In power plants a surface condenser has been conventionally employed to cool and condense a gas (steam) exhausted from a power generating steam turbine for recycling the condensed water. In a condenser of this type thousands of or tens of thousands of copper alloy condenser tubes, having ordinarily an internal diameter of 10-40 mm.phi. and a length as long as 5-40 m, are incorporated. Through the condenser tubes flows cooling water such as sea water so as to cool down the exhaust steam (vapor) passing the outside gap of the large number of condenser tubes.
The copper alloy condenser tubes are subjected to various types of corrosion because of fairly high speed flowing of corrosive water such as sea water on the order of 1-2.5 m/s and of the pollution of cooling water, for example, which necessitates coating or painting of the inner surface of the tubes with an anti-corrosive paint of organic resin over the whole length thereof for the purpose of preventing corrosion. However, the coating is strictly conditioned not to greatly inhibit the heat transfer rate of copper alloy tubes. In the case of applying anti-corrosion coating on the inner surface of copper alloy condenser tubes, the thickness of the coated film must be uniform and controlled in the range as small as 10-30.mu. from the view point of maintaining the desired heat transfer rate.
The life of coating of this type is inevitably shorter than the life of the power plant itself, which is as long as 20-30 years if the above-mentioned thin coated film is maintained; and the coating is liable to be worn away after the tube is mounted in the plant to expose sometimes the base metal. The coating may be damaged in some cases by mechanical rubbing with shells or sand grains contained in the cooling water, and its wearing may be accelerated by the so-called sponge-ball cleaning carried out to remove mud and/or sea weeds stuck on the inner surface of the tubes. Such being the case, the tubes must be recoated periodically or occassionally to maintain anti-corrosion and anti-rust condition in the plant.
Some of coating methods have been recognized and practiced widely for coating the interior of tubes of relatively small length, for example, flowing paint in a tube or directly brushing paint. Such methods are however utterly impracticable, in the case of coating a long tube of small diameter such as a condenser tube, for obtaining a coated film of uniform thickness there. And particularly in the former method the paint flowed into a tube can not be diffused in the interior of the tube unless the horizontal tubes installed in the condenser are inclined.
As a relatively practical method spray coating can be mentioned, wherein the interior of a tube is coated by a spray gun spraying atomized paint. Even in this method employing a long necked spray gun of 500 mm, a coatable area, or the length of the coated area, covered by means of inserting the gun into the tube is naturally limited (restricted) in the partial length thereof. As a variation of such spray coating, a method of coating, wherein a paint spraying nozzle is moved (retracted) from one end opening of a tube to the other end opening during spraying paint, has been developed and has succeeded in getting a coated film of uniform thickness throughout the whole length of a long tube of small diameter.
There have still been, however, several problems in the spray coating method of this type: it is a matter of course that no spray is allowed while the spray gun is moved through a long tube from one end to the other end before it reaches the destination where spraying is to begin; even a slight leakage of paint from the nozzle in the meantime will hamper the uniform coating of the tube interior; the paint passage must be absolutely tightly closed while it is not in use, otherwise the nozzle will suffer from uneven spraying or no-spraying due to hardening of paint left in the nozzle.
For the elimination of those problems conventional nozzles have employed a stopper of core bar type. In other words, a pointed portion on the tip of the stopper is fitted into an opening portion of the paint passage of a nozzle insert, so that the opening portion of the paint passage is opened and closed by the advancing and retracting of the stopper. Such a type of stopper is liable to abnormally work or in some cases become entirely paralysed, in the event that a tube to be coated is very long, due to unexpected deflection or friction of the stopper.
The inventors of this invention proposed, in JITSU-GAN-SHO-54(1979)-147332 and JITSU-GAN-SHO-54(1979)-147333 (Japan, etc., a method of eliminating the above-mentioned disadvantages by means of closely and tightly covering the paint spraying nozzle by a stopper of a cap shape type. In a nozzle of this type the tip portion thereof is covered by a cap type stopper until it reaches the other end of a tube to be coated. Upon the nozzle reaching the other end of the tube, the cap is removed from the nozzle to allow the paint to be sprayed while the nozzle is being retracted along through the tube inside. This enabled coating of the interior of a long tube. It still leaves, however, something to be desired, such as the necessity of arranging an individual operator on either end of the tube to be coated for the purpose of putting on and removing the cap type stopper, and particularly in the event of coating copper alloy tubes in a condenser at a power station the stopper removed on one side of a condenser must be brought back each time to the other side of a condenser where the nozzle is inserted. It not only provides a great problem of coating efficiency, but also another serious problem of deteriorating the operation environment owing to a remarkable rising of density of the harmful organic solvent, such as toluene, on the stopper removing side at the beginning of paint spraying.
In the spray coating of copper alloy tubes in a condenser there are some unavoidable restriction conditions, especially in the event of coating or re-coating of the already installed tubes in place, from the view point of operation mode, operation environment, or operation term allowed, etc. A first problem of restriction lies in an extremely narrowly limited space for the coating operation; it is often limited in the condenser water box, the dimensions of which are 2-3 m in depth, 2-3 m in width, and 2-5 m in height. In the case of coating tubes in such a restricted space, ordinary operational apparatus, techniques and conditions used in a satisfactorily broad manufacturing plant can not be applied as they are.
A second problem of restriction lies in deterioration of the operational environment, due to gradual pollution of the atmosphere in the condenser water box where the coating operation must be carried out, because the evaporation of thinner (solvent) from the paint in the narrow operation space makes it undesirable to keep the operators staying there for a long time.
A third problem of restriction lies in that the re-coating operation must be finished during the term of inspection of the whole power generating plant, so the term allowed for the operation of re-coating the heat exchanger tubes is relatively short. Even when an established operation mode in an already installed plant may be introduced there, the number of coating apparatuses permitted to be worked in the narrow space is naturally limited. Elongation of the operation term is very inconvenient under such circumstances. A coating method and apparatus of high efficiency has thus been badly looked forward to.