How to Build a Centrifugal Pump Curve Part 5: Net Positive Suction Head
/By Mark Bingham
We are almost finished building our e-1510 pump curve! In this final blog of our series, we’ll introduce positive suction head (NPSH) to our curve and explain what NPSH is and why it matters in pump selection. This blog is intended as a basic introduction to NPSH. If you are ready for a deeper dive, please see links to additional articles previously published in the JMP Study Hall.
In the simplest terms, NPSH is the minimum pressure required at the pump suction for safe and satisfactory pump operation. The manufacturer creates an NPSH curve by testing the pump with 68°F water. (We explain the relevance of temperature further down in the article.) The NPSH curve is included with the published pump curve, as shown in Figure 1.
Figure 1. Centrifugal pump curve with net positive suction head (NPSH) curve.
As the scale at the bottom right of the pump curve shows, the NPSH required is specified in feet of absolute pressure. Absolute pressure is the sum of the gauge pressure at the pump (which is sometimes negative) and atmospheric pressure. Table 1 converts a few points of the NPSH requirement for our e-1510 4BD pump in feet to psi absolute for a sea-level installation.
Table 1. Conversion of feet absolute to psig at sea level.
As the flow through a pump increases, so does the required pressure at the pump suction. Notice that our e-1510 4BD pump's highest NPSH requirement is less than 25 feet at the end of the 9.5-inch curve. (As previously discussed, we never want to operate a pump at the end of its curve. We reference this maximum pressure requirement for the sake of example.)
Table 1 shows that 25 feet of absolute pressure equals -3.9 psig at sea level. Note that the maximum NPSH required (NPSHR) for this pump is a vacuum. Lower flow rates allow safe operation with an even deeper vacuum. While operating a pump with a suction pressure below atmospheric pressure could result in other problems, such as air coming out of the solution, cavitation won’t occur as long as the suction pressure is above the value specified on the curve, ideally with a margin of safety. Bell and Gossett recommend a margin of 5 feet, or 40 percent of the NPSHR, whichever is greater.
Since a pump can operate in a vacuum without cavitation, operation in a closed system is rarely problematic as long as the system is adequately pressurized. We'll introduce the effect of temperature on the NPSHR later; for now, we'll say that temperatures above 180°F or so warrant an analysis of available NPSH, even for closed systems.
What Happens When Suction Pressure Is Insufficient?
When a pump operates with inadequate suction pressure, some water will boil, creating steam bubbles. The pressure increases as the fluid moves through the pump, causing these bubbles to collapse. This collapse exerts large forces on the internal surfaces of the pump, which can irreparably damage the impeller. Figure 2 shows a pump impeller destroyed by cavitation. As you might imagine, a pump operating in this condition is very noisy. The sound is often described as gravel passing through the pump.
Figure 2. Impeller severely damaged by cavitation.
It’s important to note that water temperatures above 68°F require higher suction pressures to prevent cavitation because the fluid is closer to boiling. Again, cavitation results from the collapse of bubbles formed by boiling the water entering the pump.
How Atmospheric Pressure Impacts NPSH
Table 2. Atmospheric pressure at increasing elevations
Atmospheric pressure, usually the largest positive contributor to available NPSH, is lower at higher elevations. At sea level, we have 34 feet of NPSH from atmospheric pressure (14.7 psi x 2.31 feet/psi.). If we operate a similar pump at an elevation of 5300 feet, say in Denver, Colorado, atmospheric pressure is only 28.4 feet. Table 2 shows atmospheric pressure at altitudes of 10,000 feet.
The JMP Study Hall has published several other articles that discuss NPSH. Please see the following blogs for a more detailed exploration.
NPSH Demystified: What Is It and Why Is IT So Critical?
Avoiding Pump Cavitation in Open Systems (3-Part Series)
How to Read a Pump Curve (3-Part Series)
We hope you have enjoyed this series on How to Build a Centrifugal Pump Curve. While we’ve used Bell & Gossett’s e-1510 4BD as our example throughout this series, the principles we've discussed generally apply to all centrifugal pumps.
