Boilers
The boiler domain tests scope definition, classification, safety accessories, water-level and pressure protection, and the periodic inspection cycle - plus your ability to read a boiler's history and say what is due now.
Scope and classification
A boiler is pressure equipment using fuel combustion or another heat source to heat a working medium: a steam boiler with design normal water-level volume ≥30 L; a hot-water boiler with outlet pressure ≥0.1 MPa gauge and rated power ≥0.1 MW; or an organic heat-carrier boiler. Steam boilers are graded by rated steam pressure: low ≤1.6 MPa, medium 1.6–6.0, high 6.0–14, ultra-high 14–18, subcritical 18–22.1, supercritical above 22.1 MPa.
Safety accessories and protection
- Safety valve, pressure gauge, water-level gauge, thermometer, blowdown valve.
- Overpressure alarm and interlock; high/low water-level alarm with low-water interlock; flame-failure protection; programmed ignition control.
- Steam boilers need two independent water-level gauges, except rated evaporation ≤0.2 t/h where one suffices.
- Safety valves are calibrated at least annually.
Periodic inspection
External inspection annually; internal inspection generally every 2 years, with the first internal inspection within one year of commissioning; hydrostatic test generally every 6 years, usually run concurrently with an internal inspection at about 1.25 times rated working pressure.
Worked example
A 4 t/h, 1.25 MPa steam boiler had its last internal inspection in May 2019 and its last hydrostatic test in 2019; the current year is 2025. Reasoning: external inspection is due every year regardless; the 2-year internal cycle has been exceeded several times, so internal inspection is due and overdue; the 6-year hydrostatic interval also matures, so schedule the hydrostatic test with the internal inspection, verify safety-valve calibration date and water-level interlock function, and check for prior 检验意见通知书 items.
Traps that cost marks
Mixing the 2-year internal cycle with the annual external one; forgetting the hydrostatic interval; overlooking the low-water interlock as a testable item; assuming one water-level gauge is acceptable for all steam boilers.