Design conditions in ASME B31.3 are specifically intended for pressure design. The design pressure and temperature are the most severe coincident conditions, defined as the conditions that result in the greatest pipe wall thickness or highest required pressure class or other component rating. Design conditions are not intended to be a combination of the highest potential pressure and the highest potential temperature, unless such conditions occur at the same time. Although it is possible for one operating condition to govern the design of one component in a piping system (and be the design condition for that component) and another to govern the design of another component, this is a relatively rare event. If it is encountered, the two different components in the piping system should have different design conditions.
Design Pressure
In determining the design pressure, all conditions of internal pressure must be considered. These include thermal expansion of trapped fluids, surge, and failure of control devices. The determination of design pressure can be significantly affected by the means used to protect the pipe from over pressure. Process piping systems are permitted to be used without protection of safety relief valves. However, in the event that none are provided on the pipe (or on attached equipment that would also protect the pipe), the piping system must be designed to safely contain the maximum pressure that can occur in the piping system, including consideration of failure of any and all control devices. Such a pressure could be, for example, the dead-head pressure of a pump. Unlike some industry standards, there is no consideration given to whether the source of failure depends on single- versus double-contingency events.
Although all these pressures must be considered, they do not necessarily become the design pressure. The Code permits pressure and temperature variations per para. 302.2.4. If the event being considered complies with the Code requirements of 302.2.4, the allowable stress and/or component pressure rating may be exceeded for a short time, as discussed below in Section 3.5. Although this is often considered to be an allowable variation above the design condition, the variation limitations are related to the maximum allowable working pressure of the piping, not the design conditions, which could be lower than the maximum allowable pressure at temperature. #Little_PEng