Introduction to mechanical seals: concepts, operation and criteria

Mechanical seals are fundamental components in pumps and rotating equipment, responsible for preventing leaks and maintaining process integrity. Although they often go unnoticed, their correct selection can prevent failures, reduce maintenance costs, and ensure continuous and safe operation. In this blog, we explain, clearly and practically, how mechanical seals work, their main configurations, and the criteria you should consider when choosing the right seal for your application and operating conditions.

What is a mechanical seal?

A mechanical seal is a device used to prevent fluid leakage between rotating and stationary parts of machinery. This device provides a seal at the entry or exit point of a rotating shaft, where the shaft passes through a stationary housing.

How does a mechanical seal work?

The primary seal is essentially spring-loaded, consisting of two extremely flat faces, one fixed and one rotating, that move against each other. The seal faces are pushed together using a combination of hydraulic force from the sealed fluid and spring force from the seal design. The surfaces of the seal faces are overlapped to a high degree of flatness; typically 2–3 bands light (0.00003” / 0.0008 mm). In this way, a seal is formed to prevent process leakage between the rotating and stationary areas of the pump.

Characteristics of materials for mechanical seals

The materials used in mechanical seals, including the materials of the end faces of the rotating and stationary rings, are fundamental to their performance. Therefore, ensuring the parallelism of the sealing surfaces and selecting the correct materials are key to guaranteeing the long-term, stable operation of mechanical seals.
Mechanical seal materials considered qualified must possess the following characteristics:

• Mechanical resistance.
• Wear resistance
• High load capacity with excellent self-lubricating properties
• Good heat dissipation
• Resistance to thermal deformation
• Resistance to thermal cracking.
• Corrosion resistance.
• Low coefficient of linear expansion.
• Good dimensional stability.

Construction materials, springs

The spring is a critical component in a mechanical seal, as it provides the necessary force to maintain contact between the seal faces, accommodating wear and misalignment and ensuring reliable and effective sealing performance under various operating conditions. The most common construction materials include the following:

304 stainless steel

304 stainless steel is the most common and widely used stainless steel for mechanical seal springs. Because it contains nickel, it is more resistant to corrosion and heat than steel alloys. It is commonly used in household appliances, vehicle parts, the food industry, and more.

316 stainless steel

316 stainless steel is a type of steel containing molybdenum and has good resistance to chloride attack. Its corrosion resistance is superior to that of 304 stainless steel, and it also exhibits high resistance to corrosion in aggressive industrial and marine environments. It is primarily used in heat exchangers and food processing equipment.

Hastelloy C Alloy

Hastelloy C alloys are nickel-molybdenum-chromium alloys with added tungsten, designed for excellent corrosion resistance in a wide range of harsh environments. Hastelloy C alloys also exhibit good high-temperature resistance. They are widely used in demanding environments such as chemical processing, pollution control, pulp and paper production, and industrial waste treatment.

Construction materials for rotating and stationary surfaces

Rotating and stationary rings are the main components of a mechanical seal. These rings are pushed against each other, sealing the system or mechanism and preventing leakage. Stationary seats are non-rotating primary sealing rings housed within the stationary structural parts of rotating equipment. The rotating ring rotates with the main shaft. Common construction materials include the following:

Ceramics

Generally, it's a 99.5% aluminum oxide that offers excellent wear characteristics due to its hardness. It's chemically inert and can be applied to almost any product. However, ceramics cannot withstand thermal shock.

Silicon carbide

It is a bluish-black material created by fusing silica and coke. It belongs to the same family as ceramics (due to the silica content), but it has better lubricating qualities and is harder. The most common type is reaction-bonded silicon carbide. However, in chemical applications, sintered silicon carbide may be recommended. Silicon carbide also has the advantage that it can be re-lapped and polished for reuse.

Tungsten carbide

It is a very versatile sealing material, similar to silicon carbide. It is as hard as silicon carbide, but has a very heavy feel, making it easy to distinguish. It is ideal for high-pressure applications due to its high modulus of elasticity, which helps prevent face distortion. Tungsten carbide also has the advantage of being re-lapped and polished for reuse.

Carbon graphite (coal)

It is a black mineral with a metallic luster, composed of crystallized carbon. In mechanical seals, it is used as one of the most common surfaces due to its low friction, heat dissipation capacity, and self-lubricating properties, which even allow for some dry operation. It is typically manufactured from molded and baked carbon and graphite powders, then impregnated with resins or metals (such as antimony) to increase its strength and make it waterproof.

PTFE (Polytetrafluoroethylene)

It is a white polymer known for its high chemical resistance, thermal stability, and excellent self-lubricating properties. PTFE can operate in contact with almost any aggressive chemical, including acids, bases, and solvents, without degrading. Furthermore, its low coefficient of friction allows it to function easily against hard surfaces, although its mechanical and thermal strength is lower than that of ceramic or SiC materials. For these reasons, its use is primarily reserved for specialized chemical applications where absolute chemical compatibility and low friction are required.

Stainless steel (304/316)

It is a corrosion-resistant material with good mechanical strength. 304 fixed faces are an economical option for low-stress applications, while 316 is used in food-grade or aggressive chemical applications. Due to its limited wear resistance, stainless steel is typically combined with a softer face, such as graphite or PTFE. It is considered a relatively soft sealing face and is therefore not recommended for environments with solid particles. Its main advantages are durability, dimensional stability, and corrosion resistance.

Elastomers

Elastomers are critical components in a mechanical seal, acting as secondary sealing surfaces that ensure the seal maintains a tight fit against the equipment housing or shaft. Their elasticity allows them to accommodate misalignment, thermal expansion, and vibration, guaranteeing reliable performance under diverse operating conditions. Common materials include:

Nitrile (NBR)

This general-purpose material offers good resistance to mineral oils, greases, and fuels, though it has limited resistance to strong acids and polar solvents. Its operating temperature range is approximately -30°C to 120°C. It is ideal for oil pumps, hydraulic systems, water applications, and standard industrial processes, and is notable for its excellent cost-performance ratio in general-purpose applications.

EPDM

A synthetic elastomer known for its resistance to steam, hot water, and alkaline solutions, as well as dilute acids. It is incompatible with hydrocarbons and mineral oils. Its temperature range is -50°C to 150°C. It is primarily used in hot water pumps, steam, heating systems, and alkaline chemical processes, standing out for its excellent resistance to heat, ozone aging, and UV radiation.

Viton / FKM

High-performance elastomer with excellent chemical and thermal resistance, capable of withstanding strong acids, bases, organic solvents, and oils. Its temperature range is from -20 °C to 205 °C. It is used in aggressive chemical processes, hydrocarbons, petrochemicals, and pharmaceuticals, making it ideal for critical applications where maximum chemical compatibility and thermal stability are required.

Criteria for selecting a mechanical seal

Selecting the right mechanical seal depends on many factors related to the fluid, operating conditions, and the pump's characteristics. To ensure safe operation and a long service life, it is essential to carefully evaluate aspects such as fluid type, temperature, pressure, rotational speed, construction materials, and the type of seal required for the application. Proper selection prevents premature failure and ensures equipment efficiency.

Fluid type

The nature of the fluid is one of the most critical factors in selecting a mechanical seal because it directly determines material compatibility and sealing efficiency. The fluid's chemical properties, such as acidity, alkalinity, or corrosiveness, affect the durability of sealing faces, elastomers, and metal components; a chemically aggressive fluid can rapidly degrade unsuitable materials, leading to leaks and premature failure.
General recommendations based on the type of fluid being handled:

1. Clean, non-corrosive water
   1. Applications: water pumps, cooling systems, HVAC.
   2. Recommended seal: simple seals or elastomeric bellows, carbon, silicon, stainless steel, ceramic faces and NBR or EPDM elastomers.
2. Wastewater or water with abrasive particles
   1. Applications: water treatment plants, sludge pumps.
   2. Recommended seal: balanced or double seals, hard faces (SiC or tungsten), chemically resistant elastomers such as EPDM, VITON/FKM.
3. Acids and corrosive solutions (HCl, H₂SO₄, citric acid, etc.)
   1. Applications: chemical industry, metal processing, food.
   2. Recommended seal: double or balanced seals with SiC/SiC or Tungsten/SiC faces, FKM/Viton or FFKM elastomers for high chemical resistance.
4. Viscous or abrasive fluids (oils, syrups, industrial sludge)
   1. Applications: pumping syrups, lubricating oil, resins, molasses.
   2. Recommended seal: balanced or double seals, silicon carbide or tungsten faces, elastomers such as EPDM, VITON/FKM

Operating temperature

Temperature is another determining factor because it affects the integrity of the materials, but it primarily impacts elastomers, which have much lower thermal limits than sealing surfaces (carbon, silicon carbide) or metallic components such as stainless steel. While elastomers can degrade, lose elasticity, or crack at high temperatures, hard surfaces and metals maintain their integrity over a much wider range. Therefore, selecting the appropriate elastomer is critical: NBR or EPDM for low and medium temperatures, FKM/Viton for medium to high temperatures, and FFKM or bellows-type metal seals for very high temperatures. Secondarily, temperature also affects fluid viscosity and surface lubrication, which can lead to premature wear of hard materials such as sealing surfaces.

Operating pressure

Operating pressure primarily impacts the sealing faces and the load applied to them, making it one of the most critical factors in ensuring the tightness and durability of the mechanical seal. As fluid pressure increases, the contact force between the seal faces also increases, which can lead to various adverse effects if the seal is not designed to withstand these conditions. Common problems include accelerated wear of the faces, abrasion, and loss of flatness, all of which reduce sealing effectiveness.
The following is a guideline pressure range for mechanical seals. These indicative pressure values ​​are derived from practical industry experience and mechanical seal design criteria, not from specific regulatory standards.

1. Low pressure ≤10-12 bar
   1. Recommendation: Simple seals are usually sufficient, as the load on the faces is less.
   2. Examples: John Crane Type 21, Burgmann MG11 / MG12/MG13
2. Average pressure ≥ 12–25 bar
   1. Recommendation: There are unbalanced mechanical seals that allow a working pressure up to 15 bar, but it is recommended to consider balanced seals for applications above 13 bar to improve durability and reduce the load on the faces.
   2. Example: Roten R580 in balanced version, Roten 902 cartridge type seal
3. High pressure ≥ 25 bar
   1. Recommendation: It is necessary to use balanced or reinforced seals, which reduce the load on the faces and allow maintaining a watertight seal without compromising durability.
   2. Example: Roten EHS, Roten E, ROTEN 85E

Conclusion

Although not always visible, mechanical seals are essential for the smooth operation of many industries. From manufacturing plants to agricultural facilities, these small devices prevent leaks, protect equipment, and maintain efficient processes.
Choosing the right seal and providing proper maintenance can make all the difference: it reduces risks, prevents unexpected downtime, and saves time and money. For example, a well-selected seal can withstand corrosive fluids, high pressures, or extreme temperatures—vital in many industrial operations. Need personalized advice or more information on sealing solutions? Contact us, and our team of experts will be happy to assist you!
https://grindmel.com/collections/sellos-mecanicos

Sources:

• Mechanical Seals for Pumps: Principles and Applications – Heinz P.Bloch

• Pump Handbook – Igor J. Karassik

• Mechanical seal selection guide – EagleBurgmann

• Mechanical Seals for the Pumps Industry-Hydraulic Institute