Self-priming pump cavitation allowance and self-priming height

1、Positive suction head (NPSH) ‌

 Effective net positive suction head (NPSHa) ‌ : Device characteristic, reflecting the actual energy of the liquid at the pump inlet that exceeds the vaporization pressure. 

Required net positive suction head (NPSHr) ‌ : a characteristic of the pump itself, referring to the minimum energy required to avoid cavitation, which is determined experimentally .

Self-priming height ‌
It refers to the vertical distance from the center of the pump impeller to the liquid surface. The theoretical maximum value is 10.33 meters (corresponding to standard atmospheric pressure), but in practice, due to factors such as pipeline resistance and net positive suction head, it usually does not exceed 7 meters ‌

2、The relationship between self-priming height and net positive suction head
When the altitude increases, the atmospheric pressure decreases and the self-priming height reduces.
When the temperature or volatility of the liquid increases, the net positive suction head (NPSH) requirement increases, and the self-priming height needs to be further reduced ‌
 actual limit ‌
If the installation height exceeds the allowable range, the liquid will vaporize at the pump inlet to form bubbles, causing cavitation, which is manifested as noise, vibration and impeller corrosion ‌.
In the design of Self-Priming Pumps, the relationship between the two needs to be balanced by optimizing structural parameters such as the impeller clearance and the volume of the liquid storage chamber ‌.

3、Application suggestions
 Selection and installation ‌
The nominal self-priming height should reserve a safety margin (for example, when 6 meters are required, select a model of 8 meters or more) ‌.
For high-temperature liquids (>40℃) or volatile liquids, the self-priming height should be further reduced by ‌.

Maintenance points ‌
Avoid having too many elbows in the suction pipeline or clogging of the bottom valve to reduce additional resistance loss ‌.
Regularly inspect the wear of the impeller to prevent performance degradation caused by cavitation ‌.

Our company produces a variety of self-priming pump with high suction lift and large flow rate.