Page 18 - Pressure Vessel Design Manual

Stresses in Pressure Vessels 5

For thick-walled vessels (R,,,/t < lo), the radial stress designing these vessels. For this reason, this text has limited
becomes significant in defining the ultimate failure of the its application to thin-walled vessels where a biaxial state of
vessel. The maximum stress theory is unconservative for stress is assumed to exist.

FAILURES IN PRESSURE VESSELS

Vessel failures can be grouped into four major categories, 2. Brittle fracture-Can occur at low or intermediate tem-
which describe why a vessel failure occurs. Failures can also peratures. Brittle fractures have occurred in vessels
be grouped into types of failures, which describe how made of low carbon steel in the 40’50°F range
the failure occurs. Each failure has a why and how to its during hydrotest where minor flaws exist.
history. It may have failed through corrosion fatigue because 3. Excessive plastic deformation-The primary and sec-
the wrong material was selected! The designer must be as ondary stress limits as outlined in ASME Section
familiar with categories and types of failure as with cate- VIII, Division 2, are intended to prevent excessive plas-
gories and types of stress and loadings. Ultimately they are tic deformation and incremental collapse.
all related. 4. Stress rupture-Creep deformation as a result of fa-
tigue or cyclic loading, i.e., progressive fracture.
Creep is a time-dependent phenomenon, whereas fa-
Categories of Failures
tigue is a cycle-dependent phenomenon.
1. Material-Improper selection of material; defects in 5. Plastic instability-Incremental collapse; incremental
material. collapse is cyclic strain accumulation or cumulative
2. Design-Incorrect design data; inaccurate or incor- cyclic deformation. Cumulative damage leads to insta-
rect design methods; inadequate shop testing. bility of vessel by plastic deformation.
3. Fabrication-Poor quality control; improper or insuf- 6. High strain-Low cycle fatigue is strain-governed and
ficient fabrication procedures including welding; heat occurs mainly in lower-strengthhigh-ductile materials.
treatment or forming methods. 7. Stress corrosion-It is well known that chlorides cause
4. Seruice-Change of service condition by the user; stress corrosion cracking in stainless steels; likewise
inexperienced operations or maintenance personnel; caustic service can cause stress corrosion cracking in
upset conditions. Some types of service which require carbon steels. Material selection is critical in these
special attention both for selection of material, design services.
details, and fabrication methods are as follows: 8. Corrosion fatigue-Occurs when corrosive and fatigue
a. Lethal effects occur simultaneously. Corrosion can reduce fa-
b. Fatigue (cyclic) tigue life by pitting the surface and propagating cracks.
c. Brittle (low temperature) Material selection and fatigue properties are the major
d. High temperature considerations.
e. High shock or vibration
f. Vessel contents In dealing with these various modes of failure, the de-
0 Hydrogen signer must have at his disposal a picture of the state of
0 Ammonia stress in the various parts. It is against these failure modes
0 Compressed air that the designer must compare and interpret stress values.
0 Caustic But setting allowable stresses is not enough! For elastic
0 Chlorides
instability one must consider geometry, stiffness, and the
properties of the material. Material selection is a major con-
sideration when related to the type of service. Design details
Types of Failures and fabrication methods are as important as “allowable
stress” in design of vessels for cyclic service. The designer
1. Elastic defi,rmation-Elastic instability or elastic buck- and all those persons who ultimately affect the design must
ling, vessel geometry, and stiffness as well as properties have a clear picture of the conditions under which the vessel
of materials are protection against buckling. will operate.