In the early 1980s, the Electric Power Research Institute (EPRI) identified 22 mechanisms that could result in the failure of a boiler tube. One of those mechanisms involved corrosion fatigue. At that time, this mechanism was not well understood. Moreover, there were no permanent solutions to address this mechanism. In 1992, approximately 10 years after it was identified by EPRI as a failure mechanism, corrosion fatigue was the leading cause of boiler tube failures (as quantified by lost availability).
Corrosion fatigue (sometimes referred to as stress enhanced corrosion fatigue) relates to the operation of a boiler. In particular, it relates to thermal cycling of the waterwall tubes (and the resultant expansion and contraction of the tubes), in conjunction with corrosion produced by the feedwater. As such, susceptibility to corrosion fatigue is influenced by the number of unit starts, the operating hours, cycle chemistry and chemical cleaning practices.
As the waterwall tubes are heated up, they will expand. The greatest expansion in any given tube will take place on the fire side. As the cold side of the tube is at a lower temperature, it will experience a commensurately reduced degree of expansion. As a result of this differential expansion, the fire side of any given tube is subject to compressive stresses, while the cold side is subject to tensile stresses.
These stresses are not problematic as long as the tube is able to expand and contract freely. However, if the tube is restrained in some manner, e.g., by a welded attachment, additional stresses are introduced into either the fireside or cold side of the tube. If these tensile stresses are of sufficient magnitude and the boiler is subject to repeated thermal cycling, cracking may result.
Corrosion fatigue is caused by the combined action of a cyclic tensile stress and a corrosive environment. It is characterized by a premature failure of a cyclically loaded part and is a serious cause of failure that could require expensive repairs. There are many ways to control the various forms of environmental cracking, including material selection, modification of the environment, protective coating, and reduction in residual stresses and by changing the design to lower tensile stress levels.
The fact that corrosion fatigue resistance of boiler tube material is influenced by the boiler water chemistry has been apparent for a number of years. Industry experience has also recognized that, while water chemistry and operation variables do influence corrosion fatigue, strain levels are of primary importance. Corrosion fatigue occurs only at selected locations in a boiler where high local stress can occur such as adjacent to the attachment. Subsection 6.3 from the EPRI Manual for Investigation and Correction of Boiler Tube Failures shows typical locations in the boiler furnace where corrosion fatigue cracking can occur. However, similar units do not always have similar corrosion fatigue behavior. This would indicate that operation and boiler water treatment can play a significant role.
Generally, strain on a boiler tube will increase during start-up and go through to a maximum. There are two sources of strain: pressure-induced strain and thermally induced strain. Pressure-induced strain will continue to increase until steady state pressure is achieved at an operating load. On top of these strains are thermally induced strains. The maximum thermal strains will occur when the largest thermal gradients occur in the boiler, typically near the end of the start-up transient.
Corrosion fatigue failures result from cyclic stressing of metal in a corrosive environment. This condition causes more rapid failure than that caused by either cyclic stressing or corrosion alone. In boilers, corrosion fatigue cracking can result from the continued breakdown of protective magnetite film due to cyclic stresses.
Examples of this type of failure include cracks in boiler components or tubes that are more highly stressed (such as bends or at other areas close to backstays or supports) when a boiler undergoes thermal fatigue due to repeated start-up and shut-down.
In a leak-type corrosion fatigue failure, the fluid escaping from the leak is limited, and as such will not result in a pressure surge in the boiler. Instead, the fluid will be carried downstream with the flue gas, ultimately passing out of the boiler through the stack. The leaking fluid does not pose a threat to operating personnel. However, if the failure is a windowpane rupture type, it could result in the release of a large quantity of fluid into the furnace or into the annulus between the tubes and the casing. It is recognized that if sufficient volume of high-pressure water or steam is suddenly released to a confined space operating at a lower pressure, the rapid expansion (flashing) of high pressure water or steam would produce a surge pressure, which, if it is of sufficient magnitude would be similar in effect to an explosion.
The fatigue cracking will initiate on the inside diameter surface of the tube. As it continues to propagate into the tube wall thickness, the newly created crack surfaces (bare metal) will be exposed to the boiler feedwater. Some corrosion of the crack surfaces will occur. The corrosion will result in the cracks becoming wider. The corrosion will also result in the formation of corrosion deposits within the crack. These deposits will continue to expand with time. The formation and subsequent expansion of the corrosion deposits within the crack will introduce additional stresses, thereby leading to crack propagation. However, the initial and primary cause of cracking is the cyclic stresses and resultant fatigue. Corrosion fatigue is relatively common in boilers subject to cycling service, and even more common in boilers subject to peaking service.
When steam explosions do occur, the resultant damage is heavily dependent upon the quality and quantity of high-pressure steam or water released. It may produce relatively limited damage, whereby the ruptured tube is subject to “whipping” but the damage is confined to the interior of the casing.