he 3 unit (25MW) of a power plant is equipped with two horizontal  split casing pumps  as circulating cooling pumps. The pump nameplate parameters are:

Q=3240m3/h, H=32m, n=960r/m, Pa=317.5kW, Hs=2.9m (i.e. NPSHr=7.4m)

The pump device supplies water for one cycle, and the water inlet and outlet are on the same water surface.

In less than two months of operation, the pump impeller was damaged and perforated by cavitation.

Processing:

First, we conducted an on-site investigation and found that the outlet pressure of the pump was only 0.1MPa, and the pointer was swinging violently, accompanied by the sound of blasting and cavitation. As the pump professional, our first impression is that cavitation occurs due to partial operating conditions. Because the design head of the pump is 32m, as reflected on the discharge pressure gauge, the reading should be about 0.3MPa. The on-site pressure gauge reading is only 0.1MPa. Obviously, the operating head of the pump is only about 10m, that is, the operating condition of the horizontal split casing pump is far away from the specified operating point of Q=3240m3/h, H=32m. The pump at this point must have a cavitation residue of , the volume has increased unpredictably, cavitation will inevitably occur.

Secondly, on-site debugging was conducted to allow the user to intuitively recognize that the fault in the pump selection head was caused. In order to eliminate cavitation, the operating conditions of the pump must be returned to near the specified operating conditions of Q=3240m3/h and H=32m. The method is to close the school outlet valve. Users are very worried about closing the valve. They believe that the flow rate is not sufficient when the valve is fully open, causing the temperature difference between the inlet and outlet of the condenser to reach 33°C (if the flow rate is sufficient, the normal temperature difference between the inlet and outlet should be below 11°C). If the outlet valve is closed again, , wouldn’t the flow rate of the pump be smaller? In order to reassure the power plant operators, they were asked to arrange for relevant personnel to separately observe the condenser vacuum degree, power generation output, condenser outlet water temperature and other data that are sensitive to flow changes. The pump plant personnel gradually closed the pump outlet valve in the pump room. . The outlet pressure gradually increases as the valve opening decreases. When it rises to 0.28MPa, the cavitation sound of the pump is completely eliminated, the vacuum degree of the condenser also increases from 650 mercury to 700 mercury, and the temperature difference between the inlet and outlet of the condenser decreases. to below 11℃. All these show that after the operating conditions return to the specified point, the cavitation phenomenon of the pump can be eliminated and the pump flow returns to normal (after cavitation occurs in the pump's partial operating conditions, both the flow rate and the head will decrease). However, the valve opening is only about 10% at this time. If it runs like this for a long time, the valve will be easily damaged and the energy consumption will be uneconomical.

Solution:

Since the original pump head is 32m, but the new required head is only 12m, the head difference is too far, and the simple method of cutting the impeller to reduce the head is no longer feasible. Therefore, a plan was proposed to reduce the motor speed (from 960r/m to 740r/m) and redesign the pump impeller. Later practice showed that this solution completely solved the problem. It not only solved the problem of cavitation, but also greatly reduced energy consumption.

The key to the problem in this case is that the lift of the horizontal split casing pump is too high.