Cooling Water Pump Failures 313
work area also posed some hazards to the people who were to execute it. We presented the proposal along with the SWOT (and our proposed actions to mitigate them) to the refinery management team. After getting their approval we did the first trial.
This was a complete failure. The force of the water was so large that the sheet was lifted till it was at about 45°. The sheet could not restrict the water flow at all, flaying about so wildly that we had to pull it out quickly.
We went back to the drawing board. The new design was a horizontal steel plate barrier, with a 3" gap along the four edges and a few vent holes in the middle. We strengthened the plate with a 6" pipe frame and decided to hang it in place using chain blocks (see Figures 41.4 to 41.6 showing the installation sequence). We checked the design calculations to ensure that the plate would withstand the expected loads and bending moments with a good safety margin. We checked the concrete channel walls to see if they would take the additional loads from the chain blocks holding the plate. We had to cut four holes 6" diameter, through the top slab, for the wire slings. An external consultant verified the wall and slab designs, to confirm that these changes were acceptable.
The SWOT on this design also exposed several threats, including the possibility of one sling alone breaking due to uneven impact loading. If this happened, the plate could get stuck at an angle in the box, perhaps blocking flow excessively. To resolve this potential problem, we decided to attach a second set of longer slings (with extra slack), to the steel frame.
The second trial was done in the presence of some members of the management team. Getting the plate in place was quite tricky, but we had a very competent and imaginative mechanical supervisor in charge. When the plate was in place, it did restrict the flow, but not adequately. The white water be-
Figure 41.4 Preparing the Plate Barrier for Installation
Figure 41.5 Progress of Installation of Plate Barrier
Figure 41.6 Plate Barrier in Place
came less frothy, but it was obvious that the flow passage was still too large. In a sense, this trial was also a failure, but we were making progress. We pulled out the plate and modified the edges, so that the gap from the walls of the box was reduced to about 2".
This time, the design worked perfectly, with the water level building up above the plate and eliminating the waterfall effect. The water in the reservoir became calm and clear. The pumps stopped their pinging noises and their capacity was back to design levels. This meant we could go back to two-pump operations.