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:
- ASME Section 8, Division I (Rules for Construction of Pressure Vessels)
- ASME Section 8, Division II (Alternate Rules)
- ASME Sections II, V, and IX are also in the scope of required codes but as supplements to the main section, Section VIII
- ASME Section I (Rules for Construction of Power Boilers) will be covered in another session
Failures in Pressure Vessels
Vessel failures can be grouped into four major categories, which describe why a vessel failure occurs. Failures can also be grouped into types of failures, which describe how the failure occurs. Each failure has a why and a how to its history. It may have failed through corrosion fatigue because the wrong material was selected! The designer must be familiar with categories and types of failure as with categories and types of stress and loadings. Ultimately they are all related.
Categories of Failures
- Material – Improper selection of material; defects in material.
- Design – Incorrect design data; inaccurate or incorrect design methods, inadequate shop testing.
- Fabrication – Poor quality control; improper or insufficient fabrication procedures including welding, heat treatment or forming methods.
- Service – Change of service condition by the user; inexperienced operations or maintenance personnel; upset conditions. Some types of service which require special attention for both selection of material, design details and fabrication methods are as follows:
- Lethal Service
- Fatigue (cyclic)
- Brittle (low temperature)
- High Temperature High shock or vibration
- Vessel contents
- Compressed air
Types of Failures
- Elastic deformation – Elastic instability or elastic buckling, vessel geometry, and stiffness as well as properties of materials are protection against buckling.
- Brittle fracture – This can occur at low or intermediate temperatures. Brittle fracture have occurred in vessels made of low carbon steel in the 40o – 50o F range during hydrotest where minor flaws exist.
- Excessive plastic deformation – The primary and secondary stress limits outlined in ASME Section VIII, Division 2, are intended to prevent excessive plastic deformation and incremental collapse.
- Stress rupture – Creep deformation as a result of fatigue or cyclic loading, i.e., progressive fracture. Creep is a time-dependent phenomenon, whereas fatigue is a cycle-dependent phenomenon.
- Plastic instability – Incremental collapse; incremental collapse is cyclic strain accumulation or cumulative cyclic deformation. Cumulative damage leads to instability of vessel by plastic deformation.
- High Strain – Low cyclic fatigue is strain-governed and occurs mainly in lower strength/high-ductile materials.
- Stress corrosion – It is well know that chlorides cause stress corrosion cracking in stainless steels; likewise caustic service can cause stress corrosion cracking in carbon steel. Materials selection is critical in these services.
- Corrosion fatigue – Occurs when corrosive and fatigue effects occur simultaneously. Corrosion can reduce fatigue life by pitting the surface and propagating cracks. Material selection and fatigue properties are the major considerations.
In dealing with these various modes of failure, the designer must have at his disposal a picture of the state of stress in the various parts. It is against these failure modes that the designer must compare and interpret stress values. But setting allowable stress is not enough. For elastic instability one must consider geometry, stiffness, and the properties of the materials. Material selection is a major consideration when related to the type of service. Design details and fabrication methods are as important as “allowable stress” in design of vessels for cyclic service. The designer and all those persons who ultimately affect the design must have a clear picture of the condition under which the vessel will operate.
Bulk transporter reported that the National Board of Boiler and Pressure Vessel Inspectors recorded the number of accidents involving pressure vessels at an increase of 24% over the course of a year between 1999 to 2000. These statistics include power boilers, steam heating boilers, water heating boilers, and unfired pressure vessels. However, the increased number of accidents was not reflected through to the number of fatalities, as these actually dropped by 33% over this period. By broadening this search, it can be seen that the reporting period of 1992 to 2001 saw a total of 23,338 pressure vessel related accidents which averages at 2,334 accidents per year. Reporting year 2000 saw the highest number of accidents at 2,686 with the lowest at 2,011 in 1998. The number of fatalities as a direct result of boiler and pressure vessel accidents has been recorded as 127 over the past 10 years. [7-9]. During the reported period between 2001 and 2008, the statistics show that the rate of accidents that were directly linked to pressure vessels is not yet on the decline.
Source: ASME Boiler & Pressure Vessel Code, Secton VIII, Division 1: Edition 2013
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