Page 359 - Centrifugal Pumps 2E

324 Centrifugal Pumps: Design and Application

Pump Corrosion

The types of corrosion encountered in a chemical environment fall into
eight typical categories.
• General uniform corrosion at a uniform rate over entire surface, either
very slow or very rapid.
« Crevice corrosion that is a localized form from small stagnant solutions
in areas such as threads, gasket surfaces, or drain holes. Crevice corro-
sion is caused by a differential in concentration of metal ions and oxy
gen added to the main body. This causes an electrical current to flow,
causing the damage.
• Pitting is localized. It is manifested as small or large holes usually pro-
duced by chlorines.
• Stress—corrosion occurs at cyclic stress on shafts.
• Intergranular corrosion—usually occurs in the presence of heat.
• Galvanic action.
• Erosion-corrosion—corrosion plus mechanical wear such as cavita-
tion.
• Selective—mat is, leaching corrosion or degraphitization usually not
found in chemical pumps.

Pump Materials
The typical material of construction to combat corrosion is either a 304
or 316 stainless steel that is superior to austenitic or ferritic steels. Other
materials would be composite plastic such as PTFE and FEE Usually fi-
ber reinforced plastic is used for strength and chemical resistance. This
includes vinyl esters, epoxies, polypropylenes, and phenolics. Ceramic
or glass is avoided because of low mechanical properties.
Some of the process liquids found in chemical plants are listed along
with pump materials used in the various environments.

Chlorine

• 65% of the chlorines are used for organic chemicals such as vinyl chlo-
ride, pesticides, fluorcarbons.
• 15% of the chlorines are used for producing pump and paper.
• 10% is for inorganic chemicals.
• The remaining 5% is for sanitation, potable water, and waste water that
are used in municipal water works and sewage plants.
• 50% of caustic soda is used for the chemical industry.