What are design pressure and design temperature, and how are they determined?

Design pressure is the pressure used for equipment mechanical design, typically set at operating pressure plus a margin (10-15% or minimum 25 psi above normal operating). Design temperature is the metal temperature used for material selection and thickness calculation, set at maximum operating temperature plus margin (usually 25-50 degrees F). These values account for: process upsets, safety valve settings, startup/shutdown conditions, and normal variations. Higher design conditions increase equipment cost and weight. Values must consider all operating cases including abnormal conditions. ASME codes specify minimum requirements for vessels and piping.