UG 45 nozzle neck check 1

[bhart192]

when UG 45 checks nozzle neck T , one of the factors used is a comparison to a chart which list nominal pipe OD and thickness
this was put in 20010 ed. we ran into this on a project , using tube instead of pipe for a small 5/8" drain tube on a small vessel. everything else passed i.e. pressure , weld size etc except for the comparison to the nominal pipe chart is anyone familiar with the intent of including a comparison to the chart just curious when all else passes this item kicks it out.

 

[SnTMan]

bhart192, it is my belief that UG-45 is intended to assure some level of mechanical strength to the nozzle. There are exceptions for manways and some services.

Prior to introduction of the table, UG-45 thickness was goverrned by some verbiage such as "Std wt + corrosion". I have found in some instances work that complied previously will not now.

Regards,

Mike

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

 

[jte]

I'd submit that UG-45 was changed not in intent but in wording (I have a bit of familiarity with this particular activity by the committee). The chart is simply the Standard wall / Sch 40 thicknesses times 0.875 to account for the typical 12.5% mill tolerance. Add CA to this value, and you have exactly the same results as you had in the prior version of UG-45. To me the way the formula for t_b is written is far more clear than the wording used previously, and this is the crucial change.

Yes, the intent of these minimum thicknesses is to create a bit of robustness. I would argue that this determination should be left to the engineer, but it's not such a big deal for new construction. And keep in mind, the Section VIII folks are strictly interested in new construction. In a post-construction environment, in many cases trying to adhere to strict compliance with an arguably arbitrary requirement such as UG-45 minimum thicknesses is not aligned with what I consider to be good engineering. In most cases the loads from attached piping are easily determined and their impact on the nozzle can be checked. For example, if Sch. 10 piping is flanged up to the nozzle, why does it suddenly need to be Sch. 40 on the vessel side of the flange? Where's the continuity of logic there? In such circumstances, I usually recommend a quick FFS and weld buildup at the next scheduled shutdown as it is easier to perform the weld buildup than it is to justify continued operation with nozzle thicknesses less than in the table.

But keep in mind, for relatively low pressures with thin wall shells, t_b may be limited by t_b1 or t_b2, not t_b3 (Table UG-45). Thus, it is entirely possible - and in my experience common - for people to think that they are out of compliance since a nozzle wall is thinner than shown in Table UG-45, although it is actually perfectly compliant with t_UG-45.

 

[SnTMan]

jte, agree w/ your entire post. I find the table and set of formulae more convenient to use as well.

Regards,

Mike

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

 

[BrotherBax]

Good insight, jte. We sometimes design small vessel shells using standard pipe sizes. After determining the required minimum thicknesses per UG-27, we subtract the 12.5% manufacturing tolerance and corrosion allowance from the nominal thickness of the selected pipe and compare that to the UG-27 values to determine if the pipe is suitable as a vessel shell. It appears because of the inclusion of the corrosion allowance in the definitions of t_a, and the t_b1,2,3 values AND the accounting for 12.5% tolerance in Table UG-45, once I've found t_UG-45, I can compare that directly to the nominal pipe thickness of my selected nozzle material and I'm finished.

To put it another way, if the UG-45 method "cooks" in the corrosion allowance and manufacturing tolerance, I shouldn't take it back off of the selected pipe size when comparing suitability. Would you folks agree with this assessment?

Thanks!
BrotherBax

 

[jte]

BrotherBax-

I think I agree with your approach. Certainly, the mill tolerance is built into the Table, so don't double-account for it.

The issue that gives me pause is that you mentioned designing a shell using pipe first, then move on to the nozzle. Just to be clear, my understanding of UG-45 is that it applies strictly to nozzles. If you are designing a shell using a pipe, then UG-45 is irrelevant to that particular process. But, as you point out, the mill tolerance of the pipe must be considered in establishing the thickness of the shell.

 

[BrotherBax]

I probably was not very clear and I agree with what you've said. Using pipe as a shell, we consider UG-27 and not UG-45. My main point was that when using standard pipe for a shell under UG-27, applicable tolerances must be considered and compared to the minimum calculated thickness. The new UG-45 takes care of those tolerances for pipe within the calculation and the calculated minimum thickness can be compared directly to the nominal pipe thickness.

Thanks!