If you’re like me, looking at a centrifugal pump curve is enough to make your eyes cross. Now, add more impellers to a single stage pump curve, as you would with a multistage pump curve, and it becomes even more overwhelming. Like most things, once you understand what you’re looking at, it’s not so difficult. I sat down with one of our application engineers, Steve Miller, to help break down the parts of a multistage pump curve.
Steve thought the best way to explain it would be to walk through an example. He brought in some application data he was using earlier in the day to size a Goulds Water Technology e-SV for a customer.
Flow is listed along the horizontal axis. On this pump curve, both GPM and m3/hr are listed. We’re interested in the GPM, listed on the bottom. In Steve’s example, the customer needs to get 75 GPM of water from the pump.
This is a critical piece of information for pump selection. How much head is the pump required to overcome at the flow rate we’ve selected at 75 GPM? For this example it’s 250 feet. Follow 250 ft across the curve until it intersects with the flow line of 75 GPM. This is the performance point, circled in magenta.
Performance Curves (Black)
A multistage centrifugal pump has multiple impellers to generate high pressures over a wide range of flows. The downward sloping curves on the top chart represent the curves of the pump’s stages. This particular pump can house up to 12 impellers.
Steve says to select the stage that is just above the performance point. That means in our example, the selected pump should have 4 stages.
Now that we have the correct performance point, we can determine the amount of horsepower required. Along the left side of the curve is HP/Stage, highlighted in yellow. This indicates the amount of horsepower needed PER STAGE. In our example, the dotted blue flow line crosses the HP/Stage curve at about 1.75 HP. Some quick math (1.75 x 4=7) would tell us that this pump would most likely require a 7.5 HP motor.
Net positive suction head required is important for proper pump operation. This is the minimum amount of pressure on the suction side of the pump to overcome pump entrance losses. If sufficient NPSH is not met the pump will cavitate, affecting performance and pump life.
When selecting the best pump for an application, efficiency is an important factor. Efficiency indicates percentage of the motor energy is being used for useful work, i.e. moving or pressurizing your process fluid. Low efficiency means that energy is being wasted and lost as heat. Sources of this loss can be: recirculation within the pump, cavitation, or friction against pump components.
Like all centrifugal pumps, multistage pumps require a minimum amount of flow to be moving through the pump to dissipate heat created. On this curve, the red line represents the minimum flow required. Operating to the left of the line severely damaging to the pump, so just don’t do it. It is also called out under the NPSH chart.
Knowing how to read a multistage pump curve is essential to the health of your pumping system. Running too far out on the curve, or too far back can damage the pump, cause excessive energy consumption, and overall poor performance.
Need help sizing a multistage pump? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.