Solution for the self-priming capacity of the cartridge centrifugal pump (VS6)
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Hello everyone: May I ask how the self-priming capability of a canned centrifugal pump is achieved? What factors are related to its self-priming capacity? What is the relationship between its self-priming capacity and NPSHr? Please also help analyze ordinary centrifugal pumps. Thank you!
Centrifugal pumps do not have self-priming capability; to start them, the pump chamber must first be filled to meet the starting conditions. But the reason it is said to have self-priming capability is that after the pump is filled and started, it does work on the liquid, which indirectly reduces the pressure at the pump inlet and thus allows the fluid at the inlet to be drawn in. NPSHR is a hydraulic characteristic of the pump itself and has nothing to do with its self-priming ability. However, increasing the NPSH margin can improve the self-priming ability. To improve the self-priming capability, I believe that, without changing the existing design, the length of the bucket bag can be increased, thereby raising the NPSHA and enhancing the pump’s resistance to cavitation
When NPSHA is greater than NPSHR by 1 m, and the pressure in the inlet tank remains constant, should the centrifugal attraction at the pump inlet be taken into consideration or not? Is it that, even if such a condition exists, it is minimal, and NPSHA can compensate for the suction at the pump inlet? In other words, is the concept of suction at the pump inlet not valid?
I’m not sure what your operating conditions are; why is it necessary for the VS6 to have self-priming capability? VS6 is generally used in situations where the inlet net positive suction head is relatively low; at such times, the inlet pressure is low, and in some operating conditions it is even negative. VS6 is itself located in a pit, which is why it is also called a “pit pump”; it is usually arranged in a reverse-flow configuration. In other words, at the time of startup, both the bucket and the inlet pipe are already filled with water. What do you consider to be the concept of self-priming?
Self-priming is the same concept as the negative pressure you mentioned, that is, relative negative pressure. Does this condition exist, and is it taken into account in the design? For example: when the pressure in the gas phase space of the tank is P1 ； The static pressure of the liquid level at the pump inlet (with pipeline resistance taken into account) is P2 ； When the pump inlet pressure is P3, what is P3=? . If it appears, P3
Self-priming and negative pressure are not the same concept. I guess what you mean is that the inlet is at negative pressure, but it’s hard to say whether a pump operating under conditions of negative pressure at the inlet has self-priming capability. P3
Yes, the inlet is at relative negative pressure. May I ask, is there any relation to the inherent properties of the pump itself? For example: NPSHR, etc.
For NPSH, according to 6.1.8 (P30/222) of Version 11 of API 610, the reference level (or reference surface) for vertically suspended pumps (such as VS6) is the midline of the pump inlet pipe. Therefore, the longer the pump shaft and the deeper it is submerged, the higher the pressure at the inlet of the first impeller at its lowest end, and the lower its NPSHr becomes. From an operational perspective, this results in an improved self-priming capability. Regarding the negative pressure and self-priming you mentioned, it should be noted that the pump’s NPSHr is an absolute pressure, not a gauge pressure. In other words, as long as a pump’s NPSHr is less than 10 meters, we can generally assume that the pump has self-priming capability; in other words, even if there is negative pressure at the inlet, the pump can still operate properly as long as it is filled with liquid. As for the factors affecting the NPSHr of a pump, it is recommended to refer to clause 6.1.8 in API 610, version 11, which explains it in detail.