Reasons for and precautions to take when closing the outlet valve during centrifugal pump startup.

Centrifugal pumps, as widely used fluid transfer devices, have a crucial starting process. A common practice during the start-up of a centrifugal pump is to close the outlet valve. This practice involves several important engineering principles and safety considerations, which we will discuss in detail below.

• The reason for closing the outlet valve when starting a centrifugal pump

1. Preventing Motor Overload

When a centrifugal pump starts, if the outlet valve is open, there is little resistance in the pump's outlet pipeline. In this case, the pump's head will be at a low level, while the flow rate will be very high. According to the pump's performance curve, the shaft power output of the motor will be very high under these conditions, easily leading to motor overload. This can damage the motor windings, burn out the wiring, and even cause safety accidents such as fires. Closing the outlet valve effectively limits the flow rate during startup, allowing the motor to start under a relatively low load, reducing the starting current, protecting the motor and related circuits, and ensuring that the pump starts smoothly.

2. Creating an Initial Vacuum Environment

The working principle of a centrifugal pump relies on the centrifugal force generated by the rotation of the impeller to create a vacuum, thereby drawing in and discharging the liquid. Before starting, the pump casing must be filled with water, and the outlet valve must be closed. When the pump starts, the impeller rotates at high speed, and the surrounding water is thrown towards the outer edge by centrifugal force, creating a certain vacuum area at the center of the impeller. As the impeller continues to rotate, the liquid is continuously pressed into the impeller by the pressure above the external liquid surface (such as atmospheric pressure), completing the suction process. If the outlet valve is not closed, it is difficult to create a sufficient vacuum in the pump, which may lead to failure to draw water normally and affect the normal operation of the pump.

• Essential conditions before starting a centrifugal pump.

Before starting a centrifugal pump, a series of conditions must be met to ensure that the pump starts correctly and operates efficiently. These conditions include, but are not limited to: firstly, the pump casing must be filled with water to create the necessary initial vacuum; secondly, the outlet valve must remain closed to prevent premature discharge of liquid during startup; furthermore, all parts of the pump must be checked to ensure they are intact and that no foreign objects have entered the pump body. Only after these conditions are met can the centrifugal pump be started to begin its suction and discharge operations.

1. Priming the Pump

Centrifugal pumps typically lack self-priming capabilities; therefore, the pump casing must be filled with the liquid to be transported before starting. The purpose of filling the pump with liquid is to fill the internal space, expel the air, and create the conditions for the impeller to generate centrifugal force and create a vacuum during rotation, allowing the liquid to be smoothly drawn in and discharged.

2. Checking Seals and Piping

Ensure that the pump's seals are in good condition and free of leaks, including mechanical seals or packing seals. Simultaneously, check that the inlet piping is unobstructed and free of blockages or debris that could prevent liquid from entering the pump. This ensures that the pump can properly draw in liquid, avoiding starting difficulties or abnormal operation due to poor suction.

• Monitoring and operation after startup

After starting the centrifugal pump and closing the outlet valve, as the pump gradually operates, the liquid around the impeller is continuously thrown towards the outer edge by centrifugal force and flows into the discharge pipe. At this time, it is important to closely monitor the readings of the pump's pressure gauge, ammeter, and other instruments, as well as the pump's operating sound and vibration. When the pump pressure gradually rises and stabilizes, and the current also stabilizes within the normal range, and there are no abnormal vibrations or noises, the outlet valve can be slowly opened. During the valve opening process, it should be done evenly and slowly, allowing the pump's flow rate and pressure to change smoothly along its performance curve, gradually transitioning to the normal operating state. This avoids sudden changes in flow rate and pressure caused by opening the valve too quickly, which could shock the pump and piping system.

• Comparison with other types of pumps

Unlike centrifugal pumps, other types of pumps such as axial flow pumps, mixed flow pumps, and vortex pumps require the outlet valve to be fully open during startup. This is due to the differences in their operating characteristics compared to centrifugal pumps. Axial flow pumps have the highest shaft power at zero flow conditions, reaching 140%-200% of the rated shaft power, while the power is lowest at maximum flow. Therefore, to reduce starting power, the valve needs to be fully open during startup; the shaft power of mixed flow pumps at zero flow conditions is also between 100%-130% of the rated power, so it is also suitable to start with the valve fully open; vortex pumps are similar to axial flow pumps, with high shaft power at zero flow conditions, reaching 130%-190% of the rated shaft power, hence their starting characteristic also involves starting with the valve fully open.