By Chad Edmondson
One of the most important lines on a pump performance curve is the Net Positive Suction Head (NPSH) curve. Discreetly applied either below or above the pump performance and efficiency curves, this single plotted line is the key to avoiding cavitation. Required NPSH or NPSHR for a given pump increases with flow. So, using the pump curve shown in Figure 1, we can see that the NPSHR for this Model 1510 B & G pump with a 8” impeller and 800 GPM and 33 Feet of Total Head is 12 feet of head.
As we discussed in an earlier blog on NPSH, cavitation occurs when the pressure at the eye of the impeller drops below the vapor pressure of the fluid inside the pump. When this occurs, vapor pockets will form on the veins of the impeller and then “implode” once they reach the higher pressure inside the pump. Cavitation is not only loud, it wreaks havoc with pump performance and can seriously damage the impeller and the pump shaft. The NPSH curve on the pump curve is critical because it tells you exactly how much pressure is required at the pump suction to keep the pump from cavitating. Figure 1
This is especially important information to have when selecting pumps for an open system like cooling towers where the water is only under atmospheric pressure. In many cases the available pressure at the suction of the pump is minimal after total head losses are subtracted. Since water boils faster under lower pressures, this increases the chance of cavitation in an open system. Note the vapor pressure of water at various temperatures in Table 1. Under a negative pressure of -14 Psig water will boil or flash to steam at only 85°F!
What Happens Between Suction and the Impeller
You may wonder why this supplemental pressure (NPSHR) at the pump suction is even necessary. The answer lies in the small space between the suction and the eye of the impeller.
Every centrifugal pump will exert a negative pressure in this small space. This pressure drop occurs because of the sudden change in velocities between the suction and discharge of the pump, the directional change of the fluid, and the increased turbulence. The negative pressure created within this space is the NPSHR value. It is the minimum amount of pressure required at the pump suction in order for the pump to operate correctly.
Figure 2 represents the pressure drop that occurs between the suction and the impeller of two pumps. Note that the lower curve dips below vapor pressure. This pump will surely cavitate, while the other is a suitable range above the NPSHR. Figure 2
This is one reason why it is so important that the design engineer accurately calculate the total head loss in the system and subtract it from whatever static head pressures that exist. This value (the NPSHA) must be equal to or greater than the NPSHR.
Finally, keep in mind that atmospheric pressures also vary geographically, so total head calculations must include the proper atmospheric values – whether it be Denver, CO or Myrtle Beach, SC.