Pressure vessels brittle fracture

Brittle fracture is probably the most insidious type of pressure-vessel failure. Without brittle fracture, a pressure vessel could be pressurized approximately to its ultimate strength before failure. With brittle behavior some vessels have failed well below their design pressures (which are about 25 percent of the theoretical bursting pressures). In order to reduce the possibility of brittle behavior. Division 2 and Sec. Ill require impac t tests. 

API RP 920, Prevention of Brittle Fracture of Pressure Vessels, 1st ed., March 1990 (ANSI/API Publ 920-1992). 

In practice, vapor release will not be spherical, as is assumed in the method. A release from a cylinder burst may produce overpressures along the vessel s axis, which are 50% lower than pressures along a line normal to its axis. If a vessel ruptures from ductile, rather than brittle, fracture, a highly directional shock wave is produced. Overpressure in the other direction may be one-fourth as great. The influences of release direction are not noticeable at great distances. Uncertainties for a BLEVE ate even higher because of the fact that its overpressure is limited by initial peak-shock overpressure is not taken into account. 

Since the aggregate risks from Process Unit 2 are largely the result of single event—brittle fracture—the qualitative hazard assessment identified potential safeguards that could be put in place to prevent a brittle fracture occurrence. It was decided that the best option would be to install an emergency shutdown system in Process Unit 2 to prevent pressuring the nitrogen vapor vessel if a cold temperature situation was present. 

Excessively low temperature may involve the hazard of brittle fracture. A vessel that is out of use in cold weather could be at a subzero temperature and well below its nil-ductility temperature. In startup, the vessel should be warmed slowly and uniformly until it is above the NDT. A safe value is 38°C (100°F) for plate if the NDT is unknown. The vessel should not be pressurized until this temperature is exceeded. Even after the NDT has been passed, excessively rapid heating or cooling can cause high thermal stresses. 

Brittle fracture is always a concern with heavy wall vessels. An 80 to 100°F (27 to 38° C) minimum hydrostatic test temperature is often specified to minimize the possibility of brittle fracture during hydrostatic testing. To minimize the possibility of brittle fracture of heavy wall reactors during start up and shutdown, reduced pressure below 200 to 300° F (90 to 150°C) is usually specified. Typical limitations are 40% of the design pressure or 20% of the original hydrostatic test pressure. With the advent of temper embrittlement resistant steels and weld metal, some refiners feel reduced pressure is only required below 100°F (38°C). 

In practice, many vessels designed for outdoor service in moderate climate are constructed of ASME SA-516 Grade 70, a fully killed, fine grain practice pressure vessel quality carbon steel plate for moderate and lower temperature service, so as to provide protection against brittle fracture. 

Most localities mandate compliance with national engineering codes such as the ASME (American Society of Mechanical Engineers) Boiler Pressure Vessel Code [1]. These codes govern mechanical design and provide the maximum allowable stresses and required low-temperature toughness, as a function of temperature, for approved materials. The codes also define requirements for fabrication procedures such as PWHT. All codes contain rules that ensure the selected material of construction will not be susceptible to brittle fracture at the minimum design temperature. Users of the codes must develop a familiarity with these rules. Eor example, code rules help avoid specifying materials at temperatures for which they are not permitted. 

ASME Section VIII, Division 1 requirements to prevent brittle fracture are 15 ft. -lb. Charpy Keyhole applied only below -20°F. Until recently, these requirements were thought to protect against brittle fracture. In the past few years, however, a considerable number of catastrophic brittle fractures in thick-wall pressure vessels have occurred throughout all industry. In each instance, the code impact values seemed to have been met or exceeded. 

The hazard of brittle-fracture is lessened as the average and local stress levels are lowered. Both the ASME Unfired Pressure Vessel Code (Par. UCS-66) and the Code for Pressure Piping (Par. 323.2.2) recognize this by allowing materials to be used below the transition temperature. Where the allowable stress is reduced to 40% of the normal allowable, the ASME Code permits such material to be used without limitation. The code for pressure piping sets an allowable stress of 15% of the maximum allowable without impact test. 

Pressure vessels are also subject to cyclic stress. Cyclic stress arises from pressure and/or temperature cycles on the metal. Cyclic stress can lead to fatigue failure. Fatigue failure, discussed in more detail in Module 5, can be initiated by microscopic cracks and notches and even by grinding and machining marks on the surface. The same (or similar) defects also favor brittle fracture. 

Minimum pressurization-temperature (MPT) curves specify the temperature and pressure limitations for reactor plant operation. They are based on reactor vessel and head stress limitations and the need to preclude reactor vessel and head brittle fracture. Figure 4 shows some pressure-temperature operating curves for a pressurized water reactor (PWR) Primary Coolant System (PCS). 

Ductility is the plastic response to tensile force. Plastic response, or plasticity, is particularly important when a material is to be formed by causing the material to flow during the manufacture of a component. It also becomes important in components that are subject to tension and compression, at every temperature between the lowest service temperature and the highest service temperature. Ductility is essential for steels used in construction of reactor pressure vessels. Ductility is required because the vessel is subjected to pressure and temperature stresses that must be carefully controlled to preclude brittle fracture. Brittle fracture is discussed in more detail in Module 4, Brittle Fracture. 

The Nil-Ductility Transition (NDT) temperature, which is the temperature at which a given metal changes from ductile to brittle fracture, is often markedly increased by neutron irradiation. The increase in the NDT temperature is one of the most important effects of irradiation from the standpoint of nuclear power system design. For economic reasons, the large core pressure vessels of large power reactors have been constructed of low carbon steels. 

The brittle fracture temperature of steel is a temperature below which its ductility has decreased so that brittle fracture of material is possible. Neutron flux irradiation of the reactor core causes damage in reactor pressure vessel (RPV) wall material and its brittle fracture temperature decreases. If the RPV is cooled below brittle fracture temperature, there is a danger of brittle fracture if there is an initial crack in the RPV wall material. The phenomenon is called pressurised thermal shock (PTS) and its worst consequence is a catastrophic failure of the RPV. PTS is more relevant for PWRs than for BWRs because PWRs generally have a narrower water gap between the reactor core and the RPV wall than BWRs. 

Kusuki A, Gotoh M, Watada M, Ohtani M and Toyomatsu H (1996), Surveillance test and integrity evalnation of PWR reactor pressure vessel in Japan against brittle fracture , Proceedings of 9" International Symposium on REACTOR... 

Margolin B, Shvetsova V, Gulendo A and Kostylev V (2007), Development of Prometey local approach and analysis of physical and mechanical aspects of brittle fracture of RPC steels, International Journal of Pressure Vessels and Piping,... 

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