A review on the large tilting pad thrust bearings in the hydropower units

Renewable and Sustainable Energy Reviews xx (xxxx) xxxx–xxxx

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A review on the large tilting pad thrust bearings in the hydropower units ⁎

Zhai Liminga,b, Luo Yongyaoa,b, Wang Zhengweia,b, , Liu Xina,b, Xiao Yexianga,b a b

State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Hydropower Thrust bearing Thermal-elastic deformation Transient Prediction Improvement

Tilting pad thrust bearings are the key components in hydropower units. As the thrust loads increase with the unit capacities of the hydro turbines, many thrust bearings failed due to rub-impacts between the collar and pads. Some plants have to be shut down for up to one month to repair the damaged pads which has lead to enormous economic losses. The lubrication processes of thrust bearings are quite complex involving fluidthermal-structural interactions between the pad, the collar, the oil film and the oil surrounding the pad. This paper reviews the bearing mechanism including the thermal-elastic deformation, the transient and dynamic characteristics, the bidirectional thrust bearing, as well as the various prediction methods on the lubrication performance. At last, an overview of some special designs including the covered materials, supporting system and hydrostatic jacking system to improve the thrust lubrication was briefly presented.

1. Introduction Energy is the fundamental requirement for economic development. The installed electricity generating capacity in China have reached nearly 1300 GW which was generated 65% through thermal, 33.5% through renewables and 1.5% through nuclear by the end of 2014. As the most important renewables for electrical power production worldwide, hydropower provides about 14% of the electricity in the world and about 23% in China. Large hydro generator units usually arrange the shaft system vertically in order to minimum the shaft deflections which cause large vertical thrust along the shaft from both the large weight of the rotating components and the axial hydraulic thrust on the turbine runner. Thus, large hydrodynamic thrust bearings are commonly used which allow substantial axial load to be transferred from the rotating components to the stationary components via a thin lubricating film with extremely low friction and practically no wear [1]. The load capacity of thrust bearings became the main factor that restricted the growth of hydro generator unit capacity after the first hydro turbine generator unit was put into operation at the end of the 19th century. In 1912, Kingsbury thrust bearing with the thrust load of 184 t was first applied in Holtwood Hydropower Plant located in Pennsylvania, USA, which made it possible to design much larger hydroelectric units with substantial axial load. Fig. 1 presents the development history of hydro power projects in the world in the recent 100 years. Weights and sizes of both the generator and hydro turbine are increasing as well as their thrust bearings. Table 1 lists several typical hydro power units and the thrust bearing parameters in China. ⁎

Now the single rated capacity of the hydropower units have reached 840 MVA with a hydro turbine of 700 MW in Three Gorges Hydropower Plant and 889 MVA with a hydro turbine of 800 MW in Xiangjiaba Hydropower Plant,. The largest load capacity of the thrust bearing has increased to about 6000 t. Fig. 2 illustrates the development of the thrust bearings for hydro generators units manufactured by Harbin motor co., LTD, one of the largest electric generator set manufacturer in China, with the thrust load increasing greatly. The thrust bearing co-produced by Harbin motor co., LTD and ALSTOM for the Three Gorges Hydropower Plant has the largest load capacity of 5410 t up to now in the world. In addition, the loads of the bidirectional thrust bearings for the pump storage plants (PSP) have reached up to about 1000 t, but are quite lower than the directional thrust bearings for conventional hydropower units. As the thrust load increases, many thrust bearing failures have happened in the world. In Russia, the thrust bearing with a capacity of 1450 t for 250 MW hydro turbine unit in Bratsk Hydropower Station was broken down in 1982. In America, the 4050 t thrust bearing for the 600 MW hydro turbine unit in the Grand Coulee Hydropower Station was burnt-out in 1981. In China, many times of “thrust bearing burning” accidents happened with the load from 650 t to 3800 t in the hydropower plants such as Wujiangdu, Longyangxia, Dahua, Gezhouba, Baishan, etc. In recent years, the accidents mainly occur in the pump storage units because of the low load capacity of the bidirectional thrust bearing and the frequent starts and stops for the grid demand [4–6]. According to the statistics, bearing failures mainly involving thrust bearings account for around 40% of operating losses in

Corresponding author at: Department of Thermal Engineering, Tsinghua University, Beijing 100084, China. E-mail address: [email protected] (W. Zhengwei).

http://dx.doi.org/10.1016/j.rser.2016.09.140 Received 29 May 2015; Received in revised form 6 July 2016; Accepted 30 September 2016 Available online xxxx 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

Please cite this article as: Zhai, L., Renewable and Sustainable Energy Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.09.140

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Fig. 1. Development of the hydro generators in the recent century [2].

one is that the bearing should have good lubrication within different speeds for both the rated condition and even the transient processes during starts and stops, the other is that the bearing should have enough load range in case of the uneven load distribution among the pads. In the past decades, many researchers have devoted themselves to the thrust bearings and promoted the development. Tanaka [8] reviewed some papers published mostly in the 1990s which is on

the hydro power units. Fig. 3 shows a damaged sliding surface of a thrust bearing due to overload [7]. Therefore, it is very important to research the lubrication mechanism of the large thrust bearing. Since the thrust bearings are one of the most crucial components in large hydropower units which greatly affects units’ safe and stable operation, whose design and manufacture have attracted much attention. There are two main demands for the large thrust bearing design:

Table 1 Main parameters of the thrust bearings for several typical hydro power units in China.

Commissioning time Unit number Output power (MVA) Rated speed (rpm) Thrust load (KN) Outside diameter (mm) Inside diameter (mm) Pad number Pad area (cm2) Single pad load (KN) Specific pressure (MPa)

Three Gorges

Shuikou

Longtan

Xiaolangdi

Gezhouba

Yantan

Xiaowan

Laxiwa

2003 8 840 75 54,100 5200 3500 24 4011 2254 5.6

1992 7 222.2 107.1 41,000 4500 2600 18 4800 2278 6

2007 7 777.8 107.1 35,257 4500 2800 18 4332 1959 4.5

2000 6 343 107.1 34,700 4150 2740 20 2870 1870 5.9

1980 13 143 62.5 33,000 3900 2450 18 3210 1833 5.6

1992 4 345.7 75 27,500 3750 2350 16 3260 1719 5.3

2008 6 777.8 150 26,460 4000 2530 16 3769 1654 4.4

2008 7 757 142.8 25,600 4240 2740 18 3198 1422 4.4

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Fig. 4. Typical structure of a thrust bearing [9].

in hydro generator units are easier to give the oil film a wedge shape in the rotational direction and generate higher dynamic pressure, which is an important reason that the tilting pad bearings are adopted for large hydro generator unit. Since the viscosity lubricant shearing stress is great, large amounts of heat are generated in the film during operation and then will transfer to its ambient environment. The details of the heat transfer are briefly summarized in Fig. 5 [10] with four types: (1) heat transferred to the runner and pad by conduction (2) heat convention lost as a result of side leakage at the inner and outer radius of the pad (3) heat carried away by the oil flow in the angular direction (4) heat transferred to the relatively cold oil in the groove between the pads, due to the intermixing with the hot oil coming out of the trailing edge of the pad (5) Heat entered the next pad by the rotating collar which accounts for about 70–90% of the total heat generated in the film. Pajączkowski et al. [11] computed the temperature distribution in the film of a centrally supported thrust bearing. Nonuniform temperature distributions were presented on the pad sliding surface with higher temperature close to the film outlet as shown in Fig. 6(a). The temperatures at an angular section show large temperature gradients in the film thickness direction although the minimum thickness is only about 100 μm as shown in Fig. 6(b). In addition, the temperatures on the runner sliding surface are almost identical in the angular direction due to the rotation of the runner. However, the side-effect of the heat generation is the consequent thermal deformations in the pads and collar. Thermal deformation usually makes pad form a convex surface, while pressure deformation makes it form a convex or concave surface, depending on the support type of the pad which greatly influence the minimum film thickness and the maximum pad surface temperature. As the thrust load increases, the specific pressure and temperature of the oil film will increase. As a

Fig. 2. Thrust load development of the thrust bearings produced by Harbin motor co., LTD [3].

thermohydrodynamic analysis of journal and thrust bearings used for turbomachinery and engines. In order to understand the lubrication mechanism of the thrust bearing and identify gaps for future studies, this paper summaries studies carried out by various investigators who focused on the lubrication characteristics of the thrust bearings in large hydropower units. The main topics are covered in the previous studies as follows: a. Mechanism studies on the lubrication characteristics of the thrust bearing. b. Development of the methods on prediction of the thrust lubrication. c. Transient and dynamic characteristics of the thrust bearing. d. Bidirectional thrust bearing for pumped storage power plant. e. Special designs to improve the lubrication. 2. Mechanism of thrust bearing lubrication In most hydro turbine thrust bearings, a tilting pad design was adopted to increase the load capacity and improve the unit stability. Fig. 4 shows a typical large hydrodynamic thrust bearing which consists of the following components: pads, collar attached to the shaft, pad support system, cooling system and the bearing housing that transfers the load to the foundation [9]. A very thin film will form between the sliding surface of the collar and each pad, and provides substantial pressure to support the axil thrust of the rotating parts in case of the following conditions: adequate viscosity of the lubricant, adequate relative speed of both sliding surfaces and favorable wedge shape of oil film. The tilting pads used

Fig. 3. Damaged sliding surface due to overload and loss of the load capacity [7].

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which usually is computed using finite differences (FDM) or finite elements methods (FEM) to solve generalized Reynolds equation for the flow and energy equation for heat generation. The other part of the model are the pad and the collar, which most commonly is computed using FDM or FEM to solve heat conduction equation for the heat conduction and elastic equilibrium equation for the deformation. According to the physical fields involved in the computation, there are mainly three calculation methods used to compute the lubricating performance of thrust bearings up to now: HD (hydro-dynamic), THD (thermal-hydro-dynamic) and TEHD (thermal-elastic-hydro-dynamic) methods. THD (thermo-hydro-dynamic) takes oil heating and temperaturedependent viscosity into account by solving energy equation which treats the temperature field either in 2D or 3D way. In the 2D approach temperature varies only along the length and width of the film but remains constant in the film thickness, while the 3D approach allows for the temperature variations in all three dimensions. The THD solution can also be divided into two cases: one is only film domain included in the model with the adiabatic assumption at the interface of the film and the pad (or the runner), the other is both film and pad (or collar included) in the model by simultaneously solving the Reynolds equation, energy equation for the film and heat conduction equation for the pad. Unlike HD and THD method, TEHD (thermal-elastic-hydro-dynamic) method calculates the lubrication considering thermal-elastic deformations of both pad and collar or pad only by simultaneously solving the Reynolds equation, energy equation for the film and heat conduction equation, elastic equilibrium equation for the pad and collar. Recent studies showed that for large thrust bearings the deformations of the collar have to be included in the computation due to its large effect on the oil film geometry and consequently on the lubrication performance of the whole bearing [14].

Fig. 5. Control volume for the heat transfer analysis [10].

consequence, the oil viscosity will decrease, and the thermal deformation of the pad and collar will increase, which usually makes the load capacity reduce, oil film thinner, and seriously leads to rub-impact between the collar and pad. lliev [7] reported a failure investigation regarding a hydro-generator thrust bearing caused by the rub-impact with the damaged area located in the middle of the pad as shown in Fig. 3. Therefore, an important object during the design phase is to reduce the thermal deflections in the pads and runner. Dąbrowski et al. [12] analyzed the thrust bearing performance with supporting discs. The influences of two main design parameters, namely the pad thickness and pivoting disk diameter, on the thermal-elastic deformation and the bearing lubrication were investigated with the results shown in Fig. 7. He summarized three main methods to reduce the thermal deformations: (1) Special design of the support aimed at compensating for thermal deformation with elastic deformation. (2) Decreasing temperature difference between the top and bottom surfaces of the pad. (3) Decreasing the size of the pad or increase the span of supported part of the pad. However, the thermal-elastic deformations in some cases are useful. Raimondi et al. [13] demonstrated that pad deformation are benefit for the load-capacity of centrally supported bearing using an isothermal model of the oil film. The results indicated that the pad has no load-capacity if there is no deformation, but can obtains maximum load-capacity (marked by ‘a′ in Fig. 8) if the deformation is almost equal to the minimum oil film thickness. Thus, not only THD analysis of the lubrication films but also thermal and pressure deformation analysis of pad are necessary for the better analysis of the thrust bearings. Therefore, a good thrust bearing is required to design to obtain adequate load-capacity, to keep the maximum pad temperature lower than an upper limit, and to keep the minimum film thickness above a lower limit for the operating conditions.

3.1.1. HD analysis In 1885, Tower [15] first observed the plug always popped out in an oil sump lubrication experiment and found fluid dynamic pressure existed in the oil film of the bearing. In 1886, according to Tower's experimental results, Reynolds [16] simplified the Navier-Stokes equations，and then derived the Reynolds equation which became the fundamental equation for the lubrication problem. Reynolds equation is a 2D equation from the N-S equations with some assumptions such as laminar and incompressible flow assumption, isothermal flow assumption, zero pressure gradient in the film thickness and so on. In 1940s, Christopherson et al. [17] and Cope et al. [18] developed HD model for the thrust bearing based on Reynolds equations and discussed the basic lubrication characteristics of bearing. Although afterwards this model was proved to have a difference from the measurement, this model opened the door to the lubrication analysis of the thrust bearing.

3. Predictions for the thrust bearing lubrication 3.1. Conventional approaches based on Reynolds equation Looking into the history of the thrust bearing analysis, the calculated model is developed from simple to complex. In order to simulate hydrodynamic bearing operating conditions in a realistic way, it is necessary to consider several physical phenomena. The most important part of the thrust bearing model is the hydrodynamic film,

3.1.2. THD analysis As we know, Reynolds equation uses isothermal models of oil film, so the temperature effects could not have been observed because of simplified film models used. In 1957, Ziekiewicz et al. [19] and

Fig. 6. Temperature distribution, (a) on the pad sliding surface, (b) on the angular section [11].

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Fig. 7. Radial cross-sections of the pad, comparing its total and thermal deformations, (a) a normally loaded pad and overloaded pad, (b) the present and a decreased diameters of the supporting disk [12].

influence of the electrical pitting of a tilting pad thrust bearing using a detailed 2D THD model. Tieu [25] used finite element method to perform a 3D THD analysis of a thrust bearing which can only tilt circumferentially. Kim et al. [26,27] developed a 3D THD model of a thrust bearing taking radial tilt into account. Comparing with the 3D THD results, they found the temperature from 2D THD model based on adiabatic theory is higher than the measurement which means that 3D THD model is closer to the real situation. However, this studies did not consider the heat transfer between the film and pad with the adiabatic boundary between the film and the pad (or the runner). In order to predict the temperature distribution in the film accurately, it is necessary to solve the Reynolds equation, energy equation of the film, viscosity-temperature equation and the heat conduction equation in the pad and runner simultaneously. The THD solution including the heat transfer between the film and pad can be divided into three methods: one is to solve the modified energy equation by adding a heat conduction effect item. Ma et al. [28] developed a modified 2D energy equation including the cooling effects of the water-cooled tubes in the pad. Wasilczuk et al. [29] carried out an optimization for a hydrodynamic thrust bearing using a special objective function. In the optimization procedure a THD model (described in [30]) of the fluid film with 2D energy equation and heat transfer through oil, bearing and rotor. The obtained results illustrate the differences of the optimum gap profiles for various operation conditions. The second method is to assume a heat exchange coefficients at the interface between the pad and collar as describe by Zhao et al. [31]. The third method is to solve the energy equation for the film and heat conduction equation for the pad simultaneously described by Chen [32] and Huang [33]. The thermal-hydrodynamic phenomena and boundary conditions considered in the THD model of the bearing are presented in Fig. 9 [12]. Almqvist et al. [34] developed a THD model of a pivoted thrust bearing that can tilt both radially and circumferentially. This model allows for 3D temperature variation in the film and pad, but 2D temperature variation in the collar. The numerical results show fairly good agreement with the experiments.

Fig. 8. Load capacity of the bearing pad as a function of relative deformation shown by Raimondi [13].

Sternlicht et al. [20] presented their studies about the thermal effects in the bearing in the same conference. In 1963, Dowson et al. [21,22] put forwards a new term “THD model” which open the door to the thrust bearing lubrication analysis with thermal effects. They concluded that the heat induced by the shear viscosity stress in the oil film will decrease the oil viscosity and density which will consequently reduce the oil film thickness under a given load. It is necessary to utilize more realistic oil film models with viscosity-temperature effects included. Liu [23] analyzed the thrust bearing in a hydro turbine unit by solving the Reynolds Eq., 2D energy equation and viscosity-temperature equation iteratively. The relationship between the load capacity and the minimum oil thickness were discussed. Recently, Steven [24] analyzed the 5

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hydro generator units. Fig. 10 illustrates a solution process of TEHD analysis for a tilting pad thrust bearing described by Heinrichson et al. [36]. TEHD analysis can also be divided into 2D TEHD and 3D TEHD analysis depending on whether to take the temperature variation in the film thickness direction into account. Ashour et al. [10] investigated a large thrust bearing with spring mattress supported using 2D TEHD method. The pad deformation was solved by the bi-harmonic bending equation with a thin plate assumption. The results show that the thermal-elastic deformation have significantly effect on the performance of the bearing. Sternlicht et al. [35] performed a 2D TEHD computation for a thrust bearing while Abde et al. [37] conducted a 3D TEHD analysis. They both treated the pad as a 2D plate. Ma et al. [28] conducted a 2D TEHD analysis for a thrust bearing in Gezhouba Hydropower Plant, China, using a 2D modified energy equation included a heat transfer term to consider the water cooling effect in the pad. A 3D pad model is solved to get the pad deformation using the finite element method. Markin et al. [38] used 2D film elements to model the oil film and 3D solid elements to model the pad and the shaft segment for a PTFE faced pad thrust bearing using the commercial FEM codes (SOLVA). The 2D element for the film is based on the Reynolds equation, so this is also a 2D TEHD solution. Jiang [39] employed a 3D TEHD model to analyze the effects of thermo-elastic deformation, rotational speed, and thrust load on lubrication performance of a tilting pad thrust bearing of a hydro turbine unit. The Reynolds equation, the viscosity-temperature of the lubricants, energy equation, film thickness equation, and heat conduction equation were simultaneously solved by the FDM. But 2D model was used for the pad with the heat distortion of the pad negligible due to the low tempera-

Fig. 9. Phenomena considered in the THD model of the bearing [11].

3.1.3. TEHD analysis In 1961, Sternlicht et al. [35] conducted a 2D adiabatic TEHD analysis for a centrally pivoted tilting-pad thrust bearing. He found that both the elastic and thermal deformations are usually several times of the minimum oil film thickness, which means the elastic and thermal deformations of the pad cannot be neglected in case of high rotational speed and heavy load especially for the thrust bearings used in the large

Fig. 10. Flow chart for the TEHD program [36].

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Fig. 11. TEHD model (a) Three domains including the pad, film and collar, (b) collar deformation due to the pressure applied and thermal expansion, (c) Temperature distribution of the pad [50].

However, the studies above only considered the pad deformation with the runner excluded. Brockett et al. [46] conducted a 3D TEHD simulation of the parallel tapered-land thrust bearing considering the deformation of both the pad and the runner. The results show that the predicted minimum film thickness with runner deformation included is lower than that with runner deformation excluded. But one drawback in this study is that the runner is assumed to be isothermal and consequently to be deformed only by pressure action. In fact, the r temperature of runner varies in the radial direction described in reference [47]. Ahmed et al. [48] studied the influences of the bearing deformation on the lubrication of a hydrodynamic fixed geometry thrust bearing. The results show that the runner thickness has a great influence on the pressure and the film thickness distribution. Ma et al. [49] conducted a TEHD lubrication analysis of PTFE thrust bearing of a hydro turbine unit considering the heat transfer in the runner. It is verified by the lubrication analysis that the plastic pad bearing is a good solution for the large and heavy-duty bearing. Borras et al. [50] built a multiphysics (TEHD) model of spring-supported thrust bearings for hydropower applications as shown in Fig. 11. In this study, the Reynolds equation is solved taking pad and collar elastic deformation and thermal expansion into account. Recently, some researchers combine their edited programming with the commercial codes to reduce the programming workload and improve the computation accuracy. Huang et al. [51] did TEHD analysis for bidirectional thrusts in pump-turbine units. He used ANSYS 11.0 to calculate the pad and collar temperatures and deformations as the inputs for the Reynolds equation and film thickness equation in the loop iterations. The results show that the thermalelastic-hydro interactions give the pad a convex sliding surface and make the collar lifted at the outer radius as shown in Fig. 12. Wu et al. also used the similar method to join the in-house codes with ANSYS to compute the TEHD lubrication for the directional thrust bearing [52] and bidirectional thrust bearing [53] in hydro power units.

ture rising; Only elastic deformation of the pad surface was taken into account in the film equation. However, these studies did not conduct 3D TEHD analysis with 3D pad model simultaneously, which is an important factor causing the calculation inaccuracy.

∂ ⎛ ∂P ⎞ 1 ∂ ⎛ ∂P ⎞ ∂ ⎛F ⎞ ⎜F2 ⎟ = ωr ⎜ 1 ⎟ ⎜rF2 ⎟ + ∂r ⎝ ∂r ⎠ r ∂θ ⎝ ∂θ ⎠ ∂r ⎝ F0 ⎠

(1)

h = h m + γp [r sin(θp − θ ) − rm sin(θp − θm )] + u m − up

(2)

lg lg(η + 0.6) = A + B lg T

(3)

⎡ 1 ∂ ⎛ ∂T ⎞ ⎛ ∂T V ∂T ∂T ⎞ 1 ∂ 2T ∂ 2T ⎤ ρcp ⎜U + θ + W ⎟ = k⎢ + 2 ⎥ + μψ ⎜r ⎟ + ⎝ ∂r r ∂θ ∂z ⎠ ∂z ⎦ ⎣ r ∂r ⎝ ∂r ⎠ r 2 ∂θ 2 (4)

∂ 2Tp ∂r 2

∂ 2Tp 1 ∂ 2Tp =0 + + 2 + ∂zp 2 r ∂r r ∂θ 2 1 ∂Tp

(5) (6)

[K ] × {U} = {r} + {R} h dz ; 0 μ

h zdz ; 0 μ

h z (z − F1 / F0 ) dz ; μ 0

F2 = ∫ F1 = ∫ P is the oil film where F0 = ∫ pressure; ω is the rotational speed; μ is the dynamics viscosity; hm is the minimum film thickness; γp is the tilt angle; um is the pad thermoelastic deformation at the minimum oil film thickness; up is the axial thermo-elastic deformation of the pad sliding surface; η is the kinematic viscosity of the lubrication oil; A and B are the coefficients decided by experiments; T is the oil film temperature; Tp is the pad temperature; [K] is the stiffness matrix of the pad; {U}is the displacement vector of the pad; {r}is the node force vector of the pad; {r}is the node temperature vector of the pad. The 3D TEHD solution with 3D pad model can be described as (1)– (6) including the Reynolds equation, oil film thickness equation, viscosity-temperature equation, energy equation, heat conduction equation and elastic equilibrium equation. In 1988, El-Saie et al. [40] used TEHD model to analyze the effects of defections due to both oil film temperature and pressure in a pivoted pad thrust bearing, and they [41] compared the computed results with experimental data, and generally good agreement was obtained for both pad surface temperature and film thickness. GLAVATSKIH et al. [42] did an investigation in the influence of oil thermal properties on the lubrication of a tiltingpad thrust bearing using a 3D TEHD model in which thermal effects were locally included considering the heat transferred into the pads. The results show that viscosity index has a more profound effect on bearing operation than oil thermal conductivity and heat capacity. Other TEHD analysis by solving 2D Reynolds equation and 2D/3D energy equation for the film, and by solving heat conducting equation for the pad, can be found in references of Ma et al. [43], Yang et al. [44], Yuan et al. [45].

3.2. Advanced approaches based on Navier-Stokes equations 3.2.1. HD analysis The HD method solving the Reynolds equation only includes the film domain without the flow around the pad, treats the bearing surface to be flat and ignores some detailed geometry such as oil grooves on the pad surface, which may have great influence on the lubrication. In addition, the common method, namely finite difference method, is limited for the pad with complex sliding surface. With the development of computational technology, the Computation Fluid Dynamics (CFD) method is popular in recent years. Yu et al. [54] used CFD method to do a HD simulation of a hydrostatic thrust bearing with sector recess multi-pad. The CFD model in this computation only includes the flow 7

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Fig. 12. Thermal-elastic deformation of the thrust bearing, (a) Pad, (b) Thrust collar [51].

field between the pad and runner. The results show a great influence of recess area on the lubrication performance. Wang et al. [55] built a 3D CFD model for the thrust bearing in Three-Gorge hydropower station. Not only the oil film but also the flow around the pad in the oil tank were modeled. The oil was considered as an incompressible isothermal Newtonian fluid. The effects of the oil film clearance size and rotational speed on the load capacity were discussed.

Table 2 Heat convection coefficient assumed for TEHD analysis of thrust bearings in different studies [59]. Authors

3.2.2. THD analysis The conventional solution computes the oil film flow using Reynolds and energy equation in which inlet temperatures are one of the main inputs. However, the flow mixture in the space between the adjacent pads greatly affect the oil temperature and velocity distribution at the film inlet. Ettles et al. [56] researches on the oil flow in a thrust bearing and found that about 85% of hot oil leaving the film enters the next oil film. Later, he proposed the ‘hot oil carry-over factor’ to consider this effect and assessed its values as a function of the sliding speed and the gap size between the bearing pads by experiments [57,58]. But there are still many unknown details about the flow mechanism in the gap between the pads. In addition, the pads in the thrust bearings for large hydro turbine units are usually fully submerged in the lubricating oil. During operation, part of the heat generated by the oil film shearing is transferred to the pad through the sliding surfaces and then to the surrounding oil through the pad free walls by convection. Thus, as a consequently a thermal gradient in a pad is caused and form its deflection, and thereby decreases the load capacity. Therefore, the heat convection coefficients for the pad free surfaces are very important for thrust bearing lubrication analysis. In the TEHD solution that solves the Reynolds equation and energy equation for the film domain and heat conduction equation for the pad domain iteratively, the heat convection coefficients are usually defined as experiential or arbitrary constants as listed in the Table 2 [59]. However, in reality, heat transfer coefficient and their corresponding ambient oil temperature at one pad surface are not uniform, and different surfaces have different thermal properties. The assumption for the heat convection coefficient is one of the main factors resulting in the errors of TEHD computation. Now CFD method is a good way to study the oil film flow, and the flow between bearing pads considering the viscosity shearing heat generation without the assumption of the temperatures at the film inlet and the heat convection coefficients at the pad free surfaces which can be solved automatically in the CFD process. Zhang et al. [63] were the first ones to use CFD techniques to carry out computations of a complete thrust bearing model include the oil film and the space between the pads. Wasilczuk et al. [64] also built a 3D CFD model

Bearing outer diameter (mm)

Heat convection coefficient hc

(W/m2K)

Vohr [59]

7 bearings in the range of 700–3100

567

El-Saie et al. [40]

2940 787 149

450 −162+16ω , for ω > 10 (1/s) 1000

1590

710 at pad bottom

Ettles [60]

Comments

Function of ω

Function of U , μ and B

1420 at pad leading, trailing, and inner radius surfaces, 2840 at pad outer radius surface Markin et al. [37]

Glavatskich et al. [41]

Heinrichson et al. [61]

228.6

950 228.6

2196

100, except pad corner near trailing edge where 500 was imposed 100 100

758.2 at pad bottom 1516.4 at pad inlet and outlet 436.1 at inner and outer radius

Received in “turning” procedure with the use of measured data

Ta changed depending operational conditions, according to measured data

on

the

At pad bottom, inlet, and outlet Ettles formula, at inner and outer radius function of pad radius

consisting of the oil film and the pads space for the thrust bearing with an oil supply system. The good results shows that the THD analysis with CFD method can improve the inputs to traditional method based on Reynold equation. It also helps to optimize the oil supply system in the thrust bearing. Later, Grzegorz et al. [65] carried out fluid film calculations in a thrust bearing using CFD software in order to study the effect of the lubricating groove on the lubricating characteristics. 8

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Fig. 13. (a) Thrust bearing with a system of direct oil supply, (b) Geometry and boundary conditions for the 3D THD calculation [64].

3D model including the oil film and the space between the pads was built with adiabatic conditions assumed for the bearing pad while adiabatic or isothermal boundary conditions applied for the runner in two cases as shown in Fig. 13. The bearing pad surface was assumed as an inclined plane. The numerical results show that different chamfer heights at the inlet groove lead to different max temperatures on the pad surface. Although the 3D flow with the viscosity shearing heat generation in the film of the thrust bearing was included using CFD techniques in the studies above, the heat transfers between the oil with the pad and runner were still not considered. Further works have been done to improve this problem. Wasilczuk et al. [66] continued to use CFD software to study the influence of the oil groove on the lubricating performance of a large thrust bearing in the hydro power unit, especially on the power loss. ANSYS MFX Solver combined ANSYS CFX for fluid calculations and ANSYS Mechanical for structural calculations is used in the study. The deformations of the pad and runner surfaces were calculated in the external bearing model described in reference [12] and were used as the mesh initial deformation. So during the computation, only temperatures and heat fluxes are transferred between the fluid and structure without mesh displacement and fluid force. In addition, steady state conditions and turbulence model k − ω based on SST were used. Presented results show that supply groove at the leading edge of the pad has a potential for improving running characteristics of large thrust bearings without any increase in cold oil flow. The CFD model and numerical results of power loss in the bearing are shown in Fig. 14. In another way, Papadopoulos et al. [67] conducted CFD simulation to characterize the THD lubrication performance of a sector-pad thrust bearing with the commercial code ANSYS CFX 14. Only conjugate heat transfer features between solid and fluid in CFX were used instead of the MFX feature in ANSYS.

3.2.3. TEHD analysis As discussed above, the thermal-elastic deformation of the pad and collar should be included in the lubrication for the large thrust bearing. Hemmi et al. [68] conducted a tilting-pad thrust bearing in a hydro turbine unit with the solution using CFD (STAR-CD) for the fluidthermal filed in the oil film and FEM (ABAQUS) for the thermal-elastic filed in the pad and collar. Firstly he used CFD method to solve the flow and heat generation in the oil and the heat conduction in the pad. Then according to the temperature distribution in the pad and pressure distribution on the sliding surfaces, he calculated the thermal and mechanical deformation and use the deformation results as the inputs for the oil film computation. This process was repeated until the pad deformation was converged. Wodtke et al. [69] used the fluid-structure interaction (FSI) technique in ANSYS to investigate the TEHD lubrication of a tilting pad thrust bearing. The technique combines CFD and FEM packages which allows for the simulation of the whole thrust bearing including oil flow, bearing components with boundary conditions like oil temperatures at the film inlet and heat exchange coefficients on the pad free walls moved away. Unlike Makoto Hemmi's solution [68] which computed the pad temperature distribution in CFD codes, Wodtke's method separated the bearing model into fluid-thermal field of oil flow using CFD and thermal-elastic field of pad (collar) using FEM as shown in Fig. 15. Both field of TEHD model are connected internally in the forms of fluid-structure interfaces through which loads are exchanged between two fields during the iterative solution. Thermal-elastic deformations of the pad and collar change oil gap geometry, while the pressure and heat flux in the oil film are then applied to solids as boundaries with the process shown in Fig. 16. Later, Wodtke et al. [70] used this FSI method to do a TEHD analysis for a thrust bearing in a hydro power plant. The numerical results are agree well with the measurement data. This study also demonstrated that one feature of the FSI technique is that the heat exchange

Fig. 14. (a) Geometrical model of the fluid part surrounding the bearings pad, (b) Components of power loss in food lubricated bearing [66].

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Fig. 15. Solid and fluid domain the FSI analysis of a thrust bearing [69].

is a popular direction which researches the changing rule of the bearing lubrication parameters from the disturbed state to the stable state considering the thermal-elastic deformation of the pads. Glavatskikh et al. [73] conducted comprehensive experiments of transient thermal effect for a laboratory-sized bearings following a sudden change in operating conditions. The effects of sudden variations in rotational speed, load and oil supply flow rate on the bearing friction torque, collar and shaft temperatures and pressure profile were presented and analyzed. Wu et al. [74] summarized the effects of abnormal operating conditions such as start-up, run away and load rejection on thrust bearing lubrication based on the operating data of Shuikou Hydro Power plant. Ettles et al. [75] summarized the results of general design studies of various thrust bearing configurations under transient operation conditions. It is shown that transient effects can cause an ‘overshoot’ of thermal deformation which would lead to “thermal ratchetting”. The studies also indicate that a peak deflection occurs well before thermal equilibrium is reached. Pajączkowski et al. [11,14] conducted the transient TEHD simulation of hydrodynamic tilting pad thrust bearings in warm and cold start-up using a new approach which combines FEM and CFD. The static oil pocket and inlet and outlet chambers were modeled thanks to this advanced technique FSI. The results show that the minimum oil film thickness value reaches to stabilization almost immediately, while the stabilization of the deformations requires much more time shown in Fig. 18.

Fig. 16. Multi-physics solver process in TEHD analysis.

coefficients on the pad free surface are not input data assumed at the beginning of the calculation, while they are automatically calculate in the course of calculations and would be different in different locations of the computation process. Zhai [71,72] also applied FSI method to the 3D TEHD analysis for a bidirectional thrust bearing in a pumped storage plant. The influence of the pivot location of the pad on the lubrication characteristics were discussed.

4. Other main features of the thrust bearings 4.1. Transient characteristics

4.2. Dynamic characteristics

Recent designs of bearing are usually very reliable, while older machines often failed within the first few minutes during start-up. Since relative motion of the sliding surfaces produce the hydrodynamic pressure in the film, so low speed during start-up and shut-down makes it difficult to generate adequate pressure to provide full film lubrication. As shown in Fig. 8, the optimum point (marked by ‘a′) gives maximum load-capacity. If transient deformations are higher than optimum ones (‘b′), the load-capacity would decrease greatly (50%) with oil film thinner and temperature higher which then leads to even larger thermal deflections of the pad (‘c′). Finally, this ‘chain reaction’ may cause bearing seizure. This is particularly the case with pump-turbine units which are often required to switch modes several times daily. Each change of mode requires a reversal in the direction of rotation. Fig. 17 shows the wiped pads by a thermal ratchetting mechanism during the early stages of a cold start. As the more and more pump storage power plant units are built, the transient thermal behavior of thrust bearings has become more important. Therefore, it is necessary to investigate the transient lubrication performance to estimate the load capacity with good accuracy during design. However, standard steady-steady analysis may not provide accurate information about transient performance because thermal fields of the bearing need at least several minutes to adapt to new conditions whereas the operational parameters are usually changed very quick. Transient thermal-elastic-hydro dynamic (TTEHD) lubrication analysis

One feature of the thrust bearing is to support the weight of the rotating parts and the hydraulic thrust, and the other important function is the ability to control the axial vibration which is usually described by dynamic characteristics of stiffness and damping. Li et al. [76] derived the Taylor expansions of the oil film thickness equation, transient Reynolds equation and calculated 28 dynamic characteristics coefficients including stiffness and damping. However, the heat generation in the film, heat transfers in the pad and consequent thermal-elastic deformation are neglected. Jeng et al. [77] employed spherical crowning model to simulate the pad elastic-thermal deformation of a pivoted pad thrust bearing and then calculated the linear stiffness and damping coefficients which were strongly affected by the degree of pad crowning. Ma et al. [28,42] conducted 2D and 3D TEHD lubrication computation for a tilting pad thrust bearing using finite element method. He calculated the stiffness coefficients using partial derivative method. Srikanth et al. [78] developed a new method to calculate the angular stiffness of the thrust bearing. He did 2D TEHD analysis using finite difference method to solve the Reynolds and energy equation, while using ANSYS to determine the pad deformation. The factor causing dynamics of the bearing elements in hydro generators were also discussed. Guo et al. [79] developed a new rig for tilting-pad thrust bearings 10

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Fig. 17. Wiped pad from pump-turbine applications: (a) at Bath County power station, (b) at Sir Adam Beck power station.

plants of 21,980 MW. Bidirectional thrust bearing is a key component of PSP units, directly influence the reliability of the unit, and then effect the stability of the power grid. The conventional thrust bearings in hydro power units are normally designed as unidirectional ones with best load capacity obtaining at the angular eccentric ratio of 58～60% [80]. However, the pads of bidirectional thrust bearings in PSP units have to be designed as centrally support to get favorable lubrication performance in both directions. Thus, this defect gives bidirectional thrust bearing much lower load-capacity than the conventional directional bearing, which is an important factor limiting the development of large PSP units. In addition, frequent start-ups and shut-downs, and higher rotational speeds than conventional hydro generators are also the special design problems for the bidirectional thrust bearings in pumped storage power stations. Luo et al. [80] illustrate the scope of application and operating performance of thrust bearings with different centrally supporting structure in various operating conditions by means of theoretical calculation and experimental analysis. Huang et al. [51,81] and Wu et al. [53] conducted experiments for a bidirectional thrust bearing in a test rig and verified the 3D TEHD numerical results. Fig. 19 shows that the pressure center is almost at the pad angular center rather than off the angular center in bidirectional thrust bearing. Wang et al. [82] used CFD method to do 3D HD analysis for a bidirectional thrust bearing. The effects of the pad inclination angle and the rotor speed on the lubrication performance were analyzed. But, only HD model was used in this analysis.

Fig. 18. Calculated results in transient process (a) pad temperature, (b) pad deformation [10].

and measured the absolute displacement vibrations of the test experiment bearing with the changes of dynamic force. Then the dynamic characteristics of the test bearing were obtained. The experimental results show that the operating conditions influence largely on the static and dynamic characteristics.

6. Special designs to improve the lubrication performance As summarized before, the main problems of the design and operation of large tilting pad thrust bearings are [1]: (1) Excessive pad deformations affecting generation of hydrodynamic pressure (2) uneven load sharing among bearing pads (3) Inadequate load-carrying capacity at transient states. Therefore, some special designs have to be used to avoid or reduce these side-effects to obtain good lubrication performance.

5. Bidirectional thrust bearings in pumped storage plants Pump storage power (PSP) station is used worldwide as a reliable peaking power due to its good performance on the grid balancing, while other rebellious power generation like wind and nuclear power plants cannot be easily and quickly adjusted to the demand of the grid. Different from the conventional hydropower unit, an important feature of the motor-generators with pump-turbines in PSP units is the operation in both rotational directions. In one direction it operates as a turbine and generates electric energy during power consumption peaks, whereas in the other direction it operates as an pump using the electric energy to pump the water to higher elevation during energy consumption valleys. From the first PSP station built in Zurich, Switzerland in 1882, many developed countries have built a number of PSP to optimize their power grid. America had increased the PSP plants’ share of the total installed capacity by 2%, while France and UK more than 4%, and Japan about 11% at the end of the 20th century. In the past 20 years, China accelerated the development of PSP plants to meet the growing grid capacity with Guangzhou, Xilongchi, Huizhou, Xianyou PSP plants built. By the end of 2014, the capacity of built PSP plants had reached up to 22,110 MW, with building and approved PSP

6.1. Covered materials on the pad sliding surface In order to prevent pad damage and embed contaminative particles, tilting pads of the thrust bearings are normally coated with a thin layer of soft alloy, which is called while metal or Babbitt and imposes a temperature limit. The most usual problem of heavy load Babbitt thrust bearings was high temperature in the center and center-trailing areas of the pad which led excessive thermal deflection, wiping of the central areas of the pads and subsequent cascading damage from pad to pad as smeared Babbitt form one pad broke away and enter the film of the following pad. To solve this problem, PTFE-faced pads were used in large hydrogenerators instead of Babbitt pads in Russia in the 1970s [83,84]and 11

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Fig. 19. (a) Bidirectional thrust bearing with double elastic plate centrally supporting structure. (b) Experimental oil pressure distribution [51].

the TEHD results are verse. Uno et al. [87] experimentally researched the influences of PTFE material on the performance of sector-shape tilting-pad thrust bearing in hydro generators. The results show that the maximum pad surface temperature decrease markedly over a wide range of loads and that the maximum film pressure decreased as shown in Fig. 21(b). In addition, PTEF coating make surface wear reduce drastically and can consequently make a simpler design of the thrust bearing moving away an oil lift system with size reduction and longer operation life. Ettles et al. [88] used the GENMAT analysis software to do TEHD lubrication computations for PTFE and Babbitt bearings. The comparison results with test verification show that the power loss and temperature distributions of the two pads are almost identical while the Babbitt-faced, spring-supported pad can less crowning than the PTFEfaced disk-supported bearing. Recently, Glavatskih et al. [85,89] numerically and experimentally studied a thrust bearings using a thin layer of PTFE (1–2 mm) directly bonded to the pads without wire mesh. Some detailed lubrication characteristics of the PTFE bearing were analyzed. Fang et al. [90] also conducted TEHD analysis for PTFE pad and Babbitt pad bearings. The results also show that the PTFE pad has less thermal deflection than the Babbitt pad. Another similar technology used as the liner of large thrust bearings is PEEK (polyether-ether-ketone) layer coated on the pad, which bonded to steel with the use of porous bronze sintered onto the pad body as an intermediate layer. Fig. 20 shows the cross sections of both types of structures.

Fig. 20. Polymer composite layers based on (a) PTFE, (b) PEEK [14].

6.2. Supporting system for reducing the thermal deformation

was then introduced into China. The PTFE (Polytetraflorethylene) is usually classified as a material for use in dry sliding application because its friction coefficient against steel can be as low as 0.05 and the rate of wear can be very low as show in Fig. 20(a). The PTFE has higher thermal resistance and cause smaller thermal distortion of the pad than Babbitt layer due to which thermal gradient across the pad thickness is much lower and provides heat insulation. In many cases the only change made for the large thrust bearing was to remove the Babbitt from pads and replace it with PTFE. At present PTFE pads can be used at specific loads up to 10 MPa, although most PTFE bearings are running at pressures only slightly higher than the Babbitt bearings (2– 3.5 MPa) that were replaced. Glavatskih et al. [85] did further TEHD analysis of PTFE-faced pad thrust bearings using the method similar to [42]. He used a simple analytical model, Winkler model [86], to consider the soft layer deformation. Fig. 21(a) compares the THD and TEHD results of pad inclination for both the Babbitt and PTFE-faced pads as a function of speed. The THD model predicts lower pad tilt for the Babbitt pad, while

As mentioned above, excessive thermal deformations of the pads can greatly change the geometry of the oil film and consequently decrease the load-capacity of the thrust bearing. Many researchers have explored the way to reduce the deformation. Ettles and Anderson [91] did a 3D TEHD simulation of thrust pad bearings whose pads are supported by a disk. 3D film and 3D pad model were built. The THD problem was solved with the finite difference method, while the pad deformations induced by temperature and pressure fields were determined with the finite element method. Fig. 22 shows that the smaller size of support disk can make the minimum film thickness thinner and also the maximum pad surface temperature higher. It also shown that partial cut-away of the support disk are applied to increase the film thickness and to reduce the pad temperature. At the same year, Ettles [61] also conducted a 3D TEHD simulation to study the effects of support spring arrangements on the lubrication of the thrust bearings with spring-supported pads. The results show that the thermal-elastic deformation of the pad must be well controlled by selecting the support 12

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Fig. 21. Analysis for PTFE pad bearings, (a) pad inclination predicted by the THD and TEHD models for the Babbitt and PTFE bearings [85], (b) basic performance of test bearing (800 rpm) [87].

Fig. 22. The effect of the load and disk support size on the characteristics of the bearing (a) minimum film thickness (b) maximum pad surface temperature [91].

Xiaolangdi Hydropower Station. A most common method to limit the pad deformation is using the elastic deformation to compensate the thermal deformation through a pad supported on a ring-shape support described by Ettles et al. [92] and further discussed by Dabrowski et al. [12]. Fig. 23 shows that the temperature gradient through the pad tends to make the pad convex, while the hydrodynamic pressure tends to make the pad concave [12]. The resulting total deflection of the pad then turns out to be fairly small. They found that the thermal deformation is mostly determined by the temperature difference between the top and bottom of the pad, and seldom by the its thickness, while the elastic deformation is strongly affected by the pad thickness, support dimension, and oil film pressure. Double-layer pad with twin support is another important application to limit the thermal deformation which is applied in Guri II Hydro Power plant. The bearing pads consist of two parts: the upper was relative thin, while the lower was very thick with cooling ducts machined on its top. During operation, the collar rotation drives the oil to flow through ducts and then to cool the bottom side of the upper pad. Consequently, high temperature difference exists between the top and bottom sides of the upper part while low temperature difference in

Fig. 23. Schematic diagram of TEHD effects in thrust bearing [11].

spring arrangements properly. Now, several pad supporting designs have been developed to reduce thermal defections of the pad in thrust bearings such as large ringshaped supports [92], spring mattress pad supports [61], double support pads [93]. The most common support types of large thrust bearings in hydro turbine units are spindle supporting system, doubledisk supporting system, elastic disk supporting system, spring mattress supporting system as well as spindle support with pin, etc. In China, the small pin mattress supporting double-layer-Babbitt pad are applied in Three Gorges, Longtan, Laxiwa, Xiaowan Hydropower Plants, while elastic beam double-disk supported pads in Shuikou Hydropower Plant, and Elastic oil tank supporting elastic metal-plastic pads in 13

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Fig. 24. TEHD analysis (a) FEM model of the pad under the load of pressure. (b) Thermal-elastic deformation of the pad [103].

Fig. 25. A pad with a hydrostatic recess [104].

concavity caused by hydrodynamic pressure in the film. In addition, the spring-supported thrust bearing has good self-adjustment and is also benefit to reduce the axial vibration in operation [96]. Ashour et al. [97], Sihha et al. [98,99], and Ettles [61] include the effect of elastic deformation in a simplified way to study the lubrication of springsupported thrust bearings. Wang et al. [100] developed a 3D TEHD model to analyze the influence of spring pattern on the lubrication of the thrust bearing and put forward suggestions to improve the performance. Yuan et al. [101,102] performed laboratory experiments for spring-supported thrust bearing, compared the measurements with the numerical predictions by using a comprehensive commercial software package (GENMAT), and obtained good agreements. The pins-supporting system with similar principle to the spring supporting system. Huang et al. [103] employed a 3D TEHD model to investigate the lubrication characteristics of the thrust bearing with pins supporting system on a spindle in Three Gorges Hydro Power Plant, China, with the load capacity of about 6000 t. He used FDM to solve the Reynolds equation, energy equation for the film and heat conduction equation for the pad and collar, and used FEM software ANSYS 11.0 to solve the thermal-elastic deformation on the solids. The results indicate that the thermal-elastic deformation of the bearing components play an important role in its lubrication shown in Fig. 24. Hydraulic equalizing system is the most sophisticated method for load equalization. Hydraulically connected pistons are usually applied to support the bearing pads. If the force acting on a certain pad is high, then higher pressure is generated in the piston below this pad. Consequently, the oil then flows from the piston with higher pressure to the piston with lower pressure until the pressure are equal to each other. Now this kind of supporting system has been applied in

the lower part which consequently results in small total thermal deformation. Huang et al. [81] used TEHD method to analyze the effects of the supporting type on the lubrication of the thrust bearing. The results show that the thermos-elastic deformation distribution on the pad surface of cutting away disk or double-disk supported thrust bearing are better than the single-disk one, which leads to prior lubricant performance. 6.3. Supporting system for sharing load among the pads Load sharing is also an important aspect of large thrust bearings which is rarely referred to in the previous references [94,95]. Various inaccuracies of assembly, manufacturing and deformations of the bearing usually cause a misalignment on the surfaces of all the pads, which means the surfaces are no longer on one flat plane. Consequently, the load on bearing pads may be uneven with some pads being overload that may cause wear on these pads and failure to the whole bearing. To equalized loads acting on the pads, the thrust bearings in the hydro generator unit are usually designed with some particular supporting system such as jacking crewing supporting, spring mattress supporting, pin mattress supporting, ring-shaped disk supporting which have another function to reduce the thermal deformation. Spring mattress supporting system is widely used for large thrust bearings which can not only equalize the load distribution on the pad but also reduce the thermal deformations of the pad. In this kind of bearings, coil springs (or Belleville springs, or rubber washer) are arranged like springs in a bed mattress next to each other. Thus, pad convexity caused by temperature gradient in the pad can be reduced by 14

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lubrication of the large thrust bearing with three dimensional model which could be a cost effective solution for an extensive analysis. A sectional model with symmetric boundary conditions is used to simplify the problem and save the computation time with the assumption of even load among the pads. In the future, it is necessary to use full model including all the pads which can analyze the effect of uneven load among the pads and inclination of the runner surface on the lubrication performance. Thus, hydraulic equalizing system below the pads is possible to be included in the model with the high pressure oil flow in the pistons although many difficulties still exist at present. In addition, the transient analysis with varying thrust load is a further direction to achieve more accuracy results to help design a better thrust bearing with good transient characteristics. Many investigators have studied the lubrication process in thrust bearings through experimental, analytical and numerical studies. Some special designs have been applied to improve the lubrication performance, but the problems about the thermal-elastic deformation, loadcapacity, transient characteristics still exist more or less. Extensive studies are required to develop a full understanding of the characteristics and influence factors of the lubrication of the large thrust bearings in hydropower units, paying more attention to the effects of thermal-elastic deformation, supporting system, covered material and hydrostatic jacking system.

Wujiangdu, Guangzhou Hydro Power Plant and achieved good results. 6.4. Hydrostatic jacking system Another practical problem in the operation of the thrust bearing is the contact rubbing when the bearing velocities approach zero during start-up and shut-down condition. Thus, a hydrostatic jacking system is usually installed in the bearing to solve this problem. Fig. 25 shows a small, circular groove are pocketed on the pad sliding surfaces and fed by a pressurized oil conduit. This kind of groove can minimize the groove dimension in order to limit hydrodynamic lubrication deviations from that of the plane pad. During startup process, the system is activated before start and remain in operation until a high rotational speed is sufficient to generate self-sustaining hydrodynamic film. Similarly, during shutdown process, the system is activated after the speed drops below a certain level and operates until the shaft stops. However, the need to expand the groove area to increase the lifting capacity of hydrostatic system conflicts with the need to preserve performance of the original plane pad design. A haphazardly designed groove may change the film pressure distribution and make the pad tilt away from its optimum position which may cause reduced oil film thickness, increased pad temperature and contact between the sliding surfaces. Therefore, it is necessary to analyze the lubrication performance of the thrust bearings using a model including the groove. Wordsworth et al. [105] found that a static recess may improve the performance of hydrodynamic bearing, while it may deteriorate the performance by tilting the pad in the wrong way [106,107]. Dennis et al. [108] used an isoviscous and isothermal model to numerically analyze the effects of the recess type and size on the performance of a thrust bearing with sector-shaped pads. He found that some types of recesses can increase oil film thickness and reduce power loss relative to the plane pad design at low ranges of lubricant viscosity and rotational speed. Heinrichson et al. [36,62,104,109] did a 3D simulation of tilting-pad thrust bearings considering the effects of highpressure injection pockets. The results show that a shallow pocket contributes positively due to the pressure buildup, while a deep pocket contributes negatively due to the loss of bearing area. Fillon et al. [110] used THD model to investigate the influence of hydrostatic groove on the lubrication characteristics of a large tilting-pad thrust bearing in Itaipu power plant and found that the lifting pocket quite significantly changes the numerical lubrication performance of the bearing. Besides the special designs above, Wasilczuk [111] summarized several other possible methods to decrease the losses in the bearing and the thermos-elastic deformation of the pads, such as no-bath lubrication, high VI oils, water lubrication and so on. It seems that the high VI lubricants can offer immediate benefits and does not need any changes for the bearing design, while no-bath lubrication requires re-design for the bearing.

Acknowledgement The authors thank National Natural Science Foundation of China (Grant no. 51439002，Grant no. 51409148), Tsinghua University Initiative Scientific Research Program (No. 20151080459), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20120002110011, No. 20130002110072) for their financial supports. References [1] Wasilczuk M, Wodtke M, Dabrowski L. Large hydrodynamic thrust bearings and their application in hydrogenerators,. Encycl Tribology 2013:1912–26. [2] International Water Power & Dam Construction, May; 2009. [3] Tao X. New development of curial technology for large hydro power generator unit,. Electr Equip 2006;7:17–21. [4] Jiang D. Analysis on the thrust bearing failure of pumped-storage generating unit of Guangzhou pumped storage plant. Water Power 2001:52–4. [5] Cheng Y. Analysis and treatment of elastic shaft-disc thrust bearing burn-out accident of pump-turbines. Adv Sci Technol Water Resour 2007;27:59–61. [6] Yang S. Treatment of thrust bearing pad erosion of unit 4 in Bailianhe pumped storage power station. Mech Electr Tech Hydropower Station 2011;34:44–7. [7] Iliev H. Failure analysis of hydro-generator thrust bearing. Wear 1999;225:913–7. [8] Tanaka M. Recent thermohydrodynamic analyses and designs of thick-film bearings. Proc Inst Mech Eng, Part J: J Eng Tribol 2000;214:107–22. [9] Liu F. Research on lubrication property of stability condition on large-scale water turbine group, [Master thesis]. Huazhong University of Science & Technology; 2008. [10] Ashour N. An investigation on large thrust bearings. In: Proceedings of the 13th international conference on aerospace sciences & aviation technology, Cairo, Egypt; 2009. [11] Pajaczkowski P, Schubert A, Wasilczuk M, Wodtke M. Simulation of large thrustbearing performance at transient states, warm and cold start-up. Proc Inst Mech Eng, Part J: J Eng Tribol 2013:1–8. [12] Dabrowski L, Wasilczuk M. Evaluation of water turbine hydrodynamic thrust bearing performance on the basis of thermoelastohydrodynamic calculations and operational data. Proc Inst Mech Eng, Part J: J Eng Tribol 2004;218:413–21. [13] Raimondi A. The influence of longitudinal and transverse profile on the load capacity of pivoted pad bearings. ASLE Trans 1960;3:265–76. [14] Pajaczkowski P. Simulation of transient states in large hydrodynamic thrust bearings. Gdansk, Poland:: Gdansk University of Technology; 2010. [15] Tower B. Second report on the friction experiments. Pro Inst Mech Eng 1885:58–70. [16] Reynolds O. On the theory of lubrication and its application to mr. beauchamp tower's experiments, including an experimental determination of the viscosity of olive oil,. Proc Philos Trans R Soc 1886;177:157–224. [17] Christopherson D. A new mathematical model for the solution of film lubrication problems,. Proc Ins Mech Eng 1942;146:126–35. [18] Cope W. A hydrodynamic theory of film lubricant,. Proc Soc Lond Ser A 1949;197:201–17. [19] Ziekiewicz C. Temperature distribution within lubricating films between parallel

7. Conclusions Thrust bearings are the key components in the hydro power units. This paper has carried out a review on the lubrication mechanism of the thrust bearings with more attention to the thermal-elastic-hydraulic interactions in the pad and collar which is not able to be completely avoided in practical operation, but can be reduced to an acceptable level by some designs such as covered material, special supports. Transient and dynamic characteristics of the bearing have also been discussed. The development of the thrust bearing computation has been summarized in detail with five aspects: (1) physical field from HD to TEHD, (2) model dimension from 2D to 3D, (3) computational method from convention solution based on Reynolds equation to FSI solution based on Navier-Stokes Eq., (4) space domain from runner collar excluded to included, (5) time from stable to transient process. FSI technique has provided a quite good way to compute the 15

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