In recent years, several catastrophic failures in the feedwater piping of utility steam power, nuclear power plants and pulp and paper facilities have been attributed to wall thinning of the piping enhanced by a phenomenon termed “Flow-Accelerated Corrosion” (FAC). FAC affects certain piping systems exposed to flowing water or wet steam. The rate of metal loss depends on a complex interplay of many parameters such as water chemistry, material composition, and hydrodynamics. Carbon steel piping components that carry wet steam are especially susceptible to FAC and represent an industrywide problem. The catastrophic failures in the industry as a result of FAC damage has brought to the attention of insurance underwriters, trade organizations and plant managers the need to implement wall thickness monitoring programs. A number of these catastrophic failures caused by flow-accelerated corrosion have been evaluated by Thielsch Engineering.
In order to understand how to evaluate the effects of FAC, it is first important to understand the damage mechanisms at play. FAC affects a large number of systems in modern steam electric generating facilities. Increases in flow rates associated with larger generating units and improved oxygen control have provided a more optimal environment for FAC to occur.
When ASTM Specification A-106, Grade B or Grade C carbon steel and other mild steels are exposed to water, the initial reaction is Fe + 2H2O → Fe+2 + 2OH-1 + H2 . This reaction is then followed by 3Fe+2 + 4H2O → Fe3O4 (magnetite) + 4H2. The magnetite forms a protective oxide layer on the surface of the metal.
The second reaction (formation of magnetite) is dependent on temperature, pH levels and oxygen concentration. In low oxygen feedwater this reaction proceeds relatively slowly. In areas of high turbulence or flow rate, such as elbows and tees, the Fe+2 produced in the first reaction, is carried from the surface of the material before the second reaction can occur. Consequently, the protective oxide layer does not develop and the underlying metal can continue to corrode at a relatively high rate (corrosion rates on the order of 0.04″ per year are considered high).
As previously mentioned, flow rates are the primary cause of the erosion. The flow patterns are inherent to the piping system and result from the piping geometry and the operational characteristics of the feed pumps. Industry wide design practices for piping geometry, fluid velocity and additional wall thickness allowances to address FAC in piping systems do not currently exist andit is generally left to the designer to use “good engineering practices” in the design of flow path geometry. “Target” velocities in feedwater piping systems are normally in the range of 10 ft. to 15 ft. per second;however, velocity is typically not a primary design consideration.
Feedwater chemistry is another factor that affects the susceptibility of piping to FAC. Two parameters in particular are oxygen and pH. Flow-Accelerated Corrosion is specifically active in piping systems containing water with the following chemical parameters:
Free Oxygen (02): 0 to 20 ppb
pH: 7.0 to 9.2
Optimizing chemistry in the condensate and feedwater systems is critical for two reasons. First, to prevent corrosion of the piping and heater tubes themselves, and second to minimize the formation and transport of corrosion products that travel to the boiler and beyond. Excursions of pH outside a relatively narrow range induce corrosion, most notably in iron-based materials. Feedwater piping and heat exchanger tubes exhibit minimal corrosion at a mildly alkaline pH. For a feedwater system of all steel metallurgy the optimum pH range is 9.2 to 9.6. Corrosion control in mixed-metallurgy systems is more complicated, so the question becomes, “What is the best pH for a system containing carbon-steel piping and copper-alloy heat exchanger tubes?” In years past, a commonly recommended pH range for mixed-metallurgy systems was 8.8 to 9.1, but this recommendation was recently raised to 9.0 to 9.3.
Finally, material composition plays an integral role in the susceptibility of a system to FAC damage. Very extensive worldwide research of over 25-30 years has shown that small additions of chromium to carbon steel will markedly reduce any FAC damage from occurring. A chromium content as low as 0.12% can reduce significantly the rate at which thinning by FAC occurs.
There are several steps that can be taken to prevent the occurrence of failures as the result of FAC. Conducting proper inspections is at the foundation of an effective FAC program. The first step of any effective inspection program is to identify those piping systems that could be susceptible to FAC, most typically feedwater and steam extraction systems. This would involve reviewing specifications, design drawings, and operating data to identify carbon steel piping systems operating between 230°F and 500°F, with the “sweet spot” for damage being 300°F – 325°F. Subsequently, the flow velocity in these piping systems should be calculated. Locations in piping systems with flow velocities greater than 15 ft. per second should be inspected.
As part of the inspection program, the piping system should be subject to a detailed visual examination. Areas exhibiting abrupt changes in direction or other conditions likely to produce turbulence should be targeted. These locations typically include bends, tees, and elbows or other components that initiate a change in the direction of flow or result in turbulence. It is important to note that the adjacent straight runs of piping downstream of these fittings should also be included. Specifically, these areas should be stripped of insulation and a detailed wall thickness survey performed. The results of the wall thickness survey should be compared to the nominal wall thickness and the minimum required wall thickness to determine its life expectancy. With the pipe surface exposed, non-destructive shaving samples should be removed and subject to chemistry to determine chromium content. Those locations that have greater than 0.12% can be excluded from future inspection.
The second step involves an on-site inspection where a grid system is permanently mapped around the entire circumference and length of the fittings. Wall thickness values are then ultrasonically measured and recorded at each grid point. The wall thickness values are very carefully transcribed into a computer database. This initial data is compared with the nominal and minimum wall thickness requirements to identify any areas of significant thinning. Areas of gross wall loss are recommended for repair or replacement. By comparing the current wall thickness data to the nominal wall thickness, rough estimates of the wear rates can be determined.
The final step of the program is to re-inspect the same areas after two to three years of service. It is important that this re-inspection be performed very carefully, such that the new results can be compared to the prior baseline results, and wall thickness wear rates can be calculated. Based upon the results of these comparative evaluations, intelligent, preventive, and predictive maintenance schedules can be derived.
The establishment and implementation of a long-term strategy is essential to the success of a plant’s FAC program. This strategy should focus on reducing FAC wear rates and focusing inspections on the most susceptible locations. Monitoring of components is crucial to preventing failures. However, without a concerted effort to reduce FAC wear rates, the number of inspections necessary will increase as the operating hours increase, due to increased wear. In addition, even with selective repair and replacement, the probability of experiencing a consequential leak or rupture may increase as operating hours increase. It is recommended that in order to achieve the long-term goals of reduced cost and increased safety, a strategy of a systematic reduction of FAC wear rates be adopted. Viable options available to reduce FAC wear rates include improvements in materials, improvements in water chemistry and, localized design changes.
Since FAC has become recognized as a potential condition for catastrophic failures, our field engineering personnel have become extensively involved in the condition assessment of feedwater and related piping systems. For more information about Thielsch’s experience with flow accelerated corrosion and how to assess the piping systems susceptible to this damage mechanism contact Peter Kennefick at firstname.lastname@example.org or Robert Smoske at email@example.com.