Selecting and Purchasing Chromium-Molybdenum Alloys for Refinery Services
One of the key processes in the refining or upgrading of crude oil and bitumen is hydrotreating or more generally, hydroprocessing. Hydroprocessing adds hydrogen to fractions of crude oil or bitumen (typically naphtha or gas oil) to remove hydrogen sulfide and/or nitrogen compounds or to modify hydrocarbons’ structures. As the processes involve high temperatures and high pressures, alloy steels are chosen to resist hydrogen attack and hydrogen sulfide corrosion. Vessels (reactors and separators), heat exchangers and furnace coils are called upon to meet these challenges. The selection of candidate alloys and stainless steels involve the use of the Nelson Curves (API 941) for resistance to hydrogen and of the Couper-Gorman curves for resistance to hydrogen/hydrogen sulfide corrosion. In the operating temperatures and pressures required, it is apparent that to resist hydrogen attack, chromium-molybdenum (Cr-Mo) alloy steels are required. Much experience with hydroprocessing has shown that embrittlement in these steels occurs with operating age.
Evaluating the candidate Cr-Mo alloy steels, there are basically five choices: 1Cr-1/2Mo, 11/4Cr-1/2Mo, 2¼Cr-1Mo, 2¼Cr-1Mo-V and 3 Cr-1Mo-V. Selecting an alloy is based upon the following factors:
- Hydrogen partial pressure
- Hydrogen and hydrogen sulfide partial pressures
- Type of hydrocarbon being processed: naphtha, gas-oil or distillate
- Embrittlement factor
- Maximum thickness required
- Desired lifetime before embrittlement would occur
Further information of alloy selection and specification may be found in the API Recommended Practices: 934 A through E.