Startup and Shutdown at Ambient Temperature
Pressure vessels that are installed at geographic locations with severe winter climates may be required to be at maximum operating pressure at low ambient temperatures during transient startup and/or shutdown conditions. The minimum design metal temperature for this situation should be the lowest ambient winter temperature that would be expected for a startup or shutdown during unusually severe winter conditions. Figure 2-2 of API 650 (available in the Tank Manual) can be used as a guideline for establishing minimum design metal temperature; however, somewhat warmer temperatures are usually satisfactory for normal startups and shutdowns, especially in locations with temperate climates. Engineering judgment should be used to determine the probable minimum temperature during vessel pressurization. The operating pressure must not exceed 40% of the MAWP until the vessel is warmed up to the minimum design metal temperature (35% of the MAWP for vessels built in 1999 and later).
It is necessary to provide adequate resistance to brittle fracture during startup and shutdown at low ambient temperatures. All of the new Code requirements must be adhered to for the minimum design metal temperature. However, transient startup and shutdown conditions are not considered to be critical service, unlike continuous operation at low temperatures.
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