HAZOP Guide Words (No, More, Less, Reverse, As Well As, Part Of, Other Than)
HAZOP guide words are the seven standard deviation modes used in a hazard and operability (HAZOP) study to systematically explore how a process parameter (flow, level, pressure, temperature, composition) might deviate from design intent. The seven guide words are NO (complete absence), MORE (quantitative increase), LESS (quantitative decrease), REVERSE (opposite direction), AS WELL AS (qualitative addition), PART OF (qualitative decrease), and OTHER THAN (substitution). Combined with the process parameters, the guide words generate the deviation tuples that drive HAZOP node analysis per IEC 61882.
How are guide words applied per parameter?.
For Flow: NO = no flow (pump failure, blocked line, valve closed); MORE = more flow (control valve open, demand increase); LESS = less flow (partial blockage, pump cavitation); REVERSE = backflow (downstream higher pressure); AS WELL AS = contamination (cross-connection); PART OF = missing component (multi-component feed); OTHER THAN = wrong material (mis-loaded feed). For Pressure: NO is rare (vacuum is more meaningful); MORE = overpressure (relief device required); LESS = underpressure or vacuum; REVERSE typically does not apply; AS WELL AS, PART OF, OTHER THAN typically do not apply to pressure. For Temperature: MORE = overheating (cooling failure); LESS = underheating (heating failure); the qualitative guide words rarely apply. Each parameter has guide words that apply meaningfully and guide words that the team marks as 'not applicable.'
What is a HAZOP node?.
A HAZOP node is a section of the process selected for analysis as a unit. Nodes are typically scoped at the equipment level (one node per major equipment item) or at the function level (one node per identified process function: feed preparation, reaction, separation, distillation). Node boundaries are drawn on the P&ID and explicitly listed. Each node has design intent (the conditions the process expects at this point), and the HAZOP team applies the guide words to each parameter to develop deviation scenarios. A typical complex process plant has 50 to 200 HAZOP nodes.
What follows the HAZOP guide-word application?.
Each (node, parameter, guide word, cause) combination that produces a credible deviation goes into the HAZOP worksheet. The team identifies the consequence (what happens if the deviation occurs), the existing safeguards (the layers of protection currently in place: instrumented protection, relief devices, mechanical integrity, procedural controls), and the risk-matrix severity and likelihood. Where the existing safeguards are inadequate, the team writes a recommendation (typically an additional layer of protection, often a new SIF). Recommendations track through to closure as the design progresses.
Frequently asked.
Why are there seven guide words and not more?
The seven guide words were chosen as a minimum complete set that covers the deviation categories meaningful in process operations. More guide words would create redundancy and slow the HAZOP team; fewer would miss important deviation modes. The original ICI methodology established the seven; subsequent work (CCPS, IEC 61882) has confirmed the seven-word set as the practical standard.
Do HAZOP guide words apply to batch operations?
Yes, with adaptation. Batch operations introduce time-based deviations (TIMING: too early, too late, simultaneous), state transitions, and recipe-phase deviations. CCPS guidance for batch HAZOP supplements the standard seven guide words with batch-specific deviations. The structure is the same; the deviation set is extended.
What is the difference between HAZOP and LOPA?
HAZOP identifies hazards and the deviation scenarios. LOPA (Layer of Protection Analysis) quantifies the risk of each scenario and determines whether the existing layers of protection meet the risk reduction target. LOPA is downstream of HAZOP: the HAZOP scenarios are inputs to the LOPA, and the LOPA outputs (target SIL per SIF) drive the SIS design under IEC 61511.
Can a HAZOP team use software to generate guide-word combinations?
Software tools (PHA-Pro, IH-LEAP, Sphera, ABS Group) generate the guide-word × parameter combinations and structure the worksheet. The software does not replace the team analysis; it ensures every combination is considered, captures the team's findings, and produces the audit-traceable worksheet. The engineering judgment about consequence severity, cause credibility, and safeguard adequacy remains with the team.