DHA Methods: HAZOP, What-If, and LOPA for Combustible Dust
Combustible dust analyses borrow from the same toolbox, but the NFPA standards do not force any one method.
If a corporate safety team, an insurer, or a process-safety auditor has asked whether your combustible dust process hazard analysis will use HAZOP, What-If, or LOPA, the question usually comes from the chemical process safety world. Those methods grew up under federal OSHA’s Process Safety Management standard and California’s PSM and CalARP rules, where a structured hazard analysis is required for covered facilities. Combustible dust analyses borrow from the same toolbox, but the NFPA standards do not force any one method.
This piece explains what each method does, where it fits a dust-focused analysis, and how to pick one without over-engineering the study. For the broader DHA picture in California, see our dust hazard analysis in California guide.
Where These Methods Came From
HAZOP, What-If, What-If/Checklist, Failure Mode and Effects Analysis (FMEA), Fault Tree Analysis, and Layer of Protection Analysis (LOPA) are all recognized Process Hazard Analysis (PHA) techniques. Cal/OSHA’s Process Safety Management standard at 8 CCR §5189 and federal OSHA’s PSM standard at 29 CFR 1910.119 both list these methods as acceptable PHA approaches for covered processes (refineries, large ammonia refrigeration systems, certain chemical operations). California’s CalARP program (19 CCR §2735 et seq.), administered by your local CUPA (which may be a county environmental health department, fire department, or another local agency depending on jurisdiction), applies the same toolset to facilities holding regulated substances above threshold quantities.
A combustible dust hazard analysis under NFPA 652 (and now NFPA 660, which consolidates the family) does not pull from the PSM standard. NFPA expects each process to be reviewed for ignition sources, dust accumulation, and deflagration safeguards, but the writer is free to choose the technique. Many DHAs are completed using a structured walkthrough and checklist tied to the applicable commodity chapter. HAZOP, What-If, and LOPA are available when the study calls for more rigor.
HAZOP
A Hazard and Operability Study (HAZOP) is a team-based, node-by-node review of a process. The team breaks the system into nodes (sections of piping, vessels, conveyors, dust collectors), then applies “guide words” (no, more, less, reverse, as well as, part of, other than) to each design parameter (flow, pressure, temperature, level, composition) to surface deviations and consequences.
HAZOP works best on a continuous chemical process with current piping and instrumentation diagrams. Applied to combustible dust, HAZOP can identify the deviations that lead to ignition or dust escape (loss of inerting, plugged ducting, reverse flow into a hopper, over-temperature on a drying surface). It is thorough. It is also resource-heavy: a multi-day workshop with a process engineer, an operator, a maintenance lead, and a trained facilitator.
For most California food, woodworking, and metal-finishing facilities, HAZOP is more than the dust analysis needs. We reserve it for sites where dust handling is integrated into a larger PSM-covered process, or where corporate safety expects a process-industry-style deliverable.
What-If and What-If/Checklist
What-If analysis is a structured brainstorming method. The team asks “what if” questions about each step of the process: what if the dust collector inlet plugs, what if static is not bonded across the duct, what if the housekeeping crew is short-staffed for a week, what if the new feedstock has finer particles than the qualified one. Each scenario is recorded with its consequences and current safeguards, then prioritized.
What-If/Checklist pairs the open-ended questions with a written checklist drawn from the applicable NFPA chapter (660 today, formerly 652/654/61/484). The checklist closes the gaps a free-form session can miss. For most dust-focused DHAs, this is the right level of structure: enough rigor to satisfy an inspector or underwriter, fast enough to fit a one- or two-day site engagement.
When we walk a California facility for a DHA, the most useful sessions we run sound like What-If/Checklist conversations. The operators know the abnormal conditions; the checklist makes sure none of the NFPA-listed hazards are skipped.
LOPA
Layer of Protection Analysis is not a standalone hazard-identification method. LOPA picks up after HAZOP or What-If, takes a defined scenario, and asks whether the independent protection layers in place (basic process control, alarms with operator response, safety instrumented functions, relief devices, dikes, fire protection) reduce the frequency of the unwanted outcome below the facility’s risk tolerance. The output is semi-quantitative: an order-of-magnitude estimate of how often the scenario could escalate, and whether more layers are needed.
For combustible dust, LOPA is occasionally useful on a high-consequence scenario (a dust collector explosion that could vent into an occupied room, a milling step that has experienced a prior flash event). It is overkill for the standard NFPA review. When LOPA does come up in a California DHA, it is usually because a PSM-covered process is adjacent or because corporate insurers want a numerical safeguard count.
Comparison: When Each Method Fits a DHA
| Method | What it does | Best fit for a DHA | Typical effort |
|---|---|---|---|
| HAZOP | Node-by-node, guide-word review of process deviations | Dust handling inside a PSM-covered process; corporate process-safety expectations | Multi-day team workshop |
| What-If | Structured brainstorming on abnormal conditions | Small or single-process facilities; a quick screen | Half-day to one-day session |
| What-If/Checklist | What-If paired with the applicable NFPA checklist | Most California food, wood, metal, and plastics DHAs | One- to two-day site engagement |
| LOPA | Semi-quantitative review of independent protection layers on a specific scenario | High-consequence scenarios; PSM-adjacent operations; insurer-driven studies | Adds days to an existing HAZOP or What-If |
| NFPA process-by-process review | Structured walkthrough against the commodity chapter of NFPA 660 | The baseline. Adequate for most dust-only facilities | One- to two-day walkthrough plus report |
How CDMS Picks a Method for a California DHA
The method is a scoping decision, not a marketing claim. Three questions drive it:
Is the dust process inside a PSM or CalARP covered process?
If yes, the PHA method already chosen for that process governs, and the dust scope folds in. HAZOP is typical.
Has a regulator, insurer, or corporate parent named a method in writing?
If yes, we match it. If no, we default to What-If/Checklist against the relevant NFPA 660 chapter.
Is there a high-consequence scenario that needs safeguard validation?
If yes, LOPA may be added on top of the base method for that one scenario.
The deliverable does not change much: a written analysis, organized by process, with findings, references to NFPA 660, and prioritized recommendations. The method changes how the hazards are identified. The report is what the fire department inspector, the insurer, or the corporate auditor actually reads.
When You Actually Need More Than a Process-by-Process Review
Most California dust-only DHAs do not need HAZOP or LOPA. The NFPA 660 process-by-process walkthrough is the recognized baseline; if no one has asked you for a specific method, that is what an inspector or underwriter expects to see. For the standards-side detail on NFPA 660 and the five-year reanalysis cycle, see combustible dust hazard analysis under NFPA 652, 654, and 660.
The cases where method matters: a PSM- or CalARP-covered facility where dust handling sits inside the larger process boundary, a corporate safety program built on HAZOP that wants the dust DHA in the same format, an insurer asking for documented protection layers on a specific scenario, or a post-incident investigation where the original analysis missed the failure mode. In those cases the right method is part of the scope. We name it in the proposal so there is no surprise.
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BSY started working with CDMS last year after our in-house EHS person departed the company. CDMS reviewed our existing operational permits as well as any additional Federal, State and Local regulations that could apply and helped us to create a comprehensive compliance calendar to track regulatory deadlines and submittal due dates. The CDMS team does an excellent job of tracking everything and can be relied upon to complete the forms accurately and assist with submittals, allowing me to focus on our business.
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