PWHT and tube repairs

[aida2011]

Hi all,

I have three questions:

1) I have a failed tube which has a longitudinal crack, about 1.5" in length. The manager doesn't want to re-tube, instead, he asked me to repair the tube. I was thinking about doing window patching, but I think window patching is more suitable for bulged tubes. What are your guys opinion?

2) Which should come first, the PWHT or NDT (radiography)?

3) BS code states that PWHT is required for plates thicker than 35 mm. What does ASME Code Sec I say about the minimum thickness for PWHT?

Thanks very much, I hope this matter can be solved.

Best regards

Aida Honey

 

[metengr]

Longitudinal crack - replace the tube or plug it! Attempting to repair this type of failure is against good engineering practice. We did this type of repair only once in a power boiler because it was needed for load - basically we heat and beat the tube shut and pad welded it. Next outage we replaced it.

A window repair will be tricky and requires enough material to be removed to ensure proper fit up. With an axial-oriented rupture of this type, I would simply plug the tube versus repair it.

 

[chicopee]

There is nothing wrong with flush widow patching as long as it is done properly. I don't have current editions of NBIC, however, if patching method is no longer discussed in current editions, get an older copy that discusses such procedure.

 

[metengr]

Flush patch or window repairs were intended for use when replacing boiler tubes where butt welding could not be performed 360 degrees. It was never intended to locally repair tubes. Instead remove the failed section and install a dutchman.

 

[chicopee]

Metengr, please see attachment from 1987 NBIC. NBIC is silent where flush window patching is to be done and the statement in the 1st paragraph of the attachment"...the defective material should be removed until sound material is reached...". "...sound material..." implies an existing tube with some defects. Granted the figure 8 does show two tube ends being welded together before the flush patch is welded, however this could be for other reasons then the one you stated such as perhaps removing 2' of tube length where the defect is located and then replacing that section to include two flush patches. Unless you have an updated NBIC confirming your reply, I'll have to stand with my reply. I would say that aida2011 should contact the Jurisdiction boiler inspector for his input.

 

[chicopee]

Ooops here is the attachment

 

[weldstan]

I would have to venture that over 98% of the tubes that we repair were made with a Dutchman, replacement or plugging and not a with a window type. It is simply too difficult to do correctly in the field and is done only when accessibility issues demand.

 

[chicopee]

With Dutchmen which are short sections of tubes you would have to follow the procedures outline in my attachment if you were to repair water wall tubes in contact with refractory walls and you need to include flush window patches to finish up when butt welding procedure.