Lecture 9 - Functional Safety: SIL Assignment and Realisation

Question 1

What is the safety requirement specification?

Answer 1

• Functional and Integrity requirements
• Clear and concise statements.
• Live document
• REFER TO SLIDES FOR TABLES/EXAMPLES

Question 1

What is a risk assessment?

Answer 1

• Qualitative, Semi-quantitative, Quantitative
• Example Methods
• Layer of Protection Analysis (LOPA) - Semi-quantitative / quantitative
• # Risk Graph – Qualitative
• Goal is to find the risk gap.
• REFER TO SLIDES FOR GRAPHS

Question 2

What is safety design?

Answer 2

• Required integrity, description
• High or Low demand? (PFD / PFH)
• Input, Logic, Output Loop –
  Select the components
• Build and Implement
• FAT / SAT
• REFER TO SLIDES FOR TABLES/EXAMPLES

Question 3

PFH and PFDavg Calculations

Answer 3

• REFER TO SLIDES

Question 4

What are the basic rules for safety related systems?

Answer 4

1. Safety functions are considered after other risk reduction measures (inherent safety, mechanical protection) have been applied. Safety functions are used to close risk gap.
2. SIL of a safety-related system is the risk gap.
3. SIL ≥ 3 should not be encouraged. Additional risk reduction measures required – typically there’s something wrong with the process.
4. Safety-related systems of SIL 2 or 3 can be replaced by combination of independent lower SIL, i.e.
   SIL 1 + SIL 1 = SIL 2; SIL 1 + SIL 2 = SIL 3
   All sensors, logic solver, final elements must be separate.