Guide: Mechanical seals – carbon vs. Sic vs. Cer vs. TC

Are you experiencing premature pump failure or struggling with repeated leaks in pumps or agitators? When choosing materials for your mechanical seal, there are two things that need to fall into place: the mechanical seal itself must fit your pump or equipment, and the material combination on the rotating part and stationary parts must match your process. To ensure optimal operation, you need to know the difference between the four primary materials: carbon, silicon carbide (SiC), ceramic (Cer) and tungsten carbide (TC). 

The wrong choice can lead to production stops and costly downtime. Making the right choice is about finding the right balance between dimensions, sealing surfaces and the gaskets that protect both your equipment and your end product from contamination. 

Before we focus on the materials, the foundation must be in order. The first step is always to ensure that the mechanical seal is designed for the specific equipment in which it is to be installed. 

Mechanical seals are often designed for specific pump and shaft dimensions and only work when the dimensions, seat and shaft are suitable for the application in question. If the parts do not fit together, even the strongest solution will fail over time. Only when basic compatibility is in place does it make sense to combine the materials on sealing surfaces and gaskets. 

When we talk about material selection, we are referring to the material that makes up the stationary part and the rotating part. It is not just a matter of preventing an acute leak, but of ensuring the long-term operational reliability of the plant. By matching the properties of the material to your operating conditions, you can minimise the risk of costly production stoppages and avoid frequent replacements. 

Carbon is a relatively soft material with low friction and good chemical resistance. It is the standard solution in many applications and is well suited for clean, non-abrasive media at moderate pressures and temperatures. 

In practice, carbon is often used as a rotating sealing surface against a harder stationary surface made of stainless steel, ceramic or silicon carbide. In many hygienic applications, this ensures stable and gentle operation. However, remember that carbon should never be used with carbon, as this causes wear. 

Silicon carbide is an extremely hard, wear-resistant and corrosion-resistant material. It is designed to handle high pressures, extreme temperatures and media containing particles or crystals. 

Do you work with sugary products? Then you know that they have a risk of crystallisation at the sealing surfaces. Here, sealing surfaces made of Sic will typically provide a significantly longer service life. Sic against SiC is also often preferred in chemically demanding pharmaceutical and CIP processes due to the material's chemical resistance. 

Ceramic is a very hard material that is often used as the stationary counterpart to a rotating carbon surface. It is an economically attractive solution for less demanding tasks, such as transporting clean water or thin liquids at moderate temperatures. 

Although Cer has a high surface hardness, it is also brittle in shape. This means that it is sensitive to thermal shock and mechanical impact. In processes with powerful CIP or SIP, where the temperature rises rapidly, ceramics may be at risk of cracking. Therefore, we often see the material used in standard pumps rather than in the most critical process steps. 

Tungsten carbide, also known as hard metal, is characterised by its extreme resistance to mechanical impact. Where Sic and Cer can be brittle, TC is far more robust against vibrations and pressure surges in the system. 

The material is often found in the refrigeration industry, specifically in pumps for ammonia, or in the chemical industry for media under high pressure. TC is the obvious choice when operating conditions require a material that does not break under mechanical stress but still retains a very high wear resistance. 

Based on your operating conditions, you can consider the friction pair itself as either ‘soft vs. hard’ or ‘hard vs. hard’. If the materials on the sealing surfaces do not match your process, failures will occur quickly. In practice, incompatibility means that you risk sudden shutdowns in the middle of production, where every hour of downtime is costly. 

This combination is well suited for clean or slightly contaminated media where the load is moderate. The advantage is low friction and minimal heat generation, which protects both the sealing surfaces and the axle. This is the typical solution in many liquid processes in dairies and breweries.

Do you instead need a robust solution for media with particles, crystals or high pressure? With this combination, you get maximum wear resistance and long service life in harsh environments. It is particularly well suited for repeated breakdowns on softer materials, e.g. in the production of syrups or in chemically demanding pharmaceutical processes.

You may have chosen the most optimal sealing surfaces, but if the packing fails, the pump will still leak. The soft components – the gaskets– are just as important for the functionality of the mechanical seal. 

Even if the friction pair fits together, the mechanical seal can have a significantly shorter service life if the packing material itself cannot withstand the process medium or cleaning chemicals. Therefore, your choice of NBR, EPDM, viton/FKM or PTFE must always be assessed in conjunction with sealing surfaces, media and cleaning routines. 

• NBR: NBR is suitable for oil and fatty media at moderate temperatures. But be aware: NBR is less suitable for hot water/steam systems and aggressive CIP media. 
• EPDM: Often the first choice for water, steam and CIP-based processes in food and pharmaceutical plants. It has good resistance to cleaning agents and steam. 
• Viton/FKM: Choose Viton/FKM for aggressive chemicals, high temperatures and oil/grease-based media. The material is ideal for chemical processes or CIP cleaning. 
• PTFE: Used where extreme chemical resistance and temperature resistance are required, e.g. in aggressive chemical environments. PTFE is less elastic than rubber, but extremely resistant. 

We know that it can be difficult to navigate the different materials and combinations when choosing a mechanical seal – but don't worry. If you are unsure about how the materials interact with the different packing materials in your particular process, it may be beneficial to get someone else's perspective on the operating conditions. 

Do you need an assessment of whether your current solution is optimally composed, or are you faced with choosing materials for new equipment? Then feel free to contact us on +45 7020 0422 or send an email to inquiry@alfotech.eu. We are happy to advise you on both material selection and specific sealing solutions that match your process.