Plants often must handle slurries in applications ranging from processing to wastewater treatment. Dealing with such mixtures of liquid and solids is challenging and difficult. Some key elements in slurry pumping are the size and nature of the solids in the liquid and the kind of the abrasive wear they cause. Another is the corrosiveness of the liquid or the mixture.
Sites frequently rely on centrifugal pumps for slurry services. These pumps (and their associated piping systems) need special provisions that call for a detailed knowledge of the solid and slurry properties to prevent wear, corrosion, erosion and other adverse effects such as settling of the solids. Specifying the optimum combination of speed, geometry and materials requires properly balancing often conflicting pump priorities; this demands consideration of stable operation, maximum wear life, operational flexibility and minimal energy consumption.
In this article, we’ll cover practical guidelines and rules for centrifugal pumps for slurries. We’ll also discuss key operational features, material selection and other considerations.
Horizontal centrifugal pumps usually are used for slurry services, although vertical and other types of pumps are favored for some specific applications. Centrifugal pumps for handling slurries have features tailored to the particular service that reflect the corrosive or abrasive nature of the slurry and the solids concentration. These may include choice of materials, use of liners and even different driver sizing.
The first major requirement of a slurry pump is to provide adequate service life. Erosion and corrosion effects of slurries, such as the impingement of high velocity flow of liquid/solid mixtures, are really challenging. In many applications, some solids in the mixture are larger than usually specified particles; so, the pump should be able to pass them without any damage or operational problems.
As a result of such requirements, a slurry pump often is larger than its clear liquid counterpart. Moreover, it generally sacrifices efficiency, both maximum efficiency and efficiencies over the whole operating range, in exchange for the ability to achieve good operation in these challenging services.
Because wear is a function of velocity, a slurry pump’s speed should be as low as possible; units usually operate at 1,200 rpm or slower. Often, direct coupling between the pump and a low-speed electric motor or other driver makes most sense. On the other hand, many other applications favor gearboxes to meet the desired speed and duty point. In services requiring variable flow, variable frequency drives are used to provide the necessary continual speed changes.
Although the emphasis on a slurry pump tends to be on the size and percentage of solids to be pumped, corrosion resistance is also an important factor for material selection in many applications. In such cases, the material chosen must provide an adequate combination of both erosion and corrosion resistance.
For slurry services, a pump operating on the left of the pump performance curve or at the best efficiency point (BEP) is usually preferred; as an indication, the rated point should lie somewhere between 85% and 100% of the BEP point.
The performance curves of many centrifugal pumps are based on handling water. So, to obtain the performance characteristics for these slurry centrifugal pumps, you must modify the results for the presence of solids. Many correlations and correction methods exist for predicting the performance of centrifugal pumps when handling slurries; these account for factors such as individual effects of particle size, particle size distribution, specific gravity and concentration of solids. They usually provide a head reduction factor and efficiency reduction factor for slurries compared to clear water. However, every pump has unique service-specific factors for a given application. You should verify these by experiments.
Wear, Erosion And Corrosion
Major factors that influence wear include the following:
• details of erosive particles (material, size, shape, etc.);
• concentration of solids;
• fluid velocity and particle velocity; and
• hydrodynamic properties of the flow (Reynolds number, etc.).
Turbulent flow analysis usually isn’t applicable for slurry pumps because the presence of solid materials will directly influence the turbulence parameters. The mechanisms of turbulence become a complex problem particularly for dense slurries. This, combined with the nature of the flow inside a slurry pump, which is characterized by unsteadiness as well as deformed velocity distribution patterns, cause a very chaotic situation. However, some simple rules have been verified both in theory and experiments. For instance, the erosive wear rate is proportional to the flow velocity. It also depends on the solids concentration; as that concentration increases, so, too, does the wear rate. In addition, the sizing and specific gravity of solids in the slurry affect wear.