Investigations on Performance Characteristics of A Centrifugal Slurry Pump Handling Water and Ash

Abstract

Power generation in India is primarily coal based in the present scenario. Coal used in our thermal power plants produces a large amount of ash. Currently, bottom ash is being transported hydraulically from thermal power plant to ash ponds through pipelines at low solid concentration. The operation of ash disposal pipelines is therefore highly uneconomical due to requirement of large quantity of water and increase in power consumption for pumping water. The basic parameters for the design of any slurry transportation system are hydraulic parameters, which include properties of carrier fluid, particle size, solid concentration, slurry rheology, specific gravity of the solids etc. The fly ash and bottom ash from different power plants differ greatly in their characteristics and hence the design of ash disposal system needs to take in to account these variations. It is therefore necessary to have knowledge of the properties of fly ash, bottom ash and their slurries. Pump is the heart of any ash disposal system. Coal ash is generally transported to the ash pond using slurry pumps and pipelines in the thermal power plant. Bottom ash particles are coarser without much fine particles. The Indian coal ash having higher specific gravity and also content large amount of non-combustible matter. Further, these also differ widely in their physical and chemical characteristics depending on the coal mines. Therefore in order to optimize the hydraulic design procedure for transporting bottom ash (B.A.) at higher concentrations, it is essential to carry out basic investigation using bench scale tests and pilot plant loop to develop the required technology. As rheology of the slurry plays a major role in flow, the rheological characteristics of the bottom ash in the concentration range of 10-50% (by weight) with and without the addition of fly ash has been studied to facilitate conveying of bottom ash slurry at higher concentrations. Addition of fly ash in the bottom ash has been varied from 10 to 30% (by weight). The flow behaviour of all the slurries was observed as Newtonian. It is seen that the shear stress value increases monotonically with increase in the solid concentration. The value of relative viscosity increases with increase in the proportion of fly ash in the bottom ash slurry. The rheology of the slurry suspension was evaluated with the variation of the temperature from 26ºC to 41ºC at different shear rates. The temperature of the slurry suspension was increased in step of 5ºC at concentration of 20% (by weight). The relative viscosity gets reduced with increase in the temperature of the bottom ash slurry, with and without addition of fly ash. The optimum modification in viscosity was found as temperature of 36 ºC. The performance characteristics of the centrifugal slurry pump have been evaluated at four different speeds namely 1000, 1150, 1300 and 1450 rpm with clear water and bottom ash slurry with and without addition of fly ash. Experiments were conducted with bottom ash slurry at four concentrations for each speed. Addition of fly ash in the bottom ash has been varied from 10 to 30% (by weight). Similarly, the experiments with addition of the fly ash in the bottom ash mixture in different ratios namely 9:1, 8:2 and 7:3 were conducted at four concentrations in the range of 12 to -48% to evaluate the performance characteristics of the pump at all four speeds. The pump total head, overall efficiency and pump input power at different flow rates have been evaluated. The performance characteristics results show that the values of head and the efficiency of the pump depend on the solid concentration. It was also observed that the performance parameter of the pump strongly depends on slurry concentration. The addition of fine particles of fly ash in the coarser particles of bottom ash slurry, leads to reduce the additional head losses in the pump. The pump performance in terms of head and efficiency has been found to improve with addition of fly ash in bottom ash slurry. The solid-liquid two-phase flow behavior in the centrifugal slurry pump has been numerically simulated using Computational Fluid Dynamics (CFD) code FLUENT for the design condition and also off-design conditions. The mixture model has been used for the simulation of the solid-liquid two-phase flow of bottom ash and water mixture. The performance characteristics of the centrifugal slurry pump are numerically predicted with water and bottom ash slurry at different speed 1000, 1150, 1300 and 1450 rpm. The solid concentration of the bottom ash slurry varied from 0% to 45% by weight. The numerical results are compared with the experimental measurements. The CFD-results show satisfactory agreement with experimental data in a complete operating range of the pump with water. The predicted simulation results show that performance characteristics of the centrifugal slurry pump are the function of concentration and viscosity of the bottom ash slurry. The deviation of predicted value of head ratio and power ratio with experimental data is lie within the limits of ±8%. Both the pump head and the efficiency are reduced with concentration of solid-liquid mixture. The flow separation has been found near the volute tongue and becomes more remarkable with increasing solid phase concentration. The predicted numerical results are helpful for improvement in the hydraulic design of centrifugal slurry pumps. The present study can be useful for transporting coarser particles like bottom ash with the addition of fine particulate fly ash at higher concentrations, so that large amount of water and power can be saved.