When the centrifugal pump is started, is the outlet valve fully open or closed?

The disadvantage of fully opening the outlet valve of the centrifugal pump when starting it

If the outlet valve is fully opened when starting the centrifugal pump, it is like letting an athlete sprint with full force without warming up, which will bring a series of serious problems.

From the perspective of the motor, centrifugal pumps have an important characteristic: when the pressure (head) is low, the flow rate will be large, and the power of the pump is proportional to the flow rate.

When starting, the outlet valve is fully open, and there is initially no pressure in the pipeline. The flow rate of the pump will instantly increase, and the power of the pump will also increase significantly.

At the same time, the rotating parts of the motor and pump accelerate from a stationary state to high-speed operation, requiring a large acceleration, which results in a very large starting current.

Such a large current is a huge burden for the motor and can easily cause overload. Once the motor is overloaded, overheating may occur, and prolonged overload operation may even directly burn out the motor.

In some factories, there have been cases where the outlet valve was fully opened when the operator started the centrifugal pump, causing the motor to overload instantly and the winding to burn out. This not only caused equipment damage, but also forced production to be interrupted, resulting in huge economic losses.

In addition to damaging the motor, starting the centrifugal pump with the outlet valve fully open can also have adverse effects on the pump body itself. The high-speed impact of high flow liquid on the impeller, pump casing and other components inside the pump body in a short period of time can cause strong vibrations and harsh noise.

This kind of vibration and noise not only affects the normal operation of equipment, but may also cause interference to the surrounding working environment. Long term exposure to such an environment may even pose a threat to the health of operators. Moreover, continuous vibration can cause loosening and increased wear of pump components, shortening the service life of centrifugal pumps.

The advantage of fully closing the outlet valve of the centrifugal pump when it is started

Compared with starting a centrifugal pump with the outlet valve fully open, starting with the outlet valve fully closed is like adding a "safety lock" to the equipment's startup, with many significant benefits.

From the perspective of motor protection, when the outlet valve is fully closed, the liquid in the centrifugal pump hardly flows at the moment of starting, and the flow rate is approximately zero. According to the characteristic curve of a centrifugal pump, the shaft power of the pump is minimized when the flow rate is zero.

This means that the motor needs to overcome the minimum load during startup, and the starting current also decreases significantly.

Taking a centrifugal pump from a certain factory as an example, under normal circumstances, if the outlet valve is fully opened and started, the starting current of the motor can reach 5-7 times the rated current. However, when the outlet valve is fully closed and started, the starting current is only 2-3 times the rated current.

In this way, the motor will not be subjected to excessive voltage and current surges due to excessive starting current, effectively protecting the winding, insulation and other components of the motor, and extending its service life.

In terms of the operational stability of the pump body, closing the outlet valve to start can keep the centrifugal pump in a relatively stable state during the initial start-up. During this process, the impeller gradually accelerates and rotates, receiving less liquid impact force, and the load and vibration are also at a lower level.

This is not only beneficial for protecting the impeller, reducing the risk of wear and damage to the impeller, but also avoiding the loosening and damage of other components of the pump body caused by the severe vibration of the impeller, ensuring the stable and reliable operation of the pump body.

In addition, closing the outlet valve can effectively prevent the occurrence of cavitation. When starting a centrifugal pump, if the outlet valve is opened, the pressure inside the pipeline is low and the liquid flow rate is fast, which can easily cause gas in the liquid to escape and form bubbles.

When these bubbles burst in the high-pressure area inside the pump, they will produce strong water hammer, causing serious erosion and damage to the impeller and pump casing, which is the phenomenon of cavitation.

Closing the outlet valve can gradually increase the pressure inside the pump, avoiding the escape of gas and the formation of bubbles, thereby effectively preventing damage to the pump body caused by cavitation.