STEAM DRUM PWHT

[TanEng]

We have a repaired WHB water drum (3" thick) that need to be PWHT. Its a vertical drum with 700 numbers of 2" tube holes, 7000 Kg vertical steam drum is resting on the water drum. Since we are going to PWHT this lower water drum only while 7000 kg steam drum is resting on it, that can deform the tube holes, what you guys think.. I'll really appreciate some help.

 

[metengr]

We have a repaired WHB water drum (3" thick) that need to be PWHT

Yes, this is a serious concern because of distortion. Here are several questions to allow us to help you;

Can you please describe the exact nature of the weld repair to the lower waterwall supply drum? What type of weld repair was performed, what is the drum base material? What was the preheat requirement?

 

[TanEng]

Orignal plan was to replace all the tubes but when we removed the tubes we found some cracks on the Carbon Steel Waterwalls. These cracks were repaired by welding and Preheating was not considered.

 

[metengr]

TanEng;
This does not help me. What was the depth of weld repair in the drum? Did you remove the carbon steel tubes and decide to weld them back in? What is the base material of the drum - SA xxx or other material? Preheat was not considered? Why? Was the weld repair to the drum performed using a qualified welding procedure? Lots more questions but I am running out of time.

 

[TanEng]

Metengr, I really appreciate you quick responses.

Yes! we removed all the Carbon steel tubes and we are going to weld all 700 new tubes with SA-516-70 waterwalls after PWHT. I'm not sure about the depth of cracks right now but for sure Qualified welding procedure have been used.

Could you please recommend any technique to aviod tube hole distortion during PWHT because of the 7000 Kg weight on the wasterwalls.

 

[metengr]

TanEng;
I would expect that these are partial penetration welds that join the stub tubes to the header in addition to any repair welds to the header bore holes. If this is the case, I would use Alternative Welding Method 1 in the NBIC and avoid PWHT altogether. You will need to elevate preheat to 300 Deg F for this welding method. However, it will get you out of PWHT.

I would not recommend any PWHT of this magnitude on an existing drum because of distortion concerns. If you must PWHT the drum because you have exceed the 3/4" nominal weld thickness PWHT exemption rules currently in PW-39 of ASME B&PV Code, Section I, you will need to temporarily support support the vertical header that is resting on this lower drum. This is not impossible to do but using the alternative welding methods in the NBIC is just for this very reason, to avoid PWHT.