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Studies relating to ignition of the fire at King's Cross Underground Station
Fire Safety Journal 18 (1992) 35-47
Studies Relating to Ignition of the Fire at King's Cross Underground Station R. K. Wharton Explosion and Flame Laboratory, Health and SafetyExecutive, Harper Hill, Buxton, Derbyshire SK179JN, UK
ABSTRA CT The damage survey after the fire at King's Cross Underground Station found a layer of grease and debris on the running track below the undamaged section of escalator No. 4. The ignition characteristics of this layer were important in relation to the role it may have played in the incident. This paper summarises tests done with a range of ignition sources on new grease and samples removed from escalator No. 4. Work with new greases mixed with fluff or inert fibrous material established that the presence of these additional components greatly enhances the ignitability of the grease by acting as a wick. The greases could be ignited by a flaming match, but not by a glowing cigarette. However, cigarettes initiated smouldering combustion in the types of dust layers found beneath the escalator.
1 INTRODUCTION Following the fire at King's Cross Underground Station on 18 November 1987, in which 31 people died, scientists from the Research and Laboratory Services Division of the Health and Safety Executive (HSE) investigated the cause and spread of the fire, initially at the request of the Chief Inspecting Ott~cer of Railways (Department of Transport), and subsequently on behalf of the Inspector of the Public Inquiry. The initial task for the HSE team was to catalogue the damage to the escalator and booking hall. The report of this work I provided evidence 35 (~) 1991Crown Copyright
36
R. K. Wharton
Fig. 1. Example of the grease layer on the running track below escalator No. 4.
to suggest that the fire started under the right-hand side of escalator No. 4, and also established that a layer of grease and debris was present on the running track below the undamaged section of the escalator (Fig. I). During the course of the investigationvarious theories for ignition of the firewere examined, including arson, electricaleffectsand frictional heating. Halliday's post-fire investigation"~ established that matches were found on the running track underneath the escalator and that initiation of the fire by discarded smokers' materials had to be considered a strong possibility. Comprehensive testswere undertaken at HSE's Explosion and Flame Laboratory at Buxton to investigatethe ignitabilityof the various types of grease used on the escalator machinery. A range of ignition sources was examined, and the results were presented to the Inquiry in a reports that forms the basis of this paper. Other theories for ignition of the fire are referred to in the assessments made by Fennell in his Investigation Report. 4 2 EXPERIMENTAL PROGRAMME Samples of new grease and chain lubricants of the type identified as being in possible use on escalator No. 4 were obtained from London
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Underground Ltd by Cremer and Warner (London, UK), technical consultants to the Inquiry. Detailed specifications are given in Ref. 3. Chain lubricants Nos 1 and 2 are referred to as 'Black Band' and 'Red Band', respectively. On-site investigations found quantities of dust present on the balustrade support trusses to the right-hand side of the undamaged section of escalator No. 4. Samples were removed for analysis and testing. Samples of the grease layer from the right-hand track under the undamaged section of escalator No. 4 were taken for use in the ignition tests. A portion of the grease layer from the track was repeatedly washed with a suitable solvent (Lec, Arrow Chemicals Limited, Burton-onTrent, UK) to extract soluble components, and the solution was filtered. A fine, dark-grey deposit was retained in the filter paper (thought to be the molybdenum disulphide additive listed in the specification for both chain lubricants), while a fluff layer remained above. The latter was repeatedly washed until the solvent remained clean. The proportion of solid foreign matter in the sample was 6%. Samples of the dust from the balustrade support trusses and the extracted fluff were examined in order to provide information on both the nature of these constituents and on a suitable material that could be added to new greases to simulate the grease/fluff mix found under escalator No. 4. Because of the short time-scale in which to do the experiments, household vacuum-cleaner debris was used as representative of the type of fluff/dust found in the analysis, and 10% grease/fluff mixtures were created by mixing nine parts by weight of new grease with one part of fluff. In order to make the mixtures as close as possible to those recovered, cigarette ends and used matches were added to the fluff prior to mixing. The mixed samples were similar in appearance to greases taken from King's Cross. A comprehensive series of tests was done using various ignition sources and a range of greases, including samples from King's Cross, new greases (lithium grease, chain lubricants Nos 1 and 2), and combinations of new grease and fluff. Since the angle of elevation of the escalator involved in the fire was 30° , the tests were done with the grease samples smeared on a 5-mm-thick metal plate, 1 m long and 60 mm wide, inclined at 30° to the horizontal. Measured local air velocities in the region of the track under the Victoria line escalators adjacent to the scene of the fire ranged from 0.1 to 0.5 m/s. s Therefore, a value of 0.2 m/s was taken as representative of the velocity that could have been present above the inclined grease
R. K. Wharton
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TABLE 1 Calibration of Ignition Sources
Mean burning time (s) Igniter
Still air
Draught ~
Single match Four-match crib Crumpled paper
39 65 27
26 58 25
° 0.2 m/s.
layer under escalator No. 4 while it was moving. Experiments with various ignition sources were done, therefore, in both still air conditions and with an applied air velocity (measured above the grease bed at the point of ignition using a Prosser Scientific Instruments Ltd (Ipswich, UK) low-flow air velocity meter Type AVM502 with ambient temperature correction). The match and crumpled-paper igniters were calibrated 3 by determining the mean burning time from 20 tests with an inert ceramic blanket on the inclined surface: ignition in these tests was then defined as burning that lasted for longer than twice the mean burning time of the igniter alone under the same set of conditions (Table 1). The four-match crib was produced by igniting three spent matches with a burning match, and v,as included in the studies because collections of match sticks were f~.~utj~ in the grease layer under the undamaged section of escalator No. 4.. 3 RESULTS
&l Tests with matches In order to avoid variations in the ignition source, one brand of matches was used for all the experiments. Experiments were done to obtain information relating to the likelihood of a dropped burning match igniting a sample of grease taken from the track under the right-hand side of escalator No. 4. The grease sample was well mixed and placed in a layer 10 mm thick on the inclined, metal test plate. Matches were struck and, once flaming of the match head had ceased, dropped from a height of 1.1 m in still air on to the grease layer. The experiment was repeated until 20 matches had hit the test sample. Nine matches were still burning after landing on the fuel bed and, of
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these, five produced ignition of the grease layer (defined as combustion growing from the point of impact of the match such that a selfsustaining fire would have been produced). Further work was done to investigate the ability of dropped matches to fall down the gap between the escalator treads and skirting boards. This gap on escalator No. 4 is reported 6 to have been 0-12 mm. On-site measurements of the undamaged section of escalator No. 4 indicated gaps of c. 8 mm. A section of escalator constructed at Buxton for use in a fire trial was used for the tests in which matches were dropped from a height of approximately 1.1m and thrown in the general direction of the right-hand edge of the treads where they met the skirting boards. The gap for these experiments was c. 10mm and the experimenter stood 20 cm from the edge of the step. The metal edge strips (fire cleats) were present at the right hand side of all the treads and the tests were done in still air conditions. One hundred unlit matches were thrown away from the body with the right hand whilst not looking at the stair treads, and, of these, eight passed through the gap. Observations made during these tests suggest that a possible mechanism for matches passing through the gap involves impact with a step cleat followed by a tumbling trajectory towards the right-hand side of the tread. During the post-fire investigation, a layer of fluff was found surmounting the grease layer in certain locations. In order to ascertain whether the presence of fluff could have influenced the ignition and burning characteristics of the grease layer, the tests were repeated with a c. 10-mm layer of vacuum-cleaner debris on a sample of grease from escalator No. 4. Most of the burning matches bounced off the fibrous layer. Of 50 that remained lodged on the surface, two instances of ignition of the fluff layer were recorded: combustion lasted for 10 and 46s, and neither case involved burning of the grease substrate. Table 2 gives results of match ignition tests with grease on a 30 ° inclined surface. For new chain lubricant, which was tested as a c. 4-mm-thick layer, it was possible to achieve ignition (as defined earlier) only in the absence of a draught, but the burning process was short-lived. V'ith the four-match igniter, ignition was obtained in all six experiments. Ignition of chain lubricant No. 2 was achieved in all the tests with a single match in still air, and in one test with four matches plus applied draught. In no case was the duration of the burning process greater than 2 min 16 s.
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R. K. W h a r t o n TABLE
2
Test Results for S a m p l e s o f N e w Grease and G r e a s e from Escalator N o . 4 Ignited by Matches Duration of burning rain :s a Test material
Chain lubricant No. 1 Black Band
Ignition source Single match
Single match and draught Four matches Four matches and draught
Chain lubricant No. 2 Red Band
Single match
Single match and draught Four matches Four matches and draught
Lithium grease No. 3
Single match
Single match and draught Four matches Four matches and draught
King's Cross samples
Single match Single match and draught Four matches Four matches and draught
Test I
Test 2
Test
3
Y N Y Y
1: 34 0: 20 >3: 00 >3:00
N N Y Y
0: 25 0: 35 > 3 : 00 2:17
Y N Y Y
3:10 0: 20 >3: 00 3:31
Y Nb N N
2:05 0: 21 1: 31 0:41
Y Nb N Y
1:40 0:16 1: 49 2:16
Y Nb N N
2:01 0:17 !: 33 0:48
N N Y Y
0:55 0:24 >3: 00 >4:00
N N Y N
1:13 0: 26 > 3 : 00 0:29
N Y Y Y
0:34 1: 03 1: 53 >4:00
Y Y Y Y
>1:00 >1:30 >2: 00 >3: 00
Y N Y Y
>1:00 0:28 > 2 : 00 > 3 : 00
N N Y N
0:34 0:20 >2: 00 0: 41
a y = ignition, N = no ignition. b Matches only burnt half-way down stick.
Chain lubricant No. 2 was less viscous than No. I, and the maximum depth that could be attained on the inclined surface was 2 ram. The observations in Table 2 can be explained in terms of the matches sinking into the grease layer and extinguishing, rather than resting on top with continued burning. Tests were also done with an 8-10-mm layer of lithium grease No. 3 on the inclined surface. Ignition by a single match was achieved in only one of the six experiments, while only one of the six tests done with four matches failed to result in ignition. With the four-match configuration, four of the six experiments produced a growing fire that had to be extinguished. The series of experiments was repeated using mixtures of new grease and fluff; the results obtained are reported in Table 3. 3.2 Tests with glowing cigarettes In order to standardise the igniter, the same brand of filter-tip king-size cigarettes was used for all the tests. The cigarette was cut at the mid point of the length of tobacco, and the portion with the filter tip was used in the experiment. The cigarette was lit and immediately placed centrally on the major axis of the inclined test sample with the tip
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TABLE 3 Test Results for New-Grease/Fluff Mixtures Ignited by Matches Duration o f burning rain :s ° Test material
Ignition source
Test 1
Test 2
Test 3
Chain lubricant No. 1 (Black Band) + fluff
Single match Single match and draught Four matches Four matches and draught
Y Y Y Y
>1:15 >2:00 >2:15 >2: 30
Y Y Y Y
>1:15 >2: 00 >2:15 >2: 30
Y Y Y Y
>1:15 >2:00 >2:15 >2: 30
Chain lubricant No. 2 (Red Band) + fluff
Single match Single match and draught Four matches Four matches and draught Single match Single match and draught Four matches Four matches and draught
Y Y Y Y N Y Y Y
>1:30 > 1: 45 >2: 30 >2: 30 0:15 > 1: 30 >2: 30 >3: 00
Y Y Y Y N Y Y Y
>1: 30 > 1: 45 >2: 30 >2: 30 0:37 > 1: 30 >2: 30 >3: 00
Y Y Y Y Y Y Y Y
>1: 30 > l : 45 >2: 30 >2: 30 >2: 30 > 1: 30 >2: 30 >3: 00
Lithium grease No. 3 + fluff
" Y - ignition, N
-
no ignition.
downward. Ignition in this series of tests was defined as any evidence of flaming combustion with no specified duration. The greases listed in Tables 2 and 3 were tested in both still air and simulated draught conditions. Each experiment was repeated three times, and in none of the 42 tests was ignition achieved. With lithium grease new grease, and new-grease/fluff tested with an applied air velocity, there was evidence of degradation of the surface of the test material. It has been reported elsewhere6 that live matches were found in the grease layer under escalator No. 4. Therefore, some experiments were done in order to determine whether such a match could be ignited by contact with a glowing cigarette. Triplicate tests were done in still air conditions with the three new.grease/fluff mixtures, as well as a sample of grease from escalator No. 4. The latter series of tests was repeated with a 5-mm layer of fluff added on top of the grease bed. Unless otherwise stated, in each experiment the live match was pushed into the grease until it was flush with the upper surface, with the stick parallel to the major axis of the inclined plate and the head lowermost. The glowing cigarette was placed on top with the tip nearer the base of the plate, so that on burning back the glow would pass over the match head. It was possible to attain ignition of the match, and subsequently the grease, with every type of sample tested. Three experiments were also done under simulated draught conditions with a fluff-covered sample of grease from escalator No. 4. In one case the match was ejected from the sample on ignition of the head and
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R. K. Wharton
smouldering of the fluff was noted for a period of c. I min. In the other experiments burning of the fluff layer was observed but this process did not last for longer than 45 s and in neither instance did the grease ignite. As well as investigating the ignition of matches in the grease by direct contact with a glowing cigarette, additional work was done to determine whether smouldering combustion could be initiated in the layer of fluff/dust on the grease by a cigarette such that ignition of a remotely located live match could occur. The inclined test apparatus was used with a sample of grease from escalator No. 4 spread 5 mm thick over a distance of 100mm. Dust from the balustrade support trusses was sprinkled on the grease layer to a depth of 5 mm, and a live match was placed in this upper layer with the head lowermost. A glowing cigarette was applied below the match such that the tip was 25 mm from the match head. An applied air velocity of 0.2 m/s was used throughout the test. Smouldering of the dust layer was observed, and this spread slowly away from the tip of the cigarette. The cigarette was removed after 2.5 min, and the dust continued to smoulder witli no visible flame or glow and the production of very little smoke. The edge of the smouldering zone could be seen as a change in colour of the de~'k-grey dust to black. This zone propagated up the incline until, at 6 rain 32 s from application of the cigarette, the match head ignited but failed to induce flaming combustion in either the dust or grease layer. Smouldering of the dust layer continued for c. 30 min. The experiment was repeated using a sample of dust supplied by the Cremer and Warner team and reported to have been collected from King's Cross escalator No. 1. This dust was observed to be more fibrous than that used in the initial test. In this experiment the match was located 50 mm up the track from the glowing tip of the cigarette. Smouldering of the dust layer was not observed until 5 min 15 s from application of the cigarette, and ignition of the match head occurred at 17 min 20 s. 3.3 Tests with cigarettes and greasy paper A series of 42 ignition tests was done using a cigarette applied to a piece of greasy paper laid on the sample surface. The paper sample was a piece of newspaper 80 x 90 ram, which was greased with the material under investigation. The tests examined three new greases, their combination with fluff, and a sample of grease from escalator No. 4. The experiments were
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done in triplicate in the presence and absence of a simulated draught, and in no case was an ignition achieved (defined as any evidence of flaming combustion). Generally, the paper was partially consumed by glowing combustion, and a char remained.
3.4 Tests with burning paper
The results obtained in tests with various greases and grease mixtures using burning crumpled paper as an igniter are listed in Table 4, Of the new greases tested, only chain lubricant No. 1 gave sustained burning with this source of ignition. The sample o2 grease removed from escalator No. 4 could be ignited by burning paper in the absence, but not in the presence, of a draught. In tests with grease/fluff mixtures the chain lubricants gave ignition in TABLE 4 Test Results for Samples of Various Greases Ignited by a Piece of Burning Paper Duration of burning (min :s) a Test 2
Ignition source
Chain lubricant No. 1 (Black Band)
Burning paper Burning paper with draught
Y
>3:00
N
0:22
N Y
0: 34 >2: 30
Y N
>3:00 0:33
Chain lubricant No, 2
Burning paper Burning paper with'draught
N N
0: 34 0: 23
N N
0: 43 0: 29
N N
0:27 0:26
Burning paper Burning paper with draught
N
0: 28
N
0: 34
N N
0: 44 0: 24
N N
0:29 0:22
King's Cross samples
Burning paper ¢,Y Burning paper N with draught
>1: 20 0: 27
N N
0: 26 0: 25
Y N
>1:20 0:19
Chain lubricant No. 1 (Black Band) + fluff
Burning paper Burning paper with draught
Y Y
>1:50 >1:45
Y
>1:50
Y
Y
>1:45
Y
>1:50 >1:45
Chain lubricant No. 2
Burning paper Burning paper with draught
Y Y
>1:45 >2:00
Y Y
>1: 45 >2:00
Y Y
>1:45 >2:00
Burning paper Burning paper with draught
N Y
0: 20 >1: 30
Y N
0: 55 0: 20
N Y
0:25 >1:30
(Red Band)
Lithium grease No. 3
(Red Bane|) + fluff
Lithium Grease No. 3 + fluff
° Y = ignition, N = no ignition.
Test I
Test 3
Test material
44
R. K. Wharton
every test, whereas with lithium grease a growing fire was only obtained when using an applied air velocity. 3.5 Additional tests with new greases mixed with inert material
Some experiments were done with mixtures of new grease and an inert material to investigate the role of the fluff in the burning of samples of grease from King's Cross. One part by weight of chopped, non-combustible, ceramic insulating material (Kaowool) was mixed with nine parts of each new grease, and the mixtures were tested on the 30° inclined surface using match ignition and an applied air velocity. In each of the tests on the mixtures produced with chain lubricants Nos 1 and 2 and lithium grease, flames spread on the surface from the point of application of the match until the whole sample was burning: experiments were terminated after >2 rain of burning. 4 DISCUSSION The simple dropping experiments with matches indicated that it would be possible for a match to remain alight and pass through the gap between the tread and skirting board when thrown from a standing position. The overall probability of this match achieving ignition of the grease layer can be equated to the product of the probability of the match passing through the gap and the probability of ignition as investigated above. However, this simplified analysis takes no accom,t of certain key factors that would decrease the likelihood of ignition being achieved; these include the effect of the moving escalator, the effect of air velocity, the presence of a movihg drive chain above the inclined grease bed on escalator No. 4 and the ability of the match to remain alight during a tumbling trajectory. Approximately one-third of the fire cleats were missing from the treads on the undamaged section of escalator No. 4. The absence of these metal strips could facilitate the ability of a tumbling match to pass through the gap between the tread and skirting board. The experiments with greases and grease/fluff mixtures showed that ignition could not be achieved by using a glowing cigarette, whereas combustion generally resulted when using a flaming ignition source. Crumpled, burning paper was included as one of the igniters
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examined because of the interest of the Inquiry in the possibility of arson. The Investigation Report 4 concludes that the evidence failed to demonstrate any basis for an allegation of arson, leaving ignition by matches as the most likely cause of the fire. Considering the number of passengers travelling on the underground and the proportion of these who would have been smoking (prior to the prohibition by London Underground), situations would have arisen where lighted matches were discarded on the escalator and were able to reach the grease bed on the running track. Evidence that ignition of this grease had occurred previously with limited burning was provided by the observation of localised char damage to the lower edge of the skirting boards. The tests with new greases and the grease taken from under escalator No. 4 indicated that the sample from King's Cross was much more easily ignited by a match. The key role of tbe fluff and detritus in affecting the ignitability of the grease is clearly illustrated by the results obtained for chainlubricant/fluff mixtures; these were very similar to those for the grease sample from King's Cross and substantially different from the new greases alone. The studies using an inert fibrous material mixed with new greases showed that ignition and subsequent burning was more easily achieved than with just the new grease. The results were similar to those obtained when testing the mixtures of grease and fluff, and indicate that the enhanced ignitability and burning found with grease/fluff mixtures is due to a wicking action by the fluff rather than to the inherent combustibility of this component. Although ignition of greases and grease/fluff mixtures by a glowing cigarette was shown to be unlikely to occur, it has been demonstrated that smouldering combustion can be initiated in fine dust on the upper surface of a layer of grease. The rate of smouldering of the dust layer evaluated from the experiment in which smouldering progressed over 25 mm was 6 × 10-2mm/s, which is in good agreement with literature values7 for the smouldering rates of various dusts (e.g. beech, cork, grass), reported to be in the range c. 1.5-7 × 10-2 mm/s. Similarly, the experiment in which smouldering was monitored over a distance of 50 mm yielded a rate of 7 × 10-2 mm/s. The advancing front of this type of smouldering was capable of igniting a live match embedded in the grease layer. However, this mode of match ignition is probably much less likely to occur than direct ignition of the layer of grease by a flaming match dropped from above.
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R. K. Wharton
5 CONCLUSIONS The experiments reported here established that (1) in no case was it possible to achieve ignition using a glowing cigarette or a glowing-cigarette/greasy-paper combination; (2) ignition by a flaming match was possible for certain of the greases tested; (3) ignition with sustained burning was attained most easily with the grease/debris from escalator No. 4; (4) of the new greases, only chain lubricant No. 1 could be ignited by pieces of burning paper; (5) burning paper could ignite the grease sample from escalator No. 4; (6) new greases with fluff were more easily ignited than new greases alone; and (7) the enhanced ignitability and subsequent burning found when fluff was present due to a wicking effect. Thus, the presence of fluff and detritus in the grease layer under the escalator at King's Cross was a key factor in the initiation of the fire. This raises wider fire safety issues concerning cleanliness and good housekeeping. It was also shown that a live match head embedded in samples of new grease/fluff a ~ grease from escalator No. 4 could be ignited by contact with a glowin~ eiF~.~ette. Tests with a layer of dust/fluff on top of a sample of grease from King's Cross indicated that smouldering could be induced by a glowing cigarette and that this could propagate over some distance to initiate ignition of a remotely located live match. The evidence presented in this paper was considered by the Inquiry and forms the basis of the conclusion 4 that the fire was probably initiated by a carelessly discarded lighted match which fell through the clearance between the steps and the skirting board on the right-hand side of escalatoT No. 4. The match fell on to the running track between the chain and t~:ailer wheels, where there was an excessive accumulation of readily ignitable grease and detritus.
ACKNOWLEDGEMENT The author is grateful to Mr A. E. Jeffcock for experimental assistance, which was provided to a tight timetable in order to meet the needs of the Inquiry.
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REFERENCES 1. Moodie, K. & Wharton, R. K., Fire at King's Cross Underground Station, 18th November 1987. Part 1: Damage assessment. RInD Incident Report No. IR/L/FR/87/54, Health and Safety Executive, Buxton, UK, 1987. 2. Halliday, D. J. X., Fire Investigation Report, King's Cross Underground Station. Metropolitan Police Forensic Science Laboratory, London, UK, undated. 3. Wharton, R. K., Fire at King's Cross Underground Station, 18th November 1987. Part 5: Ignition tests and characterisation of samples. RLSD Incident Report No. IR/L/FR/88/6, Health and Safety Executive, Buxton, UK, 1988. 4. Fennell, D., Investigation into the King's Cross Underground Fire. HMSO, London, UK, 1988. 5. Moodie, K., Fire at King's Cross Underground Station, 18th November 1987. Part 2: Interim proposals for assessment of fire. RLSD Incident Report No. IR/L/FR/87/55, Health and Safety Executive, Buxton, UK, 1987. 6. London Fire and Civil Defence Authority, Fire at King's Cross Underground Station at 19.36 hours on 18th November 1987. London Fire Brigade Fire Investigation Report, London, UK, 1988, p. 31, para 2.69. 7. Palmer, K. N., Smouldering combustion in dusts and fibrous materials. Combust. Flame, 1 (1957) 129.
Studies Relating to Ignition of the Fire at King's Cross Underground Station R. K. Wharton Explosion and Flame Laboratory, Health and SafetyExecutive, Harper Hill, Buxton, Derbyshire SK179JN, UK
ABSTRA CT The damage survey after the fire at King's Cross Underground Station found a layer of grease and debris on the running track below the undamaged section of escalator No. 4. The ignition characteristics of this layer were important in relation to the role it may have played in the incident. This paper summarises tests done with a range of ignition sources on new grease and samples removed from escalator No. 4. Work with new greases mixed with fluff or inert fibrous material established that the presence of these additional components greatly enhances the ignitability of the grease by acting as a wick. The greases could be ignited by a flaming match, but not by a glowing cigarette. However, cigarettes initiated smouldering combustion in the types of dust layers found beneath the escalator.
1 INTRODUCTION Following the fire at King's Cross Underground Station on 18 November 1987, in which 31 people died, scientists from the Research and Laboratory Services Division of the Health and Safety Executive (HSE) investigated the cause and spread of the fire, initially at the request of the Chief Inspecting Ott~cer of Railways (Department of Transport), and subsequently on behalf of the Inspector of the Public Inquiry. The initial task for the HSE team was to catalogue the damage to the escalator and booking hall. The report of this work I provided evidence 35 (~) 1991Crown Copyright
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R. K. Wharton
Fig. 1. Example of the grease layer on the running track below escalator No. 4.
to suggest that the fire started under the right-hand side of escalator No. 4, and also established that a layer of grease and debris was present on the running track below the undamaged section of the escalator (Fig. I). During the course of the investigationvarious theories for ignition of the firewere examined, including arson, electricaleffectsand frictional heating. Halliday's post-fire investigation"~ established that matches were found on the running track underneath the escalator and that initiation of the fire by discarded smokers' materials had to be considered a strong possibility. Comprehensive testswere undertaken at HSE's Explosion and Flame Laboratory at Buxton to investigatethe ignitabilityof the various types of grease used on the escalator machinery. A range of ignition sources was examined, and the results were presented to the Inquiry in a reports that forms the basis of this paper. Other theories for ignition of the fire are referred to in the assessments made by Fennell in his Investigation Report. 4 2 EXPERIMENTAL PROGRAMME Samples of new grease and chain lubricants of the type identified as being in possible use on escalator No. 4 were obtained from London
Studies relating to ignition of the fire
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Underground Ltd by Cremer and Warner (London, UK), technical consultants to the Inquiry. Detailed specifications are given in Ref. 3. Chain lubricants Nos 1 and 2 are referred to as 'Black Band' and 'Red Band', respectively. On-site investigations found quantities of dust present on the balustrade support trusses to the right-hand side of the undamaged section of escalator No. 4. Samples were removed for analysis and testing. Samples of the grease layer from the right-hand track under the undamaged section of escalator No. 4 were taken for use in the ignition tests. A portion of the grease layer from the track was repeatedly washed with a suitable solvent (Lec, Arrow Chemicals Limited, Burton-onTrent, UK) to extract soluble components, and the solution was filtered. A fine, dark-grey deposit was retained in the filter paper (thought to be the molybdenum disulphide additive listed in the specification for both chain lubricants), while a fluff layer remained above. The latter was repeatedly washed until the solvent remained clean. The proportion of solid foreign matter in the sample was 6%. Samples of the dust from the balustrade support trusses and the extracted fluff were examined in order to provide information on both the nature of these constituents and on a suitable material that could be added to new greases to simulate the grease/fluff mix found under escalator No. 4. Because of the short time-scale in which to do the experiments, household vacuum-cleaner debris was used as representative of the type of fluff/dust found in the analysis, and 10% grease/fluff mixtures were created by mixing nine parts by weight of new grease with one part of fluff. In order to make the mixtures as close as possible to those recovered, cigarette ends and used matches were added to the fluff prior to mixing. The mixed samples were similar in appearance to greases taken from King's Cross. A comprehensive series of tests was done using various ignition sources and a range of greases, including samples from King's Cross, new greases (lithium grease, chain lubricants Nos 1 and 2), and combinations of new grease and fluff. Since the angle of elevation of the escalator involved in the fire was 30° , the tests were done with the grease samples smeared on a 5-mm-thick metal plate, 1 m long and 60 mm wide, inclined at 30° to the horizontal. Measured local air velocities in the region of the track under the Victoria line escalators adjacent to the scene of the fire ranged from 0.1 to 0.5 m/s. s Therefore, a value of 0.2 m/s was taken as representative of the velocity that could have been present above the inclined grease
R. K. Wharton
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TABLE 1 Calibration of Ignition Sources
Mean burning time (s) Igniter
Still air
Draught ~
Single match Four-match crib Crumpled paper
39 65 27
26 58 25
° 0.2 m/s.
layer under escalator No. 4 while it was moving. Experiments with various ignition sources were done, therefore, in both still air conditions and with an applied air velocity (measured above the grease bed at the point of ignition using a Prosser Scientific Instruments Ltd (Ipswich, UK) low-flow air velocity meter Type AVM502 with ambient temperature correction). The match and crumpled-paper igniters were calibrated 3 by determining the mean burning time from 20 tests with an inert ceramic blanket on the inclined surface: ignition in these tests was then defined as burning that lasted for longer than twice the mean burning time of the igniter alone under the same set of conditions (Table 1). The four-match crib was produced by igniting three spent matches with a burning match, and v,as included in the studies because collections of match sticks were f~.~utj~ in the grease layer under the undamaged section of escalator No. 4.. 3 RESULTS
&l Tests with matches In order to avoid variations in the ignition source, one brand of matches was used for all the experiments. Experiments were done to obtain information relating to the likelihood of a dropped burning match igniting a sample of grease taken from the track under the right-hand side of escalator No. 4. The grease sample was well mixed and placed in a layer 10 mm thick on the inclined, metal test plate. Matches were struck and, once flaming of the match head had ceased, dropped from a height of 1.1 m in still air on to the grease layer. The experiment was repeated until 20 matches had hit the test sample. Nine matches were still burning after landing on the fuel bed and, of
Studies relating to ignition of the fire
39
these, five produced ignition of the grease layer (defined as combustion growing from the point of impact of the match such that a selfsustaining fire would have been produced). Further work was done to investigate the ability of dropped matches to fall down the gap between the escalator treads and skirting boards. This gap on escalator No. 4 is reported 6 to have been 0-12 mm. On-site measurements of the undamaged section of escalator No. 4 indicated gaps of c. 8 mm. A section of escalator constructed at Buxton for use in a fire trial was used for the tests in which matches were dropped from a height of approximately 1.1m and thrown in the general direction of the right-hand edge of the treads where they met the skirting boards. The gap for these experiments was c. 10mm and the experimenter stood 20 cm from the edge of the step. The metal edge strips (fire cleats) were present at the right hand side of all the treads and the tests were done in still air conditions. One hundred unlit matches were thrown away from the body with the right hand whilst not looking at the stair treads, and, of these, eight passed through the gap. Observations made during these tests suggest that a possible mechanism for matches passing through the gap involves impact with a step cleat followed by a tumbling trajectory towards the right-hand side of the tread. During the post-fire investigation, a layer of fluff was found surmounting the grease layer in certain locations. In order to ascertain whether the presence of fluff could have influenced the ignition and burning characteristics of the grease layer, the tests were repeated with a c. 10-mm layer of vacuum-cleaner debris on a sample of grease from escalator No. 4. Most of the burning matches bounced off the fibrous layer. Of 50 that remained lodged on the surface, two instances of ignition of the fluff layer were recorded: combustion lasted for 10 and 46s, and neither case involved burning of the grease substrate. Table 2 gives results of match ignition tests with grease on a 30 ° inclined surface. For new chain lubricant, which was tested as a c. 4-mm-thick layer, it was possible to achieve ignition (as defined earlier) only in the absence of a draught, but the burning process was short-lived. V'ith the four-match igniter, ignition was obtained in all six experiments. Ignition of chain lubricant No. 2 was achieved in all the tests with a single match in still air, and in one test with four matches plus applied draught. In no case was the duration of the burning process greater than 2 min 16 s.
40
R. K. W h a r t o n TABLE
2
Test Results for S a m p l e s o f N e w Grease and G r e a s e from Escalator N o . 4 Ignited by Matches Duration of burning rain :s a Test material
Chain lubricant No. 1 Black Band
Ignition source Single match
Single match and draught Four matches Four matches and draught
Chain lubricant No. 2 Red Band
Single match
Single match and draught Four matches Four matches and draught
Lithium grease No. 3
Single match
Single match and draught Four matches Four matches and draught
King's Cross samples
Single match Single match and draught Four matches Four matches and draught
Test I
Test 2
Test
3
Y N Y Y
1: 34 0: 20 >3: 00 >3:00
N N Y Y
0: 25 0: 35 > 3 : 00 2:17
Y N Y Y
3:10 0: 20 >3: 00 3:31
Y Nb N N
2:05 0: 21 1: 31 0:41
Y Nb N Y
1:40 0:16 1: 49 2:16
Y Nb N N
2:01 0:17 !: 33 0:48
N N Y Y
0:55 0:24 >3: 00 >4:00
N N Y N
1:13 0: 26 > 3 : 00 0:29
N Y Y Y
0:34 1: 03 1: 53 >4:00
Y Y Y Y
>1:00 >1:30 >2: 00 >3: 00
Y N Y Y
>1:00 0:28 > 2 : 00 > 3 : 00
N N Y N
0:34 0:20 >2: 00 0: 41
a y = ignition, N = no ignition. b Matches only burnt half-way down stick.
Chain lubricant No. 2 was less viscous than No. I, and the maximum depth that could be attained on the inclined surface was 2 ram. The observations in Table 2 can be explained in terms of the matches sinking into the grease layer and extinguishing, rather than resting on top with continued burning. Tests were also done with an 8-10-mm layer of lithium grease No. 3 on the inclined surface. Ignition by a single match was achieved in only one of the six experiments, while only one of the six tests done with four matches failed to result in ignition. With the four-match configuration, four of the six experiments produced a growing fire that had to be extinguished. The series of experiments was repeated using mixtures of new grease and fluff; the results obtained are reported in Table 3. 3.2 Tests with glowing cigarettes In order to standardise the igniter, the same brand of filter-tip king-size cigarettes was used for all the tests. The cigarette was cut at the mid point of the length of tobacco, and the portion with the filter tip was used in the experiment. The cigarette was lit and immediately placed centrally on the major axis of the inclined test sample with the tip
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41
TABLE 3 Test Results for New-Grease/Fluff Mixtures Ignited by Matches Duration o f burning rain :s ° Test material
Ignition source
Test 1
Test 2
Test 3
Chain lubricant No. 1 (Black Band) + fluff
Single match Single match and draught Four matches Four matches and draught
Y Y Y Y
>1:15 >2:00 >2:15 >2: 30
Y Y Y Y
>1:15 >2: 00 >2:15 >2: 30
Y Y Y Y
>1:15 >2:00 >2:15 >2: 30
Chain lubricant No. 2 (Red Band) + fluff
Single match Single match and draught Four matches Four matches and draught Single match Single match and draught Four matches Four matches and draught
Y Y Y Y N Y Y Y
>1:30 > 1: 45 >2: 30 >2: 30 0:15 > 1: 30 >2: 30 >3: 00
Y Y Y Y N Y Y Y
>1: 30 > 1: 45 >2: 30 >2: 30 0:37 > 1: 30 >2: 30 >3: 00
Y Y Y Y Y Y Y Y
>1: 30 > l : 45 >2: 30 >2: 30 >2: 30 > 1: 30 >2: 30 >3: 00
Lithium grease No. 3 + fluff
" Y - ignition, N
-
no ignition.
downward. Ignition in this series of tests was defined as any evidence of flaming combustion with no specified duration. The greases listed in Tables 2 and 3 were tested in both still air and simulated draught conditions. Each experiment was repeated three times, and in none of the 42 tests was ignition achieved. With lithium grease new grease, and new-grease/fluff tested with an applied air velocity, there was evidence of degradation of the surface of the test material. It has been reported elsewhere6 that live matches were found in the grease layer under escalator No. 4. Therefore, some experiments were done in order to determine whether such a match could be ignited by contact with a glowing cigarette. Triplicate tests were done in still air conditions with the three new.grease/fluff mixtures, as well as a sample of grease from escalator No. 4. The latter series of tests was repeated with a 5-mm layer of fluff added on top of the grease bed. Unless otherwise stated, in each experiment the live match was pushed into the grease until it was flush with the upper surface, with the stick parallel to the major axis of the inclined plate and the head lowermost. The glowing cigarette was placed on top with the tip nearer the base of the plate, so that on burning back the glow would pass over the match head. It was possible to attain ignition of the match, and subsequently the grease, with every type of sample tested. Three experiments were also done under simulated draught conditions with a fluff-covered sample of grease from escalator No. 4. In one case the match was ejected from the sample on ignition of the head and
42
R. K. Wharton
smouldering of the fluff was noted for a period of c. I min. In the other experiments burning of the fluff layer was observed but this process did not last for longer than 45 s and in neither instance did the grease ignite. As well as investigating the ignition of matches in the grease by direct contact with a glowing cigarette, additional work was done to determine whether smouldering combustion could be initiated in the layer of fluff/dust on the grease by a cigarette such that ignition of a remotely located live match could occur. The inclined test apparatus was used with a sample of grease from escalator No. 4 spread 5 mm thick over a distance of 100mm. Dust from the balustrade support trusses was sprinkled on the grease layer to a depth of 5 mm, and a live match was placed in this upper layer with the head lowermost. A glowing cigarette was applied below the match such that the tip was 25 mm from the match head. An applied air velocity of 0.2 m/s was used throughout the test. Smouldering of the dust layer was observed, and this spread slowly away from the tip of the cigarette. The cigarette was removed after 2.5 min, and the dust continued to smoulder witli no visible flame or glow and the production of very little smoke. The edge of the smouldering zone could be seen as a change in colour of the de~'k-grey dust to black. This zone propagated up the incline until, at 6 rain 32 s from application of the cigarette, the match head ignited but failed to induce flaming combustion in either the dust or grease layer. Smouldering of the dust layer continued for c. 30 min. The experiment was repeated using a sample of dust supplied by the Cremer and Warner team and reported to have been collected from King's Cross escalator No. 1. This dust was observed to be more fibrous than that used in the initial test. In this experiment the match was located 50 mm up the track from the glowing tip of the cigarette. Smouldering of the dust layer was not observed until 5 min 15 s from application of the cigarette, and ignition of the match head occurred at 17 min 20 s. 3.3 Tests with cigarettes and greasy paper A series of 42 ignition tests was done using a cigarette applied to a piece of greasy paper laid on the sample surface. The paper sample was a piece of newspaper 80 x 90 ram, which was greased with the material under investigation. The tests examined three new greases, their combination with fluff, and a sample of grease from escalator No. 4. The experiments were
Studies relating to ignition of the fire
43
done in triplicate in the presence and absence of a simulated draught, and in no case was an ignition achieved (defined as any evidence of flaming combustion). Generally, the paper was partially consumed by glowing combustion, and a char remained.
3.4 Tests with burning paper
The results obtained in tests with various greases and grease mixtures using burning crumpled paper as an igniter are listed in Table 4, Of the new greases tested, only chain lubricant No. 1 gave sustained burning with this source of ignition. The sample o2 grease removed from escalator No. 4 could be ignited by burning paper in the absence, but not in the presence, of a draught. In tests with grease/fluff mixtures the chain lubricants gave ignition in TABLE 4 Test Results for Samples of Various Greases Ignited by a Piece of Burning Paper Duration of burning (min :s) a Test 2
Ignition source
Chain lubricant No. 1 (Black Band)
Burning paper Burning paper with draught
Y
>3:00
N
0:22
N Y
0: 34 >2: 30
Y N
>3:00 0:33
Chain lubricant No, 2
Burning paper Burning paper with'draught
N N
0: 34 0: 23
N N
0: 43 0: 29
N N
0:27 0:26
Burning paper Burning paper with draught
N
0: 28
N
0: 34
N N
0: 44 0: 24
N N
0:29 0:22
King's Cross samples
Burning paper ¢,Y Burning paper N with draught
>1: 20 0: 27
N N
0: 26 0: 25
Y N
>1:20 0:19
Chain lubricant No. 1 (Black Band) + fluff
Burning paper Burning paper with draught
Y Y
>1:50 >1:45
Y
>1:50
Y
Y
>1:45
Y
>1:50 >1:45
Chain lubricant No. 2
Burning paper Burning paper with draught
Y Y
>1:45 >2:00
Y Y
>1: 45 >2:00
Y Y
>1:45 >2:00
Burning paper Burning paper with draught
N Y
0: 20 >1: 30
Y N
0: 55 0: 20
N Y
0:25 >1:30
(Red Band)
Lithium grease No. 3
(Red Bane|) + fluff
Lithium Grease No. 3 + fluff
° Y = ignition, N = no ignition.
Test I
Test 3
Test material
44
R. K. Wharton
every test, whereas with lithium grease a growing fire was only obtained when using an applied air velocity. 3.5 Additional tests with new greases mixed with inert material
Some experiments were done with mixtures of new grease and an inert material to investigate the role of the fluff in the burning of samples of grease from King's Cross. One part by weight of chopped, non-combustible, ceramic insulating material (Kaowool) was mixed with nine parts of each new grease, and the mixtures were tested on the 30° inclined surface using match ignition and an applied air velocity. In each of the tests on the mixtures produced with chain lubricants Nos 1 and 2 and lithium grease, flames spread on the surface from the point of application of the match until the whole sample was burning: experiments were terminated after >2 rain of burning. 4 DISCUSSION The simple dropping experiments with matches indicated that it would be possible for a match to remain alight and pass through the gap between the tread and skirting board when thrown from a standing position. The overall probability of this match achieving ignition of the grease layer can be equated to the product of the probability of the match passing through the gap and the probability of ignition as investigated above. However, this simplified analysis takes no accom,t of certain key factors that would decrease the likelihood of ignition being achieved; these include the effect of the moving escalator, the effect of air velocity, the presence of a movihg drive chain above the inclined grease bed on escalator No. 4 and the ability of the match to remain alight during a tumbling trajectory. Approximately one-third of the fire cleats were missing from the treads on the undamaged section of escalator No. 4. The absence of these metal strips could facilitate the ability of a tumbling match to pass through the gap between the tread and skirting board. The experiments with greases and grease/fluff mixtures showed that ignition could not be achieved by using a glowing cigarette, whereas combustion generally resulted when using a flaming ignition source. Crumpled, burning paper was included as one of the igniters
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45
examined because of the interest of the Inquiry in the possibility of arson. The Investigation Report 4 concludes that the evidence failed to demonstrate any basis for an allegation of arson, leaving ignition by matches as the most likely cause of the fire. Considering the number of passengers travelling on the underground and the proportion of these who would have been smoking (prior to the prohibition by London Underground), situations would have arisen where lighted matches were discarded on the escalator and were able to reach the grease bed on the running track. Evidence that ignition of this grease had occurred previously with limited burning was provided by the observation of localised char damage to the lower edge of the skirting boards. The tests with new greases and the grease taken from under escalator No. 4 indicated that the sample from King's Cross was much more easily ignited by a match. The key role of tbe fluff and detritus in affecting the ignitability of the grease is clearly illustrated by the results obtained for chainlubricant/fluff mixtures; these were very similar to those for the grease sample from King's Cross and substantially different from the new greases alone. The studies using an inert fibrous material mixed with new greases showed that ignition and subsequent burning was more easily achieved than with just the new grease. The results were similar to those obtained when testing the mixtures of grease and fluff, and indicate that the enhanced ignitability and burning found with grease/fluff mixtures is due to a wicking action by the fluff rather than to the inherent combustibility of this component. Although ignition of greases and grease/fluff mixtures by a glowing cigarette was shown to be unlikely to occur, it has been demonstrated that smouldering combustion can be initiated in fine dust on the upper surface of a layer of grease. The rate of smouldering of the dust layer evaluated from the experiment in which smouldering progressed over 25 mm was 6 × 10-2mm/s, which is in good agreement with literature values7 for the smouldering rates of various dusts (e.g. beech, cork, grass), reported to be in the range c. 1.5-7 × 10-2 mm/s. Similarly, the experiment in which smouldering was monitored over a distance of 50 mm yielded a rate of 7 × 10-2 mm/s. The advancing front of this type of smouldering was capable of igniting a live match embedded in the grease layer. However, this mode of match ignition is probably much less likely to occur than direct ignition of the layer of grease by a flaming match dropped from above.
46
R. K. Wharton
5 CONCLUSIONS The experiments reported here established that (1) in no case was it possible to achieve ignition using a glowing cigarette or a glowing-cigarette/greasy-paper combination; (2) ignition by a flaming match was possible for certain of the greases tested; (3) ignition with sustained burning was attained most easily with the grease/debris from escalator No. 4; (4) of the new greases, only chain lubricant No. 1 could be ignited by pieces of burning paper; (5) burning paper could ignite the grease sample from escalator No. 4; (6) new greases with fluff were more easily ignited than new greases alone; and (7) the enhanced ignitability and subsequent burning found when fluff was present due to a wicking effect. Thus, the presence of fluff and detritus in the grease layer under the escalator at King's Cross was a key factor in the initiation of the fire. This raises wider fire safety issues concerning cleanliness and good housekeeping. It was also shown that a live match head embedded in samples of new grease/fluff a ~ grease from escalator No. 4 could be ignited by contact with a glowin~ eiF~.~ette. Tests with a layer of dust/fluff on top of a sample of grease from King's Cross indicated that smouldering could be induced by a glowing cigarette and that this could propagate over some distance to initiate ignition of a remotely located live match. The evidence presented in this paper was considered by the Inquiry and forms the basis of the conclusion 4 that the fire was probably initiated by a carelessly discarded lighted match which fell through the clearance between the steps and the skirting board on the right-hand side of escalatoT No. 4. The match fell on to the running track between the chain and t~:ailer wheels, where there was an excessive accumulation of readily ignitable grease and detritus.
ACKNOWLEDGEMENT The author is grateful to Mr A. E. Jeffcock for experimental assistance, which was provided to a tight timetable in order to meet the needs of the Inquiry.
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47
REFERENCES 1. Moodie, K. & Wharton, R. K., Fire at King's Cross Underground Station, 18th November 1987. Part 1: Damage assessment. RInD Incident Report No. IR/L/FR/87/54, Health and Safety Executive, Buxton, UK, 1987. 2. Halliday, D. J. X., Fire Investigation Report, King's Cross Underground Station. Metropolitan Police Forensic Science Laboratory, London, UK, undated. 3. Wharton, R. K., Fire at King's Cross Underground Station, 18th November 1987. Part 5: Ignition tests and characterisation of samples. RLSD Incident Report No. IR/L/FR/88/6, Health and Safety Executive, Buxton, UK, 1988. 4. Fennell, D., Investigation into the King's Cross Underground Fire. HMSO, London, UK, 1988. 5. Moodie, K., Fire at King's Cross Underground Station, 18th November 1987. Part 2: Interim proposals for assessment of fire. RLSD Incident Report No. IR/L/FR/87/55, Health and Safety Executive, Buxton, UK, 1987. 6. London Fire and Civil Defence Authority, Fire at King's Cross Underground Station at 19.36 hours on 18th November 1987. London Fire Brigade Fire Investigation Report, London, UK, 1988, p. 31, para 2.69. 7. Palmer, K. N., Smouldering combustion in dusts and fibrous materials. Combust. Flame, 1 (1957) 129.