In industrial and off-shore marine seawater cooling water systems, hypochlorite derived from seawater is introduced into the cooling water system to prevent bio-fouling in piping and equipment.
In industrial plants such as power stations, chemical plants and refineries, seawater is usually drawn from a pit or sump through seawater supply pumps located close to a seawater inlet pipe or channel.
The seawater to the hypochlorite generator(s) is piped from the downstream side of the seawater supply pumps to provide the salty water from which the hypochlorite is derived.
Seawater passing through the generator is converted to sodium hypochlorite in a constant flow “once through” process providing a concentration of up to 1500 mg/l of sodium hypochlorite. The hypochlorite is collected in a degas/storage tank before being returned and diffused with the incoming seawater in the seawater supply pump(s) pit or sump. In most applications 0.1 to 0.3 mg/l of free available chlorine above the seawater chlorine demand is sufficient to control marine growth in cooling water systems. The chlorine demand of seawater can range from less than 0.5 mg/l in open ocean situations to 5 or 6 mg/l in coastal and harbor areas. In situations where the chlorine demand fluctuates, a shock treatment may be necessary from time to time. Should shock treatment be required, it should be accompanied with a suitable de-chlorination method located before the seawater return to ensure the local marine species are not affected by the release of over chlorinated seawater. The de-chlorination process can be controlled by measuring the free available chlorine with a chlorine analyzer at the outlet of the seawater return system, and then injecting the correct amount of de-chlorination chemical automatically.