Guidelines to Determine MPT for Existing Vessels
Note that “minimum pressuring temperature” (MPT) is the same as “minimum design metal temperature” (MDMT).
Most vessels built since 1969 to Company specifications should already have MPT on the vessel drawings, or on Safety Instruction Sheets (SISs).
For carbon and low alloy steel vessels without an MPT specified, Figure UCS-66, along with materials of construction and vessel thickness, can be used to establish MPT for vessels up to 6 inches thick. For vessels made with materials shown on Figure UCS-66, the MPT can be established directly from the curves if component thicknesses are known. For materials not listed in UCS-66, see the next subsection.
The MPT is determined by the intersection of component thickness with the appropriate curve on Figure UCS-66. Consider only welded parts such as shells, heads, channels, and integrally reinforced nozzles for MPT. The component with the highest MPT sets the MPT for the vessel.
Reinforcing pad nozzles or small nozzles without reinforcement do not need to be considered for MPT unless the risks of brittle fracture (i.e., low temperature, autorefrigeration, adjacent facilities, etc.) warrant extra precaution. Fractures of vessels with reinforcing pad nozzles generally occur in the shell plate at the reinforcingpad-to-shell fillet weld. Integrally reinforced nozzles, however, should be considered for MPT.
The following guidelines clarify the use of Figure UCS-66 to establish MPT:
1. The thickness of vessel components refers to the thickness at a weld.
2. An MPT does not need to be established for nonwelded parts like cover flanges or heat exchanger channel covers.
3. To use the curves for normalized material, vessel records must indicate normalized material was used.
4. For P-1 carbon steel vessels that were stress-relieved but were not required to be stress-relieved by Code, the MPT may be 30°F lower than is given by the exemption curve on Figure UCS-66. This is consistent with Code Paragraph UCS-68(c). Normally carbon steel vessels 1¼ inch and less in thickness are not required to be stress-relieved by Code rules. See ASME Code, Section VIII, Division 1, Table UCS-23, to determine whether a steel is a P-1 steel.
5. All grades of SA 285 and SA 515 steels thicker than 3/4 inch should be assigned to Curve A rather than to Curve B.
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