Pressure Vessel Alteration - Small Nozzle Addition 3

[TiCl4]

Background
I need to add some NPS 2" nozzle to the top heads of several Section VIII Div 1 pressure vessels. The vessels are older (1960s/70s) and have undergone a fitness-for-service (FFS) evaluation that de-rated them (from 50/75 psig down to 25-35 psig) due to a few thinner nozzles that did not have reinforcement pads. The shells and heads have shown no corrosion over the years from original design based on most recent thickness tests a few years ago.

During the FFS evaluation, I noticed that all nozzles 2" and under were excluded from consideration in the COMPRESS calculations. I need to add some new nozzles, and want to check with the good members of this forum to ensure I am going about this the right way.

As far as I can find it, ASME VII Div 1, UG-36 (c)(3) states that reinforcement is not need for welded connections provided:

(-a)(-1) 3.5 in diameter - in vessel shells or heads with a required minimum thickness of 3/8 in. or less

(-a)(-2) 2-3/8 in diamter - in vessel shells or heads over a required minimum thickness of 3/8 in.

The vessels are a mix of 7/16" and 3/16". The FFS report has two "required" thickness stated:

[ol 1]
[li]Minimum thickness per UG-16: In all cases the required thickness is 0.0625", well less than 3/8".[/li]
[li]Design thickness due to internal pressure (t): Ranges from 0.15-0.2", but still less than 3/8".[/li]
[/ol]


My main question(s)

Am I right in my thinking that adding these NPS 2" nozzles are covered by UG-36(c)(3) and will not need any calculations to ensure MAWP remains unchanged? I will be sure to follow the spacing provision in UG-36 that limit the proximity of multiple non-reinforced openings.

If I am correct in the above assumption, is there anything I need to do other than normal NDT post-weld and inspector sign-off to add these nozzles? My location does not require ASME VIII as law, and there is no state pressure vessel inspector that must be consulted. Despite that, I have started requiring the site to have repairs "R"-stamped, and all modifications must follow code.

Am I missing anything?

 

[Christine74]

Are these radial nozzles or hillside nozzles?

If they're radial nozzles then I agree, you don't need to run reinforcement calculations. But if these are hillside nozzles the cord length could easily be more than double the width of the opening so you would need to run reinforcement calculations.

Another thing to consider is that you would need to comply with the minimum thickness requirements of UG-45. You also might have to design the nozzles to support external piping loads.


-Christine

 

[TiCl4]

Christine74,

Hillside. However, the head angle here is well less than 45 degrees from horizontal. My rough calculation is that the length of the opening for a hillside nozzle in a 45 degree flat surface is SQRT( 2.375^2 + 2.375^2) = 3.36", still less than the 3.5" opening for part (-a)(-1). I know the head (ASME F&D) is rounded, but over a small distance like that is essentially flat.

These nozzles will have level instruments mounted on them, so maybe 5-10 lbs total weight. I'll check UG-45 to ensure the correct nozzle schedule is used.

Do you know what UG-35 means when it references "required minimum thickness"? Is this the required thickness due to internal pressure as given in the FFS COMPRESS results or something else?

 

[Christine74]

I think you're fine. I don't have Section VIII in front of me right now but as I recall the UG-36 "d" dimension is based on the nozzle ID (not the opening ID) if the nozzle is attached with a full-penetration weld, so you should have plenty of margin there. You will have to meet UG-45 though.

If the other details of the 2" nozzle (materials, weld sizes, pipe schedule) match another nozzle on the vessel you might be able to document the modification on an R-1 as a Repair form rather than an R-2 Alteration form.


-Christine

 

[TiCl4]

Sadly, we don't have similar nozzles in other reactors. I've never dealt with an R-2 form before, so I perused it for a minute. In this case, where no design updates are needed due to the exemption, who would be the "Design Peformed By:", and who would sign off on the design certificate portion?

 

[Christine74]

That part of the Alteration report would normally be signed by the responsible engineer (i.e. you).

 

[TiCl4]

I don't think I can sign that section, unless I'm misunderstanding the form.

The assumption of the form seems to be that the design is performed by someone in a company that holds an R cert (I'm getting this from the statement "name of "R" organization responsible for design" and the fact that the design certification asks for the R-cert number). We have an embedded contractor with an R-stamp, with his parent company holding the R-cert. Until now, we have only required repairs. My company does not have an R certification of authorization. Does this mean I need to go to a design firm organization that has an R-cert to be able to sign off on the R-2?

I think that's the main thrust of this thread: I am not experienced whatsoever in the design/modification of pressure vessels, and wanted to check to see if I needed to consult an "authorized" design firm; specifically, I wanted to check to see if the exemption of small diameter nozzles would exempt this from being subject to a design review, which would require an alteration rather than a repair.

 

[Christine74]

Sorry from your question I assumed that you worked for an "R" stamp holder. A representative from the "R" stamp contractor will have to sign off on the R-2 as they are ultimately responsible for the Alteration, so you would have to get the signer to concur with your assessment that reinforcement calculations are not required for these nozzles. Additionally you would need approval from the contractor's Authorized Inspector.


-Christine

 

[david339933]

UW-35 basically refers to undercut.

 

[TiCl4]

Christine,

Thanks so much for you input so far. Based on API 510, would this be considered an alteration, though?

3.1.1
alteration
A physical change in any component that has design implications that affect the pressure-containing capability of a pressure vessel beyond the scope described in existing data reports. The following should not be considered alterations: any comparable or duplicate replacement, the addition of any reinforced nozzle less than or equal to the size of existing reinforced nozzles, and the addition of nozzles not requiring reinforcement.

I thought that if the nozzle did not need to be reinforced, this could be considered a repair and not need a R-2.

Edit: Scraped from another Eng-Tips post:

From NBIC, part 3
3.3.3 (j) - "The addition of a nozzle where reinforcement is a consideration may be considered to be a repair, provided the nozzle is identical to one in the original design, located in a similar part of the vessel, and not closer than three times its diameter from another nozzle."

The vessels already have multiple 2" un-reinforced nozzles on the top heads. If I'm following this correctly, another nozzle of the same design should only require a repair, correct?

 

[Christine74]

Interesting. I wasn't aware of that provision but I agree it sounds like this could be documented on an R-1 as a Repair, but I would check with the contractor's A.I. just to confirm.


-Christine

 

[r6155]

If this old pressure vessel was not stamped during construction (1960s /70s), then no R stamp is applied.
Use your good engineering judgment and get on with the job.

Regards

 

[TiCl4]

R6155,

Thanks - all vessels have U-stamps.

 

[Louisiana Inspector]

You can add the 2" nozzle as long as it meets the design requirements of the applicable construction code. This will fall under a 510 repair and will not require an R-Stamp UNLESS your company or jurisdiction requires it to be. Even if an R-Stamp is not required the company doing the work must hold a current NBIC R-Stamp and a API 510 Inspector must be involved.

 

[SnTMan]

Do you know what UG-35 means when it references "required minimum thickness"? Is this the required thickness due to internal pressure...

TiCl4, assume you meant UG-36. "required minimum thickness" means thickness required for all applicable loadings ala UG-22. In practice it often just means thickness due to pressure loadings, internal and/or external.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand