Storage tank shell-to-bottom leak during hydrotest
A 20,000 m³ atmospheric storage tank is being hydrotested. The shell is 20 mm A516 Gr.70, the annular bottom plate is 12 mm, and the shell-to-bottom weld is an 8 mm external fillet with a 6 mm internal fillet. During filling to 12 m, water seeps from under the shell at one location, about 1.5 m from the shell. The tank is new; the contractor proposes to drain, clean, and repair the leak by arc gouging from inside and rewelding the fillet. The client requires the tank back in service within 5 days. The area is classified as Zone 2 because the tank will store gasoline; the tank has been water-filled, not gas-free. The foundation is a concrete ring wall with sand cushion. The contractor's WPS covers 8 mm fillet welding with 100 °C preheat, but not gouging or backing.
1. Identify possible causes of the leak and explain why arc gouging from inside may not be the correct first action.
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The leak may be a weld defect (porosity, lack of fusion, crack), a lamination in the 12 mm annular plate, or water tracking through the sand cushion. Arc gouging from inside is risky because it may enlarge the defect, introduce gouges, and cannot prove the leak path.
First drain and clean; vacuum-box the fillet, PT/MT the visible weld, and UT the annular plate for laminations. If the leak is under the shell, excavate from outside or inside only after the defect is located. The WPS does not cover gouging, so a repair procedure is needed.
2. Set out a safe repair sequence, including hot-work controls and weld procedure requirements.
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Sequence:
- isolate, drain, gas-free and monitor;
- hot-work permit; continuous LEL and oxygen monitoring;
- ventilation and fire watch;
- excavate the defect to sound metal with a qualified gouging/grinding procedure;
- PT; preheat 100 °C;
- weld with a qualified WPS for the actual thickness and restraint;
- control interpass;
- inspect by PT/MT and vacuum box;
- protect with coating.
Because the tank will store gasoline, the repair must not leave a crevice; consider a full-penetration weld detail if the design allows.
3. What acceptance tests would you require before refilling?
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Before refilling:
- visual and dimensional check;
- PT/MT of the repair;
- vacuum box or leak test of the fillet;
- UT of the annular plate;
- hydrotest to the specified level with settlement readings;
- documented repair and NDT records.
If hydrotest is repeated, monitor the shell-to-bottom movement.