Hazardous Area Classifications & Protections: SIL, LOPA, and Safety Instrumented Systems
Safety Integrity Level (SIL):
Safety Integrity Level (SIL) is a study implemented in hazardous areas to protect against accidents, fires, or blasts in process industries. It is a recent study that complies with the IEC 61508 (Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems) and IEC 61511 (Safety Instrumented Systems for the Process Industry Sector) standards.
Layer of Protection Analysis (LOPA):
The Layer of Protection Analysis (LOPA) is a method used to analyze all process hazards and layers of protection. It acknowledges that a single safety measure alone cannot eliminate risks, and multiple layers of protection are necessary. LOPA is performed after the Hazard and Operability Study (HAZOP) to assess if existing protection measures can reduce risks to an acceptable level.
HAZOP and SIL Study:
While a HAZOP study makes the overall process robust by adding redundant instruments and additional alarms, it does not address the required reliability of the system. SIL studies are essential to determine the necessary reliability and identify if additional safety measures, beyond the HAZOP study, are required.
SIL Levels:
SIL levels, such as SIL 1, SIL 2, SIL 3, and SIL 4, are determined based on hazard frequency and severity. SIL 4 is used for high-frequency and high-severity fault conditions.
Safety Life Cycle:
The Safety Life Cycle provides a structured approach to identify and analyze process hazards and determine if a Safety Instrumented System (SIS) is required. The IEC 61508 standard offers a safety lifecycle model that helps guide new projects in selecting and using the appropriate version for their applications.
SIL Identification, Verification, and Validation:
SIL identification and SIL verification studies are conducted during detailed design engineering for projects, while SIL validation is performed on-site after the installation of all components in the SIL-certified loop.
Application of SIS:
A Safety Instrumented System (SIS) is designed to prevent hazardous events by taking a process to a safe state when predetermined conditions are violated. SISs are also known as safety interlock systems, emergency shutdown systems (ESD), or safety shutdown systems (SSD).
Example:
Consider a pressure vessel containing flammable gas controlled by a PLC-based process control system. Implementing an SIS as an additional safety measure will reduce the risk of overpressure. The SIS may include a pressure transmitter to sense intolerable pressure levels, a logic solver for system control, and a solenoid valve to vent the vessel's contents to a safe location, thereby bringing the pressure vessel to a safe state.
Understanding SIL, LOPA, and the application of SIS is crucial for ensuring safety and mitigating risks in hazardous areas of process industries.