Fire & Explosion Risk Assessment

An Introduction to Fire and Explosion Risk Analysis (FERA)

Introduction

Fire and Explosion Risk Analysis (FERA) is a systematic process used in process safety engineering to understand and manage the potential for fires and explosions at industrial facilities. In simple terms, it's a detailed study that identifies what could go wrong, how likely it is to happen, and what the consequences would be. The primary goal of a FERA is to quantify these risks to protect the facility's equipment and assets, ensuring a safer operational environment.

1. The Core Purpose of a FERA

A FERA is conducted with three primary objectives in mind, each contributing to the overall safety and integrity of a facility.

• Quantify accidental loads: The analysis identifies and calculates the potential forces (loads) that fires and explosions could exert on structures and equipment. This is critical for ensuring that the facility is designed to withstand worst-case scenarios.
• Inform safety system design: It provides essential data needed to design effective fire-fighting systems and Passive Fire Protection (PFP). This helps prevent a single incident from escalating and causing a chain reaction that affects adjacent equipment.
• Recommend risk reduction measures: The study makes specific recommendations to lower the overall risk. This can involve adding new safety features or optimizing existing systems to be more effective.

These objectives guide the entire analysis, which begins by carefully defining the scope and the core assumptions of the study.

2. Setting the Stage: Scope and Assumptions

Before the analysis can begin, its boundaries must be clearly established through a defined scope and a set of foundational assumptions.

2.1. Defining the Boundaries (Scope)

The "scope" of a FERA defines exactly what is being studied and under what conditions. This ensures the analysis is focused and its results are correctly interpreted. For this type of study, two key limitations are established:

1. The analysis is for a specific facility (the Central Processing Facility or CPF) during its normal operation phase only. Other phases, like construction or commissioning, are excluded.
2. The focus is strictly on risks to assets, such as equipment and structures. Risks to people are addressed in separate, specialized studies like a Quantitative Risk Assessment (QRA), ensuring that all aspects of facility safety are covered by the appropriate analysis.

2.2. The Role of Assumptions

A FERA is a complex modeling exercise that relies on a set of approved assumptions to proceed. These assumptions provide a consistent foundation for the calculations and cover three main areas:

• Description and background data
• Consequence and frequency analysis methods
• Risk and impact criteria (what is considered a significant risk)

With the scope and assumptions in place, the analysis can move forward with a detailed, step-by-step methodology.

3. The FERA Process: A Step-by-Step Breakdown

The FERA methodology is a structured process that moves from identifying potential leaks to evaluating their final impact on the facility.

Step 1: Identifying Potential Problems (Failure Cases)

The first step is to break down the facility into "isolatable sections"—areas of piping and equipment that can be sealed off by safety valves. Within each section, analysts define specific "failure cases," which are potential leak scenarios. Each failure case is defined by four key factors:

• Material/phase: What substance would be released (e.g., gas, pressurized liquid).
• Release condition: The nature of the leak (e.g., from a pump or a stored inventory).
• Process conditions: The temperature and pressure of the substance at the time of release.
• Release location: Where in the facility the release occurs, including its height.

Step 2: Sizing the Leak

Analyzing every possible leak size would be impractical. To make the analysis manageable, potential leaks are grouped into three representative sizes: Small (S), Medium (M), and Large (L).

Step 3: Estimating the Likelihood (Frequency Analysis)

"Leak frequency" is the estimated probability that a leak will occur in a given year. This is calculated by performing a detailed "parts count" of every component within an isolatable section from engineering drawings (P&IDs). This includes counting items like valves, flanges, and lengths of pipe, each of which has an associated failure rate based on industry data.

Analysis of this data reveals a key insight: small leaks are far more common than large ones.

Step 4: Considering the Spark (Ignition Probability)

A flammable release only becomes a fire or explosion if it finds an ignition source. The analysis models the probability of this happening, which is divided into two types:

• Immediate Ignition: The released substance ignites instantly at the source.
• Delayed Ignition: The substance forms a flammable cloud that disperses before finding an ignition source later.

The chance of ignition depends on several factors, including whether the product is a gas or liquid, the rate of release, and the type of facility. In a typical analysis, the total probability of ignition is further divided, for example, with 40% of ignitions assumed to be immediate and 60% delayed.

Step 5: Calculating the Consequences

This step uses specialized software (PHAST/PHAST RISK) to model the physical effects of an ignited release. The outcome depends on whether the release is a liquid or vapor and whether ignition is immediate or delayed.

Step 6: Mapping the Scenarios (Event Trees)

An "Event Tree" is a diagram that visually maps out all the possible outcomes that can follow a single leak event. Starting with the initial leak, the tree branches out to account for different probabilities at each stage: immediate vs. delayed ignition, success or failure of the emergency isolation system, and the effectiveness of the blowdown (depressurization) system.

By multiplying the probabilities along each branch, analysts can calculate the final frequency of a specific outcome, such as a "Jet fire, not isolated." These diagrams are essential tools for understanding the full spectrum of possibilities for both vapor and liquid releases.

Once all the potential consequences have been modeled and their frequencies calculated, the results must be evaluated against established safety criteria.

4. Understanding the Results: Damage Thresholds

To determine if a modeled fire or explosion poses a significant risk to assets, its effects (like heat radiation or overpressure) are compared against predefined "thresholds." If an event is predicted to exceed a threshold, it is considered a credible threat to equipment integrity.

Crucially, for any of these hazards to be considered in the final risk assessment, they must also be predicted to occur more frequently than a minimum threshold, typically once every 10,000 years (a frequency of 10⁻⁴ per year).

The final results of a FERA are often presented in tables that show the maximum distance at which these damage thresholds would be exceeded for various failure cases, providing a clear picture of the risk zones within the facility.

5. Key Takeaways

The Fire and Explosion Risk Analysis is a comprehensive safety study that can be summarized in three major phases:

• Identification: FERA begins by identifying all credible leak scenarios (failure cases) within a facility and estimating how frequently they might occur based on a detailed parts count and historical data.
• Modeling: It then models the consequences of these leaks, considering ignition probabilities and system responses to determine the potential for hazardous events like jet fires, pool fires, and explosions.
• Evaluation: Finally, the severity of these events is measured against established damage thresholds to quantify the risk to assets and inform the design of critical safety systems.