[Chapter 1] What are Stainless Steel Tanks?
Stainless steel tanks are commonly used in food, beverage, dairy, medical, cosmetics, and other production processes that require cleanliness and purity. These are also used in industrial operations to store chemicals and gases that require a high level of chemical resistance. Stainless steel is a form of iron alloy that contains a specific amount of chromium, which gives the metal corrosion resistance. Corrosion resistance is obtained by coating a surface with a thin layer of metal oxides that protects it from corrosive elements.
Tanks made of stainless steel come in a variety of designs and sizes. Depending on the location and transportation limits on dimensions, these can be arranged vertically or horizontally. Some tanks with only a few litres of capacity can hold modest amounts of liquids and compressed gases, while others can hold thousands of gallons. Simple stainless steel water tanks typically feature an inlet, outlet, and manhole, as do domestic water tanks. Several nozzles are fitted in stainless steel tanks used in industrial operations for mixing different liquids and gases, as well as for the installation of various monitoring sensors. Some are used for reactors and mixers that can feature agitators and mixing heads for substance blending. Cooling jackets and double walls can be added for added insulation and temperature control. Because its construction can easily be adjusted to meet specific end requirements, stainless steel tanks have a wide range of applications.
What Characterizes Corrosion-Resistant Stainless Steel?
Stainless steelworks on the principle of passivation, in which corrosive chemicals in the atmosphere and process fluids cause metals to become "passive" or unreactive to oxidation. The passive film is a thin layer of metal oxides that coats the surface of stainless steel.
Stainless steel is primarily ferrous, with a minimum of 10.5 per cent chromium alloyed in. Chromium oxides are the metal oxides that make up the passive film. Carbon, nickel, manganese, and molybdenum are among the alloying elements present. Carbon is the most important alloying element in the production of steel from pure iron. Steel becomes harder and stronger when it has a particular quantity of this element. Nickel and manganese are metallurgical stabilisers that promote an austenitic structure. Stainless steel's austenitic structure prevents heat treatment from hardening it. This allows stainless steel to withstand higher temperatures while still preserving mechanical qualities like ductility. Austenitic stainless steels are also more robust at low temperatures than ferritic stainless steels. It's worth noting that manganese has only half the effect of nickel and is commonly used as a cheap alternative. On the other hand, molybdenum serves the same purpose as chromium. It also improves the material's corrosion resistance. Because molybdenum has a bigger atom than chromium, it is more effective at strengthening steel, particularly at higher temperatures. The use of Molybdenum has the disadvantage of making stainless steel ferritic, which is characterised by brittleness. More nickel is added to counteract this.
Passivation
Passivation is achieved by exposing a base material, stainless steel, to air, where metal oxides form on its surface. To aid in the creation of the passive coating, the stainless steel is subjected to a chemical treatment that involves extensively cleaning it in nitric acid-based passivation baths. To avoid interfering with the formation of the passive layer, contaminants such as exogenous iron or free iron complexes are eliminated. The metal is cleaned in an acidic bath and then neutralised in an aqueous sodium hydroxide bath. Other oxide layers generated by high-temperature milling processes such as hot-forming, welding, and heat treatment are also removed by descaling.
[Chapter 2] Types of Stainless Steel Used in Tank Fabrication
Stainless steel comes in a variety of grades that are used to handle various chemicals. Corrosion resistance, strength, hardness, and high and low-temperature performance vary by grade. Three types of stainless steel are commonly used in the manufacture of stainless steel tanks. 304/304L, 316/316L, and duplex are the three options.
304 and 304L stainless steel
Aside from the steel forming alloys, stainless steel 304 contains 18–20 per cent chromium, 8–11 per cent nickel, and 2% manganese. Because it offers enough corrosion resistance for most applications and is less expensive than other grades, this is the most often used stainless steel. Because of its austenitic metallurgical structure, it is ductile and may be used to manufacture a wide range of goods.
The chromium, nickel, and manganese content of stainless steel 304L is comparable to that of 304. It differs from stainless steel 304 in that it has a reduced carbon concentration, which prevents sensitization. When chromium and carbon atoms alloyed in steel react at high temperatures to generate chromium carbides, sensitization occurs. Because some of the chromium has already been utilised to make chromium carbides, there is less chromium available to make the passive film. These occur at the steel structure's grain boundaries, making it vulnerable to intergranular corrosion. When stainless steel is subjected to high-temperature processes or applications, this process becomes troublesome. The generation of chromium carbide is reduced when the carbide content is reduced, and corrosion resistance is maintained even at high temperatures.
316 and 316L stainless steels
16–18 per cent chromium, 10–14 per cent nickel, 2–3 per cent molybdenum, and 2% manganese make up stainless steel 316. This grade is more corrosion resistant than stainless steel 304 due to the addition of molybdenum. It has a larger nickel content to counteract the increased molybdenum's ferritic forming ability. Stainless steel 316 is commonly used in highly corrosive settings, such as chemical storage tanks and tanks near seawater. Stainless steel 316 has a low carbon grade, 316L, similar to stainless steel 304. To avoid sensitization, lower carbon content is also used in high-temperature applications.
Stainless Steel Duplex
This stainless steel has a metallurgical structure that is a mix of austenitic and ferritic. In terms of corrosion resistance and mechanical qualities, austenitic stainless steel outperforms ferritic stainless steel. They are, nevertheless, extremely vulnerable to stress corrosion cracking. When a fracture propagates when a material is exposed to a highly corrosive environment, it is known as stress corrosion cracking. This can cause ductile materials to fail suddenly. Stress corrosion cracking is not a problem for ferritic metallurgical structures. Stress corrosion cracking resistance is increased when the ferritic and austenitic phases are combined. This is best suited for tanks used in environments and process fluids that contain chlorides, such as domestic water. Duplex stainless steel has a chromium content of 20–28 per cent, a molybdenum content of 2–5%, and nickel content of 5–8%. Duplex stainless steels offer superior corrosion resistance and mechanical strength due to their increased chromium and molybdenum content. Due to the lower nickel percentage and higher strength for a given thickness, duplex is less expensive than 316, allowing thinner plates or sheets to be employed. Standard duplex stainless steel, often known as 2205 stainless steel, is the most widely used duplex stainless steel grade.
[Chapter 3] Types of Stainless Steel Tanks
Stainless steel tanks are extremely adaptable. A variety of attributes might be combined to fulfil a specific purpose. Single-walled or double-walled tanks, horizontal or vertical tanks, insulated or heated tanks, and so on are all possibilities. Stainless steel tanks are classified in the following ways based on their function and construction.
- Single-walled Stainless Steel Tanks: These are the most basic stainless steel tanks, and they are usually cylindrical in shape. Depending on the application and size constraints, these can be arranged vertically or horizontally. The tank's internal and external surfaces are both protected from corrosive substances by having a single wall. These are mostly used to store water for residential consumption and in liquid raw material processing operations.
- Double-walled Stainless Steel Tanks: These are used for applications that require secondary containment in the event of a leak, as defined by the Environmental Protection Agency (EPA) in its oil spill prevention programmes, particularly the SPCC (Spill Prevention, Control, and Countermeasure). Stainless steel can be used on one or both sides of a double-walled tank, depending on the corrosion resistance required. Other stainless steel tanks with double walls have insulation between them. The insulation protects the product from temperature changes in the environment.
- Stainless Steel Clad Tanks: Large stainless steel tanks with strong walls are both costly and impracticable to construct. A solution is to build the tank out of carbon steel plates that are subjected to the process fluid's static load and pressure while being protected from corrosion by a thin strip of stainless steel cladding. The two metals are pressed and heated together to generate stainless steel clad carbon steel plates. During this process, a metallurgical bond is produced. Stainless steel sheets can be bonded on one side (single-side cladding) or both sides (bi-directional cladding) (double side cladding). Other techniques, such as hot roll bonding, cold roll bonding, and explosive bonding, are available in addition to pressing.
- Jacketed Stainless Steel Tanks: Are utilised for procedures that require additional heating, cooling, or thermal stability. The area between the two stainless steel plates or sheets is filled with a heating or cooling fluid. The inner wall is where heat is transferred. The heating or cooling liquid returns to the utility systems after heat transfer (boiler or cooling towers). There are three types of stainless steel jacketed tanks:
- Conventional Jacket: The outer wall of this stainless steel jacketed tank is only supported by baffles soldered between the two metal sheets or plates. An annular space exists between the walls. Low-pressure applications are best served by conventional jacketed stainless steel tanks. Due to the greater thickness required for the outer wall when used at higher pressures, the cost increases dramatically.
- Dimple Jacket: The exterior wall of this sort of jacketed tank is spot-welded or plug-welded into the tank. This form of attachment to the inner wall results in depressions or "dimples" on the outer wall's exterior surface. The dimples can be arranged in a staggered or in-line pattern. It is stronger than traditional jackets because it has a wider effective area of attachment. This allows for thinner sheets to be employed. However, because thermal shock can degrade welds, this is not relevant to operations that involve fast heating and cooling cycles.
- Half-pipe Coil Jacket: A split pipe is wound and welded around the tank wall instead of an outside metal sheet wall. This technique of connection is more durable than the other two, making it suitable for high-pressure applications without suffering from thermal fatigue. This is more expensive than a dimpled jacket for big tank capacity applications but less expensive than conventional.
- Process Tanks Made of Stainless Steel: In most manufacturing and industrial operations, this type of stainless steel tank is utilised for mixing, dissolving, or homogenising process components or ingredients. The agitator or mixing head is the most prominent characteristic of these tanks. There are a variety of agitator designs to choose from. High-shear mixing heads, mixing paddles, impellers, and helical agitators are examples. Temperature, pressure, and level are among the parameters monitored by instruments. By utilising double walls, stainless steel process tanks can include additional functions such as heating and insulation.
- High-pressure Stainless Steel Tanks: Tanks used for household and commercial purposes typically have modest pressures of up to 1 barg. When the operating pressure is greater than 1 bar, design and construction must adhere to ASME Boiler and Pressure Vessel Code. This is done to ensure that the energy stored in the vessel does not endanger workers or the environment. Design considerations, design factors, material selection, fabrication procedures, and testing criteria are all outlined in the ASME code. Compliance is indicated by the addition of certification marks to the tank specifications. All vessels, regardless of kind, must meet high-pressure vessel specifications.
- Compressed Air Tanks: Because most compressed air systems for industrial and manufacturing plants have pressures around 5 to 6 barg, this type of tank is also covered by the ASME Boiler and Pressure Vessel Code. Air receivers and air buffer vessels are frequently made of stainless steel tanks. Wet-type air receivers hold air while reducing moisture by allowing water vapour to condense inside the vessel, which is subsequently drained to the tank blowdown. Because moisture and air are prevalent, corrosion-resistant materials are required.
[Chapter 4] Surface Finishes for Stainless Steel Tanks
Stainless steel is normally smooth and slightly reflective, however, it can be grainy or brushed in one direction in other cases. Surface finishes refer to the various profiles. Surface roughness is an important feature of surface finish. The deviation of microscopic peaks and troughs from a metal's perfect surface. The surface smoothness and roughness of stainless steel tanks are essential characteristics because they affect moisture retention and material adherence on the tank's surface.
- Mill Finish: This is the standard supply state for stainless steel sheets and plates after they have been manufactured at a steel mill. As a result, a mill finish has not yet been mechanically or chemically altered to fit a particular function. Hot and cold rolling are the most common methods of production. Secondary milling processes, such as pickling, are used to improve the corrosion resistance of stainless steel. The No. 2B mill finish is the most common for stainless steel tanks. The ASTM designation No. 2B is defined by a smooth and slightly reflective surface. Ra, or average surface roughness, is often in the 0.30 - 0.50 m range. Skin pass rolling is the method utilised to attain this smoothness. Mill finishes like No. 1D, No. 2D, and BA are also available (bright annealed).
- Polished, Ground and Brushed (Mechanical) Finishes: Additional procedures are performed to a conventional mill finish to create the desired surface smoothness. To reduce polishing effort, the mill finishes grade closest to the desired surface characteristics is chosen. Fine abrasive compounds bonded to belts and discs that cut in a unidirectional manner are used for polishing and brushing. Depending on the application, the average roughness for this finishing varies. This is typically done on stainless steel process tanks with a particular surface roughness for the inner walls in order to achieve optimal material flow while being mixed or agitated. The mechanical finishes No. 3 and No. 4 are very popular.
- Electropolished: Electropolishing is an electrochemical procedure that eliminates or levels tiny peaks on the metal's surface. Submerging the metal in a hot electrolyte bath does this. After that, the metal is connected to a DC power supply and a cathode is submerged in the electrolyte. The surface of the metal dissolves into the electrolyte solution when the current travels through it. The microscopic surface peaks degrade more quickly than the flat areas. The surface eventually grows smoother, with a smoothness of less than 0.2 m. Where product contamination is a concern, electropolished stainless steel tanks are employed in the food and pharmaceutical industries. Moisture stimulates the growth of bacteria, thus the tank's surface must be smooth enough to prevent moisture from lingering. A highly smooth surface also prevents any product from clinging to the surface, making the rinse phases easier for tanks with clean-in-place (CIP) capability.
[Chapter 5] Tank Specifications and Extra Features
Specifications, also known as ordering information, are design considerations given to the manufacturer that is specific to the application. Also, check to see if the provider can fabricate the essential characteristics. The fundamental information needed to define a stainless steel tank is listed below.
- Capacity: As defined by the process owners, this is one of the most essential design concerns. This is the tank's nominal capacity for storage or processing. This is distinct from the overall or gross volume, which includes space set aside for vapours and expansion.
- Pressure Rating: Apart from capacity, pressure rating is a crucial design element that influences the thickness of the metal sheet or plate that will be employed. According to the tank's experienced pressure, further standards such as welding processes and inspection methods are required.
- Size Restrictions: This is the tank's preferred size as determined by the process owner. This, however, is contingent on the manufacturer's capabilities and any regulations. It's better to start with commercially available or conventional sizes and make adjustments as needed.
- Stainless Steel Grade: As previously stated, there are three stainless steel grades that are usually used for tanks. 304/304L, 316/316L, and duplex are the three options. The cheapest stainless steel is 304/304L, which is suitable for slightly corrosive conditions and chemicals. Although 316/316L has better characteristics than 304/304L, it is much more expensive. Duplex stainless steel offers equivalent qualities to 316/316L stainless steel (better for more expensive grades). Although duplex is more expensive, developments in manufacturing technology are lowering costs.
- Finish Type: Standard mill finish and electropolished are the most common stainless steel tank finishes. For agitation and mixing procedures, mechanical polishing is used to obtain a certain surface roughness.
- Nozzles and Manholes: Nozzles are welded-in stub-in connectors for joining, coupling, or bolting inlet, outlet, and instrument pipes to the stainless steel tank. Manholes allow access to the interior of the tank for cleaning and maintenance. The process owner normally specifies the nozzle and manhole diameters.
- Pressure Relief or Liquid Overflow Valves: These serve as a safety net in the case of equipment failure or process disruption. Excessive pressure and flow can cause an explosion and leakage if they exceed the tank's limits. This is especially crucial for pressure tanks and high-throughput process tanks.
- Ground Flush Welding Requirement: For tanks used in the food, beverage, pharmaceutical, and dairy industries, there must be no areas where the product can stagnate. Weld caps can obstruct the flow of liquid. Contamination can occur if microbes proliferate in these places. Ground flush welds can be specified by process owners. This procedure, however, severely weakens the joints and should be taken into account when determining the thickness of the sheet walls.
- Application-specific Features and Accessories: Application-specific Cooling jackets, insulation, agitator and mixer connections, brackets, supports, lifting lugs, and interior structures such as baffles, trays, and ladders are among the features and accessories.
Conclusion
- Stainless steel tanks are frequently used in food, beverage, dairy, medicine, cosmetics, and other manufacturing operations that require cleanliness and purity.
- The fundamental benefit of stainless steel is its resistance to corrosion.
- Stainless steel employs the passivation principle, which causes metals to become "passive" or non-reactive to oxidation. In stainless steel, the passive film is made up of chromium oxide.
- Chrome, nickel, molybdenum, and manganese are the principal alloying constituents in stainless steel. Different grades of stainless steel, such as 304/304L, 316/316L, and duplex, are produced by varying the composition.
- Single-walled, double-walled, cladded, and jacketed stainless steel tanks are the various constructions. Process tanks and storage tanks are two types of stainless steel tanks that can be categorised based on their application.
- The surface smoothness and roughness of stainless steel tanks are essential characteristics because they affect moisture retention and material adherence on the tank's surface.
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