Pump School

The performance of centrifugal pumps is affected when dealing with viscous liquids. Increase of power, reduction of height and reduction of flow occur with moderate and high viscosities. Figures 1 and 2 provide means of determining the performance of a conventional centrifugal pump pumping a viscous liquid when its performance with water is known. Figures 1 and 2 can also be used as a help in the selection of a pump for a certain application. The values shown in figure 2 are calculated from the average of tests of single-stage conventional pumps pumping oil. The values of figure 1 were prepared from other tests on several smaller pumps. Therefore the correction curves are not exact for a particular pump. When precise information is essential, performance tests should be conducted with the viscous liquid to be pumped.

LIMITATIONS OF THE CURVES FOR CORRECTION OF PERFORMANCE WITH VISCOUS LIQUID

As these curves are based on empirical considerations, extrapolations beyond the limits shown are not recommended. Use only for conventional design hydraulic pumps, in the normal operating range, with open or closed rotors. Do not use for mixed or axial flow pumps or for special design hydraulic pumps for viscous or non-uniform liquids. Use only when suitable NPSH is available, in order to avoid cavitation. Use only with Newtonian (uniform) liquids.
Non-uniform liquids can produce varied results, depending on the particular characteristics of the liquids.

SYMBOLS AND DEFINITIONS

The following equations are used to determine the viscous performance when the performance with water is known.

CQ, CH and Ch are determined from figures 1 and 2, which are based on the performance with water. Figure 1 is to be used for pumps with flows less than 25 m³/h at the point of best efficiency (of performance with water).

1 – Performance correction curves for viscous liquids (up to 25 m³/h). Ref.: ANSI/HI 1.3

Figure 2 – Performance correction curves for viscous liquids. Ref.: ANSI/HI 1.3

INSTRUCTIONS FOR PRELIMINARY SELECTION OF A PUMP FOR A GIVEN HEIGHT, FLOW AND VISCOSITY

Given a desired flow and height of a viscous liquid to be pumped, and the viscosity and density at the pumping temperature, figures 1 and 2 can be used to find equivalent flow and height when pumping water. Enter with the desired flow (QVIS) at the bottom of the appropriate figure, and proceed, going up to the desired viscous height (HVIS). For multi-stage pumps, use height per stage. Proceed horizontally (to the left or right) until the viscosity of the fluid, and then go up to the correction curves. Divide the viscous flow (QVIS) by the flow correction factor (CQ) to get the flow in equivalent water (QW approximately). Divide the viscous height (HVIS) by the height correction factor (CH) of the curve identified as "1.0 x QNW" to get the height in equivalent water (HW approximately). Using this new water equivalent height x flow point, select a pump in the usual manner. The viscous efficiency and viscous power can be calculated.

Reference: ANSI/HI 1.3

INSTRUCTIONS FOR DETERMINING THE PERFORMANCE OF THE PUMP WITH A VISCOUS LIQUID WHEN THE PERFORMANCE WITH WATER IS KNOWN

Given the complete characteristics of a pump pumping water, its performance can be determined when pumping a liquid with a specified viscosity.
On the pump efficiency curve, locate the water flow (1.0 x QNW) at which the maximum efficiency is obtained.

From this flow, determine the flows (0.6 x QNW), (0.8 x QNW), (1.2 x QNW).
Enter at the bottom of the appropriate figure with the flow at the best efficiency (1.0 x QNW), go up to the height developed (in a single stage) (HW) at this flow, then follow horizontally (left or right) until the desired viscosity, and then go up to the several correction curves.
Read the values of (Ch), (CQ) and (CH) for the four flows.
Multiply each height by its corresponding correction factor to obtain the corrected heights. Multiply each yield value by (Ch) to obtain the yield values applicable to the corresponding corrected flows.