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(a) Maximum. Except as required in UW-2(d)(3),the maximum temperature used in design shall be not less than the mean metal temperature (through the thickness)expected under operating conditions for the part considered (see 3-2). If necessary, the metal temperature shall be determined by computation or by measurement from equipment in service under equivalent operating conditions. See also U-2(a).
NOTE: The user and Manufacturer are cautioned that certain fabrica-tion details allowed by this Division may result in cracking at weldsand associated heat-affected zones (HAZ) for vessels designed foruse at elevated temperature. WRC Bulletin 470,“Recommendationsfor Design of Vessels for Elevated Temperature Service”has informa-tion that may prove helpful to the vessel designer. WRC Bulletin 470contains recommended design details for use at elevated tempera-ture service, which is for the purposes of this Division, when the al-lowable stresses in Section II, Part D are based on time-dependentproperties. The use of these details does not relieve the Manufac-turer of design responsibility with regard to consideration of stress-es associated with both steady state conditions and transient events,such as startup, shutdown, intermittent operation, thermal cycling,etc., as defined by the user.
(b) Minimum. The minimum metal temperature used in design shall be the lowest expected in service except when lower temperatures are permitted by the rules of this Division10 (see UG-116, UCS-66,and UCS-A-3). The minimum mean metal temperature shall be determinedby the principles described in(a) above. Considerationshall include the lowest operating temperature, operational upsets, auto refrigeration, atmospheric temperature, and any other sources of cooling [except aspermitted in(f)(3)below for vessels meeting the require-ments of(f) below]. The MDMT marked on the nameplate shall correspond to a coincident pressure equal to the MAWP. When there are multiple MAWP’s, the largest value shall be used to establish the MDMT marked on the nameplate. Additional MDMT’s corresponding with other MAWP’s may also be marked on the nameplate.
(c)Design temperatures that exceed the temperature limit in the applicability column shown in Section II, PartD, Subpart 1, Tables 1A, 1B, and 3 are not permitted. In addition, design temperatures for vessels under externalpressure shall not exceedthe maximum temperatures given on the external pressure charts.
(d)The design of zones with different metal tempera-tures may be based on their determined temperatures.
(e)Suggested methods for obtaining the operating tem-perature of vessel walls in service are given in Nonmandatory Appendix C.
f)Impact testing per UG-84 is not mandatory for pressure vessel materials that satisfy all of the following:
(1)The material shall be limited to P‐No. 1, Gr. No. 1 or 2, and the thickness, as defined in UCS-66(a)[see also Note (1) in Figure UCS-66.2], shall not exceed that given in (-a) or (-b) below:
(-a)1/2in. (13 mm) for materials listed in Curve AofFigure UCS-66;
(-b)1 in. (25 mm) for materials listed in Curve B,C, or D of Figure UCS-66.
(2)The completed vessel shall be hydrostatically tested per UG-99(b)or UG-99(c) or 27-4. Alternatively, the completed vessel may be pneumatically tested in accordance with 35-6.
(3)Design temperature is no warmer than 650°F (345°C) nor colder than −20°F (−29°C). Occasional operating temperatures colder than −20°F (−29°C) are acceptable when due to lower seasonal atmospheric temperature.
(4)The thermal or mechanical shock loadings are not a controlling design requirement. (See UG-22.)
(5)Cyclical loading is not a controlling design requirement. (See UG-22.)