Hydro Electric Power Plants

Effect of oil pressure

 At low sliding speeds a bearing performs satisfactorily when immersed in an unpressurized oil bath, but at higher speeds the centrifugal effects of the collar on the surrounding oil become substantial. Recirculation slots are provided at the back of the carrier ring to equalize the pressure gradients created by the collar, but their success is only partial.

Tests were carried out at constant speeds to measure the static pressures at the outer and inner peripheries of the pads for given oil supply pressures. Fig. 13.7 shows the results for 9000 rev/min, which was the maximum possible test rig speed when the tests were performed. It will be seen that pressures at the outer periphery of the pads were substantially less than the supply pressure, and pressures below atmospheric were measured at the inner periphery of the pads with low oil supply pressures. This was particularly noticeable at high speeds where higher temperature readings indicated oil starvation at the pad inlet. These sub-atmospheric pressures can be avoided by increasing the supply pressure. Other workers (4) have shown that the safety factor of a tilting pad thrust bearing

is increased by raising the housing pressure to relatively high values (10 bar),  but from tests performed it is concluded that supply pressures in the range of  1-1.5 bar are sufficient for the majority of modern applications.

Effect of oil supply arrangement

 Different arrangements of oil inlet and outlet positions in conjunction with different oil flow patterns inside the bearing were studied to see how they affect the supply of fresh cool oil to the pads. The merits of each arrangementwere judged by the measured pad temperatures. The principal arrangements tested are shown in Fig. 13.4. The main oil flow path through the bearing was varied by using a combination of different oil inlet and outlet positions in conjunction with full or partial recirculation slots in the carrier rings. From simple considerations of oil flow it could be thought that the best arrangement is given by (V) where the total flow is supplied at the inner periphery of the loaded face and discharged at the surge face. On the other hand, the worst arrangement would seem to be (111), in which the oil enters opposite the collar and has to flow against the collar’s centrifugal gradient to lubricate the pads; furthermore, most of the oil could pass around the collar and be discharged at the top without ever reaching the pad inlet edge.

The chailpc in maximum pad temperature versus speed is shown in Fig. 13.5 for each of the six  arrangements in Fig. 13.4, and it will be seen that the different oil flow arrangements have only a marginal influence on maximum pad temperatures. This was attributed to the extensive churning and mixing of the oil within the casing which was induced by the rotating thrust collar, and by this method a fairly uniform oil bath temperature is maintained irrespective of oil entry arrangements or oil flow inside the bearing.

Effect of oil quantity

 The oil temperature rise through a tilting pad bearing assembly (i.e. the difference in housing oil inlet and outlet temperatures) is a parameter used extensively in design and operation. Fig. 13.6 shows the effect of oil quantity on the maximum pad temperature of a bearing operating at 6000 and 8000 rev/min with the corresponding temperature rise marked against each test point. It will be seen that increasing the oil flow to give temperature rises of less than 15-20 degC has little beneficial effect on bearing performance as measured by pad surface temperature. Conversely, however, relatively large changes in pad surface temperature can occur with small oil flows,  .e. at high temperature rises.

 

GENERAL CHARACTERISTICS OF TILTING PAD THRUST BEARINGS

 The work described in this section was aimed at confirming, under controlled test conditions, the effects of different operating parameters on the performance of the bearing.

Effect of speed and load

 Fig. 13.3~ shows the effect of speed on the bearing

sliding speeds the power absorbed increases nearly as the square of the speed. In addition to shearing losses beneath the pads, the power absorbed also includes drag losses on the exposed parts of the thrust collar and shaft. The drag losses (or churning losses) can amount to 50 per cent or more of the total losses at high sliding speeds.

There is also a rapid increase with speed in maximum pad temperature, which arises partly from the higher shearing rate and partly from the increased hot oil carry-over from one pad to the next. The high pad temperature represents a real limit to bearing loading, not only because the white-metal may lose its strength and begin to flow plastically but also for reasons concerned with thermal fatigue, sometimes known as thermal ratcheting.

The effect of pad specific loading at a constant high speed (12 000 revlmin) is shown in Fig. 13.3b. The relatively slow increase in power absorbed with increasing load will be noted, showing that a considerable proportion of the power reduction is attributable to churning losses. It follows from Figs 13.3~ and b that the rate of increase >f pad temperature due to speed is of the same order as that due to load. This has the practical effect of making it difficult to reduce maximum pad temperatures by using a larger bearing as any decrease in temperature due to lower loading can be largely offset by the increase in mean sliding speed at the thrust pad.

* 1 bar = 14.50 Ibf/in2 = lo5 N/m2

INTRODUCTION

 TILTING PAD THRUST BEARINGS operating at high sliding speeds (in excess of 60 m/s) may fail owing to high pad surface temperatures. The object of the work described in this paper was primarily to investigate the effectiveness of current methods used for the design of high-speed assemblies, and to evaluate improvements. The paper also studies the effects of speed, load, oil quantity, oil pressure, misalignment, and oil flow arrangement on a flood lubricated double thrust bearing. The final section considers the problem of power loss in high-speed tilting pad thrust assemblies and shows how significant savings can be made by using a different system of lubrication (directed lubrication).

 EXPERIMENTAL EQUIPMENT

 The Orion test rig shown in Fig. 13.1 was used for the majority of the work described. A 124-mm 0.d. double tilting pad thrust bearing assembly was used, in which each thrust ring incorporated eight white-metal faced, steel backed, centre pivoted thrust pads, 28 mm wide. To enable accurate friction torque measurements to be made, the double thrust bearing was supported in a test head mounted on hydrostatic journal bearings with the thrust load applied by means of hydrostatic pistons. The 55-mmtest shaft was driven at speeds up to 15 000 rev/min by a 56-kW variable-speed d.c. motor. Lubricating oil was supplied to the test head from an external circuit which incorporated heate’rs, coolers, pressure and temperature controls, etc., to maintain the lubricant within the required parameters for each test. Shaft speed was measured with an electronic counter, and friction torque by means of a spring balance.

For the early tests the bearing performance was estimated mainly by means of copper-constantan thermocouples embedded within about 0.05 mm of the bearing surface. Later tests used film thickness probes embedded in selected thrust pads. These probes were of the capacitance type with the signal being read on an inductive ratioing instrument (I)+. Up to eight thermocouples were used in some pads, and a typical set of instrumented pads is shown in Fig. 13.2.

For many of the initial tests on the directed lubrication system of oil supply described later in this paper, a relatively simple test rig (Scylla) was used. This rig permitted larger thrust pads (68 mm) to be tested on a 750-mm 0.d. t h m t Cbllar at speeds up to 3000 revlmin, which enabled a mean sliding speed of 100 m/s to be achieved compared with the maximum of 76 m/s that was possible on the Orion rig. To minimize power requirements and to simplify loading arrangements the Scylla rig incorporated two pairs of thrust pads, each pair being hydraulically loaded against opposite sides of the thrust collar so that

there was no resultant axial thrust. Instrumentation consisted of thrust pad temperature thermocouples, oil flow quantity, pressure, temperature, shaft speed, and applied thrust. It was found that results obtained on this simplerig could be subsequently confirmed on the Orion rig.

All the work described in this paper used a mineral oil with a viscosity of 41 cS at 50°C, 8.4 cS at 100°C. The majority of the tests were carried out with an oil inlet temperature of 50°C.

The Cabinet Committee on Economic Affairs today approved the constitution of an Empowered Committee headed by the Ministry of Power comprising senior level representatives from concerned Ministries to finalise the financial compensation, measures for maintaining the fragile eco-geological balance in the discontinued project site of Loharinag Pala in Uttarkhand. An agency would be identified to implement environmental safety measures and rejuvenate losses of ecology in and around the project site.

The Ministry of Power will bear the expenses for compensation to NTPC and also for safety measures and as per the recommendations of the Empowered Committee.

The National Ganga River Basin Authority (NGRBA) decided in its meeting in November, 2010 to discontinue all under-construction hydro electric projects in the upper riches of the Bhagirathi in view of the necessity of maintaining “aviral dhara” (continuous flow) in the Bhagirathi. 135 km stretch towards Gomukh and Uttarkashi is a declared eco-sensitive zone. With the decision of NGRBA, Loharinag Pala (600 MW) hydro electric project, which was being implemented by NTPC was discontinued.

The project site falling under the eco-sensitive zone will have necessary protection measures in terms of ecology and compensation to NTPC will ensure that the country’s largest public sector thermal power generator does not suffer any financial loss because of discontinuance of the project which it started implementing after obtaining all statutory clearances.