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Volume II: ASME Pressure Vessels

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  • Volume II: ASME Pressure Vessels

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 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 three 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.
  • 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 provide custom solutions to our customers.

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 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.

Pressure vessel metallurgy design will be discussed in the next installment of Fabrication News.

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