CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION.

The following paper is an excerpt from my book "Bill Mc Nally's Centrifugal Pump And Mechanical Seal Reference Manual". The words marked in blue reference you to the alphabetical section of the book or directly to the subject on the CD for a detailed explanation of that particular topic. If you do not have a copy of the book or CD you can find a lot of the information in the individual papers I have published on the web.Check out the index for a list of my papers by subject.

We will begin by deciding what operating conditions our pump has to meet and then we will approach pump suppliers to see how closely they can satisfy these needs. Unfortunately no comprehensive theory which would permit the complete hydrodynamic design of a centrifugal pump has evolved in the many years that pumps have been around, so the pump manufacturer will be doing the best he can with the information you supply to him.

To clearly define the capacity and pressure needs of our system we will construct a type of graph called a system curve. This system curve will then be given to the pump suppliers and they will try to match it with a pump curve that satisfies these needs as closely as possible.

To start the construction of the system curve I will assume you want to pump some fluid from point "A" to point "B". To do that efficiently you must make a couple of decisions:

Since we are just getting into the subject, one of the first things we should learn is that centrifugal pump people do not use the word pressure. As mentioned in an earlier paragraph they substitute the word head, so you will have to calculate the three kinds of head that will be combined together to give you the total head of the system required to deliver the needed capacity. Here are the three kinds of head you will be calculating:

You will be calculating these heads on both the suction and discharge side of the pump. To get the total head you will subtract the suction head from the discharge head and that will be the head that the pump must produce to satisfy the application. It will become obvious in the calculations, but I should mention here that if the suction head is a negative number, the suction and discharge heads will be added together to get the total head. If you subtract a minus number from a positive number you must add the numbers together. As an example: 4 - (-2) = + 6

The total head of a pump seldom remains static. There are a number of factors that can change the head of a pump while it is operating and you should be familiar with most of them. You can find these factors in the alphabetical section of my book labeled "head, the reasons for changes in the discharge head of a centrifugal pump".

All of this head information is calculated from charts and graphs you will find in the appendix and articles in the alphabetical section of the manual and on these web pages. This head data will be plotted on a set of coordinates called a system curve. Since we will not be operating at a single point all of the time we will make the calculations for a range of different capacities and heads that we might expect to encounter. This range is described as the operating window we will need to satisfy the application.

Making these calculations is not an exact science because the piping is seldom new, pipe inside diameters are not exact and the charts and graphs you will be consulting cannot compensate for corrosion and solids built up on the piping, valve and fitting walls. Life is never simple. This is the point where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost always guarantee the selection of an oversized pump that will run off of its best efficiency point (BEP) most of the time.

The final calculations are then plotted on the system curve that describes what the pump has to do to satisfy the requirements of the application. You can find examples of these calculations in the alphabetical section of my book and within these web pags. Look for:

The pump manufacturer requires a certain amount of net positive suction head required (NPSHR) to prevent the pump from cavitating. He shows that number on his pump curve. When you look at the curve you will also note that the net positive suction head required (NPSHR) increases with any increase in the pump's capacity.

You will also be calculating the net positive suction head available (NPSHA) to be sure that the pump you select will not cavitate. Cavitation is caused by cavities or bubbles in the fluid collapsing on the impeller and volute. In the pump business we recognize several different types of cavitation. :

Pump cavitation is experienced in several different ways

Remember that the net positive suction head required (NPSHR) number shown on the pump curve is for fresh water at 68° Fahrenheit (20°C) and not the fluid or combinations of fluids you will be pumping. When you make your calculations for net positive suction head available (NPSHA) the formula you will be using will adjust for your fluid.

When the pump supplier has all of this in-exact information in his possession he can then hopefully select the correct size pump and driver for the job. Since he wants to quote a competitive price he is now going to make some critical decisions:

He might begin with the type of pump he will recommend:

There are additional decisions that have to be made about the type of pump the supplier will recommend:

There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them:

After carefully considering all of the above, the pump supplier will select a pump type and size, present his quote and give you a copy of his pump curve. Hopefully you will be getting his best pump technology. To be sure that is true you should know what the best pumping technology is.

At this stage it is important for you to be able to read the pump curve. To do that you must understand:

If all of the above decisions were made correctly the pump supplier will place his pump curve on top of your system curve. The required operating window will fall within the pump's operating window on either side of the best efficiency point (BEP). Additionally, the motor will not overheat and the pump 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 close to, or at the best efficiency point, producing radial impeller loading problems that will cause shaft deflection, resulting in premature seal and bearing failures. Needless to say the motor or driver will be adversely affected also.

With few exceptions pump manufacturers are generally not involved in mechanical sealing. You will probably be contacting separate seal suppliers for their recommendation about the mechanical seal. Recent mergers between pump and seal companies unfortunately does not produce the instant expertise we would like sales and service people to posses.

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