PWHT, Sec VIII Div 1 vs ASME B31.3 5

[jtseng123]

Hello all,
I asked several people the following question, no one knows the answer. Does anyone know it ?

" Why in B31.3 thickness greater than 3/4", PWHT is required for P1 material, while in Div 1, thickness greater than 1.5", PWHT is required ? That is a lot of difference, and how/why the committee came up with that magic thickness ? What is the quantitative measurement to come up these different thickness ? "

PWHT is to remove welding residual stress, but how high is high to decide PWHT is required ? I do not think surface hardness test is a measurement to decide you need PWHT because obviously those two codes are in conflict, and there are many ways you can get the hardness test within the criteria. I haven't heard how the residual stress can be measured. Maybe I am not aware of it,

 

[stanweld]

Different user groups on the B&PV Code vs the B31.3 piping Code account for some of the rational, also the different design Safety factors and expected added piping loads (other than due to pressure) play a role. Residual welding stress becomes more germane to safe operation when the design safety factor is 3:1 for B31.3 vs 4:1 (now 3.5:1) for ASME VIII and additive piping loads may readily exceed 5 KSI.

 

[metengr]

,,,and now B31.1 has reviseed the PWHT thickness exemption for P-No 1 base material up to 1.5" with preheat and using carbon equivalent. Trying to harmonize codes and standards is like herding cats.

 

[stanweld]

In addition to the above, in the early days of B31.3, the use of E6010 electrodes was common place and carbon equivalents of carbon steel pipe were typically greater than those of today; thus, hydrogen embrittlement was much more a concern. As metengr stated, it is also incredibly difficult to revise codes, especially in regard to loosening of requirements.

 

[jtseng123]

Thanks for your reply.
However, that does not solve my question why both codes made the difference.
I have many welded duct to design and purchase, ranging from 24" up 100" in diameter for petrochemical plant. All the length are more than 100' long, routing around the plant from one vessel to the other. Normally it is designed per B31.3. The thickness easily gets above 3/4", that PWHT is mandatory, which adds more cost and schdule. But if I call it Div 1, which hardly done by people, I can save cost and schedule tremendiously.

So in this real situation, why B31.3 ask for PWHT ? If residual stress is a concern in addition to bendingstress from dead weight and thermal expansion, what is the quantitive measurement of the residual stress so I can measure or test it, and if within certian range, I do not need PWHT.

CE can be checked, but code does not say under what CE you can waive PWHT. Still, why B31.3 is so stringent than Div 1 ? and how do they come up with the thickness on both codes ?

 

[metengr]

However, that does not solve my question why both codes made the difference.

stanweld has indeed answered your query. I will provide a similar answer. The Code and Standards committees are their own consensus bodies and are made up of individuals that bring experience and knowledge. Rules are developed based on knowledge and practical experience and are not meant to be identical to other existing codes and standards. They can be identical in some cases.

So, each committee develops its own rules. If you don't like it start attending meetings and voice your concerns. That is what I do.

 

[stanweld]

Depending on operating pressure/temperature parameters, ductwork need not be subject to either Code. We have installed large quantities of ductwork where AWS D1.1 was invoked by the Owner/Engieer and no PWHT required.

 

[jtseng123]

Stanweld,
Do you measn your duct neither B31.3 nor Div. 1 ?

I am dealing with petrochemical plant in Houston. I can say that 99% of the duct will be ASME B31, regardless carbon steel or alloy. And we follow code strictly without exception.

The question I posted is wanting to get the inside of both codes and why since they have so much difference. Until now, seems no solid answer.

I will post another new thread regarding the thickness.

Jeff

 

[TGS4]

What is the pressure in your "duct"? What stanweld is saying is that, depending on the pressure, you may fall outside of either Code.

I don't understand your confusion about the difference between the Codes. Stanweld and metengr have very explicitly explained to you why there are differences. It has absolutely NOTHING to do with anything fundamentally difference between piping and pressure vessels. It has EVERYTHING to do with there being different PEOPLE who volunteered to write the two different Codes. There is no edict from ASME that any two different Codes have to be compatible, cross-wise coherent, or even agree on anything. There are clear bright lines hat delineate the realm of one Code vs another, and the individual Code committees are free to do as they see fit (consensus and all that stuff INSIDE the committees ONLY...). What part of this do you not understand?

 

[jtseng123]

Sorry TGS4, you sound you are not technically qualified to asnwer the question.

My duct is 650 F with 60 psig, where inside the 1450 degF design temperature carrying flue gas and catalyst, with refracotry lined to keep the steel wall tempertaure around 300-400 F.

Again, I did not see anyone really has technical inside about my question. The answer I got so far is, putting all together, it is the preference and the feeling (or experience if you like to say so) of these two committees to set what they want, no real science behind.

 

[metengr]

jtseng123;
Without trying to sound like I am putting you down - TGS4 and myself among others are various ASME code volunteers. TGS4 is more than technically qualified to provide a reply and has to your question. I believe you do not comprehend what Codes and Standards are all about. I would suggest you sit down and read the Preface of the various Code books, and work with someone that can mentor you with Code books. An internet forum is not the place.

 

[jtseng123]

Thanks.
Before I posted the thread, I have asked many senior pressure vessel engineers in different companies in Houston, 20-40 years experience. No one has the clue and eager to know the answer, even the lead people in Coade who makes the PV Elite pressure vessel program would like to know.
Here is one I got recently:

"My guess is that somebody decided that PWHT was a good idea at some point in time. The vessel guys looked at what they were building and it approached 1-1/2 inches and the piping guys looked at what they were building and it approached 3/4 inches, so they picked a thickness just beyond those points.
There is no solid technical justification for selecting either thickness. At best, one could argue that pipe was mostly stick welded and had lower heat input than that seen in vessels which were largely sub arc welded. Higher heat input corresponds to slower cooling rates, and that corresponds to softer microstructures and less sensitivity to hydrogen.
Any speculation on why we have different thickness is an EWAG (Educated Wild-Ass Guess A guess where you don't know the answer, but you do know something about the subject in general, so your answer ...) "

What he said about welding on pipe is not really correct, as I have many large diameter duct the fabrication has no differnce to vessel.

 

[stanweld]

jtseng123,
Having been on Code committees and being mentored by individuals who were involved in Codes in the late 1940s and the 1950s, I can assure you that there were valid technical reasons for the requirements set therein based on the state of the art of steel making, welding and inspection methods of the time. I also fully understand that the aforementioned methods have substantially changed (for the most part improved) over the past 45 years that I have been involved in the industry.

As stated previously, it is extremely difficult to amend codes to loosen historically justified requirements, which now may be outdated. It is much easier to incorporate additional requirements when justified through experience with certain alloys; e.g., witness the rapid changes to the Codes regarding the enhanced ferritic, P91 type alloy steels (it is noted that B31.3 is woefully behind B31.1 and ASME I in this regard - its principal use being in the power industry.

 

[TGS4]

It's really unfortunate that a new member (jtsengr123) to this community of over 1 million members would besmirch the reputation of highly-respected members. stanweld and metengr have been here for almost 9 years and myself for almost 7. Take a quick look at the MVP list for this forum - you have received answers from 3 of the top 20 (not to mention the #1) MVP, as voted by our colleagues. I would dare say that your inability to comprehend what is being said to you places you highly into the category of "not technically qualified to understand the answer". Don't worry - it happens all the time. Keep it up, though, and you will just be ignored.

Here's a bit of free advice - don't bite the hand that feeds you. Oh, and it's "technical insight" not "technical inside".

 

[jtseng123]

Excellent comment on me. I have designed and/or purchased more than 400 pressure vessels for world wide projects in the past 20 years, and still do not know much about the code since it grows to more than 700 pages, very complicate reading subject to all kinds of interpretations, code cases, addendum and changes all the time. I always ask friends/ colleagues for things I do not understand.
Majority of pressure vessel engineers I have seen, just try to interpret the code and implement it. When asking why code is doing that, they stunned. But do I know why? sure I don't.
I am beyond the stage of interpreting and implementing the code. Someone can do that. Just want to get any Technical Basis I face that someone may know in this public forum, that may also benefit to others who want to know more. I do not attend any ASME code meeting, that is for people who have deep knowledge beyond me. I am only a little guy making ends meet.