Definition: The NPSHr (“Net Positive Suction Head required“) refers to the minimum pressure at the pump inlet, determined by the pump manufacturer, that is necessary to prevent cavitation within the pump. It specifies the required discharge pressure on the suction side to prevent the pumped medium from vaporizing and causing damage to the impeller or casing.
What is the NPSHr exactly?
Cavitation can occur in centrifugal pumps if the pressure at the pump inlet drops too low and the pumped medium begins to boil locally. The resulting vapor bubbles collapse in areas of higher pressure within the pump, causing shock loads, material wear, and, in the worst case, complete pump failure. To prevent this, it is crucial that the NPSH value available (NPSHa) at the system is always higher than the NPSH value (NPSHr) required by the pump.
The NPSHr is determined by the pump manufacturer through test bench trials. This involves investigating the pressure drop at the impeller inlet at which a defined efficiency loss (usually 3%) occurs. This point is expressed as the NPSHr value. It is important to understand that the NPSHr does not include a safety margin – it is the absolute minimum value. In actual plant operation, a safety margin of approximately 0.5 to 1.0 m (or 10–20%) is therefore generally recommended to compensate for fluctuations, temperature changes, medium properties, and operating variations.
Key factors influencing the NPSHr are the impeller geometry, rotational speed, pump design, as well as the viscosity and vapor pressure of the pumped medium. Pumps with high rotational speeds or very small impeller inlet cross-sections usually exhibit higher NPSHr values. Therefore, selecting the right pump – including considering its NPSH behavior – can be crucial for long-lasting and trouble-free operation.
