P91 and inspection nozzles

[athomas236]

I am involves with a project that has superheater outlet headers and pipes made from P91.

The headers have inspection nozzles that are also made of P91. There are some reservations regarding the cleanliness of these headers and the contractor has been asked to remove the end caps on the inspection nozzles for a borscope inspection.

The contractor has responded to this request by saying that it is not practical to remove the end caps because rewelding the end caps after the inspection will need the nozzles to be filled with argon backing gas which would mean filling the boiler with argon.

I would be pleased to hear of other peoples experience with the problem.

Last week I was in a meeting with another contractor on another project, so I asked him the same question. His response was that backing gas is not requred so I am really confused now.

Regards,

athomas236

 

[metengr]

The contractor has responded to this request by saying that it is not practical to remove the end caps because rewelding the end caps after the inspection will need the nozzles to be filled with argon backing gas which would mean filling the boiler with argon.

In this situation, backing gas for GTAW welding is NOT required.

 

[athomas236]

metengr,

Thanks for the information, but why is backing gas required.

Regards,

athomas236

 

[davefitz]

It sounds as though the contractor is mistaken. In my opinion, the boiler mfr should be contacted and his opinion on this matter be polled.

The purpose of the boroscope inpsection nozzle is, well, inspection by boroscope, or to allow surveilance of the interior of a major weld during initial assembly ( in EU codes, such surveilance allows use of a more liberal weld fatigue life curve), or to allow insertion of a source for X-ray. Small component P91 welds are often made without having the entire pressure part pressurized with argon.

Maybe the contractor suspects some debirs is inside. I can recall a steam blow of a new plant- the initial lift steam blow sent large rocks thru the main steam line which launched the steam silencer like a rocket . Inspection of the duplicate unit main steam line revealed lots of debris placed inside by disgruntled workers.

 

[metengr]

Backing gas is normally used for austenitic stainless steels to avoid surface oxidation in the root and surrounding base material. I have seen an AWS paper on the evaluation of comparing backing gas and no backing gas on austenitic stainless steel. The results were rather dramatic, using backing gas for austentic stainless steels resulted in a better weld root bead profile, and no surface oxidation and crevices (meaning better corrosion resistance in aqueous service).

 

[Welder4956]

We have made a few repairs on P91 pipe welds with no backing gas for the GTAW root pass. The best description of the root appearance is either "stalagtites" or "stalagmites" depending on whether it is top or bottome of the pipe. It is much more than sugaring, the molten metal solidifies with a crystal-like surface contour that is readily visible on the RT.

I would suggest using a water soluble purge dam and a football needle to purge through the root opening. I have also heard of some success with SMAW root passes. Pat Patrick of Fluor published a good paper in the AWS Welding Journal on this.