When discussing cartridge filtration and process filtration systems, most conversations focus on micron rating, filter media, flow rates, differential pressure, or chemical compatibility. However, one critical component is frequently overlooked: the cartridge filter endcap configuration.
In industrial filtration systems, the end fitting of a filter cartridge is the only seal between the filter and the housing. If that seal fails, process fluid can bypass the filter media entirely, making even the highest efficiency membrane filter ineffective.
For critical filtration applications — especially membrane filtration below 5 micron — cartridge endcap selection becomes just as important as the filter media itself.
During recent customer site visits, we identified several final filtration systems using 0.45 micron and 0.2 micron membrane filters installed in double open ended (DOE) configurations.
While DOE cartridge filters remain widely used across industrial water filtration and general process filtration applications, final microfiltration and sterile-grade filtration typically require higher sealing integrity.
For membrane filtration applications below 5 micron, single open ended (SOE) cartridge filters are generally considered best practice due to:
Cartridge filter endcaps are the connection points between the filter cartridge and the filter housing.
Their primary purpose is to ensure all process fluid passes through the filtration media without bypassing around the cartridge.
In industrial filtration, there are two primary cartridge configurations:
DOE filters are open at both ends and rely on:
The seal integrity depends heavily on:
DOE cartridge filters are commonly used in:
The biggest weakness of DOE filtration systems is bypass risk.
The slightest damage to a housing knife-edge seal can allow fluid to bypass the filter media completely.
This issue is especially common in:
Bypass has been demonstrated at differential pressures around 1.8 bar where sealing pressure depends entirely on the installer’s applied force.
As filtration ratings become finer, this becomes increasingly critical.
Single Open Ended (SOE) filters use engineered sealing mechanisms such as:
Unlike DOE filters, SOE cartridge filters lock securely into the housing and create a controlled, repeatable sealing arrangement.
SOE filters are widely used in:
Membrane cartridge filters are often used for critical process filtration applications where absolute-rated retention performance is essential.
These include:
In these systems, even minimal bypass can completely compromise filtration performance.
A 0.2 micron membrane filter can only achieve true retention performance if the entire filtration system maintains integrity throughout operation.
That includes:
This is why most membrane filter manufacturers primarily produce membrane cartridges in SOE configurations.
The Code 2 filter uses:
This configuration has become an industry standard for:
Advantages include:
Code 3 filters feature:
These are commonly used in:
Benefits include:
Code 7 filters are considered one of the most secure and widely accepted process filtration configurations.
They feature:
Code 7 filters are widely used in:
The fin adaptor helps stabilise cartridges within multi-round housings and improves installation consistency.
Code 8 filters combine:
These are commonly used in:
They provide stronger sealing security than DOE systems while maintaining simpler installation.
One of the most overlooked issues in industrial filtration is supplier lock-in caused by proprietary filter end fittings.
Some filter housing manufacturers intentionally design non-standard cartridge fittings that only accept their own replacement filters.
This creates significant long-term risks:
A common tactic is supplying free starter filters with a housing purchase, effectively locking customers into a single supplier for future replacement cartridges.
The “A” end fitting uses non-standard BS224 seals and is only compatible with specific housings.
While technically effective, this design creates dependency on a single filter supplier.
For new filtration system installations, it is generally recommended to use universally available standards such as:
This ensures long-term sourcing flexibility and competitive filter pricing.
DOE cartridge filters are not inherently wrong.
In fact, they remain highly effective in many industrial filtration applications including:
DOE systems remain popular because they are:
However, as filtration ratings become finer, sealing integrity becomes increasingly important.
As a general rule:
One of the biggest misconceptions in industrial filtration is that micron rating alone determines filtration performance.
In reality, effective process filtration depends on:
A membrane filter is only as effective as the sealing arrangement surrounding it.
Because in filtration, performance is not just about what the filter media can remove — it is also about ensuring the process fluid has no way around it.
Many industrial process filtration systems evolve over time through:
As a result, it is common to find membrane filtration stages operating in housings originally designed for general DOE filtration.
A professional filtration system review can help identify:
Endcaps may appear to be a small component within a cartridge filtration system, but in critical process filtration applications, they play a major role in system integrity and filtration performance.
As micron ratings become finer, sealing security becomes increasingly important.
For membrane filtration systems — particularly below 5 micron — selecting the correct cartridge endcap configuration is essential for:
In industrial filtration, true performance is achieved not only by the filter media itself, but by ensuring there is absolutely no bypass path around it.
If you'd like your filtration system reviewed to make sure it's optimal, reach out to PoreFiltration and we'd be happy to come and visit your site.
And if you have any other questions about endocaps or membrane filters then give us a call or send us an email - we’d be more than happy to help.
And here are a few more blogs and links that you might find useful:
PoreFiltration – Making your filtration systems work harder