Risk Assessment Using Fault Tree Analysis $($FTA$)$

Step $1$: Defining the Top Event

Top Event: Explosion at FCC Unit of ExxonMobile Refinery

On February $18,2015,$ an explosion occurred at the Fluid Catalytic Cracking $($FCC$)$ Unit of the ExxonMobile Refinery. This incident caused minor injuries to two workers and significant property damage. Catalytic dust from the explosion was scattered up to a mile away, affecting nearby communities.

The FCC unit is crucial for converting heavy hydrocarbons into products like gasoline. It involves reactions in the "hydrocarbon side" and pollution control processes in the "air side." On the incident day, a slide valve failure allowed hydrocarbons to enter the air section and ignite in the electrostatic precipitator $($ESP$),$ triggering a massive explosion.

This event revealed several weaknesses in the plant's process safety management, such as reliance on uncertified protection systems, reuse of outdated procedures, and failure to ensure critical equipment functionality. These issues highlight the need for thorough risk analysis and regular maintenance in the oil refining industry.

A visual representation of the incident is available on YouTube in the video titled "Animation of $2015$ Explosion at ExxonMobil Refinery in Torrance, CA$."$ This animation illustrates the event's mechanism and impact, emphasizing the importance of preventive measures in occupational safety and health risk management.

Step $2$: Identify Direct and Indirect Causes

$1.$ Slide Valve Failure

$$ The slide valve in the FCC unit failed, causing an uncontrolled flow of hydrocarbons. This failure could be due to inadequate maintenance, outdated equipment, or design flaws. The malfunction allowed hydrocarbons to enter the FCC air section, creating a dangerous situation.

$2.$ Hydrocarbons in the FCC Air Section

$$ Hydrocarbons, which should not be in the FCC air section, entered due to the slide valve failure. The air section is designed for pollution control, not for handling hydrocarbons. The presence of hydrocarbons in this section created a flammable mixture, significantly increasing the risk of explosion.

$3.$ Ignition in an Electrostatic Precipitator $($ESP$)$

$$ The ESP, which removes solid particles from waste gas using electrostatic charges, was not designed to handle hydrocarbons. However, hydrocarbons reached the ESP due to the previous failures. An ignition source within the ESP, likely from electrostatic currents or sparks, ignited the hydrocarbons, causing a powerful explosion and spreading catalytic dust into the surrounding area. This combination of flammable materials and ignition sources in a confined space led to the destructive explosion.

Step $3$: Identify Root Causes

To understand the true causes behind the explosion, we need to identify the root causes of each direct cause:

$1.$ Slide Valve Failure:

$$ Insufficient Maintenance: Poor or incorrect maintenance can lead to dirt accumulation, wear, and mechanical failure. Without proper maintenance, the slide valve may fail, allowing uncontrolled hydrocarbon flow.

$$ Lack of Periodic Testing: Regular testing and verification are critical to detect mechanical problems early. Without these checks, malfunctions go unnoticed, leading to complete failure.

$$ Inadequate Valve Design: If the valve is not designed for current operating conditions, it may fail under pressure or temperature variations. Process changes without corresponding equipment upgrades can also cause failure.

$2.$ Hydrocarbons in the FCC Air Section:

$$ Poor Detection and Control: Inadequate detection systems or lack of automatic controls can allow hydrocarbons to enter the air section unnoticed, creating a high explosion risk.

$$ Ineffective Monitoring System: Without an effective monitoring system, the presence of hydrocarbons in the air goes undetected, increasing operational risks.

$$ Emergency Procedure Failures: Weak emergency procedures and insufficient training can prevent proper shutdown of hydrocarbon flow in emergencies, escalating the risk of serious incidents.

$3.$ Ignition in an Electrostatic Precipitator $($ESP$)$:

$$ Inappropriate ESP Design: ESPs not designed to handle hydrocarbons can ignite flammable materials, leading to explosions.

$$ Lack of Warning and Automatic Shutdown: Without warning systems or automatic shutdown mechanisms, hydrocarbons can ignite undetected, causing severe incidents.

$$ Insufficient Maintenance and Testing: Regular maintenance and safety function tests are crucial. Inadequate upkeep can lead to undetected damages, increasing failure risks during critical operations.

By identifying these root causes, we can better understand the factors contributing to the explosion and implement more effective preventive measures, enhancing safety and reducing the likelihood of future incidents.

Build a Fault Tree, with refer with the above data