Periodically we PWHT an existing vessel to get a lower minimum design metal temperature. One issue often overlooked is that the original welds were qualified without a PWHT. Is there anywhere in the codes or recommended practices (API? ASME? BPVC? etc.) that address this issue? What, if any, testing of the welds should be performed?
With regards to ASME Section IX, all welds that recieve PWHT have to be tested that way. (Per QW-253 for example, which calls QW-407.1 an essential variable) If the material has impact requirements, (I would assume that it might as you are talking about lower temperatures) our customers even require us to qualify our welds with a longer PWHT time at temperature than we need in case they need to add additional PWHT time at their plant. (With notch toughness requirements, a the PWHT time of the test weld has to be at least 80% of the PWHT time of production welds per QW-407.2) Not only is it a code requirement, but it is very important. Some of the products that we weld, have to get PWHT above 1100F because if they are tempered below 1100F, they get embrittled. (changes a 40-50 ft-lb material tempered at 1120F to a 5-9 ft-lb materail when tempered at 1050F.) How are you to know this or other effects the PWHT might have on previous welds unless they are tested? It seems to me that putting a vessel through PWHT without having all welds qualified is opening up a can of worms with regards to safety and liability if anything ever happens.
It also depends on the wall thickness. Look at ASME Sec. VIII for the requirements. Is it above 2" or less than 2"? Is it a Div.1 vessel or Div. 2? Does it have a "U" Stamp from ASME? If so, check with an Authorized Inspector for the requirements. If it has an ASME Code stamp, you should be coordinating any repair, welding or PWHT with an Authorized Inspector anyway.
By -
Date 01-02-2002 19:25
This is an interesting issue to which there are many different opinions.
Firstly, the ASME boiler and PV code (e.g. ASME 8 & 9) is written for new fabrications. As such, it would only really be a "guide" when it comes to repairs and later modifications. Typically the governing code for these situations would be the National Board code, or other code that deals with maintenance issues.
Just from a "first principles" perspective, I would like to know why you want to PWHT the vessel. Did you perform weld repairs, or are you taking an existing vessel and re-rating it to handle lower temperatures?
In these situations, it would be important to have a welding engineer have a look at the real problem from a technical perspective. Typically he would consider the material that the original vessel was manufactured from, the welding processes used, the operating history of the vessel etc.
The issue raised regarding the qualification of welding procedures using multiple PWHT cycles, as raised by another respondent, is just such a hotly debated issue. My personal opinion is that the situation that would be conservative from the impact properties point of view, is much more important than being conservative from the material strength point of view. This is so because I have never seen a vessel designed properly to an accepted code fail from pure overstress. Typically there is about a factor of safety of more than 4 built into the design. I have however seen numerous vessels fail from poor impact properties, and depending on the material, the more conservative case would be either with more or with less PWHT. There is no hard and fast rule.
Regards
Niekie Jooste
