PREVENTION OF CAVITATION IN THE OPERATION OF WATER SUPPLY FACILITIES

Abstract

The article analyses the case of cavitation in centrifugal pumps of the 1st lift pumping station (PS-1) of the water supply system of one of the cities of Ukraine, supplied with water from a reservoir on the Dnipro River. In the classical scheme of a water supply system from a surface source, the operating mode of water treatment facilities should be uniform throughout the day, considering the water treatment’s physical and chemical processes. According to this, the operating mode of PS-1, which supplies ‘raw’ water to the water treatment plant, is considered uniform throughout the day. However, one caveat is that fast filters are washed with drinking water from clean water reservoirs. Its replenishment occurs due to the accelerated operation of the water treatment plant, and, at this time, PS-1 should increase the water supply to the water treatment plant. However, if the influx of water from the water intake structures is fixed, it occurs when the centrifugal pumps PS-1, operating under the bay, begin to capture some air. A two-phase flow of water forms in the pumps, which breaks away from the impeller blades, and cavitation occurs, destroying this impeller.

It was necessary to establish the cause of cavitation and develop measures to eliminate it during the further operation of pumps at PS-1. The reasons for cavitation may be the following: failure of the operating point according to the Q–H characteristic at both low and high water flow rates; water flow breaks away from the blades with the formation of cavities and two-phase flow; violation of the requirement of the permissible cavitation reserve – the minimum pressure at the pump inlet if the pump is installed above the water level in the tank; violation of the requirement that there must always be more water at the pump inlet than at the outlet when installing the pump under the bay. The main reason for pump cavitation at PS-1 is the discrepancy between the water inflow from the water intake structures and the water supply regime to the water treatment facilities. The influx of water to PS-1 from the water intake structures is fixed, and the water use at the water treatment plant is uneven, as it depends on the technological process of water purification.

The need to coordinate the modes of water supply into the suction pipeline of the pump and the mode of water supply to the pressure pipeline becomes urgent. In this case, there will be no cavitation.