Sprinkler skipping - Revision history

Jan: Remove links to pages that do not exist; retain displayed text (wiki-wide link audit).

2026-11-02T21:34:57Z

Remove links to pages that do not exist; retain displayed text (wiki-wide link audit).

← Older revision		Revision as of 22:34, 2 November 2026
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	Many factors influence how sprinklers respond. They include the temperature rating, spacing and depth of the sprinklers below the ceiling, and the ceiling height.<ref name="N-10-2">Iqbal and Salley (2004), p. 10-2.</ref> Openings, joists and air currents at ceiling level also change the flow of hot gases.<ref name="N-10-2" /> Fleming adds that heat conduction to the sprinkler frame and the distance of the element below the ceiling have a significant effect on response.<ref name="F-85" /> In one of his examples, a computer model shows that quick response sprinklers 0.46 m below a basement ceiling keep a sensitivity comparable to standard sprinklers at the usual maximum of 0.30 m.<ref name="F-85" /> For the chosen fire, the standard sprinkler operates after 200 s and the quick response sprinkler after 172 s.<ref name="F-85" />		Many factors influence how sprinklers respond. They include the temperature rating, spacing and depth of the sprinklers below the ceiling, and the ceiling height.<ref name="N-10-2">Iqbal and Salley (2004), p. 10-2.</ref> Openings, joists and air currents at ceiling level also change the flow of hot gases.<ref name="N-10-2" /> Fleming adds that heat conduction to the sprinkler frame and the distance of the element below the ceiling have a significant effect on response.<ref name="F-85" /> In one of his examples, a computer model shows that quick response sprinklers 0.46 m below a basement ceiling keep a sensitivity comparable to standard sprinklers at the usual maximum of 0.30 m.<ref name="F-85" /> For the chosen fire, the standard sprinkler operates after 200 s and the quick response sprinkler after 172 s.<ref name="F-85" />

	An [[ESFR sprinkler]] (early suppression fast response sprinkler) is designed to extinguish high-challenge fires, not only to control them.<ref name="N-10-3">Iqbal and Salley (2004), p. 10-3.</ref> It combines a fast response with large droplets and a high droplet velocity.<ref name="N-10-3" /> Its element has an RTI of 50 (m·s)<sup>1/2</sup> or less, against 80 (m·s)<sup>1/2</sup> or more for standard sprinklers.<ref name="N-F-21">Iqbal and Salley (2004), p. F-21.</ref> Large drop ESFR sprinklers protect high-piled storage in wet pipe systems and were developed by Factory Mutual Research Corporation in the late 1970s and early 1980s.<ref name="N-F-21" /> Their design criteria come from full-scale testing, and very strict design and installation rules apply to them.<ref name="F-73">Fleming (2002), p. 4-73.</ref><ref name="N-10-3" />		An ESFR sprinkler (early suppression fast response sprinkler) is designed to extinguish high-challenge fires, not only to control them.<ref name="N-10-3">Iqbal and Salley (2004), p. 10-3.</ref> It combines a fast response with large droplets and a high droplet velocity.<ref name="N-10-3" /> Its element has an RTI of 50 (m·s)<sup>1/2</sup> or less, against 80 (m·s)<sup>1/2</sup> or more for standard sprinklers.<ref name="N-F-21">Iqbal and Salley (2004), p. F-21.</ref> Large drop ESFR sprinklers protect high-piled storage in wet pipe systems and were developed by Factory Mutual Research Corporation in the late 1970s and early 1980s.<ref name="N-F-21" /> Their design criteria come from full-scale testing, and very strict design and installation rules apply to them.<ref name="F-73">Fleming (2002), p. 4-73.</ref><ref name="N-10-3" />

	== Skipping in performance-based analysis ==		== Skipping in performance-based analysis ==

Jan: Add 4 watermarked SVG illustrations from the Expert visual plan

2026-11-02T16:34:22Z

Add 4 watermarked SVG illustrations from the Expert visual plan

← Older revision		Revision as of 17:34, 2 November 2026
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	The droplets of the first sprinklers to open meet the vigorous upward flow of the fire plume.<ref name="C-239" /> Droplets that leave a sprinkler slowly are accelerated by the plume until they move with it, and the larger of them follow the plume well before they evaporate.<ref name="C-239" /> The plume then turns into the ceiling jet, which carries random numbers of droplets past the sprinklers further away.<ref name="C-239" /> This is the same flow that should heat those sprinklers until their links fuse or their bulbs break.<ref name="C-239" />		The droplets of the first sprinklers to open meet the vigorous upward flow of the fire plume.<ref name="C-239" /> Droplets that leave a sprinkler slowly are accelerated by the plume until they move with it, and the larger of them follow the plume well before they evaporate.<ref name="C-239" /> The plume then turns into the ceiling jet, which carries random numbers of droplets past the sprinklers further away.<ref name="C-239" /> This is the same flow that should heat those sprinklers until their links fuse or their bulbs break.<ref name="C-239" />
			[[File:fse-sprinkler-skipping-v1.svg\|thumb\|upright=1.5\|alt=Cross-section of a room: a fire plume rises under an operating sprinkler; the ceiling jet carries droplets to a second sprinkler that stays closed.\|How skipping arises: small droplets from the first operating sprinkler are carried up by the plume and along the ceiling jet, where they strike and cool the link of the next sprinkler. Based on Cooper (2002b), p. 3-239, and Iqbal and Salley (2004), p. 10-11.]]

	Iqbal and Salley explain why small droplets behave in this way. Water from an operating sprinkler must penetrate the rising plume to reach the burning fuel, so its droplets need enough size and velocity.<ref name="N-10-11">Iqbal and Salley (2004), p. 10-11.</ref> Droplets that are too small evaporate or are carried upward by the plume.<ref name="N-10-11" /> For fast-growing fires with a high heat release rate, large drop sprinklers are therefore sometimes needed.<ref name="N-10-11" />		Iqbal and Salley explain why small droplets behave in this way. Water from an operating sprinkler must penetrate the rising plume to reach the burning fuel, so its droplets need enough size and velocity.<ref name="N-10-11">Iqbal and Salley (2004), p. 10-11.</ref> Droplets that are too small evaporate or are carried upward by the plume.<ref name="N-10-11" /> For fast-growing fires with a high heat release rate, large drop sprinklers are therefore sometimes needed.<ref name="N-10-11" />
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	=== Worked example, continued ===		=== Worked example, continued ===
	At the jet velocity of 1.35 m/s from the example above, the link stops heating at a liquid volume fraction of only 3.9 × 10<sup>−6</sup>.<ref name="M-107" /><ref name="N-10-17" /> That is about 4 cm³ of water in each cubic metre of gas. For an assumed volume fraction of 1 × 10<sup>−6</sup>, the heating slows by 26 percent and operation is delayed from 2.9 to 3.9 min. For an assumed 2 × 10<sup>−6</sup>, the heating falls by 52 percent, and the sprinkler operates only after about 6.0 min.		At the jet velocity of 1.35 m/s from the example above, the link stops heating at a liquid volume fraction of only 3.9 × 10<sup>−6</sup>.<ref name="M-107" /><ref name="N-10-17" /> That is about 4 cm³ of water in each cubic metre of gas. For an assumed volume fraction of 1 × 10<sup>−6</sup>, the heating slows by 26 percent and operation is delayed from 2.9 to 3.9 min. For an assumed 2 × 10<sup>−6</sup>, the heating falls by 52 percent, and the sprinkler operates only after about 6.0 min.
			[[File:fse-sprinkler-skipping-v2.svg\|thumb\|upright=1.5\|alt=Line chart: link temperature rising towards 88 °C; it reaches 74 °C at 174 s without droplets, 234 s and 361 s with droplets.\|Calculated temperature of a standard response sprinkler link in a 1,000 kW fire, without droplets and with liquid volume fractions of 1 and 2 millionths in the ceiling jet. Based on Iqbal and Salley (2004), pp. 10-15 to 10-18, and McGrattan et al. (2026), p. 107.]]

	The heating term and the droplet term are both divided by the RTI, so it follows from the equation that a quick response link is slowed by the same fraction. At 1 × 10<sup>−6</sup> its operation moves from about 45 s to 61 s. The two volume fractions are assumed values chosen for illustration, not measurements, so the results show how sensitive a link is to wetting rather than what happens at a real sprinkler. The volume of water that reaches a particular link remains unknown in practice.<ref name="C-239" /> The FDS model also ignores thermal radiation to the link.<ref name="M-107" />		The heating term and the droplet term are both divided by the RTI, so it follows from the equation that a quick response link is slowed by the same fraction. At 1 × 10<sup>−6</sup> its operation moves from about 45 s to 61 s. The two volume fractions are assumed values chosen for illustration, not measurements, so the results show how sensitive a link is to wetting rather than what happens at a real sprinkler. The volume of water that reaches a particular link remains unknown in practice.<ref name="C-239" /> The FDS model also ignores thermal radiation to the link.<ref name="M-107" />
			[[File:fse-sprinkler-skipping-v3.svg\|thumb\|upright=1.5\|alt=Line chart: activation time against liquid volume fraction, rising steeply towards 3.9 millionths for standard and quick response links.\|Calculated activation time against the volume fraction of liquid water in the ceiling jet, for a standard (RTI 130) and a quick response (RTI 34) link in the 1,000 kW example. Based on Iqbal and Salley (2004), pp. 10-15 to 10-18, and McGrattan et al. (2026), p. 107.]]

	== Skipping in sprinkler design ==		== Skipping in sprinkler design ==
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	A more reliable alternative is to open all vents of the fire compartment together, triggered by smoke detectors or by a signal that a sprinkler has discharged.<ref name="C-239" /> One strategy opens every vent of the compartment immediately after the first sprinkler discharges.<ref name="C-239" /> A refined strategy opens at once all vents beyond the first or second ring of sprinklers around the first one.<ref name="C-239" /> The other vents then open when the sprinklers have controlled the fire, and extra vent area compensates for the vents that stay closed at first.<ref name="C-239" />		A more reliable alternative is to open all vents of the fire compartment together, triggered by smoke detectors or by a signal that a sprinkler has discharged.<ref name="C-239" /> One strategy opens every vent of the compartment immediately after the first sprinkler discharges.<ref name="C-239" /> A refined strategy opens at once all vents beyond the first or second ring of sprinklers around the first one.<ref name="C-239" /> The other vents then open when the sprinklers have controlled the fire, and extra vent area compensates for the vents that stay closed at first.<ref name="C-239" />
			[[File:fse-sprinkler-skipping-v4.svg\|thumb\|upright=1.5\|alt=Plan of a roof with a sprinkler grid and vents; vents beyond the second ring of sprinklers around the first sprinkler are marked open.\|Vent ganging after the first sprinkler discharge: vents beyond the second ring of sprinklers open at once, the vents closer to the fire after the sprinklers have controlled it. Based on Cooper (2002b), p. 3-239.]]

	Sprinkler sprays also stir the smoke layer, and this mixing can lead to smoke logging, in which the smoke is mixed through the whole space.<ref name="C-238">Cooper (2002b), p. 3-238.</ref> Cooper identifies six configurations of spray and smoke layer, of which four favour a combined design and two do not.<ref name="C-238" /> Which one occurs depends mainly on the thickness and temperature of the layer and on the momentum, spread angle and droplet size of the spray.<ref name="C-238" /> Shortly after rapid-response sprinklers discharge, the layer is still thin and not too hot, and smoke logging is then never observed.<ref name="C-238" /><ref name="C-239" />		Sprinkler sprays also stir the smoke layer, and this mixing can lead to smoke logging, in which the smoke is mixed through the whole space.<ref name="C-238">Cooper (2002b), p. 3-238.</ref> Cooper identifies six configurations of spray and smoke layer, of which four favour a combined design and two do not.<ref name="C-238" /> Which one occurs depends mainly on the thickness and temperature of the layer and on the momentum, spread angle and droplet size of the spray.<ref name="C-238" /> Shortly after rapid-response sprinklers discharge, the layer is still thin and not too hot, and smoke logging is then never observed.<ref name="C-238" /><ref name="C-239" />

Mark: FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5)

2026-11-01T23:01:43Z

FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5)

Mark: New FSE wiki article: Sprinkler skipping (written by Claude Opus 5.5)

2026-11-01T22:24:17Z

New FSE wiki article: Sprinkler skipping (written by Claude Opus 5.5)

New page

'''Sprinkler skipping''' is an unpredictable delay in the operation of sprinklers near a fire, caused by water droplets from sprinklers that have already opened.<ref name="C-239">Cooper (2002b), p. 3-239.</ref> The fire plume carries some of these droplets up to the ceiling, where the hot ceiling jet takes them past the sprinklers that should open next.<ref name="C-239" /> Droplets that strike the heat-sensitive link or glass bulb of such a sprinkler cool it by evaporation, so it heats more slowly and discharges later.<ref name="C-239" /> Smoke vents that open on fusible links suffer from the same effect, which Cooper calls vent skipping.<ref name="C-239" />

Conventional sprinkler design accounts for skipping only empirically, through the experimental basis of its design rules.<ref name="C-239" /> A sprinkler system has many heads and only a few are likely to be skipped, which makes such systems naturally tolerant of the effect.<ref name="C-239" /> Vent systems have far fewer units, so skipping matters more where sprinklers and vents are combined.<ref name="C-239" /> The Fire Dynamics Simulator (FDS) adds a droplet-cooling term to the equation that predicts when a sprinkler link reaches its operating temperature.<ref name="M-107">McGrattan et al. (2026), p. 107.</ref>

== What skipping is ==
In the ''SFPE Handbook of Fire Protection Engineering'', Cooper defines sprinkler skipping as the unpredictable and harmful delay in sprinkler discharge that results when droplets carried by the ceiling jet strike a link or bulb.<ref name="C-239" /> The term therefore describes a delay in operation, not a defect of the sprinkler itself.<ref name="C-239" /> A sprinkler spray consists of droplets of many sizes and velocities, and it varies even between nominally identical [[sprinkler head]]s.<ref name="C-239" /> A full description of such a spray is beyond current technology.<ref name="C-239" />

The droplets of the first sprinklers to open meet the vigorous upward flow of the fire plume.<ref name="C-239" /> Droplets that leave a sprinkler slowly are accelerated by the plume until they move with it, and the larger of them follow the plume well before they evaporate.<ref name="C-239" /> The plume then turns into the ceiling jet, which carries random numbers of droplets past the sprinklers further away.<ref name="C-239" /> This is the same flow that should heat those sprinklers until their links fuse or their bulbs break.<ref name="C-239" />

Iqbal and Salley explain why small droplets behave in this way. Water from an operating sprinkler must penetrate the rising plume to reach the burning fuel, so its droplets need enough size and velocity.<ref name="N-10-11">Iqbal and Salley (2004), p. 10-11.</ref> Droplets that are too small evaporate or are carried upward by the plume.<ref name="N-10-11" /> For fast-growing fires with a high heat release rate, large drop sprinklers are therefore sometimes needed.<ref name="N-10-11" />

== How a sprinkler is triggered ==
An automatic sprinkler serves two functions: it is a heat detector as well as a nozzle that distributes water.<ref name="F-85">Fleming (2002), p. 4-85.</ref> Experiments show that convective heating by hot gases is the most important factor in its activation.<ref name="N-10-9">Iqbal and Salley (2004), p. 10-9.</ref> The hot gases rise from the fire as a plume, strike the ceiling and spread out as a [[ceiling jet]], a relatively fast and shallow flow driven by buoyancy.<ref name="N-10-9" /> This jet is about 5 to 12 percent as thick as the height of the ceiling above the fire, with its highest temperature and velocity close to the ceiling.<ref name="N-10-9" />

The sensitivity of a sprinkler is expressed by its [[response time index]] (RTI), a concept developed by Factory Mutual Research Corporation.<ref name="N-10-7">Iqbal and Salley (2004), p. 10-7.</ref> The index is measured in plunge tests with a gas flow of constant temperature and velocity, and it assumes that heat conduction to the sprinkler frame is negligible.<ref name="N-10-7" /> It depends on the mass and surface area of the heat-sensitive element, and elements with little mass per unit area respond quickly.<ref name="N-10-7" /> Typical values are 130 (m·s)1/2 for a standard response link and 34 (m·s)1/2 for a quick response link.<ref name="N-10-8">Iqbal and Salley (2004), p. 10-8.</ref> Sprinklers with an ordinary temperature rating operate at 57 to 77 °C.<ref name="N-10-6">Iqbal and Salley (2004), p. 10-6.</ref>

For a steady fire, the time needed to heat the element from room temperature to its operating temperature grows with the RTI and falls with the square root of the gas velocity.<ref name="N-10-9" /> It also grows as the operating temperature approaches the temperature of the ceiling jet.<ref name="N-10-9" /> A low RTI and a hot, fast ceiling jet therefore give early operation.<ref name="N-10-9" />

<math display="block">t_\mathrm{act} = \frac{\mathrm{RTI}}{\sqrt{u_\mathrm{jet}}} \ln \left( \frac{T_\mathrm{jet} - T_a}{T_\mathrm{jet} - T_\mathrm{act}} \right)</math>

{| class="wikitable"
! Symbol !! Description !! Unit
|-
| <math>t_\mathrm{act}</math> || activation time of the sprinkler || s
|-
| <math>\mathrm{RTI}</math> || response time index of the sprinkler || (m·s)1/2
|-
| <math>u_\mathrm{jet}</math> || velocity of the ceiling jet at the sprinkler || m/s
|-
| <math>T_\mathrm{jet}</math> || temperature of the ceiling jet at the sprinkler || °C
|-
| <math>T_a</math> || ambient temperature || °C
|-
| <math>T_\mathrm{act}</math> || operating temperature of the sprinkler || °C
|}

The temperature and velocity of the ceiling jet come from correlations based on large-scale fires of 668 kW to 98,000 kW.<ref name="N-10-10">Iqbal and Salley (2004), p. 10-10.</ref> Separate expressions apply where the plume strikes the ceiling and where the flow has become horizontal further out.<ref name="N-10-10" /> The method assumes a smooth, flat and unconfined ceiling, considers only convective heating and ignores radiation from the flames.<ref name="N-10-12">Iqbal and Salley (2004), p. 10-12.</ref> It does not apply to sprinklers far below the ceiling, because most of the heat then rises past them and they may not activate.<ref name="N-10-12" />

=== Worked example ===
Iqbal and Salley consider a steady fire of 1,000 kW, 3.0 m below a ceiling, with a standard response link sprinkler 3.0 m from the axis of the plume.<ref name="N-10-15">Iqbal and Salley (2004), p. 10-15.</ref> The sprinkler has an RTI of 130 (m·s)1/2 and an operating temperature of 74 °C.<ref name="N-10-15" /> Their spreadsheet uses an ambient temperature of 20 °C and a convective heat release rate of 700 kW for the jet temperature.<ref name="N-10-17">Iqbal and Salley (2004), pp. 10-17 to 10-18.</ref> The ceiling jet at the sprinkler then reaches 88 °C at 1.35 m/s, and the sprinkler operates after about 2.9 min.<ref name="N-10-17" /><ref name="N-10-16">Iqbal and Salley (2004), p. 10-16.</ref> Under the same conditions, a quick response link with an RTI of 34 (m·s)1/2 would operate after about 45 s.

== How water droplets delay operation ==
The Fire Dynamics Simulator estimates the link temperature from an equation by Heskestad and Bill, with an added term for cooling by water droplets from previously activated sprinklers.<ref name="M-107" /> In words, the link gains heat from the hot gas in proportion to the square root of the gas velocity and loses some heat by conduction to its mount. It loses further heat in proportion to the amount of liquid water carried in the gas.<ref name="M-107" /> The constant of the droplet term comes from experiments by DiMarzo and co-workers and is relatively constant for different types of sprinkler.<ref name="M-107" />

<math display="block">\frac{\mathrm{d}T_l}{\mathrm{d}t} = \frac{\sqrt{|u|}}{\mathrm{RTI}} \left( T_g - T_l \right) - \frac{C}{\mathrm{RTI}} \left( T_l - T_m \right) - \frac{C_2}{\mathrm{RTI}} \left( T_g - T_l \right) \beta \, |u|</math>

{| class="wikitable"
! Symbol !! Description !! Unit
|-
| <math>T_l</math> || temperature of the sprinkler link || °C
|-
| <math>T_g</math> || gas temperature near the link || °C
|-
| <math>T_m</math> || temperature of the sprinkler mount (ambient) || °C
|-
| <math>u</math> || gas velocity at the link || m/s
|-
| <math>C</math> || conduction factor (C-factor) of the sprinkler || (m/s)1/2
|-
| <math>\beta</math> || volume fraction of liquid water in the gas || none
|-
| <math>C_2</math> || droplet cooling constant, 2.23 × 105 || (m/s)−1/2
|}

If conduction to the mount is ignored, as in the activation time equation above, the droplet term reduces the heating of the link by the fraction <math>C_2 \beta \sqrt{|u|}</math>. When this fraction reaches one, the link stops heating at all, however hot the gas. The constant is large, so very small amounts of water are enough to slow a link down.<ref name="M-107" />

=== Worked example, continued ===
At the jet velocity of 1.35 m/s from the example above, the link stops heating at a liquid volume fraction of only 3.9 × 10−6.<ref name="M-107" /><ref name="N-10-17" /> That is about 4 cm³ of water in each cubic metre of gas. A volume fraction of 1 × 10−6 slows the heating by 26 percent and delays operation from 2.9 to 3.9 min. At 2 × 10−6 the heating falls by 52 percent, and the sprinkler operates only after about 6.0 min.

The heating term and the droplet term are both divided by the RTI, so in this model a quick response link is slowed by the same fraction. At 1 × 10−6 its operation moves from about 45 s to 61 s. These results show how sensitive a link is to wetting; they are not measurements, and the volume of water reaching a particular link remains unknown in practice.<ref name="C-239" /> The FDS model also ignores thermal radiation to the link.<ref name="M-107" />

== Skipping in sprinkler design ==
Sprinklers are designed to control a fire. If the first sprinkler cannot control it, a second one opens and adds cooling, and the process continues until the fire is under control.<ref name="N-10-1">Iqbal and Salley (2004), p. 10-1.</ref> Design areas usually include 10 to 15 sprinklers, while incident data show that more than half of all fires are controlled or extinguished by one or two.<ref name="H-18">Hopkins et al. (2005), p. 18.</ref> Cooper notes that traditional sprinkler design accounts for skipping empirically, through the experimental basis of the design rules.<ref name="C-239" /> The large number of heads in a typical installation, compared with the few sprinklers likely to be skipped, makes sprinkler systems naturally tolerant of the effect.<ref name="C-239" />

Many factors influence how sprinklers respond. They include the temperature rating, spacing and depth of the sprinklers below the ceiling, and the ceiling height.<ref name="N-10-2">Iqbal and Salley (2004), p. 10-2.</ref> Openings, joists and air currents at ceiling level also change the flow of hot gases.<ref name="N-10-2" /> Fleming adds that heat conduction to the sprinkler frame and the distance of the element below the ceiling have a significant effect on response.<ref name="F-85" /> In one of his examples, a computer model shows that quick response sprinklers 0.46 m below a basement ceiling keep a sensitivity comparable to standard sprinklers at the usual maximum of 0.30 m.<ref name="F-85" /> For the chosen fire, the standard sprinkler operates after 200 s and the quick response sprinkler after 172 s.<ref name="F-85" />

An [[ESFR sprinkler]] (early suppression fast response sprinkler) is designed to extinguish high-challenge fires, not only to control them.<ref name="N-10-3">Iqbal and Salley (2004), p. 10-3.</ref> It combines a fast response with large droplets and a high droplet velocity.<ref name="N-10-3" /> Its element has an RTI of 50 (m·s)1/2 or less, against 80 (m·s)1/2 or more for standard sprinklers.<ref name="N-F-21">Iqbal and Salley (2004), p. F-21.</ref> Large drop ESFR sprinklers protect high-piled storage in wet pipe systems and were developed by Factory Mutual Research Corporation in the late 1970s and early 1980s.<ref name="N-F-21" /> Their design criteria come from full-scale testing, and very strict design and installation rules apply to them.<ref name="F-73">Fleming (2002), p. 4-73.</ref><ref name="N-10-3" />

== Skipping in performance-based analysis ==
Performance-based analyses often use sprinkler activation times to estimate the size of a controlled fire. The data sheets of the Australian Building Codes Board are an example from Australia. They take the maximum heat release rate of a controlled fire as the rate at the activation of the last sprinkler within the assumed area of operation.<ref name="ABCB-74">Australian Building Codes Board (2020a), Data Sheet C2, p. 74.</ref> They assume that the fire activates no more than the design number of sprinklers for the hazard, or the number in the room if that is smaller.<ref name="ABCB-74" /> In a growing fire, any delay in the activation of these sprinklers therefore raises such an estimate.

Simple fire models do not capture this effect. The suppression model of CFAST (Consolidated Fire and Smoke Transport) records the activation of every sprinkler but ignores the effect of a second sprinkler on the fire.<ref name="P-37">Peacock et al. (2026), p. 37.</ref> Fleming warns that the use of such calculations for sprinkler actuation times has not been fully established.<ref name="F-85" /> He considers them useful mainly on a comparative basis.<ref name="F-85" />

== Vent skipping and combined systems ==
Smoke vents that open when a fusible link melts work on the same principle as sprinklers, so they suffer from the same random delays.<ref name="C-239" /> Cooper calls this vent skipping, and it is a central problem of [[sprinkler-vent interaction]].<ref name="C-239" /> Unlike sprinkler design, the design of automatic vent systems does not account for it.<ref name="C-239" /> Far fewer vents than sprinklers are installed per unit floor area, so Cooper suggests increasing the total vent area of a compartment by a safety factor.<ref name="C-239" />

A more reliable alternative is to open all vents of the fire compartment together, triggered by smoke detectors or by a signal that a sprinkler has discharged.<ref name="C-239" /> One strategy opens every vent of the compartment immediately after the first sprinkler discharges.<ref name="C-239" /> A refined strategy opens at once all vents beyond the first or second ring of sprinklers around the first one.<ref name="C-239" /> The other vents then open when the sprinklers have controlled the fire, and extra vent area compensates for the vents that stay closed at first.<ref name="C-239" />

Sprinkler sprays also stir the smoke layer, and this mixing can lead to smoke logging, in which the smoke is mixed through the whole space.<ref name="C-238">Cooper (2002b), p. 3-238.</ref> Cooper identifies six configurations of spray and smoke layer, of which four favour a combined design and two do not.<ref name="C-238" /> Which one occurs depends mainly on the thickness and temperature of the layer and on the momentum, spread angle and droplet size of the spray.<ref name="C-238" /> Shortly after rapid-response sprinklers discharge, the layer is still thin and not too hot, and smoke logging is then never observed.<ref name="C-238" /><ref name="C-239" />

== Limits and common misconceptions ==
Skipping cannot be predicted for a particular sprinkler. The droplet paths depend on the fire plume, which varies with the location and growth of the fire.<ref name="C-239" /> Even a perfect knowledge of the spray as it leaves the sprinkler would therefore be of little use in practice.<ref name="C-239" /> Activation models add their own simplifications: the method of Iqbal and Salley treats the element as a single lumped mass and ignores radiation.<ref name="N-10-12" />

A common misconception is that a skipped sprinkler has failed. Cooper describes a delay in discharge, caused by cooling, not a fault in the sprinkler.<ref name="C-239" /> Another misconception is that quick response sprinklers cannot be skipped. In the FDS model, droplets slow a quick response link by the same fraction as a standard link.<ref name="M-107" /> A third misconception is that only the first sprinkler matters. Control often depends on further sprinklers opening in turn, and these are the ones exposed to droplets from the first.<ref name="N-10-1" /><ref name="C-239" />

== See also ==
* [[Wet pipe sprinkler system]]
* [[Sprinkler design density]]
* [[Hydraulic calculation]]
* [[Critical water application rate]]

== Notes ==
<references />

== References ==
* Australian Building Codes Board. (2020a). ''Data sheets: Fire Safety Verification Method'' (Handbook annex, Print version 1.1).
* Cooper, L. Y. (2002b). Smoke and heat venting. 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. 3-219–3-242). 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.
* 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.
* McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). ''Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model'' (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
* Peacock, R. D., McGrattan, K. B., Forney, G. P., & Reneke, P. A. (2026). ''CFAST – Consolidated Fire and Smoke Transport: Vol. 1. Technical reference guide'' (Version 7, NIST Technical Note 1889v1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.TN.1889v1

[[Category:Fire Suppression]]

← Older revision		Revision as of 00:01, 2 November 2026
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	The temperature and velocity of the ceiling jet come from correlations based on large-scale fires of 668 kW to 98,000 kW.<ref name="N-10-10">Iqbal and Salley (2004), p. 10-10.</ref> Separate expressions apply where the plume strikes the ceiling and where the flow has become horizontal further out.<ref name="N-10-10" /> The method assumes a smooth, flat and unconfined ceiling, considers only convective heating and ignores radiation from the flames.<ref name="N-10-12">Iqbal and Salley (2004), p. 10-12.</ref> It does not apply to sprinklers far below the ceiling, because most of the heat then rises past them and they may not activate.<ref name="N-10-12" />		The temperature and velocity of the ceiling jet come from correlations based on large-scale fires of 668 kW to 98,000 kW.<ref name="N-10-10">Iqbal and Salley (2004), p. 10-10.</ref> Separate expressions apply where the plume strikes the ceiling and where the flow has become horizontal further out.<ref name="N-10-10" /> The method assumes a smooth, flat and unconfined ceiling, considers only convective heating and ignores radiation from the flames.<ref name="N-10-12">Iqbal and Salley (2004), p. 10-12.</ref> It does not apply to sprinklers installed far below the ceiling on heat collectors, flat shields above the sprinkler, because most of the heat then rises past them and they may not activate.<ref name="N-10-12" />

	=== Worked example ===		=== Worked example ===
	Iqbal and Salley consider a steady fire of 1,000 kW, 3.0 m below a ceiling, with a standard response link sprinkler 3.0 m from the axis of the plume.<ref name="N-10-15">Iqbal and Salley (2004), p. 10-15.</ref> The sprinkler has an RTI of 130 (m·s)<sup>1/2</sup> and an operating temperature of 74 °C.<ref name="N-10-15" /> Their spreadsheet uses an ambient temperature of 20 °C and a convective heat release rate of 700 kW for the jet temperature.<ref name="N-10-17">Iqbal and Salley (2004), pp. 10-17 to 10-18.</ref> The ceiling jet at the sprinkler then reaches 88 °C at 1.35 m/s, and the sprinkler operates after about 2.9 min.<ref name="N-10-17" /><ref name="N-10-16"~~>Iqbal and Salley (2004), p. 10-16.<~~/~~ref~~> Under the same conditions, a quick response link with an RTI of 34 (m·s)<sup>1/2</sup> would operate after about 45 s.		Iqbal and Salley consider a steady fire of 1,000 kW, 3.0 m below a ceiling, with a standard response link sprinkler 3.0 m from the axis of the plume.<ref name="N-10-15">Iqbal and Salley (2004), p. 10-15.</ref><ref name="N-10-16">Iqbal and Salley (2004), p. 10-16.</ref> The sprinkler has an RTI of 130 (m·s)<sup>1/2</sup> and an operating temperature of 74 °C.<ref name="N-10-15" /> Their spreadsheet uses an ambient temperature of 20 °C and a convective heat release rate of 700 kW for the jet temperature.<ref name="N-10-17">Iqbal and Salley (2004), pp. 10-17 to 10-18.</ref> The ceiling jet at the sprinkler then reaches 88 °C at 1.35 m/s, and the sprinkler operates after about 2.9 min.<ref name="N-10-17" /><ref name="N-10-16" /> Under the same conditions, a quick response link with an RTI of 34 (m·s)<sup>1/2</sup> would operate after about 45 s.

	== How water droplets delay operation ==		== How water droplets delay operation ==
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	\| <math>\beta</math> \|\| volume fraction of liquid water in the gas \|\| none		\| <math>\beta</math> \|\| volume fraction of liquid water in the gas \|\| none
	\|-		\|-
	\| <math>C_2</math> \|\| droplet cooling constant, 2.23 × 10<sup>5</sup> \|\| (m/s)<sup>−1/2</sup>		\| <math>C_2</math> \|\| droplet cooling constant, 2.23 × 10<sup>5</sup> (the source prints the unit as (ms)<sup>−1/2</sup>; the equation is consistent only with (m/s)<sup>−1/2</sup>) \|\| (m/s)<sup>−1/2</sup>
	\|}		\|}

	If conduction to the mount is ignored, as in the activation time equation above, the droplet term reduces the heating of the link by the fraction <math>C_2 \beta \sqrt{\|u\|}</math>. When this fraction reaches one, the link stops heating at all, however hot the gas. ~~The~~ constant is large, so very small amounts of water ~~are enough to~~ slow a link ~~down~~.~~<ref name="M-107" />~~		The source prints the unit of the constant as (ms)<sup>−1/2</sup>, but the equation is dimensionally consistent only with (m/s)<sup>−1/2</sup>, which is used here.<ref name="M-107" /> If conduction to the mount is ignored, as in the activation time equation above, it follows from the equation that the droplet term reduces the heating of the link by the fraction <math>C_2 \beta \sqrt{\|u\|}</math>. When this fraction reaches one, the link stops heating at all, however hot the gas. Because the constant is large, the equation implies that very small amounts of water can slow a link.

	=== Worked example, continued ===		=== Worked example, continued ===
	At the jet velocity of 1.35 m/s from the example above, the link stops heating at a liquid volume fraction of only 3.9 × 10<sup>−6</sup>.<ref name="M-107" /><ref name="N-10-17" /> That is about 4 cm³ of water in each cubic metre of gas. A volume fraction of 1 × 10<sup>−6</sup> ~~slows~~ the heating by 26 percent and ~~delays~~ operation from 2.9 to 3.9 min. At 2 × 10<sup>−6</sup> the heating falls by 52 percent, and the sprinkler operates only after about 6.0 min.		At the jet velocity of 1.35 m/s from the example above, the link stops heating at a liquid volume fraction of only 3.9 × 10<sup>−6</sup>.<ref name="M-107" /><ref name="N-10-17" /> That is about 4 cm³ of water in each cubic metre of gas. For an assumed volume fraction of 1 × 10<sup>−6</sup>, the heating slows by 26 percent and operation is delayed from 2.9 to 3.9 min. For an assumed 2 × 10<sup>−6</sup>, the heating falls by 52 percent, and the sprinkler operates only after about 6.0 min.

	The heating term and the droplet term are both divided by the RTI, so ~~in this model~~ a quick response link is slowed by the same fraction. At 1 × 10<sup>−6</sup> its operation moves from about 45 s to 61 s. ~~These~~ results show how sensitive a link is to wetting~~; they are not measurements, and the~~ volume of water ~~reaching~~ a particular link remains unknown in practice.<ref name="C-239" /> The FDS model also ignores thermal radiation to the link.<ref name="M-107" />		The heating term and the droplet term are both divided by the RTI, so it follows from the equation that a quick response link is slowed by the same fraction. At 1 × 10<sup>−6</sup> its operation moves from about 45 s to 61 s. The two volume fractions are assumed values chosen for illustration, not measurements, so the results show how sensitive a link is to wetting rather than what happens at a real sprinkler. The volume of water that reaches a particular link remains unknown in practice.<ref name="C-239" /> The FDS model also ignores thermal radiation to the link.<ref name="M-107" />

	== Skipping in sprinkler design ==		== Skipping in sprinkler design ==
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	== Skipping in performance-based analysis ==		== Skipping in performance-based analysis ==
	Performance-based analyses ~~often~~ use sprinkler activation times to estimate the size of a controlled fire. The data sheets of the Australian Building Codes Board are an example from Australia. They take the maximum heat release rate of a controlled fire as the rate at the activation of the last sprinkler within the assumed area of operation.<ref name="ABCB-74">Australian Building Codes Board (2020a), Data Sheet C2, p. 74.</ref> They assume that the fire activates no more than the design number of sprinklers for the hazard, or the number in the room if that is smaller.<ref name="ABCB-74" /> In a growing fire, any delay in the activation of these sprinklers therefore raises such an estimate.		Performance-based analyses can use sprinkler activation times to estimate the size of a controlled fire. The data sheets of the Australian Building Codes Board are an example from Australia. They take the maximum heat release rate of a controlled fire as the rate at the activation of the last sprinkler within the assumed area of operation.<ref name="ABCB-74">Australian Building Codes Board (2020a), Data Sheet C2, p. 74.</ref> They assume that the fire activates no more than the design number of sprinklers for the hazard, or the number in the room if that is smaller.<ref name="ABCB-74" /> In a growing fire, any delay in the activation of these sprinklers therefore raises such an estimate.

	~~Simple fire models do not capture this effect. The suppression model of~~ CFAST (Consolidated Fire and Smoke Transport) records the activation of every sprinkler but ignores the effect of a second sprinkler on the fire.<ref name="P-37">Peacock et al. (2026), p. 37.</ref> Fleming warns that the use of such calculations for sprinkler actuation times has not been fully established.<ref name="F-85" /> He considers them useful mainly on a comparative basis.<ref name="F-85" />		CFAST (Consolidated Fire and Smoke Transport) is a two-zone fire model.<ref name="P-1">Peacock et al. (2026), p. 1.</ref> Its suppression model records the activation of every sprinkler but ignores the effect of a second sprinkler on the fire.<ref name="P-37">Peacock et al. (2026), p. 37.</ref> Fleming warns that the use of such calculations for sprinkler actuation times has not been fully established.<ref name="F-85" /> He considers them useful mainly on a comparative basis.<ref name="F-85" />

	== Vent skipping and combined systems ==		== Vent skipping and combined systems ==
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	Sprinkler sprays also stir the smoke layer, and this mixing can lead to smoke logging, in which the smoke is mixed through the whole space.<ref name="C-238">Cooper (2002b), p. 3-238.</ref> Cooper identifies six configurations of spray and smoke layer, of which four favour a combined design and two do not.<ref name="C-238" /> Which one occurs depends mainly on the thickness and temperature of the layer and on the momentum, spread angle and droplet size of the spray.<ref name="C-238" /> Shortly after rapid-response sprinklers discharge, the layer is still thin and not too hot, and smoke logging is then never observed.<ref name="C-238" /><ref name="C-239" />		Sprinkler sprays also stir the smoke layer, and this mixing can lead to smoke logging, in which the smoke is mixed through the whole space.<ref name="C-238">Cooper (2002b), p. 3-238.</ref> Cooper identifies six configurations of spray and smoke layer, of which four favour a combined design and two do not.<ref name="C-238" /> Which one occurs depends mainly on the thickness and temperature of the layer and on the momentum, spread angle and droplet size of the spray.<ref name="C-238" /> Shortly after rapid-response sprinklers discharge, the layer is still thin and not too hot, and smoke logging is then never observed.<ref name="C-238" /><ref name="C-239" />

	== Limits and ~~common misconceptions~~ ==		== Limits and practical points ==
	Skipping cannot be predicted for a particular sprinkler. The droplet paths depend on the fire plume, which varies with the location and growth of the fire.<ref name="C-239" /> Even a perfect knowledge of the spray as it leaves the sprinkler would therefore be of little use in practice.<ref name="C-239" /> Activation models add their own simplifications: the method of Iqbal and Salley treats the element as a single lumped mass and ignores radiation.<ref name="N-10-12" />		Skipping cannot be predicted for a particular sprinkler. The droplet paths depend on the fire plume, which varies with the location and growth of the fire.<ref name="C-239" /> Even a perfect knowledge of the spray as it leaves the sprinkler would therefore be of little use in practice.<ref name="C-239" /> Activation models add their own simplifications: the method of Iqbal and Salley treats the element as a single lumped mass and ignores radiation.<ref name="N-10-12" />

	~~A common misconception is that~~ a skipped sprinkler has failed. Cooper describes a delay in discharge, caused by cooling, not a fault in the sprinkler.<ref name="C-239" /> ~~Another misconception is that~~ quick response sprinklers ~~cannot be skipped~~. In the FDS ~~model~~, droplets slow a quick response link by the same fraction as a standard link.<ref name="M-107" /> ~~A third misconception is that only~~ the first sprinkler ~~matters. Control often~~ depends on further sprinklers opening in turn, and these are the ones exposed to droplets from the first.<ref name="N-10-1" /><ref name="C-239" />		Three practical points follow. First, a skipped sprinkler has not failed: Cooper describes a delay in discharge caused by cooling, not a fault in the sprinkler.<ref name="C-239" /> Second, quick response sprinklers are not protected against skipping. As derived above from the link equation of FDS, droplets slow a quick response link by the same fraction as a standard link when conduction to the mount is ignored.<ref name="M-107" /> Third, where the first sprinkler cannot control a fire, control depends on further sprinklers opening in turn, and these are the ones exposed to droplets from the first.<ref name="N-10-1" /><ref name="C-239" />

	== See also ==		== See also ==