Page 20 - Pressure Vessel Design Manual

Stresses in Pressure Vessels 7
I e. Moment load-Longitudinal or circumferential.
Loadings can be outlined as follows: d. Tangential load.

A. Categories of loadings f. Thermal load.
1. General loads-Applied more or less continuously
across a vessel section. B. Typey of loadings
a. Pressure loads-Internal or external pressure 1. Steady load-Long-term duration, continuous.
(design, operating, hydrotest. and hydrostatic a. InternaVexternal pressure.
head of liquid). b Dead weight.
b. Moment loads-Due to wind, seismic, erection, c. Vessel contents.
transportation. d. Loadings due to attached piping and equipment.
c. Compressive/tensile loads-Due to dead weight, e. Loadings to and from vessel supports.
installed equipment, ladders, platforms, piping, f. Thermal loads.
and vessel contents. g. Wind loads.
d. Thermal loads-Hot box design of skirthead 2. Nonsteady loads-Short-term duration; variable.
attachment. a. Shop and field hydrotests.
2. Local loads-Due to reactions from supports, b. Earthquake.
internals, attached piping, attached equipment, c. Erection.
Le., platforms, mixers, etc. d. Transportation.
a. Radial load-Inward or outward. e. Upset, emergency.
b. Shear load-Longitudinal or circumferential. f. Thermal loads.
c. Torsional load. g. Start up, shut down.

STRESS

ASME Code, SectionVIII, Division 1 vs. Types, Classes, and Categories of Stress
Division 2
The shell thickness as computed by Code formulas for
~~
ASME Code, Section VIII, Division 1 does not explicitly internal or external pressure alone is often not sufficient to
consider the effects of combined stress. Neither does it give withstand the combined effects of all other loadings.
detailed methods on how stresses are combined. ASME Detailed calculations consider the effects of each loading
Code, Section VIII, Division 2, on the other hand, provides separately and then must be combined to give the total
specific guidelines for stresses, how they are combined, and state of stress in that part. The stresses that are present in
in
allowable stresses for categories of combined stresses. pressure vessels are separated into various cla.~.sr~s accor-
Division 2 is design by analysis whereas Division 1 is dance with the types of loads that produced them, and the
design by rules. Although stress analysis as utilized by hazard they represent to the vessel. Each class of stress must
Division 2 is beyond the scope of this text, the use of be maintained at an acceptable leL7el and the combined
stress categories, definitions of stress, and allowable stresses total stress must be kept at another acceptable level. The
is applicable. combined stresses due to a combination of loads acting
Division 2 stress analysis considers all stresses in a triaxial simultaneously are called stress categories. Please note
state combined in accordance with the maximum shear stress that this terminology differs from that given in Dikision 2,
theory. Division 1 and the procedures outlined in this book but is clearer for the purposes intended herc,.
consider a biaxial state of stress combined in accordance with Classes of stress, categories of stress, and allowable
the maximum stress theory. Just as you would not design stresses are based on the type of loading that produced
a nuclear reactor to the niles of Division 1, you would them and on the hazard they represent to the structure.
not design an air receiver by the techniques of Division 2. Unrelenting loads produce primary stresses. Relenting loads
Each has its place and applications. The following discussion (self-limiting) produce secondary stresses. General loadings
on categories of stress and allowables will utilize informa- produce primary membrane and bending stresses. Local
tion from Division 2, which can be applied in general to all loads produce local membrane and bending stresses.
vessels. Primary stresses must be kept lo~er than secondary stresses.