How to Build a Centrifugal Pump Curve Part 1: Elements of the Pump Curve
/By Mark Bingham
Understanding centrifugal pump curves is essential for designers and installers of hydronic HVAC and plumbing systems. These handy charts provide a wealth of information about a pump's performance capability and help us make better pump selections. They may look intimidating, but we break through the complexity in this blog series by demonstrating how to build a centrifugal pump curve line-by-line.
We begin with a blank graph (Figure 1). Although this graph does not yet contain specific values, it is scaled for a Bell and Gossett e-1510 4BD, one of the more common pumps used in HVAC applications. As we progress through the series, we will add all the essential elements of the curve, explaining each as we go.
Like all pump curves, this graph has an x-axis (horizontal) representing pump capacity and a y-axis (vertical) representing the pump head. Notice that this graph uses US Customary Units (feet and gpm) for these measurements. Some curves have dual scales with System International (SI) units (metric) as well as US Customary Units.
FIGURE 1. A blank pump curve scaled for a Bell and Gossett e-1510 4BD pump.
FIGURE 2. Dual scale pressure gauges that indicate both psi and feet will save you the effort of converting psi to feet of head when taking readings on centrifugal water pumps.
Why Feet Instead of PSI?
If you are familiar with the instrumentation used to measure pressure in mechanical systems, you may wonder why the y-axis is scaled in feet when most pressure gauges are scaled in pounds per square inch (psi). Pump manufacturers publish curves with feet of head because the curves are universally applicable for any fluid with a similar viscosity to water. If they published curves scaled in psi, they would have to create a different curve for each fluid with a specific gravity different from water. This is because the specific gravity of the pumped liquid must be known to convert feet to psi. For water, which has a specific gravity of one, 2.31 feet equates to 1 psi. For ethylene glycol, which has a specific gravity of 1.1, 2.1 feet equates to 1 psi.
In the field, a gauge reading on any centrifugal pump handling water can easily be converted from psi to feet of head by multiplying by 2.31. Just note this conversion should only be used for water. If you're lucky, the gauge has a dual scale showing both psi and feet. (Figure 2). In a future blog, we'll show how to calculate the feet per psi conversion and discuss head calculations in detail. For now, just think of pump head as the energy a given pump imparts to the fluid.
Net Positive Suction Head Required (NPSHr) Scale
There is also a compressed vertical scale on the lower right of the graph. This scale (also shown in feet) indicates the pump's net positive suction head required (NPSHr). NPSHr is the minimum pressure required at the pump suction for the pump to operate properly. The scale and corresponding curve are explained later in this series.
Determining Required Capacity (Flow)
When selecting a pump for a specific application, we need to know the desired system flow rate, which can be found in the equipment specifications for the system we are pumping. Figure 3 is an excerpt from a chilled water coil datasheet for a specific design. It indicates that a flow of 60.5 gpm is required to achieve design cooling capacity through this single coil. However, remember that a chilled water pump may be serving many similar coils. If so, we can determine the maximum required flow rate by totaling the flow requirements for all the coils. A diversity factor is usually applied to this total, as it is unlikely that all the coils will operate at full capacity simultaneously.
FIGURE 3. Maximum flow rates required for specific coils can be found in the equipment specifications for a pumping system.
In our next blog, we'll discuss the relationship between head and flow for a pump with a specific impeller size and how this relationship is represented on the pump curve.
