Ketchup can be very aggravating. At first it will not come out of the bottle at all, despite all efforts, then it suddenly comes out in rush and floods your hamburger. The reason is that ketchup becomes more viscous (think thicker) when sitting. When we shake and strike the bottle, this shears (think stirs) the ketchup and it becomes less viscous and then will suddenly pour. Many Sludges/Slurries are like ketchup. If they sit for a while they become more viscous and hard to get moving, but when sheared, they thin out and will move easily. Fluids that change viscosity when sitting are referred to as Non-Newtonian and do not behave according to normal water hydraulic rules. They require extra force to overcome the initial resistance to moving. This extra resistance is referred to as Shear Stress. Most Sludges/Slurries with 3% + solids are Non-Newtonian. In some cases, 10+ times more horsepower may be required to pump these materials. The science of Sludge/Slurry Engineering is to optimize the entire pumping system to require the least amount of extra horsepower that is practical in a particular application.
Special Problems of Non-Newtonian Sludge/Slurry Pumping
- Non-Newtonians thicken at rest, so getting them to the suction inlet of the pump requires special attention.
- A sludge/slurry that has been sitting in a discharge pipe requires extra force. When pumping force is applied Non-Newtonians initially compress instead of moving. This is different than water which will not compress.
- Pumping Non-Newtonian at slow velocities in a pipe may require more horsepower than running faster. This is counter intuitive and results in specifying a discharge pipe that is often too large.
Happily, there is well established science for designing a good sludge /slurry pumping system. The extra horsepower required to overcome Shear Stress can be predicted by a Static Shear Stress Test. We highly recommend this relatively inexpensive test ($500). An alternative is predicting extra horsepower required using the EPA method.
We stand ready to use our experience and knowledge to help make your Sludge/Slurry pumping system a success.