FAC in Power Plants: The Silent Threat
On December 9, 1986, an elbow in the condensate system at the Surry Nuclear Power Station in Virginia ruptured unexpectedly. The failure caused four fatalities and tens of millions of dollars in repair costs and lost revenues. The culprit? An insidious phenomenon known as Flow-Accelerated Corrosion (FAC) 1 .
While this incident occurred in a nuclear facility, FAC remains a persistent threat in gas-fired combined cycle power plants, particularly in the complex systems of heat recovery steam generators (HRSGs). These plants, which harness both gas and steam turbines to generate electricity efficiently, face unique challenges from FAC—especially in their low-pressure evaporator systems where delicate balances of pressure, temperature, and chemistry interact to either accelerate or inhibit this destructive process.
What makes FAC particularly dangerous is its stealthy nature—it gradually thins pipe walls without visible signs until catastrophic failure occurs. The phenomenon represents a complex interplay between physics and chemistry, where water velocity, material composition, temperature, pH, and system pressure combine to determine corrosion rates. Among these factors, drum pressure has emerged as a critical control variable that plant operators can manipulate to extend equipment life and prevent dangerous failures 2 3 .
FAC Impact
- Stealthy corrosion with no visible signs
- Causes catastrophic failures without warning
- Costs millions in repairs and downtime
- Threatens worker safety
Understanding FAC: The Invisible Erosion
Flow-accelerated corrosion represents a peculiar form of material degradation where the protective oxide layer on carbon steel surfaces dissolves into fast-flowing water or wet steam. Unlike conventional corrosion that occurs in stagnant conditions, FAC thrives in dynamic fluid environments where the flowing medium constantly removes protective films, exposing fresh metal to further attack 4 .
FAC Mechanism
The process begins when carbon steel components naturally form a protective magnetite (Fe₃O₄) layer when exposed to water and heat. This dark protective coating, following the Schikorr reaction, normally shields the base metal from further corrosion. However, under certain conditions of flow, chemistry, and temperature, this layer dissolves into the water stream. The flowing water then carries away the dissolved iron, exposing fresh metal to continue the cycle of dissolution 3 .
Critical Factors Influencing FAC Rates
- Water chemistry: pH levels dramatically affect iron solubility
- Temperature: FAC peaks typically between 130-150°C
- Material composition: Carbon steel is most vulnerable
- Flow dynamics: Turbulence and velocity are major contributors
- System pressure: Affervescent conditions in evaporators
The most vulnerable locations are typically flow disturbance areas—elbows, reducers, valves, and anywhere flow patterns become turbulent. In HRSGs, the low-pressure evaporator sections with their short-radius harp tubes are particularly susceptible due to their design and operating conditions 1 .
The Drum Pressure Connection: The Balancing Act
In the complex ecosystem of a heat recovery steam generator, drum pressure exerts a profound influence on FAC through multiple mechanisms. The relationship isn't straightforward—it involves interacting effects on fluid properties, flow dynamics, and chemical equilibria 2 .
Void Fraction Changes
The most significant effect occurs through void fraction changes. In evaporator tubes, water and steam coexist in a two-phase mixture. As pressure decreases, steam occupies more space for the same mass, resulting in higher mixture velocities at the same flow rate. These increased velocities enhance the erosive capacity of the fluid, accelerating the removal of the protective magnetite layer. Research shows that reducing drum pressure by just 10% can increase steam-water mixture velocity by 15-20% in typical evaporator tubes, significantly increasing FAC potential 2 .
Ammonia Distribution
Drum pressure also affects ammonia distribution in the system—a critical factor in pH control. Most combined cycle plants use ammonia for pH control in their water chemistry programs. Unfortunately, ammonia distributes preferentially to the steam phase rather than the water phase. At lower pressures, this distribution becomes more pronounced—with 70-90% of the ammonia escaping with the steam in low-pressure systems. This leaves the water phase with insufficient ammonia to maintain proper pH, creating acidic conditions that dramatically accelerate FAC 3 .
How Drum Pressure Affects FAC-Related Parameters in HRSG Systems
| Parameter | Effect of Increased Drum Pressure | Impact on FAC |
|---|---|---|
| Steam-water mixture velocity | Decreases | Reduces erosion force |
| Ammonia retention in water | Increases | Maintains protective pH |
| Saturation temperature | Increases | Can move toward optimal FAC temperature |
| Steam quality | Decreases | Reduces droplet erosion |
| Density of fluid | Increases | Reduces acceleration in bends |
Case Study Examination: The Evidence Mounts
A compelling case study published in Power Technology and Engineering examined a 795 MW combined cycle power unit with a P-132 type HRSG that had experienced repeated tube failures in its low-pressure evaporator circuit. The researchers embarked on a comprehensive investigation to understand how operating conditions, particularly drum pressure, influenced FAC rates 2 .
Research Methodology
The research team employed a multidisciplinary approach, combining detailed hydraulic calculations, examination of failed components, and analysis of operational data. They calculated steam-water mixture velocities under different operating scenarios using established thermal-hydraulic models approved by industry standards. Their methodology followed these steps:
- Field measurement of actual tube thicknesses at various locations in the LP evaporator
- Computer simulation of flow parameters under different pressure scenarios
- Laboratory analysis of water chemistry and its relationship to pressure changes
- Long-term monitoring of corrosion rates under adjusted operating conditions
Key Findings
The researchers specifically examined the effect of increasing the low-pressure drum pressure from its design value of 3.14 kg/cm² to elevated levels while monitoring corrosion indicators. They conducted tests under both constant and variable feedwater temperature conditions to isolate the pressure effect 2 .
The results were striking. The study found that increasing the drum pressure by approximately 25% (while making complementary adjustments to maintain proper system operation) reduced steam-water mixture velocities by 18-22% in the most vulnerable locations. This velocity reduction translated to a corrosion rate decrease of nearly 30% based on ultrasonic thickness measurements taken over a six-month period 2 .
Case Study Results - Drum Pressure Impact on FAC Parameters
| Operating Parameter | Original Value | Modified Value | % Change | FAC Impact |
|---|---|---|---|---|
| Drum pressure | 3.14 kg/cm² | 3.92 kg/cm² | +25% | Highly beneficial |
| Mixture velocity | 4.2 m/s | 3.3 m/s | -21% | Reduced erosion |
| Ammonia in water | 0.15 ppm | 0.38 ppm | +153% | Better pH control |
| Corrosion rate | 0.25 mm/year | 0.18 mm/year | -28% | Extended tube life |
Perhaps more importantly, the researchers demonstrated that these pressure adjustments allowed the continued use of standard carbon steel (Grade 20) rather than requiring more expensive corrosion-resistant alloys. This finding has significant economic implications for plant operators facing the dilemma of balancing reliability costs with generation efficiency 2 .
The case study further revealed that pressure management worked most effectively when combined with design modifications such as replacing outlet bends with straight tubes, revising the riser system design, and installing anti-shunt partitions in heating surfaces. This holistic approach addressed both the hydraulic and chemical aspects of FAC simultaneously 2 .
Mitigation Strategies: Fighting Back Against FAC
While drum pressure optimization represents a powerful tool against FAC, effective corrosion management requires a multifaceted approach that addresses all contributing factors. Modern combined cycle plants employ several complementary strategies to control FAC, based on research from leading institutions worldwide 2 3 1 .
Water Chemistry Optimization
The transition from All-Volatile Treatment Reducing (AVT(R)) to All-Volatile Treatment Oxidizing (AVT(O)) has proven particularly effective. AVT(O) maintains a slight oxygen residual (5-10 ppb) while eliminating oxygen scavengers, promoting the formation of a protective hematite layer rather than the more soluble magnetite. This approach requires ultra-pure water quality (cation conductivity ≤0.2 μS/cm) but can dramatically reduce FAC rates when properly implemented 3 1 .
Film-Forming Amines (FFA)
These specialty chemicals have shown remarkable effectiveness in combating FAC, particularly in low-pressure systems where ammonia depletion problems occur. FFAs work by creating a hydrophobic barrier on metal surfaces, preventing contact between the water and the oxide layer. Case studies demonstrate that properly applied FFA treatment can reduce iron concentrations in LP drums from 1000 ppb to less than 5 ppb—a 99.5% reduction 3 .
Design Modifications
Engineering changes to HRSG geometry can significantly reduce FAC susceptibility. These include replacing sharp bends with gradual curves, increasing tube diameters to reduce velocity, and redesigning riser systems to allow more independent movement of steam-water mixtures. Such modifications address the hydrodynamic drivers of FAC rather than just the chemical aspects 2 .
Materials Upgrading
In severely susceptible areas, switching to more corrosion-resistant materials represents a definitive solution. Even small chromium additions (0.05-0.1%) dramatically reduce FAC susceptibility. For extreme cases, stainless steel components may be warranted despite their higher cost 2 .
FAC Mitigation Strategies Comparison
| Strategy | Mechanism | Effectiveness | Cost | Implementation Complexity |
|---|---|---|---|---|
| Drum pressure increase | Reduces mixture velocity | High | Low | Moderate |
| AVT(O) chemistry | Forms protective oxide layer | High | Moderate | High |
| Film-forming amines | Creates barrier film | High | Moderate | Moderate |
| Design modifications | Reduces flow turbulence | Very High | High | High |
| Material upgrades | Increases inherent resistance | Complete | High | Low-Moderate |
| Operational adjustments | Optimizes parameters | Moderate | Low | Low |
The most successful FAC management programs employ multiple strategies simultaneously, tailored to the specific design and operating conditions of each HRSG. For example, a plant might implement optimized drum pressure along with AVT(O) chemistry and targeted use of film-forming amines, while planning design modifications during major outages 2 3 .
Conclusion: Future Directions and Importance
The battle against flow-accelerated corrosion in combined cycle power plants represents a classic example of how scientific understanding applied to industrial operations can yield dramatic improvements in both reliability and safety. The relationship between drum pressure and FAC rates, once poorly understood, has now become a fundamental principle in HRSG design and operation 2 1 .
Ongoing Research
Ongoing research continues to refine our understanding of these complex interactions. Advanced computational fluid dynamics models now allow engineers to predict FAC susceptibility with increasing accuracy before equipment is even built. Meanwhile, new monitoring technologies including real-time wall thickness monitoring and advanced water chemistry sensors provide unprecedented visibility into corrosion processes as they occur 4 .
Broader Implications
The implications extend beyond combined cycle plants to virtually any facility with complex water-steam systems—including conventional fossil plants, nuclear facilities, and industrial steam systems. The lessons learned about how pressure affects corrosion have universal applicability in these environments 1 .
As the energy transition progresses toward more flexible operation of power plants, FAC management becomes even more critical. Cycling operations introduce additional variables that can accelerate corrosion, making understanding of pressure-related effects even more valuable. The research continues, but already the findings have made significant contributions to plant safety and reliability worldwide 2 3 .
What began as a mystery following a tragic accident has evolved into a sophisticated engineering discipline that continues to protect lives and infrastructure. The careful management of drum pressure stands as just one example of how seemingly small technical adjustments can yield outsized benefits in the complex systems that power our modern world.