Detailed Product Description
MEASUREMENT OF OILFILM THICKNESS IN HYDRODYNAMIC THRUST BEARINGS USING FIBREOPTIC SYSTEM AND CLOSED CIRCUIT TELEVISION MONITOR
The performance of vertical rotor support systems as employed in Engineering products like Hydro-generators, vertical pumps, etc, depend to a large extent on the proper operation of the bearings and related tribology systems, which are subjected to frequent variations in load as well as operations of the machine. Depending upon the type, size and operating conditions of the unit, the Bearings are suitably designed and located. Like other parts and subassemblies of the machine, the bearings too are subjected to overloading conditions leading to abnormal wear and accelerated fatigue conditions, finally resulting in early failures. Bearings being critical components, rigorous performance analysis of the tribological system, encompassing the Bearings behavior and that of oil and its coolers for such abnormalities, are a must to enable correct assessment of the loading pattern and failure prevention. The analysis will also help in undertaking life extension program studies on certain critical tribological aspects.
The usual factors responsible for the overloading of bearing segments and related tribological systems are
Increase in the thrust and radial loads of the bearing segments, brought about by the corresponding increase in the Axial and Radial loads of the Machine.
Reduction in the load carrying capacity of the bearing due to deterioration in the lubricant properties, because of aging, contamination, etc.
Additional unexpected loads due to mal-operations of the machine under Runaway speeds, misalignments etc.
Overheating of bath oil temperature, and consequent degradation due to high heat concentration and poor performance of coolers, etc.
The deciding operating parameter of the hydro dynamic bearing segments is the oil-film thickness separating the thrust runner surface and the bearing segments. The oil-film Thickness in Hydrodynamic thrust bearings for vertical applications during normal operation vary from 10 to 85 microns .This film thickness drastically changes during the coast-up and coast-down periods of the machine when rotational speed changes . It is also sensitive to load and speed fluctuations of the machine as outlined in a to d above. Above all the performance parameters, the creation and retention of the requisite oil film thickness in the bearing segments decide the successful operation of the machine. Typical vertical section of a hydro-machine is shown in fig 1.The location of guide bearings and thrust bearing is clearly shown. Details of thrust bearing assembly are separately shown with instrumentation details necessary for oil film thickness measurement. The systems response to overspeed conditions is discussed
The other mal-operation of the machine is during runaway speeds, when all other functional aspects of the machine come in to play in addition to the tribological parameters, and these are to be taken in to account for a realistic analysis. As this mode of operation is a time dependent function together with speed, correct time reckoning is important to quantify the effects of mal-operation. An average of three minutes of operation can be considered for a realistic analysis. The approach followed is similar to the one already described for cooling water cutoff conditions. Part of the heat exchanging capabilities can be built within the system by holding additional quantity of oil as described, so that exigencies as and when they arise can be tackled from within, the oil acting as the main heat sump. In addition, the other practical / economical alternative is to install suitably designed heat exchangers inline with additional capacity, to be looped in depending upon the requirements from time to time. This approach involves provision of additional burden in terms of storage and heat exchangers with in the system. The other alternative should be a system which takes care of this additional burden from within, as and when called for, when the machine operation over speeds during runaway speed conditions, and other load fluctuations. Such a system should have direct heat exchanging capabilities as and when called for when the machines run off the normal operating speed. Direct cooling of the hydrodynamic thrust bearing pads, through a suitable cooling jacket embedded with in the thrust pad would be ideal. Such a system should be totally leak proof to the coolant from mixing with the lubricating oil.
Mal-operations contribute substantial variations in the performance of the machine, leading to over loading of critical components and support systems of the machinery. The additional forces/losses/temperature effects, generated during the period, are to be taken care of in terms of adequate design right from the beginning. The whole exercise needs a systematic approach, and strength considerations, right at the initial design stage itself. Tribological study of these mal-operations have given additional data with reference to the types and intensities of forces generated during mal-operations, which needs a detailed examination from other disciplines like Rotor dynamics, Heat transfer, Structural dynamics, Flow dynamics, and Governing aspects of the machine etc. Direct cooling of the Hydrodynamic thrust bearing segmental pads, through suitably designed water jacket embedded with in the pad segment is discussed here. The advantage being the design can be incorporated to existing machines as well. Detailed description of the design and the test results arrived on prototype bearing pads is described with informative graphs, showing the behavioral trends and time response effects of the system.