Intro4u2u

SIL & TUV AK Approvals

Safety Integrity Levels (SIL) pertain to the degree of process hazard and the required system reliability. TUV is a certification for individual components for use in certain SIL system applications. For more detail, search for an article by Dave Harrold, CONTROL ENGINEERING regarding CERTIFICATION.

ANSI/ISA S84.01 and IEC 61508 address safety system requirements. IEC 61508, “Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems,” is a European standard. ANSI/ISA S84.01-1996, “Application of Safety Instrumented Systems for the Process Industries,” is a U.S. standard. Another IEC standard 61511 is also in the works for process plants.

S84.01 defines Safety Integrity Levels (SIL) 1 through SIL 3, while IEC 61511 defines SIL 1 through SIL 4. The higher the number the more critical the process shutdown demand. Portions of a process facility such as a pressure vessel are evaluated and rated. Some areas may require SIL 1 and others SIL 2 or 3. The SIL pertains to safety-instrumented system function and not to components. The SIL 3 applications may require a TMR system with triple sensors for each measurement. TUV pertains to the reliability of programmable logic controller components, field sensors, pushbuttons, solenoid valves, etc.

Organizations certifying safety system and sensor products to meet IEC and ANSI/ISA safety standards include Factory Mutual and TÜV Rheinland. Products are tested and found to assure that they cause a trip. This does not mean that they are reliable but if they fail it will cause a trip. The product is certified fit for use for a specified SIL or application within certain limits. The limits are indicated within the certification approval report. The user should obtain the report for safety system applications.