Module 6 - SIS-SIL

Module from the Instrumentation Engineering Curriculum curriculum.

SAFETY INSTRUMENTED SYSTEMS (SIS) & SIL · Learning Objectives · 1. Define Safety Instrumented Systems (SIS) and Safety Instrumented Functions (SIF) 2. Apply IEC 61511/ISA 84 lifecycle for SIS design and management 3. Determine SIL requirements using risk graph or LOPA methods 4. Understand PFD (probability of failure on demand) targets per SIL level 5. Specify proof test intervals and common cause failure mitigation · SIL Level Definitions and Requirements

SIL Level	PFD Range	Risk Reduction Factor	Availability	Architecture (typical)	Proof Test Interval	Typical Refinery Application
SIL 1	0.01 to 0.1	10 to 100	90-99%	1oo1 (single sensor, single valve)	Annual	Low-pressure alarm trip, level switch interlock
SIL 2	0.001 to 0.01	100 to 1,000	99-99.9%	1oo2 or 2oo3 (redundant sensors)	Semi-annual to annual	Emergency shutdown (ESD), HIPPS, reactor trip
SIL 3	0.0001 to 0.001	1,000 to 10,000	99.9-99.99%	2oo3 or 2oo4 (triple modular redundancy)	Quarterly to semi-annual	Gas detection shutdown, high-integrity pressure protection
SIL 4	0.00001 to 0.0001	10,000 to 100,000	99.99-99.999%	Special (not typical in process industry)	Very frequent	Nuclear industry — rarely applied in refining
SIF Design Elements
Element	Function	Requirement	Common Failure Mode	Mitigation	Testing
Sensor (initiator)	Detect abnormal condition	SIL-rated transmitter or switch, fail-safe on loss of signal	Sensor drift, plugged impulse line, wiring fault	Redundancy (voting), diagnostics, scheduled calibration	Proof test: apply known input, verify output change
Logic Solver	Process sensor inputs, execute logic, drive outputs	SIL-rated safety PLC (not DCS), watchdog timer, self-diagnostics	CPU failure, memory corruption, power supply	Redundant CPU, voted I/O, separate from BPCS	Full function test: simulate all inputs, verify all outputs
Final Element	Take safe action (close/open valve, trip motor)	SIL-rated valve + actuator, spring-return fail-safe, partial stroke test capable	Valve stuck, actuator failure, solenoid coil failure	Partial stroke testing, redundant solenoids, regular exercising	Full stroke test: verify valve goes to safe position
Power Supply	Provide reliable power to SIS	UPS + battery backup, separate from BPCS power	Battery depletion, UPS inverter failure	Redundant UPS, battery monitoring, diesel generator backup	Battery load test quarterly, UPS test annually
Communication	Signal path from sensor to logic to final element	Hardwired preferred (4-20mA), if fieldbus: SIL-rated protocol	Cable damage, marshalling error, communication timeout	Cable segregation from power, redundant paths, surge protection	End-to-end function test per proof test schedule
Source: IEC 61511-1/2/3, ISA 84.00.01, FOS Chief Files — Relief Valves folder, IPE-EP-12 series, PSSR_Checklist_Template_v1.xlsx

Source: Instrumentation_Engineering_Curriculum_v1.xlsx · Sheet: Module 6 - SIS-SIL

© 2026 Inflection Point Engineering, LLC. All rights reserved. The content of this page — including calculation methods, reference data, written analysis, interactive tools, and source code — is the intellectual property of Inflection Point Engineering, LLC and is protected under applicable copyright, trademark, and trade secret laws. Unauthorized reproduction, redistribution, modification, or derivative use in whole or in part is prohibited without prior written consent.

Disclaimer. This material is provided for informational and educational purposes only and does not constitute professional engineering advice. Calculations, reference data, and methodologies are based on published standards and accepted engineering practice but are not a substitute for engineering judgment, site-specific analysis, or review by a licensed Professional Engineer. Inflection Point Engineering, LLC makes no warranties, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of any content presented here, and shall not be liable for any direct, indirect, incidental, or consequential damages arising from its use. Users assume all risk associated with applying this content to real-world design, operations, or decisions.