THe F factor in area of reinforcement calcs 8

[jgibbs22]

Hey everyone:

Little confused on the F factor in the area of reinfrocement equations. The only examples and discussion I have read about only look at non-circular openings. Does anyone have a good paper on this topic? Can circular openings have a F factor less then 1 (assumung they are integrally reinforced and on the shell)? Thanks everyone!

 

[SnTMan]

jgibbs22, the F factor accounts for the difference between hoop stress and longitudinal stress in a cylinder, notice the F factor is 1.0 in the longitudual direction (or the plane on which the hoop stress acts), and 0.5 in the circumferential direction.

An F smaller than one for a reinforcement calculation in the transverse direction allows more of the shell to be considered as available for reinforcement, since the longitudinal stress is half the hoop stress.

Of course the use of F less than one is subject to restrictions that must be followed.

Regards,

Mike

 

[jgibbs22]

Mike,

Thank you for the reply! From some of the history of the code I was reading, I do understand the reasoning for F, but I am having a hard time understanding whether the F factor depends on the nozzle location on a cylindrical vessel, or if it is the nozzle position/orientation (assuming a non-circular nozzle) that allows the use of the F-factor. If anyone has a detalied reference out there, or if I am making this to hard on myself, let me know. Thanks everyone!

Jason

 

[jte]

Jason-

Take a look at the paper available at

http://store.asme.org/product.asp?c...tegory_name=&product_id=PVP2006-ICPVT11-93596

Not sure I understand your question correctly, but I think the answer is that you need to look down through the nozzle into the shell, and determine the F factor from that perspective. The actual location of the nozzle on the shell should not matter. I think you may be making it too hard on yourself...

jt

 

[jgibbs22]

JT,

Thanks, that does actually help! So if a nozzle is always installed so that its center line is radial to the vessel, the F value will be 1. But, if there is an angle away from the centerline that makes the nozzle not be isntalled radially, you can have an F factor less then one?

jason

 

[SnTMan]

jgibbs22, yes you can. The most usual way is to have a "hillside" or offset nozzle. The finished opening is equal to the diameter in the longitudinal direction, but is greater in the circumferential direction. Use of an F factor of 0.5 helps to compensate for the larger opening dimension, given that the smaller F can be used in the first place. See the definition of F in UG-37, see also Appendix L, L-7.7 for this.

Regards,

Mike

 

[TomBarsh]

Jason,

Mike and jt have provided accurate descriptions of the F factor. But it still seems there's some misunderstanding of what this factor does.

It took me a long time to finally understand Figure UG-37 with its circle and angle theta. The circle represents a radial nozzle on the shell. As jt said, this figure looks "down through the nozzle into the shell". Thus the angle theta is actually a measure of the plane cross-section cut through the nozzle at various angles from the longitudinal vessel axis. Now I always make a little note each time I get a new Code Edition/Addenda indicating that the circle is the nozzle neck.

A plane cross-section cut through the nozzle and oriented parallel to the vessel axis will interrupt the circumferential stresses in a cylindrical shell. The circumferential stress (PR/t) is twice the longitudinal stress (PR / 2t). Thus this cross-section is (in most cases) the critical cross-section; Figure UG-37 imposes the F factor to be 1.0 at this cross-section.

But as the plane cross-section is cut at other angles around the nozzle's circumference the effect of circumferential and longitudinal stresses trade places until at theta = 90° the effective stress is fully longitudinal. ASME Code gives a nod to this issue by allowing the F factor to be used. At theta = 90° F = 0.5 as for the ratio of longitudinal stress to circumferential stress. As the Code describes it, F is a correction factor that compensates for the variation in internal pressure on different planes cut with respect to the axis of the vessel.

The F factor can be used only for nozzles with "integral reinforcement" as defined in UW-16(c)(1). This could be as simple as a straight neck with full penetration groove weld to the shell, but you cannot use a reinforcing plate and have integral reinforcing.

A radial nozzle with integral reinforcing would require approximately 1/2 as much reinforcing area at theta = 90° as at the plane at theta = 0°. But practically speaking there would be no effect on the design or analysis. The fact that the cross-section at theta = 90° requires only half as much area does not result in any net reduction of the reinforcing area that is provided (eg: you aren't going to reduce the shell thickness in that direction, etc). That is unless for some reason you resort to some weld build-up to reinforce the nozzle for this plane for an existing vessel that has seen deterioration due to service.

But for non-radial (offset or tilted) nozzles the critical cross-section (that one which governs the design of the nozzle reinforcing) may be at either theta = 0° or theta = 90°. Both cross-sections would typically be checked and either could govern.

 

[SnTMan]

Bravo, TomBarsh, well and thoroughly explained.