Wet pipe sprinkler system

A wet pipe sprinkler system is an automatic sprinkler system in which the pipes are filled with water under pressure at all times. When the heat of a fire opens a sprinkler, water flows from it at once and keeps flowing until the system is shut off. Only the sprinklers that are heated enough to operate discharge water; the others stay closed. The wet pipe system is by far the most common of the four basic sprinkler system types, which also include dry pipe, deluge and preaction systems.

Wet pipe systems are described as the oldest and most reliable type of sprinkler system. They are simple, have few mechanical parts and need no human action to operate. Most are designed to control a fire rather than to extinguish it: they limit its growth and keep it within the room of origin until the fire service completes extinguishment. Their main limitation is that the water in the pipes must not freeze, so the protected spaces must be heated. For residential and general buildings, US statistics show that the systems operate in about 90 to 95 % of fires large enough to activate them, and are effective in 96 to 97 % of those cases.

How the system works

A sprinkler system consists of a water supply, a network of distribution pipes and individual sprinklers, supported by control valves, pumps and water flow alarms. In a wet pipe system, the whole network from the supply to the sprinklers is permanently charged with water. An alarm check valve is fitted where the water enters from the supply. It has connections for local and remote alarms, and it acts as a non-return valve that lets water flow only towards the sprinklers.

A wet pipe sprinkler system. All pipes are permanently filled with water under pressure; only the sprinkler heated by the fire opens. Based on Iqbal and Salley (2004), p. 10-1 and Hopkins et al. (2005), pp. 13 to 14.

Each sprinkler head is held closed by a heat-sensitive element. The most common elements are a soldered metal link, which melts at a set temperature, and a glass bulb filled with liquid, which expands and shatters the bulb. When the element releases, water flows out of the orifice and strikes a deflector, which breaks it into a spray over the burning material. A sprinkler therefore acts both as a heat detector and as a nozzle that distributes water. The spray sprinkler, introduced in the early 1950s, was a major step forward because it spread the water much more evenly than earlier designs.

Operation is local. When a fire grows large enough, a sprinkler close to it opens and discharges water at a set rate. If the fire keeps growing, further sprinklers open, until the fire is controlled or extinguished or the water supply runs out. Water cools the burning materials as it evaporates. Sprinklers are designed to control a fire, but they often extinguish it, because the surrounding materials can no longer heat up to their ignition temperature. One gallon of water, about 3.8 litres, absorbs 2,586.5 kJ when it is heated from 21 °C and turned into steam at 100 °C.

Most wet pipe systems in offices are control-mode systems. Their aim is to limit the heat release rate and keep the fire in the room or area of origin. Full extinguishment may need the fire service. A separate class of sprinklers is designed for early response and extinguishment, for example in storage buildings and dwellings. In general, an active extinguishing system intervenes while the fire is still growing, so the fire does not become fully developed.

Comparison with other system types

All sprinkler systems, whatever their purpose, can be placed in one of four basic types. These types differ in the most basic aspect: how water reaches the area of the fire. In a wet pipe system, water is discharged as soon as the first sprinkler opens.

The four basic sprinkler system types: wet pipe, dry pipe, deluge and preaction. Based on Fleming (2002), pp. 4-72 to 4-73.

A dry pipe sprinkler system contains compressed air or nitrogen instead of water. A dry pipe valve holds the water back until an open sprinkler releases the air pressure. Such systems are used where water in the pipes could freeze, for example at loading docks, in cold stores and in attics. The air must first escape and the water must then travel through the pipes, so the discharge is delayed. In the United States, NFPA 13 limits this delay to 60 seconds and increases the design area of dry systems by 30 % above that of wet systems. Alternating wet and dry periods also cause corrosion inside the pipes.

Deluge systems use open sprinklers. A separate detection system opens a deluge valve, after which water flows from all sprinklers at once; they are used for rapidly spreading, high hazard fires. Preaction systems use closed sprinklers, but water enters the pipes only after a detection system has opened the valve. They are used where accidental discharge is a particular concern, such as in computer rooms. A preaction system is more expensive and complicated than a wet pipe system. Where some freezing risk exists, a wet pipe system can be filled with an antifreeze solution, which is discharged first and is followed by water.

The World Trade Center towers illustrate the usual pattern in office buildings. The office and support spaces were protected by wet pipe systems. Dry pipe and preaction systems covered less than 10 % of the floor area, mainly in computer rooms and other special hazard areas.

Design

In the United States and Canada, sprinkler systems are designed and installed to NFPA 13, which most building codes reference. In Australia, the effectiveness values of the national verification method assume wet pipe systems built and maintained to AS 2118 and AS 1851. Fleming notes that calculation methods are well developed only for the flow of water through the pipes. The ability of a spray to suppress a fire cannot yet be calculated, so the fire suppression side of sprinkler design is empirical.

Design starts with the hazard classification of the occupancy, which Fleming calls the most critical step. If the hazard is underestimated, the fire may overpower the sprinklers. NFPA 13 uses the classes light hazard, ordinary hazard groups 1 and 2, and extra hazard groups 1 and 2. Offices are light hazard, laundries ordinary hazard group 1 and flammable liquid spray booths extra hazard group 2. The class sets a minimum water density, a flow per unit area, over a design area in which the sprinklers are expected to open.

The designer then shows by a hydraulic calculation that the pipes deliver this density to every sprinkler in the hydraulically most remote design area. In one example from Fleming, a density of 6.1 mm/min over 221 m² is chosen. With each sprinkler covering 16.72 m², the design area contains 14 sprinklers. Before the 1970s, systems were sized from pipe schedule tables; area and density curves then made hydraulic calculation possible and allowed more economical systems on strong water supplies.

Flow from a sprinkler

The flow through a sprinkler depends on the water pressure at its orifice. All properties of the orifice are lumped into one experimentally determined constant, the K-factor:

$$Q=K\sqrt{P}$$

Symbols

Symbol	Description	Unit
$Q$	flow rate from the sprinkler	L/min
$K$	K-factor of the sprinkler, set at listing	L/(min·bar1/2)
$P$	pressure at the sprinkler	bar

The relation applies to a single sprinkler orifice and uses the pressure acting at the orifice. A standard sprinkler with a nominal orifice of ½ inch has a K-factor of 5.6 in US units, gallons per minute per psi^1/2. In SI units this corresponds to about 80.7 L/(min·bar^1/2). In Fleming's example, a sprinkler covers 15.6 m² at a density of 6.1 mm/min, so it needs at least 95.4 L/min. The pressure required is then (95.4/80.7)² ≈ 1.4 bar, the value Fleming gives.

The calculation then works back towards the supply. Each step adds the friction and elevation losses, so the required pressure and flow grow towards the riser. Sprinklers nearer the supply discharge more than the minimum, because they receive a higher pressure. NFPA 13 also requires a pressure of at least 0.48 bar at the end sprinkler, to ensure a proper spray pattern.

The result is compared with the fire water supply. A test of a public main gives a static pressure without flow and a residual pressure at a known flow. Between these points, a supply curve can be drawn, and the system demand, including an allowance for fire service hoses, must lie below it. In the WTC towers, standard ½ inch sprinklers with a K-factor of 5.6 and a temperature rating of 165 °F (about 74 °C) were used in most areas.

Performance and reliability

Sprinkler systems are designed for a density over a design area that usually includes 10 to 15 sprinklers. In practice, incident data show that more than half of all fires are controlled or extinguished by one or two sprinklers. A study of fires in New York City buildings from 1969 to 1978 gives the following distribution:

Cumulative share of fires controlled or extinguished by a given number of operating sprinklers, New York City, 1969 to 1978. Based on Hopkins et al. (2005), p. 19.

Cumulative share of fires controlled or extinguished, New York City, 1969–1978

Sprinklers operating	High-rise buildings (%)	Low-rise buildings (%)
1	65.4	57.1
2 or fewer	84.5	74.3
3 or fewer	91.3	82.3
6 or fewer	97.3	92.6

The Australian Building Codes Board splits sprinkler effectiveness into two parts. Reliability is the share of fires, large enough to activate a sprinkler, in which the system operates. Efficacy is the share of operating cases in which the system contains and controls the fire until the fire brigade arrives. Effectiveness is the product of the two. If efficacy is achieved, flashover is assumed to be prevented and the fire is unlikely to spread beyond its room. Based on US statistics for wet pipe systems, the Board gives these typical design values:

Reliability, efficacy and effectiveness of wet pipe sprinkler systems used as typical values in Australian performance-based design. Based on Australian Building Codes Board (2020a), Data Sheet C2, p. 71.

Typical values for wet pipe sprinkler systems in Australian performance-based design

Building use	Reliability (%)	Efficacy (%)	Effectiveness, typical (low–high) (%)
Residential (NCC Classes 2, 3 and 4)	95	97	92 (87–97)
General (NCC Classes 5, 6, 7a, 8 and 9)	90	96	86 (81–91)

Most recorded failures have causes outside the sprinklers themselves. In US data, a system that was shut off accounted for 59 % of failures to operate. Water that did not reach the fire caused 51 % of the cases in which an operating system was ineffective, and too little water caused 30 %. Other causes include manual intervention, lack of maintenance, damaged components and a system unsuited to the fire. NUREG-1805 adds closed supply valves, empty tanks, pumps that fail to start, corroded or painted sprinklers and inadequate water supplies.

A fault tree study of Australian high-rise offices found reliability values between 86.6 and 97.9 %. Isolation of sprinkler zones during tenancy changes and out-of-specification sprinklers were the main factors. A zone isolation valve on each floor reduced the failure probability to between 10 and 63 % of its original value. In probabilistic risk assessment for US nuclear power plants, NUREG/CR-6850 cites an unreliability of 0.02 for wet pipe systems, against 0.05 for deluge and preaction systems. A wet pipe system needs no signal from a detection system to operate, so a detection failure does not affect its performance.

Use in fire safety engineering

In performance-based design, a wet pipe system affects both the design fire for life safety and the fire load used for structural design. In New Zealand, C/VM2 (second edition, 2025) treats a fire in a sprinklered building as controlled at a constant heat release rate from the moment a sprinkler activates, for life safety analysis. Activation is calculated with prescribed sprinkler properties. A standard response sprinkler has a response time index of 135 m^1/2s^1/2, an activation temperature of 68 °C and a radial distance of 3.25 m. A quick response sprinkler has a response time index of 50 m^1/2s^1/2.

For structural design, C/VM2 assumes burnout of the firecell without sprinkler intervention. The fire load energy density may, however, be multiplied by 0.5 for a sprinklered firecell for several applications, including the fire duration. The maximum fire resistance rating that needs to be specified is 180/180/180 for a sprinklered firecell, against 240/240/240 for an unsprinklered one. In Australia, the verification method compares a proposed design with a reference building. Its sprinkler effectiveness values are used with a sensitivity analysis between the low and high estimates.

The collapse of World Trade Center 7 in 2001 shows the importance of the water supply. Sprinklers on floors 1 to 20 were supplied directly from the city main through an automatic fire pump. A loss of power or damage to the main could therefore interrupt the supply. NIST concluded that if water had been available and the sprinklers had operated as designed, the fires would probably have been controlled and the collapse prevented.

Limitations and misconceptions

A common misconception is that all sprinklers in a building open when a fire breaks out. In reality, only those exposed to enough heat operate and discharge water. A related misconception is that a sprinkler system is a fire suppression system. Hopkins and co-workers call this a misnomer for most office installations, which are designed for control.

The water-filled pipes are both the strength and the weakness of the system. The pipes must be kept above freezing; in the United States, NFPA 13 requires at least 40 °F (about 4 °C) wherever wet pipe systems are installed. A broken sprinkler or a failed pipe or fitting releases water onto building contents that may be sensitive to water damage. Water is also unsuitable for materials that react with it, and burning hydrocarbons can float on water and keep burning.

A controlled fire is not a clean fire. A fire held back by sprinklers burns less completely and produces more soot per kilogram of fuel. Because less fuel burns, the total amount of smoke still remains limited. Finally, a sprinkler system can be overwhelmed or disabled. NIST therefore stresses that structures should be designed for fires in which sprinklers are absent, do not function or are overwhelmed. In the US data, a system that was shut off accounted for 40 % of all effectiveness failures, against 7 % for damaged system components.

See also

• Deluge system
• Sprinkler hazard classification
• Sprinkler design density
• Response time index

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
• Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-72–4-87). National Fire Protection Association; Society of Fire Protection Engineers.
• Grill, R. A., & Johnson, D. A. (2005a). Documentation of the fuel system for emergency power in World Trade Center 7 (NIST NCSTAR 1-1J). National Institute of Standards and Technology.
• Hopkins, M., Schoenrock, J., & Budnick, E. (2005). Fire suppression systems (NIST NCSTAR 1-4B). National Institute of Standards and Technology.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• National Institute of Standards and Technology. (2008). Final report on the collapse of World Trade Center Building 7 (NIST NCSTAR 1A).
• Nowlen, S. P., Najafi, B., Wyant, F. J., Forester, J., Kazarians, M., Kolaczkowski, A., Joglar, F., Funk, D., Hannaman, G., & Anoba, R. (2005b). EPRI/NRC-RES fire PRA methodology for nuclear power facilities: Vol. 2. Detailed methodology (NUREG/CR-6850; EPRI 1011989). Electric Power Research Institute; U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.