Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-11-29
2003-06-17
Bell, Bruce F. (Department: 1746)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S196010, C204S196030, C204S196370
Reexamination Certificate
active
06579429
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antifouling system for preventing marine organisms from attaching themselves to surfaces exposed to seawater of a structure exposed to seawater. More particularly, the present invention relates to an antifouling system including an electrical catalyst coat formed on a surface, exposed to seawater, of a structure, and capable of generating oxygen to prevent marine organisms from attaching themselves to the surface.
2. Description of the Related Art
Mussels, barnacles, hydrozoan and the like (hereinafter, referred to inclusively as “marine organisms”) attach themselves to the inlet and the outlet tube plate supporting heat transfer tubes of a heat exchanger installed in a power plant using seawater as cooling water. These marine organisms clog end parts of the heat transfer tubes to obstruct the insertion of cleaning swabs in the heat transfer tubes and/or cover the inner surface of the heat transfer tubes. Therefore, the power plant is unavoidably subject to frequent shutdown for work to remove the marine organisms from the heat transfer tubes. Those marine organisms are more likely to attach themselves to titanium tube plates and titanium heat transfer tubes, which are corrosion resistant in seawater, than to attach themselves to tube plates and heat transfer pipes which are made of copper alloys.
Larval marine organisms pass through a strainer to penetrate into a rubber-lined steel water box of the heat exchanger. They adhere to the rubber-liner of the steel water box, grow thereon, and fall off therefrom. This result in clogging of heat transfer tubes.
For the purpose of exterminating those marine organisms and preventing marine organisms from attaching themselves to the tubes (i.e., “antifouling”), various measures are taken. Such measures include: pouring chlorine or a chlorine compound into environmental seawater; coating surfaces exposed to seawater with an antifouling paint containing a toxic ion generating pigment; and generating toxic ions, such as chlorine ions or copper ions, through the electrolysis of seawater.
Although these measures exercise effective antifouling functions, the management of the quantity and concentration of those chemicals is not simple when dealing with quantities of seawater and, therefore, the chemical concentration of seawater is liable to be excessively large. Consequently, it is highly possible that the seawater containing an antifouling chemical causes environmental contamination. Thus, there is a trend in recent years to inhibit or control the use of the aforesaid methods.
Recently, many researchers and technicians are engaged in research-and-development activities to develop safe antifouling measures which will not cause environmental pollution. For example, antifouling silicone paints are nontoxic and do not cause environmental pollution. However, collision of shells and foreign substances with the silicone paints shortens the effective antifouling life of the silicone paints. Coating work using antifouling silicone paints requires a high cost. Antifouling silicone paints cannot be applied to structures having large surfaces and existing structures by simple, easy coating work. The antifouling effect of antifouling silicone paints is reduced in still seawater. Due to the above disadvantages, antifouling silicone paints have not been prevalently applied to practical uses.
JP-B (Kokoku) No. Hei 01-46595 discloses another antifouling method. In this method, a film of an electrical catalyst, such as a mixed crystal of platinum group metals or a mixture of such a mixed crystal and oxides of such platinum group metals, is formed on the surfaces of structural members. Water or seawater is electrolyzed using the electrical catalyst as an anode to generate sufficient oxygen substantially without producing chlorine gas in order that the adhesion of organisms living in water to and the deposition of scales on the surfaces of the structural members are suppressed.
However, in this known antifouling method, the electrical catalyst is directly coated on the titanium structural members, which are immersed in water or seawater as an anode. Thus, metallic members of a heat exchanger electrically connected to the titanium structural members, such as members of the water box and water tubes usually formed of steels and lined with rubber, are subject to anodic loading. Therefore, if a part of the rubber lining should be accidentally broken, a current flows through a part of the steel member corresponding to the broken part of the rubber lining and the steel member is corroded abnormally.
This known antifouling method subjects a structure having structural members coated with the film of the electrical catalyst to an electric resistance heating process to heat the component members at a temperature in the range of 350 to 450° C. for several hours to activate the electrical catalyst. Such a heating process is possible to damage the structure and costly and hence this known antifouling method has not been prevalently applied to practical uses.
Generally, only the heat transfer tubes and the tube plates of a titanium heat exchanger are formed of titanium, and the body, the water box, the supply pipes for carrying seawater to the heat exchanger, and the discharge pipes for discharging seawater into the sea are formed of steels. Since the steel water box, the steel seawater supply pipes and the steel discharge pipes are electrically connected to the titanium members, those steel members are subject to galvanic corrosion when immersed in seawater and are corroded intensely. Therefore, the surfaces to be exposed to seawater of those steel members are coated with rubber linings to protect the same from corrosion.
If the rubber lining coating the steel member should be broken, the titanium member electrically connected to the steel member must be loaded at a cathodic potential by a cathodic protection method which reduces the potential of the steel member to a protective potential. However, since the cathodic protection method uses the titanium member as an anode, the steel water box, the steel supply tubes and steel discharge tubes electrically connected to the titanium member are loaded at a cathodic potential. Consequently, it is theoretically impossible to use the cathodic protection method, and an electric current flows through a part of the steel member corresponding to a broken part of the rubber lining to cause the abnormal corrosion of the steel part.
Japanese patent laid-open publication No. P2000-119884A (Kokai) discloses an antifouling system that generates oxygen on the surfaces to be wetted with seawater of a structure exposed to seawater to suppress the adhesion of marine organisms to the surfaces exposed to seawater. In the antifouling system, a titanium sheet, on which the electrical catalyst is pre-coated, is used as an anode-forming member. The titanium sheet is fixed on a titanium tube plate, via an insulating adhesive layer. A conductive member disposed on a rubber lining coating a wall of a water box (usually made of steel) of the heat exchanger. A positive electrode of a dc power unit is connected to the anode-forming member or the electrical catalyst, and a negative electrode of the dc power unit is connected to the conductive member. The dc power unit is internally provided with an automatic potential controller that adjusts potential difference between the positive and the negative electrode such that oxygen is generated while generation of chlorine in seawater is suppressed.
In the above system, since a titanium sheet is provided with the pre-coated electrical catalyst, such titanium sheet can be easily bonded to the surface of the titanium tube plate at an ordinary temperature using the insulating adhesive. Thus, any destructive thermal stress will not be induced in the components and the assembled structure of the heat exchanger. In addition, the anode-forming member is electrically insulated from a structural member, such as a titanium tube plat
Inagaki Shuichi
Nakashima Shoji
Ooba Tadahiko
Sakurada Shigeru
Bell Bruce F.
Kabushiki Kaisha Toshiba
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