Page 255 - Centrifugal Pumps Design and Application

228 Centrifugal Pumps: Design and Application

large degree of very hard chromium carbide (1800 Knoop hardness) pre-
cipitation that can be achieved in high chrome iron. Wear increases:

• When the particles are angular.
» With particle density.
• With increasing particle velocity such that the rate of wear is directly
m
proportional to V where m can vary from 2,5 to 4.

Parts life can be significantly extended if the system head requirements
are reduced and a lower-rotational-speed pump is selected.

Where pumps are applied to a slurry that is both corrosive and abrasive
the predominant factor causing wear should be identified and the materi-
als of construction selected accordingly.

To make the correct pump selection the following factors must be spe-
cified:

• Particle size distribution—From which can be determined the average
particle size.
• Particle shape—State whatever particles are angular or smooth.
• Solids concentration—For convenience a nomograph relationship of
concentration to specific gravity of aqueous slurries is given in Figure
13-2,
« Article hardness—Given in terms of Mohs or Knoop scale.
• Particle specific gravity—The mixture specific gravity can be deter-
mined from Figure 13-2 if the concentration is known. Note the pump
BMP is directly proportional to the mixture specific gravity.
» Conveying liquid—State viscosity, temperature, and corrosiveness.
« System requirements—Total head and capacity. It may be necessary to
correct the pump performance for the effects of the solids in the liquid,

Pump Materials to Resist Abrasive Wear

Tough materials are used to resist gouging abrasion (caused by the im-
pingment of large dense particles). Toughness is the amount of plastic
deformation a material can withstand without fracture. Generally the
larger the difference is between the yield and tensile strength, the tougher
the material will be.