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Contents
  1. 1Sources of water
  2. 2Flow, pressure and duration
    1. 2.1Worked example
  3. 3Water for the fire service
  4. 4Reliability and redundancy
  5. 5Limits and practical issues
  6. 6See also
  7. 7Notes
  8. 8References

A fire water supply is the source, storage and distribution of the water that sprinkler systems, hydrants, risers and hoses need to fight a fire. It can come from a public water main, from tanks on site, or from pumps that draw on either of them.[1][2] Its strength is described by how much water it can deliver, at what pressure, and for how long. Fleming calls the strength of the water supply the key to whether it will adequately serve a sprinkler system.[3]

The supply serves two groups of users. Automatic systems such as sprinklers need water at once and without human action.[1] The fire service needs water at the building for its hoses, from a fire hydrant, from a riser inside the building or from its own pumping appliances.[4][5]

In New Zealand, Building Code clause C5.5, as reproduced in C/VM2 (second edition, 2025), requires buildings to be provided with the means to deliver water for firefighting to all parts of the building.[6] A water supply can also be a single point of failure. A closed valve, an empty tank or a pump that does not start can disable every system that depends on it.[7] Redundant supply paths and independent sources are used to reduce this dependence.[8][9]

Sources of water

Water can come from several kinds of source. In the United States, a wet standpipe system needs a direct connection to an automatic water supply. Examples are a city water distribution system, a tank and pump, an elevated tank or a pressure tank.[1] Sprinkler systems draw on the same kinds of source. NFPA 13, the US sprinkler standard of the National Fire Protection Association (NFPA), refers to separate standards for the supply components. These are NFPA 14 for standpipes, NFPA 20 for fire pumps, NFPA 22 for water tanks and NFPA 24 for private fire service mains.[10]

Schematic of a building fed by a street main and a roof tank, with fire pump, riser, sprinklers, hose reel, street hydrant and fire service connection.
Typical parts of a fire water supply for a multistorey building. Based on Fleming (2002), pp. 4-82 to 4-83, Hopkins et al. (2005), p. 21 and Evans et al. (2005), pp. 14 and 18.

If not enough water is available at a suitable pressure, a stored water supply becomes necessary.[11] The basic choice is between an elevated (gravity) tank, a pressure tank, or a suction tank combined with a pump.[2] Without a pump, the energy for the system must come from the height of a gravity tank or from the air pressure in a pressure tank.[2]

Elevation adds or removes pressure at 0.433 psi per foot, or 0.099 bar per metre.[12] The pressure of a gravity tank must therefore be calculated from the lowest expected water level, which is normally the level at which the tank counts as empty.[2] A pressure tank must contain enough air to push out the last water at an adequate pressure. A common rule of thumb has been one-third air at a minimum pressure of 75 psi (5.17 bar), but this rule does not hold for systems with high pressures.[2]

Fire pumps make up for a lack of pressure where enough water is available at a suitable suction pressure.[11] Plumbing codes sometimes set a minimum suction pressure of 10 to 20 psi (0.69 to 1.38 bar).[11] Listed fire pumps have diesel or electric drivers and capacities from 25 to 5,000 gpm (95 to 18,927 L/min). The most common sizes lie between 250 and 2,500 gpm (946 to 9,463 L/min).[11] NFPA 20 requires each horizontal fire pump to deliver at least 100 % of its rated pressure at its rated flow and at least 65 % of that pressure at 150 % of the flow. With no flow, the pressure must not exceed 140 % of the rated pressure.[11]

The World Trade Center complex in New York shows how these elements are combined in a large building. Its primary supply was a 12 in. fire main that looped the complex and was fed from the municipal supply by two redundant 16 in. connections.[13] Each tower had reserve tanks of 5,000 gal (about 19 m³) on several mechanical floors, and the tanks on the 41st floor had automatic fill lines from the domestic water system. Twelve fire pumps moved the water up the building.[4] Fire department engines could also pump water from street hydrants into the buildings through 32 fire department connections at 14 locations.[4]

Flow, pressure and duration

A water supply is assessed by testing it. Each test gives at least two values: the static pressure without flow and the residual pressure while a known flow is taken.[3] The static pressure is read from a gauge on a hydrant. The residual pressure is read on the same gauge while water flows from another hydrant, preferably downstream.[3]

Two hydrants on a water main: the first with a pressure gauge, the second, downstream, discharging water measured with a pitot tube.
Water supply test: the gauge on one hydrant reads the static pressure with no flow and the residual pressure while a downstream hydrant flows. Based on Fleming (2002), p. 4-81.

The static pressure reflects elevated tanks and running pumps, while the drop to the residual pressure reflects friction losses in the pipes.[3] Because friction loss grows with the flow to the power 1.85, the two points can be drawn as a straight line on special graph paper. This line, the water supply curve, gives the available pressure at any flow.[3]

The required flow and pressure come from a hydraulic calculation of the system. For an adequate design, the system demand point, including the allowance for fire service hoses, must lie below the water supply curve.[3] Outside the design area of a sprinkler system, no flow is added other than the hose stream allowances.[14] For standpipes, US practice based on NFPA 14 (2000) requires 500 gpm (1,893 L/min) at 100 psi (6.9 bar) at the hydraulically most remote standpipe. Each additional standpipe adds 250 gpm (946 L/min), up to a maximum of 1,250 gpm (4,732 L/min).[15]

Duration is the third requirement. Tanks must be sized to provide the minimum durations set by NFPA 13 or another applicable standard, and the required pressure must still be available when the tank is nearly empty.[2] The stored volume follows from the flow and the duration:

V=Q×t

Symbols
Symbol Description Unit
V usable stored water volume m³ (or L)
Q water demand (flow) of the systems supplied m³/min (or L/min)
t required duration of supply min

The relation assumes a constant demand and no refill during the fire. Any automatic refill, for example from the public main, lengthens the actual duration. The volume is the usable volume above the level at which the tank counts as empty.[2][4]

Worked example

Two 5,000 gal tanks and a 5,000 gal standpipe reserve tank on the 110th floor of each World Trade Center tower supplied the booster pump for the highest sprinkler zone. Their combined capacity was 15,000 gal (56.8 m³).[4][16] The fire pump was rated at 500 gpm (1,893 L/min), and the maximum permissible flow was 150 % of that, 750 gpm (2,839 L/min).[16] Rearranging the relation gives the duration of supply:

Falling curve of supply duration against flow for a 56.8 m3 tank, with marked points 30 min at 1,893 L/min and 20 min at 2,839 L/min.
Duration of a 56.8 m³ (15,000 gal) stored supply as a function of the demand, t = V/Q. Based on Evans et al. (2005), pp. 18 to 19.

t=VQ=56.8 m31.893 m3/min=30 min

At the maximum flow of 2.839 m³/min, the same tanks last 20 minutes. The National Institute of Standards and Technology (NIST) gives the same values: a minimum of 30 minutes at 500 gpm, without any supplemental water from the domestic system, and 20 minutes at 750 gpm.[16] Fleming's example for a pressure tank shows the same logic. A 30-minute supply for a demand of 140 gpm (530 L/min) requires at least 4,200 gal (15.9 m³) of water, plus the air needed to keep the pressure up.[2]

Hydraulic calculations by NIST for the light hazard sprinkler systems of the towers gave delivered densities of 0.10 to 0.27 gpm/ft² with up to 16 open sprinklers. The primary water supply could maintain these flows for about 10 to 89 minutes, depending on the location and number of open sprinklers.[8] NIST concluded that the storage tanks gave enough duration for normally expected fires to allow the fire department to respond and supplement the supply.[17]

Water for the fire service

The fire service draws its water from hydrants, from risers and from its own vehicles. In a fire engineering analysis, the time to set up a water supply is one of the steps in the fire brigade intervention model of the International Fire Engineering Guidelines.[18] The amount of firefighting water available at the scene is one of the resources to be assessed.[19] If the water supply or the number of firefighters is insufficient for interior firefighting, the strategy may change from offensive, fighting the fire, to defensive, stopping its spread to neighbouring buildings.[20]

National rules set the facilities. In Australia, Table 2.8 of the Fire Safety Verification Method Standard 2022 requires external fire hydrants for all buildings in the fire brigade intervention scenario. Internal hydrants are required in sprinklered buildings if any point is more than 100 m away or the building has more than three levels, and in unsprinklered buildings from 70 m.[21] The scenario applies only to buildings within 50 km road travel of a fire station.[22] In New Zealand, C/VM2 (second edition, 2025) accepts water from a pumping appliance parked so that any point in the building lies within 75 m, about three hose lengths. Otherwise an internal hydrant to NZS 4510 is needed.[5]

Inside tall buildings, risers bring the water to the floors. Standpipe systems are classed as wet, damp or dry by their water supply. Wet systems are filled with water at all times, while manual dry standpipes have only fire department connections and no automatic supply.[1] Removing the need for long hose lays from the fire engine to an upper floor improves the initial attack.[1] For Dutch indoor car parks, NIPV notes that a dry riser can shorten the attack time, because less hose has to be rolled out.[23]

Occupants can use the same water. The International Fire Engineering Guidelines mention suppression by building occupants with extinguishers and a hose reel.[24] The New York City building code had no requirement for fire extinguishers, because it required occupant hose reels.[25] In World Trade Center Building 7 (WTC 7), hose reels were located in the stair enclosures, with hose cabinets throughout the floors.[26]

Reliability and redundancy

Several common causes of sprinkler failure lie in the water supply. The US Nuclear Regulatory Commission (NRC) lists a closed water supply valve, a broken supply header, an empty water tank and a fire pump that fails to start among the causes.[7] An electric pump can fail through a power failure, and a diesel pump through poor maintenance, dead batteries or lack of fuel.[7] The Australian data sheets summarise US statistics by Ahrens. A system that was shut off accounted for 59 % of the cases in which a sprinkler system failed to operate. When systems did operate but were ineffective, 30 % of the cases involved not enough water being discharged.[27]

Bar chart of why sprinklers failed to operate: shut off 59 %, manual intervention 17 %, maintenance 10 %, damage 7 %, wrong system 7 %.
Causes of sprinkler systems failing to operate in US fires, as summarised in the Australian data sheets. Based on Ahrens (2017), as cited in Australian Building Codes Board (2020a), Data Sheet C2, p. 79.

The reliability of fire protection systems therefore depends strongly on valves and their management. An Australian fault tree study of sprinkler systems in high-rise offices found reliabilities between 86.6 % and 97.9 %. Shutting off sprinkler zones during tenancy changes was one of the main factors in failure.[28] According to the fault tree analysis, a zone isolation valve for each floor could reduce the failure probability by 10 % to 63 % of its original value.[28]

Redundancy reduces the dependence on a single source. In New Zealand, C/VM2 (second edition, 2025) states that a sprinkler system with two independent supplies, of which only one may depend on a town's main, has a very high degree of reliability. Such a system, with a Class A or Class B2 water supply to NZS 4541, can alone satisfy clause C3.6, which limits the radiation received at the relevant boundary.[29][9] The World Trade Center towers were supplied from loops at two remote locations, with isolation valves that allowed either main to operate independently.[30] WTC 7, by contrast, was fed by two mains within 12 ft (3.7 m) of each other from the same city main.[30]

Redundant sources can still share a weak point, which then becomes a common cause failure. NIST found that the risers of WTC 1, 2 and 7 provided redundant supply. Each floor's sprinkler system, however, had a single connection to the riser, a single point of failure for the sprinklers on that floor.[31] The emergency fire pumps lacked remote redundancy in their power lines, which could have affected the sprinklers and standpipes once power was lost.[32]

At one chemical plant, the fire pump house 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 systems. Barry gives target availabilities for such systems of 0.98 to 0.999.[33]

Limits and practical issues

A design water supply covers the expected fire, not every fire. In the World Trade Center, the secondary water supply for the high and mid-level zones depended on manually started fire pumps. NIST notes that an automatic supplemental supply is required by NFPA 14 and represents current best practice.[31] The lower floors of WTC 7 were supplied directly from the city system, so there was no secondary source if that system failed.[17][31] A long run of 4 in. pipe from the 110th floor tanks restricted the flow to several floors, although it was sufficient for the design.[31]

New hazards can change the demand. NIPV reports that fires in electric vehicles in car parks need large amounts of water for a long time to cool effectively. The extinguishing water supply and water drainage should be designed for this, and contaminated cooling water needs attention.[34] The operating time of a sprinkler system, typically 30 to 120 minutes depending on the standard, limits how long it can support the fire service.[35] Firefighting water can also harm the environment. The International Fire Engineering Guidelines name run-off of firefighting water from a chemical warehouse as an example.[36]

Statistics on sprinkler performance can hide supply problems. Australian and New Zealand data in the International Fire Engineering Guidelines show that 65 % of fires were controlled by one sprinkler head. These data exclude fires in which sprinklers failed to operate or water supplies were not available, so they may not reflect the overall reliability of sprinkler systems.[37]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Hopkins et al. (2005), p. 21.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Fleming (2002), p. 4-83.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Fleming (2002), p. 4-81.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Evans et al. (2005), p. 18.
  5. ↑ 5.0 5.1 Ministry of Business, Innovation and Employment (2025b), p. 54.
  6. ↑ Ministry of Business, Innovation and Employment (2025b), p. 53.
  7. ↑ 7.0 7.1 7.2 Iqbal and Salley (2004), p. 10-2.
  8. ↑ 8.0 8.1 Evans et al. (2005), p. 32.
  9. ↑ 9.0 9.1 Ministry of Business, Innovation and Employment (2025b), p. 44.
  10. ↑ Fleming (2002), p. 4-73.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 Fleming (2002), p. 4-82.
  12. ↑ Fleming (2002), p. 4-80.
  13. ↑ Evans et al. (2005), p. 14.
  14. ↑ Fleming (2002), p. 4-75.
  15. ↑ Hopkins et al. (2005), p. 20.
  16. ↑ 16.0 16.1 16.2 Evans et al. (2005), p. 19.
  17. ↑ 17.0 17.1 Evans et al. (2005), p. 40.
  18. ↑ National Research Council of Canada et al. (2005), p. 2.9-5.
  19. ↑ National Research Council of Canada et al. (2005), p. 1.9-5.
  20. ↑ National Research Council of Canada et al. (2005), p. 2.9-4.
  21. ↑ Australian Building Codes Board (2022), p. 16.
  22. ↑ Australian Building Codes Board (2022), p. 15.
  23. ↑ Van der Graaf et al. (2021/2023), p. 66.
  24. ↑ National Research Council of Canada et al. (2005), p. 1.7-1.
  25. ↑ Lew et al. (2005), p. 151.
  26. ↑ Hopkins et al. (2005), p. 128.
  27. ↑ Ahrens (2017), as cited in Australian Building Codes Board (2020a), Data Sheet C2, p. 79.
  28. ↑ 28.0 28.1 Moinuddin and Thomas, as cited in Australian Building Codes Board (2020a), Data Sheet C2, p. 81.
  29. ↑ Ministry of Business, Innovation and Employment (2025b), p. 43.
  30. ↑ 30.0 30.1 Evans et al. (2005), p. 30.
  31. ↑ 31.0 31.1 31.2 31.3 Evans et al. (2005), p. 41.
  32. ↑ Evans et al. (2005), p. 42.
  33. ↑ Barry (2002), p. 5-203.
  34. ↑ Van der Graaf et al. (2021/2023), p. 6.
  35. ↑ Van der Graaf et al. (2021/2023), p. 47.
  36. ↑ National Research Council of Canada et al. (2005), p. 2.9-3.
  37. ↑ National Research Council of Canada et al. (2005), p. 3.7-3.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Australian Building Codes Board. (2022). Fire Safety Verification Method Standard 2022.
  • 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.
  • 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.
  • 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.
  • Lew, H. S., Bukowski, R. W., & Carino, N. J. (2005). Design, construction, and maintenance of structural and life safety systems (NIST NCSTAR 1-1). National Institute of Standards and Technology.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • 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.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
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