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 often overlooked, their correct selection can prevent failures, reduce maintenance costs, and ensure continuous and safe operation. In this blog, we will explain, in a clear and practical way, how mechanical seals work, what their main configurations are, and what criteria you should consider to choose the right seal according to the application and working conditions.

What is a mechanical seal?

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

How does a mechanical seal work?

The primary seal is essentially spring-driven, consisting of two extremely flat faces, one stationary 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 lapped to a high degree of flatness; typically 2-3 light bands (0.00003” / 0.0008mm). This creates a seal to prevent process leakage between the rotating and stationary areas of the pump.

Characteristics of materials for mechanical seals

Mechanical seal materials, including rotating and stationary ring face materials, are the foundation of mechanical seal performance. Therefore, ensuring the parallelism of the sealing surfaces and choosing the correct materials are key to ensuring the long-term stable operation of mechanical seals.
Qualified mechanical seal materials must possess the following characteristics:

• Mechanical strength.
• 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 for 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. Among the most common construction materials are the following:

Stainless Steel 304

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

Stainless Steel 316

Stainless steel 316 is a type of steel that contains molybdenum and has good resistance to chloride attack. Its corrosion resistance is superior to that of 304 stainless steel, as well as high corrosion resistance in aggressive industrial and marine atmospheres. It is mainly used in heat exchangers, food processing equipment.

Hastelloy C Alloy

Hastelloy C alloys are nickel-molybdenum-chromium alloys with tungsten addition designed to have excellent corrosion resistance in a wide range of severe environments. Hastelloy C alloy also has good high-temperature resistance. It is widely used in the most severe environments such as chemical processing, pollution control, pulp and paper production, and industrial waste treatment.

Construction materials for rotating and stationary faces

The rotating and stationary rings are the main components of a mechanical seal. These rings are pushed against each other to join the system or mechanism to prevent leakage. Stationary seats are non-rotating primary sealing rings housed in the stationary structural parts of the rotating equipment. The rotating ring rotates with the main shaft's rotation. Among the most common construction materials are the following:

Ceramic

Generally, 99.5% aluminum oxide, which offers excellent wear characteristics due to its hardness. It is chemically inert and can be applied to almost any product. However, ceramic cannot withstand thermal shock.

Silicon Carbide

It is a bluish-black material created by melting silica and coke. It belongs to the same family as ceramic (due to silica), but has better lubricating qualities and is harder. The most common 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 relapped and repolished for reuse.

Tungsten Carbide

It is a very versatile sealing material like silicon carbide. It is very hard like silicon carbide, but it is very heavy to the touch, making it easy to distinguish from silicon carbide. Ideal for high-pressure applications due to a high modulus of elasticity that helps prevent face distortion. Tungsten carbide also has the advantage of being able to be relapped and repolished for reuse.

Carbon Graphite (Carbon)

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

PTFE (Polytetrafluoroethylene)

It is a white polymer known for its high chemical resistance, thermal stability, and excellent self-lubricating properties. PTFE can work in contact with almost any aggressive chemical product, including acids, bases, and solvents, without degradation. In addition, its low coefficient of friction allows it to function easily against hard surfaces, although its mechanical and thermal resistance is lower than that of ceramic or SiC materials. For these reasons, its use is mainly reserved for special 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-demand applications, while 316 is used in food or aggressive chemical applications. Due to its limited wear resistance, stainless steel is usually combined with a softer face, such as graphite or PTFE. It is considered a relatively soft seal face, so it is not recommended in media with solid particles. Its main advantages are durability, dimensional stability, and corrosion resistance.

Elastomers

Elastomers are critical components in a mechanical seal, as they act as secondary sealing surfaces, ensuring that the seal maintains a tight fit against the housing or shaft of the equipment. Their elasticity allows for accommodation of misalignments, thermal expansions, and vibrations, guaranteeing reliable performance under various operating conditions. Among the most common materials are:

Nitrile (NBR)

General-purpose material with good resistance to mineral oils, greases, and fuels, although limited against strong acids and polar solvents. Its temperature range is approximately -30°C to 120°C. It is ideal for oil pumps, hydraulic systems, water, and standard industrial processes, standing out for its excellent cost-performance ratio in general applications.

EPDM

Synthetic elastomer known for its resistance to steam, hot water, and alkaline solutions, as well as diluted acids. It is not compatible with hydrocarbons or mineral oils. Its temperature range is between -50°C and 150°C. It is mainly 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 pharmaceutical industries, being ideal for critical applications where maximum chemical compatibility and thermal stability are required.

Criteria for selecting a mechanical seal

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

Fluid type

The nature of the fluid is one of the most critical factors in the selection of a mechanical seal because it directly determines the compatibility of the materials and the sealing efficiency. The chemical properties of the fluid, such as its acidity, alkalinity, or corrosive capacity, affect the durability of the sealing faces, elastomers, and metallic components; a chemically aggressive fluid can rapidly degrade unsuitable materials, leading to leaks and premature failures.
General recommendations based on the type of fluid to be handled:

1. Clean, non-corrosive water
   1. Applications: water pumps, cooling systems, HVAC.
   2. Recommended seal: single or elastomeric bellows seals, carbon, silicon, stainless steel, ceramic faces, and NBR or EPDM elastomers.
2. Wastewater or abrasive particles
   1. Applications: wastewater treatment plants, sludge pumps.
   2. Recommended seal: balanced or double seals, hard faces (SiC or tungsten), chemical-resistant elastomers such as EPDM, VITON/FKM.
3. Acids and corrosive solutions (HCl, H₂SO₄, citric acid, etc.)
   1. Applications: chemical industry, metal treatment, 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 sludges)
   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

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

Operating pressure

Operating pressure primarily impacts the sealing faces and the load applied to them, being one of the most critical factors for 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 several adverse effects if the seal is not designed to withstand these conditions. Among the most common problems are accelerated wear of the faces, abrasion, and loss of face flatness, reducing sealing effectiveness.
Below is an indicative pressure range for mechanical seals. These indicative pressure values come from practical industry experience and mechanical seal design criteria, and not from a specific regulatory standard.

1. Low pressure ≤10-12 bar
   1. Recommendation: Single seals are usually sufficient, as the load on the faces is lower.
   2. Examples: John Crane Type 21, Burgmann MG11 / MG12/MG13
2. Medium 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 tightness without compromising durability.
   2. Example: Roten EHS, Roten E, ROTEN 85E

Conclusion

Although not always seen, mechanical seals are essential for many industries to run smoothly. From manufacturing plants to agro-industrial plants, these small devices prevent leaks, protect equipment, and keep processes efficient.
Choosing the right seal and performing proper maintenance can make all the difference: it reduces risks, prevents unexpected shutdowns, and saves time and money. For example, a well-selected seal can withstand corrosive fluids, high pressures, or extreme temperatures, which is vital in many industrial operations. Do you need personalized advice or more information on sealing solutions? Contact us and our team of experts will be happy to help 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