A frequently stated expression around the pump industry is that viscosity is the Kryptonite of centrifugal pumps. Pardon the allusion to Superman, but it is a reference most of us not only relate to, but also understand. Further, viscous fluids do have a negative and weakening effect on centrifugal pump performance.
Viscosity is a measure of a fluid’s resistance to flow at a given temperature. You can also think of it as fluid friction. A more technical definition would explain viscosity as a force required to move a liquid plane (think plate) of some unit area, over some distance above another plane of equal area in a defined time period. In training classes, I simply define viscosity as a fluid’s resistance to pour but, more importantly, a resistance to be pumped.
Isaac Newton was probably the first person we know to quantitatively define a coefficient of viscosity. His concept and related work was not completed, but was later refined by Jean Leonard Marie Poiseuille (see Poiseuille’s Law).
Why are we concerned about viscosity in regards to centrifugal pumps?
Mainly because viscosity has such an extraordinary and often negative effect on centrifugal pump performance. An increase in viscosity will dramatically reduce a pump’s efficiency in conjunction with marked reductions in head and flow. The net result is an increase in the brake horsepower required for the driver.
All centrifugal pump performance curves are based on pumping water, unless stated otherwise. When I started in the pump business, there were no computer programs to cipher the necessary viscosity corrections and the manual methods could take hours to complete. With the advent of computerized programs for pump selection, it is now simple to correct the pump’s performance for viscosity in one keystroke, but we often overlook the details and effects of what viscosity changes do to the pump performance and especially the required brake horsepower.
Prior to computer programs, there were basically three methods to correct a centrifugal pump’s performance from water to viscous.
- The A.J. Stepanoff model was viable at the best efficiency point (BEP) for head and flow but reliability and validity diminished with increased departure from the BEP.
- The Paciga method was slightly better than the Stepanoff model because it could be more accurate across a wider range of flows. Paciga had incorporated specific speed and a flow ratio (actual flow as compared to BEP). The down side was that as viscosity increased the reliability diminished. This was mostly due to the effect of the Reynolds number in the formula calculations.
- Hydraulic Institute original method using viscous correction charts to obtain viscous correction factors (for head, flow and efficiency). The method was an improvement over previous ones because of the ease, accuracy and wide range of applicability. For people who have been in the business for some time, it would be prudent to review the newer methods presented by the Hydraulic Institute (Refer to ANSI/HI guideline 9.6.7-2010). The new method uses a formula called parameter B to yield viscous correction factors. The newer method also eliminates some of the confusion and inaccuracy in the 100 gallons per minute (gpm) range.
Corrections to Pump Curves
In a perfect world, a centrifugal pump performance “curve” would actually be a straight line, but in the real world, it is curved due to losses in the pump. The major factors are a combination of mechanical, leakage, shock and disc friction losses. Disk friction is the major contributor and most important factor when quantifying the losses. The curves as mentioned are based on water performance, but with applications on viscous fluids those water curves must be corrected for the viscosity to be accurate. The head, flow, efficiency and brake horsepower (BHP) curves will all require modification (viscous corrections).
At what minimum value of viscosity to start corrections?
The pump manufacturer is the best source for this value, as it will depend on the application, fluid personality and the pump geometry. Note that at 100 centipoise, the viscous effects will be significant. I will state that at 30 to 40 centipoise or greater, you should use the corrections or risk adverse effects. I also recommend that somewhere in the area of 5 to 10 centipoise, you must at least be aware and conscious of the effects however minor.
Since checking the correction curves is so easy these days it would be unwise not to check.
Impeller Shape & Size Effects
The lower the specific speed (Ns) of an impeller, the higher the disc friction will be. This is simply due to the geometry of the impeller and the 90 degree flow angle that the fluid enters and then exits the impeller. As an impeller’s specific speed increases, the entrance-to-exit angle becomes lower and the interaction with the fluid is less.
The smaller an impeller is, the more likely the disc friction effects will be higher simply because the surface area of the impeller and casings have more interaction with the fluid than in a larger pump.