TANK FLOOR BOTTOM REPLACEMENT

[gasoperations]

We are replacing a API 650 tank botom. The welding is under going but it can be seen that there are many bulges on the tank bottom . It appears like air pockets under some places. The bulged sections also move up and down with weight movement over them.

It does not appear to be normal. Is it caused by wrong welding sequence / weld method / soil compacting underneath etc.

Any help will be appriciated.

 

[IFRs]

It may be because the contractor used de-coiled plate without a temper pass and the residual stresses are being activated by the heat of welding. Ultimately it will be weighted by the product and may be of little or no significance. On the other hand we have no real information on how-what-where-when-how much, etc.

 

[gasoperations]

Thanks for reply.

But what is meant by " de-coiled plates without temper pass". I am under the impression the tank bottom plates are flat only and not like curved plates for tank shell.

Also this is acceptable as per code or there is some limiting specification indicated some where

 

[IFRs]

De-coiled plate comes from the mill in large coils (like paper towels). The fabricator or the service center removes it from the coil and mechanically flattens the plate before cuttng it to length for shipment. This process results in residual stresses in the plate. These stresses can be removed by heat treating. There are lots of google hits, below is one:

http://www.stormingmedia.us/26/2682/A268214.html

 

[JStephen]

Welding sequence can affect it. If you're replacing a bottom, you may be working with smaller than normal plates, and also may have limitations on weld sequencing.

One of the tank standards has or used to have a statement that out-of-plane distortion equal to 1% of the radius was not uncommon. There's not a specific limitation on it, though.

The alternative to decoiled plate is "mill flat" plate, except that much of the mill flat produce comes off coil lines anyway- only it's decoiled at the mill instead of at a processor's facility.

 

[IFRs]

For customers who express a strong quality concern, I usually require de-coiled plate to have a temper pass no matter where it comes from.

 

[JoeTank]

What is a temper pass?

Joe Tank

 

[IFRs]

A light roll after de-coiling. See

http://www.thefabricator.com/RollForming/RollForming_Article.cfm?ID=98

 

[chicopee]

JStephen has also a good point. Back in the 70's when I designed for a microwave antennae manufacturer, we were welding flat plates of aluminum and they buckled too. Solution was to cut holes at the center of the buldges and the plates straightened out.

 

[FrCivEng]

It could also result from a backfill compaction defect since the origin of after years of loading / offloading cycles ; in such a case, API 653-Appendix B gives the procedure for bulge assessment to make the decision to replace or not the bottom plates and for backfill repair ; note that API653-B also recommends to proceed to inside levelling measurement of the whole bottom plate when the tank is out of service