SIL Risk Assessment & Hazard Analysis 2 — Questions and Answers
Question 1: In a Hazard and Operability Study (HAZOP), what does the guideword 'MORE' signify?
- Complete absence of the design intent
- Quantitative increase of a parameter above its intended value (Correct answer)
- Partial achievement of design intent
- Application of something other than the intended action
Correct answer: Quantitative increase of a parameter above its intended value
The HAZOP guideword 'MORE' indicates a quantitative increase in a process parameter such as flow, temperature, or pressure beyond its design value.
Question 2: Which risk reduction technique involves eliminating the hazard entirely rather than controlling it?
- Engineering controls
- Administrative controls
- Inherently safe design (Correct answer)
- Protective equipment
Correct answer: Inherently safe design
Inherently safe design (ISD) eliminates hazards at the source rather than adding protective barriers, representing the highest level in the safety hierarchy.
Question 3: What does a Fault Tree Analysis (FTA) 'OR gate' indicate?
- All input events must occur simultaneously for the output event to occur
- Any single input event is sufficient to cause the output event (Correct answer)
- Exactly two of the input events must occur
- The output event occurs only when inputs alternate
Correct answer: Any single input event is sufficient to cause the output event
An OR gate in FTA means that any one of the input events alone is sufficient to cause the output (top or intermediate) event.
Question 4: In IEC 61511, the term 'Process Demand' refers to:
- The frequency at which the SIS is tested
- The rate at which the process requires the SIF to take action (Correct answer)
- The maximum allowable PFD of the SIF
- The number of safety instrumented functions in a loop
Correct answer: The rate at which the process requires the SIF to take action
Process demand rate is the frequency at which a hazardous condition arises that requires the safety instrumented function to respond.
Question 5: Which layer of the Layers of Protection Analysis (LOPA) is NOT considered an Independent Protection Layer (IPL)?
- Basic Process Control System (BPCS) (Correct answer)
- Safety Instrumented System (SIS)
- Pressure relief valve
- Dike or bund wall
Correct answer: Basic Process Control System (BPCS)
The BPCS is generally not credited as an IPL when it is also the initiating cause of the hazard scenario being analyzed.
Question 6: What is the primary output of a quantitative risk assessment (QRA)?
- A ranked list of hazards by severity only
- Numerical estimates of risk expressed as frequency and consequence (Correct answer)
- A qualitative ranking matrix of risk scenarios
- A list of safeguards without numerical likelihood
Correct answer: Numerical estimates of risk expressed as frequency and consequence
QRA produces numerical risk estimates combining the frequency of an event with its consequences to support risk-informed decision making.
Question 7: In the context of SIL determination, 'Risk Tolerance Criteria' defines:
- The minimum hardware fault tolerance for a sensor
- The maximum level of risk that an organization deems acceptable (Correct answer)
- The proof test interval for a safety function
- The diagnostic coverage of the safety system
Correct answer: The maximum level of risk that an organization deems acceptable
Risk tolerance criteria establish the maximum residual risk that an organization or regulator considers acceptable after all safeguards are applied.
SIL Risk Assessment & Hazard Analysis 2
In a Hazard and Operability Study (HAZOP), what does the guideword 'MORE' signify?