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 Section8,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.
What is a Pressure Vessel?
General reference to pressure vessel will be to ASME for this discussion. Definitions vary, but generally: “A metal container capable of withstanding various loadings.”
Sizes and shapes are generally spherical or cylindrical but can also be constructed of flat plates. Materials can also be non-metallic, such as fiberglass, etc. A pressure vessel includes a number of devices, such as some heat exchangers, storage tanks, process vessels, etc.
ASME stands for “American Society of Mechanical Engineers."
A pressure vessel is defined as "a vessel in which the pressure is obtained from an indirect source or by the application of heat from an indirect source or a direct source. The vessel proper terminates at: (a) the first circumferential joint for welded end connections; (b) the face of the first flange in bolted flange connections; or (c) the first threaded joint in threaded connections." Pressure vessels include but are not limited to compressed gas storage tanks (i.e., air, oxygen, nitrogen tanks, etc.), anhydrous ammonia tanks, hydro pneumatic tanks, autoclaves, hot water storage tanks, chemical reactors and refrigerant vessels, designed for a pressure greater than 15 psi and a volume greater than 5 cubic feet in volume or one and one-half cubic feet in volume with a pressure greater than 600 psi.
Design Criteria Basis
ASME Section VIII, Division 1
Criteria are mostly by formula that has a built-in safety factor. This section is the most widely used and does not call for detailed analysis of higher of more localized stresses that usually exist. Stresses are within the safe range by built-in safety factors. Temperature gradient is not normally considered in this section. The Maximum Stress Theory is the basis of design.
ASME Section VIII, Division 2
Criteria for Section VIII, Division 2 is alternate rules and require consideration of local stresses, temperature gradient, and cyclic pressures, etc. Generally, lower stress values are allowed and thinner wall thicknesses are resulted due to more detailed analysis. The Maximum Shear Stress Theory is the main basis of design.
ASME Section VIII, Division 3
For design of pressure vessel with high design pressure (>10,000 psig) requires fracture analysis.
Design: Other Required Expertise & ASME Standards (Partial List)
There are other areas of knowledge needed to complete pressure vessel design as well as many other ASME codes that are both needed for pressure vessel design and many that are outside of pressure vessel design.
Areas of knowledge needed for pressure vessel design:
- Structural (AISC, etc.): Basics of structural design is essential since a pressure vessel is a structural design subject to stresses produced from types of loadings, such as thermal, weight, pressure, outside forces (earthquake, wind, motions, etc.) The supports (skirt, saddles, base ring, etc.) are all designed through the use of structural calculations.
- Metallurgy: Properties of materials (stress and strength qualities, corrosion properties, etc.) is essential for pressure vessel design.
- Welding knowledge
- WRC (Welding Research Council) Bulletins 107 and 297: Additional method of calculating nozzle stresses at nozzle to shell junction
- Other ASME-related codes, such as B31.3 (Process Pressure Piping), B31.1 (Power Piping), B16.5 (Pipe Flanges and Flanged Fittings, and B16.47 (Large Diameter Steel Flanges – 26”-60”)
Pressure vessel design parameters will be discussed in the next installment of Fabrication News.