Fun with UG-36(c)(3)(a) Exemptions

[TomBarsh]

Here is an interesting trick I have seen in the past and that has come up twice this week.

"Small" openings may be exempt from reinforcing area requirements based on rules of UG-36(c)(3)(a). But UG-36(c)(3)(d) limits the number of exempted nozzles that may exist in a cluster of nozzles.

COMPRESS automatically exempts nozzles from reinforcing area based on these rules. Some users ask why one nozzle isn't exempt when it would seem to be exempt based on its size. The COMPRESS nozzle report contains descriptive text that the subject nozzle is "too close" to the other nozzles per UG-36(c)(3)(d).

It gets interesting when the "unexempt" nozzle does not have sufficient reinforcing area. Rather than add a reinforcing pad or increase the neck thickness, etc, it may be possible to apply the UG-36(c)(3)(a) exemption to the "problem" nozzle and force another nozzle to meet the area replacement requirements...hopefully that nozzle will meet the area requirements as it is currently constructed, so that no additional work is necessary.

COMPRESS provides an option on the nozzle dialog to specify that the the UG-36(c)(3)(a) exemption should not be applied to the current nozzle: click the Areas button on the nozzle dialog [Edit: this is now the Calculation Options button] and click the option No UG-36(c)(3)(a) Exemption. When calculations are run again this nozzle will not receive the exemption, which allows another of the adjacent small nozzles in the cluster to receive exemption. If we're lucky, that nozzle will be exempt without any further ado. I have seen this a few times!

Tom Barsh
Codeware Technical Support

 

[TomBarsh]

Special Reminder!! There is a break point at 3/8" shell thickness between the size of "exempt" openings being either 2 3/8" or 3 1/2". ie: 2 3/8" in 3/8" or less thickness, or 3 1/2" in greater thicknesses.

In years past, the Code was not clear on whether the 3/8" referred to nominal thickness or to required thickness. COMPRESS took the conservative approach and used the nominal thickness.

But with a recent Edition, the Code now explicitly refers to the exemption being based on "required minimum thickness". Thus the break point at 3/8" is based on required thickness, not nominal thickness.

COMPRESS now exempts nozzles more readily as a result.

(If my memory serves me well, this change took place in the 2004 Edition)

Tom Barsh
Codeware Technical Support

 

[unclesyd]

Here is a link to a pdf by Randall W. Whiteside. P.E. that has a pictorial explanation of this rule.

http://www.pdhonline.org/courses/m205/m205content.pdf

 

[unclesyd]

TomBarsh,
I've seen several of their courses and they look pretty good. I've never taken any but the people I know who have were quite satisfied.