Tier 1 and Tier 2 Process Safety Events: Classify Events Without Losing the Lesson

An event tier answers a reporting question. It does not explain why a loss of containment happened or what must change to prevent another one. Treat classification and investigation as connected steps with different purposes.
API published the fourth edition of Recommended Practice 754 in August 2026. The practice defines a tiered approach for process safety performance indicators in refining and petrochemical operations. Its update includes revised direct-cost thresholds, clarified definitions and expanded examples. Use the current edition when a site has chosen to report against API RP 754; do not rely on old threshold tables copied into a spreadsheet.

What the tiers do

Tier 1 and Tier 2 describe process safety events for external or industry-level reporting. Tier 1 captures the more consequential events under the practice’s criteria, while Tier 2 captures events meeting a lower consequence threshold. The criteria depend on factors such as material released, quantity, location, consequences and event type. The standard contains the detailed decision rules.
Tier 3 and Tier 4 indicators focus on site-specific control performance and learning. They can show warning conditions and safeguard weaknesses that a count of Tier 1 and Tier 2 events cannot. The API RP 754 fact sheet describes the four tiers as a leading-to-lagging continuum.
API RP 754 is a recommended practice, not a substitute for local law. In South Africa, determine reporting duties from the Major Hazard Installation Regulations and official explanatory notes, applicable permits and other binding requirements. A company may use API definitions for consistent performance reporting while separately meeting its legal reporting duties.

A classification workflow that holds up

1. Stabilise the event and notify the right people. Follow the site emergency plan and applicable reporting requirements first.
2. Record verified facts. Capture material, quantity, release point, duration, location, ignition, injuries, damage and process conditions. Mark estimates clearly.
3. Apply one controlled method. Use the current standard edition, document borderline decisions and have a competent reviewer check the classification.
4. Keep the investigation open. A tier does not identify failed barriers, management-system weaknesses or the conditions that shaped decisions.
5. Trend the result with other evidence. Review similar events, near misses, equipment impairments and overdue actions to find patterns.
   Do not use event tiers to rank individual workers or to decide whether an event deserves learning. A low-tier event can expose a serious weakness in a safeguard. A higher-tier event may have several interacting causes. Preserve the facts and investigate the system.
   If your facility needs help aligning event data with major-hazard scenarios, see MMRisk’s MHI risk assessment services and our guide to MHI risk assessment. Contact MMRisk to discuss a review.