Safety Instrumented Systems (SIS) are designed to implement the Safety Instrumented Functions (SIF) to safeguard critical processes in the case of an unacceptable deviation in the process. An SIS system is comprised of sensors, logic solvers, and final elements and the logic solver is programmed to act on the deviation to place the chemical process in a safe state.
Once SIL Selection (performance target) is completed, achievement of the SIL Selection is verified by a process commonly referred to as SIL Verification. SIL Verification includes determination of probability of failure on demand, spurious trip rates, and other performance metrics. SIL Verification considers the type of equipment employed, advanced voting arrangements, diagnostics, and testing frequency.
Engineering well defined Safety Instrumented Functions (SIF) are a critical step in the Safety Lifecycle, and the source of many errors in SIS design as a result of common misconceptions about what constitutes a SIF. SIF Definition requires an understanding of hazards associated with the chemical process, and the specific instruments that are utilized to protect against those hazards. SIFs are intended to protect against specific and identifiable hazards instead of general hazards, such as fire and gas explosion. These hazards are not always clearly defined in the Process Hazards Analysis (PHA), which is often used as the primary means for defining SIFs.
Ensuring the ongoing integrity of a SIS throughout its lifecycle requires vigilant maintenance and testing practices that are aligned with the risk analysis assumptions and requirements specifications. We develop Maintenance & Test Procedures for SIS, and assist in the implementation of those tests. Additionally, we provide Continuing Engineering Support to monitor and track actual equipment performance against assumptions, audit system activity and changes so that system performance and equipment changes are reconciled with initial assumptions and specifications.
All of the requirements developed during the risk analysis and conceptual design phases of the Safety Lifecycle, and other safety critical requirements, need to be collected and presented in a Design Basis Document, referred to as the Safety Requirements Specification (SRS), that is used for subsequent detailed design engineering of the SIS System.
In addition, we provide Independent Functional Safety Assessment and Certification to ensure that the SIS is designed in compliance with the ISA/IEC 61511 and best practices of peer organizations.