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Animation of 2015 Explosion at ExxonMobil Refinery in Torrance, CA

WEBVTT
Kind: captions
Language: en

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

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