NATURAL STEAM PLANTS

Press, Volume XCII, Issue 27733, 10 August 1955, Page 7

NATURAL STEAM PLANTS

LIMIT ON SIZE OF TURBINES EFFECT OF CORROSION CRACKING “The Press’* Special Service WELLINGTON, August 9. Ideas of huge turbines driven by New Zealand’s geothermal steam have, for the time being, been abandoned. Because of an enemy called stresscorrosion cracking, turbines driven by natural steam in any future project will be limited to 25,000 kilowatts generating capacity. Mr T. Marshall, of the Department of Scientific and Industrial Research, told the Wellington branch of the Institute of Chemistry in an address that separated geothermal steam contained small quantities of water droplets. These were very difficult, and almost impossible economically, to eliminate. “Separated geothermal steam is slightly more corrosive than boiler steam and most alloys used in plants would be satisfactory if condensation and air contamination were avoided in the pipelines and generators,” said Mr Marshall. Corrosion effects are due in order of importance, to hydrogen sulphide, water droplets, carbon dioxide and chlorides present in the tapped steam. “Stress-corrosion, is a highly important factor in the building of any power plant. This is caused by the formation of a hard brittle corrosion film on the outside of the metal equipment and should any part of the plant be under stress ♦his will result m cracking or breaking apart. “The effect of this occurring in a heavy turbine would be disastrous,’ said Mr Marshall. “Experiments have <hown that only alloys with tensile strengths below 40 tons a square inch will not undergo this cracking. As a result the turbine size will be limited to 25,000 kilowatts.”