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The Explosion Hazard in a Smoke Shaft

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Publisher’s version / Version de l'éditeur:

Technical Note (National Research Council of Canada. Division of Building Research), 1971-11-01

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The Explosion Hazard in a Smoke Shaft

McGuire, J. H.

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DIVISION OF BUILDING RESEARCH

No.

NATIONAL RESEARCH COUNCIL OF CANADA

562

NOTlE

'fEClHIN II CAlL

PREPARED BY J. H. McGuire CHECKED BY G. W.S. APPROVED BY N. B. H. DATE November 1971 PREPARED FOR Limited Distribution

SUBJECT THE EXPLOSION HAZARD IN A SMOKE SHAF T

Many firefighters in North America have pressed for the provision of smoke shafts in high buildings to ameliorate smoke problems. There has been some response to this pressure and, in recognition of their in-creasing use, the 1970 National Building Code (1) has included reference to smoke shafts as one means of venting a high-rise building. Their use and effectiveness has also been the subject of a DBR study (2).

Although there is no record of a so-called "smoke explosion" in a smoke shaft, the potentiality for damage is so high should one occur, that the likelihood of such an event is worth some consideration.

SMOKE EXPLOSIONS

To create an explosion during the course of a fire, a mixture of air and fuel distillates within the lilrnits of flammability must be established in a confined space and a source of ignition provided. During the normal course of a fire high temperature igniting sources abound and as fuel dis-tillates issue from the basic liquid or solid fuel they form a mixture (with the surrounding air) that comes within the limits of flammability close to the surface of the fuel. Ignition thus occurs while the volume of mixture that is within the flammable limits is quite small and cannot be described as confined. The combustion process that results does not, therefore, constitute an explosion.

"Smoke explosions" do occur, however and the most common se-quence of events giving rise to them originates with an oxygen deficient atmosphere in the fire region. Provided fairly high temperatures are sus-tained' volatiles continue to be distilled off until the proportion of fuel distillates in the atmosphere is of the same order as the vitiated air, the oxygen deficiency of which was responsible for the suppression of ignition in the first place.

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-The final stage in the sequence of events is a rapid introduction of air following, for example, the breakage of a number of windows in the area. If flow and mixing conditions are propitious a large volume of gas could be transformed from an over -rich to a highly flammable mixture and, subject to the opportune provision of a spark or other pilot ignition source, the potential for a substantial explosion exists.

It has been shown that for certain enclosure geometries the maxi-mum pressure developed in an explosion is proportional to the square of the fundamental burning velocity of the gas (3). With the notable exception of hydrogen (11 ft/sec) and acetylene (5.8 ft/sec). the maximum funda-mental burning velocities of most flammable gases (and volatiles) that have been investigated range between 1 and 2 ft/ sec. Measurements (4) indicate エィ。セ over a wide range of mixture strength within the limits of flammability, there will not be a sharp variation of burning velocity. Although measurements related to many of the distillates issuing from common fuels have not been made, existing knowledge suggests that

the pressures that could be developed by a "smoke explosion" are almost of the same order as those prevailing during explosions involving other common gaseous fuels. The principal factor that would suggest lower maximum pressures is that a smoke mixture would essentially include some oxygen deficient air. Extrapolating results quoted by Gaydon and Wolfhard (4), an action that would probably be valid for oxygen deficiencies of a few per cent (i. e., 0 concentration> 14 per cent), suggests that reduced oxygen concentrcftion would give roughly linearly reduced burning velocities.

Reports from firefighters confirm that substantial smoke explosions are possible although. in general., they are more moderate. The two

features that usually moderate the explosions are (1) that only the gases within a portion of a confined space corne within the flammable limits at the time of the incident, and (2) that the enclosure usually incorporates vents of some sort (e. g., windows).

SMOKE EXPLOSION IN A SMOKE SHAFT

Noting that violent smoke explosions can and do occur (albeit rarely) during the course of fires in buildings the question to be investigated is the likelihood of a similar type of explosion in a smoke shaft. The sequence of events would again be expected to originate with an oxygen deficiency in the fire region. In the most unfortunate circumstances a mixture of fuel distillates and air just above the flammable limits would flow into the

shaft. It would be brought down to within the flammable limits by infiltration of air through the shaft walls and around the dampers that exist at every floor. An explosion could then occur if a hot ember or other ignition source

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-time of the explosion and the effect of this in reducing the overpressure m.ust be discussed. For sm.all ducts (diam.eter not exceeding 2 ft 6 in. ) quite accurate form.ulae predicting this effect have been developed (3) and for the purposes of this Note they can be taken to apply. to a reason-able approximation, to virtually all sizes of sm.oke shaft. In the case of propane the form.ula giving m.axim.um. pressures (lbf/in. 2) is 0.04 L/D where D is the diam.eter of the shaft and L its length, the upper lim.it-ing value of L/ D to which the form.ula was intended to apply belim.it-ing 60.

lt is assum.ed that the shaft will be com.pletely open at both ends, which approximates the conditions prevailing in the sm.oke shaft under discussion.

The choice of propane as the fuel is reasonable as discussed previously. The m.axim.um. length/diam.eter ratio quoted (i. e., 60) is quite likely to prevail in a sm.oke shaft, hence an overpressure of at least 2.4 psi would appear possible. The m.aximum. overpressure that can be withstood by building com.ponents is. of course, dependent on the com.-ponent but a com.m.only accepted generality (5) is that m.ost ordinary build-ing walls will not withstand a "sustained internal pressure" (in the con-text of explosions) as great as 1 psi. Giving m.ore detail, Glasstone (6) suggests that windows will usually shatter at overpressures of between 0.5 and 1 psi and unreinforced concrete or cinder-block walls (8 to 12 in. thick) at 2 to 3 psi.

It is to be hoped that in m.any cases sm.oke shafts would withstand sm.oke explosions within them. prim.arily because the overpressures de-veloped would be of a lower order than the value just quoted. The lower intensity would be associated with the fact that only the gases in a portion of the shaft would be expected to be within the flam.m.able lim.its just prior to the initiation of an explosion.

RETURN .AIR SHAFTS

Return air shafts in certain configurations can correspond closely to sm.oke shafts as far as the likelihood of a sm.oke explosion is concerned. Thus the discussion concerning sm.oke shafts can be considered directly applicable to the case of a return air shaft exhausting directly to the exterior and serving only the fire floor. all other connections being darnpered.

Where none of the dam.pers are closed conditions differ in two respects. Firstly, the connecting ducts at each floor would serve as ex-plosion vents to som.e extent. The m.axim.um. overpressure to be expected in a vented duct is a function of L/D (where L is the distance between ex-plosion reliefs and D is the diam.eter of the duct) and of the K factor of the vent (the ratio of the cross-sectional areas of the duct and the vent). To generalize, and neglecting the influence of the K factor within practical lim.its, the threshold value of L/ D is of the order of 10. The hazard with vertical ducts of 2 ft or m.ore in diam.eter. with open branch ducts at every floor. will therefore usually be acceptable. In turn where the ducts are

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-sITlaller the likelihood of severe daITlage to the building is not great, pro-vided the duct walls are not structural ITleITlbers.

The second significant difference between having daITlpers open as com-pared with closed is that diluent air will vary, with height, the ITlixture within the shaft. It is thus unlikely that a ITlixture will be within the ITlost

hazardous ranges of the flamITlability liITlits over ITlore than a few storeys in height. In general, therefore, a vertical return air duct serving a nUITlber of floors is less likely to give rise to as great a hazard as a smoke shaft although it is desirable that its walls should not constitute struc-tural components of the building.

OTHER SHAFTS

The title of this Note indicates that its scope is confined to sITloke shafts. This li.m.itation was chosen because it was thought that sITloke explosions are ITlore likely to occur in sITloke shafts than in other shafts. The basis of this belief ITlust be briefly presented. SITloke shafts will be characterized by a large opening cOITlITlunicating with the fire region but only sITlaller leakage areas around closed daITlpers cOITlmunicating with other floor spaces. Other shafts should have only relatively sITlall but si.m.ilar leakage areas at all floors.

With regard to the existence of an opening at the top of a shaft, a sITloke shaft would always be expected to be top vented following the outbreak of a fire while other shafts would not be expected to be effectively vented. Provision for venting ITlight exist but, if perITlanent. would probably not be very significant in the present context and, if not, would probably be fusible-link operated, i. e., opening only when fire occurred in the shaft itself. In buildings constructed recently. however, such shafts ITlight be heavily vented shortly after the outbreak of fire.

The principal effect of both the above factors would be that ig-nition sources, such as glowing eITlbers. would be much more likely to enter and be transported up a sITloke shaft than any other shaft. The large opening at the fire floor would give unrestricted passage and top venting would ITlaxiITlize the flow velocities associated with stack action.

The only obvious circUITlstances in which an ignition source could be expected to penetrate a siITlple shaft. at a time when it con-tained a flaITlITlable atmosphere, would be during the course of a smoke explosion in a floor space. In this event the pressure wave generated by the explosion could tranSITlit the associated flame into the shaft, high velocities being established at the cOITlITlunicating leakage area. Such an event would only provide an ignition source in the shaft at a particular tiITle which ITlight not be the ITlost propitious froITl the ITlix-ture point of view. .

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-PRECAUTIONAR Y MEASURES

Any hazard associated with an explosion in a smoke shaft could be eliminated by providing explosion reliefs at various locations up the height of the shaft. The extent to which the reliefs should be provided could be assessed from the "Guide to the use of Flame Arresters and Explosion

Reliefs" (3) following a- decision as to the maximum tolerable overpressure (6). The latter would of course be dependent on the choice of construction

ma-terial and technique. 8-to 12-in. unreinforced brick wall panels being capa-ble of withstanding 7 to 8 psi (6) and blast resistant construction being generally capable of withstanding at least 10 psi. In view of the fact that only the gases in a portion of a shaft would usually be within the flammable limits at the time of an incident. design to an overpressure of 0.04 (length)/ (diameter) (psi) is hardly called for. In most cases sturdy construction of a shaft would. of itself. provide adequate protection against a smoke ex-plosion and the provision of vents other than at the top of the shaft would not be necessary. Should a vent prove to be desirable at some location up the height of the shaft. and the shaft to have no exterior wall, the venting could be effected by way of a service area unlikely to be occupied. The service area itself would then in turn require some measure of venting. The likelihood of a smoke explosion in a smoke shaft is remote and even if a design does not comply with the above recommendation it could be considered acceptable. In such cases the potentialities of an explosion should be minimized by avoiding the use of the shaft walls as structural components. Although not as effective, this suggestion could in fact prove more restrictive than the previous recommendation.

CONCLUSION

Although unlikely, a serious smoke explosion in a smoke shaft is a possibility. Elimination of associated hazard by sturdy design and venting provision is sufficiently free of complication and expense as

to make it, in general. practical and worthwhile. REFERENCES

1. National Building Code of Canada 1970. Article 3.2.6.7.

2. Tamura. G. T. Analysis of smoke shafts for control of smoke move-ment in buildings. Trans •• ASHRAE Vol. 76, Part II. 1970.

3. Guide to the use of flame arresters and explosion reliefs. Ministry of Labour; Safety. Health and Welfare New Series #34. HMSO. 4. Gaydon. A. G .• and H. G. Wolfhard. Flames. their structure.

radiation and temperature. 1953.

5. Fire Protection Handbook. National Fire Protection Association)Boston, 13th Edition, 1969, p. 17-57.

6. The Effects of Nuclear Weapons. Samuel Glasstone (Editor). U. S. Atomic Energy cッュュゥウウゥッセ Revised Edition, 1962. p. 163.

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