By Paulo Cyranka, Flavio Ivan Barbier Rolim, Marcia Ferreira FortesÁguas, and Hermann Friedenberg de Lemos
When personnel with Furnas Centrais Elétricas S.A. began modernizing the 1,216-mw Furnas plant, they discovered damage to Unit 6 caused by alkali-aggregate reaction. The bottom stationary wearing ring was out of round, and the wicket gate stems were out of alignment. The plant owner and contractor worked together to design and implement an on-site machining effort to bring the unit back into alignment.
The 1,216-mw Furnas hydroelectric facility on the Grande River is the first plant built by Furnas Centrais Elétricas, a utility serving central Brazil. The Furnas plant, built in the early 1960s, contains eight 154-mw Francis turbines with a rated net head of 94 meters. The first unit was commissioned in September 1963 and the eighth and final one in March 1974. After more than 40 years of operation, the plant required modernization.
Complicating the situation was a problem with alkali-aggregate reaction. Furnas personnel first discovered the problem in the plant in the mid-1970s. At the time, they instituted a program to track the situation but did not make any changes to operation or maintenance of the facility.
During routine testing performed before disassembly of Unit 6, the first unit to be modernized, plant personnel discovered the bottom stationary wearing ring was out of round and the wicket gate stems were out of alignment. These alignment problems were attributed to the effects of alkali-aggregate reaction.
To bring the unit back into alignment, the plant owner and the contractor for the turbine work collaborated to design and implement a multi-step on-site machining effort. The effort brought the unit back into round. The same machining steps are being performed on the other units as modernization work progresses at the plant.
Discovering problems caused by alkali-aggregate reaction
Furnas personnel first noted signs of alkali-aggregate reaction at the plant in the mid-1970s. Gel began leaking from the concrete structure, leading Furnas personnel to investigate further. Alkali-aggregate reaction in concrete structures results when the aggregate used reacts with the alkali hydroxides in the concrete. This reaction results in expansion and cracking of the concrete over a period of many years. There are two types of reactions: alkali-silicate reaction and alkali-carbonate reaction.
At that time, the following events were detected:
- Unleveled central wall blocks and adjacent blocks between the water intake and spillway;
- Stress cracking on powerhouse structures, mainly the turbine pit concrete; and
- Stress cracking on top of spillway pillars.
On average, the cracks were 3 millimeters long.
To monitor this situation, in 1974 Furnas personnel performed a geodetic leveling measurement to determine displacement of blocks in the central powerhouse wall and adjacent blocks. In addition, they used finite element modeling to analyze the cracks. This analysis indicated that the powerhouse cracks were of thermal origin, while those on the pillars resulted from the concrete drying and soaking process. However, the information could not help Furnas determine the possible extend of the alkali-aggregate reaction.
In September 1976, Furnas personnel detected the first measurable displacements through geodetic leveling measurement. Personnel determined that the average annual rate of expansion was 22 micrometers per meter.
In 1980, Furnas personnel installed several tri-orthogonal meters to measure joint displacements on the dam crest and dam drainage gallery. Beyond routine monitoring, no further action was taken on this situation.
Gel leaking from concrete structures at 1,216-mw Furnas led to the discovery of problems with alkali-aggregate reaction at the facility.
Then, in 1992 and 1993, visual assessments by Furnas personnel indicated a rubbing process between the spillway sluice gates and their fixed parts. Personnel attributed this rubbing to the gradually reducing clearances observed between the moving and fixed parts of the spillway gates. This movement was attributed to alkali-aggregate reaction.
In 1995, Furnas personnel performing leveling measurements detected a significant reduction in the average annual rate of concrete expansion, down to 8 micrometers per meter. Personnel attributed this reduction to a decrease in the quantity of active elements in the concrete that were susceptible to alkali-aggregate reaction.
At this point, Furnas personnel determined that work was needed to assess the extent of alkali-aggregate reaction in the plant. So, in 1995, Furnas instituted a program of concrete core drilling for the structures being monitored. This program included extraction of concrete specimens to characterize its physical and thermal properties. Results indicated the plant was suffering from an expanding type of alkali-silicate reaction.
Based on these results, Furnas personnel performed statistical modeling of the temporal series of concrete block displacements. The goal was to gain information on the evolution of the reaction. Results from this modeling indicated that, since 1997, the phenomenon has been following the same pace, evidencing a low-intensity evolution at decreasing speeds. Furnas personnel interpreted from these findings that the phenomenon was becoming stationary.
To complement the measuring devices already in place, in 1997, Furnas personnel implemented a first-order leveling line and planimetric trilateration network. Strain gages were installed on the central wall and spillway pillars. Wire strain gages and fissure width measurement instruments also were installed in the drainage galleries for the purpose of monitoring concrete expansion due to alkali-aggregate reaction. Aside from taking regular readings from all equipment, no further work has been performed.
In June 2003, Furnas personnel decided to modernize the plant. This decision was based on expectations by Furnas that the incidence of failures in generating units, such as ground failures due to stator winding aging and aging of unit auxiliary equipment, would increase. Furnas awarded the modernization contract to a consortium of Voith Siemens Hydro Power Generation, Alstom Brasil, Engevis Engenharia S.A., and Contrutora Norberto Odebrecht S.A.
Before releasing the units for decommissioning, Furnas personnel performed an extensive battery of tests. The goal was to gain a comprehensive portrait of the status of the units. More than 25 different tests were performed over 30 days. These included measurements of wearing ring circularity, wicket gate leaks, and wicket gate clearance. These tests were fundamental to assess the influence of alkali-aggregate reaction on the behavior of the units.
Furnas personnel chose Unit 6 as the first to be decommissioned because modernization of this unit would not have a significant effect on the daily operation of the plant. During decommissioning tests performed on Unit 6, measurements of various equally spaced diameters along the circumference of the bottom stationary wearing ring were found to be out of round.
Verifying problems with Unit 6
During disassembly of Unit 6, which began in May 2005, the contractor took measurements from the distributor bottom ring. Analysis of these measurements revealed important dimensional and geometric variations in the unit. These variations included:
- Lack of circularity in the housing of the bottom stationary wearing ring;
- Lack of top surface flatness of the distributor bottom ring, in the region where the wicket gates operate;
- Variation of the measured radius values for the 20 bottom wicket-gate bearing housings; and
- Manufacturing deviations causing dimensional variations from the wicket-gate center lines to the seal edges, on the inlet and outlet edges.
A device, or trammel (see arrow), installed during disassembly of Unit 6 at the 1,216-mw Furnas plant confirmed the lack of circularity of the bottom stationary wearing ring housing.
These variations include those relative to the top and bottom trunnions and indicate probable adjustments made on wicket gates after unit assembly, possibly to offset bottom ring distortions resulting from concrete expansion.
When contractors removed the stationary wearing ring during unit disassembly, they discovered that the underlying bottom ring onto which it was bolted also was out of round by more than 2 millimeters. This indicated that the bottom ring, upon which the stationary wearing ring is mounted, also was out of round. Further investigation during disassembly revealed that all 20 wicket gate stems on this unit were out of the vertical plumb line by various amounts.
Developing a plan tosolve alignment problems
To restore the proper geometrical and dimensional conditions in Unit 6, a series of interventions and services was required. Personnel from Voith Siemens Hydro proposed a multi-step recovery procedure that consisted of:
- Grinding the high points in the bottom stationary wearing ring housing and the upper face of the stainless steel bottom ring, to provide surface uniformity;
- Machining to standardize the diameter of the bottom wicket gate bearing housings, using intermediate and top bearing centers on the turbine cover as reference points, then adding a bushing to the external diameter of the lower bearing to compensate for the remachining of the bottom ring housings; and
- Machining to regularize the wicket gate seal surfaces, at the inlet and outlet edges.
Based on the above recommended procedure, Furnas personnel and the contractors developed a multi-step plan to solve the alignment problems on Unit 6. Work began in September 2005 and was completed in January 2006.
Restoring the bottom stationary wearing ring housing diameter
Grinding was used to remove material and standardize the diameter of the bottom stationary wearing ring housing. The method used involved mapping each region by using metal punch tools of calibrated depths to make holes of varying depths, depending on the amount of surface to be smoothed. The punched holes then were filled with colored tracing ink so that the areas were easy to visualize. During the grinding, the high points are smoothed until all the ink is removed.
The distance between the punched points was about 30 millimeters in the longitudinal/vertical directions. The distances were decreased in areas where accuracy of material removal increased.
To guarantee uniformity of the surface being smoothed, contractors used straight, high-frequency disc and sanding wheel grinders (of various grit sizes) made of zirconia and aluminum grit.
In addition, Voith Siemens Hydro developed a circularity device (trammel) to control the diameter of the housing. The trammel was fitted with two dial gages on its two extremities, to control the stationary wearing ring housing diameter. The device was centered with the help of a plumb line, referred to points determined during the disassembly procedure. Voith Siemens Hydro used the upper stationary wearing ring as a reference for the center because this surface was in a better condition compared with any other surface.
Regularizing the bottom ring stainless steel surface
Grinding also was used to remove material and standardize the bottom ring stainless steel surface. Machining work on the surface was divided into two steps: rough pass and finish.
- The rough pass operation was carried out in a manner similar to that applied to the wearing ring housing, consisting of mapping of irregular areas and punching using punches with calibrated depths. High-frequency portable grinders with sanding discs and cup-type grindstones (adequate for stainless steel) were used for material removal.
- For finish work, a straight portable grinder fitted with a cup grindstone was adapted to the end of the trammel. The grinder was axially mounted on the extremity of the trammel and run over the stainless steel surface to remove small variations left by the rough pass process. The manually driven trammel swiveled over the whole perimeter of the bottom ring stainless steel surface, both clockwise and counterclockwise. Contractors achieved a bottom ring flatness of station 666 291.0, which is 1.2 millimeters in elevation below the theoretical design elevation (666 292.2). This new elevation took into account the average clearance needed between the wicket gates and bottom ring.
Correcting verticality of the holes for the wicket gate shafts in Unit 6 at the 1,216-mw Furnas plant involved installation of a tool used to remachine the diameters.
The wicket gate distributor height was adjusted by removing shims from or adding shims to the stay vane supporting structure. Voith Siemens Hydro used an optical N3 level to control the flatness of the bottom ring stainless steel surface, based on at least eight leveling points. Dial gages were axially fastened to the trammel to help detect any remaining high or low points around the perimeter of the bottom ring stainless steel surface.
Measuring concentricity of bottom ring/turbine cover holes
To obtain the concentricity of the 20 bottom ring/turbine cover holes that house the wicket gate shafts, contractors launched a plumb line hole by hole that was centered on the turbine cover hole. The plumb line was positioned in accordance with the original turbine cover mounting and centered by the upper stationary wearing ring.
Because the lower wicket bearings have two different diameters along the height of the bottom bearing housing, measurements for bottom ring hole center localization relative to the plumb line were made at two heights, both in the radial and tangential directions. In the radial direction, deviations were assigned a negative sign toward the center of the unit and a positive sign in the opposite direction. In the tangential direction, deviations were assigned a positive sign in the clockwise direction and a negative sign in the counterclockwise direction. Using these measurements, Voith Siemens Hydro performed calculations to determine the final position needed for the centers of each pair of holes that housed a wicket gate shaft.
Wicket gate bottom bearing housing hole machining
Machining of the holes for the wicket gate shafts was aimed at correcting verticality of the shafts. Previous measurements of the lack of concentricity of the holes established that the problem could be solved by widening the diameters of the holes by 5 millimeters.
The first step in this process involved positioning a template for the new machined hole on the existing wicket gate hole in the bottom ring. With the turbine cover properly positioned over the stay vane pre-distributor, the contractor passed a plumb line through the center of the hole in the bushing holder housing on the turbine cover, lowering it down to the bottom ring access chamber. Next, the template was centered over the existing hole, relative to the plumb line. The template was then fastened to the upper surface of the outlet ring by spot-welding. This work was repeated for all 20 holes for the bottom ring wicket gates.
Next, the contractor removed the turbine cover. The contractor then installed a model ABV machine tool from Rottler Manufacturing Inc. The unit chosen was type 550 (550 millimeter travel) with an automatic feed and frequency inverter.
After centering the Rottler machine relative to the inside diameter of the templates, the contractor machined the diameter of the holes.
Using a flat tool for axial machining, Voith Siemens Hydro machined the axial surface of the 20 holes, just enough to ensure surface uniformity.
This process was then repeated for the other 19 holes.
The main purpose of this work was to re-establish the vertical plumb of the wicket gate shafts. The final benefit for Furnas is less time for the machine to get its balance during start up, which means the spiral case is filled with water in less time.
During decommissioning of Unit 6, average clearance between the faceplates was 0.66 millimeter, and distributor leaks were measured at 940 liters per second. After rehabilitation of the bottom stationary wearing ring and wicket gates, clearance decreased to 0.5 millimeter, and leakage dropped to 640 liters per second.
Plans for future work
Based on positive results from work on Unit 6, Furnas is incorporating this machining work during modernization of the other five units at the plant. Work on Unit 5 began in June 2007 and was completed in August 2007.
To ensure greater precision relative to the grinding process, a better finish, and more precise dimensional control during the surface recovery process, Voith Siemens Hydro engineers have developed a machine to be used for standardizing the following surfaces:
- Bottom stationary wearing ring housing;
- Upper face of bottom ring (stainless steel); and
- Turbine cover support surface on the pre-distributor.
This machine is to be used to regularize surfaces, permitting machining both axially and radially. It is composed of two brackets (upper and lower), a hydraulic unit, a main axle, and a tool set for axial or radial machining work. The machine can be used at other facilities with some adaptations.
Messrs. Cyranka and Rolim and Ms. Águas may be reached at Furnas Centrais Elétricas, Rua Real Grandeza, 219 Sala 506, Botafogo, Rio de Janeiro 22283-900 Brazil; (55) 21-25283397 (Cyranka), (55) 21-25285170 (Rolim), or (55) 21-25284041 (Águas); E-mail: pcyranka@ furnas.com.br, email@example.com, or firstname.lastname@example.org. Mr. de Lemos may be reached at Voith Siemens Hydro Power Generation, Rua Friederich Von Voith, 825, Jaragua, Sao Paulo 02995 Brazil; (55) 11-39445233; E-mail: email@example.com.
Paulo Cyranka is a senior engineer in the mechanical engineering department, Flavio Ivan Barbier Rolim is head of the rotary equipment mechanical division, and Marcia Ferreira Fortes Águas is an engineer in the civil engineering department at Furnas Centrais Elétricas S.A. in Brazil. Hermann Friedenberg de Lemos is a project engineer in the area of generating unit modernization with Voith Siemens Hydro Power Generation in Brazil.