Deluge system

A deluge system is a water-based fire suppression system in which every nozzle or sprinkler is permanently open, so that water discharges from all of them at once when a valve in the supply opens. The valve, called a deluge valve, is opened by a separate fire detection system rather than by heat at the nozzles. The deluge system is one of the four basic types of sprinkler system, next to the wet pipe, dry pipe and preaction systems.

Its purpose is to deliver the largest amount of water in the shortest time. Deluge systems are used against rapidly spreading, high hazard fires, such as fires in highly flammable fuels that develop quickly and reach very high temperatures. Typical applications in the sources include aircraft hangars, transformers, turbine-generator bearings, fan units and water curtains at openings. The price of this speed is a large water demand and extensive water damage, both when the system is needed and when it operates by mistake. In nuclear power plant risk studies, a deluge system is assumed to fail on demand about five times in a hundred.

How a deluge system works

A deluge system consists of a network of pipes with open sprinklers or spray nozzles, connected to a water supply through a deluge valve. The open nozzles contain no fusible elements that could hold back the water. Instead, the deluge valve holds the water back from the piping until it is activated. The deluge valve sits on the water supply side of the system and opens automatically when the detection system calls for water. Once it has opened, water flows into the pipes and discharges from every outlet on the system at the same time. In the words of the definition, there are no closed sprinklers in a deluge system.

Main parts of a deluge system. A separate detector opens the deluge valve, after which water flows from every open nozzle at once. Based on Fleming (2002), p. 4-72 and Iqbal and Salley (2004), p. 10-2.

The detection devices are installed in the same area as the nozzles. In the nuclear guidance of the United States Nuclear Regulatory Commission (NRC), the system is described as typically actuated by heat detectors mounted above the open sprinklers. Most deluge systems can also be started by hand. A glossary in the same report defines the deluge sprinkler system simply as a system with open sprinklers or nozzles, in which all water flows out when the deluge valve actuates. The single word "deluge" is defined there as the immediate release of a commodity, usually a water spray released for fire suppression.

A water spray system works on the same principle. It is a fixed pipe system with spray nozzles chosen for a specific discharge and distribution over the surface or area to be protected. Its control valve is opened automatically by fire detectors in the same area, or by hand. In the United States, the National Fire Protection Association (NFPA) covers water spray systems in NFPA 15, Standard for Water Spray Fixed Systems for Fire Protection. Foam-water sprinkler and spray systems fall under NFPA 16. The same open-nozzle principle is used in total compartment application water mist systems, where water is discharged from all nozzles as soon as a control valve opens.

Comparison with other sprinkler systems

All sprinkler systems can be placed in one of four basic types, which differ in how the water reaches the area of the fire. In a wet pipe system, the pipes are filled with water under pressure. Heat opens a single sprinkler, which then discharges water over its own area. In a dry pipe system, the pipes contain air or nitrogen under pressure, and water enters only after a sprinkler has opened and the air pressure has dropped.

The four basic sprinkler system types. Only in the deluge system do all outlets discharge, because its nozzles are open. Based on Fleming (2002), pp. 4-72 to 4-73.

These two types use automatic sprinklers, which open one by one in response to heat. Only the sprinklers exposed to enough heat operate; the idea that all sprinklers open in a fire is a common misconception. For a deluge system that idea is correct, because the whole system discharges together.

The preaction system is the closest relative. It is similar to a deluge system, but it uses automatic sprinklers instead of open ones. A separate detection system opens a deluge valve and fills the pipes, yet water flows only where heat has also opened a sprinkler. The NRC report describes it as a deluge system with closed heads, in which two conditions must be met before water is released. Preaction systems are used where accidental discharge of water is a special concern, as in valuable computer rooms. In the World Trade Center towers, the preaction systems used hydraulically operated differential diaphragm deluge valves with manual and electric detectors.

The four basic types of sprinkler system

Type	Sprinklers	Pipes before operation	What releases the water
Wet pipe	automatic (closed)	water under pressure	heat opens individual sprinklers
Dry pipe	automatic (closed)	air or nitrogen under pressure	an opened sprinkler lets the air escape, then the dry pipe valve opens
Preaction	automatic (closed)	usually air at a small pressure	detection opens the valve, heat opens individual sprinklers
Deluge	open	no water (held back by the deluge valve)	detection or manual action opens the deluge valve; all outlets discharge

The difference also shows up in the hydraulic design. For pressure losses in steel pipe, the Hazen–Williams method assigns a roughness coefficient C of 120 to wet and deluge systems, but only 100 to dry and preaction systems. A lower value means a rougher pipe and a larger friction loss for the same flow. The values are intended to simulate the expected interior roughness of aged pipe.

Detection and actuation

The nozzles of a deluge system cannot sense a fire, so the system relies on a separate detection system. The NRC report names heat detectors above the open sprinklers as the typical choice. In aircraft hangars, the systems were historically activated by rapid-response detection systems. The International Fire Engineering Guidelines list heat, smoke, gas sensing and flame detectors as the main detector types. A flame detector is activated by electromagnetic radiation emitted by flames. It can be treated as a point that receives radiation from the flame, and its sensitivity can vary with the direction of the incoming radiation.

A deluge valve can also be tied to more than one detector. In a coincident detection system, two or more detectors must operate before an alarm is recognised. These can be two similar detectors at different locations or two different types of detector. The air handling units of the World Trade Center towers used this approach. The first smoke detector to operate shut down the unit, and a second detector within the unit then activated its dedicated deluge system for a filter fire.

False activation has been a serious problem in practice. All military service branches in North America experienced false activation of foam-water deluge systems over aircraft with open cockpits. The causes included lightning strikes, water hammer in aging underground mains, accidental releases by maintenance staff and vandalism. Accidental use of manual pull stations, failed pressure relief valves, roof leakage into overhead heat detection and false detector signals also played a part.

Optical fire detectors were found to be prone to false alarms and were tested with fuels not typical of aviation. The concept that the U.S. Navy developed in response used low-level deluge nozzles for aqueous film-forming foam (AFFF), operated by improved optical detectors. These detectors were designed to be highly immune to false alarms and to detect fuel spill fires rapidly.

Water demand

In a wet pipe system, water is supplied only for the sprinklers in a design area, since the others are not expected to open. In a deluge system all outlets discharge together. The water supply must therefore feed the whole system at once. The minimum flow from one sprinkler is its area of coverage multiplied by the required density, the depth of water applied per minute. Applied to a deluge system, the same product over the whole protected area gives the flow that the deluge valve must pass:

$$Q=D\times A$$

Symbols

Symbol	Description	Unit
$Q$	total water flow through the deluge valve	L/min
$D$	design density (application rate)	L/(min·m²)
$A$	total floor area covered by all open nozzles of the system	m²

The relation assumes an even discharge over the area. In reality, each outlet flows in proportion to the square root of the pressure at that outlet, so nozzles near the supply discharge more than the most remote one. The result is therefore a lower bound on the flow, before any pipe friction is taken into account. The density itself is not derived from theory; sprinkler design criteria are empirical, because the mechanisms by which water suppresses fire have not been characterised well enough for calculation.

Worked example

Factory Mutual Research Corporation tested deluge arrangements for aircraft hangars. One arrangement used standard sprinklers at a spacing of 12.1 m² per head, discharging 6.6 L/(min·m²) of AFFF solution at an end head pressure of 97 kPa. Each head therefore delivers at least 6.6 × 12.1 = 79.9 L/min. Assume a deluge zone of 40 such heads, covering 484 m². The deluge valve must then pass at least:

$$Q=6.6\mathrm{L}/\mathrm{(}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}\mathrm{)}\times 484{\mathrm{m}}^{\mathrm{2}}\approx 3,200\mathrm{L}/\mathrm{m}\mathrm{i}\mathrm{n}$$

This is about 53 L/s for the zone, before any friction losses are added. In the same test series, this arrangement extinguished an 83.6 m² JP-4 pool fire in 1 minute 45 seconds. A wet pipe system, by contrast, needs water only for the sprinklers in its design area, because the other sprinklers are not expected to open.

Applications

Aircraft hangars are the best documented application in the sources. Hangar protection aims to protect the aircraft and to prevent collapse of the roof, which is usually unprotected steel. Before foam was developed, hangars were protected by water deluge systems discharging about 10.4 L/(min·m²) (0.25 gpm/ft²), together with sloped floors and drains. Even then, protection of aircraft fuselages against burnthrough within about 1 minute could not be ensured. In one 121 m² JP-4 fuel fire, ceiling temperatures of 927 °C were recorded within 30 seconds of ignition, before the deluge system discharged.

The introduction of firefighting foam changed this design. Protein foam systems discharging 8.2 L/(min·m²) were an improvement on water alone. With AFFF through standard sprinklers, the NFPA 409 rate for overhead deluge systems may be reduced from 8.2 to 6.6 L/(min·m²), which is 20 % less foam. NFPA 16 requires a minimum of 6.6 L/(min·m²) for deluge and closed-head AFFF systems. This gives a safety factor over the 4.1 L/(min·m²) at which AFFF from sprinklers is effective on pool fires.

Application rates for overhead hangar deluge protection with water, protein foam and AFFF. Based on Scheffey (2002), pp. 4-107, 4-108 and 4-112.

Tests with closed-head sprinklers instead of open heads showed that a large number of sprinklers opened during the fire tests, so that this alternative was not feasible for the hangar scenario. The later U.S. Navy concept combined a quick-response, closed-head wet pipe system in the hangar ceiling with low-level AFFF deluge nozzles for fuel spill fires. More generally, the foam-water deluge system is one type of fixed low-expansion foam system, used in bunds and on spills.

In nuclear power plants, deluge systems protect equipment such as transformers and turbine-generator bearings. At North Anna Unit 2 in 1981, a main transformer failed and its oil burned. The deluge systems on two transformers activated, but the fire was too severe for the system and took the fire brigades about one hour to control. The NRC drew the lesson that a fixed suppression system can be overwhelmed even when the fire starts in the equipment it protects. At Salem Unit 2 in 1991, deluge systems on two generator bearing housings actuated as designed during a turbine failure. Together with other automatic systems, they extinguished some of the fires.

In buildings, deluge systems are used for local protection and for water curtains. A water curtain is a screen or wide-angle spray of water that protects exposures from radiated heat, smoke and flames, and may also be called a water screen. The New York City building code applied to the World Trade Center covered exit ramps in public garages. Their vehicle openings had to be protected by a deluge-type sprinkler water curtain of 3 gpm per linear foot (about 37 L/min per metre). In the towers themselves, local application deluge systems protected the fan filters in the air handling units.

Offshore platforms also use deluge nozzles. Their spray is much coarser than water mist, and in British Gas explosion tests such large-drop sprays increased turbulence and could make a gas deflagration worse. The editors of the industry report concluded that more work was needed with sprays of finer drop sizes.

Deluge systems are specified where highly flammable fuels can produce a fast, very hot fire. In industrial processes, many substances that are gases at ambient conditions are stored under pressure as a liquid with vapour above it. The failure of such a container while its liquid is above its normal boiling point is called a BLEVE, a boiling liquid expanding vapour explosion. A BLEVE typically occurs in a metal container that has been overheated above 538 °C. The liquid absorbs heat, so the wetted part of the shell is usually not at risk; the danger lies in the surfaces of the vapour space.

Reliability and limitations

Unlike a wet pipe system, a deluge system needs both a detection signal and a deluge valve that opens before any water flows. Unreliability values from a 1994 data report by Parkinson and co-workers are recommended in the fire risk method for United States nuclear power plants. They are 0.05 for deluge or preaction sprinkler systems, compared with 0.02 for wet pipe systems. These values do not include unavailability during maintenance, credit for manual actuation, dependent failures or plant-specific data. Earthquakes add a further failure mode, because deluge trip valves may not withstand an earthquake and may open spuriously or jam shut.

Recommended unreliability values for automatic suppression systems in fire risk studies of US nuclear power plants. Based on Parkinson et al. (1994), as cited in Nowlen et al. (2005b), p. P-6.

The main disadvantage named in the NRC report is extensive water damage, because all of the open sprinklers discharge. Unwanted operation makes this worse. At Fort Calhoun in 2001, smoke from a fire in another area actuated a deluge water curtain in a stairwell. The water sprayed onto safety-related motor control centres and caused short circuits in safety-related circuits. The curtain had been installed because there was no fixed fire barrier between the two fire areas.

A deluge system also depends on its water supply, which can be a shared weak point. At one chemical plant, the fire pump house was flooded three times in five years, and the pump controllers were shut off each time. The pumps thus formed a common cause failure for numerous deluge water spray systems. Finally, a deluge system may control a fire rather than extinguish it. The International Fire Engineering Guidelines distinguish three possible effects of suppression on a design fire: no effect, control and extinguishment. The North Anna transformer fire shows that a system which operates can still be overwhelmed by the fire it is meant to protect against.

See also

• Jet fire
• Wet pipe sprinkler system
• Water mist system
• Fire water supply

References

• Barry, T. F. (2002). Quantitative risk assessment in chemical process industries. 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. 5-176–5-213). National Fire Protection Association; Society of Fire Protection Engineers.
• 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. (2005b). Fire protection and life safety provisions applied to the design and construction of WTC 1, 2, and 7 and post-construction provisions applied after occupancy (NIST NCSTAR 1-1D). 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.
• Keough, R. J., & Grill, R. A. (2005). Fire alarm systems (NIST NCSTAR 1-4C). National Institute of Standards and Technology.
• Mawhinney, J. R., & Back, G. G., III. (2002). Water mist fire suppression systems. 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. 4-311–4-337). National Fire Protection Association; Society of Fire Protection Engineers.
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• 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.
• Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. 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. 4-88–4-122). National Fire Protection Association; Society of Fire Protection Engineers.