Centrifugal pump
Thread Content
Why close the outlet valve before starting the centrifugal pump
To reduce the starting current, which is related to the load on the transformer, larger motors have a significant impact on the transformer’s load during startup.
It is mainly to protect the motor; when a centrifugal pump starts, the current can momentarily reach 7 times the rated current. If the outlet is opened, the current will be even higher, which can damage the motor
The main purpose is to establish back pressure as quickly as possible, in order to avoid excessive flow at the start of pump operation, which could lead to prolonged motor overload and eventual damage to the motor. In other words, if the power of your motor is sufficient, there is no problem at all in starting the pump with the outlet valve open; for example, fire pumps typically have motors with much higher power than process pumps, and in such cases it is possible to start the pump with the outlet valve open. As for the issue of 7 times the current, this cannot be controlled by closing the outlet valve, although it is possible to reduce this current to some extent.
It is mainly for the purpose of protecting the motor. When a centrifugal pump starts, its current level is already relatively high. If the outlet valve is opened, work is done on the fluid, which results in the current being under overload for a longer period of time and with a greater load. With the outlet valve closed, there is no issue with work being done, so the current will quickly return to a low-load level. Furthermore, closing the outlet valve allows for a rapid establishment of backpressure. During system operation, the pressure behind the outlet valve is generally high. If the outlet valve is opened first, and the check valve does not function properly, it can lead to backflow of the fluid, causing the pump to rotate in reverse and the impeller nuts to loosen, thereby damaging the pump
Prevent high-volume output, overload-induced tripping, and even motor damage! And it has a significant impact on production fluctuations!
Other things aside, if it’s not closed, no pressure can be generated, and cavitation is also likely to occur.
The outlet valve is closed to protect the motor. When the outlet valve is opened wide and the pump is started, the motor already has a high starting current; performing work on the medium will inevitably cause excessive flow through the motor, leading to its damage.