NPSH test with Viscous Liquid

[Vic123]

I am looking to test a single stage centrifugal pump for NPSH using a viscous oil instead of water.
Would the results be the same as that of water and is there anything I need to be concerned about?

 

[monaco8774]

For my sins a long time ago I used to actualy test rotary lobe pumps for NPSH in a simple test loop for different viscosities, the NPSH does change on the pump substantially, we used to use Versicole for the testing diluted with water, this would give us viscosities up to 5000 cP. Water is around 1 cP and oil if unmixed with water is typically around 5 cP. For this range there is no significant effect on NPSH. However if you get an emulsion formed between oil and water at what they call the inversion point you can easily get viscosities up to 5000 cP which would seriously effect the NPSH.

Oil is usually newtonian like water so technically unless you expect emulsions forming I dont see any problems aside the usual fact that you are now using a flamable fluid rather than a benign water.

 

[Jalink]

Vic123,
Just to give you an idea of the differences:
Nett positive suction pressure required = Atmospheric pressure - vapor pressure - liquid friction pressure loss in pump - liquid accellerations in pump.

Density of liquid
Nett positive suction pressure required = gravity constant x density x NPSH. Here you see the large role of the density of the liquid. This is why NPSH is normally determined for water service, except when the liquid density is above that of water.

Vapor pressure
The vapor pressure of hydrocarbons is normally quite different from that of water and depends on the hydrocarbons used. Hydrocarbons will normally not have a specific (one component) vapor pressure, so correction could be difficult.

Viscosity
When the viscosity of the hydrocarbons is quite high, internal friction in the pump will lead to locations inside the pump where the vapor pressure of the liquid will be reached earlier, than will be the case with water.

Temperature and shear effects
With water the physical properties stay quite the same with differences in shear and the temperature effects due to heat input (friction and shear) in the pump are quite predictable. Hydrocarbons however can have significant changes in their physical properties, e.g. viscosity and vapor pressure, due to temperature en shear effects.