What is the operational principle of Anti-fouling systems?
23 Jun '17, 08:30
Anti-fouling systems help avoid problems of blockages in water intakes. The main cause of the fouling problems are barnacles and mussels which are drawn into the intake of vessels as larvae and then attach themselves to the surfaces of pipes where they grow and multiply. There is also considerable evidence that marine growth accelerates corrosion. Previously the only remedy has been mechanical removal and the replacement of damaged parts. Preventive measures have been used such as chlorination, chemical dosage or an electrolytic system.
The electrolytic system consists of pairs of copper and aluminium or iron anodes which are mounted in the ship's sea chest or strainer, and a control panel, either a LED panel or an analog digital meter showing the output of each anode.
During the operation a DC current is applied to the copper anode which produces ions which are then carried throughout the pipework system by the seawater flow. Although the concentration of copper in solution are said to be extremely small - less than two parts per billion - they create an environment where marine organisms cannot settle or multiply. This also gives continuous protection to valves, condensers engine cooling systems and ancillary equipment.
A second anode is employed to combat corrosion. All metals have layers of protective oxide films which are prone to breakdown by natural means. Seawater which is carrying corrosive agents such as sulfur can break down these protective films on all metal surfaces. Soft iron anodes are used for protection of alloyed pipework such as yorcalbro ( an aluminum brass).
This involves metering in quantities of an anti-foulant into the sea water boxes. A typical chemical is a Ferrous chloride which coats the pipework with a protective ferrous layer.
Ultrasonics are said to have a twofold effect on anti-fouling: a disturbance action, caused by the high-frequency waves, which renders the habitat unacceptable and a mechanical action, which operates on organisms trying to deposit adhesive, by preventing it from solidifying and on already anchored organisms of 4-5mm. A reduction in fouling of as much as 80% is claimed. A generator produces and then sends electrical impulses at high frequency via a coaxial cable to transducers mounted externally to the sea chests or strainers. Each transducer contains piezoelectric ceramic crystals, which when excited by the electrical impulses generate the ultrasonic beam. Power levels are said to be low, with an input of around 300-600w for each generator, which can supply four transducers. The main advantages of this system is that it is non-invasive, no parts are in contact with sea water so require replacing, and that no toxic substances are produced.
Chlorine is used as an effective pollution control. However, its application raises difficulties in the form that it is used
Chlorine gas is highly toxic and attacks the mucous membranes in the respiratory tract
Anhydrous liquid chlorine has a very high thermal coefficient of expansion and places high hydrostatic loading on the container. Also, it is a vigorous oxidizing agent and can cause instant auto-oxidation of metal surfaces when ignited by a spark. Water in trace amounts can lead to rapid corrosion of the container. The release of a 50-ton tank requires evacuation of a 5-mile radius.
Sodium Hypochlorite is available as a 15% high concentration liquid manufactured by chemical industry It is odorless and requires no special handling. However the economics of use is poor. A sewage plant requires 6 ton of chlorine per day would require 83 tons of sodium hypochlorite (13-15% solution).
Bulk storage is impractical due to the 100 day half life. On-site production removes the costs of transportation.
a water will kill all marine life quickly, 1 PPM will prevent fouling. This may be tested on board.
The total output of chlorine is a function of current rather than flow through the unit, adequate flow is required to ensure cooling and to prevent calcareous deposits.
A typical 1 Kg unit requires a minimum flow of 100 liters per minute. Less than this will mean regular acid cleaning is required, less than 50Litres per minute will lead to overheating and heavy fouling. Cell damage occurs at greater than 9v, high voltage alarm/shutdown occurs at 8 v.
This system is designed to be used in sea water only and not in fresh water.
23 Jun '17, 08:45