What Is Design Temperature?
Design temperature is the maximum (or minimum) metal temperature that a piping component is expected to reach during any operating, startup, shutdown, or upset condition. It is the temperature used, together with design pressure, to select materials, determine allowable stresses, and choose pressure ratings for all piping components.
Design Temperature vs. Operating Temperature
| Parameter | Definition | Determined By |
|---|---|---|
| Normal operating temperature | Fluid temperature during steady-state operation | Process simulation |
| Maximum operating temperature | Highest fluid temperature during any foreseeable condition (startup, regeneration, steam-out, upset) | Process engineering |
| Design temperature (high) | Maximum metal temperature used for component design; typically equals or exceeds maximum operating temperature | Piping engineer per ASME B31.3 |
| Design temperature (low / MDMT) | Minimum design metal temperature; lowest temperature the piping may experience (autorefrigeration, ambient, hydrotest in cold weather) | Piping engineer per ASME B31.3 |
How to Determine Design Temperature
| Condition | ASME B31.3 Guideline |
|---|---|
| Insulated piping | Design temperature equals the maximum fluid temperature (metal temperature approximately equals fluid temperature) |
| Uninsulated piping | For fluid temperatures above 38 C (100 F), design temperature may be taken as 95% of fluid temperature for NPS 4 and larger, or fluid temperature for smaller sizes |
| Externally heated piping | Design temperature equals the maximum temperature achievable by the heating medium |
| Steam tracing | Design temperature accounts for both process fluid and tracing steam temperature |
| Solar radiation | For uninsulated outdoor piping, add up to 15 C (30 F) to the maximum ambient temperature for equipment selection |
Impact on Material Selection
Design temperature directly determines which materials may be used and their allowable stress values:
| Temperature Range | Material Consideration | Example |
|---|---|---|
| Below -46 C (-50 F) | Austenitic stainless steel or nickel alloys required; carbon steel not permitted | Cryogenic LNG, ethylene plants |
| -46 C to -29 C (-50 F to -20 F) | Impact-tested carbon steel (killed, fine-grain) or low-temperature grades (ASTM A333 Gr.6) | Cold climate, autorefrigeration |
| -29 C to 427 C (-20 F to 800 F) | Standard carbon steel (ASTM A106 Gr.B, A234 WPB) is acceptable | Most process piping services |
| 427 C to 593 C (800 F to 1100 F) | Chrome-moly alloy steels required (ASTM A335 P11, P22, P91) to resist creep | High-temperature steam, heater outlets |
| Above 593 C (1100 F) | High-alloy or nickel-based alloys (Inconel, Incoloy) | Furnace piping, reformer outlets |
Impact on Pressure Rating
The allowable pressure for flanges, valves, and fittings decreases as design temperature increases. The ASME B16.5 pressure-temperature rating tables define these relationships:
| ASME B16.5 Class | Max. Pressure at 38 C (100 F) | Max. Pressure at 260 C (500 F) | Max. Pressure at 427 C (800 F) |
|---|---|---|---|
| Class 150 | 19.6 barg (285 psig) | 12.1 barg (175 psig) | 8.4 barg (122 psig) |
| Class 300 | 51.1 barg (740 psig) | 33.3 barg (483 psig) | 23.1 barg (335 psig) |
| Class 600 | 102.1 barg (1480 psig) | 66.6 barg (965 psig) | 46.2 barg (670 psig) |
Values shown for ASTM A105/A216 WCB (Group 1.1). Actual ratings depend on material group.
Both design temperature and design pressure are recorded in the piping specification header and in the title block of every piping isometric. They form the basis for all mechanical design calculations and material selections throughout the project.
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