Estratégias de controle da velocidade de rotação de unidades de bombeamento de pivô central

Abstract

Center pivot systemspumping units that operate with a fixed rotational speed are designed according to the topographic differences of the angular position that requires the highest total dynamic head.In areas where the topographic differences in the different angular positions of the lateral line vary greatly, it is possible to reduce pumping energy consumption with the use of a frequency inverter. However, the use of inverters requires the development of strategies for varying the pump rotation speed that do not induce pressure deficiencies along the moving lateral line that affect irrigation quality. Two central pivots with 69kPa (10psi) pressure regulating valves, which operate in terrain with uneven topography, were considered in this study: one of 15 towers, in Vila Propício-Goiás; and another, with 7 towers, in Formiga-Minas Gerais. These pivots were submitted to five strategies to control the rotation speed of the pumping unit, which involve different combinations between (i) the location of the pressure control point, or points, and (ii) the reference pressure value at the control point. Each strategy was evaluated by the adequacy of the spatial distribution of pressure in the lateral line, according to the EPANET2.2 simulation, and the corresponding specific energy consumption value (kWh/m3). For the two pivots, the irrigation quality was impaired by the strategy that associated the control at the ends of the lateral line with a pressure reference value equal to the commonly recommended one (130kPa) for the regulating valves used. However, when pressure control at the extremities was associated with higher than recommended reference values (250 kPa in V. Propício and 200 kPa in Formiga), irrigation quality was not affected. The process of optimizing the position along the lateral line in which pressure control should be carried out indicated different positions for the two pivots: 70emitter of 130 span in V. Propício and 70 emitter of 60 span in Formiga. However, the recommended reference value for the two center pivots was similar: 192kPa in V. Propício and 198 kPa in Formiga. At V. Propício, the reduction in energy consumption obtained with the strategy based on optimized values of the control position and pressure reference value was greater (21%) than that obtained (17%) with the control at the extremities with a reference value of 192kPa. In Formiga, the reductions in energy consumption obtained with these same strategies were practically the same (34.3 and 33.9%). It is concluded that varying the pump rotation speed reduces energy consumption, but it must be done based on appropriate criteria, which are characteristic of each situation considered.