Page 14 - Pressure Vessel Design Manual

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Stresses in Pressure Vessels

DESIGN PHILOSOPHY

In general, pressure vessels designed in accordance with It is general practice when doing more detailed stress
the ASME Code, Section VIII, Division 1, are designed by analysis to apply higher allowable stresses. In effect, the
rules and do not require a detailed evaluation of all stresses. detailed evaluation of stresses permits substituting knowl-
It is recognized that high localized and secondary bending edge of localized stresses and the use of higher allowables
stresses may exist but are allowed for by use of a higher in place of the larger factor of safety used by the Code. This
safety factor and design rules for details. It is required, how- higher safety factor really reflected lack of knowledge about
ever, that all loadings (the forces applied to a vessel or its actual stresses.
structural attachments) must be considered. (See Reference 1, A calculated value of stress means little until it is associ-
Para. UG-22.) ated with its location and distribution in the vessel and with
While the Code gives formulas for thickness and stress of the type of loading that produced it. Different types of stress
basic components, it is up to the designer to select appro- have different degrees of significance.
priate analytical procedures for determining stress due to The designer must familiarize himself with the various
other loadings. The designer must also select the most prob- types of stress and loadings in order to accurately apply
able combination of simultaneous loads for an economical the results of analysis. The designer must also consider
and safe design. some adequate stress or failure theory in order to combine
The Code establishes allowable stresses by stating in Para. stresses and set allowable stress limits. It is against this fail-
UG-23(c) that the maximum general primary membrane ure mode that he must compare and interpret stress values,
stress must be less than allowable stresses outlined in material and define how the stresses in a component react and con-
sections. Further, it states that the maximum primary mem- tribute to the strength of that part.
brane stress plus primary bending stress may not exceed 1.5 The following sections will provide the fundamental
times the allowable stress of the material sections. In other knowledge for applying the results of analysis. The topics
sections, specifically Paras. 1-5(e) and 2-8, higher allowable covered in Chapter 1 form the basis by which the rest of
stresses are permitted if appropriate analysis is made. These the book is to be used. A section on special problems and
higher allowable stresses clearly indicate that different stress considerations is included to alert the designer to more com-
levels for different stress categories are acceptable. plex problems that exist.

STRESS ANALYSIS

Stress analysis is the determination of the relationship governing stresses and how they relate to the vessel or its
between external forces applied to a vessel and the corre- respective parts, attachments, and supports.
sponding stress. The emphasis of this book is not how to do The starting place for stress analysis is to determine all
stress analysis in particular, but rather how to analyze vessels the design conditions for a gven problem and then deter-
and their component parts in an effort to arrive at an mine all the related external forces. We must then relate
economical and safe design-the rllfference being that we these external forces to the vessel parts which must resist
analyze stresses where necessary to determine thickness of them to find the corresponding stresses. By isolating the
material and sizes of members. We are not so concerned causes (loadings), the effects (stress) can be more accurately
with building mathematical models as with providing a determined.
step-by-step approach to the design of ASME Code vessels. The designer must also be keenly aware of the types of
It is not necessary to find every stress but rather to know the loads and how they relate to the vessel as a whole. Are the

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