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== Notes ==
== Notes ==

Latest revision as of 22:32, 2 November 2026

Dry and wet risers are fixed vertical water pipes in a building with a hose outlet on each floor. They allow firefighters to connect their hoses close to a fire instead of laying hose from the street.[1] A wet riser is filled with water at all times and connected to an automatic water supply. A dry riser contains air, and in its simplest form has no water supply of its own: the fire service pumps water in through an inlet at street level.[2] In the United States the same installations are called standpipe systems, and the vertical pipes in them are called standpipe risers.[1]

Risers exist because laying hose from an appliance in the street to an upper or underground floor takes time and effort.[3] Removing the need for long hose lays from the appliance to an upper floor is generally accepted to improve the initial attack, even though no performance statistics are kept for these systems.[2] In a high-rise building, the fire service depends on fire pumps and standpipe systems that can supply enough water for fires beyond the reach of appliances on the ground.[4] The pressure needed rises by about 9.8 kPa for each metre of height, which is why tall buildings divide their risers into vertical zones with tanks and pumps.[5][6]

Terminology and components

A standpipe system is a fixed piping system that delivers water to designated areas of a building to support manual firefighting.[1] In a high-rise building it consists of risers with hose connections at each floor, supplied by pumps and by at least one fire department connection at street level.[1] The connections may or may not have hoses permanently attached, depending on the type of system and the local code.[1] The number of risers depends on the size and layout of the building, and the systems are usually pressure-monitored to show that they are ready for use.[1]

The main parts are the riser itself, the hose valve or outlet on each floor, and the inlet through which the fire service pumps water into the system. In American usage this inlet is called a fire department connection or "siamese" connection.[1][7] In the World Trade Center towers, each hose station had a valve, a 125 ft (38 m) hose and a nozzle, and the racks were mounted about 4.5 ft (1.4 m) above the stair landing.[8][9] This height allowed firefighters to reach the valve without walking up half a flight of stairs.[9]

The basic concept has changed little in about 90 years. NFPA 14, the American standard for the installation of standpipe and hose systems, was first adopted in 1915.[1] New Zealand documents speak of "fire hydrant systems for buildings" and of an internal fire hydrant designed to NZS 4510.[10][11] A Dutch guideline for car parks, translated into English, uses the term "dry riser".[12]

Wet, dry and other types

NFPA 14, as summarised in the NIST report on the World Trade Center suppression systems, describes standpipe systems by their water supply as wet, damp or dry.[2] Wet systems are the preferred arrangement. They are filled with water at all times and connected directly to an automatic water supply, such as the public water main, a tank with a pump, an elevated tank or a pressure tank.[2] Because the pipes are full, every part of the building that contains them must be heated to prevent freezing.[2]

Two building sections: left, a water-filled riser fed by a tank and pump; right, an empty riser fed by a fire appliance at a street-level inlet.
A wet riser (left) is always full and connected to an automatic water supply; a manual dry riser (right) contains air until the fire service pumps water in through the inlet at street level. Based on Hopkins et al. (2005), pp. 19 and 21.

Dry systems contain air at atmospheric pressure or under pressure, and NFPA 14 divides them into manual, automatic and semi-automatic types.[2] A manual dry standpipe has only fire department connections and no connection to an automatic water supply: water reaches the outlets only when a fire service pump feeds the inlet.[2] An automatic dry standpipe holds pressurised air or nitrogen behind a dry-pipe valve; when a hose valve is opened, the air escapes and the water supply opens the valve and fills the pipe.[2] A semi-automatic dry standpipe has a connection to an automatic water supply that is kept closed until it is needed.[2]

The choice between wet and dry involves a trade-off. A wet riser gives water at once, but needs a reliable automatic supply and protection against frost.[2] A manual dry riser needs no permanent supply, but delivers nothing until the fire service has connected a pumping appliance to its inlet.[2] Its use therefore depends on fire service access to the inlet. The New York City building code, in both editions compared by NIST, provides for wet standpipes.[13]

Standpipe systems are also classified by user.[1] Class I systems have 2½ in. (63.5 mm) hose valves for heavy streams used by the fire service in advanced stages of a fire.[14] Class II systems have 1½ in. (38 mm) hose stations with hoses and nozzles for trained occupants or fire brigades to fight small fires.[14] Class III systems combine both and serve the fire service, trained occupants and fire brigades.[14]

Whether occupants should rely on pre-connected hoses has long been debated.[2] Some jurisdictions permit only Class I systems, for use by the fire service alone.[2] The pre-connected hoses of the World Trade Center towers were intended for the trained fire brigade or the fire department.[8]

Pressure and building height

Water in a vertical pipe presses down with its own weight, so the pressure at the bottom of a riser is higher than at the top.[5] The pressure caused by a column of water alone is called static pressure, and the height of the column is called the head.[5] The SFPE handbook gives this relation from fluid statics: the pressure difference between two points equals the density of the water, times the acceleration of gravity, times their difference in height.[5]

Δp=ρgΔh

Symbol Description Unit
Δp static pressure difference between two levels Pa
ρ density of water (about 1,000 kg/m³) kg/m³
g acceleration of gravity (9.81 m/s²) m/s²
Δh difference in height between the two levels m

The relation applies to water at rest or to the elevation part of the pressure in a flowing system; friction losses in pipes and fittings come on top of it.[5][14] Titus gives the same rule in American units: a column of water of h feet corresponds to 0.433h psi, and 1 psi corresponds to 2.3 ft (0.7 m) of water.[5] In SI units this is about 9.8 kPa per metre of height. The pump ratings of the World Trade Center standpipe system follow this rule: a pump rated at 360 psi was also listed as 831 ft of head, and 0.433 × 831 ≈ 360.[15]

Worked example. A standpipe must deliver its design flow at 100 psi (689 kPa) at the most remote outlet.[14] If that outlet is 300 ft (91.4 m) above the pump, the height alone costs:

Δp=1000×9.81×91.4≈897 kPa (130 psi)

The pump must therefore supply at least 689 + 897 ≈ 1,590 kPa, about 230 psi, before any friction loss is added (rounded up). The height of 300 ft is the limit used in the New York City building code, which allows an automatic fire pump to serve only the lowest 300 ft of a taller building.[16] Zones above that height must be supplied by gravity or pressure tanks.[16]

Straight line of static pressure against height, rising about 9.8 kPa per metre, with a marker at 91.4 m and 897 kPa.
Static pressure of a water column against height, about 9.8 kPa per metre; the marker shows the worked example at 300 ft (91.4 m). Friction losses and the outlet pressure come on top. Based on Titus (2002), p. 4-45.

The same physics explains the vertical zoning of tall buildings. The standpipes in each World Trade Center tower were divided into four vertical water supply zones: floors 1 to 8, 9 to 41, 42 to 76 and 77 to 110.[6] Check valves between the zones stopped water from flowing down from one zone into the next.[6] Because the working pressure varied with elevation, the riser pipes were heavier at the bottom. The lowest zone had extra-strong steel pipe with fittings rated at 1,000 psi, the top zone standard pipe with 350 psi fittings.[17] Adjustable pressure-restricting devices were fitted to the hose racks on floors 77 to 94.[9]

Tall tower schematic split into four standpipe zones, with water tanks near the top of zones, pumps on mechanical floors and inlets at street level.
Vertical zoning of the standpipe system in a World Trade Center tower: four supply zones, gravity tanks for the first minutes, manual fire pumps on mechanical floors and fire department connections at street level. Based on Hopkins et al. (2005), pp. 10, 30 and 38.

High pressures also limit what the fire service can supply. The 1997 high-rise firefighting procedures of the Fire Department of the City of New York require building fire pumps to deliver their rated capacity at 50 psi (345 kPa) at the highest outlet.[18] The same procedures warn that these pumps, and gravity tanks, may not give adequate nozzle pressure on the upper floors, so the fire service's own pumpers are the primary supply in most operations.[18] As a rule of thumb, they recommend one pumper supplying the standpipe for each hand line in use.[18]

Design requirements

In the United States, NFPA 14 sets the design flow for Class I and Class III systems at 500 US gal/min (about 1,900 L/min) at 100 psi at the most hydraulically remote standpipe.[14] Each additional standpipe adds 250 US gal/min (about 950 L/min), up to a total of 1,250 US gal/min (about 4,700 L/min).[14] This assumes two 2½ in. hose lines on the most remote standpipe and one on each of the others.[14] "Remote" means hydraulically remote: the outlet needing the highest pressure at the source, which may be the top floor or the far end of a building, so the design is checked with a hydraulic analysis.[14] Class II systems are designed for 100 US gal/min (about 380 L/min) at 65 psi (448 kPa).[14]

The spacing of the outlets is expressed in lengths of hose. When the World Trade Center was built, NFPA 14 required Class I hose valves to be placed so that every part of a building lay within 30 ft (9.1 m) of a nozzle. That nozzle was assumed to be on a 100 ft (30.5 m) hose.[14] The New York City building code, in its 1968 and 2003 editions, requires that every point on every floor can be reached by a 20 ft (6.1 m) stream. The stream comes from a nozzle on not more than 125 ft (38 m) of hose.[13] The risers and their 2½ in. hose valves must be inside the stair enclosures, with an outlet on each floor 5 to 6 ft above the landing.[13][19]

The same code sets the riser size by height: at least 4 in. (about 100 mm) in diameter up to 150 ft (46 m) from the entrance floor to the highest outlet, and 6 in. (about 150 mm) above that.[16] Where a system has several risers, they must be cross-connected at or below street level.[19] Buildings over 300 ft (91 m) high need an additional water supply for their standpipes, supplemented by manually operated fire pumps.[20]

In New Zealand, the second edition of C/VM2 (2025) requires vehicle access to a hard-standing from which an unobstructed path leads to within 20 m of the inlets to sprinkler or fire hydrant systems.[10] It also requires the means to deliver water for firefighting to all parts of the building.[10] That water comes either from a pumping appliance parked so that every point in the building lies within 75 m, about three hose lengths, or from an internal hydrant installed to NZS 4510.[11] For intermediate floors, a commentary in the same document accepts a hydrant in a safe path within 40 m, about two hose lengths, of the most remote point.[21]

In the Netherlands, guidance on indoor car parks advises placing a dry riser where the fire service enters, because it shortens the hose to be laid out and so the attack time.[3] It recommends a smoke-free stairwell as an attack route to underground storeys, with a connection point for a dry riser, and notes that most car parks have such a stairwell.[12] The International Fire Engineering Guidelines (2005) list the location of hydrants and other fire service facilities, and the amount of firefighting water available, among the factors that govern the efficiency of fire service intervention.[22]

The World Trade Center, 2001

The towers of the World Trade Center show how a large riser system works and how it can fail. Each tower had three standpipe risers per zone, one in each exit stair, with Class III hose stations on every floor.[8][17] In the initial operating mode, water came by gravity from 5,000 US gal (about 19,000 L) tanks placed above each zone, enough for 500 US gal/min for about 10 minutes.[23][24] During that time, staff were expected to start manual 750 US gal/min fire pumps on the 7th, 41st and 75th floors, which drew on the city fire water supply.[8][24] Fourteen fire department connection stations at street level allowed the fire service to add water and pressure.[8]

On 11 September 2001 the aircraft impacts damaged the stairwells around the risers. In WTC 1, NIST concluded that two of the three standpipe risers were probably lost and the third possibly, so the standpipe water supply would have been lost after a limited time.[25][26] Isolation valves on the 88th and 99th floors could in principle have allowed a partial restoration, but the status of the systems was unknown.[26] The fire service's first size-up found the standpipe systems inoperable at the impact zones.[27] Restoring them would have required building staff to close an isolation valve between the towers and to go up with firefighters to seal the broken pipes and run the pumps.[27]

The scale of the task shows why risers matter. NIST estimated that about 4,700 L/min would be needed to extinguish half a floor of a tower, using about five 63.5 mm hose lines, and about 9,400 L/min and ten lines for a whole floor.[27] Such an operation would involve at least 15 to 20 engine companies, and if the building's pumps were not working, special high-rise pumpers would have to feed the standpipes.[27] Fire departments also rely on lifts, such as a firefighting lift, to carry personnel and equipment up tall buildings, and lifts are often an integral part of high-rise firefighting.[27] In 7 World Trade Center later that day, a chief officer opened a standpipe on the 4th floor and found no water.[28]

Limitations

No performance records are kept for standpipe and hose systems, so their reliability cannot be quantified from the sources.[2] They are not an alternative to automatic sprinklers.[2] A riser only supplies water if its source works. In the World Trade Center towers the extra water depended on manually started pumps, whereas NFPA 14 requires an automatic supplementary supply as current best practice.[8][29] A dry riser depends entirely on the fire service reaching and charging its inlet.[2]

Because risers run through the stairs, damage that affects the stair cores can disable several risers at once, as it did in 2001.[25] The NIST emergency response report lists the distance between stairwells containing standpipes, and redundant standpipe and water supply systems, among the difficulties seen in the responses of 1993 and 2001.[30] Sectional isolation valves, such as those on floors 88 and 99 of the World Trade Center risers, offer a way to shut off a damaged section and restore part of a system.[24][26]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Hopkins et al. (2005), p. 19.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 Hopkins et al. (2005), p. 21.
  3. ↑ 3.0 3.1 Van der Graaf et al. (2021/2023), p. 66.
  4. ↑ Lawson and Vettori (2005), p. 76.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Titus (2002), p. 4-45.
  6. ↑ 6.0 6.1 6.2 Hopkins et al. (2005), p. 38.
  7. ↑ Hopkins et al. (2005), p. 35.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Hopkins et al. (2005), p. 10.
  9. ↑ 9.0 9.1 9.2 Hopkins et al. (2005), p. 40.
  10. ↑ 10.0 10.1 10.2 Ministry of Business, Innovation and Employment (2025b), p. 53.
  11. ↑ 11.0 11.1 Ministry of Business, Innovation and Employment (2025b), p. 54.
  12. ↑ 12.0 12.1 Van der Graaf et al. (2021/2023), p. 51.
  13. ↑ 13.0 13.1 13.2 Razza and Grill (2005c), p. 46.
  14. ↑ 14.00 14.01 14.02 14.03 14.04 14.05 14.06 14.07 14.08 14.09 14.10 Hopkins et al. (2005), p. 20.
  15. ↑ Hopkins et al. (2005), p. 34.
  16. ↑ 16.0 16.1 16.2 Razza and Grill (2005c), p. 48.
  17. ↑ 17.0 17.1 Hopkins et al. (2005), p. 37.
  18. ↑ 18.0 18.1 18.2 Lawson and Vettori (2005), Appendix I.
  19. ↑ 19.0 19.1 Razza and Grill (2005c), p. 47.
  20. ↑ Razza and Grill (2005c), p. 49.
  21. ↑ Ministry of Business, Innovation and Employment (2025b), p. 55.
  22. ↑ National Research Council of Canada et al. (2005), p. 1.9-5.
  23. ↑ Hopkins et al. (2005), p. 30.
  24. ↑ 24.0 24.1 24.2 Hopkins et al. (2005), p. 39.
  25. ↑ 25.0 25.1 Evans et al. (2005), p. 37.
  26. ↑ 26.0 26.1 26.2 Evans et al. (2005), p. 38.
  27. ↑ 27.0 27.1 27.2 27.3 27.4 Lawson and Vettori (2005), p. 77.
  28. ↑ Lawson and Vettori (2005), p. 110.
  29. ↑ Evans et al. (2005), p. xxxi.
  30. ↑ Lawson and Vettori (2005), p. l.

References

  • 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.
  • Hopkins, M., Schoenrock, J., & Budnick, E. (2005). Fire suppression systems (NIST NCSTAR 1-4B). National Institute of Standards and Technology.
  • Lawson, J. R., & Vettori, R. L. (2005). The emergency response operations (NIST NCSTAR 1-8). 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.
  • Razza, J. C., & Grill, R. A. (2005c). Comparison of the 1968 and current (2003) New York City Building Code provisions (NIST NCSTAR 1-1F). National Institute of Standards and Technology.
  • Titus, J. J. (2002). Hydraulics. 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. 4-44–4-71). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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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