Preliminary Hazard Analysis

Timing and depth are the main differences. A preliminary hazard analysis is performed early, on conceptual or preliminary design information, and produces a coarse, ranked hazard list using broad severity and likelihood categories. A HAZOP is performed later, on detailed design representations such as piping and instrumentation diagrams, and systematically examines deviations node by node using guide words. The CCPS guidelines present the PHA as a front-end screening method whose findings identify which hazards deserve the more rigorous, resource-intensive HAZOP or other detailed analyses once the design has matured.

A hazard risk index is a single risk level obtained by combining a hazard's severity rating with its likelihood rating through a matrix, in the system-safety tradition the PHA descends from. Severity categories run from negligible to catastrophic and likelihood from improbable to frequent, and each pairing maps to a level such as low, medium, high or unacceptable. The index lets hazards be ranked on a common qualitative scale even without quantitative data, which is exactly what an early-stage analysis needs to prioritize design attention and decide which hazards must be addressed now versus studied later.

No. The PHA is deliberately a screening and prioritization step, not a complete risk analysis. Its ratings are coarse and based on limited design detail, so while it reliably surfaces and ranks hazards early, the significant ones it identifies should be re-examined with detailed methods — HAZOP, FMEA, or quantitative risk assessment — as the design matures. The CCPS guidelines frame hazard evaluation as a tiered program in which the PHA is the entry point; relying on it as the final word would leave high-risk hazards without the rigorous analysis they need.