Increased temperature, pressure and purity of centralstation feedwater causes corrosion-erosion in feed pumps. Studies by operators, pump manufacturers and technical groups have led to certain conclusions: 1. Reducing corrosion by modifying the water is rarely feasible. 2. Materials must be used which resist corrosion-erosion. 3. Cast iron and carbon steel are generally unsuitable. 4. Five percent chromium steels prevent corrosion-erosion. Higher alloy concentrations do not provide a marked improvement. Relative service life is about 1 for iron and steel, 10 to 90 for 1.% chromium steel, 100 for 5.% chromium steel, 100 to 105 for 13.% chromium steel and 100 to 150 for 18/8. General use of chromium steels presents manufacturing problems. Clearances must be adjusted and different materials and/or hardnesses are important to prevent seizure. Preheating and stress-relief annealing are important after welding. Stainless or “K” Monel shafts are recommended.
Heater drip pumps need all-bronze or 13.% chromium steel construction if dissolved gases are present. If there is no contamination, or if the temperature is low as in the case of condensate pumps, iron or steel casings with bronze impellers are used. Circulating pump materials vary with the water quality.
Cast iron casings and bronze impellers are typical for fresh water. For sea and harbor waters all-bronze construction is desirable. With polluted harbor waters, stainless steel impellers in bronze casings are sometimes necessary. Commercial considerations sometimes dictate the use of cast iron casings for very large circulating pumps. For the corrosive applications, Monel or stainless shafts are recommended to prevent failures by corrosion fatigue.
