High-temperature process filtration: How to match the seal, media and cycle

In a filtration process, it's not the steel you need to worry about. A stainless steel filter housing can withstand 150 °C without any problems. The small union gasket that keeps the joint tight operates under exactly the same conditions – but with entirely different material properties at play. It can fail long before the steel shows any signs of wear. So when the filter does start to leak, it's rarely due to the steel. It's the seal. 

In our ultimate guide to filtration in the process industry, we reviewed common filter types and the general recommendations. Here, we go a step further. We look at what actually happens to the seal as the temperature rises – from typical temperature limits and material selection to thermal expansion and practical recommendations. 

Before we turn to the materials, it's worth bearing the actual figures in mind. In the process industry, operations typically take place within four quite distinct temperature ranges, and each of these places its own demands on the filter: 

• Pasteurisation: Typically 85–90 °C for high-temperature pasteurisation. The filter is rarely located within the unit itself, but immediately before or after it.  
• CIP cleaning: Typically 75–85 °C using caustic soda or acid. Runs daily or several times a day. The load is cyclical, not constant. 
• Clean steam and SIP: Up to 140–150 °C. The most demanding thermal stress the filter typically encounters. 
• Hot water treatment: 80–95 °C, often used as an alternative to chemical CIP in dairies. 

The point is that it's not just the peak temperature that matters. It's the combination of temperature, chemical load and the number of cycles over time that determines how long the seal will last. 

The rubber materials are the filter’s thermal bottleneck, and the choice between them sets the limit for what the filter can withstand. The four common materials – EPDM, NBR, FPM and PTFE – each have their own temperature range and strengths: 

EPDM is the standard choice in the vast majority of food applications and strikes a balance between heat resistance and chemical resistance to CIP liquids. However, whichever material you choose, you must specify the seal based on two parameters simultaneously. If you select based solely on the medium, you may overlook the thermal limit. If you select solely on the basis of temperature, the medium may degrade the seal at the very temperature it would otherwise have withstood. 

All materials expand when heated, but by no means at the same rate. Rubber expands approximately 10 times more than steel for the same temperature rise, and it's precisely this difference that creates the hidden stress in your joint. 

When heated, the elastomer is pressed more firmly against the sealing surface, and the sealing force increases. It's rarely a problem in itself. However, as it cools, the elastomer contracts more quickly than the steel, the sealing force decreases, and the risk of leakage is greatest during this transition phase. After a certain number of cycles, the elastomer no longer springs back. This is known as permanent deformation (compression set), and the result is that the seal no longer closes properly – not even when the system reaches operating temperature. 

A calculation example illustrates this: If your filter undergoes two daily CIP cycles at 85°C followed by cooling to 5°C, the seal undergoes approximately 700 thermal cycles per year. It's the sort of stress that wears down seals, even when they remain within their specifications. That is why the number of cycles is a better measure of your replacement interval than the calendar itself. 

The industries we typically work with subject their filters to their own unique combinations of temperature, chemistry and cycle frequency – and this influences the choice of material. In breweries, the process itself is moderate, with wort boiling at around 100 °C and cold fermentation and storage tanks at the other end of the scale. It's the daily CIP at 75–85 °C that's the major factor causing wear on the seals. 

In dairies, the CIP temperature is similar to that in breweries, but the chemistry is different. Milk scale requires more aggressive caustic cleaning than beer residues, and pasteurisation – particularly UHT (ultra-high temperature) at up to 135 °C – imposes a higher thermal limit. EPDM remains the standard choice in both sectors, but in dairy lines with frequent UHT or particularly aggressive CIP, the material is pushed closer to its limits, and replacement intervals often need to be shorter. Here, union gaskets with screens are an obvious solution, as the seal and filter element are replaced simultaneously. 

In the pharmaceutical and bioprocess sectors, the picture is different. Here, it's not cleaning but sterilisation that sets the thermal limit. SIP with saturated steam typically runs at 121–135 °C for a minimum of 15–30 minutes per cycle, which brings EPDM close to its upper limit. In supply lines carrying pure steam, FPM or PTFE are therefore often the obvious choice, as they have more headroom up to the operating temperature and better withstand repeated exposure to steam. If you choose EPDM for SIP applications, the supplier should document the service life for the specific cycle time, temperature and frequency you are working with. Otherwise, you risk reaching the material’s limit sooner than expected.

Choosing the right filter for high temperatures starts with three decisions you can take to the supplier: 

1. Specify the thermal cycle, not just the peak temperature: Explain what the filter is exposed to over an entire operating cycle in your plant – heating, operation, cooling and standby. 
2. Match the sealing material to both the medium and the temperature: Request documentation of compatibility at the actual operating temperature, not at 20 °C. 
3. Replace seals based on cycles, not calendar time: Tailor the maintenance schedule to the plant’s actual thermal load – in many dairies, this means short intervals compared to a brewery. Elastomers are wear parts and should be included in your spare parts stock so that you don’t end up with a leak and an empty shelf. 

The filter housing easily withstands high temperatures. However, it's the seal itself that determines how long your filter can withstand the temperature without failing. And the choice between EPDM, NBR, FPM and PTFE is just as much about the cycle as it is about the peak temperature. 

If you’re unsure about material selection or temperature tolerance, you’re more than welcome to get in touch with us. Call us on +45 7020 0422 or email inquiry@alfotech.eu, and we’ll help you find the right solution. 

Choose FPM when the operating temperature exceeds 150 °C, or when the medium is chemically aggressive in a way that EPDM cannot withstand. For SIP above 121 °C and in combinations of high temperature and oil or grease, FPM is often the right choice for your process. Please note, however, that FPM is not compatible with all CIP liquids. Hot, concentrated caustic soda, in particular, can be a problem. 

Yes, provided you have selected the appropriate material. EPDM can withstand 85 °C without any problems within its specification, but it's the cyclic stress that causes wear – not the temperature itself. With two cycles a day, you will easily reach 700 cycles a year, and even a seal operating within its temperature range will suffer permanent deformation over time. Monitor the differential pressure and plan your replacement intervals based on the number of cycles. 

There is no single standard interval, as the load varies so much from plant to plant. The rule of thumb is that your replacement interval should be based on the number of cycles and on observations: differential pressure, signs of deformation, hardening or leakage on start-up.  

Yes. SIP typically runs at 121–140 °C, pushing EPDM right to its limit. If you carry out repeated SIP cycles, you should consider FKM or PTFE, and the material should be verified for the specific cycle you are running. At the same time, thermal expansion becomes more pronounced, and the risk of leakage during the cooling phase increases.  

Yes, thermal shock is a real strain. When a hot filter is exposed to cold water at the end of the CIP programme, the elastomer contracts much faster than the steel, and this accelerates permanent deformation in the seal. Over time, micro-cracks may also develop in the welds. You can rarely avoid this sort of shock entirely, but you can account for it by using sealing materials that can withstand the temperature differences encountered in your production process.