Pressure Vessel Head Thickness 1

[Shmulik]

Hi all,
Head of a pressure vessel made of ASME B16.9 Cap,
NPS: 24" ; Nominal W.T: XS (12.7 mm) ; Material: SA-234 WPB
Design wall thickness calculated acc. to the requirements of ASME VIII-1 / UG-44 is: 10.75 mm
Actual wall thickness measured on cap: 12.4 mm at welding end ; 9.6 mm (min) at spherical portion.
The question is whether this cap complies with ASME VIII-1 requirements and may be permitted to use?

 

[TGS4]

See UG-44. Yes, provided that the pressure-temperature rating of the fitting is suitable for the design temperature and pressure of the vessel.

 

[Shmulik]

This is the point of the question: Pressure temperature rating for B16.9 fittings shall be calculated. So, where shall we measure fitting's thickness?
From the following interpretation of ASME B16.9 it seems that thickness shall be measured at welding end:

Interpretation 1-4:
Subject: B16.9 (1978 Edition);
Date Issued: February 27, 1990
File Number: B16-88-002
Question: Does ANSI B16.9-1978 place any minimum dimensional requirements on the inside of fittings at a position other than welding ends?
Reply: No.

On the other hand, from the following one (Question 1) we may understand that entire fitting has a required minimum wall thickness:

Interpretation: 14-1
Subject: B16.9-2012, Tolerances
Date Issued: September 24, 2014
File Number: 14-1150
Question 1: Does ASME B16.9 establish a minimum wall thickness?
Reply 1: A minimum wall thickness of 87.5% applies unless the purchaser specifies a different wall thickness tolerance. See Table 13.
Question 2: Does ASME B16.9 establish a maximum wall thickness?
Reply 2: No, except at the ends. End preparation must conform to the requirements of para. 8.
Question 3: In ASME B16.9 Table 12, what is meant by the expressions t<22(.88) and t>22(.88)?
Reply 3: The expressions describe the nominal wall thicknesses for which the plain and compound bevels are required.
Question 4: In ASME B16.9, are the tolerances in Table 13 and the tolerances in Table I-12 different?
Reply 4: Yes. The tolerances in Table 13 are to be used when fittings are manufactured using SI Units, and the tolerances in Table I-12 are to be used when fittings are manufactured using US Customary Units. See para. 1.4.
Question 4: In ASME B16.9, are the angular tolerances in Table 13 in millimeters?
Reply 5: Yes. See Note (a).

Another thought: why does ASME B16.9 (Table 10) require that cap's shape shall conform to the requirements of ASME BPVC?

GENERAL NOTES:
(b) The shape of these caps shall be ellipsoidal and shall conform to the requirements given in the
ASME Boiler and Pressure Vessel Code.

 

[Jano6924]

My understanding is that mínimum thickness applies to entire fitting. If nominal thickness is 12.7 mm then theminimum thickness would be 12.7*0.875 = 11.1125 mm, since you state that measured thickness in spherical portion is 9.6 mm then this Cap is out of compliance.

Hope this helps.

 

[metengr]

In my view, the head is being supplied as a pressure part under an ASME pressure part specification ASME B16.9. If you review Section VIII, Div 1 Part UG-11,PREFABRICATED OR PREFORMED PRESSURE PARTS FURNISHED WITHOUT A CERTIFICATION MARK, the head would be in compliance as long as the pressure-temperature rating for working pressure of the head meets or exceeds the design conditions for the vessel.

 

[Shmulik]

Thanks, metengr
Your approach, as well as the other posts here, is B16.9's required minimum wall thickness is for entire fitting surface.
How may you refer to the phrase: "minimum dimensional requirements" in Interpretation 1-4 (quoted above)?
Should it exclude wall thickness? Namely: does a minimum wall thickness required in any portion of a fitting, not only in weld ends?

 

[metengr]

If you review B16.9, the fittings are designed and either proof tested or by calculation to ensure they meet the required pressure temperature rating.

2.2 Design of Fittings
The design of fittings shall be established by mathematical
analyses (e.g., ACME B16.49 for bends) contained
in nationally recognized pressure vessel or piping codes,
or at the manufacturer?s option by proof testing in accordance
with para. 9 of this Standard. In order to meet
design or manufacturing requirements, it is expected
that some portion of formed fittings may have to be
thicker than the pipe wall with which the fitting is
intended to be used. The mathematical analyses, if used,
may take into account such thicker sections. Records of
mathematical analysis and/or successful proof test data
shall be available at the manufacturer?s facility for
inspection by the purchaser.

Why are you worried about wall thickness of a fitting outside of the weld end prep location, manufactured to an ASME standard and allowed for use by ASME Section VIII, Div 1? You cannot use the rules from Section VIII, Div 1 to establish a minimum wall thickness of a pressure part supplied under a different standard. The fitting is designed under different rules as I showed above.

 

[Shmulik]

Thank you metengr
Very well explained!

 

[MrPDes]

I hope you are not placing a nozzle in the pipe end cap. The reinforcement of the nozzle including crown minimum thickness would need to be in accordance with ASME VIII Div 1.

 

[r6155]

I agree with Jano6924 and MrPDes

Regards
r6155

 

[metengr]

Agreed, placing a nozzle in a pipe cap would require an evaluation for minimum thickness using ASME Section VIII for reinforcement. If a nozzle or penetration is not being installed there is no need for a minimum thickness evaluation as long as the P and T rating meets the design of the vessel.

 

[r6155]

Shmulik. Please, can you tell us which is the class marked on the cap?

Regards
r6155

 

[Shmulik]

Hi friends,
Thanks for your concern.
No, nothing is placed on this end cap (the vessel is horizontal).
I am very well aware about that any operation on a butt weld fitting outside it's welding end made it been considered as "Unlisted component" and it is no longer accepted under the provision of UG-44.

r6155: kindly, what do you mean?
There is no class designation under ASME B16.9 (Buttwelding Fittings).

 

[r6155]

Shmulik
I consider that pressure-temperature rating implies a class number, similar to ASME B16.5 flanges.

ASME B 16.9 2.1 Basis of Ratings
The allowable pressure ratings for fittings designed in accordance with this Standard may be calculated
as for straight seamless pipe of equivalent material (as shown by comparison of composition and mechanical
properties in the respective material specifications) in accordance with the rules established in the applicable
sections of ASME B31, Code for Pressure Piping. For the calculation, applicable data for the pipe size, wall
thickness, and material that are equivalent to that of the fitting shall be used. Pipe size, wall thickness (or
schedule number), and material identity on the fittings are in lieu of pressure rating markings.

ASME B16.5 say “pressure-temperature rating” and ASME B16.9 2.1.say “pressure rating”, Are different.

Which is the pressure rating of your welding cap NPS 24?.

Your calculation according to ASME VIII Div 1 is correct.
Remember "The allowable pressure ratings for fittings designed in accordance with this Standard MAY be calculated...."

Regards
r6155

 

[Shmulik]

r6155
Material grade designation marking includes the prefix “WP” as it required for standard fittings (B16.9 4.4.1)

Pressure rating designation is not applicable for fittings such as caps. There is no rating similarity to ASME B16.5 flanges since it's class designation is a dimensionless number.
Pressure rating designation has only referenced for fittings that are connected with ASME B16.5 flanges, namely: Table 9 Lap Joint Stub Ends
(ASME B16.9 1.10)