## NET POSITIVE SUCTION HEAD (NPSH) 101

By Dan R. Benjamin

Anyone who has studied a pump performance curve has seen several components on the graph below. There is usually a vertical axis that depicts pressures and a horizontal axis that depicts flow rates. Additional components include efficiency, horsepower and impeller diameters. There may also be a curve labeled NPSH or NPSHr.

NPSH = Net Positive Suction Head
NPSHr = required
NPSHa = available

Every pump installation has a certain amount of NPSHa and every pump has a certain amount of NPSHr at each given performance point. Simply put, pumps need positive pressure on the suction side in order to “push” the liquid being pumped into the suction of the pump. That push comes from the sum of all the positive influences on the liquid to be pumped at the source. These positive influences include atmospheric pressure and the actual height of the liquid above the pump suction. Additional factors to consider include both temperature and characteristics of the liquid.

#### NPSH PROBLEMS

Pump performance problems can occur if NPSHa is not properly considered when selecting a pump for a given application. The NPSHa has to be calculated and compared to the NPSHr, as supplied by the pump manufacturer.

If there is not enough NPSHa, numerically expressed as a number greater than the NPSHr, you can expect the pump to cavitate which shortens its useful life and limits its performance. If the pump you have selected cannot be reconsidered, there are several solutions. The feasibility of these solutions can only be considered when evaluating specific installations.

#### POSSIBLE SOLUTIONS

• Raise the liquid level on the suction side which increases the vertical height
• Lower the temperature of the liquid being pumped (vapor pressure)
• Physically lower the pump
• Physically raise the tank on the suction side
• Increase the diameter of the suction piping