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William Stroman,San Diego Gas & Electric Co.Presented at the Western Turbine Users Group 1998 Annual Meeting, March 22-25, 1998, San Diego, Calif.High Pressure Corrosion Control Most corrosion and corrosion by-products are generated during cyclic operation as a result of improper lay-up and startup with poor water chemistry resulting in corrosion, corrosion by-product migration and accumulation onto down stream components. Corrosion by-products have a tendency to deposit on steam turbine blading and in high heat flux sections of the boiler. The concern for steam turbine blade deposition is the potential development for the mechanism leading to corrosion fatigue failures of blades and in-service performance losses due to surface roughness from blade deposition. Boiler deposition accumulation can lead to tube overheating and corrosion failures. As a result of the deposition and scale build up, periodic cleaning and maintenance are required to prevent corrosion related outages and to restore performance. Failure to note the potential problems associated with cycling will be realized when corrosion-related problems result in forced outages, reduce reliability, and higher maintenance costs. Some of the chemistry concerns associated with cycling operation being observed, are as follows:
Boiler Corrosion FatigueStress corrosion fatigue is a concern when cycling. Corrosion fatigue is considered to be a reaction between metal fatigue and water side corrosion at temperatures ranging from 302 to 550 degrees F. The failure mechanism occurs as a result of boiler tube temperature gradients varying from ambient to high metal temperatures (thermal/mechanical stresses are induced); and chemistry environment influences (corrosion fatigue) such as pH, chlorides, sulfates, dissolved oxygen and carbon dioxide that promote localized breakdown and crack initiation of the protective magnetite scale. Studies have found that dissolved oxygen had the greatest influence on crack initiation among the environmental factors evaluated. The studies noted that the corrosion fatigue crack length increased by a factor of 50 for the same number of cycles exposure when the dissolved oxygen concentration was increased from <10 to 1,000 ppb (1 ppm). Breaking vacuum during off periods allows for air ingress, resulting in rapid metal oxidation and contamination of condensate and boiler water by oxygen, carbon dioxide, and metal oxides. Dissolved oxygen can reach parts per million levels to exceed desired control limits during start up for periods from several hours to as long as 12 hours before reaching desired feedwater specifications. To reduce the stress corrosion fatigue propagation rate, measures should be considered for lowering the thermal/mechanical stresses during cycling operation. Following are some examples for reducing stress:
Thermal stress is influenced by the arrangement of HRSG sections, headers, tubes, interconnecting pipes and operating practices as related to shut down and startup. Consider installing strain gauges and thermocouples in selected sites to monitor strains and temperatures over a variety of operating conditions. This will help to identify the potential sources of cyclic strain associated with pressure and temperature ramp from pre-start to normal operation and signs of boiler sub-cooling typically associated with the initiation of flow in the water circuits during the early stages of the boiler startup. Boiler chemistry influences that effect corrosion fatigue rate, such as low dissolved oxygen content, maintaining pH within parameters, and insuring good feedwater quality, should be incorporated into the storage and return to service program.
Flow Accelerated Corrosion (FAC)Flow accelerated corrosion (FAC) is a process whereby the normally protective magnetite [black oxide] (Fe3O4) layer on carbon or low-alloy steel dissolves into a stream of flowing water (single phase flow) or a water-steam mixture (two phase flow). Both the pH at temperature and the level of dissolved oxygen in the stream influence the stability and solubility of the magnetite oxide layer. If the oxidation reduction potential (ORP) is negative, a reducing environment exists that can reduce or eliminate the magnetite protective oxide layer that leads to FAC. As the magnetite oxide layer becomes thinner and less protective the corrosion rate is increased. Over time, general reduction of wall thickness damage is caused by FAC. The damage is localized in the sense that it typically occurs down stream of elbows, fittings, and bends within a limited area of a pipe. FAC is thinning from the inside out; therefore, it cannot be detected except through non-destructive testing (i.e., ultrasonic or radiographic or visual examination). A thinned component will typically fail due to overstress from operating pressure excursions, or abrupt changes in conditions such as water hammer, start-up loading, etc. Large rupture occurs suddenly rather than providing warning of their degraded condition by first leaking.. The concern for tube failures led OSHA to issue a warning, October 1996, regarding FAC catastrophic tube failures and recommended that employers of electrical power generation facilities establish a FAC program to:
Oxygen removalPast industry water chemistry practices believed that all the dissolved oxygen must be eliminated from the feedwater to control corrosion. To deoxygenate the feedwater the oxygen was mechanically removed by the condenser/deaerator with supplemental additions of an oxygen scavenger. This led to a trend toward lower and lower oxygen levels with increased amounts of oxygen scavenger (such as hydrazine) being applied to maintain a 40 to 100 ppb hydrazine (or equivalent) residual. This practice, is also applied into the start-up chemistry where the potential to overfeed the scavenger can lead to a significant reducing environment. Maintaining an oxygen scavenger residual causes the feedwater to become more and more reducing (e.g., ORP <-300 mV) and has, for all-ferrous systems, actually produced the opposite desired effect of producing a protective oxide film to one where erosion-corrosion of iron based materials increased. This mechanism is active on condenser shells, feedwater, and wet steam piping, deaerator, feedwater heaters shells and tube inlets, and in economizers. FAC can occur in many different metals although it has been of most concern in the carbon steel piping and equipment that operate in the temperature range of 212 to 482 degrees F. and specifically at 300 degrees F. (e.g., low pressure heaters, high pressure heaters, deaerator, economizer inlet, feedwater piping). courtesy of : By William Stroman, San Diego Gas & Electric Co.
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