Balance line (bypass line). Throttle devices positioned between the running mechanism of the pump and the mechanical seal lower or raise the pressure on the mechanical seal. The necessary flow is conveyed through balancing lines linking chambers of varying pressure. Examples:

• multi-stage centrifugal pumps with or without a balance disk or piston (balancing):
  The balance line connects the stuffing box on the pressure side with the one on the suction side and/or a second intermediate stage.
• low-temperature pumps with thermal isolation of the shaft seal (cryogenic mechanical seal): The balance line connects the pump's pressure-relief chamber positioned between the throttle and the mechanical seal with the gas chamber of the (storage) tank.

Only the pump manufacturer is able to give precise details of the balance line's effect on the magnitude of process pressures under given operating conditions.