Flow-accelerated corrosion is one of the few failure mechanisms in power plants that can progress from acceptable wall thickness to catastrophic pipe rupture with almost no warning — and with no visible surface indication until the wall is already critically thin. The Surry Unit 2 accident in 1986, the Mihama Unit 3 fatalities in 2004, and the Iatan fossil plant rupture in 2007 all share the same root cause: FAC wall thinning that outpaced the inspection program. A structured FAC management program — susceptibility ranking, UT thickness mapping, chemistry optimisation, and CMMS-tracked multi-cycle records — is not optional for any plant operating carbon steel piping in wet steam or two-phase flow service. OxMaint's CMMS platform gives power plant integrity and maintenance teams the asset-level inspection records, thinning rate calculations, and outage work order integration to run an EPRI-aligned FAC program that closes the gap between inspection data and documented corrective action. Book a free demo to see how FAC programs are structured and tracked in OxMaint.
Power Plant Integrity · FAC · UT Inspection · CMMS · OxMaint
FAC (Flow-Accelerated Corrosion) Programs for Power Plants
FAC is a silent, progressive wall-thinning mechanism that has caused fatal pipe ruptures, multi-million dollar forced outages, and regulatory shutdowns across fossil, nuclear, and combined cycle plants worldwide. The only reliable defence is a structured inspection program — susceptibility-ranked, UT-mapped, chemistry-optimised, and documented in a CMMS with multi-cycle trending that turns measurement data into remaining life predictions before the component fails.
75%
FAC rate reduction from 1% Cr content in low-alloy steel vs. carbon steel — material selection is the most effective single control
~150°C
Temperature of peak FAC susceptibility — LP economiser and evaporator circuits in HRSGs operate directly in this window
100:1
Benefit-cost ratio range for FAC inspection programs — UT inspection cost versus avoided forced outage and pipe rupture consequence
3 Factors
Material, chemistry, and fluid conditions — all three must align to produce damaging FAC; optimising any one reduces risk significantly
The Mechanism
What FAC Is, How It Progresses, and Why Conventional Inspection Misses It
FAC is not corrosion in the conventional sense — there is no visible rust, no surface pitting, no early warning that the material is dissolving. The mechanism is fundamentally electrochemical: iron from the magnetite layer on carbon steel pipe surfaces dissolves into the flowing fluid, is carried downstream, and is replaced by fresh metal from the pipe wall. Repeat this process continuously at high flow velocity and the pipe wall thins, invisibly, from the inside.
Year 1
Normal wall thickness
No visible symptoms
Year 3–5
Measurable thinning begins
UT detectable if measured
Year 6–8
Accelerating wall loss
Still no surface indication
Year 9–12
Critical thickness reached
Rupture risk: high
Why Visual Inspection Fails
FAC wall thinning occurs on the inner pipe surface, producing a characteristic smooth "orange peel" texture — invisible to any external visual inspection. The outer pipe surface looks completely normal until the wall is critically thin. Only ultrasonic thickness measurement from the outside — or direct inspection of the bore — can quantify remaining wall thickness.
Why Annual UT Alone Is Not Enough
FAC thinning rates vary by location, fluid conditions, and operating profile. A component thinning at 0.3 mm/year needs inspection every 3–4 years. One thinning at 1.5 mm/year from a 5.0 mm wall needs inspection every 12–18 months. Without a CMMS-tracked thinning rate calculation per location, every component gets the same inspection interval — either wastefully frequent or dangerously infrequent.
Why CHECWORKS Needs a CMMS
EPRI's CHECWORKS software predicts FAC susceptibility rankings — but prediction alone does not prevent failures. The inspection results, thinning rates, and remaining life calculations from CHECWORKS need to be stored in a CMMS as asset-level records, converted into outage inspection work orders, and tracked to closure with documented as-found measurements. Without that connection, CHECWORKS rankings sit in an engineer's laptop and never reach the maintenance crew.
Risk Variables
The Three Variables That Determine FAC Rate at Any Location
FAC rate at a specific pipe location is determined by the interaction of three independently controllable variables. A plant FAC program must assess each variable at every susceptible location — and structure the monitoring and corrective actions accordingly.
Variable 01
Material Composition
Carbon steel is maximally susceptible to FAC. Chromium additions dramatically reduce susceptibility — 1% Cr reduces FAC rate by approximately 75%, and 2.25% Cr provides near-complete immunity. Stainless steels and nickel alloys are essentially immune. This is why material substitution — replacing carbon steel bends and tees at the highest-risk FAC locations with Cr-Mo alloy components — is often the most cost-effective long-term control where chemistry optimisation is not sufficient.
Carbon Steel — Maximum susceptibility
1% Cr steel — ~75% reduction
2.25% Cr+ / Stainless — Near immune
Variable 02
Chemistry Conditions
pH and dissolved oxygen are the two dominant chemistry variables. FAC rate peaks at approximately pH 8.5 and decreases sharply above pH 9.0 in single-phase flow — which is one of the primary reasons AVT(O) targets feedwater pH 9.2–9.6. Dissolved oxygen at OT levels (30–150 ppb) establishes a dense protective FeOOH layer that essentially eliminates single-phase FAC. Reducing conditions (AVT(R) with oxygen scavenger) promote the less-protective magnetite layer and sustain FAC susceptibility, which is why AVT(R) is avoided where possible in modern all-ferrous combined cycle designs.
pH 9.2–9.6
AVT(O) target — significant reduction
OT (30–150 ppb DO)
Single-phase FAC essentially eliminated
Variable 03
Fluid and Flow Conditions
Temperature, flow velocity, and fluid phase all determine FAC rate. Temperature peaks at ~130–160°C — the LP economiser and evaporator range in most HRSGs. Two-phase flow (wet steam) is significantly more aggressive than single-phase flow at the same temperature and velocity. Geometric turbulence at elbows, tees, reducers, and components downstream of orifices and control valves creates localised high-velocity zones where FAC rate can be 3–5× the straight-pipe rate — which is exactly why these locations dominate the high-priority inspection list in any CHECWORKS susceptibility ranking.
Elbows and bends
Tees and wyes
Downstream of orifices
Control valve exits
Reducers and expanders
Header connections
Storing FAC inspection UT readings in spreadsheets with no thinning rate calculation or outage work order linkage?
OxMaint stores UT readings by location against each pipe asset, calculates thinning rates across outage cycles, flags locations approaching minimum acceptable thickness, and integrates inspection findings into outage work packages — automatically. Free to start.
Program Architecture
The Six Elements of an Effective FAC Program — Per EPRI NSAC-202L
EPRI's NSAC-202L-R4 identifies six interrelated elements required for a fully effective plant FAC program. Corporate commitment and data sharing are preconditions — the four operational elements below are where OxMaint provides the structural and documentation backbone.
01
Susceptibility Ranking and Scope Definition
CHECWORKS or equivalent susceptibility modelling produces a ranked list of locations by predicted FAC wear rate — prioritising inspection resources toward the components with highest risk. OxMaint receives the susceptibility ranking output as the basis for the inspection PM work order structure: highest-ranked components get the most frequent inspection schedule, lowest-ranked components the longest interval. The susceptibility ranking is stored against each pipe component asset, along with the model version, run date, and input parameters used — creating a defensible audit trail for the inspection scope decision.
CHECWORKS Integration · PM Frequency Assignment · Scope Documentation
02
UT Thickness Mapping and Baseline Establishment
Baseline UT readings at defined measurement grids — minimum 5-point grid per elbow, per EPRI guidance — are the foundation of the entire FAC program. Without a documented baseline, no thinning rate can be calculated and no remaining life can be predicted. OxMaint stores baseline readings with full grid location identifiers, outage date, technician, and measurement equipment calibration record. Subsequent outage readings are entered against the same grid identifiers — so the thinning rate calculation is automatic across cycles, not a manual spreadsheet exercise before each outage.
Grid-Level UT Records · Baseline Storage · Calibration Documentation
03
Thinning Rate Calculation and Remaining Life Assessment
Thinning rate (mm/year) calculated from two or more outage inspections is the key input for remaining life assessment. A component with a known thinning rate and a known minimum acceptable wall thickness has a calculable date of failure. OxMaint displays the thinning rate per measurement location and flags components where the calculated remaining service life is shorter than the interval to the next planned inspection — triggering either an earlier inspection or a repair/replacement work order before the threshold is reached. This is the difference between a reactive inspection program and a predictive one.
Thinning Rate Display · Remaining Life Flags · Predictive Scheduling
04
Corrective Action — Repair, Replace, or Rerank
When a UT reading crosses the minimum acceptable wall thickness threshold or the thinning rate indicates imminent failure, one of three corrective actions is required: weld repair, component replacement with higher-Cr material, or a model rerank to confirm susceptibility predictions are being met. OxMaint converts the inspection finding into a formal corrective work order — with failure mode, wall thickness data, material specification for replacement, and outage window flag — so the repair reaches the outage work package register with all required technical information already attached. No engineering-to-maintenance communication gap.
Corrective WO Generation · Material Spec · Outage Package Integration
High-Risk Location Map
Where FAC Occurs in a Combined Cycle HRSG and Conventional Fossil Plant
FAC susceptibility is not uniformly distributed — it concentrates at specific temperature ranges, flow geometries, and pipe system locations. These are the locations that must be in any FAC inspection program as a minimum starting scope, before susceptibility modelling refines the priority ranking further.
| Location / System |
Plant Type |
Primary FAC Driver |
Inspection Method |
Priority |
| LP Economiser bends and headers |
Combined Cycle HRSG |
Peak FAC temperature (~150°C) + two-phase flow in carbon steel |
UT grid mapping every outage — non-negotiable minimum |
Critical |
| LP Evaporator / riser circuits |
Combined Cycle HRSG |
Two-phase flow at peak dissolution temperature, high velocity risers |
UT mapping, C-scan for broad coverage at evaporator passes |
Critical |
| HP Economiser bends |
Combined Cycle HRSG |
Single-phase FAC below saturation temperature; still significant in carbon steel |
UT spot checks at high-velocity bends; more frequent if AVT(R) |
High |
| Moisture separator drain and extraction lines |
Fossil / Nuclear / Combined Cycle |
High-velocity two-phase flow from turbine extraction steam at saturation conditions |
UT at all elbows, tees, and downstream of control valves |
High |
| Feedwater heater drain cascades |
Fossil / Combined Cycle |
Two-phase flow from high-pressure drain to lower-pressure heater shells; geometric turbulence at drain control valves |
UT at drain valve exits and downstream piping; calibrate against CHECWORKS prediction |
High |
| Condensate and feedwater pump suction piping |
All thermal plants |
Single-phase FAC at moderate temperature; often overlooked because the consequence is pump damage not pipe rupture |
UT at bends and reducers; iron transport monitoring as indirect indicator |
Medium |
Multi-Cycle Records
Why FAC Program Records Are the Most Valuable Asset in Your CMMS
A single UT reading is a snapshot. A sequence of UT readings from the same location across four or five outage cycles is a prediction engine. The quality of FAC program documentation determines whether your next outage team can make a calculated remaining life assessment — or whether they are starting from scratch because the previous measurements are in a retired engineer's spreadsheet.
Typical Spreadsheet Program
UT readings in per-outage Excel files — different column headers each year
No consistent location identifier — "elbow near pump" vs "CW-E-42-A"
Thinning rate calculated manually before each outage — if anyone remembers
Engineer retires — records partially reconstructable from email attachments
Regulator requests 10-year inspection history — week-long reconstruction exercise
Program risk: High — records cannot support remaining life calculation
OxMaint FAC Program
UT readings stored by grid point against standardised asset location IDs — every outage, same structure
Thinning rate calculated automatically from successive readings — displayed on asset record
Remaining life flag triggered when calculated RUL falls below next inspection interval
Engineer retires — all records intact in OxMaint, next engineer inherits full history
Regulator requests 10-year inspection history — exported from OxMaint in minutes
Program quality: Fully documented — supports predictive remaining life management
Common Questions
What Integrity and Maintenance Engineers Ask About FAC Program Management in OxMaint
Does OxMaint integrate with CHECWORKS susceptibility ranking outputs?
OxMaint does not replicate CHECWORKS modelling — it receives the output. CHECWORKS susceptibility rankings are imported into OxMaint as asset-level data, where they drive PM inspection frequency assignments. When CHECWORKS is re-run with updated operating data, the updated rankings are imported and inspection frequencies adjusted accordingly — maintaining full traceability between susceptibility model version and inspection scope decision.
Book a technical call to map the CHECWORKS-to-OxMaint data workflow against your program.
How does OxMaint handle the grid-level UT measurement structure required for FAC inspection?
Each pipe component susceptible to FAC is set up in OxMaint with its defined measurement grid — minimum 5 points per elbow per EPRI guidance, more for components with previous high thinning rates. Each grid point is a sub-location on the parent asset with its own measurement history. Technicians enter readings at the grid point level during outage inspection, and OxMaint automatically calculates and displays the thinning rate for each point across successive readings.
Sign up free to explore the asset hierarchy and measurement record structure before your next outage.
What happens in OxMaint when a UT reading crosses the minimum acceptable wall thickness?
A corrective work order is automatically generated — flagged with the measured thickness, the minimum acceptable thickness, the asset location ID, and the recommended action (repair or replacement with material specification). The work order is immediately visible to the maintenance planner and, if the reading was taken during an outage, is automatically elevated into the active outage work package register for same-outage resolution where possible.
Book a demo to see the threshold-to-work-order workflow for FAC inspection results.
Can OxMaint support both HRSG FAC locations and conventional fossil boiler external piping in the same program?
Yes. OxMaint's asset hierarchy supports any combination of plant systems — HRSG pressure circuits, condensate and feedwater piping, turbine extraction lines, and BFW heater drain cascades can all be included in the same FAC program with separate susceptibility rankings, inspection frequencies, and UT record structures per system type. Multi-unit plants can run separate FAC programs per unit while sharing a single OxMaint instance for fleet-level trend visibility.
Create a free account and start building your FAC asset hierarchy today.
FAC · Flow-Accelerated Corrosion · UT Mapping · CHECWORKS · CMMS
A Pipe That Fails From FAC Was Almost Always Inspectable. The Question Is Whether the Records Existed to Predict It.
OxMaint gives power plant integrity and maintenance teams the CMMS infrastructure to build and sustain an EPRI-aligned FAC program — susceptibility-ranked, UT-mapped, thinning-rate-calculated, and documented from first baseline reading to every corrective action and replacement decision that follows.
