1 vfd / 1 soft start

[Brandogreen]

Hi, I am having a little trouble with my system. I have 2 300hp motor's powering 2 1,000 gpm pumps. Pump 1 has a vfd drive and pump 2 has a soft start. My system flow is constantly changing but pressure must reamin the same. Pump 1 will run on the vfd fine, and when it reaches 100% pump 2 will kick on and take over the flow. Pump 1 will then ramp down and take over the remainder......my problem is that when the system flow is at 1050-1150 it is in the middle of both pumps and excesive cycling occures on pump 2.
How can i narrow this margin? Would lowering the max. output on pump 1 vfd help?

 

[BigInch]

That's telling you that pump 1's discharge pressure cannot match the pressure being discharged by pump 2 running at full speed and (both pumps running at) 50% flow, as the pump 1 head would be relatively high. A pump with a VFD control flowing at 50% really only wants to produce 25% of full speed head, so there's probably somwhere around a 75% pressure mismatch. For pump 1 to reach pump 2's pressure, it must recirculate more % of flow internally until internal pressure builds up enough to eject it into the manifold, but then it can't sustain that flow any longer, so it goes back into its internal pressure buildup cycle again. Pump 2 is surging, causing the cycling I think you are describing. Pump 2 also probably cannot run parallel at any lesser flowrate either without surging even more.

Solutions / workarounds.

1.) Don't operate at that problematic range.
2.) Remove the VFD and add discharge pressure control valves.
3.) Keep the VFD and add a recirculation line to pump 2 back to the suction manifold to increase the suction pressure of both units, lessening the load and discharge pressure of pump 2 while enabling a more balanced discharge pressure to be produced by pump 1.
4.) Junk the VFD and add a recirculation line from discharge manifold to suction manifold that includes a pressure control valve, which is adjusting flow from discharge manifold to suction manifold to control the pressure in the discharge manifold's to whatever pressure you need to flow downstream.