scope:

ASPECTS OF CONSTRUCTION COVERED BY THESE RULES

(a) Subsection NH contains rules for materials, design, fabrication, examination, testing, and overpressure relief of Class 1 components, parts, and appurtenances that are expected to function even when metal temperatures exceed those covered by the rules and stress limits of Subsection NB and Tables 2A, 2B, and 4 of Section II, Part D, Subpart 1.

(b) The rules of Subsection NH are applicable to Class 1 components independent of the type of contained fluid - water, steam, sodium, helium, or any other process fluid.

(c) The stress limits and design rules of Subsection NB are applicable only to service conditions where creep and relaxation effects are negligible. Consequently, the rules of Subsection NB only guard against the time-independent failure modes-ductile rupture, gross distortion (buckling and incremental collapse), and fatigue. Therefore, those portions of the component, part, or appurtenance that are at all times experiencing temperatures within the range covered by Tables 2A, 2B, and 4 of Section II, Part D, Subpart 1 may be designed in compliance with the rules of NH-3000 in Subsection NH, or alternatively, in compliance with the rules of NB-3000. In addition, the rules of Subsection NH extend specific rules of NB-3000 to elevated temperature service, provided the designer can demonstrate that the combined effects of temperature, stress level, and duration of loading do not introduce significant creep effects.

(d) At temperatures and loading conditions where creep effects are significant, the design analysis shall also consider the time-dependent material properties and structural behavior by guarding against the four modes of failure shown below:

(1) ductile rupture from short-term loadings

(2) creep rupture from long-term loadings

(3) creep fatigue failure

(4) gross distortion due to incremental collapse and ratcheting Brief guidelines are also provided in Subsection NH for the three modes of failure shown below:

(5) loss of function due to excessive deformation

(6) buckling due to short-term loadings

(7) creep buckling due to long-term loadings

(e) Design procedures and materials data not contained Subsection NH may be required to ensure the integrity the continued functioning of the structural part during the specified service life. For example, the rules do not provide methods to evaluate deterioration which may occur service as a result of corrosion, mass transfer phenomena, radiation effects, or other material instabilities. Nor do the rules ensure continued functional performance of deformation-sensitive structures such as valves and pumps.

TEMPERATURE AND SERVICE LIFE LIMITS

The rules of Subsection NH shall not be used for structural parts which will be subjected either to metal temperatures or to times greater than those values associated with the Smt data for the specified material (see Appendix I-14).

ORGANIZATION OF SUBSECTION NH RULES

In general, the numbering of rules in Subsection NH follows the numbering system used in Subsection NB. References to Appendices are to this Subsection unless otherwise identified.