Standard and Vanadium enhanced 2.25Cr1Mo plate steels (ASTM A387 gr. 22 and A542 type D) are commonly used for the manufacturing of heavy reactors for use in petroleum refining plants. These reactors are fabricated from heavy wall plates, forged shells (thickness up to 300-350mm), forged nozzles and fittings. They are subjected to thermal cycles (stop and go) and to severe service conditions (high pressure, high hydrogen partial pressure). A primary concern for end-users is defining the Minimum Pressurizing Temperature (MPT). This temperature is the lowest temperature at which the vessel can be repressurized after shutdown. This minimum temperature insures no risk of brittle fracture, and is defined by fracture mechanics and/or CVN approaches and calculation. This paper presents the methodology of MPT determination and the impact of ageing and exposure to hydrogen on material mechanical properties and on the value of the MPT. The MPT determination method is explained by using a virtual pressure vessel that is representative of reactor vessels found in petroleum refineries.
One of the leading risks in the petrochemical / refining industries is the risk of brittle (i.e. unstable) failure of pressure vessels. Heavy wall low-alloy pressure vessels are designed to operate at high temperatures, and at high pressures and hydrogen partial pressures leading to many problems. During unit shutdowns, pressure and temperature are reduced to levels much lower than exposed to in operation. After conclusion of the maintenance procedures, the operation staff will restart the unit, raising the vessel pressure and temperature to the operational set points, thus starting a new production cycle. During the early stages of the start-up process, strict attention must be paid to metal temperature and internal pressure to insure sufficient toughness of the reactor material. This is done by controlling the heating and pressurization rate of the vessel, avoiding conditions that could cause brittle failure. This dramatic and unacceptable situation could occur if the low-alloy steel temperature is not high enough to achieve a level of fracture toughness that is sufficient to mitigate crack instability and catastrophic failure. Ensuring sufficient toughness at every temperature deals with definition of Minimum Pressurizing Temperature (MPT) which then serves as a guideline for future operation of a given reactor. A vessel start-up program that is based on MPT concept includes a system of steps of temperature-pressure couples that must be abided by when the production cycle is restarting. In the case of a newly fabricated reactor, the definition of MPT is quite easy because all needed material properties can be required at the delivery of the pressure vessel and then the reactor is designed to avoid the risk of brittle failure. End-users are aware of the problems that can occur and safety margins are used. Evaluation of alloy embrittlement is also taken into consideration. Steel makers provide guarantees to their customers that the steels they produce have a satisfactory resistance to temper embrittlement. This guarantee stems from confidence gained in testing of the steels that comprise the pressure boundary.
