Torispherical Heads 1

[sme75]

I am working on the design of a vessel that has torispherical heads and have a couple questions. With a design load of 75 psi the calculated head thickness is around 5/16". This was calculated using the general vessel formulas from table 2-1 of the Pressure Vessel Design Manual by Dennis Moss. In procedure 2-4 of the same book there are equations for stress in the head at four locations. If I use the equations for the stresses in the knuckle I get approximately 9,800 psi and 132,000 psi in the two directions. Can anyone give me a good reference or a brief explanation on why the stress of 132,000 psi is acceptable? It is many times greater than the yield stress. Also, if a fatigue analysis is required on this vessel, I would assume that this area of the vessel would give me the maximum stress range. Is that correct? Any thoughts would be appreciated. Thanks.

 

[vesselsanonymous]

Please advise of the diameter, material and design temperature and I will reply.

Dennis R. Moss

 

[sme75]

Dennis,

vessel diameter = 90"
material = 516 Gr. 70
temp = 350 deg. F

Thanks in advance for any comments you have.

 

[TD2K]

SME75, I wonder if you are having your leg pulled with someone posting as "Dennis R. Moss".

 

[jte]

TD2K and SME75-

Well, I think you at least know me and my postings. I worked with Dennis for ten years prior to accepting an opportunity to move on from an E&C company to an operating company. Coincidentally, I had a phone conversation yesterday with another former co-worker who used me as a reviewer for a paper which he is publishing. Turns out that he was the one who showed Dennis this question and did some searches for him to see where else he was quoted.

I have little doubt that "vesselsanonymous" is the real Dennis R. Moss. Welcome aboard, Dennis. It will be fun sparring with you once again!

jt

 

[LSThill]

Hi vesselsanonymous (Mechanical) Mr. Dennis R. Moss and jte (Mechanical):

Referende to Pressure Vessel Design Manual by Dennis R. Moss

Procedure 5-4 (ATTACHENT PARAMETERS) on Page 174 should this be replace with the following:

WRC BULLETIN 448 Evaluation of Welded Attachments REPLACE WRC 198

Introduction

The purpose of this report is to summarize the background of Code Cases N-122-2 and N-318-5. Case N-122-2 is included in this report as am appendix. Case N-318-5 is Appendix E of report 2. Frequent reference will be made in the following text to these Cases. The definitions of symbols used in this report are those of the Cases. Other symbols are defined where used in the text and/or table.

N-122 and N-318 were first published around 1980; some four years after the publication of Welding Research Council Bulletin 198. The motivation if the development of N-122 is described by the following extract WRC198:

"A piping system design requires the specification of a corresponding support system whose function is to transmit force to the piping system or to limit its deformation. Structural attachments which connect the pipe system with its supporting elements are classified into two general categories, those integral to the piping system and those that are not . Lugs welded to the pipe are examples of integral attachments. whereas roller supports and various types of clamps are examples of nonintegral attachments. Article NB-3645 of Section III of the ASME Boiler and Pressure Vessel Code requires that the effects of attachments in producing thermal stresses, stress concentrations, and restraints on pressure-retaining members shall be taken into account in checking for compliance with design criteria."

The preceding quote is directly relevant to N-122 (Class 1 piping); WRC 198 covered only CLASS 1 piping. However,having a relatively simple method of calculating stresses at lugs, the analogous N-318, of the Class 2 and 3 piping, was developed.

During the years between 1980 and 1994, N-122-1 and N-318-1,-2,-3,-4, were published. These mostly involved clarifications of how to use the Cases. in 1994. N-122-2 and N-318-5 were published. These editions included a major change in that the coefficients of the equations of CT, CL and CN were divided by two. This report deals, in large part, with the bases for making this change.

The review of this report leads to several suggestions for revision N-122-2 and/or N-318-5. These are discussed in Sections 4.3, 4.4 and 4.5 of this report and are summarized in Section 8.


Regards to FE/Pipe "bend with Stanchion" page 1.8.4 compare to WRC Bulletin 448 January 2000 Evaluation of Welded Attachments on Pipe and Elbows

Report 1: Background and Suggested Revisions:
ASME Code Cases N-122-2 and N-318-5 "Lugs on Straight Pipe" , E.C. Rodabaugh & E.A. Wais
Report 2: Evaluation of Welded attachments on Pipe and Elbows
E.C. Rodabaugh
E.A. Wais
G.B. Rawis

Leonard Stephen Thill
Leonard@thill.biz

http://www.thill.biz

 

[sme75]

vesselanonymous,

Just wondering if you were going to be able to respond to my question?

sme

 

[prex]

sme75,
trying to reply to your original questions, I would comment as follows:
1)Your calculated thickness (for a 6% knuckle with no welds in the head) seems correct (didn't check it)
2)Don't worry about local stresses: these are bending discontinuity stresses that are unrelevant to thickness calculation
3)However the very high stress you find seems really excessive and should be erroneous
4)Local stresses may be relevant to fatigue analysis, but I would be surprised if the head stresses were limiting the fatigue life of a real vessel, normally this is due to nozzle stresses

prex

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