How to Build a Centrifugal Pump Curve Part 2: Trimmed Impellers

By Mark Bingham

In Part 1 of our Building a Pump Curve series, we discussed the parts of an unplotted pump curve. Now it's time to add some curves!

In Figure 1, we've added a single head/capacity curve for a 9.5-inch diameter impeller--the largest impeller that can be used with our example pump, a Bell & Gossett e-1510 4BD. During laboratory tests on an actual e-1510 4BD pump, the manufacturer recorded the head and flow characteristics shown on this curve. All pump manufacturers are responsible for creating pump curves that comply with the testing protocol established by the Hydraulic Institute (ANSI/HI 14.6 − 2022).

Figure 1. Curve for a Bell & Gossett e-1510 4BD pump with a 9.5-inch diameter impeller.

Here are a few points in tabular form read from the 9.5” curve:

We visually selected these points from the published pump curve. Bell & Gossett Systemwize pump selection software can be used to determine these points more accurately. These coordinates are for pure water, however, the same selection software can and should be used to determine pump performance when pumping other fluids, like a glycol solution. As we touched on in the previous blog, different fluids have different characteristics, including viscosity, which impacts the position of the curve, and specific gravity, which affects the power requirements and the feet to psi conversion. We’ll delve deeper into this later in the series.

Centrifugal pumps must always operate on the head capacity curve when running at the pump's specified speed. For example, the e-1510 4BD with a 9.5-inch impeller operating at 1770 rpm must always operate on the curve shown above. But suppose the head and flow we need don't correspond with a point on the 9.5-inch curve? In that case, we must trim the impeller or operate the pump at a reduced speed if a variable-speed drive controls the motor. We can sometimes operate at a point slightly above the curve by increasing the pump speed. In another blog, we explore how variable speed control impacts pump curves.

For now, let’s say we want to use this specific constant speed pump for a flow rate of 600 gpm at a head of 68 feet. This head/capacity combination is not on the 9.5-inch impeller curve. However, we can see by the curve in Figure 2 that we can achieve this head and flow with a 9-inch impeller. In this case, we would simply request that the manufacturer or representative trim the diameter by ½ inch on a lathe. They will also rebalance the impeller before reassembling the pump to ensure smooth operation with minimal vibration.

If our desired head and flow lie somewhere between the 9- and 9.5-inch impeller curves, we can trim the impeller in 1/8-inch increments. We should use pump selection software to determine the exact diameter our conditions require.

Figure 2. The curve shows us that a 9-inch impeller meets our needs for 600 gpm at 68 feet of head.

The graph in Figure 3 shows the manufacturer’s head/capacity curves for this pump with five impeller sizes, from 7.25 to 9.5 inches in diameter.

Figure 3. Manufacturers typically publish composite curves showing the tested performance of pumps with various-size impellers. Pump software can accurately estimate the performance of pumps with impeller sizes in between what the published curves show.

It is important to note where each head/capacity curve ends on a published curve. The manufacturer has already determined during testing that operation beyond this point will be unstable and will likely damage the pump. For this reason, we should plan to operate at a flow rate that is somewhat less than that at the end of the curve. Just keep in mind that as we reduce the impeller diameter, the end of the curve occurs at decreasing flows and heads, decreasing our range for satisfactory operation. Trimming the impeller also reduces the pump’s efficiency.

Next, we add power curves to our pump curve and discuss the relationship between power, flow, and head.