Hydrostatic Test After a Staybolt Repair
A shell boiler with a design pressure of 10 bar has had 12 staybolts replaced in the combustion chamber. The repair was made by a coded welder, and the insurer's inspector has asked for a hydrostatic test before the boiler is returned to service. The boiler has been drained, the manhole is open, and the safety valves are still fitted. The attendant suggests filling the boiler with cold mains water and bringing it up to 15 bar with the feed pump, then holding it there for 30 minutes. He also proposes to leave the safety valves in place and to sign the boiler back into service himself once the pressure holds.
1. Assess the proposed test pressure and duration against normal practice for a hydrostatic test after a repair.
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The proposal needs correction. The hydrostatic test pressure is normally set by the applicable code or the insurer's instruction, commonly 1.25 to 1.5 times the design pressure, so 15 bar is plausible for a 10 bar boiler, but the figure must be confirmed from the code or the inspector's written requirement rather than assumed. The hold time should be that specified, commonly 30 minutes, but the pressure must be measured with a calibrated gauge and the test witnessed by the inspector.
The key omission is that the safety valves must be removed or isolated and replaced with blanks or test plugs before the test. Otherwise the safety valves will lift at their set pressure and the test cannot reach the required pressure; worse, they may be damaged. The boiler should also be filled slowly and vented to remove air, and the water temperature should be close to ambient to avoid thermal stress.
2. What checks should be made during and after the hydrostatic test, and who may authorise return to service?
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During the test, inspect all repaired staybolts, seams, tube ends, and the manhole and handhole joints for weeping, sweating, or distortion. Pressure should be raised in stages and held while the inspector examines the work. After the hold, the pressure is reduced before a close visual examination, and any leak is recorded and repaired before a repeat test.
The attendant may carry out the mechanical work under supervision, but return to service must be authorised by the competent person, normally the insurer's inspector or an authorised inspector, not by the attendant alone. The test certificate, repair records, welder's details, and the inspector's signature should be filed, and the safety valves must be refitted and re-set or proven before steam is raised.