Leaking Shell Pit at 30 Inches: Can the Tank Stay in Service?
An operator finds a persistent gasoline weep at a shell pit 30 in. above the shell-to-bottom weld of an 80 ft diameter x 40 ft high fixed-roof tank holding 22 ft of product (G = 0.74). The last thickness survey recorded 0.17 in. in that locally thinned area; the calculated t_min at that elevation is 0.23 in. A sheen is visible inside the dike and was reported. The terminal has no spare tank and wants to wait 9 months for a planned turnaround, sealing the weep in the meantime with a welded lap patch placed by a contractor "next week while the tank is still full." The terminal argues that pumping the level down to 30 in. would stop the leak and let a crew weld the patch with the tank "empty." The tank has 30 years of gasoline service and has never been internally cleaned.
1. Evaluate the shell condition against the code criteria and state whether the tank can remain in service pending the turnaround.
Read the worked answer
Condition: 0.17 in. against t_min of 0.23 in. is 26% below the required thickness in the locally thinned area, and the metal has already breached, so product is escaping through the shell.
Conclusion: the tank is not fit for continued service at the current level. A shell below t_min must be repaired, replaced, or the tank rerated to a level the remaining metal supports before it continues in service. A leak changes the urgency: the pit that weeps today can grow into a larger opening, and the release is already reaching the dike, with soil and groundwater exposure at the ringwall.
Acceptable near-term actions:
- Pump down below the pit, but only if the shell at and below the new level is shown to satisfy the calculation. Simply dropping to 30 in. leaves a thinner zone below it. The correct check is the shell thickness over the whole remaining wetted height at the reduced level.
- Contain and absorb the released product, keep the dike drained, and characterize the ringwall soil where the product ran down the shell.
- Notify per the release-reporting procedure and the owner's management of change, and schedule the permanent repair as the next work, not the turnaround.
2. Is the welded lap patch acceptable, either with the tank full or with the level dropped to 30 in.? Give the technical and safety basis.
Read the worked answer
No, on both counts.
Hot work on a tank containing gasoline: welding on the shell is permitted only after the tank is emptied, cleaned, positively isolated, gas-freed, and verified below 10% LEL at the work location by a qualified person, under a documented hot-work permit with a fire watch. Dropping the level to 30 in. does not achieve this. The tank has never been cleaned, so scale and residue are saturated with product; the vapor space behind the patch is flammable regardless of liquid level, and the 22 ft of product becomes 30 in. of product plus a large flammable atmosphere.
The metal cannot take the weld: 0.17 in. plate is already breached. Welding on it will burn through, enlarge the opening, and put an ignition source directly against escaping gasoline. The lap patch also creates a crevice that traps product and cannot be UT-inspected, hiding the corrosion that caused the problem.
The correct repair is an insert plate or shell section replacement with the tank out of service and cleaned. Permissible interim measures are non-hot-work: a mechanical clamp or clamp-and-sealant applied under a written temporary-repair procedure with the inspector's approval, verified daily, with inventory kept below the pit if the calculation supports it, and a fixed repair date. Temporary measures do not close out the deficiency.
3. Describe the permanent repair and the records that close it out.
Read the worked answer
Preparation: remove from service; drain and remove sludge/scale; positively isolate every connected line by blind or spool piece; clean; gas-free with mechanical ventilation; verify below 10% LEL continuously during hot work; confined-space entry controls; hot-work permit; bond and ground the welding machine to the tank.
Repair: insert plate in sound, verified-thickness metal, with rounded corners and full-penetration welds sized to the design; PT/MT on the root and final passes; UT of the surrounding shell to map the full extent of the pitting (pits cluster, so survey above and below the pit and along the horizontal seam). Because the leak ran down the shell, inspect the bottom and annular plate in that quadrant and check for corrosion under the tank.
Testing: hydrostatic test to the design liquid level after the repair, which also proves the new welds.
Records: approved repair procedure, WPS/PQR and welder qualifications, NDE reports, hydro test report, revised thickness survey and remaining life, updated inspection plan and next inspection date, the inspector's acceptance, and the release-reporting documentation and environmental assessment of the dike/ringwall area. Update the tank record so the insert plate and its location are known to future inspectors.