Sprinkler Systems: Fighting Fire with Air and Water

You can find both systems in one structure in some buildings across the country.

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:

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