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Articles: SIL 101 | SIL 102 | SIL 103 | SIL 104
   
SIL 105

Calculating SIL Suitability Levels
Functional Safety Standards
Choosing a SIL System
LOPA: Layer of Protection Analysis



Choosing a SIL System
The identification of risk tolerance is subjective and site-specific. The owner / operator must determine the acceptable level of risk to personnel and capital assets based on company philosophy, insurance requirements, budgets, and a variety of other factors. A risk level that one owner determines is tolerable may be unacceptable to another owner.

When determining whether a SIL 1, SIL 2, or SIL 3 system is needed, the first step is to conduct a Process Hazard Analysis to determine the functional safety need and identify the tolerable risk level. After all of the risk reduction and mitigation impacts from the Basic Process Control System (BPCS) and other layers of protection are taken into account, a user must compare the residual risk against their risk tolerance. If there is still an unacceptably high level of risk, a risk reduction factor (RRF) is determined and a SIS / SIL requirement is calculated. The RRF is the inverse of the Probability of Failure on Demand for the SIF / SIS (see table below).

Selecting the appropriate SIL level must be done carefully. Costs increase considerably to achieve higher SIS / SIL levels. Typically in the process industry, companies accept SIS designs up to SIL 2. If a Process Hazard Analysis indicates a requirement for a SIL 3 SIS, owners will usually require the engineering company to re-design the process to lower the intrinsic process risk.

Safety Integrity Level Risk Reduction Factor
SIL 4 100,000 to 10,000
SIL 3 10,000 to 1,000
SIL 2 1,000 to 100
SIL 1 100 to 10



Copyright 2006 General Monitors, Inc. All logos, brand and product names are registered trademarks of their perspective owners. All rights reserved. Questions or comments to etech@generalmonitors.com



              

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General Monitors, Inc.
26776 Simpatica Circle
Lake Forest, CA 92630

Phone: 949-581-4464
Toll Free: 1-866-686-0741
Fax: 949-581-1151
etech@generalmonitors.com

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