Effect of Automatic Sprinkler Protection on Smoke Control Systems

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Ipressurized zone before opening door F7. This failure toprevent smoke spread into the stairshaft indicates that thestrategy <strong>of</strong> not actively pressurizing the stairshaft in thesame way as other zones to be protected, but relying on themechanical exhaust <strong>of</strong> the fire floor to hold the smoke onthe fire floor, does not reliably protect the stairshaft againstsmoke entry. Given that smoke spread occurred underconditions <strong>of</strong> sprinklered fires, in spite <strong>of</strong> low tempemturesand low buoyancy pressures and full pressurization toAPdesign = 0.1 in. H20 (25 Pa), it is certain that smokefrom a nonsprinklered fire would also spread into thestairshaft under open-door conditions.Two tests were conducted with no zoned smoke controlfeatures in effect, i.e., with AP desl 8n = 0. The first <strong>of</strong> thesetests involved an unshielded wood crib, in which the firewas extinguished in less than 12 minutes at a sprinklerdensity <strong>of</strong> 0.13 gpm!ft2 (5.3 L/minrn 2 ). A small amount<strong>of</strong> smoke did spread through interzonal leakage openingsinto the floor above the fire floor and into the stairshaftwhen the door was opened. The total amount <strong>of</strong> smokeproduced by the short-dumtion fire was low, and temperaturesand gas concentrations outside the fire floor did notreach dangerous levels. For the case <strong>of</strong> an unshielded fire,then, with no other smoke control features in effect, thesprinkler system performed as a successful smoke controlmeasure. This test supports the established expectation that,for unshielded fires, sprinklers provide a sufficient level <strong>of</strong>safety against smoke hazards in high buildings.The second test with a shielded wood crib, with nozoned smoke control features in effect, did not performnearly as well. Because <strong>of</strong> the prolonged burning <strong>of</strong> thefire,. a large volume <strong>of</strong> smoke was produced. The smokewas high in CO content ( > 4,000 ppm) and thereforedangerous. Smoke spread to floors above the fire floorthrough leakage openings before the stair door was openedand into the stairshaft when the door was opened. Thestairshaft became smokelogged within minutes <strong>of</strong> openingthe door, with complete loss <strong>of</strong> visibility and tempemtures<strong>of</strong> more than 150°F (65°C), These conditions were considereduntenable. This test clearly demonstmted that smokeproduced by a shielded. fire represents a serious threat tolife safety. Unless measures are taken to confine the smoketo the fire floor, smoke will spread through the building.<strong>Effect</strong> <strong>of</strong> <strong>Sprinkler</strong>s on Smoke ProductionIt is sometimes argued that sprinklers increase theamount <strong>of</strong> smoke produced in a fire. The shielded, sustainedfires in this series <strong>of</strong> tests produced large volumes <strong>of</strong>smoke. It is a moot point, however, as to how much moresmoke would have been produced if the fires had not beenrestmined. For an unshielded fire, the sprinklers are usuallysuccessful in extinguishing the fire, which then stops smokeproduction. For a shielded fire, the fuel bums in wetconditions at a reduced burning mte. It is impossible to sayfrom data collected in these tests whether the actual massproduction mte <strong>of</strong> soot and other products <strong>of</strong> combustionASH RAE Transactions: Researchwas greater when the sprinklers were operating. There isevidence, however, from at least one <strong>of</strong> the Tower tests,that smoke density in the exhaust shaft increased when thesprinklers were shut <strong>of</strong>f. It is believed that this increase wasdue to increased smoke production as the fire regrew,became hotter, and involved more fuel.Based on the observations in the tests in the one-storyroom and the Tower, it is noted that the fire itself is theactual source <strong>of</strong> smoke. Reducing the burning rate <strong>of</strong> a fireby application <strong>of</strong> sprinkler spray reduces the total production<strong>of</strong> smoke. However, the injection <strong>of</strong> sprinkler spmy. into the hot gases coming from a fire creates steam, whichdisrupts the flow <strong>of</strong> the gases in the hot layer. The turbulencecreated by the mpid cooling <strong>of</strong> the hot gases and theexpansion <strong>of</strong> steam deepens the smoke layer, even dmwingceiling gases down to floor level. The optical properties <strong>of</strong>the smoke and steam mixture are such that visibility ismpidly reduced at lower levels in the room. These rapidchanges in smoke-layer depth and visibility brought aboutby sprinkler opemtion do not necessarily mean that the mte<strong>of</strong> production <strong>of</strong> smoke has increased, however.A mpid reduction in visibility in the immediate vicinity<strong>of</strong> the fire when sprinklers activate may have negativeimplications for persons exiting the fire floor. However, ifa fire is allowed to continue to grow, visibility and temperatureconditions will sbon become unacceptable within aneven larger area on the fire floor and beyond. Therefore,the negative aspect <strong>of</strong> reduced visibility in a small areashould be viewed in the context <strong>of</strong> the benefit <strong>of</strong> elimination<strong>of</strong> the fire threat to a much larger area.The one-story test room used for some <strong>of</strong> these testswas located inside the Bum Hall, which is a very largespace similar to an atrium. During severn! tests in which thecapacity <strong>of</strong> the collector hood was surpassed, it wasobserved that the smoke from the sprinklered fires did notrise to the 40 ft (13 m) high ceiling <strong>of</strong> the Bum Hall.Instead, the smoke accumulated at floor level, leaving theupper portions <strong>of</strong> the Bum Hall clear.IMPLICATIOIIIS FOR DESIGN OFZONED SMOKE CONTROL SYSTEMSThe purpose <strong>of</strong> these experiments was to investigate theeffects <strong>of</strong> automatic sprinklers on the conditions <strong>of</strong> heatrelease mte, temperature, pressure, and levels <strong>of</strong> oxygen,carbon dioxide, and carbon monoxide in a building underfire conditions and to assess the effect that changes in theseconditions might have on the performance <strong>of</strong> a zoned smokeCOI)trol system. The results <strong>of</strong> the test progmm haveimplications for the design <strong>of</strong> zoned smoke control systemsin sprinklered buildings. These implications are summarizedhere.1. <strong>Sprinkler</strong>s have a good record <strong>of</strong> reliability in controllingfires and, by limiting the amount <strong>of</strong> combustiblesinvolved in a fire, also restrict smoke production. Asit constitutes both a detection system and a fire-fighting509
measure that is automatically initiated early in a fire, asprinkler system would enhance the effectiveness <strong>of</strong>fire department operations. For these reasons, a fullysprinklered building is considered by many to be themost effective and reliable smoke control measure <strong>of</strong>all those recommended in the Supplement to the NationalBuilding Code <strong>of</strong> Canada (NRCC !990b; see alsoNRCC 1990a).One <strong>of</strong> the tests in the Tower indicated the effectiveness<strong>of</strong> a sprinkler system in extinguishing anunshielded fire. The smoke contamination <strong>of</strong> the Towerwas light, with C0 2 and CO concentrations outside thefire floor at levels that would not be life-threatening.Temperatures outside the fire floor were nonthreatening,and even on the fire floor, the potentially threateningpeak temperatures were <strong>of</strong> short duration.2. The installation <strong>of</strong> a sprinkler system in a building doesnot eliminate the possibility <strong>of</strong> a fire producing largevolumes <strong>of</strong> smoke. In its definition <strong>of</strong> fire control,NFP A Standard 13, the standard for design <strong>of</strong> sprinklersystems, recognizes that sprinkler systems do notalways completely extinguish a fire; it is accepted that"control" <strong>of</strong> the burning rate and temperatures is sufficientto prevent fire from growing and spreading. Thestandard does not specify what burning rate or temperaturedefines the limits <strong>of</strong> that control, however. Evenwhen sprinkler spray is unobstructed, the fire may burnfor some time at a restrained rate before being completelyextinguished. Smoke will be produced throughoutthat burning period.NFPA Standard 13 rules for locating sprinklers inthe vicinity <strong>of</strong> obstructions are intended to ensure thatshielded areas are kept to an absolute minimum.Conditions that would produce a shielded fire are, to alarge extent, due to the manner in which the occupantuses the building, such as "temporarily" storing materialsin areas where they were not expected, e.g., undertables. A shielded fire could occur due to a buildingmodification. As an example, compact mobile shelvingunits for storing recorda <strong>of</strong> various types are beinginstalled with increasing frequency in commercialoccupancies. When in the closed position, the materialson the shelves are shielded from sprinkler spray.Once a shielded fire bas grown to involve all <strong>of</strong> thefuel within the shielded area, it is likely to be poorlyventilated, due to close packing <strong>of</strong> fuel materials orbecause <strong>of</strong> confmement inside or under the obstacle tosprinkler spray. Under certain circumstances, carbonmonoxide concentration in the smoke may be dangerouslyhigh. To prevent the spread <strong>of</strong> this smoke beyondthe fire floor, some active smoke control measures arerecommended.3. The maximum size <strong>of</strong> a shielded fire, in terms <strong>of</strong> heatrelease rate (HRR), depends on the amount and natore<strong>of</strong> the fuel, the ventilation conditions, and the adequacy<strong>of</strong> the sprinkler system design, expressed in terms <strong>of</strong>spray density. Assuming that the sprinkler system510design density meets the minimum requirements <strong>of</strong>NFPA Standard 13, it is likely that the shielded firewill be kept to less than 50% <strong>of</strong> its potential (unsprinklered)intensity. If the operating density for the sprinklersystem is significantly greater than the minimumdensity specified by NFPA Standard 13 for the unshieldedfuel load, a greater reduction in fire intensityis likely. Full"scale fire tests may be required todetermine bow much higher than the minimum thespray density must be to reduce the HRR <strong>of</strong> a givenfuel package by a given amount, under shielded conditions.For a sprinkler system that provides a densitysuitable for the unshielded fuel load, increased ventilationdue to opening doors to the fire floor will notsignificantly increase the intensity <strong>of</strong> the fire. Thismeans that concern about increasing fire severity byincreasing airflow to the fire floor (by opening doors,for example) may be much reduced for sprinkleredfires.4. The wood crib fires used in the tests contained asubstantial weight <strong>of</strong> dry fuel, ideally arranged to burn.It may be that shielded fuel arrangements likely to befound in typical <strong>of</strong>fice occupancies would not bum atthe same intensity. Noting this, it was demonstratedthat the HRRs <strong>of</strong> the wood crib fires were reduced bymore than 50% to less than S70 Btu/s (600 kW) in theTower tests (at a density <strong>of</strong> 0.13 gpmlft2 [53Llmin·m 2 ]), and to less than 97S Btuls (l,02S kW) forthe much larger cribs in the one-story room at a density<strong>of</strong> 0.11 gpmlft 2 (4.S L/min·m2). These reductions inthe beat release rate were achieved at minim11m spraydensities. This suggests that heat release rates greaterthan 950 Btu/s (1,000 kW) are unlikely for probableshielded fire scenarios in <strong>of</strong>fice occupancies.S. <strong>Sprinkler</strong>s are effective in reducing both temperatoresand iadiant heat on the fire floor. This encourages ahigh degree <strong>of</strong> confidence that the fire will not spreadand that windows on the fire floor are not likely tobreak, which bas important implications with respect tothe adverse pressures that could be induced across thefire-floor enclosure by wind and building stack actionif the windows break.6. Tests in the one-story test room indicated that pressurizationto exactly match the fire pressure can preventsmoke flow into the protected space. <strong>Sprinkler</strong> spraysreduce temperatures, thereby reducing buoyancypressures on the fire floor to essentially negligiblelevels. Accepted practice for design <strong>of</strong> zoned smokecontrol systems for sprinklered buildings, which is todesign for fl.P <strong>of</strong>0.05 1in. H 2 0 (12.5 Pa), is more thanadequate to prevent the spread <strong>of</strong> smoke across smallopenings. Pressure differentials created by buildingstack action and wind should also be considered alongwith buoyancy pressure generated by a sprinklered fire.7. The smoke control system tested in the fire tower involvedcreating positive pressure in ·the zones adjacentASH RAE Transactions: Research
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Effect of Automatic Sprinkler Protection on Smoke Control Systems

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