## Posts Tagged ‘specific weight of water’

### Reducing Cavitation by Raising Tank Elevation

Monday, May 7th, 2018
 Last time we learned that the risk of damaging cavitation bubbles forming at a centrifugal pump’s inlet can be eliminated by simply increasing the water level inside the tank.   Today we’ll do the math that demonstrates how reducing cavitation can be accomplished by raising tank elevation. Reducing Cavitation by Raising Tank Elevation–Before         In our example we’ll suppose that we’re having a problem with cavitation bubbles forming at the inlet, where water temperature is 108ºF and water level inside the tank stands at 33 inches.   We are using the formula, P = γ × h                                                                                    (1)     Equation (1) was introduced previously to correlate water pressure, P, with the specific weight of water, (0.036 pounds/inch3), and the height, h, of the water surface in the tank.   If h is 33 inches, then we obtain, P = (0.036 pounds/inch3) ×  (33 inches) = 1.2 pounds/inch2         (2)     So, the weight of the water in the tank exerts a pressure of 1.2 pounds per square inch (PSI) at the bottom of the tank and the pump inlet when it sits at the same elevation as the tank.     We know that if we increase the water depth in the tank relative to the pump inlet, we can raise the pressure at the pump inlet in accordance with equation (1).   Raising the pressure will eliminate the cavitation bubbles that can form there.   But, our tank is of fixed volume, and we can’t add more water to raise water depth beyond 33 inches.    However, we can increase the elevation of the tank with respect to the inlet, which will produce the same effect.   We’ll use equation (1) to determine the tank elevation, h, that will provide the needed increase.     Referring to the thermodynamic properties of water as found in tables appearing in engineering texts, we determine that if we keep water temperature at 108ºF but raise the pressure at the pump inlet from 1.2 PSI to 1.5 PSI, while maintaining current water depth in the tank, cavitation will cease.   In other words, we need to increase P by 0.3 PSI. Example of Reducing Cavitation by Tank Elevation–After         Plugging our known values into equation (1) we solve for h, 0.3 PSI = 0.036 pounds/inch3 × h                                                  (3) h = 0.3 PSI ÷ 0.036 pounds/inch3                                                  (4) h = 8.3 inches                                                                              (5)     Cavitation will cease when we elevate the tank by 8.3 inches with respect to the pump.     Yet another means of increasing inlet pressure is to install a booster pump.  We’ll talk about that next time. Copyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________

### One way to Reduce Cavitation by Increasing Water Pressure

Monday, April 16th, 2018
 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 ____________________________________