Paint, large tanks

For cylindrical items, a commonly used method is dipping. Here the paint is held in a large tank and the object to be painted is slowly lowered into the tank and then withdrawn. Many complex items can be dip painted provided that the drainage points (the places where the excess paint drips off) can be located where they are not noticeable. 

In dip coatings the article is completely immersed in a large tank containing a certain quantity of paint to be applied. The article is then pulled out excess coating drains back into the dip tank. This technique is used for metal primer application such as motorcar bodies. 

Large steel aboveground storage tanks (ASTs) are located on large tank farms of oil producers (Fig. 3.15). Maintenance teams take care of external painting and both internal and external corrosion inspections. Corrosion inspection and protection of ASTs is important for the preservation of large capital investments, the reduction of maintenance and inspection costs, and the assurance of system integrity for release... 

Ships belong to the largest moving structures ever made. For example Very-Large Crude Carriers (VLCC) can be up to 500 m long and 60 m wide which means that the exposed area of the outer hull in some cases will exceed 50.000 m2 and the exposed area in e.g. the ballast tanks will be even larger - up to 230.000 m2. This implies the need for an enormous amount of paint. The vessel should be designed so that it is solid and robust and in a way so it can be maintained properly. 

When round robin tests were performed to test the reproducibility of these standard procedures, large coefficients of variation between laboratories were obtained for tin-free paints (78-80% and 24-32% for the ISO and ASTM methods respectively Haslbeck and Holm, 2005). These discrepancies have multiple sources such as the analytical method (Haslbeck and Holm (2005) report 4-54% deviations when different laboratories measuring samples of known concentration), the sea water conditions both in the holding tank and the measuring tank (Haslbeck and Holm, 2005), the sample preparation for analysis... 

Cleaning of tanks Tanks containing the paints are cleaned only when a batch of colour has been discontinued or found defective. This requires large amounts of solvent but does not happen often (once every 6-8 months). 

Sometimes it is desired to have a large working area in a tank where, for example, a conveyor belt containing car bodies can be passed through for electrostatic painting. One way to accomplish this is to put a series of propeller mixers in a side arm of the long side of the tank, so that the flow is directed into the middle zone, but there are no mixer shafts or impellers in the center to impede the flow of the parts through the equipment. 

Spray nozzles are used for dust control, water aeration, dispersing a particular pattern of drops, coating, paintings, cleaning surfaces of tanks and vats, and numerous other applications. They develop a large interface between a gas and liquid, and can provide uniform round drops of liquid. Atomization occurs by a combination of gas and liquid pressure differences. The Figure below (courtesy of Misonix Inc.) compares the particle sizes from the ultrasonic nozzle with those from the conventional nozzle. 

Storage of Liquids. Bulk storage of liquids is generally handled closed tanks to prevent escape of volatiles and contamination. In so instances, such as water storage, where contamination and dilution are n a factor, large open reservoirs can be employed. Natural terrain, coi crete-walled excavations, or concrete tanks without tops are typica construction. Reinforced-wall design is required and the concrete must be waterproofed with a suitable paint to prevent any possibility of leaking. 

Most of the time, cellulosic diickeners are pre-dissolved in a separate tank and used under a liquid form in the manufacturing process. In general, this premix is a blend of cellulosic thickener, water, biocide and defoamer. This colloid solution is made to be pumpable and large enough to be used in several paint batches. 

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