(147c) Commonly Used Correction Factors in Safety Integrity Level Analysis

The petroleum and petrochemical industry is a special industry characterized by high temperature and high pressure, deep cold and negative pressure, flammability and explosiveness, strong corrosiveness, and highly continuous production processes. These characteristics pose many potential safety risks in the production process of the petroleum and petrochemical industry. Once an accident occurs, it will not only cause casualties, but also bring huge property losses, and causing serious losses and negative impacts on enterprises and society. In order to address these potential safety risks, the petroleum and petrochemical industry has introduced Safety Instrumented Systems (SIS). The main responsibility of SIS is to take emergency measures against potential hazards and respond promptly to deteriorating conditions to ensure the safety of production facilities, the environment, and personnel.

At present, SIS has been widely applied in process industries such as petrochemicals. In order to measure the safety performance of safety instrumented systems, the Safety Integrity Level (SIL) for safety instrumented functions has been introduced. The SIL level is usually determined based on the international standard IEC 61508 or the industry specific standard IEC 61511 (Safety Instrumented Systems for Process Industries). When conducting SIL level assessment, multiple factors will be comprehensively considered to determine the safety integrity level that the system needs to achieve. In this process, it is inevitable to apply some correction factors (also known as adjustment factors or compensation factors) to adjust the risk assessment results to reflect the actual risk level in specific contexts. The determination of these correction factors usually requires a combination of specific application scenarios and risk assessment results.

The correction factor is the probability used in calculating accident scenarios, including but not limited to usage probability, hazardous environment probability, ignition probability, explosion probability, exposure probability, fatality probability, equipment damage or other economic loss probability, etc. In actual SIL level assessment projects, the author found that different analysts have different understandings of whether to use correction factors and how to use them during SIL level assessment. This difference results in SIL rating results that cannot truly reflect the actual working conditions. Of course, not all SIL level assessments require correction factors, and correction factors must also have characteristics such as independence, reliability, and auditability. The correction factor is only required when the correction condition is a necessary factor for the occurrence of the accident scenario. The purpose of the correction is to make the analysis process more consistent with the actual situation on site, try not to let the uncertainty of the correction affect the uncertainty of the entire accident scenario, and avoid excessive use of correction factors to achieve the expected risk level.

This article will analyze and introduce the commonly used correction factors and their usage conditions in the SIL assessment process based on the guidance of relevant standards and the experience and judgment of professional engineers. By deeply understanding and applying these correction factors reasonably, we can better ensure the production safety of the petroleum and petrochemical industry, reduce the risk of accidents, and protect the safety of personnel, facilities, and the environment.