Flow Accelerated Corrosion (FAC) is a fundamental problem for nuclear, fossil and combined-cycle power plants that can result in the loss of power generation, damage to equipment and personnel injury. These documented events and failures have attracted the attention of utilities, industry groups and regulatory agencies. The economic impact of FAC in terms of lost power, lost revenue, damaged equipment and components, and personnel injury has gained increased attention. The mechanism of FAC involves the formation and removal of the protective oxide layer from the inside surface of the pipe or equipment. This process occurs in carbon steel piping systems, tanks and vessels. The FAC process is influenced by flow rate, pH, oxygen content, operating temperature, material of construction and piping configuration.
To oversee and manage FAC in power plants, utilities have assigned personnel the responsibility of managing the FAC Program either at the corporate level or a site representative or both. One of the keys in managing FAC is the relationship, interface and communication with
In the mid-1980s, a condensate system elbow ruptured at the Surry Power Station, killing 4 workers and generating many millions of dollars in repair cost, as well as the plant being shut down for an extended period of time . The failure was attributed to flow accelerated corrosion. Flow accelerated corrosion is a “process whereby the normally protective oxide layer on carbon steel or low-alloy steel dissolves into a stream of flowing water or a water-steam mixture” . The chemical process is present under certain operating conditions, water chemistry, thermodynamic conditions and material of construction.
The FAC phenomenon has occurred in power plant systems such as condensate, extraction steam, feedwater, heater drains, heater vents, moisture separator reheater drains, miscellaneous drains, etc. Significantly worn locations have occurred in a variety of components, such as elbows, to straight pipe components downstream of level control valves and flow elements, as well as equipment exit and inlet nozzles.
Since the Surry event, there have been additional failures at nuclear power plants, fossil power plants and combined-cycle plants (Tables 1 and 2). All these events have been attributed to flow accelerated corrosion either in single-phase or two-phase systems.
The flow accelerated corrosion wear pattern for single phase flow is generally scalloped and smooth.
The scallop shapes may vary in size and take on an “orange peel” surface. For the smaller sized scallops, magnification may be required to view the shapes .
The flow accelerated corrosion wear pattern for two phase flow displays different color shading that is an indication of FAC and is called “tiger striping.” The shading and the plateaus are a result of the turbulent flow of the steam and the moisture.
The various departments within the company’s organization and the FAC Engineer are dependent on each other to maintain an effective FAC Program. Each department has its responsibilities to support the FAC Program. In turn, the FAC Engineer has responsibilities to communicate with each of the departments. Table 3 shows a sampling of the interface between the departments and the FAC Engineer. It should be noted that a number of the departments listed are representative of a nuclear power plant facility. A fossil or HRSG facility might have several of the departments consolidated under a single department, with one individual having the responsibility of coordinating the activities.
Discussion and pitfalls
Based on a review of the events summarized in Tables 1 and 2, who has the responsibility to avoid an FAC component failing at their facility where it impacts plant operations and personnel safety: Management, Operations, Maintenance, Chemistry, System Engineering, Design Engineering, Plant Documentation, Non-Destructive Examination, the FAC Engineer or Training?