Balancing Safety and Availability: The Importance of Spurious Trip Levels (STL)

2024-10-24

In safety-critical industries, balancing safety and availability is essential. Whether you’re in oil and gas, chemical processing, or power generation, your systems must be both safe and reliable. While most engineers focus heavily on safety integrity, process availability is equally important.

This is why we at Risknowlogy invented the Spurious Trip Level (STL)—to help companies measure and control the risk of unnecessary system shutdowns. In this article, we'll explain why spurious trips are not just an inconvenience but a significant safety risk, and how STL can help optimize both safety and reliability.

What Is a Spurious Trip, and Why Should You Care?

A spurious trip occurs when a safety system activates without a real hazard, leading to an unintended shutdown. This can happen due to faulty sensors, over-sensitive settings, or other system misconfigurations.

The problem with spurious trips is threefold:

1. Lost Production and Revenue: Every unplanned shutdown can result in hours, or even days, of downtime, leading to significant financial losses. In industries like oil and gas, where every minute counts, even a short shutdown can have a massive financial impact.

2. Erosion of Safety Culture: Frequent false alarms can cause operators to become desensitized to safety signals. If alarms and trips are too frequent, there’s a risk that operators may ignore them, leading to dangerous complacency. When a real safety event occurs, this could result in life-threatening delays in response.

3. Increased Risk During Startup and Shutdown: Perhaps most importantly, a spurious trip triggers a shutdown and subsequent restart of the system. In many industries, startup and shutdown phases are the most hazardous operations. Much like airplanes, which face the greatest risk during takeoff and landing, industrial processes are more vulnerable to accidents during these transitions.

In fact, many major industrial accidents have occurred during startup or shutdown phases due to the complex coordination required during these operations. Let’s look at a few notable examples.

Real-World Examples of Accidents During Startup or Shutdown

1. Texas City Refinery Explosion (2005)
   One of the deadliest industrial accidents in the U.S., the Texas City Refinery explosion, occurred during startup operations. Due to poor coordination, equipment failure, and operator error, highly flammable hydrocarbons were released and ignited, causing a massive explosion that killed 15 people and injured over 180. This disaster highlighted how dangerous startup phases can be if not properly managed.

2. Milford Haven Refinery Explosion (1994)
   In another example, the Milford Haven Refinery in the UK experienced a significant explosion during a startup operation following a maintenance shutdown. An error in the startup sequence caused a buildup of hydrocarbon gases, which eventually ignited. Five workers were injured, and the plant suffered extensive damage, underlining the risks associated with restarting complex processes.

3. Flixborough Disaster (1974)
   Though not directly tied to startup, the Flixborough disaster is an example of how process mismanagement during changeovers or shutdowns can lead to catastrophic outcomes. A poorly designed temporary modification during a maintenance shutdown led to the release of flammable cyclohexane, resulting in an explosion that killed 28 people and destroyed the plant.

These cases demonstrate how spurious trips not only disrupt operations but also increase the likelihood of accidents during the most dangerous phases of process operation: shutdown and startup.

Why We Invented STL: Process Availability Matters

At Risknowlogy, we saw the need for a structured approach to managing the risk of spurious trips. Process availability—keeping your system running smoothly—is just as important as ensuring safety integrity. That’s why we created the Spurious Trip Level (STL) to give companies a measurable way to assess how well their systems are designed to avoid these unintended trips.

Traditionally, safety systems are designed with a focus on preventing dangerous events, but until now, there was no structured way to measure how well a system prevents unnecessary shutdowns. STL helps you optimize your system’s design by:

• Quantifying the likelihood of spurious trips and helping you understand the impact these trips could have on production and safety.
• Balancing safety and availability by reducing the frequency of shutdowns while maintaining compliance with safety standards like IEC 61508 and IEC 61511.
• Minimizing the risks during startup and shutdown, which are often the most hazardous phases in process operation.

STL in Action: The Key to Safer, More Reliable Systems

Consider the oil and gas industry, where unplanned shutdowns are not only disruptive but can also increase the risk of accidents during startup. By implementing STL, companies can:

1. Reduce the number of spurious trips, preventing unnecessary shutdowns and ensuring that the plant remains operational.
2. Enhance safety response by reducing the occurrence of false alarms, ensuring that operators remain focused and responsive when a real safety event occurs.
3. Optimize proof testing and maintenance schedules by adjusting intervals based on STL metrics, reducing unnecessary shutdowns for maintenance while maintaining safety.

STL provides a structured, standards-driven way to design systems that are both reliable and compliant, giving engineers and safety managers confidence that their operations are optimized for both safety and availability.

The Benefits of STL for Safety-Critical Industries

Implementing STL offers numerous benefits for safety-critical industries, including:

1. Fewer Shutdowns and Improved Availability: Reduce the number of spurious trips, keeping your plant running smoothly and minimizing costly downtime.
2. Safer Startup and Shutdown Phases: By preventing unnecessary trips, STL reduces the need for frequent startups and shutdowns, decreasing the risk of accidents during these critical phases.
3. Better Operator Trust in Safety Systems: With fewer false alarms, operators are more likely to respond appropriately during actual safety events, increasing the overall effectiveness of the safety system.
4. Increased Confidence in Compliance: STL ensures that safety systems are designed to meet international safety standards while maintaining high levels of availability.

STL: The Future of Safe, Reliable Operations

At Risknowlogy, we believe that the risk of spurious trips should be managed just as carefully as any other safety risk. Spurious Trip Levels (STL) offer a unique approach to balancing safety and availability, ensuring that your operations remain safe, compliant, and efficient.

By designing your system with STL in mind, you can reduce the risk of unnecessary shutdowns, minimize the hazards associated with startup and shutdown phases, and maintain high levels of productivity.

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Interested in learning more about STL?
Contact us today to find out how STL can help you reduce spurious trips and improve both safety and reliability.
https://go.risknowlogy.com/CONTACT