When selecting a centrifugal pump, the consideration of net positive suction head
Thread Content
When selecting centrifugal pumps, I have some doubts regarding certain aspects: typically, design firms provide the value of NPSHa, and the manufacturers then calculate NPSHr. Generally speaking, the higher the NPSHa, the better, and the lower the NPSHr, the better. So there must be some standard for evaluation. If NPSHa is lower than NPSHr, will the pump vibrate violently under normal operating conditions? If NPSHa is greater than NPSHr, by how much should it be larger?
NPSHa: The net positive suction head available for a pump system, also known as the effective or usable net positive suction head, refers to the net positive suction head determined for that oil pump system. Its value is determined by the parameters of the pump’s suction pipeline system and the flow rate in those pipes, and it is independent of the pump’s structure； NPSHr: Required net positive suction head, determined by the pump’s own structure and established by the pump manufacturer through experiments. Under normal circumstances, it is required that NPSHa be no less than NPSHr; as a practical guideline, NPSHa should be 1.3 times greater than NPSHr. This 1.3-fold requirement is quite stringent – it is specified in some foreign standards, while in China it is generally accepted that NPSHa = NPSHr + (0.5 m to 1 m). If NPSHa is lower than NPSHr, cavitation occurs, which damages the impeller, affects its performance, and causes vibrations.
If NPSHa is lower than NPSHr, this issue needs to be resolved by adjusting the installation height.
NPSHa is the net positive suction head available of the unit, which is calculated by the design institute based on factors such as the properties of the medium, system pressure, and flow rate； NPSHr is the required net positive suction head of a pump; it is a parameter inherent to the pump itself, not calculated, but determined through testing after design. When both values are relatively accurate, NPSHr must be lower than NPSHa for the pump to operate properly; if NPSHr is higher than NPSHa, the pump will experience increased vibration and noise during operation, and prolonged use can lead to corrosion and damage of the impeller and pump casing ； In severe cases, there will be intense vibration, leading to scraping between moving and stationary parts, or even seizure. Generally, NPSHr is more than 0.5 m lower than NPSHa; the commonly used difference at present is 0.8 m or 1.0 m.
How can it be adjusted through the installation height – by lowering or raising it? Thank you
The net positive suction head available (NPSHa) of the unit is calculated based on the installation height. If NPSHa is lower than NPSHr, cavitation will definitely occur in the pump; in such cases, the installation height of the pump’s inlet equipment should be increased, or measures should be taken to lower the pump further
The API standard requires that NPSHa-NPSHr be not less than 0.6 meters. Of course, there must be a cavitation margin, and it’s not the case that the smaller it is, the better. The relatively low net positive suction head of the pump itself results in a high suction specific speed, which implies a larger impeller inlet size or a larger impeller inlet angle; this leads to internal circulation and hydraulic losses at the inlet. Therefore, the suction specific speed is generally not greater than 11,000