Turbine protection at Gezhouba23 September 2008
During operation of Gezhouba hydro plant in China, the surface of turbine flow passage components have clearly been damaged by abrasion and cavitation erosion. Cui Lin Li, Jin Yu Lu, Yun Zhu and Jie Liu present the results of research undertaken to find a solution to these sediment-induced problems
Gezhouba hydro power plant is the first hydro station constructed on the Yangtze River in China. Located in the suburbs of Yichang City in central China, it consists of 21 turbines and has a total capacity of 2715MW. The first turbine unit was assembled to the network and began generating electricity in 1981, with all units geared up to full capacity by the end of 1988. The units had an annual maximum of 8611 operating hours, and 6000 hours on average. After more than 20 years of operation, the turbine flow passage components have been clearly damaged by sediments, highlighting health and safety issues in the power plant. In order to find out the pattern of damage by sediments on turbines, various tests were conducted at the plant into the erosion speed of slow hyper-concentration flow through turbine flow passage components.
Power plant statistics
Gezhouba hydro power plant has an average flow passage of 14300m3/sec, and an annual average runoff of 451Bm3. The average annual sand-carrying load is 530M tons, and the average sediment concentration is 12kg/m3. Approximately 90% of the sediment is brought during flood seasons. Gezhouba power station is located in the lower position around the corner of Nanjin Pass – which is considered the dividing point of the middle reaches and the upper reaches of the Yangtze river. Due to the circumfluence effect, the sediment concentration and flow passage sand volume are small and thin for Gezhouba Erjiang hydro plant, and big and rough for Gezhouba Dajiang hydro plant (both of these plants are sub hydro plants related to Gezhouba). According to actual measurements, the sediment concentration in Erjiang is 94%-98% of the standard concentration value (the Yichang value), while unit #18 and unit #21 have 1.37 and 1.6 times the concentration, respectively. The diameter of flow passage sediment grain passing through Unit #18 is 1.2-2 times the diameter in the Erjiang station, and for Unit #21 the value is 1.2-2.9 times. The maximum grain diameter reaches 0.62mm. The annual sand passage volume is about 15M tons per unit. The Gezhouba plant parameters are shown in Table 1.
Condition of the turbine blades
The damage caused to the Gezhouba power plant turbines share some obvious characteristics with other hydro power plants on sediment-laden rivers in China. The pattern shows that greater erosion results from greater sediment concentration, harder and coarser sediments and longer running hours of turbines. Erosion on flow passage components is normally the combined result of sediment abrasion and cavitation erosion.
Looking at the conditions of the turbine units after running for 20 years, despite the fact that the runner blades had been applied with anti-cavitation material such as OCr13Ni4-5Mo (125MW unit) and OCr13Ni6Mo (170MW unit), the level of cavitation and sediment erosion was still serious. In particular, the abrasion is apparent on the flow passage area of the turbines. The erosion regions could be categorized into the following: back of leaf heads, the outer 500mm edge on the back of the blades (the whole back leaf in a particularly serious situation); outer edge of the blades (damage gets greater closer to the edge); and the outer outlet edge of the runner blade.
After running for more than 40,000 hours, intensive pits appeared all over the back of the blade head of the two model ZZ560 turbines in Erjiang hydro plant. In addition the blade surface lost the metal luster and acquired a gray-dotted appearance. Both the interval between blades and the thickness of blades changed from 7.69 and 11.81mm when first assembled, to 8.7 and 13.4mm after over 40,000 hours of operation (including erosion on the outer edge of the blade runner and the runner chamber), the outer blade section appeared to have encountered faviform cavitation and corrugated shaped erosion, honeycombed corrugated abrasion and corrosion damage (see Figure 1). The blade runners encountered obvious corrugated shaped erosion, and the erosion appeared to be increasing in width, reducing the thickness of the outer edge by 2.7mm. The outer edge of the head of the blade upper face had an abrasive trench of 2 to 3mm deep.
Upon examination of turbine #3 after 63,367 hours of operation, serious ridging was discovered on the outer edge on the back of blade, due to both cavitation erosion and abrasion. Faviform damage appeared in between the outer edge and the lifting hole, with a depth of 9-15mm and an area approximately 1000x150mm2. The thickness of the outlet edge on the runner blade was reduced by over 13mm.
A boring test was conducted on turbine blade #15. After operating for nearly 40,000 hours, it was found that on the blade outer head there was abrasion of no less than 16mm, and 2.4mm on the upper face. Similar observations were made on the outlet edge of the runner blade, with the abrasion getting heavier closer to the edge, and appearing even worse on the outlet edge. (See figure 2)
On unit #20, after 24,000 hours of operation, the blade thickness was reduced by 21mm; and after running for 38,000 hours, the gap between the blade and the turbine chamber were hardly distinguishable or measurable. Continuous penetrating corrosion pits over an area of 200x70x60mm appeared on the outer section of the blade. As a result, eroded-sectional welding through the mid-ring had to be carried out.
In order to reduce gap cavitation erosion and abrasion on the outer edges of the blade, since 1993 a rim has been added to every blade back outer edge during maintenance. After many years of operation, it was shown that the protective rim could effectively reduce abrasion on the outer edge of the blade. However abrasion on the rim was still very serious. Unit #20 had been equipped with the rim since 1999 and after running for less than 20,000 hours, heavy abrasion appeared, reducing the bottom by 15mm and turning it into a ‘sharp blade’ (see figure 3). The root segment has partial abrasion through the whole 40mm thick segment, and the rim had to be reshaped again.
Over the years, there have mainly been two approaches to research on anti-abrasion protection of turbine flow passage components: one is to use flexible polymer chemicals as a protective cover over the surface of flow passage components; the other is to find a hard and durable metallic or non-metallic material, which is harder than sediments and has small and dense metallographic structure, to cover the surface of flow passage components, reducing the degree of abrasion on turbine blades.
In order to find suitable protective materials for Gezhouba turbines, the plant and related research departments were involved in many discussions and carried out numerous tests. In March 1983 an abrasion test was carried out in which two Epoxy mortars were painted on the runner hub and the medium ring of turbine #3. The coating layer was brushed off after 30,000 hours operating. In July 1983 a repair welding test was carried out in which the 0Cr18Ni6Mo three-phase electrode and the 0Cr13Ni6Mo base material electrode were welded on to blade #2 of turbine #2. The 0Cr13Ni6Mo base material electrode operated with good performance in the examination in February 1984. In August 1984, a nickel protective layer was painted onto the front hanging-hole plate of turbine blade #7. The layer disappeared after 30,000 hours of operation. In February 1986, turbine #5 was surfaced with a small area of nickel-based Ni1, Ni2, Ni3 metal powder protective layer on the turning body, blades and middle ring respectively; a Castor oil-IPN polymer protective layer was also painted on the inlet edge of the runner blade. After running for 22,000 hours, it was found in an examination that part of the Ni3 welding layer was still on the turning body. A 650x180mm crack appeared on the front inlet edge of blade #2 with Ni1 welding? and the layer could be peeled off easily by knocking with a hammer. The Ni2 layer was completely gone from the mid ring; the Castor oil-IPN polymer protective layer had washed off.
In 1987 a protective layer 0.1-0.2mm thick was painted on unit #2 and #3. After running for 8000 hours, there was only a thin layer left on the bottom run and the blade, and it was totally brushed off after running for about 20,000 hours. The power station and Yangtze River Basin Planning research department decided to paint cold epoxy 8021 on both sides of the outlet edge of runner blades and hanging hole in the same year. After 20,000 hours of operation, the layer was completely gone, and the abrasion damage was not reduced at all. In April 1988, an epoxy carborundum protective layer was painted on unit #1 flow passage components such as the front and back of the blade, wheel, connector, runner cone, movable guide vane, bottom ring, base ring and lower ring. After running for 4600 hours, 12300 hours and 15000 hours under inspection, except for the front side of the blade, a 100mm high layer on the outlet edge and the top of the base ring dropped off, all the other parts were well protected. In March 1989, several research departments conducted comparative trials on various anti-abrasion materials, including the double layer nylon, polyurethane rubber layer, Resin Mortar. HH896-1 epoxy mortar, S-80 US polyurethane rubber, Ni57 and Ni50A metal powder spray welding, metalloceramics tablet, stainless steel electrode, and titanium alloy tablet.
After running all the above tests for 17,919 hours, in 1992 the power station was closed down for inspection. It was found that the flow passage components were the most severely damaged, and almost all of the anti-abrasion materials in the contrast test had disappeared. Since the adhesive forces were different for these various anti-abrasion materials, the fall-off periods were different, on the front and back of blade from inlet to outlet, from root to edge, the various sized corrosion pits formed a terrace-like shape with partial corrosion pit size of 60x35mm; corrosion pits also appeared on the turning body, with partial size of 40x20mm.
Due to these abrasions, comprehensive repair had to be carried out. Protective epoxy carborundum was applied on the flow passage components and all five blades were replaced in 1999.
In late 1989 anti-abrasion coating tests were conducted on a large scale on flow passage components on other units. The coating mainly consisted of compound resin carborundum, and was painted on the bottom of the blade, runner room front leaf and the hub. A polyurethane tablet test was also conducted on the front and back of the blade – with the testing area measuring approximately 450m2 for every unit tested. The power station inspected the fall-off of protection layer and blade abrasion on each unit and the results are as follows:
• The front side of blade: no protective layer was found on a small area (5-10%), the blade encountered severe abrasion, with partial pits.
• The back of blade: the protective layer disappeared from 15-25% of the total area of the blade; severe abrasion occurred, with intensive partial pits.
In March 1992 the US super strength anti-wear coating DP.DL test was placed on blade #3 of unit #20, and blade #4 of unit #9. The other eight blades were painted with a carborundum epoxy layer developed by Yellow River Conservancy Commission hydro power research department. After 6000 hours of operation, coating on the #20 unit all disappeared; over 90% of the carborundum epoxy layer remained on the unit; over 90% of the super strength anti-wear coating on unit #9 also stayed on, and the carborundum epoxy layer was totally undamaged. In November 1996, an anti-abrasion test was conducted on the four blades of unit #16, at the head front and back, with two types of elastomer materials (polyurethane). After running for about 25000 hours (the main maintenance period) it was closed down for maintenance. It was found that the creamy elastomer material on the back blade had fallen off, whereas the front side encountered a similar situation but no abrasion. It was observed that the front side had only recently lost the protective coating.
In 1997, carborundum epoxy coating was used as a base on unit #29 and unit #21 mid rings, and the surface was painted with elastomer material (polyurethane). After running for 17,919 hours, the elastomer material almost entirely disappeared, and severe abrasion appeared on the blade. (See figure 4)
In 2001, the Shaanxi Yufeng M&E (YuFeng Coating Centre) applied HVOF coating on the new blades of unit #18, This involved coating the 300mm wide blade back edge with carbide cermet powder (product Ref. HP143) Up to December 2006 the #18 unit had passed six flood seasons, and operated for 30000 hours. There had been no obvious abrasion on the blade edge and rim during maintenance, thus the HVOF coating protection was considered very effective. (See figure 5)
The flow passage components encountered abrasive damage mainly due to sediment abrasion and cavitation erosion. From the micro-cutting theory by L.Finnie, the abrasivity of sediment on metal material of turbine flow passage components surface can be expressed as:
where Vd = abrasivity (kg/mm2), M = flow sediment mass (kg), a = erosion angle, V = sediment speed (m/sec), and P = stress yield of metal material.
From the above equation, the abrasivity is proportional to the square of sediment mass, the sediment speed to the power of n, and erosion angle of sediment to metal surface. The abrasion rate on metal material is greater when the erosion angle is small, and vice versa. The ability to resist abrasion however is determined by the stress yield of the material, hence it can be deduced that the higher the elasticity modulus, the more robust the material is in terms of anti-abrasion. In addition, the cavitation erosion happened frequently during turbine operation, and caused cavitation damage on flow passage components. The experiments showed that when water columns stroke the flow passage components, the resistance percussion force is calculated as:
Where T = resistance percussion force (N), E1 = water flow elasticity modulus, and ? = fluid density (kg/m3).
The above equation explained that the resistance percussion force T is positively related to strike speed V and the material of flow passage components. The anti-cavitation ability is proportional to material hardness.
The turbine unit in hyper-concentration flow normally encounters damage from both cavitation erosion and sediment abrasion, and the combination of the two accelerates the degree of damage. When sediment flows through the turbine, the sediments with certain Kinetic Energy collide with the surface of turbine, meaning abrasion may occur, but not always. From the equation above abrasion is strongly related with property of sediment, concentration of flow and property of surface material. For certain materials, the sediment has to have energy, and strike at a certain angle. Only when the pressure of the sediment exceeds the elasticity limit can abrasion occur. When the sediments are sharp, the pressure is much higher when applied with the same energy and abrasion angle, causing abrasion. For certain materials, when the flow of sediments has a speed exceeding the critical flow velocity, abrasion will occur after a long period of operation. Since most of the turbines in China run in concentrated flow, severe abrasion is likely to occur. Hence anti-abrasion materials need to be applied on top of the surface to prevent damage.
Having conducted numerous research on turbine anti-abrasion, the Gezhouba situation has shown that sediment abrasion damage rate is closely related to concentration of flow, size and diameter of sediments. More concentrated and rougher sediments would take less time to cause abrasion. The damage on turbines is not caused only by sediment abrasion or cavitation erosion, but rather as a joint effect of the two.
In recent years, the Wanjiazhai, Xiaoliangdi and Three Gorges power stations have brought in anti-abrasion technology on flow passage components from France and Germany. The HVOF coating applied on turbine blade #18 of the Gezhouba station has efficiently prevented sediment abrasion. After construction of Three Gorge Dam, the upper stream became wider, and the flow speed reduced, meaning the sediment concentration is also reduced. This has meant that the conditions at Gezhouba station have improved. However, the Three Gorges Dam cumulates clean water while evacuating high sediment concentration flows – in the summer these sediments are exposed to Gezhouba, meaning abrasion of the project’s turbines still exists.
The authors are Vice Professor LI Cui Lin; Post doctorate LIU Jie; Professor LU Jin Yu; Vice Professor ZHU Yun, Northwest Electric Staff University
This paper has been translated by REN Hongqi, General Manager of Shaanxi Yufeng Coating Centre, who assisted Vice Professor LI Cui Lin.