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(What steel alloy is no longer recommended for services susceptible to HTHA?)
Comprehensive and Detailed Explanation From Exact Extract:
According to API RP 571 and API RP 941 (Nelson Curves), 1 Cr--0.5 Mo steel is no longer recommended for services susceptible to High-Temperature Hydrogen Attack (HTHA).
API RP 941 documents industry experience showing that 1 Cr--0.5 Mo steels have suffered HTHA damage below previously assumed safe operating limits, due to insufficient carbide stability. As a result, this material has been removed from the Nelson Curves as an acceptable choice for new construction in hydrogen service.
By contrast:
Mn--0.5 Mo, C--0.5 Mo, and 1.25 Cr--0.5 Mo steels retain higher resistance due to more stable carbide structures.
Referenced Documents (Study Basis):
API RP 571 -- Section on High-Temperature Hydrogen Attack
API RP 941 -- Updated Nelson Curves and Material Recommendations
Which of the following statements is most accurate with regard to thermal fatigue?
API RP 571 on Thermal Fatigue emphasizes:
''Thermal fatigue results from cyclic thermal stresses during startup, shutdown, or transient operations.''
''The best mitigation strategy is to control heating and cooling rates to reduce temperature differentials and resulting stress.''
(Reference: API RP 571, Section 4.2.1.5 -- Thermal Fatigue)
Hence, option D correctly identifies the preventive action.
To avoid cooling water scaling, process side inlet temperatures should be below:
API RP 571 under Cooling Water Corrosion and Scaling:
''Cooling water scaling tends to occur more rapidly as water temperature increases. To minimize scaling, inlet temperatures to exchangers should be kept below 140F (60C) where possible.''
''Above this temperature, calcium carbonate and other salts tend to precipitate more readily.''
Thus, option A is correct.
(Deaerators that have not been postweld heat-treated are commonly subject to:)
Comprehensive and Detailed Explanation From Exact Extract:
Per API RP 571, deaerators often operate in environments where concentrated caustic solutions can form locally due to oxygen scavengers and water chemistry control additives.
When postweld heat treatment (PWHT) is not performed:
High residual welding stresses remain
Conditions are ideal for caustic stress corrosion cracking (Caustic SCC)
API RP 571 identifies deaerators as classic examples of equipment that has experienced caustic SCC when PWHT was omitted.
Referenced Documents (Study Basis):
API RP 571 -- Section on Caustic Stress Corrosion Cracking
Which damage mechanism will not benefit much from PWHT mitigation?
API RP 571 discusses Hydrogen-Induced Cracking (HIC) and Blistering:
''Postweld heat treatment (PWHT) does not significantly reduce susceptibility to HIC, as the mechanism is primarily driven by hydrogen charging in wet HS environments and steel cleanliness, not residual stresses.''
''PWHT is more effective for SSC, SOHIC, and other stress-driven mechanisms.''
Thus, HIC will not benefit much from PWHT, making option C correct.
