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FSE wiki: Write out abbreviations at first use (NFPA, NIST, NRC, WTC, AFFF, PMMA) (written by Claude Opus 5.5)
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A '''dry pipe sprinkler system''' is an automatic [[sprinkler system]] whose pipes contain compressed air or nitrogen instead of water, so that it can be installed where water in the pipes would freeze.<ref name="FL-72">Fleming (2002), p. 4-72.</ref> A special dry pipe valve holds the water back. The gas pressure in the pipes keeps this valve closed until a sprinkler opens in a fire.<ref name="FL-72" /> The escaping gas lowers the pressure, the valve opens, and water flows through the pipes to the sprinklers that have operated.<ref name="FL-72" /><ref name="IS-10-2">Iqbal and Salley (2004), p. 10-2.</ref>
A '''dry pipe sprinkler system''' is an automatic [[sprinkler system]] whose pipes contain compressed air or nitrogen instead of water, so that it can be installed where water in the pipes would freeze.<ref name="FL-72">Fleming (2002), p. 4-72.</ref> A special dry pipe valve holds the water back. The gas pressure in the pipes keeps this valve closed until a sprinkler opens in a fire.<ref name="FL-72" /> The escaping gas lowers the pressure, the valve opens, and water flows through the pipes to the sprinklers that have operated.<ref name="FL-72" /><ref name="IS-10-2">Iqbal and Salley (2004), p. 10-2.</ref>


The dry pipe system is one of the four basic types of sprinkler system, next to the [[Wet pipe sprinkler system|wet pipe system]], the preaction system and the [[deluge system]].<ref name="FL-72" /> Like a wet pipe system, it uses closed automatic sprinklers that open one by one in response to heat.<ref name="FL-72" /> The price for frost protection is a delay. Before water reaches a fire, the gas must first escape and the water must travel through empty pipes, a process that may take up to 60 seconds at the most remote point of the system.<ref name="HO-15">Hopkins et al. (2005), p. 15.</ref> In the United States, NFPA 13 compensates for this delay by requiring a design area 30 % larger than for a wet pipe system.<ref name="HO-15" /> Dry pipe systems are also more expensive to install and maintain, and they are therefore used only where they are really needed, such as in unheated loading docks, cold stores and attics.<ref name="NO-F59">Nolan (2000), as cited in Iqbal and Salley (2004), p. F-59.</ref><ref name="HO-14">Hopkins et al. (2005), p. 14.</ref>
The dry pipe system is one of the four basic types of sprinkler system, next to the [[Wet pipe sprinkler system|wet pipe system]], the preaction system and the [[deluge system]].<ref name="FL-72" /> Like a wet pipe system, it uses closed automatic sprinklers that open one by one in response to heat.<ref name="FL-72" /> The price for frost protection is a delay. Before water reaches a fire, the gas must first escape and the water must travel through empty pipes, a process that may take up to 60 seconds at the most remote point of the system.<ref name="HO-15">Hopkins et al. (2005), p. 15.</ref> In the United States, NFPA 13, the sprinkler standard of the National Fire Protection Association (NFPA), compensates for this delay by requiring a design area 30 % larger than for a wet pipe system.<ref name="HO-15" /> Dry pipe systems are also more expensive to install and maintain, and they are therefore used only where they are really needed, such as in unheated loading docks, cold stores and attics.<ref name="NO-F59">Nolan (2000), as cited in Iqbal and Salley (2004), p. F-59.</ref><ref name="HO-14">Hopkins et al. (2005), p. 14.</ref>


== How it works ==
== How it works ==
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When the heat of a fire opens a [[sprinkler head]], the air escapes through the open sprinkler and the pressure in the pipes falls.<ref name="HO-15" /> At a certain point the air pressure no longer outweighs the water pressure below the valve. This point is called the trip pressure, and it is typically 20 psi (138 kPa).<ref name="HO-15" /> The valve then opens, and the pressure of the water supply drives water into the pipes and out of the sprinklers that have opened.<ref name="HO-15" /><ref name="IS-10-2" /> Sprinklers that the fire has not heated stay closed, just as in a wet pipe system.<ref name="NO-F59" />
When the heat of a fire opens a [[sprinkler head]], the air escapes through the open sprinkler and the pressure in the pipes falls.<ref name="HO-15" /> At a certain point the air pressure no longer outweighs the water pressure below the valve. This point is called the trip pressure, and it is typically 20 psi (138 kPa).<ref name="HO-15" /> The valve then opens, and the pressure of the water supply drives water into the pipes and out of the sprinklers that have opened.<ref name="HO-15" /><ref name="IS-10-2" /> Sprinklers that the fire has not heated stay closed, just as in a wet pipe system.<ref name="NO-F59" />


A dry pipe valve can hold back water at a higher pressure with air at a lower pressure. NIST describes the principle of the dry pipe valve used in automatic dry standpipes. The valve contains a differential clapper. The lower air pressure on the system side acts on a larger surface than the higher water pressure on the supply side.<ref name="HO-21">Hopkins et al. (2005), p. 21.</ref> When air is released, the force of the air falls until the water pressure can overcome it, and water then fills the system.<ref name="HO-21" />
A dry pipe valve can hold back water at a higher pressure with air at a lower pressure. The National Institute of Standards and Technology (NIST) describes the principle of the dry pipe valve used in automatic dry standpipes. The valve contains a differential clapper. The lower air pressure on the system side acts on a larger surface than the higher water pressure on the supply side.<ref name="HO-21">Hopkins et al. (2005), p. 21.</ref> When air is released, the force of the air falls until the water pressure can overcome it, and water then fills the system.<ref name="HO-21" />


Each automatic sprinkler acts both as a heat detector and as a water distribution nozzle.<ref name="FL-85">Fleming (2002), p. 4-85.</ref> The most common sprinklers open when a soldered metal element melts or a liquid-filled glass bulb breaks at a set temperature.<ref name="IS-10-3">Iqbal and Salley (2004), p. 10-3.</ref> In a dry pipe system the sprinkler therefore still decides where water is discharged; the dry pipe valve only decides when water enters the pipes.<ref name="FL-72" />
Each automatic sprinkler acts both as a heat detector and as a water distribution nozzle.<ref name="FL-85">Fleming (2002), p. 4-85.</ref> The most common sprinklers open when a soldered metal element melts or a liquid-filled glass bulb breaks at a set temperature.<ref name="IS-10-3">Iqbal and Salley (2004), p. 10-3.</ref> In a dry pipe system the sprinkler therefore still decides where water is discharged; the dry pipe valve only decides when water enters the pipes.<ref name="FL-72" />

Revision as of 23:58, 1 November 2026

A dry pipe sprinkler system is an automatic sprinkler system whose pipes contain compressed air or nitrogen instead of water, so that it can be installed where water in the pipes would freeze.[1] A special dry pipe valve holds the water back. The gas pressure in the pipes keeps this valve closed until a sprinkler opens in a fire.[1] The escaping gas lowers the pressure, the valve opens, and water flows through the pipes to the sprinklers that have operated.[1][2]

The dry pipe system is one of the four basic types of sprinkler system, next to the wet pipe system, the preaction system and the deluge system.[1] Like a wet pipe system, it uses closed automatic sprinklers that open one by one in response to heat.[1] The price for frost protection is a delay. Before water reaches a fire, the gas must first escape and the water must travel through empty pipes, a process that may take up to 60 seconds at the most remote point of the system.[3] In the United States, NFPA 13, the sprinkler standard of the National Fire Protection Association (NFPA), compensates for this delay by requiring a design area 30 % larger than for a wet pipe system.[3] Dry pipe systems are also more expensive to install and maintain, and they are therefore used only where they are really needed, such as in unheated loading docks, cold stores and attics.[4][5]

How it works

The overhead pipes of a dry pipe system are normally filled with pressurised air, or sometimes nitrogen.[3][2] The system uses the same closed automatic sprinklers as a wet pipe system. In the description of the US Nuclear Regulatory Commission (NRC), compressed air replaces the water of a wet pipe system. A dry pipe valve replaces its alarm check valve.[2] The compressed air holds the dry pipe valve shut and so keeps water out of the system.[2]

When the heat of a fire opens a sprinkler head, the air escapes through the open sprinkler and the pressure in the pipes falls.[3] At a certain point the air pressure no longer outweighs the water pressure below the valve. This point is called the trip pressure, and it is typically 20 psi (138 kPa).[3] The valve then opens, and the pressure of the water supply drives water into the pipes and out of the sprinklers that have opened.[3][2] Sprinklers that the fire has not heated stay closed, just as in a wet pipe system.[4]

A dry pipe valve can hold back water at a higher pressure with air at a lower pressure. The National Institute of Standards and Technology (NIST) describes the principle of the dry pipe valve used in automatic dry standpipes. The valve contains a differential clapper. The lower air pressure on the system side acts on a larger surface than the higher water pressure on the supply side.[6] When air is released, the force of the air falls until the water pressure can overcome it, and water then fills the system.[6]

Each automatic sprinkler acts both as a heat detector and as a water distribution nozzle.[7] The most common sprinklers open when a soldered metal element melts or a liquid-filled glass bulb breaks at a set temperature.[8] In a dry pipe system the sprinkler therefore still decides where water is discharged; the dry pipe valve only decides when water enters the pipes.[1]

Where dry pipe systems are used

A wet pipe system needs enough heat to keep the water in its pipes from freezing.[5] In the United States, NFPA 13 requires a minimum temperature of 40 °F (4.4 °C) at all times and in all places where wet pipe systems are installed.[5] The NRC guidance therefore calls wet pipe systems unsuitable for automatic fire protection in unheated buildings.[9] Dry pipe systems were developed for parts of buildings where water-filled pipes could freeze. They are usually installed where heating the building, or part of it, is impractical or undesirable.[5]

Typical locations named by NIST are loading docks, cold storage warehouses and attics.[5] Building codes can offer the dry pipe system as one of several options. The New York City building code applied to the World Trade Center required wet pipe systems in all sprinklered areas. In areas subject to freezing, the system had to be protected by insulation, heat tracing or an antifreeze solution, or a dry pipe system had to be provided.[10]

Car parks that are open to the outside air face the same climatic problem. A report by the Netherlands Institute for Public Safety (NIPV) defines an open indoor car park as a naturally ventilated car park whose sides are all open to the outside air.[11] It notes that weather influences make sprinkler systems less suitable for such car parks. Sprinklers nevertheless remain the most obvious way to contain a car park fire and to stop it spreading from one car to the next.[12] In the Netherlands, a consultation on making sprinkler systems mandatory for certain indoor car parks started on 10 July 2021.[13]

Delay in water delivery

The main drawback of a dry pipe system is the delay between the operation of a sprinkler and the discharge of water.[3] Fleming splits the total water delivery time into two parts. The trip time is the time needed for the air pressure to bleed down until the dry valve opens. The transit time is the time the water then needs to flow from the valve to the open sprinkler.[7] The water delivery time starts when the first sprinkler opens.[7]

NFPA 13 limits this delay by limiting the size of the system. As described in 2002, the standard contained no maximum water delivery time if the system volume was no more than 750 US gallons (2,839 L).[7] Larger systems were permitted only if water reached the test point within 60 seconds.[7] The rule of thumb is that no more than 60 seconds should be tolerated, and that systems should be divided into smaller systems where necessary.[7] NIST describes the 60-second limit as a performance-based alternative to the volume limit.[3]

The delivery time is checked in the field with an inspector's test connection. This connection sits at the most remote point of the system from the dry valve and has an orifice that simulates a single sprinkler.[7] The time measured in this test is not a realistic indication of the delay in a real fire, for two reasons. The first sprinkler to open is likely to be closer to the dry valve, which shortens the transit time. If more sprinklers open, the air escapes faster, so that both the trip time and the transit time become shorter.[7]

Researchers at Factory Mutual Research Corporation derived a relation for the trip time. It grows with the dry volume of the system and with the ratio between the initial air pressure and the trip pressure. It falls with a larger flow area of open sprinklers.[7] The relation also contains the air temperature. Transit time is harder to calculate and requires a numerical model in which the system is divided into sections.[7]

The delay adds to the time that the sprinkler itself needs to respond. The International Fire Engineering Guidelines allow suppression to be assumed to start at sprinkler activation only in limited cases. Examples are a low, small enclosure with a relatively small fire that one or a few sprinklers can extinguish.[14]

Design area and water demand

A dry pipe system is expected to open more sprinklers than a wet pipe system, which discharges water immediately after the first sprinkler responds.[3] In a sprinkler design, water is supplied for the sprinklers in a design area, which is chosen together with a design density (the depth of water applied per minute) from the hazard of the occupancy.[15] NFPA 13 contains special changes to the design area for dry systems.[15] According to NIST, it requires the design area of a dry pipe system to be 30 % larger than that of a wet pipe system, with the same density:[3]

Adry=1.3×Awet

Symbols
Symbol Description Unit
Adry design area of the dry pipe system m²
Awet design area required for a wet pipe system with the same hazard and density m²

The factor applies to the design area only. The density stays the same, so the required water flow rises in proportion to the area. The minimum flow of each sprinkler is still its area of coverage multiplied by the density.[16] The factor is a design rule of NFPA 13 in the United States; it is not derived from a calculation of the actual delay.[3]

Worked example

The design criteria for World Trade Center Building 7 (WTC 7), designed in the mid-1980s, show the rule in practice.[17] For Ordinary Hazard Group 1 areas with wet pipe systems, the criteria gave a density of 0.16 gpm/ft² (6.5 mm/min) over 1,500 ft² (139 m²).[17] The loading berth had a dry pipe system with the same hazard and density, designed over 1,950 ft² (181 m²).[18] The rule reproduces this value:

Adry=1.3×139 m2≈181 m2

At 6.5 mm/min, the minimum flow over the design area rises from about 910 L/min for the wet system to about 1,190 L/min for the dry system. The loading dock had a layout with about 12 m² (126 ft²) per sprinkler.[19] At that spacing, the design area grows from 12 to 16 sprinklers.

The dry system also loses more pressure in its pipes. For friction loss calculations with the Hazen–Williams formula, steel pipe in dry and preaction systems is given a coefficient C of 100, against 120 for wet and deluge systems.[20] A lower C means a larger friction loss for the same flow. The values are chosen to simulate the expected interior roughness of aged pipe.[20]

Related system types

Comparison of sprinkler system types[1][4]
Type Contents of the pipes What starts the water flow Main reason for use
Wet pipe water under pressure heat opens a sprinkler most common type; heated buildings
Dry pipe air or nitrogen under pressure an opened sprinkler releases the air, the dry pipe valve trips risk of freezing
Preaction air, under pressure or not detection opens the valve; heat opens sprinklers concern for accidental discharge
Deluge no water; open nozzles detection opens the deluge valve rapidly spreading, high hazard fires

The preaction system was designed in part to overcome the weaknesses of the dry pipe system. It uses automatic sprinklers in pipes containing air, which may or may not be under pressure, together with a separate detection system in the same area.[4] The detectors open the valve, so that the pipes fill with water before a sprinkler opens. Preaction systems are meant to counteract the delay of dry pipe systems and to prevent damage from a broken pipe or sprinkler head.[4]

A combined dry pipe and preaction system joins both principles. Its pipes contain air under pressure, and a detection system opens the dry pipe valves without a loss of air pressure. At the same time it opens air exhaust valves at the end of the feed main, so that the system normally fills with water before any sprinkler opens.[4] In a fire risk analysis for United States nuclear power plants, the detection-suppression event tree of NUREG/CR-6850 treats a preaction or dry pipe system in one example case. It notes that a preaction system needs two signals to succeed: an automatic detection signal and the opening of the sprinkler itself.[21]

Limitations

Dry pipe systems operate more slowly than wet pipe systems and are more expensive to install and maintain.[4] They are therefore used only where there is an absolute necessity, such as a risk of freezing.[4] The NRC report names two reasons to restrict them to low-temperature areas. The first is the delay between the release of the compressed air and the arrival of water. The second is internal corrosion and tuberculation of the pipes caused by alternating wet and dry periods.[2] Corrosion or tuberculation in the sprinkler piping is also listed among the general causes of sprinkler failure.[2]

Sprinkler systems in general fail for reasons that also apply to dry pipe systems. The NRC report lists a closed water supply valve, an empty water tank, a fire pump that does not start, corroded or painted sprinkler heads and an inadequate water supply.[2] Most sprinkler systems in office buildings are designed to control a fire, and full extinguishment may require the fire service.[3][22] In a dry pipe system, the delay means that more sprinklers are expected to open before control is reached.[3]

Examples

In the World Trade Center towers and WTC 7, wet pipe systems protected the main office and support spaces. Dry pipe and preaction systems were used on a limited basis for computer rooms and other special hazards, in less than 10 % of the floor area.[5] In WTC 7, a dry pipe system protected two 12,000 gal (about 45,000 L) fuel oil storage tanks beneath the loading berths.[23] The fuel oil pump rooms also had dry pipe systems, designed as Ordinary Hazard Group 3 with 0.21 gpm/ft² (8.6 mm/min) over the entire pump room.[18]

A worked example in the SFPE Handbook shows a dry pipe system combined with another agent. A warehouse with crude rubber in piles has a dry pipe system with a maximum discharge of 0.2 gpm/ft² (8.1 mm/min) over 2,000 ft² (186 m²).[24] This density is not considered adequate for the hazard, so a fixed high-expansion foam system supplements the sprinklers.[24] The foam system is arranged to start when the dry pipe valve trips because a sprinkler head has operated.[25]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Fleming (2002), p. 4-72.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Iqbal and Salley (2004), p. 10-2.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 Hopkins et al. (2005), p. 15.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Nolan (2000), as cited in Iqbal and Salley (2004), p. F-59.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Hopkins et al. (2005), p. 14.
  6. ↑ 6.0 6.1 Hopkins et al. (2005), p. 21.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 Fleming (2002), p. 4-85.
  8. ↑ Iqbal and Salley (2004), p. 10-3.
  9. ↑ Iqbal and Salley (2004), p. 10-1.
  10. ↑ Grill and Johnson (2005b), p. 29.
  11. ↑ Van der Graaf et al. (2021/2023), p. 48.
  12. ↑ Van der Graaf et al. (2021/2023), p. 49.
  13. ↑ Van der Graaf et al. (2021/2023), p. 6.
  14. ↑ National Research Council of Canada et al. (2005), p. 2.7-7.
  15. ↑ 15.0 15.1 Fleming (2002), p. 4-74.
  16. ↑ Fleming (2002), p. 4-75.
  17. ↑ 17.0 17.1 Hopkins et al. (2005), p. 64.
  18. ↑ 18.0 18.1 Hopkins et al. (2005), p. 65.
  19. ↑ McAllister et al. (2008), p. 76.
  20. ↑ 20.0 20.1 Fleming (2002), p. 4-76.
  21. ↑ Nowlen et al. (2005b), p. P-5.
  22. ↑ Hopkins et al. (2005), p. 16.
  23. ↑ Hopkins et al. (2005), p. 128.
  24. ↑ 24.0 24.1 Scheffey and Hickey (2002), p. 4-141.
  25. ↑ Scheffey and Hickey (2002), p. 4-143.

References

  • 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.
  • McAllister, T. P., Gann, R. G., Averill, J. D., Gross, J. L., Grosshandler, W. L., Lawson, J. R., McGrattan, K. B., Pitts, W. M., Prasad, K. R., Sadek, F. H., & Nelson, H. E. (2008). Structural fire response and probable collapse sequence of World Trade Center Building 7 (NIST NCSTAR 1-9, Vols. 1–2). National Institute of Standards and Technology.
  • 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., & Hickey, H. E. (2002). Foam 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. 4-123–4-148). 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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