How to Read and Use a Pump Curve
Pump performance curves provide engineers with a plethora of information to guide pump selection, but the sheer volume of data is often intimidating and overwhelming for those new to the game. Understanding pump curves is integral for anyone specifying hydronic equipment.
Manufacturers publish curves for all of their pumps, reflecting thorough and rigorous testing in the laboratory or factory. This testing allows manufacturers to communicate how a pump curve will perform under a given set of conditions and which pumps will be the most efficient solution for any given application.
Pump Curve Elements
The two main elements of a pump performance curve are the total head and pump flow capacity, which run on the vertical and horizontal axis of a pump curve graph.
The total head is shown on the y-axis. This number, measured in feet or meters, represents the total pressure a pump can overcome to achieve a specific flow. It encapsulates all resistance values, including friction head, static head, and more. The pump flow capacity is shown on the x-axis. This number measures gallons or liters per minute (gpm or L/min) of pumped fluid at any given point during operation.
Flow capacity and head are plotted on the main curve at various impeller sizes, communicating how many gpm or L/min of fluid a pump will transfer based on total head conditions.
What factors to look for
Pump efficiency lines tell us the efficiency at which a pump with a certain impeller size and head capacity will operate. Although a pump’s calculated efficiency will vary according to flow and head, the pressure in a system is typically less than the pump curve suggests. For this reason, the best pump selection will be slightly to the left of the Best Efficiency Point (BEP). The preferred selection range is 85% to 105%.
Below are other curve factors that influence pump performance:
- Impeller Size - The impeller is the moving element inside a pump that drives the fluid. The impeller size (also known as ‘trim’) on a pump performance curve helps engineers specify the best-performing impeller size for their specific application.
- Brake Horsepower - Brake horsepower (BHP) is the amount of horsepower being consumed by the pump at any point on the curve as measured on a pony brake or dynamometer. The lines typically slope downward from left to right on the curve and vary along with the impeller trim.
- Required Net Positive Suction Head - The available net positive suction head (NPSHa) is the real amount of static head pressures minus total head loss in the system. If NPSHa is less than required (NPSHr) for the pump to achieve the desired flow and head, it means that the pressure at the eye of the impeller drops below the vapor pressure of the fluid inside the pump. This can cause cavitation, which results in noise, damage to the pump, and loss of efficiency. NPSHr is shown in feet of head above the performance curve for each operational point and is positively correlated to flow.
NPSHr is an integral part of understanding the pump performance curve and one of the most important calculations for specifying the correct pump for an application. All centrifugal pumps exert a negative pressure into the small space between the suction and the eye of the impeller because of sudden change in velocities between the suction and discharge of the pump, directional change of the fluid, and increased turbulence. The negative pressure is NPSHr –the minimum amount of pressure required at the pump suction for the pump to operate properly without cavitating. Atmospheric pressures must also be taken into consideration when calculating total head for the final NPSHr value. For more information on NPSH, click here.
Use pump curves to identify the most efficient option
When selecting a pump, it is best practice to consult several pump curves first to identify the most efficient option, which will minimize operating costs. Pump efficiency is the ratio of energy delivered by the pump in liquid horsepower to the energy supplied to the pump shaft in brake horsepower. So, a pump that delivers 75% efficiency at a given point on the pump curve is converting 75% of the brake horsepower it uses into hydronic energy or liquid horsepower.
Several factors influence pump efficiency, all of which can be found by examining a pump curve. These factors include:
- Flow and Head - Pump efficiency varies based on total head (vertical axis) and the flow (horizontal axis). When selecting a pump, use a pump curve to ensure that its primary operating range falls within or near its best efficiency range. Every pump curve has a Best Efficiency Point (BEP) at any given impeller trim. ASHRAE recommends pump selection between 66% to 115% of flow at BEP, though 85% to 105% of flow is the preferred range.
- Impeller Size - Pump efficiency is highest when the largest possible impeller is installed in the pump casing. A large impeller minimizes the fluid that escapes through the space between the impeller blade tips and the pump casing.
- Vibration - Pump efficiency decreases as the pump operates further away from the BEP because the deflection of the pump shaft, subject to axial and radial forces, increases the amount of vibration.
- Pump Size - Pump efficiency tends to increase in larger pumps. This is because the losses associated with bearing, mechanical, and internal hydraulic friction decrease in proportion to the required brake horsepower to drive the shaft as the pump gets bigger. This general rule does not support oversizing pumps in a given system, however, which decreases system pumping efficiencies significantly.
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