API 653 Floor Tmin fro 20 y/r interval

[WVW71]

When performing an internal on a tank floor on areas that have been scanned by the magnetic flux leakage (or exclusion) process, do you use the Tmin of one plate, or lowest averaged recording of the floor to try to establish a 20 y/r interval?

 

[Duwe6]

Use the lowest Tmin, period. Think about adding repair plates to any really low areas, to bring your Tmin up to a better level.

May need to reread API-653: the tank only needs 0.10" calculated remaining thickness at double your Inspx interval [makes your half-life Inspx happen at 20-years]. 2 * 20-years = 40 years to have a Tmin of 0.10"

And if this tank has full containment [well-made dike] and leak detection [sits on full concrete pad, or had double-floor, etc] the Tmin for floors is only 0.05".

 

[zelgar]

What section of API 653 allows the Tmin to get as low as 0.05"?

 

[Duwe6]

Table 4.4

Please remember, this is only allowed for a "used" tank, not a new one. For minimums, see API-650.

Also, for large tanks with annular plates, 4.4 does not apply -- have to use Table 4.5.