Master Functional Safety: SIS Design and SIL Calculations

Explore the scope of IEC 61508, detailing functional safety, safety related systems, and the role of electrical, electronic, and programmable electronic devices in achieving SIL levels.

Explore functional safety through SIS design and SIL calculations, and prepare to proceed to the next section with practical insights.

Learn how accidents arise from multiple overlapping failures and apply lopar concepts, the safety instrumented system, and sil calculations to design robust safety layers—prevention and mitigation.

Identify and mitigate hazards in industrial plants through hazop (hazard and operability studies), fault tree analysis, and what-if reviews, emphasizing simplicity and risk reduction for safe design.

Explore process control system (BPCS) as safety layer that keeps process variables within safe limits to optimize energy use and production quality, while promoting operator involvement to avoid automation overreliance.

Explore mitigation layer seven: scrubbers neutralize toxic releases and flare towers burn off excess gas, illustrating their role as physical protections and mitigative safeguards.

Mitigation layer #9 outlines the internal emergency plan—an organizational structure with skilled staff and equipment to mitigate hazards inside and outside the plant, including evacuation alarms and sirens.

Define availability and unavailability for repairable systems, show how they depend on MTTF, MTBF and MTTR, and illustrate with a transmitter example achieving 99.984% availability and 0.016% unavailability.

Show how MTBF equals MTTF plus MTTR and that MTBF applies to repairable systems while MTTF reflects successful operation, and differentiate safe and dangerous failure rates in safety instrumented systems.

Understand failure rate (fit) as a reliability metric, meaning failures per unit time per exposed components; use its reciprocal as mean time to failure and apply the exponential probability approximation.

Learn how common cause failures affect the safety instrumented system and apply the beta model to split failure rates into normal and common mode components.

Two out of three systems use majority voting to enhance safety; one out of two with diagnostic enables channel diagnosis, balancing nuisance trips and fail-to-function with 2oo3, 1oo2D, and 2oo2.

Analyze how manual proof test duration impacts PFDavg in 1oo1 architectures, using bypass testing and the revised equation with d and TD. Discover how longer tests degrade sea level.

Demonstrate practical cell calculations for safety functions using two on/off shut down valves, one-out-of-two configuration, calculating ppfd average, lambda values, risk reduction, and SIL three suitability per IEC standards.

Calculate BFD average of total safety loop, combining valves, thermocouples, and a PLC with BFD 0.0005 per year; sum FDS to yield a risk reduction factor of 673 for Cl2.

Prepare for the next section by reviewing core concepts in functional safety, SIS design, and SIL calculations.

Design requirements for safety instrumented systems per IEC 61,508 and 61 511, cover sensors, logic solvers, final elements, and redundancy across SIL 1–3.

Identify the four safety integrity levels from SIL 1 to SIL 4, the risk reduction they provide, and the three modes—low demand, high demand, and continuous—used by SIS.

Calculate hazard frequency and consequences to compare risk with tolerable levels and select a C level using the IEC risk graph for determining the required SIL.

Review prerequisites before proceeding to the next section in a course on functional safety, SIS design, and SIL calculations.

Demonstrates a diverse redundant level measurement design for a gas kiln cooling water system, using guided microwave analog measurement and a tuning fork level switch to achieve sil 3.

Explore a temperature monitoring safety function in a chemical paper process, using a downstream flow meter for diagnostic coverage to detect valve shaft damage and prevent hot medium leaks.