Seal Selection & Comparisons

Proper Selection of Seals 

There are a large number of seals that are available in industry, each with their own structure and intended function. Selecting the most appropriate seal based on the given system it is being applied to and the conditions it will face will ensure the seal can operate at peak performance and will last for many years without fail [2]. The best way of selecting a seal is to identify the specific system or device the seal is being applied to and then assessing the operating conditions to best match a seals characteristics that satisfy such conditions. Lastly, the selection can then be completed by reviewing costs to ensure the most efficient seal is chosen. If an incorrect seal is selected for the application, it can result in entire system/device failures such as a large leakage of fluids, explosions and high temperatures. 

Important Operating Conditions 

When choosing a seal, it is important to consider the operating conditions of the seal as they are factors that can dictate whether a certain type of seal is suitable for the specific operating environment and ensures that an ineffective seal is not used [2]. These operating conditions are factors that would directly correspond to the structure and material of the seal. A few of the most important operating conditions to take into consideration include [3]: 

• • Liquid 
  • Pressure 
  • Temperature 
  • Wear Resistance & Abrasion 
  • Reliability & Life Expectancy 

Liquid

Identifying the liquid and its properties that the seal may be in contact with during its operation is very important as the seal material must be able to withstand the fluid in contact. Such possible liquids that a typical seal may encounter are lubricants (oil or grease), salts, acids or abrasive fluids. Ensuring that the seal material is compatible with the liquid will make sure that the seal does not fail prematurely due to the liquid degrading the material. O-rings that are typically made from nitrile are not recommended to be used where strong acids are present but can be used around mineral oils whereas o-rings made from Ethylene Propylene are able to resist strong acids but are not recommended to be around mineral oils [4]. Liquid characteristics such as viscosity should also be taken into consideration. 

Pressure 

Pressure plays a major role in the function of various seals. As a result, the structure and material of the chosen seal should be able to withstand the internal pressure of the system that it needs to retain. If the pressure is too high, it can cause excessive wear on the seal's structure, causing it to deform and eventually fail, which can lead to a huge release of internal pressure. The material of the seal should also be able to handle constant cycles of low and high pressures (such as those experienced in cylinders) and the material should be able to prevent blow-by, which is where the fluid bypasses the seal and leaks out. The use of an incorrect head gasket in a internal combustion engine can cause the gasket to deform due to high pressure, causing it to fail completely. This can lead to the mixing of coolant in the combustion chamber [5].  

Temperature 

Temperature highly dictates the material that a seal can be made from for a certain application since some materials such as rubbers, cannot handle high temperature environments even with the presence of lubricants. Seals that are not used within the material's temperature range can cause the seal to degrade very quickly, which would cause the sealing affect to reduce greatly. This can eventually lead to leakage of fluids and outright system failure. It can also cause other components within the system to fail, such as shafts or bearings, as the seal is not generating a sufficient fluid film to keep such components cool. Seals made from typical synthetics are usually rated up to 110 celsius with some being more. Although, PTFE can be used over a wide temperature range from -240 to 260 celsius [2]. 

Wear Resistance 

The wear resistance of the material the seal is made from should be considered. Much like the factor of liquid, the material should be able to withstand any contact with fluid at any given range of operating temperatures. The seal materials should be impervious to corrosion from the sealed fluid especially at high temperatures, as sometimes high temperatures can cause fluids to become abrasive, and wear down the face of the seal. This can cause an improper sealing effect and reduce the lifetime of the seal itself [3]. 

Reliability & Life Expectancy 

Identifying the reliability standards of the company can affect the seal selection process as each company has their own standards of reliability when it comes to seal manufacturers [2]. This can influence the seal that is selected as the company may want a seal from a certain company that they trust. Additionally, life expectancy of the seal itself and its material composition should be considered. This includes the seal's shelf life and operational life. An example would be that the shelf life of a metal bellows seal is unlimited but a similar seal made from an organic material has much shorter shelf lifes [2]. 

Advantages & Disadvantages of Common Seals 

O-Rings

O-rings can be made from a range of materials and so each material has its own advantages and disadvantages. A few of the notable advantages of o-rings include their corrosion resistance, long shelf-life, its ability to be applied to a wide range of pressures, and it is a cost effective solution for most applications [6]. A few of the disadvantages come down to the material it is made from. Some rubbers are not able to withstand certain acids and some materials that have higher hardness (such as PTFE) and  lower seal-ability since the material cannot be easily compressed and conform to the mating surfaces [7]. 

Gaskets 

The advantages of gaskets depend on the material and shape the specific gasket is made from. A few of the common advantages of gaskets that make them a better choice over other seals are that they come in various shapes and sizes allowing for versatility when it comes to applications in various systems. Gaskets such as sheet gaskets can be cut from most synthetic rubbers or sheet metal into any configuration that is required. These seals are also better when two surfaces need to be mated together and can fill in the gaps between surfaces. They can also be more useful in a wider range of pressures and temperatures compared to most other types of seals [8]. 

Mechanical Seals 

There are many advantages that mechanical seals have. Some of the common advantages include their tight tolerances, meaning that they eliminate almost all leakage of fluids and can prevent most contaminants from passing through [9]. These seals also have a very long operational life due to their high durability, meaning they require minimal maintenance, compared to seals such as o-rings that should be replaced regularly. These seals can also handle a wide range of liquids and lubricants (including acids and salts). A major disadvantage of this seal is that its applications are very narrow, as they are made specifically to be used in pumps and compressors, compared to other types of seals that have a wider range of applications [10]. They also have a high initial cost due to their tight tolerances. 

Bellow Seals 

Bellow seals are specialized seals meaning they do not have a wide range of applications compared to gaskets or o-rings. Although, a few benefits of bellow seals are their long operational and shelf life, meaning they require minimal maintenance and have low operating costs. They also can withstand a range of harsher conditions such as corrosive chemicals and they can operate in a wide temperature range. This makes them a versatile seal that can be used in various operational conditions. Bellow seals have similar disadvantages to mechanical seals as they have high starting costs and and they have very specialized applications around pipes and shafts [11].

Rotary Shaft Seals (Lip Seals) 

These seals are most comparable to mechanical seals as they can be used in similar applications. Unlike mechanical seals, these seals are less complex in structure but cannot be used in a wide range of temperatures due to their mainly synthetic rubber construction. High temperatures can degrade the surfaces of the rubber and can make the seals wear out faster than expected. Although, they are still compatible with different types of lubricants and they have low friction properties much like mechanical seals, meaning they still effectively minimize friction between components. One unique advantage of this seal is the increased elasticity due to the spring sealing lip which allows the seal to actively prevent contaminants from passing through better than mechanical seals [12]. 

Labyrinth Seals 

Labyrinth seals are most comparable to rotary shaft seals (lip seals) due to their classification as a shaft seal. The main advantage of labyrinth seals is their complex path built into the housing that focuses on preventing contaminants and liquids from passing through the seal, better than any other shaft seal. Another advantage of is their ability to withstand more wear than a lip seal due to its non-contacting nature of the labyrinth seal, meaning reduced friction between other components as well. One major disadvantage of this seal is the high starting costs due to manufacturing such a complex seal as well as instillation [14]. 

V-Ring Seals 

The main advantage of v-ring seals is their simple one piece yet durable construction. They have strong grip strength onto rotary shafts and are very durable, being able to withstand most lubricants and some harsh chemicals. They also are advantageous since they can be combined with other shaft seals such as rotary shaft seals and protect them from abrasive contaminants and fluids. The major disadvantage of this seal is that it is mainly paired with another shaft seal and shouldn't be used alone as a primary seal on shafts due to the single piece structure [15].  

References

[1] Westshore Engineering, Seals. Accessed on: Feb. 25, 2021. [Online]. Available: http://www.westshoreengineering.com/seals

[2] Machine Design, Selection Factors for Seals, Nov. 15, 2002. Accessed on: Feb. 24, 2021. [Online]. Available: https://www.machinedesign.com/machine-design/article/21812738

[3] V. Frahm, 7 Considerations for Mechanical Seal Selection, Crane Engineering, Dec. 29, 2014. Accessed on: Feb. 24, 2021. [Online]. Available: https://blog.craneengineering.net/mechanical-seal-selection

[4] Macro Rubber & Plastics, O-Ring Chemical Compatibility Chart. Accessed on: Feb. 24, 2021. [Online]. Available: https://www.marcorubber.com/o-ring-chemical-compatibility-chart.htm#null=&chemId=93

[5] L. Castleman et al., The Do's and Don'ts of Seal Engineering, Machine Design, Aug. 20, 2014. Accessed on: Feb. 24, 2021. [Online]. Available: https://www.machinedesign.com/mechanical-motion-systems/hydraulics/article/21833677

[6] Cliff, The Advantages and Disadvantages of PTFE O-Rings, Eclipse Engineering Inc, May 22, 2018. Accessed on: Feb. 25, 2021. [Online]. Available: https://eclipseseal.com/blog/seals/advantages-disadvantages-ptfe-o-rings/

[7] Barnwell, Encapsulated O Ring Advantages. Accessed on: Feb. 25, 2021. [Online]. Available: https://www.barnwell.co.uk/encapsulated-o-ring-advantages/

[8] Dichtomatik, Gasket Types and Applications, Oct. 10, 2020. Accessed on: Feb. 25, 2021. [Online]. Available: https://www.dichtomatik.us/find-out-the-differences-between-different-gasket-types-their-possible-applications/

[9] Leak-Pack, Advantages/Disadvantages of Mechanical Seals. Accessed on: Feb. 25, 2021. [Online]. Available: https://leakpack.com/advantages-disadvantages-mechanical-seals/

[10] Cary Industry, Advantage and Disadvantage of Mechanical Seals, Apr. 15, 2014. Accessed on: Feb. 25, 2021. [Online]. Available: http://www.cary-seal.com/i-basics-of-mechanical-seal-754535/advantage-and-disadvantage-of-mechanical-seals-754948.html

[11] American Seal & Packing, Metal Bellows. Accessed on: Feb. 25, 2021. [Online]. Available: https://www.mechanicalseals.net/Metal_Bellows.htm#:~:text=Metal%20bellows%20seals%20feature%20a,with%20synthetic%20rubber%20sealing%20members

[12] AbbeySeals, Oil Seal (Lip Seal) vs. Mechanical Seal: Pros and Cons of Each, May 22, 2019. Accessed on: Feb. 25, 2021. [Online]. Available: https://www.abbeyseals.ie/oil-seal-lip-seal-vs-mechanical-seal-pros-cons-of-each/

[13] Cornell, Packing vs. Mechanical Seal. Accessed on: Feb. 25, 2021. [Online]. Available: https://cornellpump.com/packing-vs-mechanical-seal/

[14] W. Cash, How Labyrinth Seals Control Contaminant Ingression, Machinery Lubrication, Noria Corporation. Accessed on: Feb. 25, 2021. [Online]. Available: machinerylubrication.com/Read/29452/using-labyrinth-seals

[15] Allied Metrics Seals & Fasteners Inc, V-Rings. Accessed on: Feb. 25, 2021. [Online]. Available: https://www.alliedmetrics.com/products/o-rings/v-rings/