SIL Risk Assessment & Hazard Analysis 4 — Questions and Answers
Question 1: In IEC 61511 SIL verification, the 'Average Probability of Failure on Demand (PFDavg)' for SIL 2 must be:
- ≥10⁻¹ and <10⁰
- ≥10⁻³ and <10⁻² (Correct answer)
- ≥10⁻⁴ and <10⁻³
- ≥10⁻² and <10⁻¹
Correct answer: ≥10⁻³ and <10⁻²
SIL 2 in low-demand mode requires a PFDavg between 10⁻³ (inclusive) and 10⁻² (exclusive), corresponding to a risk reduction factor of 100 to 1,000.
Question 2: The 'Demand Mode' classification of a Safety Instrumented Function is based on:
- The number of sensors in the logic solver
- The frequency of demands relative to the proof test interval (Correct answer)
- The response time of the final element
- The diagnostic coverage of the SIF
Correct answer: The frequency of demands relative to the proof test interval
A SIF operates in low-demand mode when demands occur less frequently than once per year or less than twice the proof test interval; otherwise it is high-demand or continuous mode.
Question 3: Which element is NOT typically included in a Process Hazard Analysis (PHA) report?
- Identified hazard scenarios and consequences
- Recommended safeguards or risk reduction measures
- Detailed financial cost-benefit analysis of all recommendations (Correct answer)
- Documentation of team members and their qualifications
Correct answer: Detailed financial cost-benefit analysis of all recommendations
PHA reports document hazards, consequences, safeguards, and recommendations, but a detailed financial cost-benefit analysis is not a standard required PHA deliverable.
Question 4: In Event Tree Analysis (ETA), a 'success' branch on a safety system node means:
- The safety system fails to respond
- The safety system functions as designed, preventing escalation (Correct answer)
- The initiating event does not occur
- The consequence is always a fatality
Correct answer: The safety system functions as designed, preventing escalation
A success branch in ETA indicates that the safety barrier or system functions correctly, typically leading to a less severe outcome.
Question 5: What is the purpose of a 'Risk Graph' in SIL determination per IEC 61508?
- To calculate the exact PFDavg of the SIF numerically
- To qualitatively assign a required SIL based on consequence, frequency, probability, and demand rate parameters (Correct answer)
- To define the hardware architecture of the SIS
- To document proof test procedures for sensors
Correct answer: To qualitatively assign a required SIL based on consequence, frequency, probability, and demand rate parameters
A risk graph is a qualitative/semi-quantitative tool that uses parameters like consequence severity, occupancy, and demand rate to determine the required SIL for a safety function.
Question 6: The concept of 'ALARP' (As Low As Reasonably Practicable) requires that risk be reduced until:
- Risk reaches zero
- The cost of further reduction is grossly disproportionate to the benefit gained (Correct answer)
- The risk drops below the broadly acceptable level automatically
- All identified hazards are eliminated by design
Correct answer: The cost of further reduction is grossly disproportionate to the benefit gained
ALARP requires risk reduction until further reduction would cost more than the benefit it provides (gross disproportion test), balancing safety investment against residual risk.
Question 7: Which scenario would most likely increase the required SIL of a safety function?
- Reducing the consequence severity from catastrophic to critical
- Increasing the demand rate from once every 10 years to once per year (Correct answer)
- Adding an independent non-SIS safeguard to the scenario
- Reducing the number of exposed personnel
Correct answer: Increasing the demand rate from once every 10 years to once per year
A higher demand rate means the SIF must respond more frequently, increasing the required risk reduction and thus potentially requiring a higher SIL.
SIL Risk Assessment & Hazard Analysis 4
In IEC 61511 SIL verification, the 'Average Probability of Failure on Demand (PFDavg)' for SIL 2 must be: