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#111 - 06/10/00 02:04 PM An Inadvertence regarding Caesar II WRC 107 Module
Dorin Daniel Popescu Offline
Member

Registered: 06/05/00
Posts: 124
Loc: Middle East
As it is known, Caesar II performs the elastic analysis of the "Shell - Nozzle" junction through the WRC 107 and WRC 297 modules.
The piping stress analyses that I accomplished for several piping projects, requested the vessel stress assessment by the WRC 107 method. Following this procedure (implemented within the Caesar II software package - 4.00 and 4.20 versions), I have noticed a major inadvertence regarding the interpretation of the material strength safety criteria. The main problem is the WRC 107 stress summation routine.
As it is known, following the ASME Code (Section VIII, Division 2) provisions, Caesar II provides the following three well-known stress intensity checking criteria: 1) Pm < k Smh; 2) Pm + Pl < 1.5 k Smh; 3) Pm + Pl + Q < 1.5 (Smc + Smh), where the meaning of the involved notations may be found in the Caesar II documentation (Technical Reference Manual) or in the Pressure Vessel ASME Code, Section VIII, Division 2 (see Appendix 4, Article 4-1, par. 4-130...4-134, Fig. 4-130.1).
The major problem appears in connection with the first and the second stress intensity checking conditions. The first restriction involves the general primary membrane stresses (i.e. the normal hoop and the normal longitudinal primary membrane stresses for the cylindrical shells) developed in the unpenetrated vessel/shell wall, under the internal pressure loading exclusively. The nozzle orifice discontinuity and the shell-nozzle interaction are excluded.
The second restriction involves the (algebraic) sum of the general and of the local primary membrane stresses. The local primary membrane stresses are developed in the near vicinity of the "Shell - Nozzle" junction, as a result of the combined internal pressure and applied sustained forces&moments loading. The nozzle orifice discontinuity is taken into account, but the shell-nozzle interaction is excluded.
As a conclusion, we may say that the first and the second stress intensity checking conditions, involve the primary membrane stresses, or, in other words, the primary stresses that are uniformly distributed along the (shell) wall thickness.
Now, the WRC 107 analysis module (included in the Caesar II software package) contains the "Include Pressure Stress Indices" option. When this option is activated, the vessel stress analysis takes into account the stress indices provided by the Pressure Vessel ASME Code, Section VIII, Division 2 (see Appendix 4, Article 4-6, par. 4-612). The programme multiplies the general primary membrane stress components by these indices and the results are interpreted as being the effective general primary principal (hoop and longitudinal) membrane stresses. In the WRC 107 Stress Summation Report, the effective "general primary membrane stress" components (calculated with the ASME stress indices) are not the same for the inside and for the outside vessel wall surfaces, and, thus, these stresses don't satisfy the main membrane stress requirement: the invariance along the vessel wall thickness.
In my opinion, this approach manner is inadequate.
As the ASME Code (Section VIII, Division 2, Article 4-6) stipulates, the stress indices provided at par. 4-612, quantify the PEAK STRESS components induced by the internal pressure action, at the inside and at the outside "Shell - Nozzle" junction wall. Therefore, when we multiply the general primary membrane stress components by the ASME stress indices, we obtain the peak stresses developed under the internal pressure loading, in the near vicinity of the "Shell - Nozzle" junction.
The peak stresses quantify the following effects, developed under the internal pressure loading: a) the nozzle orifice discontinuity effect; b) the general stress concentration effect (i.e. the gross structural discontinuity effect, at the "Shell - Nozzle" junction level - the so-called "edge effect"); c) the local stress concentration effect (i.e. the local structural discontinuity effect, at the "Shell - Nozzle" junction level - small fillet radii, partial penetration welds, etc.). Briefly, the peak stress components quantify the result of the general and of the local stress concentration effects, developed under the internal pressure loading. These peak stresses deal with the fatigue analysis stress intensity checking condition: Pm + Pl + Q + F < Sa (see the ASME Code, Section VIII, Division 2, Appendix 4, Article 4-1, par. 4-135, Fig. 4-130.1). Thus, it's obvious that the peak stresses cannot be interpreted as being the effective general primary membrane stress components, because they contain the general and the local bending stress components, induced by the internal pressure action.
The consequence of this inadequate approach, is in erroneous overestimation of the general and of the local primary membrane stresses (i.e. the normal hoop and the normal longitudinal primary membrane stresses for the cylindrical shells), that may cause the unjustified failure of the membrane stress intensity checking criteria.
In my opinion, this inadvertence can be reasonably amended if the "Pressure Stress Indices" concept utilisation will be slightly modified. The most conservative, but acceptable approach of the "Pressure Stress Indices" utilisation, could be organised as follows:
1. The general and the local primary membrane stress components (i.e. the normal hoop and the normal longitudinal primary membrane stresses for the cylindrical shells) remain unmodified, as they have been assessed without applying the ASME stress indices. These stress components are: a) the hoop/circumferential general membrane stress - Circ/Pm(SUS); b) the longitudinal general membrane stress - Long/Pm(SUS); c) the hoop/circumferential local membrane stress - Circ/Pl(SUS); d) the longitudinal local membrane stress - Long/Pl(SUS).
2. The secondary/bending stress components, developed by the combined internal pressure and applied sustained forces&moments loading, have to be corrected in order to include the general and the local stress concentration effects, induced by the combined sustained actions. Therefore, instead of Circ/Q(SUS) and Long/Q(SUS) initial assessed values, we should have Circ/Q(SUS) + (Kcirc - 1) * Circ/Pm(SUS) and Long/Q(SUS) + (Klong - 1) * Long/Pm(SUS) values respectively, where Kcirc and Klong are the ASME pressure stress indices, for the hoop/circumferential and longitudinal normal principal stresses respectively (the inside and the outside wall surfaces).
3. All the other stress components remain unmodified.
In this way, although on the one hand, the total stress intensity (i.e. Pm+Pl+Q) includes the pressure peak stress components, and, on the other hand, the pressure local primary membrane stress components and the pressure secondary/bending stress components are somehow twice taken into account, this new alternative of the vessel stress summation is much closer to the ASME Code precepts than the existing procedure is.
_________________________
Dorin Daniel Popescu

Lead Piping Stress Engineer

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#112 - 06/12/00 03:19 PM Re: An Inadvertence regarding Caesar II WRC 107 Module
Mandeep Singh Offline
Member

Registered: 12/15/99
Posts: 594
Loc: Houston, Tx, USA
Dorin,

You are 100 % correct. It is amazing that this also happens to be the topic of an article in COADE's July 2000 newsletter (about to be published) - "WRC 107: Elastic Analysis v/s Fatigue Analysis". Main points are listed here,

WRC 107 module can be used to perform elastic and/or Fatigue analysis. Users should decide if they require Fatigue analysis or if elastic analysis will suffice (using the guidelines of section AD-160 Section VIII Div. 2). If only elastic analysis is required then:

  • Set up different types of load cases (Sustained, Expansion and Occasional).
  • Include Pressure Thrust if needed.
  • Do not include Pressure Stress Indices (they intensify the internal pressure stress, used for computing peak stress due to internal pressure).
  • Do not include stress concentration factors Kn and Kb (this is done by omitting the fillet radius between the vessel and nozzle).
  • Use stress summation to compare the actual stress to the allowable stress.


If fatigue analysis is required then an elastic analysis is still required. Additionally, the following procedures should be followed:

  • Set up the range pair and load cycles (e.g. - Installed to operating, pressure fluctuations) for the fatigue loading.
  • Evaluate each load range using the WRC 107 module one by one. Enter each cyclic load as a sustained load and leave the other types of loads as blank.
  • Include Pressure Thrust if needed.
  • Include stress concentration factors Kn and Kb, by entering the fillet radius between the vessel and nozzle. This will gives peak stress due to applied loads.
  • The easiest way to combine the stresses due to external loads with the ones due to internal pressure is to include the Pressure Stress Indices and perform stress summations. Use the results for Stress Intensity (Pm + Pl + Q + F) (though in the current version it reads as Pm + Pl + Q, but will be corrected in future) and ignore the results for first two equations (Pm and Pm+Pl) and ignore the comparison to the allowable stresses.
  • Then take that stress intensity (thermal or pressure cycles result in the range of stress) and refer to the rules of Appendix 5 in Section VIII Div. 2 for performing a fatigue analysis.


This topic is discussed in more detail in the WRC 107 article appearing in the next newsletter. This edition of the newsletter will also be available next month on our website, www.coade.com/newsletters/jun00.pdf (Note: this has not been posted at this time).

Your suggestion about combining the intensified stresses from the internal pressure under the category Q loads is very interesting, we will investigate it further.

CAESAR II is a tool, which can be used incorrectly. In areas where this possibility is identified, we make every effort to document the proper procedure, or modify the software. The WRC107 analysis is one area where future refinement is planned.

Regards,
Mandeep Singh
COADE, Inc.



[This message has been edited by rich_ay (edited July 28, 2000).]
_________________________
Best Regards,
Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM

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