Why can't the regulating pump flow be regulated from the water inlet side, but only from the water outlet side?

The adjustment of the water pump can only be carried out from the outlet part, because

Any valve will produce local resistance loss,

Adjust at the inlet end, and the local resistance is just in front of the pump.

However, the pump has a capacity called the maximum suction vacuum height. After exceeding this height,

Cavitation will form in the pump, which will cause great damage to the pump and usually cause failure.

Increasing local resistance is equivalent to using part of the height and reducing the original capacity of the pump.

But if it is adjusted from the water outlet, there will be no such problem.

However, attention should be paid to the damage of water hammer to the pump when adjusting from the outlet section.