Flow-accelerated corrosion (FAC) is the combined actions of corrosion and erosion in the presence of a moving fluid (liquid water or wet steam), resulting in the loss of material and localized reduction of wall thickness. The geometry of each piping component and the temperature of the fluid are the major influences on the rate of FAC. The highest wear rates are around 150°C (300°F). Other variables that effect FAC are material composition (chromium, copper, and molybdenum), flow velocity and turbulence, and water and steam chemistry (pH, oxygen, oxygen scavengers).
Flow-accelerated corrosion has been experienced worldwide and can occur in virtually all types of nuclear units, fossil-fired units, and industrial steam systems. Both nuclear and fossil power plants have had fatalities caused by FAC as the initiator. Damage from FAC has occurred in virtually every high energy water and steam/water system in a power plant.
Any carbon steel piping system with the correct thermodynamic conditions may experience FAC. In utility and industrial power plants, systems that are commonly susceptible to FAC include turbine condensate and feedwater piping, return condensate piping, feedwater and other heater drains, and deaerator vessel shells.
The susceptibility of a system to FAC can be reduced by controlling the pH and oxygen concentration of the water. Periodic inspections are performed to determine the remaining wall thickness of susceptible components. These inspections are costly and time consuming and often require the unit to be out of service for the entire time of the inspections.
Jonas, Inc. developed an FAC Monitor/Simulator that allows the user to periodically obtain estimates of the FAC wear rate in their system without the need for costly inspections or outages. This inexpensive and compact device consists of a stainless steel housing with a carbon steel insert. The housing is connected to an existing sample line and the carbon steel insert is installed in the housing.
Flow through the sample line is then restarted and normal sampling and analysis of the stream can be performed since the device does not interfere with the analysis of the samples from that line. Periodically, the carbon steel insert is removed, weighed, measured, and inspected for signs of FAC. The insert is made of two separate pieces, so it can be opened up, inspected, closed and reinserted into the Housing.
Based on the results of the measurements and visual inspection, the estimated
FAC wear rate for the piping can be determined. This data can be used to determine inspection intervals and for comparison against
the results from FAC simulation software. In addition, the device can be used to determine the effects of changes in water chemistry
on the FAC wear rate.