The San Juanico disaster was a series of deadly fires and explosions that occurred at a Pemex-owned liquefied petroleum gas (LPG) terminal on 19 November 1984, in San Juan Ixhuatepec, a town of Tlalnepantla de Baz, within the State of Mexico, Mexico. The fires and explosions were caused by the rupturing of a pipeline within the facility, which released vaporized LPG that ignited and damaged several storage tanks.
The terminal and surrounding town were destroyed by the damage, and at least 503 people were killed, with another 5,000 to 7,000 suffering a range of injuries.
It is one of the deadliest industrial disasters in history, and the deadliest industrial accident involving fires and/or explosions from hazardous materials in a process or storage plant since the Oppau explosion in 1921.
Background
The terminal, locally named San Juanico, was a storage and distribution terminal for liquified petroleum gas (LPG), owned and operated by the Mexican state-owned oil company Pemex. The terminal mostly stored propane and butane products, and received its LPG from other Pemex facilities across southern Mexico through three underground pipelines:
- a 12-inch (305 mm) pipeline from the Lazaro Cardenas Refinery in Minatitlán, Veracruz (about 576 km (358 mi) away),
- a 4-inch (102 mm) diameter pipeline from the Poza Rica Gas Complex in Veracruz (about 235 km (146 mi) away), and
- another 4-inch (102 mm) line from the Azcapotzalco Refinery in Mexico City (about 8 km (5 mi) away).
Terminal capacity and configuration
The terminal’s gas capacity consisted of 54 storage tanks for the LPG: six large Horton sphere tanks and 48 smaller, horizontal bullet tanks of various sizes. Four of the six Horton tanks had a capacity of 1,600 cubic metres (57,000 cu ft), which the other two had a capacity of 2,400 cubic metres (85,000 cu ft).
These tanks gave the facility a total capacity of 16,000 cubic metres (570,000 cu ft), representing one-third of Mexico City's entire LPG supply. Tanks were divided into several clusters by concrete walls about 1 metre (3.3 ft) tall. The bullet tanks were arranged in a stacked tower configuration.
The facility also had two ground flare pits and a fire protection system which included a reserve pond, fire pumps, and firewater spray distribution system.
Surrounding facilities and town
Neighboring the San Juanico terminal to the east was a Unigas facility with additional LPG storage and distribution capacity, and a Gasomático gas-bottling and distribution facility, where residential gas bottles were distributed by truck. The San Juanico terminal distributed LPG to these two facilities directly via underground pipelines. Five more gas distribution companies in the surrounding area obtained gas from the San Juanico terminal through truck shipments.
The town of San Juan Ixhuatepec surrounded the terminal and had a population of about 40,000 residents, with an additional 60,000 more living in nearby communities. The settlement of San Juan Ixhuatepec existed before the facility was built, but housing immediately surrounding the facility itself began only after the construction of the terminal began in 1962, although this is disputed. Houses were built as close as 130 metres (430 ft) away from the LPG storage tanks, and most of the houses in the area were simple brick or wooden buildings. At least two sphere tanks were added to the plant when it was already surrounded by the densely populated neighborhood.
Previous safety concerns
The plant was said to have been built in conformance to API standards, but this was later questioned by investigators. In the two months leading up to the disaster, inspections for the local plant safety committee revealed multiple issues with several of the facility’s mechanical and passive safety systems:
- Between 30 and 40% of safety devices, including the firewater spray system, were being bypassed or were non-operational;
- housekeeping was substandard;
- Multiple pressure gauges were physically defective and inaccurate;
- a relief valve on one of the manifolds for an LPG-receiving pipe was missing; and
- the support legs of the Horton sphere tanks lacked fireproofing.
It was also found that an additional relief valve would be needed for the Minatitlán pipeline, after the pipeline’s operational flow rate had been increased up to 11,900 cubic meters per day (about 75,000 barrels of productivity per day).
Accident
Site filling and initial leak
In the early hours of 19 November 1984, the San Juanico facility was being filled with LPG from the Minatitlán refinery’s pipeline, as the terminal’s tanks had been almost empty since the day prior. By 5:30 a.m. CST, two of the Horton tanks and all 48 of the bullet tanks were filled up to 90% capacity, while the rest of the Horton sphere tanks were at 50%; at this point, the plant held between 11,000–12,000 cubic metres (390,000–420,000 cu ft) of LPG, around 70% of the plant’s total capacity and with an energy equivalent over 300 TJ, roughly five times the energy released by the Little Boy atomic bomb.
At around 5:30 a.m., a 20 cm diameter pipe between a Horton tank and the bullet tanks ruptured, releasing vaporized LPG into the air. Control room operators at both San Juanico and the pipeline pumping station 40 kilometres (25 miles) upstream noticed a substantial pressure drop in the pipeline, but neither set of operators could identify the cause or source of the leak for 5 to 10 minutes after the initial pressure drop. As the gas cloud continued to build up inside the terminal, it reached an estimated maximum volume of 66,000 cubic metres (2,263,800 ft3).
Fire and explosions
At 5:45 a.m, the vapor cloud eventually reached one of the waste-gas flare pits and ignited. A flash fire ensued, which immediately transitioned to a violent vapor cloud explosion (VCE), likely due to its flame front’s acceleration being funneled by the congested geometry of the plant. The blast was felt and recorded by a seismometer at a National Autonomous University of Mexico (UNAM) laboratory located 32 km (20 mi) away, in Mexico City. The initial VCE created a domino effect accident, where damage from the VCE cause other pipes and storage tanks to leak, causing a massive conflagration that damaged other storage tanks, priming them to explode as well.
About 90 seconds after the VCE blast, at 5:46 a.m, the first boiling liquid expanding vapor explosion (BLEVE) occurred in one of the tanks. The explosion was reportedly visible throughout Mexico City, and the pilot of a Pan Am flight on approach to Mexico City International Airport who managed to see the explosion communicated to air traffic control that he believed a nuclear bomb may have exploded in the city. Eight more BLEVEs would occur for the next two hours, as each explosion damaged more tanks in the terminal and adjoining gas facilities; each BLEVE was recorded by the UNAM seismometer, with the last one recorded at 7:01 a.m.
The first and sixth BLEVEs are believed to have been the most violent explosions, with both registering a peak of 0.5 on the Richter scale. The BLEVE fireballs were up to 300 metres (980 ft) in diameter, and had a duration of some 20 seconds. Smaller explosions continued at the terminal and surrounding facilities until 11 a.m., when the flames on the last Horton sphere tank was extinguished.
Emergency response
The first call to the emergency services was made at 5:45, immediately after the VCE. More than 200 firefighters were deployed to the affected area in the first six hours after the first explosion. Water for firefighting was provided to the site by tank trucks normally used for domestic portable water distribution. After the last BLEVE at 7:01, firefighters prioritized cooling the two larger, intact Horton sphere tanks in order to avoid additional explosions. A major rescue operation was declared, and the majority of rescue operations occurred between 8:00 and 10:00 a.m. Around 4,000 people, including 985 medics, 1780 paramedics, and 1332 volunteers, participated in rescue and medical activities. 363 ambulances and five helicopters were involved. As many as 200,000 people were evacuated from the area. The Mexico City Metro system and public buses were commandeered to transport the wounded to hospitals and the evacuees to evacuation centers.
Casualties and damage
Death toll and injuries
The Mexican government would put the official death toll of the disaster at 503, although some reports claimed between 500 to 600 deaths, and other unofficial sources cite up to 2,000 fatalities. Between 5,000 and 7,000 injuries were recorded. Five workers at the San Juanico terminal were killed in the disaster, while the overwhelming majority of the victims were residents living in the housing area surrounding the plant, mostly in buildings within 300 metres (980 ft) of the LPG storage tank area. Most of the casualties were killed in their sleep by the initial explosions.
The heat and fire generated by explosions incinerated most of the victims’ corpses, with only 16 (2%) of the recovered remains left in recognizable condition and which contributed to uncertainty in the death toll. The American Institute of Chemical Engineers (AIChE) estimated that the thermal radiation produced by the BLEVEs exceeded the threshold of pain, 4.0 kW/m2, within a radius of 1,850 metres (1.15 mi) of the accident site.
Property damage
The San Juanico terminal was almost entirely destroyed by the disaster. The four smaller Horton sphere tanks were completely destroyed, with fragments propelled up to 350 metres (1,150 ft) away from the facility, into residential areas. The two larger Horton sphere tanks remained intact, but has collapsed onto the ground as their support legs has weakened and buckled under the intense heat of the fires. Only four of the 48 bullet tanks were found intact after the fire. 12 of the tanks that had exploded were launched from their supports and become missiles, with the furthest landing 1,200 metres (3,900 ft) off-site; the total weight of the fragments of these tanks weighed up to 30 metric tons.
Several secondary gas explosions occurred inside the neighboring plant buildings and surrounding houses. At the Gasomático site, 100 parked trucks loaded with household gas cylinders weighing 20–40 kilograms (44–88 lb) each were completely burned out, with hundreds of secondary explosions taking place.
An area of several square kilometers was affected, with varying degrees of damage from the fires and projectile tanks. Around 150 homes were completely destroyed, with a few hundred sustaining other levels of damage.
Investigation
Mexican government investigation
Initially, Director-General of Pemex Mario Ramón Beteta and other government officials attempted to shift responsibility for the accident onto the gas-distributing businesses adjoining the terminal, as well as blaming the people of San Juan Ixhuatepec for settling next to the gas plant. No results from a public inquiry or a Pemex investigation have ever been made available, except for a declaration of the Attorney General released on 22 December 1984, which identified the origin of the initial gas release as having occurred in the row of bullet tanks next to the Horton sphere tanks. In this press release, it was also stated that the responsibilities were not to be sought in the lack of plant maintenance. The Attorney General ultimately found Pemex responsible for the disaster, and ordered the company to provide the funds necessary for the Mexican Government to pay for indemnifications. The Mexican Petroleum Institute (IMP), who provides training for Pemex employees, stated that the cause of the accident was a disgruntled employee that had allegedly sabotaged the plant. The IMP also added that further investigation would not necessarily be useful.
TNO investigation
Through an internal initiative and mediation of the local Dutch embassy, a team of the Netherlands Organisation for Applied Scientific Research (TNO) reached the former terminal site two weeks after the accident. This investigation was not to determine the cause of the accident and was not assisted by Mexican authorities or Pemex, rather it was a scientific mission whose purpose was to test physical models used in safety studies for the prediction of damage from industrial explosions and examining the emergency response to the accident. The investigation was accompanied by the IMP.