Animation of 2015 Explosion at ExxonMobil Refinery in Torrance, CA

Narrator: The Torrance Refinery is a 750 acre facility,
located just outside of Los Angeles, California. At the time of the explosion, the
refinery was owned by ExxonMobil. An important part of the refining process takes place
in the facility’s fluid catalytic cracker, or FCC Unit. In the FCC Unit, heavy hydrocarbons from crude oil
are broken or cracked into smaller hydrocarbons, which can then be processed into
gasoline and other fuel products. The heavy hydrocarbons are first fed into
a reactor, where they mix with a catalyst. The heavy liquid hydrocarbons are converted into
lighter hydrocarbon vapors, as they travel up the reactor. At the top of the reactor, the lighter
hydrocarbon vapors are separated from the catalyst. The hydrocarbon vapors then flow
to the main distillation column. The catalyst falls down the side of the reactor,
where it moves through a slide valve, to a piece of equipment
called the regenerator. During the reaction, a layer of carbon, called
“coke” forms on the catalyst, that must be removed. Inside the regenerator, air is added and
the coke on the catalyst is burned off. The catalyst is then fed back to the reactor
through a slide valve and the cycle is repeated. When the coke is burned off the catalyst, this
creates products of combustion called “flue gas”. The flue gas flows out the regenerator and enters
a system comprised of multiple pieces of equipment, which remove any remaining
catalyst particles present. The regenerator and flue gas system
comprise the air side of the FCC Unit. The last piece of equipment in the flue gas system
is called the “electrostatic precipitator” or ESP. The ESP removes small catalyst particles,
using static electricity. While the ESP is energized, it creates
sparks, which are sources of ignition. It is critical that the flammable hydrocarbons in the
reactor do not flow into the air side of the FCC Unit, as this could create
an explosive atmosphere. To avoid this hazard, the two slide
valves connecting the reactor and regenerator are used to maintain a catalyst barrier
between the pieces of equipment. The sequence of events that eventually led to the explosion
at the refinery began on Monday, February 16th, 2015, when a piece of equipment in the air side of the FCC Unit,
called the “expander” vibrated forcefully enough that the refinery’s control system automatically transitioned
the FCC Unit to a standby mode known as “safe park”. During safe park mode, the flow of
hydrocarbons into the reactor is turned off. The flow of air into the
regenerator is also stopped. The two slide valves connecting the
reactor and regenerator are closed, to ensure a catalyst
barrier is maintained. Steam is then forced into the
reactor to prevent hydrocarbons in the main distillation column
from flowing back inside. The ESP remains energized
during safe park. One slide valve, however, had
eroded over six years of operation. And even though it closed, it could not
maintain a catalyst barrier in the reactor. Within seven minutes of the
Unit going into safe park, all of the catalyst in the reactor fell
through the slide valve, into the regenerator. A direct pathway was created for hydrocarbons
to flow between the reactor and the regenerator. But the pressure of the steam flowing
into the reactor as part of safe park mode was high enough to prevent hydrocarbons in
the main column from flowing back inside. With the Unit in safe park mode, operators
attempted to restart the expander several times, but were unable to do so. Refinery personnel met to identify a strategy to
repair the expander and bring the FCC Unit back online. Operations personnel predicted the expander could not
restart, because catalyst had likely accumulated inside. On Tuesday, February 17th, a meeting took
place involving a group of refinery personnel. The group discussed a similar
expander outage that occurred in 2012, for which the refinery had
developed what is called “a variance”. A variance is a management
approved deviation from procedure. The group decided to
use the 2012 variance, which allowed a departure from the
typical requirements for isolating the expander. Part of that process involved installing a
blind in one of the expander’s outlet flanges. On the morning of Wednesday, February 18th,
ExxonMobil maintenance attempted to install that blind, but were unable to do so, because
steam was escaping through the open flange. Steam from the reactor had traveled through the
leaking slide valve, into the air side of the FCC Unit. Using the variance as a guide, the flow
of steam into the reactor was decreased in an attempt to reduce the
amount escaping from the expander. But the variance did not evaluate
whether this flow rate was sufficient to prevent hydrocarbons from flowing into
the reactor, from the main distillation column. And unknown to the operators,
light hydrocarbons from a separate Unit had flowed through a leaking heat exchanger
into the main column, increasing pressure inside. With the steam reduced and
less pressure in the reactor, nothing could prevent the hydrocarbons from
flowing back from the main distillation column. The hydrocarbons flowed into the reactor,
where they escaped through the leaking slide valve, into the air side of the FCC Unit. At 8:07 a.m., a maintenance supervisor,
working in the FCC Unit, received an alarm on his
personal hydrogen sulfide monitor, warning him that
hydrocarbons were leaking nearby. By 8:40 a.m., multiple workers around the expander
received the same alarm and the FCC was evacuated. In an attempt to
mitigate the problem, a supervisor ordered the flow of steam to the
reactor to be increased, but it was too late. A flammable hydrocarbon mixture was
flowing through the air side of the FCC Unit and moving toward the ESP,
with its multiple ignition sources. There, the flammable hydrocarbon mixture
violently exploded. [Sound of explosion]


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