A calculation involving a centrifugal pumping from vacuum  pump vessel.Fluid:
Water @ 180 DegF.Fluid Density: 0.970
Vapor Pressure @ 180 DegF:  7.51 PSIA.Vessel: 28 in Hg Vacuum
Approx Friction Losses:  0.5 ft;Water Operational level above pump centerline: +7.5 ft;Net Positive Suction Head Available = Pressure on Surface of Liquid + Static Suction Head – (Vapor Pressure + Friction Loss)
                Where all the terms are in feet of liquid.
With the Vessel @ 28" Hg Vacuum and the liquid vapor pressure @ 7.51 PSI Absolute, you would not have any liquid in the vessel (only vapor)!!!
Since you will have liquid in the tank, the pressure on the surface of the liquid will be equal to the vapor pressure.
Therefore, the NPSHa = Static Suction Head – Friction Loss.
This is a distillation unit and the water in the vessel will become vapor as it re-enters the unit.

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