11 March 2016
Fabrication News
ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and
understanding of the ASME code for the design of pressure vessels for the
chemical and process industry as applicable in the United States and most
of North and South America.
The main sections for discussion are:
DESIGN: DEFINITIONS
The following conditions must be considered in the design of ASME
Pressure Vessels. These terms are defined as follows:
Elastic Failure: When marked plastic deformation has begun (the point at
which a ductile metal has considered to fail.
Ductile Metals: Metals where marked plastic deformation starts at a fairly
definite stress (yield point, yield strength, etc.) and exhibits considerable
ultimate elongation (mild steel).
Brittle Metal:
Metals where marked plastic deformation is not clearly
defined (yield point, yield strength, etc.) and exhibits little ultimate
elongation.
Maximum Stress
Theory:
States that elastic failure occurs when the maximum tensile
stress becomes equal to the yield stress, yield point (ASME Section VII,
Division 1).
Maximum Shear
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Stress Theory: Stated that elastic failure occurs when the maximum
shear stress equals ½ yield stress, yield point (ASME Section VII, Division
2).
MAP: Maximum Allowable Pressure. This is the maximum pressure the
vessel can safely stand in the new and cold condition (no corrosion
allowance applied. Note: This pressure is still with the safety factor of the
code and not the destructive point.
MAWP:
Maximum Allowable Working Pressure. This is the maximum
pressure the vessel can allow with the corrosion allowance applied, still
within the code safety factor. This is what the vessel is good for in
pressure after years of service.
BASIC STRESSES
The following stresses are defined and need to be considered in pressure
vessel design.
Primary Stresses: Stresses due to weight, pressure, and concentrated
forces due to weight and pressure. Primary stresses are separated into 3
categories:
• Pm: General primary membrane stress that is the average stress
throughout the thickness of the component considered.
• Pl: Local primary membrane stresses due to weight or pressure but
is localized around a discontinuity such as a nozzle in a vessel.
• Pb: Primary bending stress is due to primary loads (weight and
pressure, etc.) and varies about the thickness and is compressive on
one side of the neutral axis and tensile on the other of the neutral
axis.
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Secondary stresses, Q: Stresses that are self-limiting, such as
thermal stresses and local bending stresses at nozzle discontinuities.
• Q: Self-equalizing stresses necessary to satisfy continuity of the
structure. Can be caused by mechanical load or differential thermal
stresses.
• Peak Stresses: F
• Increment stress added to primary or secondary stresses by a
concentration (notch).
• Stress Allowable – ASME Section VIII, Division 1
• Sa = Stress allowable listed in the ASME material properties of
Section II, Part D. This is generally the stress entered into the
formulas. This is also the “Pm” stress defined above and resulting
from pressure as used in the Pd/2t and Pd/4t.
• Stress Allowable – ASME Section VIII, Division 2
• The allowable stresses to be applied to Division 2 of the ASME code
are also listed in Section II, Part D. The allowable stresses are higher
than those of Division 1 due to the lower safety factor needed
resulting from more detailed analysis required by Division 2.
Section VIII, Division I engineering experience, we have the unique ability to
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CODE REQUIREMENTS FOR DESIGN
Sample Sections in the Code
General reference for design of pressure vessels is contained in the “UG”
section of the ASME code for Section VIII, Division 1. Division 1 is the
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Fabrication News
primary code for design of most of the pressure vessels. The following are
a few of the general sections. References are made to other sections,
such as UW (welding) and paragraphs as needed to cover complete design
of the pressure vessel.
• UG-2: Defines general responsibilities, scope, and general
statements for the code.
• UG-16: Statement of general design requirements.
• UG-20: Defines “Design temperature” as used in the code.
• UG-21: Defines “Pressure” as used in the code.
• UG-22: Defines “Loadings” to be considered. Forces from vessel
motions are to be included here.
• UG-23: Allowable stresses to be used are referenced in this section.
• UG-27: Thickness of shells under internal pressure and the relative
formulas are in this section. Other UG sections define heads, etc., for
all other components and conditions.
• Example: t = PR / SE – 0.6P + Ca, where
• t = thickness in inches, P = design pressure,
• S = allowable stress, E = joint efficiency, Ca = corrosion
allowance
• UG-37: Reinforcement required for openings in shells and
formed heads.
• UG-84: Charpy Impact Test for cold service, etc.
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11 March 2016
Fabrication News
Pressure vessel metallurgy design will be discussed in the next installment
of FABRICATION NEWS.
This is presented to you as a service from BOARDMAN INC. located in
Oklahoma City, Oklahoma.
Since 1910, Boardman has been a respected custom fabricator. We take pride in
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high quality solution to our customers. With more than 7 ASME Section VIII,
Division I engineering experience, we have the unique ability to provide custom
solutions to our customers.
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Trayed Towers & Columns
ASME Pressure Vessels
Molecular Sieves
Rotary Dryers & Kilns
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Acid settlers
Stacks, Scrubbers
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Ducting
Bins
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The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4”
thick.
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