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Sea Water Chlorination

Use of chlorine for control of biological fouling in power plant cooling water system is the most effective way of controlling bio-fouling at reasonable concentration as chlorine is available at a low cost. Chlorine Gas feeders   are simple and easy to control at low capital cost and requires minimum maintenance. The chlorine residual for shock dosing can be relatively short and in expensive. De-toxification (dechlorination) systems can be installed at a very nominal cost to meet the environmental requirement.

Total residual chlorine (TRC) as expressed in red book of United States of Environmental Protection Agency (EPA) specifies 0.2 mg/litres TRC. It should not exceed 160 minutes for 24 hours.

Point of chlorine dosing in Sea Water
Normally the sea water for power plant is taken deep in the sea at Sea water intake head. The sea water travels through intake pipe and reaches the CW pump sump from where the water is pumped to the condenser.

Continuous and Shock dosing chlorination
In Sea water application both continuous and shock dosing chlorination are applied for effective bio fouling.

Continuous chlorination
At sea water intake head of inlet pipe, continuous dosing of approx. 1 - 2 ppm will be effective in control of Mollusks, algae, slime and weed. They constrict the flow of Sea water in the intake pipe. It also prevents sea shell deposition in the pipeline which is a most trouble-some to remove.

Continuous chlorine dosing is done near to the pump house ahead of bar racks and traveling water screen. 1-2 ppm is recommended so that the complete bar racks and traveling water screen can be kept clean and free from any bio fouling. This will also ensure the pump sump and pump suction side free from any slime growth. The free residual chlorine and the condenser outlet can be measured to determine the chlorine dosing as the chlorine demand of sea water varies according to the season and also the quality of the sea water.

Shock dosing chlorination - Dosing point No.3
Shock dosing of chlorination is done effectively at 1 - 5 ppm depending upon the quality of sea water. The recommended dosing time is one cycle time for every 8 hours shift which would be sufficient to keep the condenser cooling surface free from bio fouling and maintain the efficiency of the condenser and power generation.

Theory of Sea Water chlorination

In Sea water at pH 7.4 - 8.1 less than 50% of chlorine is available as fast acting Hypochlorous acid. However Sea water also contains 60 mg/litre (ppm) bromide ion which displaces the chlorine, being a stronger oxidizing agent to produce hypobromous acid.

Cl2 + H20 --------- HOCL + H+ + Cl-
HOCL + Br ---------- HOBr + Cl-

When chlorine is dissolved in water, it hydrolyzes rapidly according to the above equation. The rapidity of the reaction can be expressed that the chlorine molecule reacting with hydroxyl ion rather than with the water module.

Cl2 + OH- ------- HOCL + Cl

CHLORINE DOSING POINTS

This reaction is of great practical important relating to the chemistry of aqueous chlorine solution discharging from chlorination equipment. From the chlorinator outlet the chlorine solution is always limited by design to less than 3500 mg./litre of chlorine in water. At this concentration the most highly buffered solution will result pH of not more than 3. At this pH the amount of molecular chlorine in equilibrium with HOCl is substantial. Concentration higher than 3500 mg/l causes excessive chlorine gas release, at the point of application which is not desired.

Considering sea water at pH 8 the reaction by addition of chlorine with sea water will result in the presence of HOCl, OCl, HOBr and OBr and will act to create the hostile environment for living organism. We will have 70% to 90% HOBr as extremely rapid toxicant.

The aim of chlorination is to achieve Hypochlorous acid (HOCL) and hypochlorite ion(OCl which will be effective in preventing fouling of the cooling water circuits. The fundamental objective in water chlorination is to create hostile environment which will discourage marine organisms from establishing themselves and growing on the internal surfaces of PIPE WORK AND CONDENSER SURFACE. The hostile environmental is created and maintained by continuous chlorination.

Fouling control with chlorine dosage
The control of algae and slime in the pipe work and condensers can be achieved by shock/intermittent chlorination. The level of continuous chlorination and level of frequency and duration of intermittent shock chlorination varies from one area to another. It also depends upon the season of the year depending on biological activity and quality of water. The optimum treatment is achieved by operating experience.

Continuous chlorination dosing level is in the region of 0.3-2 ppm. Shock chlorination dosing level is practiced at 2-5 ppm for 1 cycle time for every 4-8 hours. The practice with natural sea water chlorination is to monitor the total 30 minutes demand. The water is dosed with certain quantity of equivalent chlorine. The residual is measured after 30 minutes. The difference between the initial dose level and the residual level is reported as the chlorine demand.

Only shock dosing chlorination is not sufficient for effectively treating the sea water. When shell forming organisms grow in the water, they attach themselves to the piping. Even if the organism is killed by Shock chlorination, the dead remains attached to the piping and subsequently can break free and clog the piping with disastrous results. Continuous chlorination prevents settlement and growth of marine organisms. Various tests conducted indicates residual of 0.1 mg/litre will prevent the growth of marine organisms.

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