In a slurry pump the impeller and inside of the casing are always exposed to the slurry and have to be protected accordingly against wear.
“Material selection for impeller and casing is just as important as the pump selection itself!”
There are three different conditions that create wear in a slurry pump:
l Abrasion
l Erosion
l Corrosion
Abrasion:
There are three major types of abrasion:
In slurry pumps we have mainly grinding and low stress abrasion.
Abrasion rate is dependent on particle size and hardness.
Abrasion only occurs in two areas in a slurry pump:
1.Between impeller and the stationary inlet.
2. Between shaft sleeve and the stationary packing.
Erosion:
This is the dominant wear in slurry pumps. The reason is that particles in the slurry hit the material surface at different angles.
Erosion wear is heavily inflfenced by how the pump is operated. Erosion wear is, in general, at a minimum at the BEP flow rate, and increases with lower as well as higher flows.
For reasons that are not well understood, erosion wear can also increase dramatically if the pump is allowed to operate on “snore”; that is, taking air into the inlet pipe.
It has been suggested that this may be caused by cavitation, due to the pump surfaces vibrating as the air flow over them. This is, however, difficult to accept as air bubbles generally suppress cavitation by moving to full the vapour cavities.
There are three major types of erosion:
Effect of erosion on pump components:
1. Impeller
The impeller is subject to impact wear (high and low) mainly in the eye, on the gland side shroud (A), when the flow turns 90°. On the leading edge of the vane (B).
Sliding bed and low angular impact occur along the vanes between the impeller shrouds (C).
2. Side liners (inlet and back liners)
Side liners are subject to sliding bed and crushing and grinding abrasion.
3. Volute
The volute is subject to impact wear on the cut water. Sliding bed and low angular impact wear occurs in the rest of the volute.
Corrosion:
The corrosion (and chemical attacks) of the wet parts in a Slurry Pump is a complex phenomenon both for metal and elastomer material.
For guidance, chemical resistance tables for metals and elastomer material are given on following and in section Chemical Resistance Tables.
Material | Physical properties | Chemical properties | Thermal properties | |||
Max. Impeller Tip Speed(m/s) | Wear resistance | Hot water, diluted acids | Strong and | Oils, hydro | Highest service temp.(oC) | |
Natural rubbers | 27 | Very good | Excellent | Fair | Bad | (-50) to 65 100 |
Chloroprene 452 | 27 | Good | Excellent | Fair | Good | 90 120 |
EPDM 016 | 30 | Good | Excellent | Good | Bad | 100 130 |
Butyl | 30 | Fair | Excellent | Good | Bad | 100 130 |
Polyurethane | 30 | Very good | Fair | Bad | Good | (-15) 45-50 65 |
Wear protection - what options?
There are some major options in selecting wear protection of slurry pumps:
Impeller and casing in Hard Metal in various alloys of white iron and steel.
Impeller in elastomers and casing protected by elastomer liners. Elastomers are normally rubber in various qualities or polyurethane.
Combination of impeller of hard metal and elastomer-lined casings.
Selection of wear materials:
The choice of wear parts is a balance between resistance to wear and cost of wear parts.
There are two strategies for resisting wear:
The wear material has to be hard to resist cutting action of impinging solids!
Or
The wear material has to be elastic to be able to absorb the shocks and rebound of particles!
Parameters for selection
The selection of wear parts is normally based on the following parameters:
l Solid size (solid S.G., shape and hardness)
l Slurry temperature
l pH and chemicals
l impeller speed
The dominant wear materials in slurry pumps are hard metal and soft elastomers. DEPUMP supplies a wide range of qualities for both.
Ceramics are available as an option for some ranges.
