Hydrostatic Test After Retubing a 200-hp Firetube Boiler
A 200-hp, 150-psig firetube boiler was retubed after a tube leak. The contractor proposes a hydrostatic test at 225 psig, using 45°F city water, with the safety valve left in place and gagged. The boiler vent is closed, and the fill line is connected to the bottom blowdown. The contractor plans to hold pressure for 10 minutes and look for leaks. The boiler has a 150-psig MAWP on the nameplate; the safety valve is set at 150 psig. The inspector has not been notified. The plant wants the boiler back in service the same day. The tube sheet was re-rolled, and two tubes were replaced.
1. Evaluate the proposed hydrostatic test pressure and safety-valve gagging.
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The test pressure must be based on the code and the inspector's requirements for the repair, not on a convenient number. A test at 225 psig is 1.5 times the 150-psig MAWP, which may be appropriate for some new construction but is not automatically correct for a repaired existing boiler. The contractor must confirm the required test pressure for this repair and must not exceed the MAWP of any component. Gagging the safety valve is not acceptable. The valve should be removed and the opening blanked, or the valve isolated with a proper blind flange, so it cannot be damaged or accidentally lifted during the test. The test must be witnessed or accepted as required, and the inspector should be notified before the test so the repair can be inspected.
2. What problems do the 45°F fill water and closed vent create?
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Cold fill water creates two problems. First, the metal may be below the temperature range allowed for a hydrostatic test, increasing the risk of brittle fracture, especially at welds and rolled joints. The water should be near ambient temperature and within the limits specified by the code or inspector. Second, the closed vent traps air in the boiler. Air compresses and makes pressure control difficult, and it can prevent water from reaching all parts of the boiler. The vent should be opened while filling so air can escape, then closed only when the boiler is full. The fill line connected to the bottom blowdown is acceptable only if it is controlled and the boiler is filled slowly. The pressure gauge should be verified, and the test should be held for the required time while all joints are inspected.
3. What is acceptable evidence that the repair passed?
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The test is acceptable only if the boiler holds the required test pressure without a significant drop, and all tube ends, rolled joints, and seams remain dry. No leaks, weeping, or permanent deformation should be found. The inspector or authorized witness should verify the gauge pressure, the water temperature, and the test duration. After the test, the safety valve should be reinstalled with a new gasket or the isolation blind removed, and the valve should be checked for proper setting. The boiler should then be prepared for service with a slow warm-up. The results should be recorded, including the test pressure, duration, water temperature, inspector's name, and any repairs made. If the test fails, the boiler must not be fired until the defective joints are repaired and retested.