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Key Differences in Sprinkler Systems
This article is an attempt to briefly cover the most common differences between types of sprinkler systems. Most sprinkler systems fall within one of four categories. These four broad categories are:
• Wet pipe systems
• Dry pipe systems
• Preaction systems
• Deluge Systems
The type of sprinkler system (except deluge) typically depends on the environmental conditions in which the system will be used and the nature of the space being protected.
Wet-Pipe Sprinkler System
This type of system is the most common and is typically the easiest to design, install, and maintain. Wet-pipe systems contain water under pressure in the overhead piping at all times and utilize a series of closed sprinklers.
When a fire occurs and produces a sufficient amount of heat to activate one or more sprinkler heads, water immediately discharges from the affected sprinkler(s). Wet-pipe systems should always be the first choice because they are more inherently reliable and less costly to maintain. However, wet-pipe systems should never be considered for installation when temperatures to which the system could be exposed fall below 40 degrees F.
Dry-Pipe Systems
These systems should be used only when the piping network will be exposed to temperatures below 40 degrees F. The sprinkler system piping does not contain any water. It is charged with air (and occasionally nitrogen) under pressure.
These systems use a dry-pipe valve that holds back the water supply and serves as the water/air interface. Most listed/approved dry-pipe valves act on a pressure differential principle, in which the surface area of the valve face on the airside is greater than the surface area on the waterside.
When a fire occurs and a sufficient amount of heat is generated, one or more sprinklers operate, causing the air in the piping to escape, causing the system air pressure to drop. Once the air pressure falls below a predetermined level, the dry-pipe valve opens, allowing water to flow through the system to the open sprinkler(s).
Dry-pipe systems are a bit more complex than wet-pipe systems. They require a reliable air supply source and, because of the delay associated with water delivery from the dry-pipe valve to the open sprinklers, are subject to certain design limitations.
These limitations can include restriction of system size; the need for additional components, such as accelerators and exhausters; and adjustments to the number of anticipated operating sprinklers.
In addition to serving as stand-alone systems, dry-pipe systems can comprise a subsystem of a larger wet-pipe system in which small coolers/walk-in refrigerators and loading docks can be protected.
Preaction Systems
Similar to a dry-pipe system, a preaction system uses piping charged with air (or occasionally nitrogen) under pressure. Typically, the air pressure associated with preaction systems is generally less than that for dry-pipe systems.
There are three variations of preaction systems:
• Single interlocked preaction system
• Double interlocked preaction system
• Non-interlocked preaction system
Single Interlocked Preaction System. For these types of preaction systems, the water supply is held back by a “preaction valve.” The area being protected by the preaction system also has a supplemental fire detection system installed, as well.
Operation of the fire detection system allows the preaction valve to automatically open and admit water into the pipe network. Water will not discharge from the system until the fire has generated a sufficient quantity of heat to cause operation of one or more sprinklers. In essence, the system appears as a wet-pipe system once the preaction valve operates.
Air pressure maintained in the piping is also used to monitor the integrity of the pipe. If the piping system develops a leak, the air pressure will drop and an alarm will sound, indicating a low air pressure condition. Because water is held back with a preaction valve rather than a dry-pipe valve, water will not flow into the system until the supplemental fire detection system operates, signaling the preaction valve to open.
The preaction valve stays in its normal position until the detection system is activated.
Double Interlocked Preaction System. The double interlocked system has characteristics of both the single-interlocked system and the dry-pipe system and typically consists of a dry pipe valve mounted on top of a pre-action valve. For water to enter the system piping, both the supplemental detection system and the sprinklers on the system must operate.
Non-Interlocked Preaction System. Another variation of a preaction system is the non-interlocked system, in which either activation of the supplemental detection system or the opening of a single sprinkler initiates water flow through the system.
Preaction systems are typically found in spaces containing computer or communications equipment, museums, and other facilities where inadvertent water damage from system piping is of major concern. The double-interlocked system is most common in freezer facilities where accidental valve operation can result in the immediate freezing and damage of system piping.
Deluge Systems
Deluge systems, as the name implies, deliver large quantities of water over specified areas in a relatively short period of time. These systems are used to protect against rapidly growing and spreading fires. Typically, sprinklers used in a deluge system do not contain thermally sensitive operating elements, and as a result are referred to as open sprinklers.
A deluge valve controls the system water supply and is activated by a supplemental fire detection system. Because open sprinklers are employed, system piping is at atmospheric pressure. As water reaches each sprinkler in the system, it is immediately discharged from the system. The nature of this system makes it appropriate for facilities in which significant amounts of highly combustible materials are present. The system is also used for situations in which thermal damage is likely to occur in a relatively short period of time.
Aircraft hangars are one area of application of deluge systems. Additionally, these are used for exterior protection of high-value equipment or equipment crucial to the continuity of operations (business interruption), such as high-value or long-lead-time-to-replace equipment. This could be combustible fill cooling towers, exteriorly installed electrical transformers, or large-scale cooking/deep fat frying operations (but not typical restaurant operations).
Other Systems
Several variations to each one of these four basic systems exist. One such system is the antifreeze system that is essentially a wetpipe system with a certain amount of antifreeze concentrate added to provide a degree of protection against freezing. Because of the cost associated with these antifreeze systems, they are typically used to protect smaller spaces exposed to cold temperatures, such as outside loading docks or exterior canopies. NFPA 13 specifies which types of antifreeze can be used and the percentage of concentrate needed.
Other types of systems could be exterior (exposure protection) systems, which would typically be a type of deluge system, and a system to protect unrated glass in an occupancy in which typically a fire-rated barrier would be required.
Today, many changes and enhancements to automatic fire sprinkler systems are coming forth at a rapid rate. Design methodologies are all hydraulic calculation oriented and are specifically tailored to protect against hazards. For further assistance and guidance regarding automatic fire sprinkler protection, contact a property insurance carrier, a local fire marshal, or a local sprinkler contractor.
This article originally appeared in the December 2008 issue of Occupational Health & Safety.
About the Author
Alan P. McCartney, is the corporate Property Technical Director for Liberty Mutual Agency Markets, which is the corporate business unit for Liberty Mutual Group’s independent agent derived business. He has been with Liberty Mutual (and previously Wausau Insurance’s Property/HPR Engineering Group) for more than 10 years. In addition to his career in the insurance industry, he has been Fire Safety Coordinator (& Co-Chair Disaster Planning) for Brigham & Women’s Hospital in Boston, Mass.; Fire Marshal for the Concord (NH) Fire Department; and Fire/Safety Analyst for Phillips 66 Chemical Company (now Chevron Phillips Chemical Co) in Borger, Texas. He is a 1980 graduate of the School of Fire Protection and Safety at Oklahoma State University and has associate degrees in fire protection from NH Technical College, Laconia, N.H., and Tarrant County Junior College in Fort Worth, Texas. He is an ASSE Professional Member (Fire Protection Practice Specialty) and also a member of the Society of Fire Protection Engineers, National Fire Protection Association, Association of Professional Industrial Hygienists, International Code Congress, and NH Fire Protection Society.