Many years ago a generalized procedure was developed by the Hydraulic Institute (HI) for correcting centrifugal pump performance when handling viscous fluids. Their procedure was universally accepted by pump manufactures and industry users. HI has recently issued a revised procedure for predicting pump performance based on new research and field test data . The revised procedure also corrects the shortcomings in the earlier method and has added mathematical formulas for use with computers. This month we will discuss how viscous fluids affect centrifugal pump performance and the changes recently made by the HI.
It is widely accepted that viscous fluids affect the performance of centrifugal pumps. Since the performance of most centrifugal pumps is determined from water, a procedure is needed to correct the performance curves when pumping viscous fluids. Figure 1 illustrates a typical pump performance curve based on water and how it is affected when pumping viscous fluids. In many applications the difference in water and viscous performance are significant.
The previous HI procedure was based on a graphical representation that provided correction factors at four operating points: 60, 80, 100 and 120 percent of the best efficiency point (BEP). Using these correction factors at the four operating points, the user was able to prepare a corrected pump performance chart of flow versus total head (Q–H) and flow versus efficiency (Q-?), when handling viscous fluids. The procedure had limitations but proved to be accurate under a wide range of conditions.
The recently published HI procedure has many of the same features as the older method. However, many improvements were made to improve the accuracy and correct problems in the earlier procedure. Some of the noteworthy changes made in the latest version are presented below:
- HI previously had two separate graphs for determining pump viscous effects, one for pump flow rates less then 100 gpm and another for flow rates over 100 gpm. The two graphs gave conflicting results if the pump operated at 100 gpm. The new procedure has only one procedure regardless of the flowrate.
- HI now uses a new basis for determining the correction factors CH, CQ, and C??. The new method is based on a performance factor, called Parameter B, and includes terms for viscosity, speed, flow rate and total head.
- The latest HI procedure allows the user to determine viscous performance from graphs or equations. With the previous method only graphical methods were used to calculate correction factors. Figures 2 and 3 illustrate the revised graphical method and gives the flow rate correction factor (CQ), head correction factor (CH) and efficiency correction factor (C?), based on the performance Parameter B given above.
- The following equations are used with the correction factors to determine the corrected pump performance for viscous fluids.
- The previous method required the user to find the pump BEP before calculating corrections factors at 60, 80, 100 and 120 percent of BEP. The revised procedure uses a similar approach of finding the pump BEP but corrections factors can be determined, with caution, at any flow rate, not necessarily at 60, 80, 100 and 120 percent of BEP.
- The revised HI procedure provides a method of estimating the Net Positive Suction Head Required (NPSHR) when pumping viscous fluids. This procedure is based on pump performance measured with water and corrected for viscous fluids.
One final comment on HI’s procedure for correcting centrifugal pump performance: The question frequently asked is, “When should a pump performance curve be corrected for fluid viscosity.” The pump manufacturer is the best source to answer this question. They can provide performance curves for any pumping condition including viscous fluids. However, with available computer software, the user should always check how fluid viscosity will affect the pump flow rate, total head and most importantly power.
The following are some factors derived from the latest addition of the HI procedure for correcting pump performance. They may be helpful when considering when to correct pump water-performance curves for viscous fluids.
- The performance factor, Parameter B, is a good indicator of how viscosity influences pump performance. Parameter B takes into consideration fluid viscosity, speed, total head and flow rate at BEP.
- Whenever the performance Parameter B is one or less the fluid has no affect on the head curve, Q-H. Figure 2 shows that the head and flow correction factors are less then three percent when Parameter B equals three and about ten percent when Parameter B equals seven.
- Viscous fluids have a more dramatic effect on pump power. Figure 3 shows that the efficiency decreases by almost 12 percent when Parameter B equals three and over 30 percent when Parameter B equals seven.
- ANSI HI 9.6.7-2004, “Effects of Liquid Viscosity on Rotodynamic (Centrifugal and Vertical) Pump Performance”