Due to changes in the water environment, the water level in the fore-pool is lowered, or the pump cannot work at a lower water-level in the fore-pool because the impeller is installed too high. In addition, some new problems encountered in the actual operation of the pumping station put forward extraordinary requirements for the submerged depth of the impeller. For example, due to the improvement of irrigation and drainage standards, it is required that the water pump can operate normally at the lowest possible water level in the fore-pond (such as reducing the water level in the inner lake, increasing the amount of water regulating and storing, and improving the drainage standard of the pumping station; another example is the lowering of the groundwater level in the drainage area. , water-lifting irrigation in dry season, etc.). On the other hand, due to the large flow rate, the axial flow pump has high requirements for the stability of the inlet flow state. Therefore, it brings many difficult problems to the safe and economical operation of the pumping station, such as cavitation, hydraulic vibration and noise, the low working efficiency of the pump, and the flow head cannot meet the normal requirements specified in the sample, etc.
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The front guide wheel is a kind of axial flow impeller with light load and special structure. It rotates coaxially with the main impeller and provides energy for its inlet water flow. Due to the low installation position of the guide wheel (opposite type) and the special structure, the guide wheel itself has strong anti-cavitation characteristics, so it can solve the problems of pump cavitation and insufficient flooding depth of the impeller to a certain extent. At the same time, since the guide wheel can forcibly improve the flow state at the inlet of the main impeller, the flow velocity and pressure distribution of its section can be adjusted, which is expected to eliminate various problems such as hydraulic vibration and poor performance quality caused by poor flow state.
The guide wheel can be composed of 3 to 4 helical blades and a streamlined hub. It only needs to be connected with the main impeller by the side walls of the hub of the two wheels with screws, without damaging the original pump and unit structure. For large vertical axial flow pumps, in order to reduce the weight of the guide wheel (to improve the stress condition of the unit), the guide wheel can be made of light materials, such as aluminum alloy, glass fiber reinforced plastic, etc. Theoretical analysis and related tests show that this method is completely feasible in technology, and its effect has been preliminarily verified in the field test of a 28ZLB-70 axial-flow pumping station in China.
