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The harmful effects of flow accelerated corrosion can be mitigated by evaluating key components, such as the deaerator and associated piping
By Otakar Jonas, P.E., and Lee Machemer, P.E., Jonas Inc.
Flow accelerated corrosion (FAC), also called erosion-corrosion, in power plants can be a serious safety problem, as well as a threat to plant reliability. It is a slow, out-of-sight process that can thin large areas of piping and pressure vessels and result in break before leak failures. It also generates corrosion products that can accumulate elsewhere in the cycle.
FAC is a joint action involving corrosion and mass transfer in moving liquid water or wet steam that leads to material loss. During the process, the protective metal oxide (magnetite) is removed from the steel surface and transported into the flowing water or wet steam.
Most plants have FAC somewhere in the deaerator and feedwater piping. The extent of FAC in deaerators depends on the design and nature of the internal water/steam flows. Shell and other internal component thinning occurs mainly at the steam and drain nozzles and where the wet steam flow turns to pass under the baffles and rise up through the trays. This thinning is often caused by direct impingement or by the deflected flow after it has struck a poorly designed impingement plate. It can also occur when the flowing steam causes the liquid film on the surface of the deaerator shell to accelerate. This results in the liquid velocities approaching the steam velocity, which increases the risk of FAC.
Severe cases of FAC-related failures have been reported. In 1995 and 1996, fatal feedwater pipe failures (break before leak) occurred in a fossil power station and in a paper mill (respectively). In 2004, a fatal pipe rupture occurred in a Japanese nuclear plant
 Photo 1. Deaerating vessel with the deaerating steam inlet (bottom plate removed) on the right. All photos courtesy of Jonas Inc. |
Fortunately, FAC-related problems are normally discovered before a fatal failure occurs. For example, in March 2003 the discovery of pin-hole leaks in the bottom shell of a horizontal tray-type deaerator (Photo 1) prompted a review of the deaerator design and an evaluation to determine the root causes of the damage. By reviewing water and steam chemistry and inspection results, and by estimating the flow conditions within the deaerator, analysts determined that FAC caused the thinning.
Deaerator Description
The damaged deaerator was installed on a 500 MW coal-fired unit with a 2,750 psig drum boiler. The holes appeared at the deaerating steam inlet end below the tray compartment. During a subsequent inspection, a scallop pattern (Photos 2 and 3) typical of FAC was observed on the deaerator shell extending beyond the middle of the deaerating vessel. No distress was found in the storage tank.
 Photo 2. Bottom section of deaerating vessel shell showing the FAC pattern (steam inlet in front). |
The original deaerator design included:
- Operating pressure: 60 to 85 psia
- Design output: 3,800 klbs/hour
- Shell material: ASME SA515-70 carbon steel plate and heads
- Internals: 304 stainless steel.
 Photo 3. Surface typical for FAC damage with two perforations. |
The “cold” condensate flow entered the tray compartment via an inlet header and spray valves. The flashing condensate, generally assumed to be oxygen-free, entered the deaerator outside the tray compartment through flashboxes and was not subject to deaeration.
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During the plant’s 22 years of service, the unit’s feedwater treatment was modified several times and included several amines, ammonia and oxygen scavengers (Table 1). Amines and organic oxygen scavengers are not appropriate for control of feedwater chemistry in high pressure drum boiler units. The feedwater pH ranged from 8.4 to 9.2 and averaged 9.0, which is low for a unit with an all-ferrous feedwater system. Dissolved oxygen concentration in the feedwater ranged from 0 to 8 parts per billion (ppb), however the average concentration was less than 1 ppb.
Determining FAC
The FAC thinning rate is a function of many variables that, when multiplied together, yield the predicted rate. The general FAC model formulation is:
where:
E = Calculated wear rate
F1(T) = Factor for temperature effect
F2(AC) = Factor for alloy content effect
F3(MT) = Factor for mass transfer effect
F4(O2) = Factor for dissolved oxygen effect
F5(pH) = Factor for pH effect
F6(G) = Factor for geometry effect
F7(α) = Factor for void fraction
Flow geometry and fluid temperature have the biggest influence on FAC rate, with the highest wear rates occurring between 250 F and 350 F (deaerator operating temperature in this case was 293 F to 315 F). Other variables that affect FAC include material composition (chromium, copper and molybdenum concentrations), flow velocity and turbulence and water and steam chemistry, such as pH, oxygen and concentration of oxygen scavengers. The piping’s shape also has a large affect on the FAC rate, with straight pipe having the lowest shape factor (lowest FAC rate), followed by an elbow and then a u-bend. These components were used to simulate the flow patterns in the deaerator.
This particular unit had a few additional specific features of the cycle chemistry control that affected its susceptibility to FAC, including:
- Use of amines, dispersant and hydroquinone, which decompose
- High concentrations of oxygen scavenger and carbon dioxide
- High air inleakage-carbon dioxide reduces pH
Liquid film flow at the bottom of the deaerator was a major factor in this case. The FAC occurred because the high velocity steam accelerated the liquid film on the bottom of the deaerating vessel as it passed under the first baffle (Figure 1). At the edges of the liquid layer, the high velocity steam pushed a thin layer of water up the wall and along the sides of the shell. This created small, intense vortices that increased the FAC rate. The bottom-most sections of the shell were covered in a thicker layer of water, so they were less affected by the steam flow and these areas had a lower FAC rate.
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The deaerator was analyzed for FAC damage using multiple regression software. The software helped determine whether FAC was likely in the areas where the deaerating vessel was thinning. Material specifications and dimensions, operating conditions (pressure, temperature and mass flow rate) at 470 MW and 560 MW and water chemistry data all were considered in the analysis.
Analysis Results
FAC analysis results showed that the deaerator wear rates under most operating conditions were high (Tables 2 and 3), with the estimated total wear over the life of the unit exceeding the original one-half-inch wall thickness. A combination of flow, material and chemistry affected the wear rate result.
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Flow Velocity. The analysts who conducted the analysis assumed that the liquid’s high flow velocity was the result of acceleration of the liquid film by the steam flow at the edges of the liquid layer. This could increase liquid film velocities close to the steam velocities of 30.3 feet per second (ft/sec) at 470 MW and 32.1 ft/sec at 560 MW. To determine the velocity’s effect on the FAC rate, the analysts also used velocities that were half of the steam velocities (15.2 ft/sec and 16.1 ft/sec). They found that when the velocity was reduced by half to 15.2 ft/sec, the FAC rates were reduced by one third.
Flow Patterns. Approximate flow patterns in the deaerator were determined using three piping components that best represented the flow conditions along the shell (Figure 1). A 90-degree short radius elbow was used to approximate the flow where it turns from the shell head and passes under the baffles. Analysts used a straight pipe to simulate the flow along the bottom of the deaerator, and a u-bend to simulate the flow as it turned from the shell head, passed under the baffle and rose up through the trays. The analysis results indicated that flow pattern had a significant affect on the FAC rate. The 90-degree elbow had a predicted FAC rate one-and-a-half times greater than the straight pipe and the u-bend was almost two-and-a-half times higher. Although the flow in the deaerator wasn’t exactly like the flow in these components, they provided a good approximation of the range of wear rates that might be present under different operating conditions.
Temperature. The temperature in the deaerator varied with load from 293 F to 315 F, which is within the range where FAC wear rates are high. For this analysis, the temperature was assumed to be 293 F, which is where the deaerator typically operated. Analysts determined that increasing the temperature to 315 F could reduce FAC wear rates by 20 percent.
Chromium Content. The analysts used the deaerator shell’s actual material composition (SA515-70 with 0.02 percent chromium) in this analysis. The replacement material (2.25 percent chromium, 1.0 percent molybdenum) was found to be much more resistant to FAC than the original material. The wear rates were 76 times higher for the original 0.02 percent chromium material compared to the replacement material under the same flow and chemistry conditions.
Amines and pH. Under two-phase conditions, such as in the deaerator, the chemistry of the liquid phase can be much different than the feedwater chemistry (one-phase liquid). Due to its high volatility under the deaerator conditions, ammonia concentration in liquid water is almost 10 times less than the ammonia concentration in steam. Assuming an average feedwater pH of 9.0, the resulting pH of the liquid water in the deaerator would be 8.3. The same conditions exist when cyclohexylamine is used to control feedwater pH. When morpholine is used, however, because of its lower volatility, the liquid pH remains approximately 9.0. To cover all expected conditions in the deaerator liquid film, the FAC analysts used two pHs-8.3 and 9.0. They used ammonia, cyclohexylamine and morpholine separately for each condition. The resulting wear rates indicated that it would take almost four times longer for the deaerator shell to perforate at pH 8.3 than at pH 9.0, and the times were lowest when ammonia was used and highest when morpholine was used (Table 3).
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Oxygen Concentration. Low concentrations (ppb) of oxygen scavenger can reduce the FAC rate. To obtain a worst-case estimate of FAC rates, the analysts assumed that the dissolved oxygen concentration was 0 ppb. They discovered that increasing the oxygen concentration in the feedwater from 0 ppb to 20 ppb could decrease the FAC wear rate by more than 50 percent. This could result in a great improvement in the remaining life of many feedwater system components.
Oxygen Scavenger Concentration. High concentrations of oxygen scavengers in the feedwater negatively affect the FAC rate. By reducing the hydrazine concentration from 50 ppb to 20 ppb, the wear rate can be reduced by 50 percent. By eliminating the oxygen scavenger (0 ppb), the FAC rate can be reduced by an additional 25 percent.
Conclusions and Recommendations
This analysis concluded that several factors contributed to the deaerator’s FAC. First, the design caused high steam flow velocity and steam impingement on the bottom shell of the deaerating vessel. Other contributing factors were the water chemistry’s low pH, its excess oxygen scavenger and the presence of amine decomposition products in the water. Operating at temperatures from 256 F to 289 F at 57 psi, which is in the range of the highest carbon steel FAC rates, was also a contributing factor. And, the use of carbon steel with almost no chromium, copper and molybdenum content contributed to the FAC. Repairing the deaerator shell with 2.25 percent chromium low-alloy steel was a good solution to this problem.
As this analysis illustrates, evaluating all deaerators’ designs and operations for susceptibility to FAC is necessary. It is also important to periodically inspect all deaerators deemed susceptible to FAC. Theoretical evaluation can be used to estimate the FAC thinning rates.
For all utility and industrial steam systems, carbon steel liquid water and wet steam piping also should be theoretically evaluated. The FAC evaluation should be followed by inspections of selected components, such as orifices, thermowells, sampling nozzles, chemical injection quills and leaking valves. It should also include:
- Feedwater piping and economizer to boiler drum piping
- Economizer
- Feedwater heater shells
- Boiler downcomers, blowdown pipes and drum internals
- Condenser shell and structural components
- Wet steam turbine extraction piping.
Authors: Lee M. Machemer, P.E., has been an engineer at Jonas Inc. for 10 years and has experience with expert systems, steam cycle and water chemistry, corrosion monitoring and failure analysis. He has published more than 20 technical papers and reports.
Otakar Jonas, Ph.D., P.E., is currently president and consultant with Jonas Inc., a consulting company specializing in steam cycle corrosion and water chemistry, failure analysis, troubleshooting and research and development.
