What is the function of the minimum return line in a shielded pump?

Prevent the flow rate at the pump outlet from being too low, which could cause damage to the pump.

Is he controlling the total flow or just the flow of return water?

The purpose of installing a minimum return line at the outlet of the shielded pump is to prevent the pump from running idle without any flow, which could lead to damage if all other pipelines in the distribution station were closed. A minimum return line is typically installed, along with a flow-limiting orifice, to direct the fluid back to the pump’s inlet tank; this setup remains open at all times, and its purpose is to protect the pump.

The bearings of shielded pumps are all lubricated by the medium itself; at low flow rates, it is necessary to open the system to prevent the pump from running dry and causing damage to the bearings； When starting the pump, it is also necessary to open the minimum return line to prevent the pump from being damaged due to prolonged operation with the outlet closed.

Since the heat generated by the motor during the operation of a shielded pump needs to be carried away by the flow of the medium, the flow of medium within the pump body must not be interrupted for an extended period; otherwise, it will cause the motor to overheat and its windings to burn out. Therefore, a minimum return line is provided to ensure that fluid continues to flow within the pump and carry away heat even after the main flow path is completely closed.

In one sentence [to prevent damage to the shielded pump]

When the flow rate at the pump inlet falls below a certain value (this flow rate is known as the pump’s minimum protection flow rate), the fluid pressure at the inlet of the impeller decreases, causing the liquid to partially vaporize and form bubbles; this phenomenon is called flashing. The bubbles formed move with the liquid from low-pressure areas to high-pressure areas; in these high-pressure areas, the bubbles shrink rapidly and condense. The liquid surrounding them rushes into the space previously occupied by the bubbles at extremely high speeds, generating intense shock waves that strike the impeller and pump casing, resulting in noise and vibrations. Due to the repeated impact forces over time and the chemical corrosion caused by trace amounts of dissolved oxygen in the liquid, the surface of the pump impeller develops scars, cracks, and even spongy damage; this phenomenon is known as cavitation. These two phenomena can cause damage to the equipment and lead to unstable operation. By setting up a minimum flow return bypass line, it is possible to ensure the minimum flow rate required by the pump, thus ensuring that the pump can operate

Hello, may I ask how the minimum reflux flow rate is generally determined?

Determined by the shielded pump manufacturer; it is designed solely to protect the pump. For heat capacity, we have chosen a shielded pump with a flow rate of 3 m3/h; the minimum return flow rate specified by the shielded pump manufacturer is 1.2 m3/h. The total flow rate of the pump is 4.2 m3/h

Promote the flow of the medium in the pump chamber, enabling lubrication and heat dissipation