ASME B31.3 Repairs on P1 material

Date 07-13-2005 16:13

From the 1999 edition:

Para 328.6 Weld Repair:
...Preheating and heat treatment shall be as required for the original welding. See also para 341.3.3 (this para has more to do with inspection than heat treatment).

I've been looking for exceptions but can't find one that allows repairs to pressure containing parts to be exempt.

Charles Hall

By Jim Hughes (***)

Date 07-13-2005 16:57

Hey John,
the 2001 edition says the same thing as Charels. For P1 material it has to be 19mm (3/4")and over for PWHT. It also depends on what is in the vessel. When I was with B.P they had their own requierments that we had to follow. Our P1 WPS said that for any thickness of P1 material we had to PWHT durring fab. or after repair if the P1 material would see Hydrogen Cyanide, which was one of our products that we produced at this plant.
Hope that helps
Jim

By jon20013 (*****)

Date 07-13-2005 18:02

Okay guy's, thanks! I may be confusing Codes again... not hard to do when one works with dozens!!! I know many of the ASME Codes do allow repair of welds which have been subjected to PWHT depending on repair depths. I'll have to do a little deeper digging, I was positive B31.3 did too.

By MBSims

Date 07-14-2005 03:17

Wouldn't the need for PWHT be based on the repair cavity depth and not base metal thickness? Preheat would be based on base metal thickness.

By jon20013 (*****)

Date 07-14-2005 12:27

Marty, thats a "yes and no" question; the original need for PWHT would have been based on the material type and thickness, the need for PWHT subsequent to the repair is what I am trying to determine; I haven't access to a B31.3 Code and I'm curious if there are exceptions as there are in many other ASME Codes based on repair cavity depth ~ (I seem to recall there are, but I'm basing this simply on perhaps faulty memory!), others who have responded to this post have been unable to find the exceptions I seem to remember!

By chall

Date 07-14-2005 12:54

Section I allows an exception based on the repair depth not exceeding 10% of the material depth.

B31.1 clearly defines "nominal thickness" as the thickness of the weld; and allows an exception based on that definition provided that thickness does not exceed the limits imposed on the applicable (P number) chart.

Section VIII defines the thickness as the depth of the weld; and again, as long as it doesn't exceed the applicable (P number) limit, PWHT is not required.

I looked at B31.3 pretty hard and could not find any similar "loop hole" allowing a different definition of thickness; or any commentary allowing repairs to be exempt based on anything.

Charles

By jon20013 (*****)

Date 07-14-2005 13:08

Thanks Chall. It may well be that I am confused again, lol!!! Old-timers jumps up every now and then and bites me in the (you know where!). Heck, I might have even been thinking repairs after hydro instead of PWHT... I guess that's one reason I don't rely on memory much anymore but try to go straight to the black and white! ;-)

By G.S.Crisi (****)

Date 07-16-2005 20:30

I wasn't going to answer this post because neither I have the B31.3 Code on hand, but since Jon, in the Education section of this Forum, has asked what do I think about it, I'm going to tell my opinion based on all of the preceedings answers and on my horse sense.

1st. Jon has asked the following question:
A weld made on a plate 20 mm thick has to be repaired. To do this, a hole 6 mm deep had to be excavated in the weld to remove the defect. Jon wants to know if the repair weld has to be PWHT.
The plate material is not specified, but it belongs to the P1 group of materials; not to be confused, for example, with A335 Grade P1.
The applicable Code is ASME B31.3 "Chemical plant and refinery piping".
Now I wish to make two comments:
a) if the applicable code is B31.3 we won't bother to other codes as B31.1 (Power piping), ASME I (Power boilers), ASME VIII (Unfired pressure vessels) and the like.
b) Jon says that the weld is located in a vessel. Are you sure, Jon, that the applicable code is B31.3?

2nd. Mr Hall says that he's gone through the 1999 Edition of B31.3 and regarding PWHT, it says that the repair is subjected to the same requirements as the base metal.

3rd. Mr. Hughes says that the 2001 Edition of the Code repeats the same as the 1999 Edition. Mr. Hughes also says that PWHT for the P1 group of materials is required when the material thickness is 3/4 inches (19,2 mm) and over.

4th. Now I'll ask the following question: what's the thickness of the repair weld? 6 mm of course, which is the deepness of the hole that has been made to contain it ! As Mr. Sims correctly says, the PWHT must be based on the depth of the repair, not the original thickness !
Conclusion: Jon's repair needs not to be PWHT.

5th. Mr. Hughes also says that when we worked for BP (British Petroleum?) all the welds containing hydrogen cyanide (chemical formula HCN) had to be PWHT regardless of their thickness.
This is because hydrogen cyanide is a lethal gas, and codes are very stringent regarding lethal gases. ASME VIII, for example, requires that all vessels welds be 100% radiographed. Isn't that so, Mr. Hughes?
The difference between lethal and poisonous gases is that lethal gases (examples: hydrogen cyanide, phosgene) provoke death even if they are as traces (very very little quantities) in the air.
On the other hand, poisonous gases (examples: chlorine, ammonia, hydrogen sulphide), must be in larger quantities to provoke death. That's why you can smell a rotten egg, which emits hydrogen sulphide (and hence the odor) and don't die.
By the way, hydrogen cyanide (also called hydrocyanic acid) was the gas Germans used to kill jews in the gas chambers during World War II.

Final conclusion: Jon, you don't need to stress relieve your repair weld, unless it contains a lethal gas. Do you Gentlemen agree?
Giovanni S. Crisi
Sao Paulo - Brazil

By MBSims

Date 07-19-2005 02:43

I think your 2nd item is where the difficulty lies. However, there is some latitude in para. 331.2.2 (exceptions to basic heat treatment requirements) to justify the deviation, provided the proper technical evaluation is performed (e.g. if lethal service conditions are present they are properly considered). Here's what 331.2.2 says:

331.2.2 Exceptions to Basic Requirements. As indicated
in para. 331, the basic practices therein may
require modification to suit service conditions in some
cases. In such cases, the designer may specify more
stringent requirements in the engineering design, including
heat treatment and hardness limitations for lesser
thickness, or may specify less stringent heat treatment
and hardness requirements, including none.
(a) When provisions less stringent than those in para.
331 are specified, the designer must demonstrate to
the owner’s satisfaction the adequacy of those provisions
by comparable service experience, considering service
temperature and its effects, frequency and intensity of
thermal cycling, flexibility stress levels, probability of
brittle failure, and other pertinent factors. In addition,
appropriate tests shall be conducted, including WPS
qualification tests.