API 653 Repiar of Caustic Tank

[timmckee]

We are replacing a manway insert on a 40 year old carbon steel API 650 storage tank. The tank has been used as a White Liquor (Caustic) Storage tank.

I'm concerned the inner shell to insert weld will fail due to caustic embrittlement of the shell material. Any suggestions as how to avoid this?

 

[JStephen]

Are you concerned with the present condition of the shell plate causing problems, or with future caustic embrittlement?

Do you know if anything was done on the original tank to avoid this problem?

You should be able to stress-relieve the weld in question, if that would eliminate the problem.

 

[ricellis]

Yes stress relieving is the way to go. If you can't stress relieve the weld get a welding engineer to write up a weld procedure that minimises residual stress. You will have to use small weld runs with low hydrogen low heat electrodes, so that each run tempers the previous run. Use a low yield electrode for the outside welds and grind off the final capping run with a flapper disc.

 

[PAN]

If the tank can be used over 40 years before repair, you should refer to the original WPS.