How to Build a Centrifugal Pump Curve Part 4: Efficiency Curves

By Mark Bingham

This blog introduces efficiency curves (islands) to our pump curve and examines the variables impacting pump efficiency. We also introduce a valuable equation for hydraulic (pump) horsepower that is helpful for determining the penalty associated with throttling an over-headed pump.

The best efficiency point (BEP) for our e-1510 4BD is 86 percent (Figure 1). This efficiency occurs at 736 gpm and 71.7 feet of head. Not surprisingly, this point is on the 9.5” impeller head/capacity curve, the largest impeller available in our e-1510 4BD pump. Remember, pump efficiency decreases as the impeller diameter is reduced. We discuss this in great detail in our Pump Optimization Series.

Figure 1. The best efficiency Point (BEP) for this model pump occurs at 736 gpm and 71.4 feet of Head. BEP always occurs on the largest impeller curve.

While the manufacturer permits impeller diameters as small as 7.25 inches, trimming the impeller increases recirculation and turbulence, reducing pump efficiency.

We’ve added the remainder of the manufacturer’s published efficiency curves for the e-1510 4BD pump in Figure 2. These curves are often referred to as efficiency islands. These islands indicate that smaller impeller diameters are less efficient. However, this does not mean we shouldn’t trim an impeller. While trimming the impeller of an over-headed pump reduces efficiency, the energy savings achieved from reducing the excess head often exceeds this penalty. Below, we will illustrate how to calculate the cost of throttling the pump when the impeller is too large.

Figure 2. Now we’ve added the manufacturer’s efficiency curves forthe e-1510 4bd pump. note that efficiency decreases with the diameter of the impeller. increasing or decreasing flow beyond the BEP also reduces efficiency.

The efficiency islands also show that as we reduce or increase flow from the best efficiency point for a given impeller diameter, the efficiency decreases. For example, if we reduce the flow rate of an e-1510 4BD pump with a 9.5-inch impeller from 736 gpm to 400 gpm, the efficiency decreases from 86% (BEP) to about 72% (Figure 2).

With the efficiency curves on our curve, we can now read the flow rate, head, and efficiency for any operating point on our curve. Using the equation below, we can use this information to determine the power required for operation at any given point.

But why bother with this equation when we can see the duty point horsepower on the pump curve or determine it with even better accuracy from the selection software? It is helpful because it helps us determine the cost of a pressure drop in a system. Let’s assume we have a system designed for 700 gpm at 61 feet. The selection for a constant speed e-1510 4BD shows a 9-inch impeller and a pump efficiency of 85.1 percent (Figure 3). When the system is installed, testing reveals that the actual head requirement is 51 feet instead of the estimated 61 feet. If we add 10 feet of head by throttling a valve to balance the flow to 700 gpm, we have added a cost as follows:

Figure 3. systemwize selection for a constant speed e-1510 4BD pump for a duty point of 700 gpm at 61 feet of head.

With the horsepower calculated above and the facility electrical cost data, we can determine the added operating cost of the throttled valve. This added cost can and should be eliminated by trimming the impeller or by adding a variable frequency drive and operating the pump at a reduced speed.

While operating a pump at BEP is a great goal, we are rarely lucky enough to have a system design with flow and head requirements that perfectly coincide with this point. Even if the design flow is at the BEP, it’s unlikely that the head estimate will be exact. ASHRAE’s guidance on pump selection is indexed to the BEP. Figure 4 shows the recommendations for satisfactory and preferred selection ranges.

Figure 4. ASHRAE guidance for pump selection range.

Selecting pumps with operating points to the left of the BEP is usually preferred. Selecting to the right of the BEP increases the risk of operating near or beyond the end of the curve if the head estimate is overly conservative. Returning to the composite curve in Figure 2, if we selected the 4BD for 900 gpm at 63 feet, but the installed head requirement turned out to be only 45 feet, we’d be operating perilously close to the end of the curve after trimming the impeller to 9-inch.

While variable speed curves look very similar to reduced diameter curves, and the affinity laws tell us that trimming and reducing the speed have the same effect, operating a larger impeller at a reduced speed results in a higher pump efficiency than trimming the impeller. As Figure 5 shows, we can maintain 86 percent efficiency at a speed of less than 1000 rpm.

Figure 5. efficiency curves for a variable speed e-1510 4bd pump. notice that reducing pump speed has a similar impact on the efficiency curves as reducing the impeller size.

Let’s say we need 600 gpm at 48 feet (Figure 6). We'll need to trim the impeller to 8-1/8 inches for a constant-speed pump. At this point, the efficiency is 84.3 percent. Note that we are operating at a flow rate and head slightly above the selection criteria. This is because manufacturers traditionally trim impellers in 1/8-inch increments, so the impeller is slightly larger than would be required if we could trim to any diameter.

Figure 6. systemwize selection for a constant speed pump at 600 gpm and 48 feet of head. This selection requires that the impeller be trimmed to 8-1/8 inches. However, because impellers are trimmed in 1/8 increments, the impeller is slightly larger than needed, resulting in an operating point of 606 gpm, slightly higher than the required 600 gpm duty point.

Optionally, we can keep the 9.5-inch impeller and use a variable frequency drive to reduce the speed to 1447 rpm. This allows us to operate at 85.6%, just below the maximum efficiency of 86%. Note that we are also operating at the exact specified condition (Figure 7).

Figure 7. variable speed selection for 600 gpm and 48 feet of head delivers a more accurate operating point and higher efficiency than trimming the impeller.

Be aware that the end of the curve for reduced speeds is similar to reduced impeller diameters. Therefore, operating an over-headed variable speed pump at a significantly reduced speed introduces the same risk of running off the curve as trimming the impeller.

In the next blog, we’ll add the final element of the manufacturer’s published curve for the e-1510 4BD pump.