Using Centrifugal Pump Technology in Complex Fluids


Written by:
Dr. Richard Holm, ITT Flygt, Sweden

This study establishes the relevant characteristics of sludge and how they affect the performance of pumps with a swept-back leading edge impeller and relief groove in the volute and head losses in straight pipes.

It is generally perceived that pump technology evolves gradually in the continuous improvements of products. The improvements are driven by Life Cycle Cost (LCC), in particular by the energy cost awareness.

Still, experience shows that innovations in product development could have a game-changing impact. In pumping complex fluids, the influence of the fluid characteristics on the pump performance must be considered. This involves both pump design knowledge and system know-how, and it is challenging.

For example, several years ago a pump with a swept-back leading edge impeller and relief groove in the volute was developed to improve the clogging resistance in sewage applications in combination with an efficiency level similar to state-of-the art clean water centrifugal pumps. An extensive development program resulted in a twin-blade semi-open impeller.

A centrifugal pump with this type of impeller reaches 80 percent pump efficiency in sewage applications. The wet end volute is traditionally designed, but with a relief groove in the insert ring. The stationary relief groove and the design of the impeller leading-edge is the reason for the excellent clogging resistance in sewage applications.

In this application, the clogging effect refers to particles in the wastewater, such as rags or flocs, causing unscheduled blockage in the pumps. The wear resistance is a core issue in pump technology. For this pump, accelerated laboratory wear tests as well as wear field tests showed promising results. The relative decrease in efficiency compared to conventional single-vane pumps was reduced by approximately 50 percent [1].

Several case studies conclude that a pump with a swept-back leading edge impeller and relief groove in the volute significantly reduces LCC in sewage applications, where displacement pumps (PC-pumps) traditionally have been used.


Rheology


Rheology is the science of fluid deformation and flow of materials. Rheological studies are when fluids are subjected to an applied force per unit area, stress, during a certain time. A fluid deformation is either due to an extensional force or a shear force that lead to a fluid stress. The resulting fluid response is of elastic or of viscous behavior. The behavior is given by a non-dimensional time ratio. In rheological studies, the behavior in the range between the ideal elastic and the viscous fluids behavior is of interest. A way to present rheological data is by a flow curve. The flow curve shows the stress ( ) versus the deformation rate ( ).

In Figure 1, this is shown for shear induced stress versus a shear deformation rate (shear rate). The flow curve slope represents the shear viscosity, thus shear stress to shear rate. In the case of linear behavior, the slope is constant and the viscosity is simply . These fluids are known as Newtonian fluids.

In the case of non-linear behavior, or non-Newtonian fluids, the slope changes and an apparent viscosity are used.In the case of non-linear behavior, or non-Newtonian fluids, the slope changes and an apparent viscosity are used [2]. The apparent viscosity is the specific slope for every shear rate value with its origin in origo. In order to describe non-Newtonian fluids, the behavior has to be described with several parameters. One frequently used model is the power law model, Equation (1):                

(1) where K is the coefficient of rigidity, n is the power exponent, is the shear stress, and  is the shear rate. In the case of n = 1, the Newtonian case, K represents the constant shear viscosity.

In Figure 1, a non-Newtonian fluid may further show an increasing apparent viscosity, so-called shear thickening behavior. The equivalent for a decreasing apparent viscosity is known as shear thinning. Finally, the yield stress is some very specific behavior for some non-Newtonian fluids. This yield stress point has to be exceeded to result in a flow motion.

Wastewater treatment plants (WWTP) involve a wide range of fluid transports of suspensions with different flow characteristics. The aqueous suspension changes due to the concentrations of particles and dissolved substances. However, the suspension of interest is the one with non-linear behavior, i.e. the sewage sludge. Additionally, the non-Newtonian characteristics of sewage sludge vary depending on its origin and process history.

Processes where polymer additives or mechanical steps like sludge drainage are used change the sludge characteristics. The way in which the origin is a municipal or an industrial sludge influences the sludge character. However, in a recent study [3], it is shown that municipal sewage sludge shows essential shear-thinning behavior.

The purpose of this project is to study the performance of a pump with a swept-back leading edge impeller and relief groove in the volute as influenced by handling the non-Newtonian fluids in WWTP applications. Also, the project involves rheological characterizations of sludge which are not further reported in this paper. The pump performance is studied in both laboratory tests and in field tests of on-site WWTP installations.    

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