Hazop Guide Words: MORE
HAZOP guide words and deviations — IEC 61882:2016 guide words applied to common process parameters across node types (pipe, vessel, reactor, heat exchanger, pump, compressor, control loop, batch step), with typical causes, consequences, and safeguards
| guide word | deviation key | context | deviation description | likelihood class | node type | process parameter | severity class | typical causes | typical consequences | typical safeguards |
|---|---|---|---|---|---|---|---|---|---|---|
| MORE | flow#control_loop#general | general | Control valve fails open — flow higher than setpoint | possible | control_loop | flow | moderate | Valve actuator failure; instrument air failure (fail-open valve); controller output saturated high; instrument loop failure | Overfilling of controlled vessel; high-flow induced erosion; product loss; environmental release | High-flow alarm; valve position feedback; independent high-level alarm on vessel; fail-safe valve position specification |
| MORE | flow#heat_exchanger#tube_side | tube_side | Heat exchanger tube-side flow rate higher than design | possible | heat_exchanger | flow | minor | Upstream control valve failure; pump overspeed; bypass valve opened; increased upstream pressure | Increased pressure drop; erosion of tube sheet; vibration-induced fatigue; fouling/scale formation on tube OD | Flow controller on tube-side inlet; differential pressure indicator; tube velocity calculation during design; inspection schedule |
| MORE | flow#pipe#general | general | Flow rate higher than design intent | possible | pipe | flow | moderate | Control valve failure open; bypass valve inadvertently open; upstream pressure surge; increased line pressure; operator error | Overfilling of vessel; line erosion at high velocity; heat exchanger fouling/damage; flooding of downstream equipment | High-flow alarm (FAH); control valve positioner feedback; flow controller with high-limit; relief device on vessel |
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