1. JUDGING STRESS ACCEPTABILITY: RULES OF THE ASME CODES

The calculations of topic C permit us to estimate pressure vessel stresses on the basis of simplified assumptions. More detailed calculations (e.g., by using a finite element computer code) would permit estimation of stresses on more realistic bases. However, in either case, a design process is not complete until we judge the acceptability of the calculated stresses. These notes (topic D) provide an overview of some design rules of the ASME code that relate to such judgments. 1.2 Basis for Judgments The acceptability of a given numerical stress value depends on an interrelated set of considerations:- the nature (or classification) of the stress;- vessel material property information;- expected operating conditions throughout the vessel lifetime; and- the degree of quality assurance employed. The dependence on stress classification is perhaps the most surprising on this list. For illustration, consider a calculated pressure stress of 200 MPa and a calculated thermal stress of 200 MPa. The thermal stress will usually be judged to be much less severe than the same value of pressure stress. So we must know how a stress is classified before judging its severity. The fact that there is a dependence on quality assurance also has counter intuitive features. The term quality assurance encompasses design analyses, material testing, fabrication controls, and inservice inspections. For example, the ASME code Section III provides requirements for nuclear components and extensive quality assurance actions are mandated. A factor of safety is employed that i