ksnar,
hope this posting does not come too late into the discussion...
steven's question hit it right on the head.
Positive displacement pumps (PDP) behave completely different from centrifugal pumps (CP).
The PDP have what is called a slip factor (similar to the slip in induction motors). The higher the viscosity of the fluid pumped the less is the slippage and the flow vs head curve tends to the "ideal curve": a vertical line at the design flow @ operating speed, at double the speed the new flow would be another vertical line at double the original flow (this is the definition of PDP: flow/cycle = constant)
Now, the system curve behaves differently: the higher the viscosity the higher the pressure drop for the same flow.
So we have two families of curves:
1. pump capacities at different viscosities.
2. system curves at different viscosities.
If all are plotted on the same graph where the pump curve intersects the system curve (for the same viscosity) a series of operating points are obtained at different viscosities.
Then, a similar plot is made for the NPSHR (required NPSH) and NPSHA (available NPSH), again a the different viscosities. Another curve is determined that joins the operating points at different viscosities, if the pump operates at flows higher than the flow determined by those points the fluid will reach the cavitation point.
If this sounds complicated already, it gets worse...
Now a 3rd plot is required: flow vs viscosity.
Each of the flows, viscosity points determined before (Operating points and cavitation points) are plotted obtaining two curves:
1. op flows at different viscosities
2. limit flows for cavitation (flow above this point will cause cavitation).
Where these last 2 curves intersect is the MAX VISCOSITY for operation without cavitation, in other words: this point defines the MINIMUM TEMPERATURE of the fluid for operation of the PDP without cavitation.
For centrifugal pumps the story is different, there are formulas and curves that correct for different viscosity: as a general rule the performance of the pump is deteriorated by higher viscosity fluid.
HTH
saludos.
a.