Functional safety ensures that industrial processes are designed to operate safely, even when unexpected failures occur. Among the critical challenges in maintaining functional safety is managing common cause failures (CCFs), which can impact multiple safety systems at once. Ignoring these failures can lead to disastrous consequences. This blog aims to break down the concept of CCFs in simple terms, providing actionable insights for engineers.
What Are Common Cause Failures?
A common cause failure occurs when a single event leads to the failure of multiple systems, devices, or procedures. This can undermine safety measures that are designed to operate independently. For example:
- A clogged pipeline can simultaneously disable a pressure relief valve and a pressure switch if they share the same connection.
- A power outage may disable both the system triggering the event and the safeguards meant to protect against it.
Within CCFs, there is a subset called common mode failures, where multiple systems fail in the same way due to shared vulnerabilities.
Why Are CCFs Critical in Functional Safety?
Functional safety relies on Layer of Protection Analysis (LOPA) to evaluate risks and determine if safeguards (Independent Protection Layers or IPLs) are sufficient. The effectiveness of this method depends on the independence of safeguards.
If safeguards are not truly independent, as is often the case with CCFs, the analysis becomes flawed. For instance:
- Two safeguards relying on the same power supply will fail together during a power outage.
- If two redundant safety devices are calibrated with the same faulty tool, they may both malfunction simultaneously.
Failing to account for such scenarios can lead to overestimating the system’s safety and under-preparing for potential failures.
Rules for Preventing Common Cause Failures
To minimize the risk of CCFs, engineers must ensure that safeguards are truly independent. Here are some practical rules:
1. Maintain Device Independence
Safeguards should not depend on the same equipment, process connection, or utility.
- Avoid shared connections like common piping or tubing. For example, a pressure relief valve and a pressure switch should not share the same nozzle. If the nozzle gets clogged, both safeguards will fail.
- Design safeguards to operate independently from the initiating event (IE).
2. Mitigate Human Error
Human factors can contribute significantly to CCFs, especially if the same team or procedure is responsible for multiple safeguards.
- Use separate personnel, tools, and schedules for maintaining redundant safety systems.
- Design user-friendly alarm interfaces and manual shutdown systems to reduce operational errors.
3. Separate Utilities and Support Systems
Safeguards should not rely on the same utility or support system as the initiating event.
- For instance, a power outage that triggers an incident should not also disable the safeguards meant to mitigate it.
Examples of Common Cause Failures
1. Calibration Errors
When the same technician calibrates multiple instruments using a faulty tool, both instruments can fail simultaneously.
2. Shared Equipment Design
Using identical valves, sensors, or other components across systems can lead to simultaneous failures if there is a design flaw or manufacturing defect.
3. Utility Failures
Power outages or utility failures can disable multiple safeguards, making the entire system vulnerable.
4. Natural Disasters
Events like floods, earthquakes, or fires can trigger failures in multiple safety systems at once. For instance, a flood could disable both the process controls and the emergency shutdown systems.
Identifying and Addressing CCF Risks
1. Analyze Process Connections
Assess whether multiple instruments or devices share connections, such as piping or tubing. These shared components can create vulnerabilities.
2. Human Reliability Analysis (HRA)
If multiple safeguards rely on human actions, ensure those actions are independent. Workers from the same team or group may not provide true redundancy.
3. Fault Tree Analysis (FTA)
Use advanced techniques like FTA to mathematically model and quantify the likelihood of common cause failures.
Managing Common Cause Failures in LOPA
In some cases, LOPA can account for CCFs by creating new scenarios where common failures are treated as initiating events. For example:
- A control loop failure could result from a malfunctioning control valve or a controller failure that affects multiple systems simultaneously. Separate LOPA scenarios can be created for each failure type.
When aggregating multiple LOPA scenarios, be cautious not to overstate the failure rate. For instance, if common failures are included in multiple scenarios, the risk contribution may be double-counted.
Addressing Systematic Errors
Systematic errors, such as design flaws or procedural mistakes, are a significant source of CCFs. These errors are often embedded in the system and can impact multiple safeguards simultaneously.
To address systematic errors:
- Follow industry standards like IEC 61511, which limits risk reduction claims for safeguards that share a common design.
- Use conservative estimates for failure probabilities when assessing safeguards with shared vulnerabilities.
Real-World Examples of CCF Risks
Pressure Relief Valves
If two redundant pressure relief valves are installed on the same nozzle and serviced by the same personnel, they are vulnerable to common cause failures. Even though each valve is independently rated, the combined risk may be higher than anticipated due to shared vulnerabilities.
Redundant Instruments
When redundant instruments are serviced by the same technician or calibrated using the same tools, errors in maintenance can compromise both instruments simultaneously.
Best Practices for Preventing Common Cause Failures
- Diversity in Design
Use different types or manufacturers for redundant systems to minimize the risk of shared vulnerabilities. - Effective Process Safety Management (PSM)
Ensure that all safety systems are well-designed, properly installed, and regularly maintained. - Independent Testing and Maintenance
Assign separate teams and schedules for redundant systems to reduce the risk of human errors. - Quantitative Risk Assessment
Incorporate techniques like Fault Tree Analysis (FTA) and Human Reliability Analysis (HRA) to supplement traditional LOPA.
