Stainless Steel Tanks

Chapter 1: What are Stainless Steel Tanks?

Stainless steel tanks are extensively utilized across various industries such as food, beverage, dairy, pharmaceuticals, and cosmetics, where maintaining cleanliness and purity is crucial. They are also employed in industrial settings for the storage of chemicals and gases, where robust resistance to chemical degradation is necessary. Stainless steel, an iron alloy with a specific percentage of chromium, provides enhanced corrosion resistance. This resistance is due to the formation of a thin oxide film that safeguards the metal from corrosive substances.

Stainless steel tanks come in various shapes and sizes, with orientations that can be either vertical or horizontal, depending on space and transport constraints. They range from small tanks with a few liters of capacity to large ones capable of holding several thousand gallons. Basic stainless steel water tanks typically feature an inlet, outlet, and manhole, common in residential settings. In industrial applications, tanks are equipped with multiple nozzles for the mixing of liquids and gases and for the installation of monitoring instruments. Some tanks are designed as reactors or mixers, incorporating agitators and mixing heads for blending materials. Additionally, they may include cooling jackets and double walls for insulation and temperature regulation. The versatility of stainless steel tanks allows for customization to meet diverse end-use requirements.

What Makes Stainless Steel Corrosion Resistant?

Stainless steel operates on the principle of passivation, where the metal becomes "passive" or unreactive to oxidation by forming a protective oxide layer on its surface. This passive film is composed of chromium oxides, which shield the stainless steel from corrosive compounds in the environment and process fluids.

Stainless steel primarily consists of iron, alloyed with at least 10.5% chromium. The chromium forms the passive film of metal oxides. Other alloying elements include carbon, nickel, manganese, and molybdenum. Carbon is crucial for transforming pure iron into steel, enhancing its hardness and strength. Nickel and manganese stabilize the austenitic structure, which prevents hardening through heat treatment and maintains ductility at high temperatures. Austenitic stainless steels also exhibit better toughness at low temperatures compared to ferritic types. Manganese, while less effective than nickel, is often used as a cost-effective substitute. Molybdenum, similar to chromium, further improves corrosion resistance and strengthens the steel, especially at elevated temperatures. However, molybdenum can make stainless steel more brittle, a drawback mitigated by adding more nickel to the alloy.

Passivation

Passivation involves exposing stainless steel to air, which facilitates the formation of a protective oxide layer on its surface. To accelerate the development of this passive film, the stainless steel undergoes a chemical treatment. This process includes immersing the steel in acidic passivation baths, typically containing nitric acid, to thoroughly cleanse it. This step eliminates contaminants like exogenous iron or free iron compounds that could hinder the formation of the passive layer. Following the acidic bath, the metal is neutralized using a solution of aqueous sodium hydroxide. Additionally, descaling removes other oxide layers that may have developed during high-temperature processing methods such as hot-forming, welding, and heat treatment.

Chapter 2: What are the different stainless steel grades used in tanks?

Stainless steel offers a diverse array of grades tailored for handling various chemicals. Each grade provides different levels of corrosion resistance, strength, toughness, and performance at both high and low temperatures. In the construction of stainless steel tanks, three specific grades are commonly utilized: 304/304L, 316/316L, and duplex.

Stainless Steel 304 and 304L

Stainless steel 304, aside from the steel forming alloys, is composed of 18-20% chromium, 8-11% nickel, and 2% manganese. This is the most common stainless steel since it has enough corrosion resistance for most applications and is less expensive than other grades. An austenitic metallurgical structure makes it ductile and well-suited for forming a wide range of products.

Stainless steel 304L contains similar amounts of chromium, nickel, and manganese as stainless steel 304. The key distinction lies in its reduced carbon content, which helps prevent sensitization. Sensitization occurs when chromium and carbon atoms in the steel react at elevated temperatures to form chromium carbides. This reaction depletes chromium that would otherwise contribute to the passive film, particularly at the grain boundaries of the steel, leading to increased susceptibility to intergranular corrosion. This issue is especially significant in high-temperature applications. By reducing the carbide content, the formation of chromium carbides is minimized, thereby preserving corrosion resistance even under high temperatures.

Stainless Steel 316 and 316L

Stainless steel 316 contains 16-18% chromium, 10-14% nickel, 2-3% molybdenum, and 2% manganese. The added molybdenum makes this grade more corrosion-resistant than stainless steel 304. It has higher nickel content to counter the ferritic forming property of the added molybdenum. Stainless steel 316 is mostly used in highly corrosive environments such as chemical handling tanks and tanks near marine environments. Like stainless steel 304, stainless steel 316 has a lower carbon grade, 316L. Lower carbon content is also used for high-temperature applications to prevent sensitization.

Duplex Stainless Steel

This type of stainless steel is composed of both austenitic and ferritic metallurgical structures. Austenitic stainless steel offers superior corrosion resistance and mechanical properties compared to ferritic stainless steel, but it is more prone to stress corrosion cracking. Stress corrosion cracking occurs when a crack develops in a material exposed to a corrosive environment, potentially leading to sudden failure. On the other hand, ferritic stainless steel is resistant to stress corrosion cracking. By combining the ferritic phase with the austenitic phase, the material gains enhanced resistance to stress corrosion cracking. This makes it particularly suitable for tanks used in environments and processes involving chlorides, such as domestic water applications.

Duplex stainless steel comprises 20-28% chromium, 2-5% molybdenum, and 5-8% nickel. The increased chromium and molybdenum content enhances the corrosion resistance and mechanical strength of duplex stainless steels. Compared to 316 stainless steel, duplex steel is more cost-effective due to its lower nickel content and higher strength at a given thickness, allowing for the use of thinner plates or sheets. The most commonly used grade of duplex stainless steel is the standard duplex, also known as 2205 stainless steel.

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Chapter 3: What are the different types of stainless steel tanks?

Stainless steel tanks offer extensive customization options to meet specific requirements. They can be designed with various features, including single or double walls, horizontal or vertical orientations, and insulation or heating capabilities. The following are general classifications of stainless steel tanks based on their function and construction.

• Single-walled Stainless Steel Tanks:

  These are the simplest stainless steel tanks, usually cylindrical in construction. These can be oriented vertically or horizontally depending on the application and size restrictions. Having a single wall offers protection from corrosive compounds on both the internal and external surfaces of the tank. These are mostly used in storing water for domestic use and in manufacturing plants with liquid raw materials.

  

  
  

• Double-walled Stainless Steel Tanks:

  These are used for applications requiring secondary containment in case of spillage, as regulated by the EPA in its oil spill prevention programs, particularly SPCC (Spill Prevention, Control, and Countermeasure). Double-walled tanks can have one or both walls of stainless steel, depending on where the corrosion resistance is needed. Other double-walled stainless steel tanks have insulation in between. The insulation protects the product from ambient temperature variations.

• Stainless Steel Clad Tanks:

  Constructing large stainless steel tanks with thick walls are very expensive and impractical. A solution to this is to construct the tank with carbon steel plates that are subjected to the static load and pressure of the process fluid while being covered with a thin sheet of stainless steel cladding for corrosion protection. Stainless steel cladded carbon steel plates are formed by pressing and heating the two metals together. A metallurgical bond is formed during this process. Stainless steel sheets can be bonded on one side (single-side cladding), or both (double-side cladding). Aside from pressing, other techniques are available such as hot roll bonding, cold roll bonding, and explosive bonding.

  

  
  

• Jacketed Stainless Steel Tanks:

  These types of stainless steel tanks are primarily used for processes that require additional heating, cooling, or thermal stability. Heating or cooling fluid flows through the space between the two stainless steel plates or sheets. Heat transfer takes place through the inner wall. After heat transfer, the heating or cooling liquid returns to the utility systems (boiler or cooling towers). There are three types of jacketed stainless steel tanks:

  • Conventional Jacket:

    This stainless steel jacketed tank has an outer wall that is only supported by baffles welded between the two metal sheets or plates. The space between the walls is an annular space. Conventional jacketed stainless steel tanks are best suited for low-pressure applications. When applied to higher pressure, cost increases significantly due to the increased thickness required for the outer wall.

  • Dimple Jacket:

    In this type of jacketed tank, the outer wall is spot-, or plug-welded into the tank. This method of attachment to the inner wall creates depressions or "dimples", as seen on the external surface of the outer wall. The arrangement of the dimples can be staggered or in-line. Since there is a larger effective area of attachment, it is stronger compared than conventional jackets, allowing thinner sheets to be used. However, this is not applicable for processes that use fast heating and cooling cycles since thermal shock can weaken the welds.

    

    
    

  • Half-pipe Coil Jacket:

    Instead of using an outer metal sheet wall, a split pipe is wound and welded around the tank wall. This method of attachment is stronger compared to the other two jackets, making this design useful for high-pressure applications without being affected by thermal fatigue. For large tank volume applications, this is more expensive than a dimpled jacket but cheaper than conventional.

    

    
    

• Stainless Steel Process Tanks:

  This type of stainless steel tank is used for mixing, dissolving, or homogenizing process components or ingredients found in most manufacturing and industrial plants. The main feature of these tanks is the agitator or mixing head. There are many different agitator designs that can be used. Examples are high shear mixing heads, mixing paddles, impellers, and helical agitators. Instruments are installed for monitoring parameters such as temperature, pressure, and level. Stainless steel process tanks can have additional features such as heating and insulation by utilizing double walls.

  

  
  

• High-pressure Stainless Steel Tanks:

  Tanks used for domestic and commercial applications have low pressures, usually rated at atmospheric pressures up to 1 barg. When operating pressure exceeds 1 barg, design and construction must follow standards such as the ASME Boiler and Pressure Vessel Code. This is to ensure that the energy stored in the vessel does not threaten workplace safety and the environment. The ASME code stipulates design considerations, design factors, material selection, fabrication methods, and testing requirements. Certification marks are added to the tank specifications indicating compliance. High-pressure vessel standards are applied to all vessels regardless of type.

  

  
  

• Compressed Air Tanks:

  This type of tank is also covered by the ASME Boiler and Pressure Vessel Code since most compressed air systems for industrial and manufacturing plants have pressures around 5 to 6 barg. Stainless steel tanks are commonly used as air receivers or air buffer vessels. Wet-type air receivers are designed to store air and cut down moisture by letting water vapor condense inside the vessel, which is then drained to the tank blowdown. Since moisture and air are present, it is necessary to use corrosion-resistant materials.

  

  
  

Chapter 4: What are the different surface finishes for stainless steel tanks?

Stainless steel typically features a smooth and slightly reflective surface, though it can also have a grainy or brushed texture in certain applications. These different profiles are known as surface finishes. Surface roughness is a key characteristic, representing the deviation of microscopic peaks and valleys from the ideal surface. The choice of surface finish and roughness is crucial for stainless steel tanks as it influences moisture retention and material adhesion on the tank's surface.

• Mill Finish:

  This is the basic supply condition of stainless steel sheets or plates after manufacture from the steel mill. Thus, a mill finish is not yet altered by mechanical or chemical means to suit a specific purpose. Manufacturing is usually through hot and cold rolling. Secondary milling operations such as pickling are done to enhance the stainless steel corrosion resistance. The most popular type of mill finish used for stainless steel tanks is No. 2B. No. 2B, as designated by ASTM, is characterized by a smooth and slightly reflective surface. The average surface roughness, Ra, is typically in the range of 0.30 – 0.50 µm. The process used to achieve this smoothness is skin pass rolling. Other types of mill finishes are No. 1D, No. 2D, and BA (bright annealed).

  

  
  

• Polished, Ground, and Brushed (Mechanical) Finishes:

  From a standard mill finish, additional processes are applied to achieve the desired surface smoothness. Mill finish grade is chosen closest to the surface characteristics desired to lessen polishing effort. Polishing and brushing are achieved by using fine, abrasive materials bonded on belts and disks that cut in a unidirectional manner. The average roughness for this finishing varies depending on the application. This is usually done on stainless steel process tanks where a specific surface roughness for the inner walls is required to achieve the proper flow of materials as they are mixed or agitated. Popular mechanical finishes are No. 3 and No. 4.

• Electropolished:

  Electropolishing is an electrochemical process that removes or levels microscopic peaks on the surface of the metal. This is done by submerging the metal into a heated electrolyte bath. The metal is then connected to a DC power supply together with a cathode submerged into the electrolyte as well. As the current passes through the metal, the surface dissolves into the electrolyte solution. The microscopic surface peaks dissolve faster than the flatter regions. Eventually, the surface becomes smoother in the order of less than 0.2 µm. Electropolished stainless steel tanks are used in food and pharmaceutical industries where product contamination is an issue. The surface of the tank must be smooth enough so that no moisture can linger since moisture promotes the growth of microbes. Also, for tanks featuring clean-in-place (CIP) capabilities, having a very smooth surface prevents any product from adhering to the surface, making the rinse phases easier.

  

  
  

Chapter 5: What are the specifications and additional features for stainless steel tanks?

Specifications, or ordering information, are design details provided to the manufacturer that depend on the intended application. Before proceeding with an order, it is crucial to confirm that the supplier can produce the required features. Below is the essential information needed for specifying a stainless steel tank.

• Capacity:

  This is one of the most important design considerations, as determined by the process owners. This is the nominal volume of the tank usable for storage or processing. Note that this is different than the overall or gross volume, which accounts for space reserved for vapors and expansion.

• Pressure Rating:

  Aside from the capacity, pressure rating is an important design parameter that affects the thickness of the metal sheet or plate to be used. Additional specifications such as welding processes and inspection methods are required according to the experienced pressure of the tank.

  

  
  

• Size Restrictions:

  This is the process owner‘s preferred dimensions of the tank. Still, this depends on the manufacturer‘s capability and governing standards. It is best to start from commercially available or standard sizes and tweak them accordingly.

• Stainless Steel Grade:

  As discussed earlier, there are three stainless steel grades commonly used for tanks. These are 304/304L, 316/316L, and duplex. Stainless steel 304/304L is the cheapest and suited for mildly-corrosive environments and chemicals. 316/316L has superior properties than 304/304L but is significantly more expensive. Duplex stainless steel has comparable properties (better for more expensive grades) than 316/316L. Duplex is more expensive; however, technological advances bring lower costs.

• Type of Finish:

  The usual stainless steel tank finishes are standard mill finish and electropolished. Mechanical polishing is employed to achieve a specific surface roughness for agitation and mixing processes.

• Nozzles and Manholes:

  Nozzles are stub-in connections welded into the stainless steel tank for joining, coupling, or bolting inlet, outlet, and instrument pipes. Manholes provide access to tank internals for cleaning and maintenance. Nozzle and manhole sizes are usually specified by the process owner.

  

  
  

• Pressure Relief or Liquid Overflow Valves:

  These are safeguards in the event of emergencies such as equipment failure or process upsets. Excessive pressure and flow can exceed the limits of the tank, causing explosion and spillage. This is particularly relevant to high throughput process tanks and pressurized tanks.

  

  
  

• Requirement for Ground Flush Welds:

  For tanks utilized in food, beverage, pharmaceutical, and dairy industries, there can be no regions where the product can stagnate. Weld caps can prevent liquid from flowing. Microbes can grow in these areas, causing contamination. Process owners may opt to specify ground flush welds. However, this process significantly weakens the joints and should be considered in designing the thickness of the sheet walls.

  

  
  

• Application-specific Features and Accessories:

  These include cooling jackets, insulation, connections for agitators and mixers, brackets, supports, lifting lugs, and internal structures such as baffles, trays, and ladders.

  

  
  

Chapter 6: What are the uses for stainless steel tanks?

Stainless steel tanks offer limitless applications due to their durability and strength. Every sector of society benefits from the positive properties of stainless steel, ensuring high-quality performance and protection for stored materials. While stainless steel is often associated with antiseptic and sterile environments, these tanks are also widely used in various industrial storage and containment applications.

• Cold Storage: Various substances and liquids react poorly to the effects of low-temperature conditions where they may crack or freeze. Stainless steel tanks are resistant to the effects of low-temperature environments. In such conditions, they are capable of maintaining their strength and resilience, which makes them ideal for use as low-temperature storage containers.
• Medical Storage: The medical field places a great deal of demand on the types of materials that it is able to approve and use. Hygienic restrictions dictate the types of metals that meet the stringent demands required to protect patients and save people’s lives. The storage and care of medical instruments and tools necessitate a sterile and hygienically clean environment that can meet the standards set by the Food and Drug Administration (FDA). Stainless steel tanks meet and exceed the necessary requirements. They can be easily cleaned, do not absorb bacteria, and cannot be contaminated. In essence, medical professionals' first choice is stainless steel tanks when designing storage units for medical devices.
• Food Storage: The conditions for food storage follow the same requirements as the medical field since contaminated and unclean conditions can lead to the same types of problems that can harm people. Stainless steel tanks are found in every part of food preparation, production, and storage. Their clean, smooth finishes and easily cleaned surfaces ensure that any harmful substances are removed. Since stainless steel is tightly sealed, it is unable to absorb toxins, ingredients, enzymes, or juices used in food preparation. Additionally, any substance found on a stainless steel surface is easily spotted and removed.
• Waste Storage: Concerns over pollutants, sustainability, and ecological conditions have been rapidly rising. New and improved methods have been developed to dispose of toxic chemicals and dangerous substances. One of the methods that is depended upon is stainless steel tanks due to their tight seal, bacterial resistance, and corrosion and rust resistance. The stability of stainless steel tanks guarantees that any harmful contents will be controlled and contained.
• Outdoor Storage: Storage tanks have been a part of manufacturing and industrial operations for many years in many shapes and forms. Unfortunately, the old metals of years ago would rust, corrode, and deteriorate, which required regular and planned replacement. With the advent of stainless steel tanks that don’t rust or corrode, the need to plan for the replacement of outdoor storage tanks has become minimal. Large tanks for oil transport are made of stainless steel because it can withstand the effects of heat, rain, cold, hail, snow, and all of the elements of nature. Regardless of the environmental conditions, stainless steel tanks can withstand and endure them.

The five uses for stainless steel tanks mentioned above represent just a small sample of their diverse applications in everyday life. Their strength and durability ensure that stored or protected materials are kept safe and secure. The different varieties and grades of stainless steel provide a wide range of solutions for manufacturing these tanks.

Conclusion:

• Stainless steel tanks are widely used in food, beverage, dairy, medicine, cosmetics, and other manufacturing processes where cleanliness and purity are important.
• The main advantage of using stainless steel is its corrosion resistance.
• Stainless steel utilizes the principle of passivation, wherein metals become "passive" or unreactive to oxidation. The passive film in stainless steel consists of chromium oxide.
• The main alloying elements to create stainless steel are chromium, nickel, molybdenum, and manganese. Varying the composition produces different grades such as stainless steel 304/304L, 316/316L, and duplex.
• The different constructions of stainless steel tanks are single-walled, double-walled, cladded, and jacketed. Stainless steel tanks can also be classified according to their application, such as process tanks and storage tanks.
• The type of surface finish and the surface roughness are important specifications for stainless steel tanks since they affect moisture retention and material adhesion on the tank‘s surface.

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