Archive for May, 2018

Master of All Pumps, the Human Heart

Monday, May 21st, 2018

    We couldn’t do without pumps. They serve up water from the tap, circulate our car’s coolant, and the master of all pumps, the human heart, keeps us alive.   Pumps are essential in countless areas of our lives, and they’re of two major types, positive displacement or centrifugal.   We’ll start our discussion with a focus on positive displacement pumps.   Our hearts belong to this category.

Master of All Pumps, the Human Heart

Master of All Pumps, the Human Heart

As their name implies, positive displacement pumps displace, that is to say they move or circulate, a set quantity of liquid with each operating cycle.   Your heart moves 2 to 3 ounces of blood with every heartbeat and up to 2,000 gallons of blood per day!

Next week we’ll introduce an industrial application for a positive displacement pump when we install one in a food manufacturing plant.   You didn’t think those jelly pastries filled themselves, did you?

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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Reducing Cavitation With A Booster Pump

Monday, May 14th, 2018

    In our last article, we looked at an example problem involving a cavitating centrifugal pump that was drawing water from a storage tank.   The bottom of the storage tank was sitting at the same level as the centrifugal pump’s inlet.   The water level in the tank could not be increased to raise the pump inlet pressure, and thus eliminate the cavitation.   So, the problem was solved by elevating the tank with respect to the pump inlet.   Okay, what if the tank could not be elevated?  How do we stop the centrifugal pump from cavitating?   Well, we can install a booster pump between the tank and the centrifugal pump.

    A booster pump is, as its name implies, a special kind of pump that is used to boost, or raise, water pressure flowing in a pipe.   With regard to our example problem in the preceding article, the cavitating centrifugal pump inlet water is at 108ºF and a pressure of 1.2 pounds per square inch (PSI).

Reducing Cavitation by Raising Tank Elevation--Before

Reducing Cavitation With A Booster Pump — Before

   

    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 we can stop the centrifugal pump from cavitating.   We can install a booster pump to boost the pressure by the required 0.3 PSI and say goodbye to our cavitation problems.

Reducing Cavitation With A Booster Pump -- After

Reducing Cavitation With A Booster Pump — After

   

    This wraps it up for our series on cavitation in pumps.   Next time, we’ll begin learning about some different topics.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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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

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

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

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