes, Butts says.

2. Know the Basic Requirements of a Fire Sprinkler System

Each fire sprinkler system has basic requirements:

• All system materials and devices essential to successful system operation must be listed by an organization such as Underwriters Laboratories (UL) or Factory Mutual (FM).
• Sprinklers are required in all spaces and areas of a building unless permitted by an exception in NFPA 13.
• Sprinklers must be positioned and located to provide “satisfactory performance” with respect to activation time and water distribution.
• Clearance between the sprinkler deflector and the top of storage or contents of the room must be 18 inches or greater.

3. Sprinkler System Requirements Are Based on Hazards and Occupancy

Each building is categorized based on hazards and occupancy, which dictates the kinds of sprinklers and systems required. These categories include “light hazard,” two groupings of “ordinary hazard,” two groupings of “extra hazard” and two groupings of “storage.”

Hazards include content quantity and combustibility, heat release rate and stockpile height. For example, a standard office environment is usually considered “light hazard” while a dry cleaner may be “ordinary hazard” and an aircraft hangar may be “extra hazard.”

4. The K-Factor Is Key

A sprinkler’s K-factor is a number expressing its discharge coefficient. It relates to the amount of water permitted through the sprinkler. The larger the K-factor, the larger the hole, the larger the sprinkler and the more water will flow, Butts says. The most common K-factor is 5.6.

The K-factor, along with flow (in gallons per minute) and pressure (in pounds per square inch) is part of an important equation used to determine hydraulic demand. It looks like this:

K = Q/ √P

Fire sprinkler system designers use discharge density requirements within a specified area of sprinkler operation to calculate the flow and pressure at any given K-factor. Several charts inside NFPA 13 illustrate this relationship.

Each K-factor also has an associated thread size and orifice size, which are other important sprinkler characteristics.

5. Sprinklers Have Two Temperature Characteristics, and One Is Color Coded

Besides discharge characteristics, sprinklers have temperature characteristics:

• Thermal sensitivity: This is a measure of how quickly a sprinkler will operate and is best measured by the response time index (RTI). An RTI of 40 meters-second^1/2 or less is considered “fast response,” while 80 meters-second^1/2 is considered “standard.” Other variables such as ambient temperature, ceiling height and spacing between sprinklers will impact operation time in true fire conditions.
• Temperature rating: Temperature ratings depend on the ambient temperature of the space. For example, in an office, where temperatures at the ceiling are maintained at 100 degrees or less, sprinklers can have a temperature rating of 155 to 165 degrees and the bulb will be colored orange or red. As ceiling temperature increases, the bulb colors change. If it’s an older system without bulbs, another part will carry the color.

6. Each Sprinkler Has an Identification Number and Data Sheet

The sprinkler identification number (SIN) — printed on the deflector of all sprinklers manufactured after 2000 — can be used to identify these sprinkler characteristics. It includes one or two uppercase letters identifying the manufacturer followed by three or four numbers to identify the K-factor, deflector characteristic, pressure rating and thermal sensitivity.

Sprinklers also have safety or product data sheets that show sprinkler materials — such as stainless steel — and finishes — such as wax, polyester or Teflon.

Any materials, coatings and paints on sprinklers can only be applied and provided by the sprinkler manufacturer and stipulated at the time of ordering, Butts says.

7. There Are Sprinklers for Controlling More Challenging Fires

Sprinklers have unique design requirements that are appropriate for different applications based primarily on the hazards present in the area they protect.

• Control mode density area (CMDA): These standard sprinklers control fires until first responders can arrive. They are most often used in non-storage buildings such as hotels, manufacturing facilities and offices, but can have some storage applications less than 12 feet in height, Butts says.
• Control mode specific application (CMSA): These sprinklers can produce characteristically large water droplets that can penetrate deeper into a fire. They provide better fire control for high-challenge fires.
• Early suppression fast response (ESRF): These sprinklers provide the highest performance and are used for very specific high-challenge fires.

8. There Are Different Types of Fire Sprinkler Systems

• Wet Pipe: This simple, reliable system is the most common type because water is readily available, inexpensive and effective. Pipes are continuously filled with water, so water discharges immediately and automatically if there is a fire.
• Dry Pipe: This system is used in areas where temperatures cannot be reliably maintained at 40 degrees or more, such as a walk-in freezer or cold storage. Gas or nitrogen sits in the pipe until activated, so there is a delay in discharge.
• Pre-Action: This system is used in places where unintended activation could be devastating, such as a data center or an art museum. Instead of having water continuously fill the pipes, a smoke or heat detector sends a signal to prefill a closed sprinkler system prior to standard sprinkler activation. This keeps the pipes clear of water but allows them to pre-fill in case of an actual fire, so there is less delay than a dry pipe system.
• Deluge: This is the only open system and is typically used in high-hazard areas like a power plant or a transformer outside a building. When this system is activated, water comes out of every sprinkler.

9. Each Fire Sprinkler System Can Be Configured in Different Ways

System design configurations are based on several factors such as water supply pressure, building geometry, the size of the building, construction elements, present hazards and associated system demands, Butts says. Configurations include:

• Tree: Branch lines separate off a main “trunk” line that has a riser assembly, defined by NFPA 13 as “the vertical supply pipes in a sprinkler system” — basically where the regular plumbing connects to the fire sprinkler system.
• Loop: Branch lines separate off a looped main line, which also has a single riser assembly.
• Grid: Branch lines connect to two mains, the “near main” connected to the riser and, on the opposite side of that, the “far main.”

Because there are area limitations to fire sprinkler systems, there may be different configurations in the same building.

10. Fire Sprinkler Systems Leave Clues About Their Design

To quickly get your bearings when assessing a fire sprinkler system you don’t know much about, look for the spare sprinkler cabinet or a hydraulic nameplate located near the sprinkler riser assembly.

A spare sprinkler head cabinet will contain extra sprinklers of each type in the system. Butts says these spare sprinklers “will tell you a lot about what kind of system you have and its design criteria.”

A hydraulic nameplate or placard will also identify the system design criteria — although some are so worn they may be hard to read, Butts says.

Another characteristic to note: Most modern sprinkler systems are activated by shattering a bulb at certain temperatures, while older ones are activated through melted metal fusible link. So, if you do not have a bulb, you’re likely dealing with an older system.

Fire sprinkler systems have many elements, but these 10 facts are some of the most important for safety professionals.

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