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.
Part UG - General Requirements for All Methods of Construction and All Materials
The requirements of Part UG are applicable to all pressure vessels and vessel parts and are to be used in conjunction with specific requirements in Subsections B and C and the Mandatory Appendices that pertain to the methods of fabrication and the material used.
Minimum Thickness of Pressure Retaining Components
The minimum thickness permitted for shells and heads after forming and regardless of product form and material shall be 1.5 mm (1/16 in) exclusive of any corrosion allowance. Exceptions are:
- The minimum thickness does not apply to heat transfer plates of plate-type heat exchangers
- This minimum thickness does not apply to the inner pipe of double pipe heat exchangers nor to pipes and tubes that are enclosed and protected from mechanical damage by a shell, casing or ducting, where such pipes or tubes are DN 150 (NPS 6) or less. (All other pressure parts of these heat exchangers must meet the minimum thickness requirement) Note: This exemption applies whether or not the outer pipe, shell or protective element is constructed to Code rules. When the outer protective element is not provided by the manufacturer as part of the vessel, the Manufacturer shall note this on the Manufacturer’s Data Report. When pipes and tubes are fully enclosed, consideration shall be given to avoiding buildup of pressure within the protective chamber due to pipe/ tube leak.
- The minimum thickness of shells and heads of unfired steam boilers shall be 6 mm (1/4 in) exclusive of any corrosion allowance
- The minimum thickness of shells and heads used in compressed air service, steam service, and water service, made from one of the carbon and low alloy steel materials listed in Table UCS-23, shall be 2.5 mm (3/32 in) exclusive of any corrosion allowance
- This minimum thickness does not apply to the tubes in air cooled and cooling tower heat exchangers if all the following provisions are met:
- Tubes shall not be used for lethal service applications (“lethal substances” are defined as poisonous gases or liquids of such a nature that a very small amount of gas or of the vapour of the liquid mixed or unmixed with air is dangerous to life when inhaled)
- The tubes shall be protected by fins or other mechanical means
- The tube outside diameter shall be a minimum of 10 mm (3/8 in) and a maximum of 38 mm (1 ½ in)
- The minimum thickness used shall not be less than that calculated by formulas in UG-27 or in Appendix 1-1, and in no case less than 0.5 mm (0.022 in)
Plate material shall be ordered not thinner than the design thickness. Vessels made of plate furnished with an under-tolerance of not more than the smaller value of 0.25 mm (0.01 in) or 6% of the ordered thickness may be used at the full design pressure for the thickness ordered.
If the specification to which the plate is ordered allows a greater undertolerance, the ordered thickness of the materials shall be sufficiently greater than the design thickness so that the thickness of the material furnished is not more than the smaller of 0.25 mm (0.01 in) or 6% under the design thickness.
If pipe or tube is ordered by its nominal wall thickness, the manufacturing undertolerance on wall thickness must be taken into account except for nozzle wall reinforcement area requirements. After the minimum wall thickness is determined, it shall be increased by an amount sufficient to provide the manufacturing undertolerance allowed in the pipe or tube specification.
UG-17 Methods of Fabrication in Combination
A vessel may be designed and constructed by a combination of methods of fabrication given in the Code provided the rules applying to the respective methods of fabrication are followed and the vessel is limited to service permitted by the method of fabrication having the most restrictive requirement.
UG-18 Materials in Combination
A vessel may be designed and constructed of any combination of materials permitted in Subsection C, provided the applicable rules are followed and requirements for welding dissimilar metals are met.
For example, if a carbon steel base metal is joined to a stainless steel base metal with a nickel filler metal, the rules of Part UCS apply to the carbon steel base metal and its HAZ (heat affected zone), Part UHA to stainless steel base metal and its HAZ, and Part UNF to the weld metal.
Because of different thermal coefficients of expansion of dissimilar materials, caution should be exercised in design and construction in order to avoid difficulties in service under extreme temperature conditions, or with
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