Area: Minning Maintenance Realibility
PUMP TYPES
Two types of pumps are used in the hydraulic transportation of solids: centrifugal pumps and positive displacement pumps.
Centrifugal pumps are used for flow rates from a few litres to thousands of litres per second, they can handle solid particle sizes from microscopic to large rocks up to 300 mm. Their main limitation is the fact that they can develop pressures of not much more than 7 MPa even when they are arranged in series, with say up to 8 pump stages. Their casings can be of unlined or lined design, i.e. with internal replaceable liners, which can be made of many materials from soft elastomers to hard metal alloys to suit the material pumped. The wearing parts are mostly impellers, volutes and side liners.

The majority of centrifugal pumps are horizontal, i.e. they are of end-suction, horizontal-shaft design.
Positive displacement pumps are almost invariably of the piston and diaphragm or piston and cylinder design with inlet and outlet poppet valves. They are employed in pumping through very long pipelines, say from 2 to 50 km because of their ability to generate high pressures, which are well in excess of multi-staged centrifugal pumps. Their design flow rate range is limited from 50 to 1000 litres per second, due mainly to their large physical sizes, both at low and high flows. They are most suitable for transporting slurries with high concentrations of fine particles with a maximum size of about 6 mm. The maximum particle size is dictated by the poppet valves, which can jam in semi-open position by large particles. Poppet valves are high wear items and they need frequent attention and replacement.

Impellers can be of closed design (with two shrouds) or open design (with one shroud). For critical duties, in order to reduce friction and to achieve the best possible pump performance, impeller surfaces are often machined all over or as much as physically possible and polished.

Let us consider the impeller once again. Its vanes must be thicker in slurry pumps than in water pumps to allow for wear. Because of this extra thickness, there must be fewer vanes, otherwise the passageways would be too narrow and would affect pump performance. The passageways must be made wide enough to let the largest planned solid particles to pass through without blockage. As a consequence, fluid in this impeller cannot be guided as closely as in a water impeller and this in turn results in reduced pumping head and efficiency. Unlike water pump impellers, slurry hard metal impellers are seldom machined outside and, like rubber impellers, never internally. This extra work would be a waste of money because impellers and liners have a finite life  even though made of hard alloys or resilient elastomers  and must be replaced when pump performance falls off.
Another factor affecting both head and efficiency in slurry pumps is the actual presence of solids in the slurry. Water flowing through a pipe changes velocity  and velocity head  when pipe diameters change. When velocity head increases (or decreases), it proportionally decreases (or increases) the static head  in accordance with Bernouillis theorem.
When we have solid particles in a flowing slurry, water flows faster than the solids because the solids are moved along only when drag forces, generated by the faster water, overcome gravity forces. The difference between water velocity and solids velocity is called slippage. [This is mentioned also in Section 7, page 1]. The average velocity of the slurry is somewhere between the velocity of water and that of the solids.
The other important part of the slurry pump is its casing, which takes all the pressure loads. There a two main types of casings: those with internal replaceable liners and those without.
An important feature of slurry pumps is the provision for some simple means of axial adjustment of the gap between the impeller and the adjoining throatbush seal face. This can be achieved either by axial movement of the shaft, bearing assembly and impeller in the pump or by axial adjustment of the throatbush. Either method assists in maintaining pump performance as the inner component parts wear.