ASME Section VIII, Division 1 :2021 Changes : UG-33: 8

[CuriousMecEng]

UG-33: Subparagraph (a)(1) revised in its entirety

2019:
(1) For ellipsoidal and torispherical heads, the required thickness shall be the greater of the following:
(-a) the thickness computed by the procedure given in UG-32 for heads with pressure on the concave side(plus heads) using a design pressure 1.67 times the design pressure on the convex side, assuming a joint efficiency E = 1.00 for all cases; or

2021:
(1)(-a)This was removed.

1. Can anyone guide me on why this was removed?
2. And is there any resource where the basis of such changes can be understood?

Thanks in advance.

 

[SnTMan]

I don't have any inside info, but I'd guess it is due the wide access these days to the software used to solve the (iterative) external pressure calculations.

Regards

Mike

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

 

[CuriousMecEng]

@SnTMan: Thank you for the response.

 

[SnTMan]

Perhaps TGS4 can provide a more accurate explanation

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

 

[r6155]

(1)(-a) was removed because it doesn't make sense. UG-33 is very clear.

Regards

 

[SnTMan]

Seems to me: It must have made some kind of sense at some time. Else it wouldn't have been there. The reasoning is likely lost in time...

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

 

[TGS4]

So, there's a story that goes along with this...

Prior to the external pressure curves being introduced (back to the 1927 Edition - this paragraph didn't exist prior to 1927), there was a paragraph in U-36 that said:

Unstayed heads with the pressure on the convex side have a maximum allowable working pressure equal to 60 per cent of that for heads of the same dimensions with the pressure on the concave side.

In the 1930 Edition, that paragraph was assigned to U-36(j). In the 1940 Edition that was U-36(i), and then in the 1943 Edition that was U-36(h).

In the 1950 Edition, this paragraph was moved to UG-33, and has essentially the same form as the earlier editions.

When the thickness of the head exceeds 0.01 times the inside diameter of the head skirt (flange), the allowable working pressure shall not exceed 60 per cent of that allowed for pressure on the concave side;

The 1965 Edition re-formed the approach to calculate the thickness as opposed to calculating the MAWP. As such 1/0.6 = 1.67, and so the form in the 1965 Edition became

The thickness as computed by the procedure in Par. UG-32 for heads with the pressure on the concave sides (plus heads) using a design pressure 1.67 times the external design pressure, assuming the joint efficiency E=1.00 for all cases;

When the external pressure curves were introduced in the 1950 Edition, and then when the UG-33(d) and (e) calculations were added in the 1965 Edition, that approach from the external pressure curves should have superseded the 0.6 or 1.67 factors. However, it took the committee ~70 years to catch this issue. ¯\_(ツ)_/¯ I guess better late than never. This was performed under item 19-2278. Apparently the editors didn't even think that this was a substantial enough change to warrant inclusion in the "Summary of Significant Changes" document, but it is noted at the beginning of ASME Section VIII, Division 1 (2021 Edition) Summary of Changes for page 31.

 

[SnTMan]

TGS4, thanks for that

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

 

[CuriousMecEng]

@TGS4: Thank you so much for sharing the history behind the clause changes.