What stress values (safety factors)

[JB23]

What stress value (safety factor)is the required reinforcement area based on? My objective is to determine the safety factor when the actual area is less than the required area. Thanks for your help.

 

[jte]

JB23-

I don't think there is any particular safety factor involved in the area reinforcement requirement for openings in vessels. Actually, it makes more sense from a column buckling perspective (maintain the moment of inertia of the column).

Someone more familiar with BS 5500 might be able to confirm this, but I'm under the impression that that code does not require any nozzle reinforcement. Some local yielding occurs and the secondary stresses are relieved.

jt

 

[prex]

If you want a formal (or legal) safety factor, then you can simply take the ratio of the reinforcement areae: with two times more area than required you are two times safer. This could be taken as meaning (in a broad and general sense...) that the burst pressure of the vessel, in the very unlikely condition where this burst pressure is limited solely by that nozzle reinforcement, will be at least two times higher that it would have been with exactly the required area.
Of course in practice that safety factor will be higher than that, as the reinforcement area approach is a quite simplistic (hence safe) procedure.
However this discussion makes little sense to me, as its conclusion is valid only if that specific nozzle is the only part of the vessel controlling its strength, and this is a very theoretical situation.

prex

http://www.xcalcs.com

Online tools for structural design

 

[chicopee]

The reinforcement procedure takes into consideration the factor of safety use in the determination of the shell thickness of the vessel.

 

[dazma]

nozzle reinforcement is done to bring down the stress concentration at the joint due to removal of material. When the area provided is more or equal to that of area required, the stress concentration is assumed to be nil. But if the reinforcement is less the stress concentration would be higher and this will be detrimental only from fatigue point of view.
So basically it depends upon application. If the vessel is to see many cycles of operation it is prudent to do a fatigue analysis to check whether it can withstand the specified no. of cycles with the increased stress concentration.
Since ASME Section VIII Div 1 does not consider fatigue as a mode of failure, Div2 can be checked for the analsysis process.

 

[jte]

dazma-

Fatigue is a failure consideration for Div. 1 vessels.
Division 2 provides guidance for evaluating fatigue which is not available in Div. 1, but a Div. 1 vessel must still be evaluated. See UG-22: "The loadings to be considered in designing a vessel shall include those from:...(e) cyclic and dynamic reactions due to pressure or thermal variations, or from equipment mounted on a vessel, and mechanical loadings;..." Since Div. 1 requires fatigue analysis but doesn't provide guidance on how to accomplish this, often a Div. 2 fatigue analysis will be used for a Div. 1 vessel.

If the vessel is in fatigue, I would argue strongly against a re-pad. The fillet welds will kill you with their stress concentrations. A vessel in fatigue should use integrally reinforced nozzles or insert plates to meet the area replacement requirement of ASME VIII.

jt

jt

 

[bvi]

Sounds like you have a special situation with a constructed vessel that does not comply with the Code. As stated by Prex, the area replacement method is a simple design procedure. You can ratio areas to get a simple estimate of reduction in factor of safety, consistent with that simple design procedure.

If you want to know what the real factor of safety is, an elastic plastic limit load analysis using finite element analysis would be more appropriate.

As an alternative, there is a Section VIII Div 1 code case that provides alternative rules for nozzle reinforcement. You may be able to show that they are perfectly adequate per those alternative rules. I do not recall the code case number, look through the index.