What is a HAZOP (Hazard and Operability Study)?

How a HAZOP is conducted

A HAZOP is performed by a multi-disciplinary team, typically including a process engineer, a safety specialist, an operations representative, a maintenance representative, and a facilitator or scribe. The team works through the process system node by node — dividing the system into discrete sections along the piping and equipment — and applies a standard set of guide words to each node.

The guide words are:

• No / None (e.g., no flow, no pressure)
• More (e.g., more temperature, more pressure)
• Less (e.g., less flow, less concentration)
• As well as (e.g., impurity present in addition to the intended substance)
• Part of (e.g., only one component of a mixture reaches the node)
• Reverse (e.g., flow in the opposite direction)
• Other than (e.g., a different substance entirely)

For each combination of guide word and process parameter at each node, the team asks: what are the possible causes of this deviation, what are the consequences, and what safeguards are in place? Gaps in safeguards become HAZOP recommendations for action.

Duration and resource intensity

A full HAZOP on a complex process unit (for example, a refinery crude distillation unit or a large chemical reactor system) typically takes two to four weeks of multi-disciplinary team sessions, usually running four-to-eight hours per day. The preparation work — gathering and confirming Process Safety Information, including current P&IDs — typically consumes significantly more time than the HAZOP sessions themselves.

HAZOP versus PHA

In everyday conversation, the terms are often used interchangeably. Technically, HAZOP is one methodology for conducting a PHA — the regulatory requirement. OSHA 29 CFR 1910.119(e)(2) lists several acceptable PHA methodologies. HAZOP is the dominant choice for complex continuous processes; simpler processes may use What-If, Checklist, or hybrid methodologies.

Revalidation cycles

Under OSHA 29 CFR 1910.119(e)(6), a PHA — including a HAZOP — must be updated and revalidated at least every five years. Major process changes (handled through the Management of Change process) can also trigger PHA revisions outside the five-year cycle.

The role of P&IDs in a HAZOP

A HAZOP cannot be conducted without current, accurate P&IDs. The drawings define the nodes, the flows, the equipment, the instrumentation, and the protective systems that the HAZOP team evaluates. Stale or inaccurate P&IDs directly degrade the quality of a HAZOP.

Because most facilities maintain their P&IDs as PDFs with ad-hoc revision control, the HAZOP preparation phase — confirming the drawings are current, aligning field walkdown findings with as-built records, and rebuilding Process Safety Information — routinely consumes more engineering hours than the HAZOP sessions themselves.