Meaning
A systematic engineering process determines the required risk reduction level for each safety instrumented function in a plant. During sil selection, engineers evaluate the potential severity and likelihood of a specific hazard to assign a target safety integrity level from one to four. This choice dictates the design complexity and testing frequency of the safety system.
The selection process stops once the target safety level is assigned, passing the requirement to the system designers.
Risk Analysis
Engineers use methodologies like layer of protection analysis to calculate risk. When doing sil selection, they analyze existing independent protection layers to see if the remaining risk is acceptable. This prevents over-designing safety systems.
Target Allocation
The safety integrity level is selected based on the risk reduction factor required to make the process safe. A sil selection session brings together process safety, operations, and control systems engineers to reach a consensus on the hazard scenario. This multidisciplinary approach ensures that the hazard scenario is realistic and that all protection layers are valid.
The outcome is documented in a safety requirements specification.
Economic Consequence
Selecting a higher level than necessary can result in high capital and operating costs. Each increase in the safety integrity level requires more redundant instrumentation, more frequent proof testing, and more complex maintenance procedures. For instance, a safety function rated as SIL 2 may require a single sensor, whereas SIL 3 could require a two-out-of-three sensor configuration.
This redundancy increases both the initial procurement cost and the likelihood of spurious trips that halt production, leading to significant financial losses over the lifetime of the plant.
