Centrifugal pumps shine when a processing line may require a wide range of flow rates. Altering the flow rate by closing or opening a discharge valve will stress a pump by causing it to operate outside of its best operating point. When the situation requires significantly more or less flow than the pump is producing net positive suction head, NPSH, cavitation, or vibrations can cause irreparable damage.
As a centrifugal pump operates pressure changes from inlet to outlet; negative pressure “pulls” fluid into the pump, and positive pressure “pushes” the fluid out of the pump. Because fluids may change phase at lower pressures, cavitation commonly occurs near the inlet of the pump. This phase change will cause the pumps to run dry, producing vibrations that wear bearings prematurely. These vibrations are caused by small bubbles returning to liquid as the pressure increases.
To prevent dangerous cavitation, net positive suction head is a measurement of the pressure at the pump inlet above the vapor pressure of the pumped fluid. NPSH is usually 3 to 5 feet, more if possible, meaning that at least 3 feet of head pressure more than the is required by the system.
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