By: Jan Stein, EPRI, Barry Syrett, EPRI, Russell Chetwynd,
Southern California Edison And John Sharp, TransAlta
Large electrical generator stators are normally cooled by water. Unfortunately, generator availability is reduced when copper corrosion products plug the hollow strands of the stator-winding coil. Such flow restrictions reduce cooling capacity, and if the temperature becomes excessive, the unit must be cleaned to avoid damage to the windings. If on-line cleaning is not possible, or if the plugging is advanced, the unit must be shut down for cleaning and repair. In extreme cases, flow restrictions can lead to plant failures.
Flow restrictions in stator cooling water systems are typically the result of accumulation of matter in a localized area, usually by deposition. The most prominent example results from the formation of copper-oxides on the strand surfaces followed by the release and subsequent deposition of oxide particles at downstream locations.
There is some evidence that copper may dissolve as copper ions and re-deposit at downsteam locations where copper solubility is lower. Flow restrictions may also result when an object larger than the free opening of a system component is flushed into the component. Plugging by objects can occur during erection/commissioning or after maintenance work.
Corrosion-related problems in water-cooled electrical generators have been increasingly reported and a number of factors are responsible. Until recently, the precursor mechanisms to the corrosion were not well understood. Most of the current operating fleet was designed and built before the seriousness of long-term corrosion effects was known. As a consequence, legacy plant instrumentation may not be adequate for detecting and monitoring the specific plant conditions that lead to a flow restriction. In addition, some cleaning methods may be effective only if applied before the restriction has become advanced.
Several years ago, Southern California Edison (SCE), TransAlta, and 10 other energy companies and major OEMs, joined together with EPRI to form a Technical Advisory Group (TAG) that would address the issue of flow restrictions. With the TAG, EPRI conducted several studies to better understand the cause of the chemistry conditions that can lead to excessive corrosion. Studies were also conducted to identify methods for accurately detecting precursor signs of flow restriction, to demonstrate effective removal techniques, and to identify tools for operating and maintaining plants to prevent restrictions.
In January 2000, SCE was planning a major generator maintenance outage later in the year at its San Onofre Nuclear Generating Station (SONGS). Having heard of flow restriction problems at other generating plants in the U.S. and abroad, SCE decided to investigate the potential for flow restrictions as part of its outage planning.
In previous operating cycles, some clogging of mechanical strainers had occurred. However, it did not result in any loss of generation because the cooling system had duplex strainer capability. SONGS has two operating pressurized water reactors, Units 2 and 3, each rated at about 1,100 MW. The generators are hydrogen cooled, with a low-oxygen stator winding deionized water-cooling system at neutral pH.
To determine if flow restrictions existed, plant and OEM design engineers conducted a number of tests. First, they collected on-line temperature data from thermocouples on the outlet stator cooling water system’s Teflon hoses. Results showed the data were in line with that of similar industry generators that had experienced a degree of strand plugging requiring corrective action.
Figure 1. Partial plugging of strands at inlet and outlet water-boxes prior to cleaning of water-cooled generator at SONGS 2. Photo courtesy of EPRI
At the start of the overhaul outage, Doppler flow measurements on the outlet Teflon hoses showed that the total flow was close to the design value. Visual inspection using a video probe revealed a significant amount of copper particles and copper oxide deposits in the stator water-cooling system, Figure 1.
In 2000, TransAlta was also investigating possible flow restrictions at its Sundance 3 plant. Sundance is a coal-fired plant with six generating units and a rated capacity of 1,981 MW. In previous years, on-line instrumentation had revealed the possibility of flow restrictions. The readings indicated overall low flow measurements and increased inlet pressures to the winding. Off-line testing confirmed the readings and the problem was resolved using hot water reverse flushing.
During a planned outage in 2000, plant engineers again conducted Doppler flow measurements at each individual half coil. These measurements showed more severe flow restrictions in a number of coils.
Based on the experience of SCE, TransAlta and the other members of TAG, EPRI conducted a study that resulted in a set of guidelines for a proactive approach to detecting flow restrictions. The guidelines recommended that operating parameters and occurrence of strainer/filter clogging be frequently assessed and system copper release be monitored by analysis of the spent ion exchange resin. For older plants with many years of operations, preventive cleaning, cleaning before problems arise, may be appropriate. Ongoing monitoring includes individual bar temperature, total stator water flow, and pressure drop and individual bar flow measurements.
The guidelines noted that visual inspections give the most immediate information and should be made on a regular basis. If severe plugging is suspected, visual inspections, supported by mechanical means such as single-hollow conductor flow testing, should be carried out.
At SONGS, plant management decided to implement a chemical cleaning of the system using Alstom’s proprietary Cuproplex process. This process is based on the complexing agent EDTA (ethylene diamine tetra acetic acid), which is circulated through the whole system. It dissolves only copper oxides and does not react with copper metal. For the SONGS plant, the Cuproplex cleaning process was applied for approximately 70 hours.
Figure 2. Videoscopic inspection after Cuproplex cleaning of SONGS 3 water-cooled generator. Photo courtesy of EPRI
At SONGS 2, results showed removal of 3.9 kilograms (kg) of copper, an amount comparable to that of the cleaning of other equivalent generators that had experienced elevated temperature problems. It also indicated that the cleaning was effective. In SONGS 3, the quantity of copper removed was much smaller, only 1.9 kg. A borescope inspection after chemical cleaning showed no visible oxide deposits at tube ends, Figure 2.
One of the limitations of the Cuproplex process is that whenever the strands within a conductor are restricted, the solution either cannot flow through at all or cannot flow quickly enough to effectively remove the copper oxides. This was the case at TransAlta’s Sundance 3 plant. Following the Cuproplex treatment, four half coils did not return to design flows. As a result, TransAlta decided to conduct individual acid cleaning of those coils with 9 percent sulfuric acid solution.
This succeeded in achieving design flows, with no known detrimental effect to the coils and no observed damage to the copper material when the water-boxes and conductor ends were inspected. Figure 3 shows the coils before and after cleaning.
The EPRI guidelines evaluate a number of possible cleaning methods including chelates, acid, cationic purification, mechanical and water flushing. The use of chelates is the preferred method, but the type of chelate used depends on the properties of copper and its oxides. Chelation cleaning can be performed on-line or off-line and works at relatively low concentrations using relatively benign reagents.
Acid cleaning should be considered when persistent oxides are a problem. Cationic purification, pioneered by Electricité de France, is reported to be a useful tool for periodic removal of oxide deposits. However, its hardware and procedures make it more an operating concept than a cleaning method.
If the generator is heavily fouled, especially if the hollow conductors are plugged, mechanical cleaning should be conducted, followed by acid cleaning or chelate cleaning. Although reverse hot water flushing is relatively simple, its effectiveness is limited. In addition, with flushing there is a risk of debris contamination in areas of the winding where it is not normally found, resulting in other flow restrictions being introduced.
Figure 3 a & b. Videoscopic inspection before and after Cuproplex cleaning of Sundance 3 water-cooled generator. Photos courtesy of EPRI.
As part of their programs to address flow restrictions in generators, SCE, TransAlta, and the other members of the EPRI TAG have initiated changes to plant procedures and practices to prevent future occurrence.
SCE revised their O&M practices to include demineralizer resin sampling for copper content analysis. This is presented as a trend of grams/sq meter of system copper surface per year. On-line oxygen Orbisphere analyzers were installed for continuous monitoring of dissolved oxygen levels.
TransAlta introduced new operating and maintenance guidelines and procedures to supplement O&M recommended practices. They also initiated on-line monitoring of stator water chemistry, conductivity, pH, and dissolved oxygen. In addition, they increased the frequency of testing and recommended new long-term maintenance practices. These included temperature monitoring of every coil, periodic use of a de-oxygenation ion exchange column to maintain low DO and the installation of a cation demineralizer for elevated pH operation.
Changes to O&M practices such as those at SCE and TransAlta were based on the findings of several additional EPRI studies, which investigated the mechanisms causing the formation of copper oxides, and from the utility investigation findings and experiences.
In one study, researchers conducted literature surveys and experimental flow loop tests to better understand plugging mechanisms. Within the current operating fleet, generator stator water cooling systems have generally been designed to operate with dissolved oxygen (DO) concentrations of either more than 2 parts per million (ppm) or less than 50 parts per billion (ppb) at neutral pH.
The results of this study found that if a system operates with DO concentration in either design range, copper corrosion product particle release rates are low and do not lead to plugging of hollow strands or clogging of strainers. In the range between the extremes, particularly when DO levels are between 200 and 500 ppb, particulate release rates greatly increase and plugging and clogging become likely.
The study identified the electrochemical corrosion potential (ECP) range in which the copper oxidation state changed from cuprous oxide to cupric oxide or vice versa. From this study it was concluded that stresses induced by the change were the principal cause of particulate release and the resulting plugging or clogging. Experimental results support the recommendation that copper strand ECP should be monitored and not be allowed to reach the dangerous range associated with a change in copper oxidation state.
Another part of the EPRI study identified specific operating guidelines to prevent occurrence of flow restrictions. The chief recommendation is the use of an on-line oxygen analyzer to the stator cooling water system. Other recommended system changes include measuring coil temperature or differential temperature, coil differential pressure and a ECP monitor to provide early warning of DO transients.
For high DO systems, recommended system changes include: installation of a large-diameter vent going directly to the roof to provide air access to the storage tank and forced aeration of the water in the storage tank.
An important finding of the EPRI research was corrosion also occurs during maintenance outages. When the water is drained following shutdown, if the system is not properly dried out, or an inert gas is not used, corrosion can take place. An EPRI report addresses the proper outage practices to avoid corrosion during maintenance outages. These practices have already been implemented at SONGS and Sundance.
Guidelines for Detecting and Removing Flow Restrictions of Water-Cooled Stator Windings, EPRI Report 1004704, June 2002.
Prevention of Flow Restrictions in Generator Stator Water Cooling Circuits, EPRI Report 1006684, February 2002.
Generator Cooling System Operating Guidelines, EPRI Report 1004004, December 2001.
Jan Stein is a senior project manager in EPRI’s Generation Department, where he oversees research on generators and large motors. He graduated from Delft Technical University in the Netherlands with BSEE and MSEE degrees.
Barry Syrett is a technical fellow in EPRI’s Science and Technology Division, where he oversees research on corrosion in power generation, transmission, and distribution systems. He graduated from University of Newcastle upon Tyne in the United Kingdom with BS and PhD degrees in metallurgy.
Russell Chetwynd has been a turbine-generator component engineer at SCE’s SONGS plant for 18 years. He has a mechanical engineering degree equivalency from Staffordshire University in the UK.
John Sharp is an electric machines (generator) specialist with the generation business services department stationed at TransAlta’s Sundance Generating Plant. He is a graduate of N. Alberta Institute of Technology, Canada, with an electrical engineering technology degree.