Appendix Mexico City

4
Contents

A4.1 The Site and the Plant A4/2

A4.2 The Fire and Explosion ! 1 A4/3
A4.3 The Emergency A4/6
A4.4 The Fire and Explosion ! 2 A4/7
A4.5 Some Lessons of Mexico City A4/7

LEES, Frank. Lees' Loss prevention in the process industries: Hazard identification,
assessment and control. Butterworth-Heinemann, 2012.
APPENDIX 4/2 MEXICO CITY

At about 5.35 a.m. on the morning of 19 November 1984, The oldest part of the plant dated from 1961 to 1962
a major fire and a series of explosions occurred at and was thus over 20 years old. In the intervening period
the PEMEX LPG terminal at San Juan Ixhuatepec (San residential development had crept up to the site. This
Juanico), Mexico City. Some 500 people were killed and the process of encroachment is clearly visible from the aerial
terminal was destroyed. photographs shown in Figure A4.3. By 1984, the housing
The accident was investigated by a team from TNO was within 200 m of the installation with some houses
which visited the site about 2 weeks after the disaster and within 130 m.
has published its findings (Pietersen, 1985). A further The terminal was used for the distribution of LPG, which
account has been given by Skandia International (1985). came by pipeline from three different refineries. The main
Selected references on Mexico City are given inTable A4.1. LPG storage capacity of 16,000 m 3 consisted of 6 spheres
and 48 horizontal cylinders. The daily throughput was
A4.1 The Site and the Plant 5000 m 3. The layout of the terminal with storage tank
capacities is shown in Figure A4.4. The two larger storage
The site of the PEMEX terminal is shown in Figure A4.1 and spheres had individual capacities of 2400 m and the four
an aerial photograph of the installation itself in Figure A4.2. smaller spheres capacities of 1600 m 3. The site covered an
area of 13,000 m 2.
The plant was said to have been built to API standards
Table A4.1 Selected references on Mexico City and much of it to have been manufactured in the United
Anon. (1985v, hh); Berenblut et al. (1985); Cullen (1985); States.
Kletz (1985p); Pietersen (1985, 1985 LPB 64, 1986a,b, A ground level flare was used to burn off excess gas. The
1988a); Skandia International (1985); Hagon (1986) flare was submerged in the ground to prevent the flame
being extinguished by the strong local winds.

Figure A4.1 Area plan of the PEMEX site at Mexico City (Pietersen 1985) (Courtesy of TNO)
 MEXICO CITY APPENDIX 4/3

Figure A4.2 PEMEX plant at Mexico City before the accident (Skandia, 1985) (Photograph: State of Mexico)

Adjoining the PEMEX plant there were distribution

depots owned by other companies. The Unigas site
was some 100!200 m to the north and contained 67 tank
trucks at the time of the accident. Rather further away
was the Gasomatico site with large numbers of domestic
gas cylinders.

A4.2 The Fire and Explosion ! 1

Early in the morning of 18 November, the plant was being
filled from a refinery 400 km away. The previous day the
plant had become almost empty and refilling started dur-
ing the afternoon. The two larger spheres and the 48
cylindrical vessels had been filled to 90% full and the four
smaller spheres to about 50% full, so that the inventory on
site was about 11,000 m 3, when the incident began.
About 5.30 a.m. a fall in pressure was registered in the
control room and also at a pipeline pumping station 40 km
distant. An 8 in. pipe between sphere F4 and the Series G
cylinders had ruptured.
The control room personnel tried to identify the cause of
the pressure fall but without success.
The release of LPG continued for some 5!10 min. There
was a slight wind of 0.4 m/s. The wind and the sloping
terrain carried the gas towards the south-west. People in
the nearby housing heard the noise of the escape and
smelled the gas.
Figure A4.3 Growth of housing near the PEMEX plant When the gas cloud had grown to cover an area, which
at Mexico City (Pietersen 1985): (a) 1962; (b) 1972; eyewitnesses put at 200 " 150 m with a height of 2 m, it
(c) 1982 (Reproduced by permission of TNO.) found the flare and ignited. It was 5.40 a.m. The cloud
APPENDIX 4/4 MEXICO CITY

Figure A4.4 Layout of the PEMEX site at Mexico City (Pietersen 1985) (Courtesy of TNO)

caught fire over a large area, giving a high flame and

causing violent ground shock. Table A4.2 Timetable of events at Mexico City
When this general fire had subsided, there remained
a ground fire, a flame at the rupture and fires in some A Seismograph readings
10 houses.
Workers on the plant now tried to deal with the escape. 1 5h 44 min 52 s 6 6h 49 min 38 s
One drove off to another depot to summon help. Five others 2 5h 46 min 01 s 7 6h 54 min 29 s
who may have been on their way to the control room or to 3 6h 15 min 53 s 8 6h 59 min 22 s
man fire pumps were found dead, and badly burned. At a 4 6h 31 min 59 s 9 7h 01 min 27 s
late stage someone evidently pressed the emergency shut- 5 6h 47 min 56 s
down button.
Disturbances 2 and 7 were the most intence with a Richter scale
In the neighbouring housing some people rushed out intensity of 5 Skandia suggest that the first violent combustion may
into the street, but most stayed indoors. Many thought it not have been recorded.
was an earthquake.
At 5.45 a.m. the first BLEVE occurred. About a minute B General timetable
later another explosion occurred, one of the two most vio- 5.30 Rupture of 8 in. pipe. Fall of pressure
lent during the whole incident. One or two of the smaller in control room
spheres BLEVEd, giving a fireball 300 m diameter. 5.40 Ignition of gas cloud.Violent combustion
A rain of LPG droplets fell on the area. Surfaces covered and high flame
in the liquid were set alight by the heat from fireballs. 5.45 First explosion on seismograph, a BLEVE
People burned like torches. Fire department called
There followed a series of explosions as vessels suffered 5.46 Second BLEVE, one of most violent
BLEVE. There were some 15 explosions over a period of an 6.00 Police alerted and civilian trafffic stopped
hour and a half. BLEVE occurred of the four smaller 6.30 Traffic chaos
spheres and many of the cylindrical vessels. 7.01 Last explosion on seismograph, a BLEVE
The explosions during the incident were recorded on a 7.30 Continuing tank explosions a
seismograph at the University of Mexico. The timing of the 11.00 Last tank explosion
readings is shown in Table A4.2 Section A. As the footnote 8.00!10.00 Rescue work at its height
indicates, it is suggested by Skandia that the initial explo- 12.00!18.00 Rescue work continues
sion, or violent deflagration, was probably not recorded. 23.00 Flames extinguished on last large sphere
The damage caused is shown in Figure A4.5, which
shows the area of main housing damage and the fall of a
Explosions of cylindrical vessels.
Figure A4.5 Area plan of the PEMEX site at Mexico City, showing damage to housing area and fall of missiles (Pietersen 1985) (Courtesy to TNO)
APPENDIX 4/6 MEXICO CITY

Figure A4.6 PEMEX site at Mexico City after the accident (Skandia, 1985) (Photograph: State of Mexico)

missiles. An aerial photograph of the plant after the dis- was dark inside and there was plenty of smoke. I could not
aster is shown in Figure A4.6. see anything. I could not breathe for the gas. I noticed
Numerous missiles were generated by the bursting of the something huge fell on top of the house and a rush of air
vessels. Many of these were large and travelled far. Twenty- threw me out of the house into the street. By then I was
five large fragments from the four smaller spheres weigh- really frightened and I started to run as fast as I could. I do
ing 10!40 te were found 100!890 m away. Fifteen of the not know where my wife and children got to. They may
48 cylindrical vessels weighing 20 te became missiles and even be unidentified in a mass grave.
rocketed over 100 m, one travelling 1200 m. Four cylinders
A timetable of events during the disaster is given in
were not found at all. The missiles caused damage both by
Table A4.2, Section B.
impact and by their temperature, which was high enough to
set houses alight.
When the fire began, people were already on their way to A4.3 The Emergency
work. Eyewitnesses spoke of a huge hot, red light, intense Accounts of the emergency give little information about the
heat, smoke and lack of air, blast waves and missiles. The response of the on-site management in the emergency and
following account by Nicanor Santiago, a mason, is typical: deal mainly with the rescue and firefighting.
The site became the scene of a major rescue operation
Around 5.30 a.m. I went to work. It was still dark when which reached a climax in the period 8.00!10.00 a.m. Some
I took my bicycle out of the house, when suddenly there 4000 people participated in rescue and medical activities,
was this huge light, red and hot. I could see nothing at all. including 985 medics, 1780 paramedics and 1332 volun-
The huge light blinded me. I could not feel anything teers. At one point there were some 3000 people in the area.
except that everything was hot. Then I heard some There were 363 ambulances and five helicopters involved.
explosions and a second blast.The walls of my house were The rescuers were at risk from a large BLEVE.The Skandia
rocking, it was an earthquake. I was lying on the pave- report states: ‘If a BLEVE had occurred during the later
ment and close to me all sorts of matter came falling out of morning, a large number of those 3,000 people who were
the sky.There was a lot of broken glass, chairs and flower engaged in rescue and guarding would have been killed.’
pots were flying all over the place. I suddenly remem- The fire services were called by surrounding plants and
bered the PEMEX gas plant and I saw a big tongue of by individual members of the public about 5.45 a.m. They
fire and a very big orange mushroom, and then a felt went into the plant area only 3 h after the start of the
another explosion. Pieces of molten metal were dropping incident. Initially, they moved towards the Gasomatico site,
out of the sky and I felt intense heat waves burning my where a sphere fragment had landed and started a fire,
clothes and my hair. I ran into the bedroom, everything which caused the domestic gas cylinders to explode.
 MEXICO CITY APPENDIX 4/7

The fire brigade also fought the fire on the two larger Table A4.3 Some lessons of Mexico City
spheres, which had not exploded. They were at appreciable
risk from BLEVE of the spheres. In the event, however, these Siting of major hazard installations
burnt themselves out. The last flames on the spheres went Layout of large LPG storages
out about 11.00 p.m. Some 200 firemen attended the site. Gas detection and emergency isolation
Planning for emergencies
Fire fighting in BLEVE situations
A4.4 The Fire and Explosion ! 2 Boiling liquid expanding vapour explosions
The TNO report gives much technical information on the
course of the disaster and on the fire and explosion phe-
nomena which occurred during it.
It discusses the effects of explosions, including vapour
cloud explosions, BLEVEs and physical explosions; the Layout and protection of large LPG storages
effects of fire engulfment and heat radiation; and the The total destruction of the facility occurred because there
effects of missiles, including fragments from bullet tanks was a failure of the overall system of protection, which
and spheres. includes layout, emergency isolation and water spray
The report gives estimates of the overpressure from systems.
BLEVE of the principal vessels. It states that the degree of
blast damage to housing was not great; that the vapour Gas detection and emergency isolation
cloud explosion effects were not responsible for major One feature which might have averted the disaster is more
damage; that the second explosion, a BLEVE, was the most effective gas detection and emergency isolation. The plant
violent and did damage houses; that the worst explosion had no gas detector system and, probably as a consequence,
damage was probably from gases which had accumulated emergency isolation was too late.
in houses; and that much of the damage was done by fire.
Films were available for many of the BLEVEs, though
not for the second, violent explosion. From this evidence, Planning for emergencies
the BLEVEs had diameters of 200!300 m and durations of One particularly unsatisfactory aspect of the emergency
some 20 s. Heavy direct fire damage was done at distances was the traffic chaos which built up as residents sought to
up to about 300 m, which agrees reasonably well with the flee the area and the emergency services tried to get in.
estimates of fireball size. Another was risk run by the large number of rescuers
Avery large fire burned on the site for about an hour and who came on site from a BLEVE of one of the larger
a half, punctuated by BLEVEs. Details are given of the spheres.
number, size and range of fragments from spheres and
bullet tanks. Information on BLEVEs and missiles in the Fire fighting in BLEVE hazard situations
TNO report is given in Chapters 16 and 17. The fire services appear to have taken a considerable a risk
in trying to fight the fire on the two larger spheres. The
potential death toll if a BLEVE had occurred was high.
A4.5 Some Lessons of Mexico City
Some of the lessons to be learned from Mexico City are lis- Boiling liquid expanding vapour explosions
ted in Table A4.3. After Flixborough the problem of vapour cloud explosions
received much attention. Mexico City demonstrates that
Siting of major hazard installations boiling liquid expanding vapour explosions are an equally
The high death toll at Mexico City occurred because the important hazard.
housing was too near to the plant. At the time the plant was Mexico City represents the largest series of major
constructed the area was undeveloped, but over the years BLEVEs which has occurred and provides much informa-
the built-up area had gradually crept up to the site. tion on these.