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P-91 fitting replacement

P-91 fitting replacement

P-91 fitting replacement

(OP)
What is the accepted amount of base metal that is to be removed when replacing a defective fitting in an 18" 2" thick steam line?

RE: P-91 fitting replacement

bear1944;
P 91 material!? What do you mean by defective (weld defect, base metal defect, crack, soft region...)? What type of repair are you doing to this main steam line?
 

RE: P-91 fitting replacement

(OP)
I wasn't involved in the discovery phase, only in the procurement and fix. There was a soft spot discovered, evidently in the HAZ of a long radius 45 deg elbow and it is being replaced. I have found little info. about the amount of old pipe to discard. One authority prefers 1/2 a pipe diameter, if possible. This seems a bit wasteful. I've also found minimun 3 inches but the pipe diameter wasn't given. Is the concern about trying to avoid the pwht of the field weld impacting an existing shop weld?  

RE: P-91 fitting replacement

So in other words, you are looking to determine how far past the original girth weld (which was the weld used to join the defective fitting to the straight pipe section) the cut line should be located?

If so, I would place the cut line just beyond the base metal HAZ width in the straight pipe section. Starting at the toe of the girth weld, measure 1" and this would be the cut line on an 18" diameter steam line (the 1" would be a safe distance to remove the original HAZ). Verify with hardness testing before cutting to assure uniform hardness with other location along the pipe spool.

RE: P-91 fitting replacement

(OP)
Thanks for the response. So you don't see a need for more than one inch between the field welds and the shop welds that added the additional material to the fitting.

RE: P-91 fitting replacement

This is slightly different than what I had envisioned from your OP because I believe you are attempting to install a pup piece to make up the difference in length because you have to replace the fitting. Have you looked at simply cutting through the center of the existing girth weld, NDT after prepping the remaining portion of the weld on the pipe and deposit a new girth weld?

There is no specific guidance related to separation distance between two girth welds for installing a pup piece - shop or field. If you need to install a pup piece, I would select a distance that avoids having any touching of base metal HAZ's but far enough to assure proper interpretation of RT results for the field weld.
 

RE: P-91 fitting replacement

(OP)
Yes, our thinking was we needed to avoid welding again in the original HAZ, otherwise we would insert the new fitting in as is. It's interesting that there is no guidance concerning the distance between welds. We currently have ordered the fitting with 4 inches additional on each end. That seems to be adequate. I really appreciate your advise on this.   

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