Wärtsilä Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings

Transcription

1 Wärtsilä Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings BUSINESS WHITE PAPER CONTENTS Introduction... 2 Plain Journal Bearing Hydrodynamic Theory, a Simple Overview... 3 Bearing and Shaft Alignment and Tolerances... 5 Bearing Testing... 6 Oil vs Water Lubrication...10 Bearing In-Service Monitoring...11 Conclusions Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

2 Introduction Enhancing bearing life with water lubrication. A bearing running truly hydrodynamic will only wear when shaft rotation is started or stopped, or the shaft speed drops below the critical hydrodynamic threshold. At all other times a fluid film is separating the two surfaces of shaft and bearing, and no contact takes place resulting in no wear. However many shaft guide bearings are believed to run in mixed friction conditions, where there is still some contact between shaft and bearing, resulting in truncated bearing life. Alignment is also key; mis-aligned or damaged shafts can increase localised bearing pressure, accelerating wear and causing premature bearing failure. 2 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

3 Plain Journal Bearing Hydrodynamic Theory, a Simple Overview Zero contact needed to run hydrodynamically. A plain journal bearing consists of a shaft completely or partially surrounded by a bearing surface, having an internal diameter slightly greater than the diameter of the shaft. The bearing is typically stationary and the shaft rotates within the confines of the bearing. The bearing is lubricated typically with either oil or water. When stationary the shaft journal (the area of the shaft in way of the bearing) will be in direct contact with the bearing inner diameter. As the shaft begins to rotate; wear will take place between the two surfaces [Figure 1], this phase is referred to as solid or boundary friction. As the rotational speed of the shaft increases the lubricating fluid flooding the bearing is drawn in between the bearing and shaft surfaces, reducing the contact between them [Figure 2]. This phase is known as thin film lubrication or mixed friction. Once the shaft reaches a critical rotational speed, the contact between the bearing and shaft reduces completely and the shaft is said to be running hydrodynamic, with a fluid film separating the two components. Fig.1 Fig.2 Fig.3 N.B. Shaft running clearance has been exaggerated for clarity The coefficient of friction (CoF) of the bearing/shaft interface changes drastically during this process, with a high CoF when there is maximum contact between the two surfaces, when stationery, reducing as the area in contact decreases, as the shaft rotational speed increases. Running hydrodynamic is accepted to be when the CoF drops to 0.01 or lower. To achieve this there needs to be zero contact between shaft journal and bearing, and the remaining friction is generated by the viscous shear of the fluid separating those two surfaces. 3 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

4 The process outlined above is explained and illustrated by Stribeck s curve [Figure 4], with friction along the y axis and ZN/P along the x axis; Z being the viscosity of the lubricating fluid in centipoise, N the rotational speed in rev/min, and P the load per unit projected area of bearing or unit load. Assuming the pressure and viscosity are constant, we can take the x axis to represent speed. (In reality the shaft load will vary somewhat during the transition from static to dynamic motion, and the viscosity of the lubricating fluid will also vary due to change in temperature, with viscosity decreasing as temperature increases.) It can be seen from the graph that the CoF drops significantly in the region b-c. However it is only when the shaft is considered to be running fully hydrodynamic (region c-d), that you can conclude that no bearing or shaft journal wear is taking place. Fig.4 Stribeck Curve The wear on the bearing and shaft journal surfaces that occurs during the start-up and stopping of the shaft will be determined by a number of factors, including: - the load applied by the shaft onto the bearing surface, the coefficient of friction between the two surfaces, the surface finish of each, and the geometrical cylindricity and parallelism of the two cylinders involved (typically referred to as shaft alignment). In an ideal world the bearing inner diameter and shaft journal would be machined perfectly concentrically, and parallel; they would both have very fine surface finishes, and the two would be perfectly aligned, spreading the shaft load evenly around the contact area within the bearing. In reality current standard machining and alignment techniques and tolerances, will mean that this will not be the case in the field, and an acceptable standard must be reached for each of the above criteria. It should be remembered that the component in the system which is deemed sacrificial is the bearing, rather than the shaft journal, which is much more expensive to machine, repair or replace. Thus, once a bearing has failed it may not be the bearing or bearing material, as such, which is at fault, but rather some aspect of the greater system which has led to the failure. It is in the interest of the turbine operator to increase the efficiency of their turbine, optimising its output. Power losses due to bearing friction will obviously reduce a turbine s efficiency. It is with all of the above in mind that a bearing should be designed to run for the vast majority of its life within the hydrodynamic region [c-d, Figure 4]. 4 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

5 Bearing and Shaft Alignment Tolerances A guide to the recommended acceptable alignment tolerances. Part of a table on shaft and guide bearing alignment provided by the United States Department of Interior Bureau of Reclamation, Denver, Colorado (Facilities, Instructions, Standards and Techniques Volume 2-1) has been included in this paper. This provides a very useful guide to the recommended acceptable alignment tolerances which can be accommodated for vertical shaft Hydro turbines [Table 1]. MEASUREMENT Upper generator guide bearing concentricity Lower generator guide bearing concentricity Seal ring concentricity Shaft straightness Static shaft runout (Orbit diameter) Plumb of centre of shaft runout TOLERANCE 20% diametrical bearing clearance (Relative to turbine and lower generator guide bearing). 20% diametrical bearing clearance (Relative to turbine and upper generator guide bearing). 10% diametrical seal ring clearance (Relative to turbine guide bearing and each other). No reading point deviates more than inch [0.076mm] from a straight line connecting the top and bottom reading point inch [0.051mm] multiplied by the length of the shaft from the thrust bearing to the point of runout measurement divided by the diameter of the thrust runner multiplied by the length of the shaft from the highest plumb reading to the lowest plumb reading. Bearing/Shaft Journal Conformance Due to the machining and alignment tolerances involved it is widely accepted, that a new guide bearing/shaft journal interface will experience some wear at the beginning of its life. This process is colloquially known as bedding in, and is the two surfaces rubbing together wearing down high spots on the softer bearing surface, providing conformance between the two surfaces. It is therefore essential that the two materials chosen for the interface are compatible, and have been validated as such through laboratory or field testing. 5 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

6 Bearing Testing Confirming short bearing theory through empirical validation. Laboratory based testing carried out by Wärtsilä at their Composites research and development and manufacturing centre, UK, over many decades and on a number of different test rigs has confirmed short bearing theory through empirical validation. Fig.5 Ø200mm Horizontal Testing Rig One such rig used is an Ø200mm shafted horizontal journal bearing test rig (pictured in Figure 5). The schematic diagram shown in Figure 6 illustrates the mechanism for applying the load to the journal bearing using hydraulic rams at either end of the rotating shaft. The rams are capable of applying up to 16 bar pressure to the journal bearing during testing. Bearing temperature, bearing load, shaft rotational torque, total friction, and shaft rpm and positioning are all measured, supplying real time data capture for a number of different test regimes. 22kW Electric Motor Drive End Ram Bearing/Housing Free End Ram Fig.6 Test Rig Schematic Design 6 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

7 One such regime is a speed up/speed down test, where the shaft is rotated from a static position, accelerating up to the maximum speed of the rig (250 rpm), and then back down, returning to a static position over a period of 2 minutes (the test is carried out after a bedding in regime). Figures 7 and 8 contain graphs showing the measured friction against the peripheral rotational speed of the shaft journal in metres per second Hydrodynamic lift point - Epoxy Composite Bearing Increasing Speed 0.12 Coefficient of Friction bar 3bar 4bar 5bar 6bar Shaft Peripheral Speed (metres/second) Fig.7 Speed Up/Speed Down Test Results increasing speed 0.14 Hydrodynamic lift point - Epoxy Composite Bearing Increasing Speed 0.12 Coefficient of Friction bar 3bar 4bar 5bar 6bar Shaft Peripheral Speed (metres/second) Fig.8 Speed Up/Speed Down Test Results decreasing speed The data shown captures the start-up [Figure 7] and stopping [Figure 8] of the shaft over a series of speed up / speed down tests carried out at different bearing pressures. In this case clean fresh water was the lubricant, with an LG4 Bronze shaft journal running against an epoxy based composite bearing. You can see how the graph matches the Stribeck curve as shown in Figure 4. You can also see the friction dropping as the rotational shaft speed increases, during the mixed friction phase, before the friction drops below the 0.01 threshold and the bearing/shaft journal runs fully hydrodynamic. 7 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

8 N.B. The acceleration of the shaft, from a stationary position, and resultant shaft climb [see figure 2] at the beginning of the test [0 - ~0.2 m/s Figure 7], over a period of approximately 3-4 seconds, occurs during the boundary friction phase, in contradiction to Stribeck s curve [Figure 4]. This is a function of this particular test rig, and would change if the power of the motor was increased significantly. The friction curve would follow Stribeck s curve more closely as a result. Bearing Design Laboratory based empirical testing of short journal bearings has revealed a maximum recommended bearing pressure exerted by the shaft journal of 6 bar, with a recommended working pressure of 3-4 bar. A simple way of working out approximate bearing pressure is to take the shaft load and divide that by the projected area of the bearing. The projected area can be calculated by multiplying the length of bearing in contact with the shaft, by the shaft diameter. The ratio between the two is known as the L[ength] : D[iameter] ratio. If the shaft load is high the pressure can be reduced by increasing the bearing length, or as seen on some oil lubricated designs increasing the shaft journal diameter (in some cases drastically). However, increasing the bearing length beyond an L:D ratio of 2:1 can have other adverse effects. As has already been mentioned, the alignment between the two cylindrical surfaces of bearing and shaft journal are critical to the operation of a journal bearing. Increasing the bearing length, makes it more difficult to ensure good alignment between the two surfaces, and also has the effect of restricting the flow of lubricant through the bearing. Poor alignment will lead to un-even shaft journal/bearing contact, effectively drastically reducing the bearing contact length, and leading to isolated areas of high bearing pressure, commonly referred to as edge loading. Most of the load of the shaft/runner on a vertical turbine is taken by the axial thrust bearing, leaving the radial guide bearings, when compared to those found on a horizontal application, relatively lightly loaded. As a result, guide bearings on vertical turbines often have an L:D ratio of 1:1 or lower. However, vertical guide bearings can still see significant loads, due to hydraulic forces from the runner, imperfect shaft alignment, unbalance from the generator and other factors. Lubrication Flow and Washway Grooving Design Empirical laboratory testing and field experience has shown a need for washway grooving running axially through the inner diameter of the bearing. This increases the volume of fluid which can be forced through the bearing to lubricate it, and allows it to flow where it is needed, between the bearing and shaft journal. However, inherently these lubrication grooves reduce the surface area of the bearing inner diameter, where the hydrodynamic fluid film is formed. As with all engineering design a compromise must be reached whereby there is enough land between the grooves for a hydrodynamic fluid film to form, and enough grooves with enough cumulative cross-sectional area to provide adequate lubrication flow through the bearing. Increasing the number and width of grooves will reduce the bearing surface area, increasing the pressure. Because of its orientation a vertical turbine shaft journal can run anywhere within the circumference of its guide bearings. As a result the lubrication groove pattern has to be equi-spaced around the circumference. 8 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

9 Figure 9 shows the forces acting on a loaded journal bearing, and the resultant pressure distribution. Interrupting that area with lubrication grooving will increase localised pressure by reducing the bearing area. Historical stave bearings have a large number of grooves around the bearing s circumference, these reduce the bearing area drastically, so the designers are often forced to increase the bearing length considerably to reduce the pressure, resulting in typical L:D ratios of 4:1. Reducing the land between each groove in this way will also increase the possibility of the bearing running in the mixed friction phase, rather than running fully hydrodynamic. Load on journal Lubricant forces supporting journal Circumferential lubricating fluid pressure distribution Fig.9 Forces Acting on a Loaded Journal Bearing Diagram Modern designs take a balanced approach and reduce the quantity of grooves, to increase the land between each enabling the bearing to run fully hydrodynamic, whilst providing enough volume for a suitable flow of lubricant. Wärtsilä have established through testing that a water lubricated bearing requires a minimum flow at a rate of 0.15 litres/minute per mm of shaft. An example being an Ø500mm shaft requiring a flow of 75 Litres/minute. Oil lubricated bearings typically require less lubricant flow. Running Clearance Minimising guide bearing running clearance, will benefit the operation of a hydro turbine. Large bearing clearances can lead to increases in vibration, having negative effects on the turbine s operation. Some bearing materials swell when immersed in oil or water, so that and the potential thermal expansion of the material, have to be taken into account when calculating bearing running clearance. Low swelling and thermally stable materials are desirable. 9 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

10 Oil vs Water Lubrication Adhering to environmental regulations. Both oil and water are effective journal bearing lubricants, but there are advantages and disadvantages to the use of each. Oil is more viscous than water at the temperatures that a journal bearing lubricant will operate at, this higher viscosity will mean the fluid film separating the two surfaces is thicker, enabling higher bearing pressures to be supported. Oil also has the added advantage of not corroding ferrous based metals that it comes into contact with, allowing the use of materials such as mild steel in the bearing assembly and lubricating apparatus. Oil however can be an expensive lubricant, and will need filtration apparatus to ensure its remains free from contaminants which will shorten bearing life. Water has been demonstrated as an effective journal bearing lubricant up to 6 bar bearing pressure through laboratory and in-service validation, and in the case of hydro turbines is readily available at minimal cost. Using water from the turbine flow to lubricate the turbine s guide bearing enables high flow rates, which help to reduce potential heat build-up in the system. Equivalent flow rates are not possible with oil lubrication, due to the cost such a large volume of lubricating oil would incur, and heat exchangers have to be employed adding complexity to the assembly. Whilst the actual lubricant itself is free, the local quality of the water will have a direct impact on the life expectancy of the bearing, and if there are high levels of abrasive particulate found in the local water supply, suitable filtration should be employed. Through experience in a number of industries Wärtsilä have found that a minimum filtration mesh size of 200 microns is suitable for most applications, however for high particulate content finer meshes up to 50 microns or cyclone separators should be employed. The main resulting issue using oil over water is the potential for contamination downstream. Environmental damage due to oil is well documented, and everything should be done to avoid such contamination by hydro turbine equipment. If oil is to be used for guide bearing lubrication, the oil bath should be kept completely isolated from the turbine main water flow, by positioning the lower guide bearing away from the head cover water seal, with no potential for oil leakage onto or near the seal. This will reduce the potential for cross contamination. As this is not an issue with a water lubricated bearing, its advantage is that it can be positioned much closer to the runner, which may provide benefits for other aspects of the turbine s operation. Additionally a water seal can be incorporated into the top of the bearing assembly, reducing the number of components and the assembly s complexity, reducing cost. 10 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

11 Bearing In-Service Monitoring Ensure accurate readings with temperature sensors. Monitoring your guide bearing s health is of paramount importance for bearing longevity. Temperature sensors (such as Pt100 dual element sensors) should be used, and should be embedded into the bearing, close to the inner diameter to give as accurate a reading as possible. Additionally shaft proximity sensors can be used to give accurate readings on shaft movements relative to the guide bearing, which, once calibrated will give an indication as to the wear experienced by the bearing. Whilst these are straightforward to install on a horizontal turbine (with only one being required at bottom dead centre), a number would need to be installed for vertical applications, where the shaft can be running anywhere around the circumference of the bearing. Conclusions Bearing design in addition to other factors are critical to the continued success of a turbine s guide bearings. We have seen that a journal bearing is part of a greater system, and bearing problems are likely to be caused by issues elsewhere. Bearing design, shaft alignment, lubrication flow and lubricant quality are all critical to the continued successful operation of a turbine s guide bearings. Bearings should be designed with suitable factors of safety, and the ancillary systems related to the bearing should have suitable levels of redundancy for continued stress free operation. Both oil and water are effective lubricants, each with their own advantages and disadvantages, whilst oil may have some tribological benefits over water, the cost both in lubricant, ancillary equipment and potential clean-up costs and fines can hinder those advantages to such an extent that water is the obvious choice for guide bearing lubrication particularly for small to medium hydro turbines. In service guide bearing monitoring can help turbine operators plan for bearing replacement and repairs, further development of monitoring technology will benefit hydro turbine operators, in planning their outages, potentially reducing costs. 11 Wärtsilä Services business white paper Hydrodynamic Performance of Water Lubricated Composite Shaft Bearings 2018

12 Wärtsilä Seals & Bearings in brief Wärtsilä provides integrated seals and bearings systems, packages and products that offer lifecycle efficiency, reduced risks through reliability and are environmentally sustainable. As a truly global organisation, Wärtsilä has a broad product and services portfolio covering the whole lifecycle of the vessel. Looking ahead, Wärtsilä s continuing development and technological leadership can ensure customers an environmentally sound solution that always complies with the latest regulations. wartsila.com/sealsandbearings 2018 Wärtsilä Corporation All rights reserved. No part of this publication may be reproduced or copied in any form or by any means (electronic, mechanical, graphic, photocopying, recording, taping or other information retrieval systems) without the prior written permission of the copyright holder. Neither Wärtsilä Finland Oy, nor any other Wärtsilä Group Company, makes any representation or warranty (express or implied) in this publication and neither Wärtsilä Finland Oy, nor any other Wärtsilä Group Company, assumes any responsibility for the correctness, errors or omissions of information contained herein. Information in this publication is subject to change without notice. No liability, whether direct, indirect, special, incidental or consequential, is assumed with respect 12 to the information Wärtsilä contained Services herein. business This publication white paper is intended Hydrodynamic for information Performance purposes only. of Water Lubricated Composite Shaft Bearings 2018