Flow-accelerated corrosion (FAC) is one of the key damage mechanisms in power plants, with the potential to present a major safety issue and to cause forced outages.
The Electric Power Research Institute has developed new software, called the Fossil FAC Advisor (FFA), that can help utilities identify areas susceptible to FAC. FFA allows plant engineers to model plant components for characterization of wall thickness, organize and manage plant data related to FAC damage, evaluate plant water treatment options and assess local flow conditions. These capabilities can reduce the likelihood of in-service equipment failures and increase safety through control of FAC.
Wall thinning of steel piping
FAC is a degradation process that causes the normally protective magnetite layer on carbon steel to dissolve in a stream of flowing water (in single-phase flow) or wet steam (in two-phase flow). The phenomenon results in wall thinning (metal loss) in carbon steel piping, components and pressure vessels, and leads to a rapid removal of the base material until,
The FAC process can become rapid: wall thinning rates as high as 0.120″/yr (3 mm/yr) have occurred. The rate of metal loss depends on a complex interplay of many parameters, including the feedwater chemistry (oxidizing/reducing potential, pH and temperature), the pipe/vessel material composition (chromium, copper and molybdenum), component geometry, other materials in the feedwater systems, and the fluid hydrodynamics (flow, steam quality and mass transfer).
If undetected, the degraded component can suddenly rupture, releasing high-temperature steam and water into neighboring plant areas. The escaping fluids can injure plant workers, sometimes severely, and damage nearby equipment. Through the years, FAC has caused hundreds of piping and equipment failures in all types of fossil, industrial steam and nuclear power plants, and tube failures in heat recovery steam generators (HRSGs). Plant failures due to FAC are most likely under-reported, but more than 15 significant plant failures have occurred during the past 25 years.
FAC occurs in about 60 percent of conventional fossil plants. Susceptible piping segments and components typically include feedwater, condensate, heater drain, attemperating, air preheater water, boiler water circuits, auxiliary steam and in some cases, extraction steam lines. According to a 2003 EPRI survey, the most common areas of FAC incidents are heater drain lines, followed by piping to economizer inlet headers, piping around boiler feed pumps and economizer inlet tubing.
With the increasing deployment of HRSGs during the past decade, FAC has quickly become the No. 1 HRSG tube failure problem, with most failures and damage concentrated in the low-pressure evaporator circuits. FAC occurs in HRSGs under both single- and two-phase flow conditions.
Single-phase FAC characteristically has horseshoe-shaped pits and scalloped markings resembling orange peel (Figure 1). Two-phase FAC often exhibits a tiger-striped appearance, with blue-red oxide film and black corrosion.
Finding susceptible components
The majority of FAC susceptible components and systems, especially those operating in single-phase water, can avoid FAC damage through operation with appropriate cycle chemistry conditions. During the last 10 years, the EPRI fossil plant cycle chemistry program has focused on optimizing the feedwater treatments to minimize single-phase FAC. Particular emphasis has been given to reducing the level of iron-based feedwater corrosion products, which are the key on-line indicator of FAC. Fossil plants have more flexibility than nuclear plants to change the oxidizing-reducing potential in the feedwater, and major efforts have been directed along these lines.
However, a subset of susceptible components cannot avoid FAC damage through a cycle chemistry program and will require periodic inspection and possible replacement. One of the challenges of FAC arises from a phenomenon known as “entrance effects,” which occurs when flow passes from an FAC-resistant material to an FAC-nonresistant material, causing a local increase in corrosion rate.