Sprinkler Systems: Fighting Fire with Air and Water

The local adopted fire and building codes specify when sprinkler systems need to be installed. These codes do not specify the system design, type, or limitations. Most model codes send the user to National Fire Protection Association 13, the Standard for the Installation of Automatic Sprinkler Systems. The most current edition is the 2007 although most building and fire codes reference the 2002 edition.

The two most common types of sprinklers systems are wet and dry pipe systems. Both have advantages and special care must be taken when evaluating each of these systems. The main reason for utilizing a dry pipe system is when the sprinklers are installed in area subject to freezing (less then 40 degrees F). Examples may include refrigerated warehouses, exterior canopies, non heated storage facilities, or exposure systems.

NFPA 13 defines these systems as:

Wet Pipe Sprinkler System: A sprinkler system employing automatic sprinklers attached to a piping system containing water and connected to a water supply so that water discharges immediately from sprinklers opened by heat from a fire (NFPA 13 3.4.10).

Dry Pipe Sprinkler System: A sprinkler system employing automatic sprinklers that are attached to a piping system containing air or nitrogen under pressure, the release of which (as from the opening of a sprinkler) permits the water pressure to operate a valve known as a dry pipe valve, and the water then flows into the ping system and out the opened sprinklers (NFPA 13 3.4.5)

The main difference is the setup of the riser assembly. The wet pipe sprinkler system riser will typically have a system control valve, paddle type flow switch, and main drain. The dry pipe system will have a listed dry pipe valve which is designed to have an air supply which holds the valve in place to restrict the flow of water until a loss of air pressure. When a sprinkler opens and there is a loss of pressure, the valve opens allowing the water to flow freely in the system. The dry pipe riser will also have various trim attachments for the flow switch, connection to the air supply and possibly a quick opening device.

If you were to look at a typical big box warehouse, the bulk of the systems are wet pipe sprinkler systems. The exterior canopies or garden center areas would then be supplied by a dry pipe sprinkler system. In fact these dry systems are usually attached to the bulk main of the wet pipe system.

Another example can be the installation of a sprinkler system in a five-story hotel. Floors one through five are supplied by a wet pipe sprinkler system and then a dry pipe riser assembly is fed from the wet pipe system on the fifth floor which supplies the system in the attic.

The size of the dry pipe system is limited based on the volume of the system. If water can not be delivered in a timely fashion then the designer would be required to make multiple dry pipe systems. When installing a dry pipe system, the inspector's test connection shall be located on the most remote branch line. During the final acceptance testing water must be delivered to this connection in less than 60 seconds.

Some systems do not need to meet the 60 second requirement. These systems are less then 500 gallons, or under 750 gallons with a quick opening device. The sprinkler contractor shall include the calculated size of the system on the documents submitted for permit plan review. The 2007 edition of NFPA 13 added language which allows the designer to calculate the sprinkler water delivery with an approved computer calculation program.

Dry pipe systems have some other specific installation concepts. A snapshot at these items include:

  • The fire will continue to grow after the activation of a sprinkler in a dry pipe system. As the water travels the pipes in the sprinkler system the fire size grows. Therefore the designer should make the dry pipe system as small as possible.
  • Sprinklers can only be installed in the upright position unless specifically listed dry pipe sprinklers are utilized or horizontal sidewall sprinklers installed so that water is not trapped.
  • A dry pipe system is not intended to limit accidental damage from broken or open sprinklers.
  • Sprinkler systems that are dry can not be gridded.
  • The location of the dry pipe riser shall be heated to a minimum of 40 degrees F.
  • Piping is pitched back to the riser so that water is not trapped.

Since the fire continues to grow while the water is traveling through the pipes, NFPA 13 requires that the design area of dry pipe sprinkler systems to be increased. This 30 percent increase is required by section 11.2.3.2.5 of NFPA 13. This added percentage is to provide additional water flow since it is likely that the fire will be larger, compared to a wet pipe sprinkler system installation.

If we compare two various systems installed in an attic area of a building. The changes in the system are either wet or dry pipe based on the lowest temperature in the space. If the attic space is maintained at 40 degrees F or higher, a wet pipe sprinkler system can be utilized. The design area may be based on a calculated density of 0.10 gallons per minute (gpm) per square foot over a design area of 1,500 square feet. If the area was not maintained at 40 degrees F, then a dry pipe sprinkler system would need to be installed. The density of 0.10 gpm per square foot remains the same, but a 30 percent increase on the design area would be utilized and a required area of 1,950 square feet would be required.

Proper installation of a dry pipe sprinkler system is critical. Improper system installation can lead to freeze ups or inadequate water delivery. The designer and inspector must work together to ensure a proper system which meets the code and the needs of the owner. As a responding fire fighter you should understand that each system initially operates differently and what the alarms associated with each system are. Next time you respond on a fire alarm at one of those big box warehouses, take a look at the systems and see if you can tell the difference between wet and dry pipe systems.

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MICHAEL O'BRIAN, a Firehouse.com Contributing Editor, is a fire marshal for the Brighton Area Fire Authority in Michigan. He is the president of Code Savvy Consultants and is the creator of the dynamic webpage www.inspector911.com that is designed to assist all types of inspectors by providing resources, information, checklists and up to date news. To read Michael's complete biography and view his archived articles, click here. You can reach Michael by e mail at mobrian@inspector911.com..

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