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mm inch1 Determine the following:

a Average depth of general corrosion area GCa 4 mm

bAverage depth of internal pitting measured fromthe original bottom plate thickness, StPa

StPa 5.8 mm

c Maximum depth of internal pitting StPm 4.3 mmNote:The value that is used for initial evaluation isbased on inspection data. Subsequent evaluationsmay be made based on maximum internal pitdepth that would still remain after any repairsthat are done

d Average depth of underside pitting, Upa 3.4 mme Maximum depth underside pitting Upm 4.3 mmf Maximum depth of general internal corrosion GCm 5 mmg Maximum internal pitting rate StPr 0.1075 mm/year #DIV/0!

( StPr = StPm/N ) N 40 yearN= years tank has been in serviceStPr = 0 if an internal bottom lining is (or will be)

h Maximum underside pitting rate UPr 0.1075 mm/year #DIV/0!( Upr = Upm/N )

UPr = 0 if tank bottom is cathodically protectedN= years tank has been in service

i Maximum general corrosion rate GCr 0.125 mm/year #DIV/0!( GCr = GCm / N )

GCr = 0 if an internal lining (or will be installed)2 Determine the desired maximum T&I interval for tank Or 3 year

3 MRT1 0.88 mm #DIV/0!

To = Original Plate Thickness To 16 mm

4 MRT2 3.28 mm #DIV/0!

5 Determine the minimum acceptable values of MRT 1 and MRT 2 as follows:

a.

Step by Step Tank Bottom Remaining Thickness Calculation For Dummy

The minimum acceptable values of MRT1 and MRT2 are shown in Table Below for bottom plates, which aannular plates.

Use the following to calculate the minimum expectedremaining thickness at next T&I, based Average internalpitting and maximum underside pitting

MRT1 =To GCa StPa UPm (StPr +UPr +GCr) x Or

MRT1 = Minimum remaining thickness at next scheduledinternal inspection due to average internal pitting andmaximum external pitting

Use the following formula to calculate the minimumexpected remaining thickness until the next T&I, basedon maximum internal pitting and average undersidepitting:

MRT2 =To GCa StPm UPa (StPr+UPr+GCr ) x OrMRT2 = Minimum remaining thickness at the nextscheduled internal inspection, due to maximum internalpitting and average external pitting

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Tank Bottom/Foundation Design

No means for detection and containment of abottom leakMeans to provide detection and containment if a

bottom leak

Applied tank bottom reinforced lining over 1.25mm (0.05 in.) thick, in accordance with API RP 652

6

7

8

9

10

11

Minimum Acceptable MRT 1 andMRT2, mm (in.)

If the entire bottom must be replaced, the requirements that are contained in Para. 7.9.2 of API-653 shall be m

2.5 (0.1)

1.25 (0.05)

1.25 (0.05)

Are the bottom (and annular plate) thicknesses, MRT 1 and MRT 2, in accordance with the acceptance criteria thcontained in Step 5? If both MRT1 and MRT2 meet these criteria, the tank bottom (or annular plate) thicknesswithout further repair; proceed to Step 8. If one or both of these thicknesses are not acceptable, proceed to St

or ons o a an o om may e repa re y we over ay or ap-we e pa c p a es w n e o ow ng resa. No welding, welded-on patch plates, or weld overlays are permitted within the critical zone, except for weldfollowing:- Widely scattered pits- Cracks that are in the bottom plates- The shell-to-bottom weld- Where the bottom or annular plate is being replaced The critical zone is defined by API-653.b. If more extensive repairs are required within the critical zone than the repairs that are listed in Step 10a, thplate (or annular plate) under the bottom shell course must be cut out and a new plate must be installed. Wel

requirements must meet Para. 3.1.5.4 and Para. 3.1.5.5 of API-650.c. The repair of sumps that are located within the critical zone shall be in accordance with Step 10b.

If Step 6 has found that the bottom and/or annular plate thickness was not acceptable, examine the individualMRT1 and MRT2 equations to determine whichfactor(s) needs to be reduced in order to increase the calculated thickness to an acceptable level. Items that mconsidered, alone or in combination, are as follows: Install an internal lining that is at least 1.25 mm (0.05 in.) thick and meets API RP 652, if one is not currently iInstallation of a lining will reduce StPr to zero, and it will allow MRT1 and MRT2 to be as low as 1.25 mm (0.05 Install a cathodic protection system if one is not already installed. A properly installed and maintained cathoprotection system will reduce UPr to zero. Weld overlay repair or use lap-welded patch plates to reduce the maximum and average remaining internal(StPm and StPa respectively). Install lap-welded patch plates in areas of general corrosion in order to reduce the average general corrosionremains in the tank (GCa). Replace the entire bottom with a new bottom.The exact approach to take depends on the extent of repairs that are required, the cost, the available time, anrequirements and limitations that are contained in API-653 with respect to bottom repairs.

If cracks or leaks were found in the shell-to-bottom weld or in the bottom plate lap welds, these defects shallrepaired.

All tank bottom weld repairs must meet the welding requirements that are contained in Section 9 of API-653.

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