SIS Design

Chapter from the Instrumentation & Control Design Guide.

SAFETY INSTRUMENTED SYSTEM (SIS) DESIGN

SIL Classification Overview

SIL Level	PFD Range	Risk Reduction Factor	Availability	Typical Application	Architecture	Testing Interval
SIL 1	0.01-0.1	10-100	90-99%	Most refinery SIFs	1oo1 or 1oo2	Annual
SIL 2	0.001-0.01	100-1,000	99-99.9%	High-consequence SIFs	1oo2 or 2oo3	Semi-annual to annual
SIL 3	0.0001-0.001	1,000-10,000	99.9-99.99%	Catastrophic prevention	2oo3 or 2oo4	Quarterly to semi-annual
SIL 4	0.00001-0.0001	10,000-100,000	99.99%+	Not typical in refining	Highly redundant	Not typical in process industry

SIS Design Lifecycle (IEC 61511)

1. Hazard & Risk Assessment: HAZOP, LOPA → identify SIF requirements
2. SIL Determination: LOPA or risk graph → assign SIL to each SIF
3. Safety Requirements Specification (SRS): document each SIF
• Process variables monitored
• Final element action (de-energize to trip, fail-safe direction)
• Required PFD and SIL
• Proof test interval
• Response time requirement
4. SIS Design: select sensors, logic solver, final elements
• Voting architecture: 1oo1, 1oo2, 2oo3
• Diagnostics coverage
• Common cause failure analysis
5. SIL Verification: calculate achieved PFD
• Include sensor, logic solver, and final element failure rates
• Account for proof test interval, diagnostic coverage, common cause
6. Factory Acceptance Test (FAT)
7. Commissioning & Validation
8. Operation & Maintenance: proof testing per SRS

Source: Instrumentation_Control_Design_Guide_v1.xlsx · sheet “SIS Design”

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