Technical and Economic Effect Caused by the Erosion Arising in Hydraulic Turbines and Pumps
Technical and economic results of the erosion, being developed in hydroturbines due to attrition process caused by suspended sediment matter and cavitation, manifest themselves in two ways. First, the development of the process leads to a decline in energy properties of turbines (a decrease in efficiency and, in certain cases, in power capacity), with the associated fall of the electric energy output. Secondly, the process necessitates repair jobs.
aimed at removal of erosion aftereffect in components composing in hydroturbines the flow-passage portion; such maintenance service requires a considerable amount of labour and material expenses. As this takes place, the total extra costs rise so high as to acquire an independent technical and economic value.
The impairment of energy properties in turbines due to cavitationalabrasive erosion taking place in the components, associated with the flowpassage portion of the devices, has been ascertained during the full-scale testing performed. A drop in the efficiency of a heavily worn-out turbine, as compared with a repaired turbine with a newly installed impeller, amounts to 12 ~ 14% within the entire operational range of the power variation limits.
The decrease of efficiency and reduction of electric power output in turnaround time, brought about by this, can be amounted, with a sufficient level of accuracy, to 6 ~ 7% value of the total output. Assuming that the total electric energy generated by the hydroelectric unit in the inter-repair time (above 12,000 h) is equal to about 75 ~ 80 million kwh, it can be estimated that the energy underproduction, through deterioration of the turbine quality, amounts to about 5 million kwh.
In assessing this aspect of the problem, special attention should be given to the fact that the greatest drop of the turbine efficiency, caused by the cavitational-abrasive erosion, appears, approximately by the end of high waters taking place in mountain rivers, therefore the worsening of hydroelectric station performances are particularly susceptible to changes arising in this winter period being most scarce both in energy and water discharge volume.
Besides, while a turbine efficiency drop, in the time of a summer flash flood, may fail to be attended, in certain occasions, with a fall in its capacity and output (due to a speed-up flow rate formed by a redundant run-off volume), in winter and spring times, however, the mentioned efficiency drop results in decreasing both the guaranteed power and the amount of the electric energy generated, owing to a deficiency in the water flow. The real cost of this under-produced energy is, of course, much higher than in the other seasons of the year.
The basic energy measure governing the operational effectiveness of a pump is its efficiency:
where the suffices "/?", "v" and "/»" are correspondingly related to the values of hydraulic, volumetric and mechanical efficiency meanings.
The analysis of the test result obtained has shown that the operation of pumps in the inter-repair time can be divided into four periods.
1) The operational period, which follows immediately after the overhaul, is characterized by a gradual improvement of a pump performance. The rough surfaces of the components, forming the flow-passage portions in the pumps, being subjected to the action of the sediment matter, get grinned and smoothed. As a result, the friction dependent losses decrease to improve the performance of the device.
2) The period distinguished by the performance stability. Worsening of the performance does not arise. It accounts for the fact that the hydraulic abrasion manifests itself in the region adjacent to the trailing edges of the blade, where the flow velocities are the highest. The sediment matter particles smooth the trailing edges, thus providing only a gradual undercutting of the latter.
3) The period of a sharp decline of the performance. Quantitative changes in an impeller result, in the final analysis, in an acute decline of the pump performance. The trailing edges of the impeller blade are undermined to the level which cannot bear the pressure differential from both sides, which result in damage of these edges.
4) The period of further decline of the performance. The value of the required cavitational margin, after breakage of the blades, increases, thus making the suction height too great, which predetermines development of cavitational phenomena.