Inspection, Testing and Maintenance
Types of inspection
External inspection while the boiler is in service checks mountings, fittings, insulation, leaks and safety valves. Internal inspection with the boiler cold, drained and opened checks shell, tubes, plates, stays, corrosion, scale and cracks. Inspections are periodic and may be statutory.
Hydrostatic and steam tests
A hydrostatic test fills the boiler with water and pressurises it, usually to 1.25 times working pressure, to prove strength and tightness; the safety valve is gagged or removed and the test pressure must never exceed the permitted limit. A steam test runs the boiler at working pressure to prove it operates correctly and safety valves lift at the set pressure. Both are done after repairs and before certification.
Maintenance and repairs
Preventive maintenance covers cleaning, scale removal, refractory repair, valve overhaul and tube replacement. Repairs to pressure parts must follow approved procedures and be inspected and tested before return to service. Records of inspections and repairs must be kept.
Traps
Questions confuse the hydrostatic test with the steam test, or state the wrong test pressure. Remember the safety valve must not be relied on during a hydrostatic test, and a boiler must not be returned to service until the certificate is valid.
Worked rule of thumb
If working pressure is 10 bar, a typical hydrostatic test is about 12.5 bar (1.25 × working pressure) - but always follow the rules and the Inspector's direction.