Piping failures caused by Flow Accelerated Corrosion (FAC) have occurred at both nuclear and fossil-fuel power plants, several with fatal consequences. For reference purposes, FAC is a type of erosion/corrosion that may affect a number of systems in modern power plants. Increases in flow rates associated with larger units and improved oxygen control have provided the environment for FAC to occur.
Industry wide design practices for piping geometry, fluid velocity and additional wall thickness allowances to address FAC in piping systems do not exist. It is generally left to the designer to use “good engineering practices” in the design of flow path geometry. “Target” velocities in Boiler Feedwater piping systems are normally in the range of 10 ft. to 15 ft. per second. However, velocity is typically not a primary design consideration.
It has been shown that even low levels of trace alloying elements, particularly chromium, can increase the resistance to FAC in carbon steel piping. A chromium content as low as 0.12% can reduce significantly the rate at which thinning by FAC occurs.
There are several steps that can be taken to prevent failures resulting from FAC. The first step is to identify those piping systems that could be susceptible to FAC. This would involve reviewing specifications, design drawings, and operating data to identify carbon steel piping systems operating between 230̊F and 500̊F. Subsequently, the flow velocity in these piping systems should be calculated. Piping systems with flow velocities greater that 15 ft. per second should be inspected. As part of the inspection, the piping system should be subject to a detailed visual examination. Areas exhibiting abrupt changes in direction or other conditions likely to produce turbulence should be targeted. Specifically, these areas should be stripped of insulation and a detailed wall thickness survey performed. The results of the wall thickness survey should be compared to the nominal wall thickness and the minimum required wall thickness to determine its life expectancy. With the pipe surface exposed, non-destructive shaving samples should be removed and subject to chemistry to determine chromium content. Those locations that have greater than 0.12% can be excluded from future inspections.
The following selections of high priority areas within the piping system should be considered those areas historically susceptible to FAC damage:
1. Downstream of control valves, block valves and flow orifices
2. Back to back fittings, tees, reducer, pump exit nozzles, high velocity bypass lines (e.g. feedwater and check valve)
3. Regions around backing rings
4. Locations subject to steam flashing and cavitations and locations of poor fit-up
Since FAC has become recognized as a potential condition for catastrophic failures, our field engineering personnel have become extensively involved in the condition assessment of feedwater and related piping systems. This included the wall thickness measurements to determine evidence of FAC as evidenced by significant decreases in the original pipe wall thickness.
For more information about Thielsch’s experience with flow accelerated corrosion and how to assess the piping systems susceptible to this damage mechanism contact Pamela Smoske at email@example.com or by phone at (561) 353-5804 ext 4501.