Problem: The outlet valve of the centrifugal pump cannot be fully opened
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There is a centrifugal pump with a flow rate of 100 to 120 cubic meters per hour. After the pump is started, the outlet valve can only be opened to 10% of its full capacity; thereafter, the motor reaches its rated current level and shuts down as a protection measure. Does anyone know what could be the reason for this? Thank you!
1. Electrical issues, such as insufficient motor power or incorrect settings for the motor’s starting frequency parameters； 2. Pump issues: such as changes in the impeller or the density of the medium, etc
First, it is necessary to determine whether the pump’s outlet flow reaches the rated value when the motor operates at its rated current. If it does, then either the piping is too large or there are empty sections in the pipeline downstream. If the rated flow rate is not achieved, the causes should be sought in the motor, impeller, medium, and other aspects.
We need to clarify the specific situation first! New pump or old pump – how is the pump performing? What are the outlet pressure and flow rate? What tests have been done, etc.
It’s not very clear. We’ve had similar problems before; it’s because when the pump starts operating, its flow rate is high. The pipes leading to the valve, as well as the equipment itself, are in a vacuum state, and since the pipes are quite long, once the outlet valve is opened after the pump starts, overcurrent can occur easily.
Fundamentally, the issue lies in an improper selection of the water pump – either the flow rate is set too low or the head is set too high. It can be determined based on the pressure difference between the inlet and outlet of the water pump.
The head selection was incorrect; this outlet valve is being used as a control valve. The actual pipeline resistance is less than the pump’s head, causing the Q-H curve to deviate from the originally designed point; as a result, the flow rate is high while the head is low.
The design parameters and actual parameters must match.
Each case must be analyzed on its own. 1. If the density of the medium being transported is higher than the designed density – for example, if C3 liquefied gas was originally planned to be used, but now water is used for testing before operation – the higher density will definitely cause an overcurrent. Either the viscous material has not been preheated, or the preheating is insufficient; in such cases, the density increases as well, which can lead to excessive current. 2. Incorrect selection of the pump model – the head required is higher than what was designed. When the head is high and the density remains unchanged, the current will be high. This issue can be resolved by reducing the outer diameter of the impeller, thereby lowering the head. If the head pressure is fine, it means the motor is too small; a motor with a higher power rating can be used instead. 3. The pump is started with no back pressure at the outlet; the resistance in the outlet pipeline is low. When the valve at the pump outlet is opened, the motor load increases, resulting in overcurrent in the motor. 4. The motor coils or wiring terminals may be damp, or there could be problems with the motor starter – such as excessive or insufficient voltage, incorrect parameter settings, unbalanced three-phase currents, or faults within the motor itself, such as stuck bearings, lack of oil, rotor rubbing against the casing, rotor imbalance, bent shafts, or an improper magnetic centerline. 5. Problems with the pump itself: stuck shaft rotation mechanisms, stuck bearings, stuck seals, excessive seal compression, friction between the dust seal and the bearing cover, friction between moving and stationary parts such as the mouth ring, poor alignment, blocked balance pipes, and increased axial forces