Design Considerations

It is important to understand that Code rules are formulated to provide minimum requirements for safety and service, and that the Code holds the owner/user and the designer/manufacturer responsible for meeting Code rules as well as the service needs. This concept is discussed in detail in Section 410.

An ASME Code stamp on a pressure vessel only provides that new and cold, the pressure vessel is good for the pressure and temperature indicated and recorded on the manufacturer’s ASME data report. The stamp also permits the legal operation of the pressure vessel in a jurisdiction that has adopted the ASME Code as a part of the law.

The ASME Code cannot include rules for all types of pressure vessels and all types of services. This is particularly true for Section VIII, Division 1, since it covers a wide scope of pressure vessels from simple water and/or air receivers constructed of low carbon steel to process pressure vessels constructed with nonferrous, high-alloy, and/or high-strength materials. Section VIII, Division 2, provides for the design of pressure vessels subjected to external and internal pressure without any limit on the pressure. The design temperature is limited by the design stress for the material listed in the applicable stress table or Code case covering the construction.

Because of the range of materials and potential services for pressure vessels constructed under Section VIII, Division 1, it is understandable that the Code must hold the owner/user and the manufacturer responsible for constructing a pressure vessel for a specific service, as discussed in Section 410.

The owner/user’s designated agent may be an engineer on his staff, a design agent specifically engaged by the owner/user, the manufacturer and/or designer of a system for a specific service, an organization which offers pressure vessels for sale or lease for a specific service, or the engineer of the stamp holder that will manufacture the pressure vessel. Design considerations must include, but need not be limited to, the following:

1. The need for corrosion allowance beyond that specified by the rules. Except for a limited number of vessels constructed of low carbon steel containing air, steam, or water, corrosion allowance is not a requirement of the Code. Corrosion allowance is the extra wall thickness available beyond the thickness computed by Code rules. This thickness is assumed to be lost to corrosion at the end of the vessel design life. (Section 500)

2. The contents defined with particular attention to toxic substances in order that the mandatory requirements of the Code can be met.

3. The need determined for impact testing of base materials and deposited weld metal if the vessel will operate at low temperature. This is to assure adequate toughness. (Section 500)

4. The need for postweld heat treatment beyond Code mandatory requirements that might be necessary for resistance to corrosion or because of the service environment. (Section 600)

5. Proper selection of the materials for the service conditions. (Section 500)

6. Nozzle locations and the external piping reactions that will occur on the nozzles during normal operation.

7. The installed orientation of the pressure vessel and the type of support required.

8. For pressure vessels generating steam, the need for piping, valves, instrumentation, and fittings required for the service as set forth in Section I, Paragraphs PG-59 through PG-61.

9. The number of openings and the size for the required safety relief devices.

10. Selection of vessel heads—Very high compressive stresses can exist in the knuckle region of torispherical heads under internal pressure. The magnitude of the stress is primarily a function of the knuckle radius, the diameter, and the diameter-to-thickness ratio. (Section 100)

11. Openings in knuckles—Caution should be exercised in locating openings or other attachments that may produce large stress concentrations in the region of the knuckle of torispherical heads or the shell-to-cone junction of conical transitions.

12. Structural, wind and earthquake—The Code does not have design rules for wind or earthquake loads. Therefore, design for wind and earthquake loads should be in accordance with the UBC or CIV-EN-100, the recommended Company design standard for wind and earthquake. (Structural design is discussed in Section 440.)

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