Steel Containment Design Criteria
The material requirements for steel containment vessels are specified in ASME BPV III Code NE-2000 and RG 1.57. Table NE-2121(a)-1 lists the materials that are allowed for steel containment vessel construction. Additional requirements regarding certification and marking of materials and certification of materials suppliers are provided in NCA-3800 .
The design loadings for steel containment vessel are specified in NE-3110 of Section III of ASME B & PV Code. In particular, each service loading to which the containment may be subjected shall be categorized as one of service levels A, B, C, and D. These four service levels are to represent the four component conditions “Normal,” “Upset,” “Emergency,” and “Faulted” which were used in earlier versions of ASME BPV III Code. These service levels are established to permit different levels of allowable stresses. In general, the less likely a given condition, the higher the permitted allowable stresses. It remains the responsibility of the owner to determine corresponding service level for each postulated loading condition. If the containment vessel is expected to operate “normally” under a given set of conditions, then service level A is specified for those conditions. On the other hand, if a given set of loads is considered to be an unlikely event for the containment, a higher service level is specified. For example, the internal pressures and temperatures associated with the design basis accident must be accommodated with service level A allowable stress intensities, while service level C allowable are permitted for these pressures acting in combination with the safe shutdown earthquake (SSE). Load combinations for each of the ASME NE service levels are given in RG 1.57 .
Rules for “design by analysis” are specified in NE-3200. The theory of failure used in NE-3200 is the maximum shear stress theory, in which the term “stress intensity” is defined as twice that of the maximum shear stress, and equals the difference between the algebraically largest and smallest principal stresses at a given point . The basic ASME BPV Code, Section III criteria are expressed in terms of allowable stress intensities. The code criteria are further complicated by the fact that the ASME BPV III defines different categories of stresses. These are denoted as :
- 1) General primary membrane stress
- 2) Local primary membrane stress
- 3) Primary bending stress
- 4) Secondary stress
- 5) Peak stress
As implied by the name, a primary stress (categories 1, 2, and 3) is one which results from an imposed loading and one which is required to satisfy the law of equilibrium. In particular, a general membrane stress is defined as a global stress which extends in such a large region that an effective redistribution is not possible if the material yields. On the other hand, a local primary membrane stress is the one which results at major structural discontinuities such as the boundary between a cylinder and the dome. Such localized stresses are able to be redistributed to the adjacent areas when material yields. A primary bending stress is the through thickness bending caused by external loads. Primary bending stresses may appear in such geometries as flat heads and stiffeners, but do not exist in axisymmetric thin shell structures subject to axisymmetric loadings such as internal pressure. A secondary stress is generally extreme fiber stress developed by self-constraint of the structure, thus it is self-limiting. That is, the conditions that cause the stress will be satisfied when the material yields or distorts locally, and one application of the load will not result in failure. A peak stress is the increment of stress addition to the primary plus secondary stresses, due to local discontinuities or local thermal, including stress concentration. The peak stress is mainly for the purpose of fatigue analysis, and no limit is specified on it . Some relaxation of allowable stress limits is allowed if a plastic analysis is performed. Rules of such relaxation related to plastic analysis are presented in NE-3228.
Details of deriving aforementioned stress categories and detailed stress limits for various service levels are presented in ASME Code Subsection NE and the Chapter 9 of Companion Guide to the ASME Boiler & Pressure Vessel Code . The stresses calculated using a 3-D finite element model mentioned in Section
3.2.2 need to be categorized before compared with allowable stresses. Table NE-3217-1 in ASME Code is provided to assist the designer for this purpose.
Besides the option of “design by analysis,” subsection NE also specifies rules for “design by formula” in NE-3300. However, application of this option in steel containment design is limited only to service levels A and B without substantial mechanical or thermal load. For service levels C and D, and also for service levels A and B with significant mechanical or thermal load, the design by analysis option must be used.