Pumping frothy mixtures

Froth is an aggregation of bubbles caused in fluids or slurries by agitation or fermentation. Excessive turbulence of flow at the pump entrance, low sump level, or chemical reactions such as flotation processes or gas release from solids are some of the few causes of froth in a slurry mixture. In slurry pumping, frothy mixtures are often encountered, despite the efforts to eliminate air from the mixture.

Froth can have a major effect on the suction of the pump. It causes an increase in the required NPSH (Net Positive Suction Head) while reducing the available NPSH. This leads to the rapid expansion of air bubbles in areas of low pressure within the pump, thus decreasing both head and efficiency. The consequences of this may be a surge in the pump, water hammer or a combination of both, possibly causing severe damage. To avoid this, air must be eliminated from the slurry as much as possible.

Different techniques can be applied to eliminate air in the mixture. For example, spray bars can be used above a flotation tank for foamy slurries, or a vacuum accumulator can be used to capture gases contained in solids. When froth is known to occur and cannot be eliminated, the pumping equipment needs to be sized properly to deal with the mixture. The ratio of volumetric flow rate with air to that with no air is termed the "froth factor." In new installations where various levels of froth are expected, froth factors such as 110%, 150%, 200% or even 400% can be used. For a decrease in froth in slurries, if modification of sump and suction configuration is not possible, then froth factor should be used in selecting a larger and slower pump.

The presence of air in slurries is often found to be the cause of pump malfunctioning. This can be assessed by using the pump’s performance chart and an estimate of the fraction of entertained air. Values of pump input power, flow rate, and head can be obtained using an ammeter, flow meter, and pressure gauge, respectively, and these values can be used to identify where the operating point of the pump lies, and hence determine whether a significant amount of air is present in the slurry.

Contact Pump & Flow if you'd like to discuss design which will entail froth, or if you need to troubleshoot a pump in a frothy service.

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