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CONSTRUCTION KNOWLEDGE  >> MECHANICAL >>

SPRINKLERS
 

1. How Do Sprinkler Heads Work?
2. What Should I Know About Sprinkler Systems?
3. What Piping Configurations are Common?
4. What are the Basics of Sprinkler System Design?
5. What Public Domain Documents are Available for Further Study?
6. Tricks of the Trade & Rules of Thumb for Sprinkler Basics:

How Do Sprinkler Heads Work?

Did you ever notice that a common gag for TV sitcoms is to have the automatic fire protection sprinklers go off? It's always the same, some smoke happens and all these sprinkler heads soak the hapless suckers standing there. Since almost none of us ever experience a sprinkler head discharge, the TV show experience seems true to many people.  Please understand that it's completely false.

Smoke sets off smoke alarms, not sprinkler heads. Only heat makes sprinkler heads flow water. A typical sprinkler head has a thermal fuse of 174 °F that must melt to release water flow. The head next to it won't go off unless it too melts. So intense heat sets off sprinkler heads, and only the actual heads that experience the intense heat.

Sprinkler heads have either a glass bulb heat sensitive mechanism or a metal fusible link. With this type of activation, only the sprinkler heads directly above the fire tend to flow water. Therefore, the maximum amount of water douses the hottest fire location.

Different temperature sprinkler heads are used for various situations. The table below shows common options.

You can find much more information on sprinkler heads at Tyco Fire and Building Products or at Reliable Sprinkler.
 

What Should I Know About Sprinkler Systems?

A wet sprinkler system has a series of water distribution pipes and sprinkler heads throughout every space in a facility. The sprinkler heads are typically located between 10' and 15' on center in both directions. The pipes are full of water, so if heat from a fire raises the temperature, the fusible link or glass bulb will break and allow water to flow onto the fire. The amount of water may be in the 15 to 40 gallons per minute range, which is much less than a fire hose that may flow 250 to 1000 gallons per minute. In theory, the sprinkler system will control the fire with much less water damage to the facility.

Generally systems have control valves for maintenance and repairs that must be monitored with anti-tamper switches. Flow controls also are typically monitored so any flow in the system, especially during non-occupied times, gets reported quickly. If you want a much more detailed explanation of sprinkler systems, go to the Stanford University Intro to Automatic Fire Protection Systems.

While sprinklers aren't required in all buildings, the current US building codes (IBC and UBC) provide many design advantages for buildings that include sprinklers. Egress requirements, building construction type, fire separation requirements and many more design items have relaxed requirements in sprinklered buildings. Hence many Design Professionals specify sprinklers and consider it an overall cost savings.

Where water may freeze in the pipes, a dry sprinkler system can be used. An air compressor must provide air at a pressure higher than the water pressure in the sprinkler system so the dry system remains full of compressed air. When a sprinkler head activates, the air rushes out and the water soon (less than one minute) follows. Dry systems have several complications not found with wet systems, so only get used where pipe freezing is a concern.

Deluge systems don't use sprinkler heads to control the water flow. The piping is open at the point of water discharge and the water flow is controlled by a valve connected to the fire alarm system. A deluge system will have water flowing from all discharge points simultaneously, as soon as the fire alarm calls for flow. Only in special occupancies in which rapid fire spread is a major concern do deluge systems get installed.

Pre-action sprinkler systems provide another layer of safety from accidental sprinkler discharge. You can imagine a museum or a library could sustain tremendous losses from water damage, so they want to assure that sprinkler heads don't discharge by mistake. Though system types vary, generally an action (the flowing of water at a head location) must be proceeded by a pre-action (a smoke or heat alarm confirming that a fire is likely in progress). So a Pre-Action sprinkler system has a double check prior to water flowing.

What Piping Configurations are Common?

The sketch below shows various common piping configurations.

The other component of the sprinkler piping is the control valving. While requirements vary based on local rules, the following US Department of Defense sketch shows a typical layout.

What are the Basics of Sprinkler System Design?

It's a good idea to understand sprinkler system design (the basics aren't too complicated). The National Fire Protection Association (NFPA) sets the design standards in the publication NFPA-13. The following few steps show the process.

1. Determine the level of fire hazard for the building.
   1. Light Hazard (offices, churches, hospitals, nursing homes, gyms, schools, theaters, residential, etc.)
   2. Ordinary Hazard Group 1 (bakeries, dairy plants, electronic plants, glass manufacturing, parking garages, etc.)
   3. Ordinary Hazard Group 2 (chemical plants, mercantile, paper mills, repair garages, woodworking shops, etc.)
   4. Extra Hazard Group 1 (aircraft hangars, plywood manufacturing, printing, textile plants, etc.)
   5. Extra Hazard Group 2 (asphalt saturating, flammable liquid spraying, manufactured home plants, plastics processing, etc.)
2. Find the Design Area and Density
   1. Design Area = the worst case area in a building where a fire could burn
   2. Design Density = the gallons per minute of water per square foot that should flow onto the Design Area
3. Sprinkler Head locations and piping design

An example always helps. An office building has a light hazard classification, the Design Area is the most remote 1,500 sf and the Design Density is 0.1 gallons per minute/sf. Therefore this system requires 1,500 sf x 0.1 gpm/sf = 150 gallons per minute to be discharged over that most remote 1,500 sf area. If we look instead to a manufacturing facility, the Design Density changes to 0.2 gpm/sf. Therefore, we'd need 300 gpm to be spray over the most remote 1,500 sf area.

The table below from the US Department of Defense provides some of the design basics. To really understand the design work, you need a copy of NFPA-13, but this table shows the basic concept.

What Public Domain Documents are Available for Further Study?

The Dept of Defense has created a manual for Fire Protection Engineering for Facilities which is an excellent introduction to sprinkler systems. This 129 page handbook is officially called UFC 3-600-01 (September 2006).

Tricks of the Trade & Rules of Thumb for Sprinkler Basics:

1. Sprinkler heads act independently, only discharging water when the temperature at that head exceeds the allowable range. 
2. A sprinkler head flows about 40 gpm, while a fire hose flows in excess of 250 gpm.
3. Pre-Action sprinkler systems provide a double check prior to water flowing.