Flow-accelerated corrosion (FAC) also called erosion-corrosion (E-C) is a well know mechanism that affects carbon steel pipes and tubing carrying hot water. Flow-accelerated corrosion and cavitation are significant and costly damage mechanisms that are common to many types of utility and industrial steam boilers and hot water cycles.
FAC is a relatively slow damage mechanism but it can lead to a quick and catastrophic leak of expanding steam and hot water flashing into steam. Carbon steel pipe thinning typically occurs in the feedwater lines, return condensate and other wet steam components of the cycle.
The FAC problem is most pronounced in carbon steels. This mechanism results in the thinning of carbon and low alloy steel components. In FAC the protective coating of oxide (which is mostly magnetite) is removed from the steel surface into the flowing water. The wear rate of this material is determined by the steel composition, temperature, flow velocity and turbulence. Other factors include the pH, dissolved oxygen and the level of certain trace elements. Temperature has a dramatic effect on the FAC wear rate and components in the 250° to 400° F should receive the highest priority. Flow velocity also has a strong effect on thinning and wet steam systems are very susceptible to FAC because the steam flow velocity is usually greater then the water flow velocity.
Water chemistry effects are strong and often not well understood. The pH of feedwater and steam droplets needs to be kept above the 9.5 threshold and this depends on the types of chemical agents used. Frequent start ups and low load operations can result in substantial changes in the boiler water chemistry and this can also increase the risk for FAC.
Any carbon or low-alloy steel piping system can experience the effects of Flow-accelerated corrosion. The history of this phenomenon suggest that steam generators and boilers, feedwater piping, steam piping and return condensate lines are all at risk for FAC. Other equipment to consider would be deaerators, condensers and other heaters, tubes and vessels. The area down stream of flow meters, control valves, thermowells and sampling nozzles are all sites where extra care and inspection points should be evaluated.
An effective program for controlling FAC should include assessment of the different plant systems to attack by FAC, the use of available software for water and steam chemistry analysis and the performance of periodic inspections.
Following this analysis, the most at risk components and areas should be selected for inspection. The nondestructive (NDT) methods typically used for these inspections include ultrasonic thickness measurements of pipe walls and profile radiography of the smaller sections with geometries that can cause turbulence.
FAC Software is available within a number of software packages. However, some of the predictions of these computer based analysis have been mixed. The main reasons for some erroneous results have been poor representation of the water chemistry in the software, over simplification of the history of the water chemistry by the user and using the wrong steel composition for the piping systems.
Cavitation (cavitation/erosion) is the repeated growth and collapse of bubbles (cavities) in a flowing liquid because of the changes in the local static pressure. These changes are usually caused by increases or decreases in flow velocity. When the pressure in a flowing liquid decreases to below its vapor pressure because of these changes in flow velocity, vapor bubbles are nucleated. The bubbles are transported downstream and when they reach a region of higher pressure they collapse suddenly and can erode the solid material in the piping system that is in their vicinity.
These types of piping systems that are susceptible to flow accelerated corrosion and cavitations should be inspected after all orifices, sampling nozzles, thermowells and leaking valves. Also to be considered are blow down lines, steam separator lines, feedwater drain valves and piping, condensate return and boiler feed pump recirculation lines. Inspection methods should include methods for analyzing wall thinning using ultrasonic wall thickness measurements, radiographic inspection of piping, pulsed eddy current and magnetostrictive sensor techniques.
