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Ever hear the old saying, “There’s more than one way to cook a goose”? The statement is meant to encourage creative thinking when problem solving. This forward thinking can be applied to the problem of destructive cavitation bubbles as well. Finding ways to reduce cavitation is something engineers are well versed in. As discussed in our last blog, one way to prevent cavitation is by lowering water temperature at a centrifugal pump’s inlet. But sometimes that isn’t possible. Today we’ll discuss another way, reducing cavitation by increasing water pressure.
One way to Reduce Cavitation by Increasing Water Pressure
If you’ve ever seen a movie featuring divers, you’ll no doubt be aware that the deeper a diver goes, the more water pressure there is bearing down on him from above. The same goes for a centrifugal pump’s storage tank. The higher the water level inside the tank, the higher the pressure bearing down on the pump’s inlet, which is located at the bottom of the tank. This is the area in which cavitation bubbles are likely to form. The mathematical equation that illustrates this relationship is, P = γ × h (1) where, P is water pressure at the bottom of the tank, γ is the Greek symbol gamma, representing the specific weight of water, (0.036 pounds/inch3), and h is the depth of the water inside the tank. Let’s see what happens when we increase the water level, h, from 72 inches, shown on the left, to 144 inches, on the right. P = (0.036 Lb/in3) × (72 in) = 2.592 PSI (2) When the water level is raised to 144 inches, P becomes, P = (0.036 Lb/in3) × (144 in) = 5.184 PSI (3) We see that by raising the water level in the tank from 72 to 144 inches, pressure at the bottom of the tank where the inlet is located is increased from 2.592 PSI to 5.184 PSI, pounds per square inch. Next time we’ll see how simply elevating the tank has an impact on cavitation. Copyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________ |
