-11-May-2006

Selecting the right slurry pump

Making informed decisions when selecting a slurry pump will save countless maintenance hours, substantially reduce downtime and save money in the future, explains Chuck Bonner, Product Manager at ITT Goulds Pumps, Ashland, Pennsylvania, USA.

Choosing the correct slurry pump for a specific task requires the analysis of a number of variables.

Firstly, the slurry characteristics must be determined. Percentage solids by weight, carrying medium, solids size and slurry pH at a minimum should be known as well as slurry temperature, solids specific gravity and viscosity. Once the slurry characteristics are established, the system must be defined.

Flow, total dynamic head and net positive suction head required (NPSHR) are required in order to pick the proper pump. Most of the major pump suppliers now have computer software which, given the basic conditions of service, will suggest a number of pumps that should work on the service. However, to determine which of the selections is most appropriate, and to select the proper materials of construction, will require additional input from the customer. Key considerations in the selection of the right slurry pump include rotational speed, materials of construction, application point and stuffing box sealing.

Rotational speed

Rotational speed is a major factor in slurry pump service life. It is generally accepted that, all other things being equal, the slower running pump on a service will outlast the faster running pump by the speed ratio to the power of 2.5 (some manufacturers use the power of 3.0).

There are numerous arguments as to why this is the case. One of the more logical explanations is that the slower running pumps generally have heavier, larger diameter impellers and other wet end parts. This spreads the energy, which causes wear, over a larger area.

If speed were not an issue, it is unlikely that slurry pump manufactures and customers would spend the money to make and buy pumps with 762-mm (30-inch), 1016-mm (40-inch) and larger diameter impellers. This is also a part of the selection process which requires some thought on the part of the buyer. If a smaller, faster running pump will give years of life on a light slurry, is it worth the extra money for a pump that will last five times longer? In the case of heavy slurries, where the service life of parts can be measured in weeks, the heavier, slower running pump will almost always be selected.

Materials of construction

Materials of construction bring us to the two main types of centrifugal slurry pumps — metal and rubber lined.

Rubber-lined pumps are often chosen for use with slurries that contain solids less than 6.5 mm (0.25 inch) in size. Newer pumps, that are thickly lined with rubber can handle slurries with solids up to 12.5 mm (0.5 inch).

Ceramics are now being used to provide additional abrasion resistance for severe applications. Ceramic inserts can be added to metal wear liners and ceramic beads or rods can be embedded in rubber parts.

Rubber lined

Rubber-lined pumps are effective at pumping fine slurries. For a rubber-lined pump the resilience, or the ability of the rubber to absorb and return the energy generated by the impact of the particles in the slurry, allows it to operate with minimal degradation of the rubber components. Properly selected, a rubber-lined pump will provide excellent wear life at a lower cost than a comparable metal pump.

Susceptible

Rubber-lined pumps are susceptible to damage to the rubber components from sharp particles and particles larger than 6.5 mm (0.25 inch). Sharp particles can cut the rubber. Rubber-lined pumps should not be used on slurries that contain particles which may have sharp edges, for example, anthracite coal. Also, tramp material, if allowed to enter the pump, can cause major damage very quickly.

Large particles can damage the rubber by transferring more impact energy into the rubber than it can return to the slurry. This causes heat to build up in areas of the rubber which can cause it to harden and crack.

The tip speed of the impeller is another limitation that must be considered with selecting a rubber material. Rubber impeller tip speeds are generally limited to 25–28 m/s (5000–5500 ft/minute) because higher speeds can cause the rubber to separate from the metal framework of the impeller.

This limits the maximum head that can be generated by a rubber-lined pump to about 45 m (150 ft). This limit can be exceeded by either operating pumps in series or replacing the rubber impeller with a metal one.

Natural rubber is the primary material used in rubber-lined pumps because it has the best wear characteristics. However, temperature, hydrocarbons and other compounds that attack rubber can limit its use. Other materials also can be used, which is why rubber-lined pumps are sometimes referred to as ‘elastomer-lined’ pumps.

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