Calculating SIL suitability levels:
To General Monitors, it is a combined effort between corporate reliability, technology, and a third party to verify estimates to determine the SIL suitability standard to individual products. The steps in suitability include:
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failure rate prediction
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FMEDA
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Failure Path Investigation
Step 1 Failure Rate Prediction:
This initial step is the basis for all measurements of the material SIL. All failure rates of individual components contribute to the overall failure rate of the product within the system.
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BOM component failure rates are calculated based on component type and in-circuit bias
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The rate of product failure is the sum of all the rate of component failure.
Step 2 Failure Mode Effects and Diagnostic Analysis (FMEDA):
Effects of Failure Mode and Diagnostic Analysis (FMEDA) is a methodology that measures and quantifies the self-diagnostic potential of the material. The failure mode of each component and its failure rate is listed.
Failure modes are determined as safe (fire / gas detectable product) or dangerous (fire / gas detectable product). Any safe and dangerous mode of failure is determined by the diagnosis of the product as Detectable or Undetectable.
Step 3 Failure Path Investigation:
Using the actual product, known failure modes not identified during the FMEDA phase are investigated. This requires leads for components to be shortened and opened and classification of failure identified. SD, DD, SU, DU.
Step 4 Calculate SFF, SIL and PFD:
The product’s rate of SIL is finally determined by the Secure Failure Fraction (SFF) and the probability of Demand Failure (PFD). The following equations are being used.
SFF = (lSD + lSU + lDD) / (lSD + lSU + lDD + lDU)
PFD = (lDU)(Proof Test Interval)/2 + (lDD)(Down Time or Repair Time)
SIL level is then determined from the Safe Failure Fraction for type B equipment per the IEC 61508.
Step 5 Third Party Review:
For analysis and confirmation, all findings from the above measures are forwarded to a third party.