Sect VIII, Div. 1 Reinforcement Question 3

[henderrj]

In regards to a simple pipe nozzle in a cylindrical shell:

Reinforcement calculations are done based on a plane that cuts through the center of the nozzle, in line with the longitudinal axis of the cylinder. What about the plane which also runs through the centerline of the nozzle, but is perpendicular to the longitudinal axis of the cylinder?

My question is this: Since the circumferential stress is roughly twice the longitudinal stress in this scenario, shouldn't you be able to have a reinforcing pad that was half the required width at those points? For example, let's say we have a vertical vessel with a 6" nozzle on the shell. The calculations were run, and they called for a 12" diameter pad, total. Speaking about bare minimums, couldn't you use a reinforcing pad that was oblong, with a height of 12" and a width of 9"?

UG-37(b) says this: "Reinforcement shall be provided in amount and distribution such that the area requirements for reinforcement are satisfied for all planes through the center of the opening and normal to the vessel surface. For a circular opening in a cylindrical shell, the plane containing the axis of the shell is the plane of greatest loading due to pressure. Not less than half the required reinforcement shall be on each side of the centerline of single openings.”

The underlined sentence goes along with my point, but it never specifically says one way or the other. For the first sentence, it could be said that the requirements for reinforcement are satisfied, because less reinforcement is required on the perpendicular plane.

I've looked all over and can't find anything covering this, including interpretations. Before anybody gets angry, this is just out of curiosity. I'm not advocating for people to start using oval repads. Thanks!

 

[TGS4]

What you are saying is covered exactly by Fig UG-37 for calculating F, which is used in all of the calculations.

 

[jte]

This paper gets into the topic in more detail:

http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1600220&resultClick=1

One thing to keep in mind is that F need not be calculated for repads [it is 1.0 by definition], so you won't see an oval repad on that basis. For integral reinforcement, however, it can be a handy tool to have in your back pocket.

 

[henderrj]

TGS4 and jte,

I looked it up, and the F factor is always 1.0 if repads are used, as jte said. So, a repad could never be an oval shape if the nozzle is circular, unless the manufacturer was using more material than required.

I just want to make sure that I'm understanding this correctly, though, for integrally reinforced nozzles. To keep it simple, let's say we have a pipe nozzle in a cylindrical shell, with no repad. If the calculations call for a 3/8" fillet weld, then, potentially (based on what the F factor is), the weld size could be 3/16" on the plane perpendicular to the longitudinal axis?

Thanks for your help, I appreciate it!

 

[SnTMan]

Use of F less than 1.0 comes into play on "hillside" nozzles. Because the opening is larger in the circ direction than in the long direction, use of F = 0.5 can mean opening in adequately reinforced in both directions. Must be integrally reinforced, otherwise F = 1.0

Regards,

Mike

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

 

[jte]

Following this thought, it would seem that a non-axisymmetric repad can be used even with F=1.0, since for a hillside or tangential nozzle the area to be replaced varies with section cut. Thus, we could design a repad which in any particular plane meets the required area for that section cut even though the area provided varies depending on the plane of interest.

Not sure how often such a design would be of value, but I could see it coming up now and again.