Case Study 4 - Pemex.pdf

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PEMEX LPG BLEVE Explosion

San Juan Ixhuatepec, Mexico City, Mexico19 November 1984

At approximately 05:35 AM on 19 November 1984, a series of catastrophic

explosions and a major fire occurred at the government owned and operated

PEMEX (Petroleous Mexicanos) LPG Terminal at San Juan Ixhuatepec, Mexico

City, in Mexico. The fires lasted until 11:00 PM. As a direct consequence of these

events, 574 individuals were killed and 7,231 were injured, of whom 144 later died

in the hospital. Most of the firefighters who responded to the scene were killed by

the continuing explosions. Additionally, some 39,000 people were renderedhomeless or were evacuated, and the terminal was destroyed. The severe traffic

jams caused by the fleeing residents near the plant blocked access to the area by

emergency and rescue teams.


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Plant Description

It was a fairly modern plant, only 25 years old, built to API standards. It functionedas an LPG bulk storage and distribution depot. The facility included six spherical

storage tanks and 48 horizontal bullets. The overall storage capacity was about

16,000 m3. Three refineries supplied the facility with LPG on a daily basis through

pipelines. At the time of the disaster, the storage tanks together contained about14,000 m3 of LPG (80% propane and 20% butane).

When the PEMEX plant was built, the nearest housing was about 300 meters from

the plant boundary, but by the time the disaster occurred, low-income, residential

housing had encroached to within 100 meters of the site perimeter. The houses were

of flimsy construction and were believed to have had an average of five inhabitants

per house.

There were also two neighboring installations engaged in the distribution of bottledLPG, and each had about 20 storage bullets. Both of these facilities were sufficiently

close to be badly damaged and contribute to the disaster.


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Number of

Vessels

Individual

Nominal

Capacity (m3)

Total

Capacity (m3)

Total

Inventory (m3)

Spheres 2 2,400 4,800 4,320

Spheres 4 1,500 6,000 3,000

Bullets 4 270 1,080 972

Bullets 14 180 2,520 2,268

Bullets 6 54 324 292

Bullets 3 45 135 121

Bullets 21 36 756 680

Total 54 15,615 13,653


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What Happened (1/4)

At the time of the accident, the plant was being filled from the Minatitlan Refinery,

about 400 km away via the 8-inch Minatitlan-Mexico LPG pipeline, as on the

previous day, the plant had become almost empty. The two large spheres and the 48

bullets were filled to about 90% of capacity, and the 4 smaller spheres, to about

50%.

The cause of the accident seems to have been the overfilling of the 4 largest bullets in

the southwest corner of the plant located immediately in front of the two larger

storage spheres and the failure of their pressure relief valves to lift, resulting inoverpressure in an 8-inch line between those 4 bullets and the nearby spheres.

At 5:25 AM, a drop in pressure was noticed in the refinery control room and also at

a pipeline pumping station. The 8-inch line between the spheres and the four bullets

had ruptured, causing the sudden pressure drop. Unfortunately, though, the

operators in the refinery could not identify the cause of the pressure drop, and

continued pumping.


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What Happened (2/4)

The release of LPG from the ruptured 8-inch line, driven by the continued

pumping, produced an expanding cloud of LPG gas that quickly grew to a size of

200 m x 150 m x 2 m high. There was a light, southwesterly breeze, estimated at

about 0.4 m/s that morning, and the expanding gas cloud slowly drifted southwest

down a slight slope. It drifted over nearby residential housing, and 10 minutesafter the rupture, at 5:35 AM, the expanding LPG cloud reached a flare pit about

100 meters West of the point of release.

When the drifting gas cloud reached the flare, it ignited, resulting in a massiveUnconfined Vapor Cloud Explosion, creating a 360 meter fireball and a violent

ground shock, which was the first of the nine explosions to be recorded on the

seismograph of Mexico City University, some 30 kilometers away.


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What Happened (3/4)

From the original point of rupture, there was a directional flame that was

impinging on one of the smaller spheres. Within 12 minutes, the small sphere had

BLEVEd, creating another fireball estimated at 300 meters in diameter. It was

this this second explosion that did most of the structural damage to the

surrounding houses. During the next hour and a half, seven more large andviolent explosions occurred, the last being recorded on the university seismograph

at around 7:00 AM.

The 4 smaller spheres and 5 of the larger bullets BLEVEd in rapid succession,with many numerous, smaller explosions continuing until about 11:00 AM due to

the BLEVE'ing of at least 10 of the smaller bullets and to further unconfined

vapor cloud explosions.

Large numbers of vessel fragments and ruptured bullets weighing up to 20 tons

were propelled hundreds of meters, with some fragments traveling more than 400

meters. One 20-ton bullet traveled 1,200 meters, and 11 more bullets were

propelled more than 100 meters. Only four of the original 48 bullets remained on

their supports.


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What Happened (4/4)

The two, larger diameter spheres did not explode, but their legs had no thermal

insulation and soon buckled due to the intense heat. The two spheres thus quickly

collapsed and catastrophically failed, releasing their contents to fuel the large

number of major ground fires within the plant. LPG rained down, and surfaces

covered in liquid LPG were set alight.

The damage to the nearby houses was severe. There were additional explosions

within the houses, and many people suffered injuries and serious burns from

flaming drops ofLPG. Virtually all the houses within 300 meters of the plant werecompletely destroyed.

It was not until very late in the disaster that somebody at the refinery pressed the

emergency shut down button, stopping the pumps and the flow of LPG to the

facility. In fact, the pumping and flow of LPG through the pipeline and into the

burning and exploding plant continued at the normal rate of 75,000 bbl per day

until 6:40 AM, more than one hour after the initial explosion. At that rate, the

raging fires were fed during that initial hour with an additional 3,175 bbl of LPG.


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Responsibility (1/2)

The total destruction of the terminal occurred because the overall basis of plant

safety was insufficient, mis-design in the layout of the plant, inadequate fire

protection and firefighting systems, a lack of gas detection systems and ineffective

emergency isolation systems. In particular,

The spheres were not equipped with adequate cooling water systems to prevent

BLEVEs and their legs were not thermally insulated to prevent collapse. The fire

pumps were too close to the spheres and bullets, and thus, the initial blast

completely disabled the terminals firewater system.

Failure to shut down the pipeline system promptly contributed greatly to the

severity of the accident.

Mis-design of the manifolding of the vessels in each row to common headers

contributed to the severity of the accident by not providing effective isolation, so

that when one vessel in a row over-pressured and failed, resulting in a BLEVE, the

others in the row failed similarly, in quick succession.


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Responsibility (2/2)

Also, either due to damage from the initial explosion and the BLEVEd bullets,

or due to over pressuring, the fill lines to the four smaller spheres leaked and the

gas ignited, creating fires which impinged on the bottoms of the spheres causing

them to overpressure and explode.

Traffic jams, which built up as local residents tried to escape from the area,

blocked the arrival of emergency services.

Case Study 4 - Pemex.pdf

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