50 wt material instead of 38 wt

[stan000]

HELLO FRIENDS
IF I USED 50WT instead of 38WT for repads on storage tank.
both have impact test -50°F.
my understanding will work
opinion appreciate.

 

[JStephen]

Per API-650, repads with higher allowable stress than the shell may be used, but you don't take advantage of the higher allowable in the design. See the "No credit may be taken" sentence down in 5.7.2.3(e).
Both materials need to be suitable for the design metal temperature, and welding needs to be adequate for each.
In some cases, it may simplify matters if the higher-strength material is dual-certified.

 

[IFRs]

JStephen is right on, as usual. No harm, no foul. 50W can not be used over 1 1/2" [40mm] 4.2.3.b but you are using 50WT (the "T" is important). It needs to meet the chemical requirements from 4.2.7.5 and table 4.4. There is some additional NDE for welding to shell plates of this material, it may not really apply to repads but you can find it in 7.2.3.6 and 5.8.1.2. You'll need low hydrogen electrodes for welding to 50WT and repads for any permanent attachments welded to the repad. You did yourself a HUGE favor by getting the toughness test. Note: I think that 50WT will not be in the next edition of API 650 but that does not affect this tank. I would look into the dual certify thing, if it is you can buy it as 38W and you're good to go. Let us know how it turns out!