12.7  Materials

 

For fresh water applications, grey cast iron is suitable for pump casings and delivery mains. However, for seawater and silt-laden water conditions, severe erosion and corrosion of the cast iron occurs. Although generous allowances can be included in the material thickness, these may not prevent the need for replacement of the cast iron parts during the life of the station.

The use of austenitic ni-resist cast irons for the casing material of cooling water pumps where severe conditions exist is now increasing rapidly. This material, particularly in its spheroidal graphite form, offers proven superior resistance to attack, with negligible wear after several years' operation.

On concrete volute pumps, the built-in metal parts must be suitable for the life of the station. Again, while grey cast iron can be used for inland freshwater sites, austenitic ni-resist cast iron is required for seawater applications.

Circulating water pump impellers are supplied in stainless steel with renewable eye rings to cater for the erosion effects in this close clearance/high velocity area. Both 13/4 chromium-nickel steel and 18/10 chromium-nickel austenitic stainless steel have been used successfully in freshwater and seawater applications.

Pump shafts are normally made of carbon steel and are fully protected by stainless steel sleeves through the waterways. Special attention is required at the sleeve/sleeve and sleeve/impeller joints to prevent ingress of water onto the shaft surface, which can cause corrosion fatigue. On horizontal pump designs, overlapping sleeves and radial O-rings have been introduced to cater for the effects of shaft static and dynamic deflections. Mounting the mechanical seal on top of the shaft sleeve permits seal refurbishment without slackening the sleeve nuts, with resultant loss of axial compression.

 

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