Process Safety PHA Methods

PHA / risk assessment methods — HAZOP, what-if, FMEA, LOPA, bowtie — comparison and selection guidance.

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Method Comparison

PHA Method Comparison Matrix

Method	Best for	Typical life-cycle phase	Team size	Effort (person-wk)	Qualitative/Quant	Deliverable	Key standard
Checklist	Well-understood process replication; small MOC	Detailed design, pre-startup, MOC	1-2	0.1-0.5	Qualitative	Completed checklist	OSHA 3132; CCPS Ch.4
What-If	Early concept, bench/pilot, simple units	Conceptual, FEL-1	2-4	0.5-2	Qualitative	What-if/consequence/safeguards table	CCPS Ch.6
HAZOP	Continuous flow process, complex units w/ P&IDs	FEL-3 design freeze, 5-yr revalidation	5-8	3-12	Qualitative	Node deviations w/ C/C/S/R and recs	IEC 61882; CCPS Ch.7
LOPA	Risk-ranking selected HAZOP scenarios; SIL target	Post-HAZOP, pre-SIS design	2-4 (with HAZOP results)	1-3	Semi-quantitative	LOPA worksheet w/ IEF, IPL, PFD, residual freq	CCPS LOPA 2001; IEC 61511-3
FMEA / FMECA	Rotating & electrical equipment reliability	Reliability, spares, RCM	3-5	2-6	Qualitative or Semi-quant	FMEA sheet w/ RPN or criticality	IEC 60812; MIL-STD-1629A
Bowtie	Major-accident hazards; communication	Risk register, LFL presentation	4-6	1-3	Qualitative	Central event + threats + consequences + barriers	CCPS Bow Ties 2018
Fault Tree (FTA)	Quantification of IEF or PFD; shared-cause	SIL verification, major-accident QRA	2-3	2-8	Quantitative	Top event probability with cutsets	IEC 61025
Event Tree (ETA)	Accident progression; QRA consequence tree	Facility siting, emergency planning QRA	2-3	1-4	Quantitative	Probability-weighted outcome tree	IEC 60300-3-9
Preliminary Hazard Analysis (PrHA)	Early FEL-1 hazard identification	Conceptual / FEL-1	3-5	0.5-2	Qualitative	PHL + ranking	CCPS Ch.2; MIL-STD-882E
Facility Siting Study	Occupied-building vulnerability from blast/fire	Site layout; FEL-2	3-5	2-6	Semi-quant	Consequence-based siting matrix	API RP 752 (2019)

LOPA IPL Credits

Typical LOPA Independent Protection Layer (IPL) PFD Credits

IPL / Safeguard	Typical PFD	Credit (orders of mag)	Notes / Source
BPCS control loop (as safeguard)	1E-1	1	CCPS LOPA 2001 Table 6.5; one BPCS credit per scenario.
Operator response 10+ min with alarm	1E-1	1	CCPS LOPA 2001 Ch.6; independent indication required.
Operator response <10 min	1	0	No credit per CCPS 2001; use only w/ careful HF analysis.
Relief valve (well maintained)	1E-2	2	CCPS LOPA 2001; validated by API 510 inspection.
Rupture disc (properly selected)	1E-2	2	CCPS LOPA; install on clean service.
Blast-resistant control room / barricade	1E-2	2	API 752; facility-siting IPL.
Dike / containment (leak+accumulation)	1E-2	2	For liquid release consequences.
SIS SIL-1 function	1E-1 to 1E-2	1-2	IEC 61511 target range for SIL-1.
SIS SIL-2 function	1E-2 to 1E-3	2-3	IEC 61511; PFD demand mode.
SIS SIL-3 function	1E-3 to 1E-4	3-4	IEC 61511; rare for refinery; fuel-gas burners, H2 crossover.
SIS SIL-4 function	1E-4 to 1E-5	4-5	Rare in process; usually nuclear / aviation.
Fire & Gas detection + shutdown (certified)	1E-1	1	Credit only if independent from BPCS & SIS.
Passive fire protection (PFP, 2h)	1E-2	2	UL 1709 rated; maintenance-dependent.
Deluge / water spray	1E-1	1	NFPA 15; consumption-rate sensitive.

Source: Process_Safety_PHA_Methods_Reference_v1.xlsx

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