Commissioning, testing, inspection and maintenance
What this domain tests
This domain examines the evidence trail from installation to safe operation: strength and tightness testing, purging, commissioning checks, inspection and planned maintenance. The exam expects practical test sequences and the reasons behind them.
Testing principles
- Strength testing proves mechanical integrity at a pressure above the design condition. Tightness testing proves leak-free assembly at or near operating pressure.
- Test with air or nitrogen, never with gas. Allow temperature stabilisation before reading the gauge, because a falling temperature mimics a leak.
- Record test medium, pressure, duration, gauge type, temperature and acceptance criterion. Isolate or disconnect appliance governors and relief devices that could be damaged.
- After testing, purge inert gas or air from the pipework with the supply gas to a safe point, and verify combustion before handing over.
- Commissioning includes gas rate, operating pressure, combustion analysis, flame picture, safety-device proving and customer instruction.
Worked example
A 30 m run of 28 mm copper tube with 1.0 mm wall has an internal diameter of 26 mm. Internal volume = pi x (0.013)^2 x 30 = 0.0159 m3, or about 15.9 litres. The tightness test is set at 150 mbar with air. After temperature stabilisation, a 5-minute test shows a fall from 150 to 149 mbar. On a U-tube this is 10 mm water gauge. The drop exceeds a negligible-leak criterion for this small volume, so the joints are checked with leak-detection fluid and the test repeated. Do not accept the result simply because the gauge movement is small on a dial gauge.
Maintenance and inspection
Use risk-based intervals: governors, relief valves, gas detectors and emergency controls need more frequent proof than static pipework. Keep records of calibration, defects, isolation and reinstatement. Inspection findings must be closed out, not merely noted.
Traps that cost marks
- Pressure-testing with gas or leaving appliances connected.
- Reading the gauge before temperature equilibrium.
- Using a dial gauge too coarse to detect the specified leak rate.
- Skipping purge verification and lighting the appliance on a flammable mixture.
- Recording a defect without a repair date, owner or retest.