If the piping is to be designed according to ASME B31.3, refer Appendix S, PIPING SYSTEM STRESS ANALYSIS EXAMPLES, and you should know how the design conditions are defined.
ASME B31.3 code defines the design conditions as follows:
Design Conditions
The design conditions establish the pressure rating, flange ratings, component ratings, and minimum required pipe wall thickness in accordance with para. 301.2.1. -- Just consider operating conditions for piping flexibility analysis.
301.2 Design Pressure
301.2.1 General
(a) The design pressure of each component in a piping system shall be not less than the pressure at the most severe condition of coincident internal or external pressure and temperature (minimum or maximum) expected during service, except as provided in para. 302.2.4.
(b) The most severe condition is that which results in the greatest required component thickness and the highest component rating.
(c) When more than one set of pressure–temperature conditions exist for a piping system, the conditions governing the rating of components conforming to listed standards may differ from the conditions governing the rating of components designed in accordance with para. 304.
(d) When a pipe is separated into individualized pres-sure containing chambers (including jacketed piping, blanks, etc.), the partition wall shall be designed on the basis of the most severe coincident temperature (minimum or maximum) and differential pressure between the adjoining chambers expected during service, except as provided in para. 302.2.4.
301.3 Design Temperature
The design temperature of each component in a piping system is the temperature at which, under the coincident pressure, the greatest thickness or highest component rating is required in accordance with para. 301.2. (To satisfy the requirements of para. 301.2, different components in the same piping system may have different design temperatures.) In establishing design temperatures, consider at least the fluid temperatures, ambient temperatures, solar radiation, heating or cooling medium temperatures, and the applicable provisions of paras. 301.3.2, 301.3.3, and 301.3.4.