A South African steel mill producing 3 to 3.5 million metric tons of steel per year had a huge volume of hydraulic oil and grease leakage from its twin strand casters. The leaks resulted in poor plant availability with frequent shutdowns costing nearly US$80,000 per hour.

Whenever biocide was added at the dissolved air flotation overflow, it helped to clean the cooling tower, but there was not enough residual to clean the rest of the system. If biocide was added to the cooling tower sump, the system was cleaner but the cooling tower was left fouled. Microorganisms had also become resistant to the biocides, and the bacterial count remained high.

GE recommended an aggressive system cleanup and microbiological control program. During downtime, the system was shock-dosed with 200 ppm sodium hypochlorite and 40 ppm Spectrus NX1100 biocide every two hours until 0.2 to 0.5 ppm Cl2-free residual chlorine was achieved in the entire recirculation system. Normal shock dosages were then administered twice a week at two locations. Spectrus NX1100/3/4 at 40 ppm and sodium hypochlorite at 100 ppm was fed into the cooling tower sump and DAF overflow, and chlorine gas added to makeup water. A satellite dosing system was installed and sodium hypochlorite was continuously fed at 20 ppm to keep strainers clean.

•  Clean cooling tower, strainers, flexibles and nozzles, including low flow areas
•  Microbiological count maintained at BIOSCAN* monitor reading of 300 relative light unit (RLU) and lower
•  Free chlorine residual maintained in makeup at 0.05 to 2.5 ppm (mg/L) Cl2
•  Minimized downtime for maintenance
•  Savings of US$19.5 million per year
•  Improved caster performance and increased equipment life