This article gives you comprehensive information for selecting and specifying stainless steel tanks. Read further to learn more about:
- Stainless steels and why they are desirable as a storage tank construction material
- Types of stainless steel tanks
- Different stainless steel tanks
- Stainless steel tank finishes
- And much more…
Chapter 1: What are Stainless Steel Tanks?
Stainless steel tanks are widely used in food, beverage, dairy, medicine, cosmetics, and other manufacturing processes where cleanliness and purity are important. These are also used in industrial plants for storing chemicals and gases where strong resistance from chemical degradation is required. Stainless steel is a type of iron alloy containing a certain percentage of chromium which imparts corrosion resistance to the metal. Corrosion resistance is achieved by creating a thin film of metal oxides that acts as protection against corrosive materials.
Stainless steel tanks vary in shapes and sizes. These can be oriented vertically or horizontally depending on location and transportation restrictions on dimensions. Some tanks can hold small amounts of liquids and compressed gases with only a few liters of capacity, while others can hold several thousand gallons. Simple stainless steel water tanks have an inlet, outlet, and a manhole which are usually seen in domestic water tanks. For stainless steel tanks used in industrial processes, several nozzles are installed for mixing different liquids and gases and for the installation of different monitoring instruments. Some are used for reactors and mixers that can have agitators and mixing heads for material blending. These can also have cooling jackets and double walls for insulation and temperature control. Stainless steel tanks have a very wide range of applications since their construction can easily be modified according to specific end-requirements.
What Makes Stainless Steel Corrosion Resistant?
Stainless steel utilizes the principle of passivation wherein metals become “passive” or unreactive to oxidation from corrosive compounds found in the atmosphere and process fluids. Stainless steel has a thin coating of metal oxides at its surface which is referred to as the passive film.
The composition of stainless steel is mostly ferrous, alloyed by a minimum of 10.5% chromium. Metal oxides that form the passive film are chromium oxides. Other alloying elements are present such as carbon, nickel, manganese, and molybdenum. Carbon is the main alloying element to create steel from pure iron. A certain percentage of this element makes steel harder and stronger. Nickel and manganese are stabilizing elements that promote an austenitic metallurgical structure. An austenitic structure prevents stainless steel from hardening through heat treatment. This enables the stainless steel to endure higher temperatures while maintaining its mechanical properties such as ductility. Moreover, austenitic stainless steels have better low-temperature toughness than ferritic stainless steels. Note that manganese produces only half of the effect of nickel and is usually used as a substitute to produce cheaper grades. Molybdenum, on the other hand, has the same function as chromium. It also enhances the corrosion resistance of the material. Molybdenum is a larger atom than chromium making it more effective in making the steel stronger, especially at higher temperatures. A downside of using Molybdenum is that it makes the stainless steel ferritic which is characterized by being more brittle. This is countered by adding more nickel.
Passivation is done by allowing a base material, the stainless steel, to be exposed to air where it builds metal oxides on its surface. To enhance the formation of the passive film, the stainless steel is introduced to a chemical treatment where it is thoroughly cleaned by submerging it in acidic passivation baths of nitric acid. Contaminants such as exogenous iron or free iron compounds are removed to prevent it from interfering in creating the passive layer. After cleaning with an acidic bath, the metal is then neutralized in a bath of aqueous sodium hydroxide. Descaling is also done to remove other oxide films formed by high-temperature milling operations such as hot-forming, welding, and heat treatment.
Chapter 2: Types of Stainless Steel Used for Tank Fabrication
Stainless steel has a very wide range of grades that are used for handling specific chemicals. Different grades have varying corrosion resistance, strength, toughness, high and low-temperature performance. In the fabrication of stainless steel tanks, three grades are widely used. These are 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. Having an austenitic metallurgical structure makes it ductile and well suited for forming a wide range of products.
Stainless steel 304L has similar chromium, nickel, and manganese content. Its difference from stainless steel 304 is its lower carbon content, thereby preventing the process called sensitization. Sensitization happens when chromium and carbon atoms alloyed in the steel react at high temperatures, forming chromium carbides. Since some of the chromium is already used to form chromium carbides, less is available for forming the passive film. These occur at the grain boundaries of the steel structure which makes it prone to intergranular corrosion. This process is problematic when stainless steel is subjected to high-temperature processes or applications. Lowering the carbide content creates less chromium carbide formation and corrosion resistance is maintained even at high temperatures.
Stainless Steel 316 and 316L
Stainless steel 316 contains16 – 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 are 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 low carbon grade, 316L. Lower carbon content is also used for high-temperature applications to prevent sensitization.
Duplex Stainless Steel
This type of stainless steel consists of a combination of austenitic and ferritic metallurgical structures. Austenitic stainless steel is far superior to ferritic in terms of corrosion resistance and mechanical properties. However, they are highly susceptible to stress corrosion cracking. Stress corrosion cracking happens when a crack propagates when the material is subjected to a highly corrosive environment. This can lead to sudden failure of ductile materials. A ferritic metallurgical structure is resistant to stress corrosion cracking. By combining the ferritic phase with the austenitic phase, an added resistance to stress corrosion cracking is obtained. This is mostly suited for tanks used in environments and process fluids containing chlorides such as water for domestic use.
Duplex stainless steel contains 20 – 28% chromium, 2 – 5% molybdenum, and 5 – 8% nickel. Having a higher chromium and molybdenum content makes duplex stainless steels have higher corrosion resistance and mechanical strength. In comparison with 316, using duplex is cheaper due to the lower nickel content and higher strength for a given thickness, enabling thinner plates or sheets to be used. The most popular grade of duplex stainless steel is the standard duplex or 2205 stainless steel.
Chapter 3: Types of Stainless Steel Tanks
Stainless steel tanks are highly customizable. Various features can be integrated to serve a specific application. Tanks can be single-walled or double-walled, horizontal or vertical, insulated or heated, and so on. Below are general classifications of stainless steel tanks according to 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 EPA in its oil spill prevention programs, particularly the SPCC or 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:
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.
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 with conventional jackets which allow for 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 is not a threat to 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 on 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: Stainless Steel Tank Surface Finishes
Stainless steel is usually smooth and slightly reflective, but in some instances, their surfaces are grainy or are brushed in one direction. These varying profiles are called surface finishes. An important characteristic of surface finish is surface roughness. This is the deviation of microscopic peaks and troughs from the ideal surface of a metal. The type of surface finish and the surface roughness are important specifications for stainless steel tanks since it affects moisture retention and material adhesion on the tank’s surface.
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 is 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. 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 being mixed or agitated. Popular mechanical finishes are No. 3 and No. 4.
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: Tank Specifications and Additional Features
Specifications or ordering information are design considerations supplied to the manufacturer which depends on the application. Also, verify if the supplier is capable of fabricating the required features. Listed below is the basic information needed for specifying a stainless steel tank.
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.
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.
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 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, advances in manufacturing technology bring the costs lower.
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:
This is a safeguard in events 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 important for high throughput process tanks and pressurized tanks.
Requirement for Ground Flush Welds:
For tanks utilized in food, beverage, pharmaceutical, and dairy, there must 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.
- 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 it affects moisture retention and material adhesion on the tank’s surface.