PHA vs LOPA: When to Use HAZOP, When to Layer LOPA on Top

PHA and LOPA Are Not Substitutes

A Process Hazard Analysis (PHA) identifies what could go wrong. Layer of Protection Analysis (LOPA) quantifies how often and decides how much protection is enough. You need both — but you do not need LOPA on every PHA scenario.

What PHA (HAZOP) Does

A HAZOP team walks each process node through deviation guidewords:

For each deviation: causes → consequences → existing safeguards → recommended actions.

Output: a qualitative ranked list of scenarios with recommendations.

When to Layer LOPA on Top

Apply LOPA only when:

1. PHA flagged the scenario as high consequence (fatality, major release, asset loss)
2. Existing safeguards are credited as Independent Protection Layers (IPLs)
3. You need to decide whether a Safety Instrumented Function (SIF) is required and at what SIL

LOPA math (simplified):

Mitigated frequency = Initiating event frequency × ∏ (PFD of each IPL)
Gap = Mitigated frequency / Tolerable frequency

If the gap > 1, you need additional IPLs (typically a SIF) to reach the tolerable target.

Typical IPL Credits

Common Scoping Mistakes

• Running LOPA on every PHA finding (waste of time)
• Crediting non-independent layers (BPCS + alarm on same logic solver = 1 IPL, not 2)
• Ignoring enabling conditions (operator on shift, valve in auto)
• No documented tolerable frequency target

Templates to Get Started

Download the PHA worksheet (HAZOP-style node-by-node format) and LOPA worksheet (scenario, IPL credits, gap, SIL target). Both align with CCPS guidance.

Software for PHA / LOPA at Scale

For multi-site programs, dedicated PHA/LOPA tools (PHA-Pro, Synergi Plant, Sphera) outperform spreadsheets. For broader EHS coverage with PHA modules, see our QHSE software directory.