LEARNING ABOUT CENTRIFUGAL PUMPS (Home)

This mini course is all about centrifugal pumps. I 've chosen this type of pump to talk about because, unlike positive displacement pumps (PD pumps) they can be altered and modified to just about satisfy what ever pumping needs you might have.

I've decided the best way to handle this subject is to write an outline narrative that will get the pumping terms and concepts into some type of perspective. Once you become familiar with the terms you can turn to the individual subjects for a detailed explanation. The phrases colored in blue are links to a more detailed explanation of that particular subject. Here we go!

You want to pump some liquid from point "A" to point "B". To do that you must first make a couple of decisions:

Some of this information is calculated from charts and graphs you will find in this web site and various other publications. Since we'll not be operating at a single pumping point all of the time we'll make the calculations for a range of different capacities and heads that we might expect to encounter. This data will then be plotted on a set of coordinates that we call a system curve. This pumping range is often described as the "operating window" we'll need for the application.

Making these calculations is not an exact science because the piping is seldom new, diameters are not exact, and the charts and graphs you'll be consulting cannot compensate for corrosion and solids built up on the piping and fitting walls. Life is never simple. This is where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost guarantee the selection of an oversized pump that will operate off of its best efficiency point (B.E.P.) adding to its operating cost and making the pump vulnerable to premature seal and bearing problems.

When the pump supplier has all of this inexact information in his possession he can then try to select the correct size pump and driver for the job. Since he wants to quote a low price he has to make some critical decisions:

After carefully considering all of the above, the pump supplier will present his quote and supply you with a copy of his pump curve. At this stage it is important for you to be able to read the curve and to do that you must be able to understand what is meant by :

If all of these decisions were made correctly, the supplier will place his pump curve on top of the system curve you supplied and these curves should intersect at the pump's best efficiency point. At this point the pump will experience minimum vibration, the motor will not overheat, and the pump will should not cavitate.

If the decisions were made incorrectly the pump will operate where the pump and system curves intersect and that will not be at the best efficiency point , causing shaft deflection. Needless to say the motor or driver will be adversely affected along with the bearings and seal.

There are a number of additional conditions that'll affect the performance and reliability of the pump. You will need to learn about all of them:

It would be nice if new pumps were trouble free. They seldom are. When we describe pump problems we usually mean:

At some point you're going to have to become familiar with all of these problems and their solution. Knowing some basic rules of thumb about pumps helps.

When it comes to analyzing these pump failures you have several opportunities that require different troubleshooting techniques:

If you're lucky, your company will purchase the correct pump the first time, but that is not going to happen so you will probably have to modify your existing pump to get the life you require from the bearings and seals.

 

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