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LOPA
Risk Calculator

Layer of Protection Analysis — the engineering backbone of SIL determination. Enter your scenario, add independent protection layers, and get an instant verdict against your tolerable risk target. Based on CCPS LOPA methodology and IEC 61511.

CCPS Methodology IEC 61511 / ISA-84 Log-Scale Risk Gauge SIL 1–4 Determination Live Equation View Print-Ready Report
LOPA Core Equation
MEL = IEF × ∏(PFDi)
MEL — Mitigated Event Likelihood/yr
IEF — Initiating Event Frequency/yr
PFDᵢ — Probability of Failure on Demand per IPL×each
∏ — Product (multiply all PFDs)= MEL
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Step 1 — Scenario & Consequence
Define what you're analyzing and what happens if it does
Scenario Description (optional — appears in report header)
Consequence Severity Consequence severity determines the Tolerable Risk Target (TRT) — the maximum acceptable annual frequency for this outcome. Select the worst-credible consequence if multiple are possible. Targets shown are typical CCPS / ALARP guidance; your company may have different corporate criteria. — sets the tolerable risk target (TRT)
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Step 2 — Initiating Event
The unmitigated cause — before any protection layers act
Select Common Initiating Event (auto-fills frequency & description)
IE Description
Frequency (/yr) Enter the initiating event frequency. Accepted formats: 1e-2, 0.01, 5e-3, 1×10^-2. For demand-based events, this is the annual demand frequency. Always use the high end of the published range — be conservative.
Initiating Event Frequency (IEF) — enter above
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Step 3 — Independent Protection Layers
Each IPL must be effective, independent, and auditable
Quick-Add Common IPL
No protection layers added yet.
Select a common IPL above or click Add Protection Layer below.
LOPA Result
New Scenario
10⁻⁷ 10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³ 10⁻² 10⁻¹
TARGET MEL
← SAFER HIGHER RISK →
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AWAITING INPUT
Enter initiating event frequency to begin
MEL
—
Mitigated event likelihood /yr
Tolerable Target
—
TRT /yr
IEF
—
Initiating event freq
RRF Achieved
—
Risk reduction factor
SIS Requirement
NO SIS REQUIRED
Enter inputs to evaluate SIS requirement
Additional Risk Reduction Needed —
📐 Live Calculation Breakdown
Enter inputs above to see the live LOPA equation...
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CCPS Reference Data

Standard LOPA Data Tables

Representative values per published CCPS guidance. Actual values must be established through engineering analysis, operating history, and your company's LOPA protocol.

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Initiating Event Frequencies
Initiating EventFreq (/yr)
BPCS / control loop failure1×10⁻¹
Pressure regulator failure1×10⁻¹
Operator error — routine1×10⁻¹
Cooling / instrument air failure1×10⁻¹
Electrical power failure5×10⁻²
Pump seal failure1×10⁻²
Heat exchanger tube rupture1×10⁻²
Operator error — non-routine1×10⁻²
Atmospheric storage tank failure1×10⁻³
Piping — full bore rupture1×10⁻⁴
Pressure vessel — full rupture1×10⁻⁵
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IPL Probability of Failure on Demand
Protection LayerPFD
BPCS control action — independent loop1×10⁻¹
High alarm — response ≤15 min1×10⁻¹
High alarm — response >15 min1×10⁻²
SIS — SIL 110⁻¹–10⁻²
SIS — SIL 210⁻²–10⁻³
SIS — SIL 310⁻³–10⁻⁴
Pressure relief valve1×10⁻²
Rupture disk1×10⁻²
HIPPS1×10⁻²
Dike / secondary containment1×10⁻²
Flame / detonation arrester1×10⁻²
Blast-rated occupied building1×10⁻²
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SIL Levels & Tolerable Risk Targets
Required PFD RangeSIL Level
≥ 0.1No SIL credit
0.01 – 0.1SIL 1
0.001 – 0.01SIL 2
0.0001 – 0.001SIL 3
< 0.0001SIL 4 / Redesign
ConsequenceTRT (/yr)
Catastrophic — multiple fatalities1×10⁻⁵
Critical — single fatality1×10⁻⁴
Marginal — serious injury1×10⁻³
Negligible — minor injury1×10⁻²
How LOPA Works

The Six-Step LOPA Process

1
Define the Scenario
Identify one initiating event → one consequence pair. A single hazardous scenario may need multiple LOPA analyses — one per IE cause. Be specific: "control loop failure" differs from "operator error opening HV-201."
2
Establish the Tolerable Risk Target (TRT)
Based on consequence severity. CCPS defaults: 1×10⁻⁵/yr for catastrophic, 1×10⁻⁴/yr for critical. Corporate TRTs may be more conservative — always use your company standard. Regulatory requirements may also apply.
3
Select the Initiating Event Frequency (IEF)
The unmitigated annual frequency of the cause — before any protection acts. Use CCPS generic data, OREDA, or IEEE 493. When ranges are given, use the conservative (high) end. Plant-specific operating history takes precedence over generic values when statistically significant.
4
Identify Independent Protection Layers
Each IPL must be effective (sufficient to prevent the consequence independently), independent (of the IE cause and other IPLs), and auditable (documented, tested, maintained). Maximum PFD credit is typically 0.01 per IPL regardless of type.
5
Calculate the Mitigated Event Likelihood
MEL = IEF × PFD₁ × PFD₂ × … × PFDₙ. Each IPL multiplies the frequency by its probability of failure on demand. This is the expected annual frequency that the hazardous consequence reaches the endpoint despite all credited layers.
6
Compare MEL to TRT — Determine SIL if Needed
If MEL ≤ TRT: the scenario passes — existing IPLs are sufficient. If MEL > TRT: additional risk reduction is required. Required SIS PFD = TRT ÷ MEL. The SIL level is determined from this required PFD per IEC 61511-1.

The IPL Independence Rule

LOPA's mathematical validity rests entirely on IPL independence. Crediting non-independent safeguards is the most common LOPA error — it produces optimistic MEL values and false PASS results.

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Independent of the Initiating Event Cause
If the same failure that causes the IE can also disable the IPL, you cannot credit it. A BPCS high-level alarm cannot serve as an IPL for a BPCS control loop failure — same system, same failure mode.
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Independent of Other IPLs
IPLs must not share failure modes. Two alarms driven by the same transmitter are not independent. A PRV and a SIS that both rely on the same pressure tap share common cause — only one may be fully credited.
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Effective, Auditable, and Maintained
The IPL must independently prevent the consequence. It must be proof-tested on a defined interval (the PTI drives the PFD calculation), documented in the management system, and maintained per the mechanical integrity program. An expired PRV inspection cannot be credited.
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Common Cause Failures
A single event — utility failure, instrument air loss, common module failure — can defeat multiple IPLs simultaneously. When multiple IPLs of the same type are credited, apply a common cause factor (β factor) per IEC 61511 Annex D or your LOPA protocol.

Need a Formal LOPA Study?

This tool helps you understand and pre-screen scenarios. For a site-wide SIL determination, formal LOPA documentation per IEC 61511, or expert review of your protection layer credit claims — our team has done this across chemical, refinery, and midstream facilities.

Talk to Our LOPA Team →

Engineering Disclaimer: This calculator implements LOPA methodology from Layer of Protection Analysis: Simplified Process Risk Assessment (CCPS, 2001) and IEC 61511-1 SIL determination guidance. It is provided as a reference tool for qualified engineers. Calculated results depend entirely on user-supplied inputs — IEF, IPL PFD values, independence determinations, and tolerable risk targets must each be established through qualified engineering review and must comply with OSHA 29 CFR 1910.119, EPA RMP, IEC 61511, and ISA-84. This tool does not substitute for a formal LOPA study conducted by a competent process safety engineer and does not constitute engineering advice or professional opinion. IPL independence and PFD credit must be verified by a qualified engineer in the context of the specific process, equipment, and operating procedures. SafeGuard Projects accepts no liability for engineering decisions based solely on outputs from this calculator without independent professional engineering verification.