That's an odd pump curve characteristic for a centrifugal pump in it's normal operation range, I guess that's why I am confused.    If your pump does indeed pump two flows at one head in its normal operating range, then it should have been doing it already and someone should have tried to make it work and in essence, follow their lead.As for the curent draw to control the pump, you are right, but this vaiable also takes into account motor performance too, which which could induce current loadings that may have nothing to do with your flow hydraulics, making you pump operate where it shouldn't.  Sounds like an odd problem.  Are you looking at the pump curves where they will normally operate?  Maybe looking at them outside thier realistic range is giving you this curve characteristic?
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 Screw pumps paralled pumps(ie, near rated and higher flowrates) then near shutoff head droop should not affect stable system operations. 
 
You are correct about your concern...the problem is a classic... there are many articles written on the subject...basically what they say is:operate the system at a pressure that ensures you are to the right of the point where the pressure is equal to the shutoff pressure (i.e. beyond the "hump" in the drooping pump head-flow characteristic).Otherwise you could have one pump at a very low flow and the other at a high flow and both delivering the same pressure...How do you ensure that you operate beyond that point?1. Select the operating pressure to be below the shutoff pressure (zero flow)2. Have a bypass line discharging back to the suction vessel (not the pump suction), to discharge the excess flow and achieve a back pressure - shutoff pressure = 10 psid3. have equalizing lines from the pump suction back to the suction vessel (this is a must for vacuum service).4. the check valves at the discharge of the pumps operate correctly and with very low differential (1~2 psid)Regarding control you can get as fancy as you'd like... yes, pump speed is a result... you should be controlling discharge pressure and then flow.e.g. control first that the pressure delivered by each pump is the same, the total flow should be higher than the required flow, then control flow to the required setpoint.Depending on what is the most critical feature (pressure or flow) the control system will follow.Also if you have VFD (variable frequence drives) the pumps will be operating on different curves... and the shutoff pressure will depend on the pump speed. That's where the check valves come into the game...unless the speed (developed pressure) is high enough to overcome the check valve the 2nd pump will not deliver any flow.

2011-09-01

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