Layers of Protection Analysis (LOPA) Worksheet
Work through LOPA scenarios: initiating event frequency, independent protection layers and conditional modifiers give the mitigated frequency, the gap to the tolerable frequency and the SIL target for the SIF.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 36 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Scenarios (frequencies per year)
| Scenario | Initiating event | IE freq | IPL 1 PFD | IPL 2 PFD | IPL 3 PFD | Modifiers | Mitigated freq | Tolerable freq | Required RRF | SIF target | Remarks |
|---|---|---|---|---|---|---|---|---|---|---|---|
| High pressure in V 101, blocked outlet | Control valve fails closed | 2.00 × 10⁻¹ | 1.00 × 10⁻¹ | 1.00 × 10⁻¹ | 1.00 | 2.00 × 10⁻³ | 1.00 × 10⁻⁵ | 200 | SIL 2 | ||
| Tank overfill | Level control failure | 1.00 × 10⁻¹ | 1.00 × 10⁻¹ | 0.50 | 5.00 × 10⁻³ | 1.00 × 10⁻⁴ | 50 | SIL 1 | |||
| Fuel gas explosion in furnace on start up | Purge failure | 2.00 × 10⁻¹ | 1.00 × 10⁻¹ | 0.30 | 6.00 × 10⁻³ | 1.00 × 10⁻⁵ | 600 | SIL 2 |
About this Calculator
LOPA turns HAZOP scenarios into a SIL target with a transparent calculation. A consistent worksheet makes reviews faster and decisions traceable.
Basis: Mitigated frequency = initiating frequency × Π PFD of IPLs × conditional modifiers. Required RRF = mitigated ÷ tolerable. SIL bands per IEC 61511 (low demand).
How to use
- Take scenarios from the HAZOP.
- Enter initiating event frequency per year.
- Enter PFD of each credited IPL and any conditional modifiers.
- Enter the tolerable frequency from your risk criteria.
- Read mitigated frequency, required RRF and SIL target.
Formulas used
- Mitigated = IE × PFD1 × PFD2 × PFD3 × modifiers.
- RRF = mitigated ÷ tolerable.
- SIL 1: 10 to 100, SIL 2: 100 to 1000, SIL 3: 1000 to 10,000.
Related tools
Frequently Asked Questions
What is a conditional modifier?
A probability such as occupancy or ignition that reduces the chance of the consequence.
Can an alarm be an IPL?
Yes, with enough time to respond and an independent sensor; often PFD 0.1.
What if RRF exceeds 10,000?
Redesign the process or add non instrumented layers rather than relying on SIL 4.
Learn more on the blog
What is SIL (safety integrity level)
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article