Virtually every processing or packaging plant has tanks. They are used to store both liquids and powders as well as to carry out blending and other processing operations.
Storage tanks have been used since ancient times and were originally made of wood or clay but modern tanks are made predominantly of metal. The least expensive type of metal tank is the steel tank. The kind of steel used in tanks is usually low carbon or “mild” steel which is easy to work with and easy to weld but higher carbon steel, such as A-36, is sometimes used in places where higher strength is required. Early steel tanks were riveted together but almost all metal tanks today are welded.
A simple storage tank is produced by forming the sides with radius rolling equipment, welding it into a cylinder then welding on a circular bottom. This would produce a flat bottom tank which, for obvious reasons, would not drain very well. To create a tank that drains, the bottom can be sloped, formed into a conical shape or, for sanitary tanks, formed into a dish shape on a press and die designed for that purpose. A valve is placed at the lowest point of the bottom to control emptying of the tank.
The major problem with using steel in a tank is that, as everyone knows, steel rusts. especially when exposed to chemicals. Various rust inhibiting coatings are used to protect steel tanks, epoxy paint being the most common. However, if a product requires sanitation or purity, steel tanks are unsuitable. The material of choice is often stainless steel. Stainless steel is an alloy of iron, chromium and nickle developed in the early Twentieth Century. It is more expensive and harder to work with than steel but does not normally rust. The most common grade of stainless steel is SAE 304 stainless which is used in most food processing operations. For more corrosive environments, marine grade SAE 316 stainless is used. This alloy contains more molybdenum than 304 stainless and is resistent to pitting. There are other grades of stainless steel but these are the ones most often encountered. There are test kits available that detect the presence of molybdenum if there is ever any question whether a tank is 304 or 316 stainless. Tanks that are 316 sell for more money than those that are 304.
Tanks are often agitated. Storage tanks sometimes have side agitators. Tanks that are designed for blending often have top mounted agitators that can be turbine or sweep design. Jacketed tanks often have scraped surface agitators to keep the product from burning.
Tanks can be open on the top of closed. Some tanks are designed to hold pressure or take vacuum. Some are double walled with jackets for heating or cooling. Jackets that are designed to take pressure are often fabricated in a dimple pattern. This allows the jacket to be welded to the inner wall at the dimple points and creates a very sturdy jacket. Such jackets can often take more than 100psi steam pressure. A flat wall is sometimes welded to the outside of the dimple jacket in sanitary tanks.
Some of the leading brand names of chemical tanks are Perma-San, Chem-Tek and Dura-Tek. Sanitary tanks often bear the names Cherry Burrell, Crepaco, Walker, Precision Stainless, Mueller and many other manufacturers.
As we know from chemistry class, the most corrosive chemicals, especially acids, are stored in glass. Glass lined iron tanks have been around since the Nineteenth Century. The Pfaudler Company of New York and DeDietrich of France both claim to have developed the process for glazing iron tanks. Glass is ground to a powder and chemicals are added to make the poweder ahdere to the tank. The interior of the tank is sprayed with the mix and then the entire tank is put in an oven and heated until the glass melts into a continuous smooth impermeable lining. Sometimes the glass must be applied in more than one layer. Glass linings can crack or deteriorate with usage over time but tanks can be reglazed. Glass lined tanks often have agitators that are also glazed.
For simple storage and even for some blending operations, fiberglass and polyethylene tanks are often used. They cost less than stainless steel and they do not rust. Fiberglass tanks were developed by Owens Corning in the 1960’s and their first use was in underground storage tanks for petroleum products. Polyethylene tanks are better suited for outdoor locations than fiberglass tanks and, although they cannot withstand petroleum products and solvents, they can hold such chemicals as caustic soda and sulfuric acid.
The size of tanks is often limited by shipping considerations. The tank has to be shipped from the shop to the location. Standard truck loads are a maximum of 8 ft or 8′-6″. Even an oversized load is limited to about 12 or 13 ft. Therefore, any tank of more about 50,000 gallons capacity has to be constructed not in a shop but onsite and it cannot be moved without being dismantled. This becomes a major consideration with used equipment. A mild steel tank that cannot be shipped is never worth more than scrap value. A stainless steel tank, on the other hand, is often valuable enough to be dismantled and moved. although, the associated costs must be taken into consideration when purchasing such a tank. Glass lined, fiberglass and polyethylene tanks are never built in the field so shipping is rarely a problem with those.
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