Table of Contents
Table of Contents
Table of Contents maxLevel 4
What is Stainless Steel?
Stainless steel is a metal alloy consisting of nickel, chromium, molybdenum, and other additives. Generally, the composition ratios of this alloy vary and depend on the target properties or use, however, a composition of at least 10.5% chromium and less than 1.2% carbon is required for it to be considered "stainless" [1][2]. As the name suggests, stainless steel is created to be more corrosion resistant to exposure compared to other materials such as carbon or alloy steels. As a result of its exposure resistance, high durability, aesthetics, and hygienic properties, stainless steel play a prominent role in a wide variety of industrial processes, and the production of consumer goods [2][3].
Alloys
Alloys are uniform metallic materials composed of at least one metal and one non-metal in its most simplest form. In order for this combination to be considered uniform and thus an alloy, it must be a solid solution, a compound, or a mixture. Alloys are preferred over other materials because of the enhanced mechanical or chemical properties associated with combining elements [7]. In addition, this combination can be accomplished with little or no reductions in the desired attributes associated with the pure metals it is comprised of; including conductivity and ductility. Such properties would mean that alloys can be used to create products with increased durability and strength, which contributes to its overall longer longevity [8].
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Figure 4: Alloys are often stronger than other metals at a lighter or equal weight. Other chemical and mechanical properties are also often improved [9]. |
Often, alloys can be seen used for both industrial and commercial purposes as a result of it's machinability [8][9]. Examples include structural components of vehicles such as aircraft, bicycles, and automobiles. Commercial applications include kitchenware and utensils such as forks, knives, and saucepans [8].
Binary alloys are created when two or more metals in metal state are combined.
Alloys retain good tensile, compression, sheer, and flexural strength.
In addition to high strength, alloys usually have high conductivity and resistance.
- Some alloys have comparable strength-to-weight ratios to ceramics while being much ductile because they do not shatter upon impact.
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| Figure 1: There are numerous grades of stainless steel that vary in composition, shapes, and mechanical strength [4]. |
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| Figure 2: The composition of a stainless steel sample measured by weight-percent (stainless steel UNS S31603) [5]. |
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| Figure 3: Stainless steel is used extensively used for engineering purposes such as piping [6]. |
Applications of Stainless Steel
As a common alloy, stainless steel sees significant usage in engineering within different industries. It can be seen used in oil and gas, water utility, medical, electrical practices and much more. Through its extensive use, multiple variants and grade of stainless steel are available based on the different environments it's intended to be used in. For example, if stainless steel were to be used in a marine environment, a chloride variant would need to be used in order to resist corrosion from salty seawater. Furthermore, the application of stainless steel is able to alleviate costs due to corrosion damage; according to the US Federal Highway Administration, it is estimated that the US spends about 276 billion dollars a year on damage caused by corrosion. If more stainless steel were used instead of other pure metals, this amount would go down with increased reliability [9].
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Figure 5: The corrosion of pipes can be caused by exposure to sulfides, oxides, and hydroxides. This deformation not only results in expensive costs, but can also be harmful to human health and the environment [10]. |
Applications of Stainless Steel Piping Within Chemical Engineering
One major application of stainless steel within Chemical Engineering is in piping. Piping allows for the transport of masses such as fluids and gases which are common in large chemical plants and processes. For example, pipes of various lengths and diameters transport raw materials, intermediary chemicals, and products between different condensers, mixers, evaporators, etc. As a result of a wide variety of pressures, temperatures, and exposure, the material that the pipe is made of is crucial [11].
Industrial Usages
Some of the most common applications of stainless steel pipes can be found in Chemical Engineering industries [11][12]:
Food and Pharmaceutical Processing
Oil, Gas, and Energy
Environmental and Water Treatment Plants
Materials and Manufacturing
Chemical Plants and Refineries
Currently, stainless steel pipes are considered to be a mainstay of fluid systems with it's heavy usage in chemical and petrochemical plants [12]. It's chemical and mechanical properties makes stainless steel a better material than carbon steel; the thin layer of chromium oxide formed on the surface by the chromium of the alloy prevents corrosion from penetrating the surface. In addition, stainless steel is 100% recyclable making it much more environmentally friendly than alternatives - about all of stainless steel products, including piping, contains up to 60% of recycled material. The advantages of stainless steel for chemical plants and refineries are as described [12][13]:
Adaptability and Ease of Use:
- Stainless steel equipment, tanks, and structural elements are easily to modify
- Fluid systems using stainless steel welded pipes can be reworked while in operation
- Rework with little risk of leakage (millibar per second or less)
Resistance to Corrosion:
- Protected by most forms of corrosion, most common being oxidation
- Best overall resistance to chemicals
- Damaged oxide layer can be self-repaired; fresh exposed chromium is oxidized
Alleviates Operation Costs:
- Higher initial purchase price
- Lower long run costs from repairs
- Less chance of failure and the need of repair
Food and Pharmaceutical
Stainless steel pipes sees heavy usage in food processing where cleanliness and sanitation is of the highest importance. Through fabrication, stainless steel pipes are smooth and crack free. This prevents the buildup of bacteria and microorganisms. The two most common types of stainless steel within the food industry are 304 and 316 stainless steel [12][14].
304-stainless steel
Excellent anti corrosion properties
Suitable for most food and biological products
Passivated surface compromised in the presence of chlorides
Chlorides can cause pitting
316-stainless steel:
Stronger resistance to chlorides compared to 304
Use for meat products
Use for products with mild salt content
Duplex-stainless steel:
Useful for stagnant or slow moving processes
Use for briny or salty foods
Applications where higher temperatures are required
Advantages and Disadvantages of Stainless Steel Pipes
Advantages | Disadvantages |
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Damage and Contamination Through Contact
The following can cause contamination if they come in contact with stainless steel [14]
Affixing a non-stainless metals with stainless steel
Contact with carbon steel can cause contamination; spot corrosion
Hypochlorites, such as bleach
Sparks from welding and grinding; contaminate rust exposed surfaces
Long Term contact with other metals; galvanic corrosion
What To Use To Disinfect Stainless Steel
Nitric Acid
Iodine and Ammonium solutions
Ease of Keeping Clean
Smooth, non-porous, robust material
Majority of different cleaning processes
Can use Sodium Hydroxide clean-in-place system
Selection Based on Requirements
Connection Types/Joining Methods
Welding
Welding is a manufacturing process where parts/components are joined together through heat. Welding stainless steel is more difficult than other materials such as carbon steel because you need to exercise more care in temperature and choice of filler material [16][17].
Tig welding to connect 2 pieces of stainless steel
- Good sanitation of pieces if done correctly
- Low amount of maintenance needed
- Connection is permanent
- High initial cost
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| Figure 6: Stainless steel joining method [16]. |
Tri-Clamp
Tri-Clamp is a connection method first developed Alfa-Laval under the product name Tri-Clover. It functions by having two tri-clamp ends connected to aluminium pipes, a gasket to seal the two ends, and a clamp to lock everything into place [17].
Used for tubing requiring high sanitation
- Easy to install and seal
- Common for food and beverage production
- Does not leak before failure
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| Figure 7: Tri-Clamp/Tri-Clover Connection [18]. |
Bevel Seat
Bevel Seat is a connection thread type that is very common in North America. It functions by having a male and female connector be joined together by a gasket, and tightened with a nut to seal [17].
ACME Standards
- Required special tooling wrench to tighten/loosen
- Will leak before failure
- Common thread type in Canada and the US
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| Figure 8: Bevel Seat Connection [19]. |
DIN
DIN is a threaded connector type first developed in Germany, and stands for Deutsche Industrial Norme. Like the Bevel Seat, it functions with a male and female connector, accompanied by a gasket and a nut to seal. The product however is more common in Europe and China [17].
- Used in food and beverage
- Type 11850: Metric Standard
- Type 11851: Imperial Standard
- Required special tooling wrench to tighten/loosen
- For German brewing, use metric standard DIN Type 11850
- For North American piping, use imperial standard DIN Type 11851
- Will leak before failure
- Used in food and beverage
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| Figure 9: DIN Connection [20]. |
SMS
A threaded connection type that is less common than DIN. But also functions with a male and female connector, accompanied by a gasket and a nut to seal. Originally developed in Sweden, SMS stands for Swedish Milk Standard [17].
- Required special tooling wrench to tighten/loosen
- Common thread type in Europe and China
- Will leak before failure
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| Figure 10: SMS Connection [21]. |
Cam and Groove
The Cam and Groove connection type is also called the Camlock fitting. It functions with a male end and a female end, and are mostly used for hosing connections [17].
- Easy to use and connect
- No tools required for installation
- Used for quick changing hoses
- Used for home brewing as opposed to industrial usage
- Used for hosing requiring low maintenance
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| Figure 11: Cam and Groove Connection [22]. |
NPT Thread
NPT Thread North American technical standard which stands for National Pipe Thread. It functions with a male end and a female end, and is more commonly used for non-sanitary piping rather than sanitary tubing [17].
- Material type requires different sealing aid
- Fluid type requires different sealing aid
- Requires a wrench to install or adjust
- Can be permanent or temporary
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| Figure 12: NPT Thread Connection [23]. |
Garden Hose Thread
Garden Hose Thread connectors are usually used for water applications such as wash down hoses. It functions with a male end and a female end, accompanied by a gasket [17].
- Slightly different than NPT Thread
- 11.5 thread per inch
- Inner diameter of 3/8", 1/2", 3/4"
- Quick-release version available
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| Figure 13: Garden Hose Thread Connection [24]. |
Hose Barb
The Hose Barb connection allows for easy push-connection for stainless steel and elastic hoses. It functions with a male barb end and a female hose end, and is sometimes accompanied by a external clamp [17].
- Purposed to be permanent; can be temporary
- Liquid and Gas applications
- Good for connecting stainless steel to a flexible material
- Usually lower operational pressures
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| Figure 14: Hose Barb Connection [25]. |
Push to Connect
Push to Connect connection type is intended for use on hydraulic and pneumatic systems. It functions with a female port end, and a male hose end [17].
- Easy to install
- Easy to uninstall/disconnect
- Sizing is very important; hoses can pop through connection
- Must accurately convert between metric and imperial
- Used for processes: air/hydraulic fluid supply
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| Figure 15: Push to Connect Connection [26]. |
Stainless Steel Pipe Sizing
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| Figure 16: Stainless steel pipe sizing chart/specifications [27]. |
Passivation
Passivation is a post fabrication treatment process where for stainless steel to increase its corrosion resistance. The inner workings of passivation is not universally agreed upon, but functions with a thin layer of protective oxide on the passive stainless steel. This layer is less than 0.0000001 inches thick, and covers the entire surface of the stainless steel piece to protect it. Methods of which passivation can be achieved varies [28].
Cleaning
Cleaning of pieces is prioritized before passivation as corrosion can occur if parts of the surface is contaminated [28].
- Cleaning of grease, oils, and coolant can be achieved through a commercial degreaser treatment
- Dirt and other foreign debris should be wiped off
- Thermal oxides can be removed through grinding or acid pickling
Nitric Acid
| Stainless Steel Grades | Treatment | |
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| Chromium Nickel Grades (300-Series) | 20% volume Nitric Acid at 120/140 degrees F or 49/60 degrees C for 30 minutes | |
| 17%+ Chromium Grades (except 440-Series) | ||
| Straight Chromium Grades (12-14% Chromium) | 20% volume Nitric Acid + 3 oz/gal or 22g/L Sodium Dichromate at 120/140 degrees F or 49/60 degrees C for 30 minutes | 50% volume Nitric Acid for 30 minutes |
| High Carbon-High Chromium Grades (440-Series) | ||
| Precipitation Hardening Stainless | ||
Nitric Acid With Sodium Dichromate
- 5% weight Sodium Hydroxide at 160/180 degrees F (71/82 degrees C) for 30 minutes
- Water rinse
- 20% volume Nitric Acid + 3 oz/gal Sodium Dichromate at 120/140 degrees F (49/60 degrees C) for 30 minutes
- Water rinse
- 5% weight Sodium Hydroxide at 160/180 degrees F (71/82 degrees C) for 30 minutes (REPEAT STEP ONE)
- Water rinse
Contributors:
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