Condensor water pumps

[1combuilder]

In an effort to balance multiple CWP's through VFD's, the suction side of the pumps is drawing air. when we balanced through the pumps one at a time, this problem was not present. any thoughts?

 

[rmw]

You need to give an awful lot more information about what is going on here if you want any help here.

rmw

 

[1combuilder]

We installed 2 new 1200 ton cooling towers. In doing so, we installed new CW piping, all of which is on a common header. There were 2 existing chillers that remained. There are 3 CWP's, 1 is standby. We installed 3 new VFD's for the pumps. We balanced the system through the pumps, one at a time. The intent was to balance through the new VFD's, while at least 2 pumps are running. However, the suction side of both pumps while 2 are running, are sucking air? If we open the suction side of any of the 3 pumps while they run one at a time, they do not suck air?