Page 21 - Pressure Vessel Design Manual

8 Pressure Vessel Design Manual

Primary plus secondary stresses are allowed to be higher is not self-limiting. Primary stresses are generally due to in-
and so on. Before considering the combination of stresses ternal or external pressure or produced by sustained external
(categories), we must first define the various types and forces and moments. Thermal stresses are never classified as
classes of stress. primary stresses.
Primary general stresses are divided into membrane and
Types of Stress bending stresses. The need for divilng primary general
stress into membrane and bending is that the calculated
There are many names to describe types of stress. Enough value of a primary bending stress may be allowed to go
in fact to provide a confusing picture even to the experienced higher than that of a primary membrane stress. Primary
designer. As these stresses apply to pressure vessels, we stresses that exceed the yield strength of the material can
group all types of stress into three major classes of stress, cause failure or gross distortion. Typical calculations of
and subdivision of each of the groups is arranged according primary stress are:
to their effect on the vessel. The following list of stresses
describes types of stress without regard to their effect on PR F MC and TC
-
the vessel or component. They define a direction of stress t ’A’ I ’ J
or relate to the application of the load.
Primary general membrane stress, P,. This stress occurs across
1. Tensile 10. Thermal the entire cross section of the vessel. It is remote from dis-
2. Compressive 11. Tangential continuities such as head-shell intersections, cone-cylinder
3. Shear 12. Load induced intersections, nozzles, and supports. Examples are:
4. Bending 13. Strain induced a. Circumferential and longitudmal stress due to pressure.
5. Bearing 14. Circumferential b. Compressive and tensile axial stresses due to wind.
6. Axial 15. Longitudinal c. Longitudinal stress due to the bending of the horizontal
7. Discontinuity 16. Radial vessel over the saddles.
8. Membrane 17. Normal d. Membrane stress in the center of the flat head.
9. Principal e. Membrane stress in the nozzle wall within the area of
reinforcement due to pressure or external loads.
f. Axial compression due to weight.
Classes of Stress
Primary general bending stress, Pb. Primary bending stresses
The foregoing list provides examples of types of stress. are due to sustained loads and are capable of causing
It is, however, too general to provide a basis with which collapse of the vessel. There are relatively few areas where
to combine stresses or apply allowable stresses. For this primary bending occurs:
purpose, new groupings called classes of stress must be
used. Classes of stress are defined by the type of loading a. Bending stress in the center of a flat head or crown of a
which produces them and the hazard they represent to the dished head.
vessel. b. Bending stress in a shallow conical head.
c. Bending stress in the ligaments of closely spaced
1. Prima y stress openings.
a. General:
0 Primary general membrane stress, P,
0 Primary general bending stress, Pb Local primary membrane stress, PL. Local primary
b. Primary local stress, PL membrane stress is not technically a classification of stress but
2. Seconda y stress a stress category, since it is a combination of two stresses. The
a. Secondary membrane stress, Q, combination it represents is primary membrane stress, P,,
b. Secondary bending stress, Qb plus secondary membrane stress, Q,, produced from sus-
3. Peak stress, F tained loads. These have been grouped together in order to
limit the allowable stress for this particular combination to a
Definitions and examples of these stresses follow. level lower than allowed for other primary and secondary
stress applications. It was felt that local stress from sustained
Primary general stress. These stresses act over a full (unrelenting) loads presented a great enough hazard for the
cross section of the vessel. They are produced by mechanical combination to be “classified” as a primary stress.
loads (load induced) and are the most hazardous of all types A local primary stress is produced either by design
of stress. The basic characteristic of a primary stress is that it pressure alone or by other mechanical loads. Local primary