Determining Design Conditions
The following design conditions for a pressure vessel must be established before the actual design begins:
• Design pressure
• Design temperature
• Wind and earthquake loads
• Corrosion allowance
• External loads
• Internal loads
Pressure and temperature are the factors that often govern the mechanical design of a pressure vessel. The ASME Code requires that a pressure vessel be designed for the “most severe condition of pressure and temperature expected in normal operation.” Pressure is directly entered into the Code equations for design calculations. Temperature indirectly influences the design through its effect on the maximum allowable design stresses for the materials of construction.
Wind and earthquake loads specific to the geographic location where the vessel will be installed must also be considered in the design. The greatest effect of wind and earthquake is usually upon the support and anchoring of a vessel, with no effect upon the design of pressure containing components, unless the shell is relatively thin with respect to its diameter (i.e., a large vessel with a low internal pressure), or the contents of the vessel are relatively heavy.
It is usually necessary to add a corrosion allowance to the minimum required thickness calculated for each component of the vessel, or to provide a corrosion-resistant cladding to protect the vessel from corrosion by the process environment.
The major external loads that must be considered are normally those from piping connections to nozzles or from structural attachments to the shell and heads, such as support clips for platforms and ladders.
Piping is usually designed to have sufficient support and flexibility so the resulting loads applied to a vessel have a negligible effect on the design of nozzles and the reinforcement of the openings in the shell. Similarly, external loads attributable to structural attachments usually have a negligible effect on the design of the pressure containing components of a vessel.
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