Posts Tagged ‘cavitation bubbles’

Reducing Cavitation by Increasing Water Tank Elevation

Tuesday, April 24th, 2018

    In our last blog we learned of one way to prevent cavitation bubbles from forming at a centrifugal pump inlet when we simply added more water to the storage tank, thereby raising the water level and with it the water pressure at the pump’s inlet.   Today we’ll discuss another way of reducing cavitation, by increasing water tank elevation.

 Reducing Cavitation by Increasing Water Tank Elevation

Reducing Cavitation by Increasing Water Tank Elevation

   

    As presented previously, water pressure, P, at the bottom of the tank is determined by the engineering formula,

P = γ  × h

where 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.

    This formula applies to another scenario as well, that of raising the entire tank’s elevation with respect to the pump, as shown in the illustration.   Here h is the height of the surface water in the tank with respect to the pump’s inlet.   The equation tells us that the higher the tank is elevated, the greater the pressure at the inlet and the less chance there is of cavitation bubbles forming.

    How high do we need to elevate the tank?   We’ll do the math next time.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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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

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

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Centrifugal Pump Impeller Action

Friday, March 2nd, 2018

    Last time we discussed how the curved features of a centrifugal pump are key to its functionality.   Today we’ll examine a centrifugal pump’s impeller action and see how it creates a volatile environment inside the pump in which cavitation bubbles flourish.

 Centrifugal Pump Impeller Action

Centrifugal Pump Impeller Action

   

    Inside a centrifugal pump both low and high pressure areas are created, chiefly due to the action of the pump’s spinning impeller.   Low pressure is created at the water inlet in a way very similar to what happens when you pull the plug on your bathtub.   With the plug removed the drain opens and a tiny whirlpool forms, causing water to get sucked into the plumbing for discharge.

    The same thing happens inside a centrifugal pump due to tumultuous internal water movement.   The spinning impeller vigorously moves water from inlet to discharge.   As water is discharged, a void, or vacuum, is created inside the pump, causing water at the inlet to get sucked inside at low pressure, very much like when you suck liquid through a straw.

    As water moves inside the housing, it comes into contact with the rotating impeller itself.  This impeller is comprised of multiple spiral curved blades with a volute shape, made to maximize efficient movement of water.   They use the power of centrifugal force to create a high pressure environment, and water is flung at high speed towards the pump’s outlet, where it is then discharged.

    Next time we’ll see how the coexistence of low and high pressures within the centrifugal pump housing create the problem of cavitation bubbles.

opyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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