tdi41 tank inspection repair & alteration api 653 sec 9

Tank Inspection: Repair & Alteration API 653 Sec 99.1 General

The basis for alteration shall be as per API Std 650 or equivalent.

All repair works must be authorized by authorized inspector or engineer experienced in storage tank design.

The inspector shall designate inspection hold points required during repair & alteration.

The proposed design, work execution, materials, welding, examination & testing shall be authorized & approved by inspector or by tank design engineer.

All NDE examination required for repair & reconstruction work shall be as per Appendix F of API 653.

Tank Inspection: Repair & Alteration API 653 Sec 9

9.2 Removal & Replacement of shell plate.9.2.1 Minimum thickness of replacement of the shell

plate shall be not less than nominal thickness of any plate in the same course.

9.2.2 Minimum dimensions of replacement plate shall be 12 in (300 mm) or 12 times the thickness of replacement plate. The plate may be circular, oblong, square or rectangular with rounded corners, unless the entire plate is changed, as shown in fig.9-1.

The vertical joint shall be welded first before welding the horizontal joints.


9.2.3 Weld joint designThe replacement plate shall be welded with butt joint with complete penetration &

fusion. Lapped patch shell repair is permitted. Minimum weld spacing dimension shall be as per fig.9-1. Proper fit up, heat input & welding sequence shall be followed to reduce distortion.

9.3 Shell repair by lap welded patch plate. Lap welded patch shell repair is acceptable. However such repair shall be limited to

shell thickness less than 12 mm. The repair plate material thickness shall be smaller than 12 mm but not less than 5 mm (3/16 in).

Maximum size of the lap patch plate shall be 48 in x 72 in ( 1200 mm x 1800 mm) & the minimum size shall not be less than 4 in square (100 mm).

Prior to application of patch repair the shell area shall be inspected by ultrasonic testing to check that no crack or defects are present in the main shell.

Such patch plates may be used to reinforce areas of severely deteriorated shell plate which are not able to resist the service load.

Such plate may also be used to repair the shell area which shows lower thickness than the retirement thickness.


9.3 Shell repair by lap welded patch plate (Cont.)The repair plate thickness shall be selected as per the

design standard using joint efficiency not more than 0.35. The perimeter shall be full fillet weld.

The repair plate thickness shall not exceed shell plate thickness by more than 1/3 or 3 mm. The repair plate thickness shall not be more than 12 mm.

The existing shell plate under the repair plate shall be evaluated at each future inspection.


9.6 Repair of defective welds.Cracks, lack of fusion & unacceptable slag inclusions & porosity

shall be removed by gouging or grinding and the resulting cavity shall be properly for re-welding.

Existing weld undercut which are unacceptable shall be repaired by weld deposition.

9.7 Repair of shell penetrationReinforcing plates may be added to the existing un-reinforced

nozzle when so required. The detail shall be as per fig.9-3.Such pads may also be added inside of the tank if sufficient

nozzle projection exists.


9.10 Repair of Tank BottomsUse of welded on patch plates for repairing

portion of the tank bottom is permitted as per fig.9-5.

The minimum dimension of patch plate shall be 12 in (300 mm) and exceed at least 2 in (50 mm) beyond the corroded bottom area. A smaller patch plate up to 6 in (150 mm) diameter is also permitted in special cases.


9.10.2 Replacement of entire tank bottomA suitable non corrosive material like sand, gravel or concrete shall be used

between the old bottom & the new bottom.Voids in the foundation below the old bottom shall be filled with sand,

crushed limestone, grout or concrete.For tanks constructed with material of yield strength of 50000 psi or less,

the existing shell penetration need not be raised. If required the shell reinforcement shall be so arranged that the weld toe of the reinforcement weld is at least 4 in above shell to bottom weld toe. The weld toe shall be blend ground to minimize the stress concentration & shall be tested by magnetic particle test before & after hydrotest.

In case of floating roof tanks the support legs can be either jacked up to maintain the original height or shortened by the amount as the thickness of the new bottom plate & the cushion.


9.11 Repair of Fixed Roof9.11.1 Supported cone roofThe minimum thickness of the new roof plate shall be

3/16 in (5mm). Roof supports like rafters, girders, columns & bases

shall be repaired if required. 9.11.2 Self supporting roofsThe minimum thickness of the new roof plate shall be

5 mm or as required as per design code.


9.12 Floating roofsAll leaks in pontoons or compartments of double deck floating roof shall be

repaired by welding the leaking joints or by use of patch plates.The rim mounted primary seals can be removed, repaired or replaced.The secondary seals may also be readily installed repaired or replaced. Seal to shell gap may be maintained by adjusting the hanger system on

the primary shoe seals and adding foam filler in toroidal seals, or increasing the length of rim mounted secondary seals or repairing all the parts of the primary seals.

Generally seals are deteriorated due to wear or corrosion on metallic element or weather deterioration of seal fabric.

If the roof rim thickness is less than 0.1 in ( 2.5 mm) the new roof rim shall be made with the thickness of 3/16 in (5 mm) minimum.

tdi41 tank inspection repair & alteration api 653 sec 9

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