Flow rate at the moment of centrifugal pump startup
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I would like to ask the experts: at the moment a centrifugal pump starts up, what is the maximum flow velocity in the inlet pipe at that instant compared to the value under steady-state conditions? Can it be expressed as a multiple of the peak starting current and the steady-state current? Where can I find similar introductions or materials?
This post was last edited by altpage on 2022-9-4 at 15:27. Since a centrifugal pump creates a vacuum by the rotation of the impeller inside the pump casing, which is necessary to draw water in, the motor does the most work during the transition from a non-vacuum state to a vacuum state; as a result, the current rises momentarily. In subsequent stages, the vacuum level is maintained, and the current remains stable. From the perspective of the motor: the power of a motor consists of two components, thermal energy and mechanical energy. That is: P=UI=I squared R+mechanical energy; at the moment of startup, the mechanical energy is zero, the resistance is at its minimum, and the current is at its maximum. When the centrifugal pump starts, the outlet valve is closed; how can there be a maximum instantaneous flow rate? ； When the outlet valve of the centrifugal pump is open and the motor is started, the current required to force the fluid out combines with the current needed to start the centrifugal pump; this combination may cause the pump to shut down due to excessive current. However, if the current level at which the pump shuts down is high enough, no problem will occur (the current level for pump shutdown is set by electrical engineers) ； The starting current of a centrifugal pump is independent of its flow rate.
It is recommended to first understand clearly the working principle and startup procedure of the centrifugal pump (gas/liquid), before moving on to study other aspects. For reference.
I’m trying to figure out from which perspective it’s possible to understand the principle behind why a temporary low pressure occurs in the inlet pipe at the moment the pump starts
There is no flow rate before startup; after startup, the fluid is accelerated by the impeller, creating a negative pressure in the area behind it, and this negative pressure draws in more fluid to fill that area. If you have used a water pressure tank in the countryside, these basic physical concepts become a bit easier to understand.