In the two previous parts, we mapped the factors that favor FAC (velocity, turbulence, temperature, pH, dissolved oxygen, steel composition) and the points in the system where these conditions combine: short-radius bends, diameter reductions, equipment outlets, post-valve regions, condensate return and wet steam. What remains is to bring this together into a formal risk map and the defenses that actually reduce the attack rate.
Defenses Against FAC
Defenses against FAC follow directly from these factors.
- Chemical control: pH above 9, a range in which magnetite solubility drops, and dissolved oxygen adjusted to the system's regime (single-phase or two-phase). Scavenger dosing needs to be calibrated; excess dosing creates reducing conditions that strip away the oxide's protection, the same mechanism as the deaerator case described in Part II;
- Material selection: chromium and molybdenum alloys in sections mapped as critical, where replacing piping with common carbon steel doesn't address the cause;
- Design modification: larger bend radii at critical curves and elimination of abrupt diameter reductions, where layout allows.
This reduces the attack rate but doesn't eliminate the risk entirely. That's why a formal risk map is still necessary. It cross-references, point by point, the probability of FAC (simultaneous presence of velocity, temperature and pH in the critical range, plus the dissolved oxygen regime) with the consequence of a potential failure (operating pressure, location, personnel exposure). Each critical point on the map carries the dominant factor that put it there and the corresponding defense — not a generic list of inspected locations.
Remaining Life (RL)
For each point on the map, the decision on when to act depends on remaining life (RL):
RL = (current thickness − minimum thickness) / corrosion rate
Current thickness comes from ultrasonic thickness testing (UT) or TOFD, an ultrasonic inspection technique that measures wall loss point by point; minimum thickness comes from the design code applicable to the steam system (ASME B31.1). The detail that matters with FAC is that the corrosion rate must be that of the specific point, not a system average: FAC is localized, and an isolated section can be losing thickness much faster than the average suggests.
Formalizing this map is what separates a real FAC control program from a case-by-case reaction to failures as they appear.