Spurious Trip Level (STL) — Balancing Safety and Availability

In safety-critical industries, balancing safety and availability is essential. While most engineers focus heavily on safety integrity, process availability is equally important. This article explains why spurious trips are not just an inconvenience but a significant safety risk — and how the Spurious Trip Level (STL), invented by Risknowlogy, helps optimise 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:

• Lost production and revenue: Every unplanned shutdown can result in hours, or even days, of downtime. In industries like oil and gas, where every minute counts, even a short shutdown can have a massive financial impact.
• Erosion of safety culture: Frequent false alarms can cause operators to become desensitised to safety signals. When a real safety event occurs, this could result in life-threatening delays in response.
• Increased risk during startup and shutdown: A spurious trip triggers a shutdown and subsequent restart. In many industries, startup and shutdown phases are the most hazardous operations — much like airplanes, which face the greatest risk during takeoff and landing.

The key question is: are you managing the risk of spurious trips with the same rigour as the risk of dangerous failures?

Real-world examples of accidents during startup or shutdown

Texas City Refinery Explosion (2005)

One of the deadliest industrial accidents in the U.S. 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.

Milford Haven Refinery Explosion (1994)

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.

Flixborough Disaster (1974)

Though not directly tied to startup, the Flixborough disaster demonstrates 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.

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 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 optimise 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
• Minimising 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. Optimise proof testing and maintenance schedules by adjusting intervals based on STL metrics, reducing unnecessary shutdowns for maintenance while maintaining safety

The benefits of STL for safety-critical industries

1. Fewer shutdowns and improved availability: Reduce the number of spurious trips, keeping your plant running smoothly and minimising 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
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 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, minimise the hazards associated with startup and shutdown phases, and maintain high levels of productivity.