First of all, slurry pumps should be designed for specific operating requirements:
- They must be able to pump high-density abrasive slurries while maintaining reasonable wear life.
- They must also be able to pass large solids.
Compared with conventional water pumps, the above requirements result in a larger pump that is less efficient.
Components in contact with abrasive slurry are expected to wear. This wear can be minimized through design and material selection and proper pump application. Additionally, pumps can be unlined or fully lined with abrasive resistant materials. In either case, thick casings are installed.
Large clearances are provided within the impeller casing to allow large solids to pass, thus reducing internal velocities and wear. Slurry pump impellers are usually larger than those of water pumps. This is meant to lower impeller speed while achieving required head. Closed impellers are used for high wear applications.
Moreover, to minimize wear and allow large particles to pass, slurry pumps have thicker and fewer pumping vanes. While water pump impellers have five to nine vanes, slurry pumps feature fewer, four to five being the most common. Heavy-duty slurry pumps have short, thick vanes, which produces a pump curve that is flatter than that of equivalent-duty water pumps and can be as much as 10% less efficient than water pumps.
Taking derating into account
Contrary to water pumps, the operating point of slurry pumps depends not only on the pump curve and the pumping system, but also on slurry being pumped, as pump head and efficiency can deviate from expected water performance, i.e. deratings. System designers should be able to predict deratings to select slurry pumps with adequate characteristics and motor horse power for the given duty.
The main causes for centrifugal pump deration when handling solids are:
- Slip between the water and solid particles during slurry acceleration and deceleration, which leads to energy loss
- Increase friction losses (the higher the concentration, the higher the losses)
- Inability of suspended particles to store or transmit pressure energy
- Mechanical friction changes in the gap between the impeller and side walls, which affects energy consumption (efficiency)
Derating is described by two terms: head ratio and efficiency ratio (HR and ER), where:
- HR = head slurry/head water
- ER = efficiency slurry/efficiency water
The above deratings depend on the following factors:
- Specific gravity of solids
- Volumetric concentration
- Ratio of particle size d50 to impeller diameter (d50/D)
Using speed reducers
Most slurry pumps operate at low speeds and require speed reducers. For low- and medium-capacity pumps (e.g., below 350 HP), belt drives are used as speed reducers. Larger pumps are fitted with gearboxes as reducers. In the case of variable flow applications, variable speed devices (most often VFDs) are used to modulate pump speed, as required.
Slurry pump shafts and bearings are also larger than the equivalent water pump components. This is to comply with a higher specific gravity of pumped liquid and associated added forces; inherent imbalance due to wear; and shock loading from large particles.
Materials commonly used for slurry pump wear components are hard metals and elastomers. Hard metals combat wear, while elastomers can absorb the energy of impacting particles.
For grinding circuit applications, where pumping of large sharp-edge particles can be very erosive, mostly hard metal impellers are used. Both metal and elastomer liners are used in other in-plant and tailings disposal applications.
Froth pumping specifics
Special pump design nowadays is gaining popularity for froth applications in flotation circuits. This design is fitted with oversized inlet, open-type impellers and flow inducers to facilitate frothy slurry feed into the pump; has lower NPSH; and prevents air-locking to successfully pump mixtures of fine particles and fine dispersion of air bubbles.
BBA experts have learned from experience that understanding the above basic principles of slurry pump design and application described in this blog article facilitates the design of slurry pumping systems.
- Mineral Processing Plant Design, Practice and Control. SME edition, 2002
- WEIR, Technical Bulletin No. 28. “Froth Pumping”.