Fatigue can lead to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material, known as the fatigue or endurance limit. Sources for fatigue include thermal/stress cycling, rotation or vibration, like that produced by reciprocating compressors and positive displacement pumps. The guidance presented in the ASME B31.3 for checking cyclic stress levels is based on low cycle/high stress fatigue, e.g. thermal stresses associated with infrequent start-up/shutdown cycles. In a vibrating system, the concern is high cycle/low stress cycling but ASME B31.3 does not explicitly address high-cycle fatigue.
High cycle fatigue is of particular importance in the presence of flaws, e.g. fabrication cold (hydrogen) cracks. Under these circumstances, the majority of the components’ life will be spent propagating the crack. In a nominally defect free welded joint, fatigue life will incorporate a substantial crack initiation period, as well as a crack propagation period. Understandably therefore, the fatigue design guidance presented in ASME B31.3 is based on nominally defect free welds. The sources of fatigue loading that have to be considered in the design of pipes are more numerous than those for pressure vessels.
In addition to internal pressure fluctuations, pipes may also be subjected to external forces from direct loads, bending moments, and torques resulting from. (These low cycle fatigue loads are accounted for in pipe flexibility design analyses). In relatively flexible small diameter pipes, a number of failures have been caused by high-cycle, resonant vibrations due, for example, to external vortexes, internal turbulent flow regimes, sustained relief valve discharge, etc. If the frequency of any of the modes of these vibrations coincide with the natural frequency of the pipe, substantial resonant vibrations can be produced. Nevertheless, in the absence of complex time-history cumulative damage analyses of the small diameter piping systems, small diameter piping support design is more often than not based on field experience.
Corrosion fatigue: In addition to design features that cause stress concentration, deep scratches, sharp corners, weld profiles etc, all can act as initiation points for fatigue. Further, in the presence of a corrosive environment, a pit can initiate fatigue. Often, multiple initiation points result dependent on the pit frequency. Short of changing the environment or stresses, shot peening is used to place the surface of a potential initiation site in compression. Stress relief, use of corrosion inhibitors and protective coatings, all have had some success in resisting corrosion fatigue.
Thermal fatigue occurs in equipment that experiences frequent changes in temperature. For instance, each start-up and shutdown induces thermal stresses, which, if significant in number, can lead to thermal fatigue. In particular, coke drums and reactors (heavy section welds) in cyclic temperature service are prone to thermal fatigue. Austenitic stainless steel is often used to clad the internal surfaces of thick walled vessels to protect the alloy steel substrate from, say, H2S/H2 environments.
Austenitic stainless steel exhibit significantly higher thermal expansion (more than 30%) than low alloy steels and start-up and shutdown can cause plastic deformation of the plastic layer and adjacent base material. Repeated thermal cycles can induce high strain, low cycle fatigue of the cladding. Roll bonded cladding is significantly more resistant to fatigue than weld overlay cladding. In the latter case, the requirement for some ferrite in the weld deposit induces sigma phase formation during post weld heat treatment that reduces fatigue resistance. To minimise the risk of thermal fatigue it is recommended that the heating and cooling rates in hydrotreater plants are slower than 40°C/hr.
