Understanding Passive and Active Protection Layers in Process Safety
Process safety is critical in industrial operations, especially in high-risk industries such as oil and gas, petrochemicals, and chemical manufacturing. One of the key components of process safety is the implementation of protection layers or safeguards. These safeguards can be classified into passive or active layers, as well as preventive (prerelease) or mitigating (post release) controls. Understanding these layers is essential to effectively managing risks and ensuring the safety of personnel, equipment, and the environment.
For more details about Independent Protection Layers please view the blog ”Independent Protection Layers in LOPA“.
1. Passive Independent Protection Layers (IPLs)
Passive IPLs are designed to reduce risk without requiring any action to achieve their function. They work based on proper mechanical or process design, and their effectiveness depends on correct construction, installation, and maintenance.
Examples of Passive IPLs
- Tank dikes: Prevent the spread of hazardous liquid leaks.
- Blast walls or bunkers: Protect equipment and personnel from explosions.
- Fireproofing: Delays the impact of heat exposure to prevent structural failure.
- Flame or detonation arrestors: Prevent fire or explosions from propagating through a piping system.
Effectiveness and Considerations
If designed and maintained correctly, passive IPLs can provide a high level of risk reduction. However, they must be assessed for potential limitations, such as:
- Fireproofing must withstand direct fire exposure and water jet impacts.
- Flame and detonation arrestors can be prone to fouling, corrosion, and maintenance issues.
- Tank dikes and blast walls must be structurally sound to contain hazardous events effectively.
Some organizations consider special materials and inspection regimes as IPLs if they effectively prevent specific consequences. However, inherently safer design features eliminate risks rather than mitigate consequences and may not always be considered IPLs. Proper maintenance and periodic audits are necessary to ensure these features remain effective.
2. Active Independent Protection Layers (IPLs)
Unlike passive IPLs, active IPLs must transition from one state to another in response to a measurable process condition. These layers typically consist of:
- A sensor (e.g., instrument, mechanical device, or human input).
- A decision-making component (e.g., logic solver, relay, or human intervention).
- An action mechanism (e.g., automatic shutdown, alarm, or mechanical response).
Types of Active IPLs
1. Instrumented Systems
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- These include sensors, logic solvers, and final control elements that work together to regulate or shut down processes.
- Instrumented systems fall into two categories:
- Continuous controllers: Maintain normal process conditions through real-time adjustments.
- State controllers: Trigger alarms or shutdown processes when predefined limits are exceeded.
2. Basic Process Control System (BPCS)
The BPCS continuously monitors and controls processes under normal conditions. It provides safety through:
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- Continuous control actions to keep the process stable.
- Alarm systems that notify operators of abnormal conditions.
- Automatic shutdowns in case of process deviations.
- BPCS has limitations due to:
- Minimal redundancy.
- Limited security against unauthorized modifications.
- Potential human errors in bypassing safety interlocks.
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3. Safety Instrumented System (SIS)
The SIS is a dedicated safety system designed to execute critical safety functions (Safety Instrumented Functions – SIFs). Key SIS design features include:
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- Independence from the BPCS to ensure reliability.
- Redundant components and self-diagnostic capabilities.
- Use of voting architectures to minimize failures and spurious shutdowns.
- De-energized-to-trip philosophy for low Probability of Failure on Demand (PFD).
- Each SIF is assigned a Safety Integrity Level (SIL) based on its risk reduction capability.
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4.Pressure Relief Devices
- Pressure relief valves open when internal pressure exceeds a predetermined limit, preventing equipment overpressure failures.
- Rupture discs are designed for one-time use to relieve excess pressure but cannot close again, potentially leading to additional risks.
- Relief systems may vent material to the atmosphere or through mitigation systems such as scrubbers, flares, or quench tanks.
- The design and maintenance of relief systems must account for potential fouling, corrosion, two-phase flow conditions, and freezing issues.
- Human involvement in relief valve installation and maintenance can introduce errors, affecting the Probability of Failure on Demand (PFD) used in risk assessments.
5. Human IPLs
Human IPLs rely on trained operators to detect and respond to process abnormalities before an incident occurs. For human IPLs to be effective, the following conditions must be met:
- The required action must be clearly indicated, available, and simple to understand.
- Operators must have sufficient time to assess the situation and act appropriately.
- Decision-making should not involve complex calculations or trade-offs between safety and production.
- The operator must be physically capable of taking action under all expected conditions.
- Regular training and drills must be conducted to ensure competence.
- The alarm and required action must be independent of other credited IPLs.
While human performance is generally less reliable than automated systems, properly trained and supported operators can be effective in reducing risks.
Effectiveness and Considerations
- The IEC 61511 standard sets minimum PFD values for BPCS IPLs.
- SIS functions must be designed with redundancy and frequent testing to ensure reliability.
- Active IPLs require regular testing and validation to confirm functionality.
- While human performance is generally less reliable than automated systems, properly trained and supported operators can be effective in reducing risks.
References
Layer of Protection Analysis Simplified Risk Assessment by CCPS
