Air being drawn into condenser water piping 1

[pesco1957]

We have a roof mounted cooling tower that is supplied by 2 pumps located 40' below it. The pumps are piped in parallel off "tees" in the piping from the tower. One pump works fine, but the other is drawing air into that section and it is causing cavitation. We have a air vent relief on the piping above the pump and it belches air all the time. Where could this air be coming from? Could the seals draw air in as the pump runs but not leak water?
Thanks, Marc

 

[MechEngNCPE]

Yes, check the seals, there was a post yesterday with this same problem. When in suction it can draw air in, but not leak when not running. Seems like a common problem.

 

[317069]

pumps are 40 feet below tower, that mean you have about 12m water colum which is translated as a 1.2 bar when system is off,
does the pressure gage inside pump's room indicate to more than 1.2 bar?
- is this a new construction or an existing one.