PVP 1279 confusion

[rww88]

PVP2002-1279 "Stiffness Coefficients for Nozzles in API 650 Tanks" (published in 2002) makes the following statement regarding the FEA model for which it is based: "Nozzles were chosen to have reinforcing pads with equivalent dimensions of Table 3-6 column 5 of API 650." Older editions of API 650 refer to Table 3-6 as "Dimensions for 30-inch Shell Manhole (inches)." I have the 9th edition of API 650 Addendum 4 (1997) which states that Table 3-6 was deleted and not replaced by Addendum 1. Addendum 1 has a publication date of December 1994. Here are my questions:

1. What Table 3-6 is the PVP2002-1279 paper referring to if in fact Table 3-6 was deleted by Addendum 1 of the Ninth edition in December 1994?
2. The sample problem published in PVP2002-1279 has a nozzle diameter of 30 inches. It lists a corresponding reinforcing element outside diameter of 49.5 inches and states that it is in accordance with API 650. The corresponding nozzle diameter for a reinforcing element of 49.5 inches is 24 inches according to current editions of the standard (Table 5-6(b) shell nozzles?). What is the reasoning behind the selection of 49.5 inches in the sample problem?

I am thoroughly confused.

 

[JStephen]

One of the past revisions of API-650 added a couple of sections early in the standard, so all those old 3-something paragraphs and diagrams got renumbered to 5-something.
Another change that may have affected that is that they deleted the repad sizes for manways and instead just said to use repad sizes for the corresponding nozzle sizes.
Regardless, look through Section 3 (older versions) or Section 5 (newer version) and find the chart showing standard repad sizes for nozzles, and that is presumably what they are referencing.

On your second question, I don't know. Generally, you're allowed to check reinforcing area and weld strength and to use a smaller-than-standard repad if it meets all the reinforcing requirements. That may be what they did. Or that could just be a typo, and they meant to use a 24" manway or a 60.75" repad or whatever.

Long ago, when I was checking through Appendix P and the technical papers it references, I found that those papers made an assumption that shell thickness was proportional to pressure. That is generally true for larger tanks (with allowance for some rounding, etc) but not true for smaller tanks where minimum thicknesses apply. When I inquired about this (and perhaps others did, too), the result was a statement in later versions of Annex P that limited its use to tanks of 100' or larger, presumably to avoid this issue. So make sure the tank in question fits the design assumptions used in that document.

 

[rww88]

Thank you JStephen for the quick response. I guess we will never know what size repads the authors of PVP2002-1279 used in their FEA models.

Upon further reading of the paper, my confusion only grows. The sample problem in PVP2002-1279 states that the example nozzle qualifies as a "regular" type nozzle because the nozzle centerline elevation is 28 inches. According to API 650, the minimum height for an NPS 30 inch regular-type nozzle is 34.625 inches; the minimum height for a low-type nozzle is 30.375 inches. (Even an NPS 24 inch has a minimum of 29 inches.) At a nominal size of NPS 30 inch, the example nozzle is very nearly a flush-type nozzle.

Apparently, some errors were introduced between the time of composition and that of publication of the paper.

 

[rww88]

The plot thickens: Numerous discrepancies exist between the correlation equations appearing in the PVP 1279 graphs and those tabulated in the paper for the same parameter.