Viscous liquids are among the most challenging applications for pumping. But pumping grease to lubricate wheel bearings is handled simply enough with a small piston pump fed by a heavy follower plate. So, there must be more to this that should be analyzed.
While it resists responding to pounding on the bottle, being gently poked with a butter knife starts the flow rather quickly.
This is a characteristic called thixotropic or sheer thinning. There, the viscosity declines once the ketchup starts moving. Sheer thickening (or dilatant) fluids such as titanium dioxide (TIO₂) coatings have the reverse quality. Even measuring the viscosity of these non-Newtonian fluids is problematic if not practically impossible.
Conventional pumps are common in that they essentially work against the effects of viscosity.
With a conventional centrifugal pump, the static volume in the bottom of impeller vane cavities grows with increasing viscosity. The “dead volume” then reduces pumping volume, thereby decreasing efficiency and performance.
Rotary positive displacement pumps typically require internal machining tolerances that are related to design viscosity. This can pose a significant challenge for design with a process of variable viscosity. Using the greater tolerances for the maximum design viscosity translates to excessive internal slip and reduced performance for thinner fluids.
Reciprocating pumps decline in performance with wear and also have the disadvantage of pulsating flow and related increase of line losses that are typically calculated at five times that of the equivalent non-pulsating flow.
Is there a pump design that works with viscosity? The disc pump technology is not new, having been invented and patented by Nikola Tesla in 1913. Tesla assumed tight spacing of the discs was necessary, and this concept was attempted in a turbine with thin tightly spaced discs that failed to hold position. A working pump was apparently never built by Tesla, and the concept sat for most of the next seven decades. It was first brought to market in the 1980s.
To illustrate the function of the disc pump, consider a potter’s wheel. If water is poured onto the middle of the rotating wheel, the water moves toward the edge and finally off the wheel as the speed is increased. The thickness of the film of water is a function of viscosity. If honey is used, the film moving toward the edge is thicker, because of the higher viscosity. In a disc pump, two discs are operated in parallel, secured to a shaft and enclosed within a pump casing. This is among the lesser known designs and typically overlooked in discussions of pumps.
The viscous drag is simply that a layer of process fluid molecules is attached to the discs. This layer rotates in synchronous speed with the discs (dark red in Image 1). The adjacent fluid is dragged along at a somewhat lesser speed with fluid directly between rotating at the lowest speed. Operating on the viscous-drag principle, the disc pump generates a laminar flow that works with the process fluids and its resulting boundary layer.
The efficiency of the disc pump actually increases with viscosity. This is illustrated in Image 2. The practical limit for the conventional centrifugal pump is 250-300 centipoise (cP). The disc pump maintains a near constant efficiency from that 300 cP limit to 10,000 cP viscosity and even higher. The disc pump is also capable of passing large, rock-hard, abrasive solids. Consider dewatering applications such as in filter press feed. The pump is required to begin the cycle at a high flow and relatively low head. As the press fills, flow falls and pressure increases. The end cycle is at high pressure and near shut-off head.
The ability to handle the extreme range in viscosity (< 1 to 10,000 cP) with a standard design pump is most advantageous, particularly in a process that starts with water-like viscosity and increases to 1,500 cP or higher. While relatively low with water (compared to a centrifugal pump designed for water), efficiency is maintained with increasing viscosity to 10,000 cP and higher (see Image 2). The large internal tolerances are advantageous in pumping materials that tend to deposit out on internal surfaces such as lime slurry.
The viscous drag and laminar flow mechanism is also inherently a low sheer pump, the lowest of any pump design tested.
The gentle fluid handling translates to addressing other problematic applications: polymers, latex, fabric softeners, fish eggs (salmon hatchery) and even live fish. Oil and water separators also benefit from the disc pump as oil droplet size is maintained to enhance separator efficiency.
As with any tool, a pump should be selected to operate as close as possible to an ideal condition for what it has been designed. Approaching the ideal will determine the expected outcome.