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Internal Inspection Results

A general arrangement of the pump can be seen in Figure 41.2. Internal inspection showed that the pumps suffered severe cavitation damage. There was extensive damage to the aluminum-bronze suction bells (inlet cones).

Cooling Water Pump House Layout

Figure 41.1 Cooling Water Pump House Layout

Pump House Cross Section

Figure 41.2 Pump House Cross Section

Cooling Water Pump Failures 311

The impellors and wear rings, made of monel metal, were, however, largely unaffected. Damage to the suction bells meant that we had to purchase new bells for all three pumps. The vendor needed 6 months to supply these, while we estimated that the bells would last only 3-4 months. Cavitation by itself had a process impact, resulting in loss of capacity.

Analysis

We sought assistance from the pump vendor and design contractor to analyze the problem, so that we could identify the physical causes of damage. There was no doubt that cavitation was the cause of damage to the suction bells, as all three pumps produced the characteristic 'pinging' noises associated with this process. The suction bells had many hemispherical cavities with metal loss up to 1/2" in diameter. This confirmed the cavitation theory.

The white water indicated significant aeration, which could account for loss of capacity. The vendor had already researched this subject. Their test data showed that 1% aeration could reduce pump flow by about 8%. We wanted to establish the level of aeration to see if we could explain the measured loss of flow. Our instrument engineer came up with an innovative way to measure the degree of aeration (see Appendix 41-A). Using his technique, we measured the level of aeration in the reservoir at 1-2% over the depth of the water. With this information, we could attribute loss of capacity as being entirely due to the white water. Bursting of the air bubbles in the suction bell was the most likely cause of the pinging noises and metal loss in the bells. Loss of capacity and suction bell damage could both be the result of the high level of aeration, seen as white water.

The white water itself originated in the waterfall. The water was clear till it reached the weir, and then became frothy (see Figure 41.3). The next ques-

The Waterfall

Figure 41.3 The Waterfall

tion was how to minimize or eliminate this aeration without disturbing the weir.

 
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