Bottom Course Below t_min: Repair, Replace, or Rerate the Fill Height?
A 60 ft diameter x 40 ft shell-height fixed-roof tank was built in 1963. There is no nameplate, no radiography record, and the material is assumed to be A36; the owner's engineer has authorized S = 23,200 psi and E = 0.70. The tank stores a petroleum product with G = 0.85 at a maximum fill of 40 ft, and the owner wants 0.125 in. of corrosion allowance carried into the next 20-year interval. A 2024 UT survey of the bottom course (readings taken 12 in. above the shell-to-bottom weld and upward) gives a minimum average thickness of 0.42 in.; there is no significant pitting. The annular plate, bottom, roof, and foundation were all found acceptable, and there is no leakage. The terminal has no spare capacity and cannot take the tank out of service for a shell replacement this year.
1. Calculate the minimum acceptable thickness for the bottom course and state whether the tank can continue at a 40 ft fill height. Show each term.
Read the worked answer
Formula (shell, one-foot method): t_min = 2.6 x D x (H - 1) x G / (S x E) + CA
- D = 60 ft, H = 40 ft (full fill height for the bottom course, not course height)
- G = 0.85, S = 23,200 psi, E = 0.70 (no radiography records, so the lowest joint efficiency applies)
- CA = 0.125 in.
Arithmetic:
- Numerator: 2.6 x 60 x 39 x 0.85 = 5,171.4
- Denominator: 23,200 x 0.70 = 16,240
- 5,171.4 / 16,240 = 0.318 in.
- t_min = 0.318 + 0.125 = 0.443 in.
Result: the measured 0.42 in. is 0.023 in. below t_min. The tank cannot be operated at a 40 ft fill height as it stands. Note the (H - 1) term: the design head is taken to one foot above the shell-to-bottom weld, which is where the survey readings begin. The deficiency is about 5% of the metal present but about 12% of the pressure-containing (CA-excluded) section, so it is not a rounding issue.
2. The inspector proposes rerating (lowering the maximum fill height) instead of shell repair. Determine the maximum fill height the calculation supports and state the value you would put into the tank record.
Read the worked answer
Set the available pressure-containing metal equal to the required amount:
- Available = 0.42 - 0.125 = 0.295 in.
- 0.295 = 2.6 x 60 x (H - 1) x 0.85 / 16,240 = 132.6 x (H - 1) / 16,240
- (H - 1) = 0.295 x 16,240 / 132.6 = 36.1
- H = 37.1 ft
Judgment: I would record 36 ft, not 37 ft. Reasons:
- 0.42 in. is an average minimum from a grid; using it with zero margin assumes the survey captured the true low point.
- The 0.125 in. corrosion allowance is a planning number, and the (H - 1) basis already carries a small built-in conservatism that should not be spent twice.
- 36 ft is a clean setpoint for alarms and procedures.
Rerating is an engineering change, not a repair: it requires the authorized inspector's written concurrence, an entry in the tank record showing the new maximum fill height and the calculation basis, and confirmation that no shell opening or appurtenance depends on operation above 36 ft.
3. Assuming the 36 ft rerate is adopted, what must be verified and documented across the rest of the tank before it returns to service?
Read the worked answer
1. Re-check every course at the new level. For a shell with five 8 ft courses, course 2 has its bottom at 8 ft, so H = 36 - 8 = 28 ft:
- t_req = 2.6 x 60 x 27 x 0.85 / 16,240 + 0.125 = 0.220 + 0.125 = 0.345 in. Compare with the measured value for that course. Repeat for courses 3 to 5. Upper courses usually pass, but they must be shown to pass, not assumed.
2. Check appurtenances against the new level. Any nozzle, overflow, gauge, or vent that was designed for 40 ft of head now has different duty; verify nothing depends on the higher level.
3. Reset the controls. Lower the high-level alarm, the independent overfill trip (if fitted), and the level transmitter range; revise the operating procedure so 36 ft is enforced procedurally, not just noted on a drawing.
4. Record the basis. Updated t_min per course, the revised maximum fill height, the corrosion rate used (0.42 in. is only a single data set, so the next interval must be shortened until a second survey establishes a rate), the next internal inspection date, and the inspector's approval. Notify the owner's management of change and insurer.