Controls, Safeguards, and Interlocks
Core concepts
Understand the hierarchy: field devices, logic solver/PLC, HMI, and safety instrumented systems (SIS). Know trip functions: low water, high pressure, flame failure, low air/fuel pressure, and high stack temperature. Learn the difference between a permissive (must be true to start) and a trip (stops operation). Understand fail-safe design and the role of the burner management system.
Traps
Questions often confuse permissives with trips or treat an alarm as a trip. Watch for items that suggest bypassing interlocks during startup - never acceptable. Also distinguish automatic vs manual reset after a trip; many trips require investigation before reset.
Worked example
A low-water trip uses redundant level switches with a time delay to avoid nuisance trips. If one switch fails, the other still trips the burner. Rule of thumb: safety functions should be independent of the basic process controller and designed to fail safe.