Subject : Using a variable speed motor to control flow in a centrifugal pump.13-8

If you operate too far off the pump's BEP(best efficiency point,) the shaft will deflect radially and that could lead to both seal and bearing problems. So, what should you do if you have to vary the capacity of a centrifugal pump? The discharge control valve is not a logical choice because, if you alter the capacity of a centrifugal pump, the head changes also, and in most cases this will guarantee you'll be operating off the pump's best efficiency point (BEP). It turns out there are several possible solutions to preventing the problem of shaft deflection, while running off the pump's best efficiency point.

You have several options when selecting a variable speed drive:

So what is the correct answer ? Is the variable speed drive a sensible choice? The only correct answer is "Sometimes"! Let's take a look at what alters when you change the speed of a centrifugal pump. In the following drawing the "H" axis is the head (feet or meters) and the "Q" axis is the capacity (gpm or M3/hr.)

Changing the speed of a centrifugal pump has just about the same affect as changing the diameter of the pump impeller. The "Affinity Laws" allow you to predict the results of this change.

The area within the curved lines (ABCD) is called the operating window of the pump. Notice that the sloping best efficiency point line intersects the capacity leg (Q) at an angle. This slope causes a problem with many pumping applications.

For the variable speed drive to be a sensible solution to your pumping application the system curve would have to fall on, or close to this best efficiency point line, or you will experience radial loads that will translate to shaft deflection. Most pump companies want you to operate within 5% to 10% of the BEP. Heavy duty pumps that have a low L3/D4 (shaft diameter to shaft length ratio) have a much larger operating window.

The above diagram shows that the head is going to have to increase at a predetermined rate as the capacity increases. In Technical Paper 7-01 you learned that there are three kinds of head that will have an affect on the pump's capacity:

In paper 5-12 you learned that a system curve is constructed by the end user of the pump and describes the head/ capacity relationship over the desired operating range of the pump that is going to be specified. The pump manufacturer places his pump curve on top of this system curve, and the point where they intersect is where the pump is going to operate.

Lets take a look at a system curve for a typical boiler feed pump, or any pump that will be discharging into a constant pressure vessel or tank:

 

The boiler is running at a constant pressure, but the steam demand is changing. The boiler feed water capacity must vary with the steam demand, but the pressure or head must remain constant.

The system curve is a straight horizontal line because the dominant head is the pressure head. The amount of piping and elevation is minimal.

Laying the best efficiency point (BEP) sloping line from a varying speed drive on top of the system curve (EF) would show that we are at the best efficiency point only at one point.

Allowing the tolerances of the operating window (ABCD) you can see that we are operating efficiently over only a portion of the desired system curve. A similar application would be pumping a varying capacity to a very high tower or elevation where the static head is the dominant head.

A hot or cold water circulating system describes a different type of system curve. The dominant head in this example is the friction head and that varies by almost the square of the capacity.

In other words, two times the capacity gives you four times the head, or three times the capacity would give you nine times the head. If you plot this on a piece of chart paper you would get an "exponential curve" as shown on the left.

If you lay the best efficiency point line on this "exponential curve E-F" you would get a pretty good match and just about all of the system curve falls within the operating window (ABCD), so this becomes the ideal variable speed application.

In other words you use a variable speed drive any time the system head is dominated by friction in the piping, fittings and valves.

You will find this last curve in many common pumping applications:

Many systems are a combination of all three types of heads. You're going to have to decide which head is the dominant one.

One of the most common methods of varying pump shaft speed is to use a Variable Frequency Drive (VFD). These drives take advantage of the fact that torque, speed and horsepower of an AC electric motor are related to the frequency and voltage of the electrical power supply. Here is the relationship:

Horse Power Capability =f(Torque x speed)

VFDs convert incoming alternating current (AC) to direct current (DC) and then invert the DC power into variable frequencies and voltage AC power. Most VFDs produce a constant voltage/frequency (hz) ratio.

A low L3/D4 shaft is still your best protection against damage caused by operating off the pump's best efficiency point. Any pump that experiences frequent starts and stops has this problem.

If the dominate head in the system is pipe friction losses a variable speed device can have some advantages:

 The bad news is that pumps with variable speed drives have several potential problems:

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