Adding a Nozzle to a PWHT Vessel Shell

[Sharik]

We are having a Contractor install a new nozzle (4" NPS) into a vessel shell. The vessel was stress relieved as part of the fabrication, for process reasons. The shell is P1, carbon steel, nominal thickess 0.4375".

We are under the impression in order to stress relieve the nozzle attachment, a circumferential band would have to be heated all around the vessel. This is according to ASME Section VIII, Div. 1, Paragraph UW-40(5).

The Contractor though is telling us instead of heating a band they can heat a 'donut' around the nozzle. According to them there are ASME Code cases where this has been allowed. Of course they can't show us or tell us which Code case(s) they are. It is also quite difficult to find a specific ASME Code case.

Does anyone have any knowledge of this alledged Code case?

I know API 510 allows a 'donut' instead of a circumferenital band but this repair/alteration is being completed to the Code of Construction, ASME Section VIII, Div. 1.

I don't have a copy of the National Board Code with me but this Contractor also insists the use of a 'donut' is allowed in the NBIC.


Thanks

 

[unclesyd]

You should get immediately with your AI and discuss the problem with him.

We have done the exact same thing as you describe.

My old NBIC book states that the postweld heat treatment will be by the ASME Code except as described below.

Alternate Methods: Under certain conditions, postweld heat treatment as outlined above maybe inadvisable or impractical. In such instances, any other method of postweld heat treatment or special welding method acceptable to the inspector maybe used.

As you state you doing PWHT for process reasons I can’t see where he would have any objections to the donut.

 

[Sharik]

unclesyd,

Even though the vessel was stress relieved for process reasons, there is an 'HT' on the nameplate. So all repairs/alterations have to meet the applicable ASME requirements (UW-40, UCS-56, etc.). These do not allow a donut unless the nozzle is on a head or a spherical vessel.

I'd rather not get the AI involved just yet. He is a little weak and usually just bends in the direction of the strongest wind.

 

[jte]

Sharik-

The new nozzle's design is governed by VIII-1. Its testing and acceptance is up the the AI. The AI will go by NBIC or by API-510. As unclesyd pointed out, alternate methods of PWHT may be used. Be careful, however, with a donut approach. I've performed enough FEA's of proposed donut PWHT's to know that they can result in very high stresses. It will all depend on your geometry. One compromise that works is to have a donut area at the full PWHT temperature with a band around the rest of the vessel at some lower temperature. See a paper published at last year's ASME Pressure Vessels and Piping conference, "Three Turnaround Heat Treating Studies" at

http://store.asme.org/product.asp?c...Papers&category_name=&product_id=PVP2003-1934

for an example case study of a replacement of a 16" nozzle in a CS vessel requiring PWHT for process reasons.

jt

 

[metengr]

Sharik;
If this vessel is in service and you are adding a nozzle, the repairs will be performed in accordance with the NBIC, unless your Jurisdiction has other specific requirements.

The NBIC was revised in 2003 to provide alternative methods of PWHT, specifically the "bulls eye" method. Local post weld heat treatment using the "bulls eye" method is suitable for carbon steel shells. As part of the task group that assembled the field information to justify this method for the NBIC main committe, we cited over 40 field repair applications where this method was used and has been in service for over 25 years.

I will caution you to follow the temperature gradient requirements in Part RD of the NBIC to assure proper stress relief.

 

[metengr]

As a follow-up, I agree with Unclesyd, you should get the AI on board ASAP for any weld repairs to an in-service vessel. All repairs or alterations must be approved by the AI in accordance with the NBIC.

One thing I have noticed over the years is that a well formulated repair or alteration plan for a pressure retaining item that is documented and contains rationale for the AI to follow will fly the first time every time.

 

[deanc]

Think you will find what you need in NBIC RD-1070. Your Contractor should have this,if not,they should not be doing the work.

 

[Sharik]

Thanks everyone for the help so far. A little more information may help you. In this Canadian province, the API 510 and the NBIC are not recognized Codes. They are used as reference material and the AI's are supposed to have their National Board Certification. Contractors are not required to be National Board registered and 'R' stamps are not required. All they need is a QC Manual/Program, meeting the requirements of ASME Section VIII, Div. 1, Appendix 10 and the requirements of the AI.

So repairs and alterations are to be completed to the Code of Fabrication.

We will have the AI involved with this alteration, we are just trying to get all our ducks in a row first and then submit the detailed procedure. We'd rather have all this figured out prior to submission.

So does anyone know of these alledged ASME Code Cases this Contractor is talking about?

Thanks

 

[deanc]

No Code Case that I can find- No interps.in your favor.

Suggest you contact your Jurisdiction-explain what you wish to do and back it with the NBIC. Tell the AI your plans and ask him to contact his supervisor if he needs backing. Write your procedure out and have everyone involved sign it-or email,print copies and attach to same.

Good luck-have a good reason for your argument.

 

[Sharik]

We aren't really too worried about this. Right now I'm telling the Contractor to provide us with these Code cases. If they can't, then we will do the circumferential band around the vessel as per ASME Section VIII, Div. 1. We've done the calculations and there are no problems with tower support.

Kind of an interesting situation though.

 

[bmoorthy]

Assuming only ASME sec VIII Div 1 is applicable, it is perfectly permitted to perform the PWHT as described (Donut/Patch heating and not full circumference band), provided the contractor has previous experience in carrying out such heat treatment.

Good Luck
B.Moorthy
moorthykar@yahoo.com

 

[bmoorthy]

Refer to UW 40 a (8), which goes some thing like this

" Heating of other configurations. Local area heating of other configurations not addressed in a(1) to a 97) above is permitted provided (based upon suffieciently similar documented experience or evaluation) are taken that ........"

We have performed such operation in the past after taking the approval of AI. But in our case the changes (addition of the nozzle was requested by our process guys during the manufacturing itself, but by that time the job was already heat treated, hence we resorted to this after careful eveluation by our Heat treatment expert and our material Engineer.)


Yours is a slightly different situation wherein the vessel is in operation, but as per the postings, i understand you have more flexbility, since NBIC is not applicable. Any way check out UW 40 a (8) and try to sell the idea to AI. This could be cost effective as compared to Circumferential band. But the flip side is, if you are not careful ther could be buckling/out of shape resulting from local patch heating. if the contractor has previous experience and the \heat treatment procedure is OK , then you may go for it.

Ask for a detailed Procedure.






Good Luck
B.Moorthy
moorthykar@yahoo.com

 

[VeryPicky]

Sharik
Pleas consider the following:
1. The design code does not cover the service.
2. A donut is always resulting in higher stresses, you might actually harm the vessel by inducing more stress in place of residual welding stress.
3. As a vessel owner (I assume) you may require any preheat configuration as you want provided it is acceptable to code - go for a complete wrap.
4. Welding Research Council Bulletin 452 is a great resource for local PWHT of pressure vessels.

 

[deanc]

bmoorthy-Sorry,but I do not agree with you.

UW-40(a)(8) says"Not addressed in (a)(1)through (a)(7)". Think in this case,it is addressed,therefore one must follow the requirements as listed.

There are other good reasons as some of the others have pointed out.

But-this is not new construction and I would defer back to the NBIC and good engineering judgement-documented of course.

 

[Sharik]

We agree with DeanC's interpretation of UW-40. The configuration we are dealing with conforms to UW-40(a)(5) and so we PWHT a circumferential band around the vessel. The Contractor is the one who doesn't want to do the band and is proposing the donut. But, unless they can show us the Code Cases or Interpretations from ASME which specifically address the donut, we will be developing the repair/alteration procedure for the AI based on UW-40(a)(5).

Thanks to everyone who responded.

 

[GenB]

It is my understanding that the Code will allow the 'donut'
PWHT around the nozzle, but in reality may not be an acceptable eng. practice. the final alt. procedure involves the vessel details as Diameter, wall thickness and nozzle design.
ER

 

[bvi]

When you say for process reasons, I assume it is for corrosion reasons, where high residual stresses may result in stress corrosion cracking. A bulls eye stress relief will leave high residual stresses in the vessel after PWHT. These will be around the perimeter of the bulls eye. If my assumption is correct, I suggest doing the full circ PWHT.

 

[unclesyd]

Full circumferential PWHT can do the same, high residual stress. Also if near a weld you have to be careful about where you terminate the HT. We had a similar situation were we had four amine columns that required stress relief for corrosion problems. One inservice PWHT was terminated too near the weld and we had problems with the weld later on. We went to the donut
Also PWHT of a standing column has caused troubles in the past. One of our sister companies has a 30" dia. column with and unwanted 6" bustle. This was caused from problems with the thermocouples.
Just make sure you have people well versed in the art doing the job.