LOPA SIL Target Calculator
Use layer of protection analysis (LOPA) to find how much risk reduction a safety instrumented function must provide, and the SIL that corresponds to it.
- Mitigated frequency = fIE × ΠPFDIPL × CM = 0.0005 per year
- Required PFD = tolerable ÷ mitigated = 1.00 × 10^−5 ÷ 0.0005 = 0.02
LOPA Calculation
PFDSIF,required = ftolerable ÷ fmitigated
Typical credits: BPCS control loop PFD 0.1, operator response to alarm 0.1, relief valve 0.01. CM combines conditional modifiers such as probability of ignition or presence of people.
Worked example
An event at 0.1 per year with two IPLs of 0.1 and a modifier of 0.5 gives 5 × 10⁻⁴ per year. Against a target of 10⁻⁵ the SIF needs PFD ≤ 0.02, a SIL 1 function with RRF 50.
Frequently Asked Questions
What is LOPA?
A semi quantitative risk method that multiplies initiating event frequency by the failure probabilities of independent protection layers to estimate the remaining risk.
What is a typical tolerable frequency?
Company specific; values like 10⁻⁵ per year for a single fatality to an employee are common in industry.
Can the BPCS be both initiating event and IPL?
Usually not in the same scenario, and IEC 61511 limits total BPCS credit; check your company rules.
Learn more on the blog
What is SIL (safety integrity level)
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article