Balancing line in vacuum pumps

A balancing line is provided in suction of vacuum pumps and connected to condensate storage tank.

This line is used to remove external air continuously and ensure that pump has enough suction ie NPSH, to drag water from the condenser with out cavitation.

The balancing line is to maintain the suction pressure in negative even though if any air leak is happening through. Its acts as a vent to remove air pockets while charging the pump.