I'm looking to run 2 large (600kW) centrifugal pumps in parallel (duty / assist). One of the pumps is new the other is 50 years old but has similar head / flow characteristic although lower efficiency now. I'm concerned that the pump curves are not continuous rising to closed valve. Are there any recommendations on controlling these pumps, particularly when the assist pump starts to prevent hunting? I should add that the pumps will be speed controlled (via VSDs) and my intention at the moment is to always start the pumps at minimum speed and use current set-point control to run the pumps at the same speed but would be grateful for any advice.

I don't think running the pumps at the same speed will work, they are both different and will behave diffenently in the system, making speed a variable that shouldnt be used for control.  I am assuming that you are adding a second pump to maintain system pressures that have fallen for some reason.  Having the pumps controlled to maintain a set pressure would make more sense.  How are you designing the second pump to come on?  Based upon pressure?

I also dont understand what you mean when you say the pump curves are not continuous rising to closed valve?  Please give us some more information if you can...

Drooping head-flow curves toward shutoff flow are not all that uncommon in mid and low specific speed centifugal pumps. In my opinion, parallel operation of pumps with low flow head droop should not be a problem unless you're at or close to two-phase suction flow conditions where fluid compressibility effects can get you into system flow instability and flow surging problems. Boiler feed and condensate pumps are in that category while pumping hot water with relatively low suction pressures. In pressurized systems with conservative Minimum Suction Pressure requirements that include dissolved air, other gases and water vapor suppression provisions or even unpressurized systems with adequate NPSH available, low flow operation of pumps with drooping characteristics can usually succeed. If you're operating on the negative slope portion of the head flow curve of all paralled pumps(ie, near rated and higher flowrates) then near shutoff head droop should not affect stable system operations.

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