Bypass is the most expensive kind of filter failure because it is silent. A blinded screen announces itself with a rising pressure differential. A bypassing screen performs perfectly on every measured parameter while letting contamination through – until something downstream is damaged or a product fails.
How bypass happens
- A seal that is not seated. A fold, a twist or a particle trapped under the seal creates a path. This is the most common cause and it usually dates from installation.
- Over-tightening. Crushing a seal deforms it permanently, and it never seals properly again even when it is fitted correctly the next time.
- A scored or corroded seat. The housing seat itself is the sealing surface. Once it is damaged, no seal will close it.
- Wrong seal material. A seal that hardens, swells or shrinks in the process fluid loses compression and stops sealing. It may take weeks or months to show.
- A deformed frame. A screen frame that is bent or twisted will not sit flat, leaving a gap on one side.
- Pressure. At high differential, a poorly supported screen can deflect enough to lift away from its seat.
- Interrupted seats. Welds, machining marks or debris across the seat face create a channel.
Design features that reduce it
- Positive location – a groove, a shoulder or a step that holds the seal in position rather than relying on friction.
- A defined compression so the seal is compressed by a known amount rather than tightened by feel.
- Replaceable seals so a degraded seal can be changed without replacing the frame or the housing.
- Support on the downstream side so pressure pushes the screen onto its support and seat rather than off.
- A single flow path – every additional joint is a potential leak path.
Seal material selection
Silicone, EPDM and nitrile behave very differently across temperature and chemistry, and the wrong one fails slowly rather than immediately. Temperature range, chemical compatibility with the full stream including cleaning agents, and compression set are the three properties that matter.
Compression set deserves a mention: it is the tendency of a seal to stay deformed after compression is removed. A seal with poor compression set works once and then does not. If your housing seals work when new and fail on reuse, compression set is a likely cause.
How to detect bypass
You cannot detect it from pressure drop, because a bypassing screen often shows a lower differential than it should. The reliable methods are:
- Particle counting or analysis downstream, compared with upstream. This is the definitive test.
- Visual inspection of the seal and seat at every change-out, looking for a clean path where fluid has passed, which shows as a polished or stained track.
- A dye or tracer test on a clean system, where a path will show immediately.
- Comparison with a known-good housing running the same duty.
The visual check is the cheapest and it costs nothing if it is folded into the normal change-out routine. A track across a seal face is unmistakable once you know what you are looking at.
The other thing that goes wrong at change-out is orientation, covered here: installing a filter screen correctly.
What we can do
We supply PP filter screens with frames designed for positive seal location and defined compression, and we can supply seals matched to your temperature and chemistry. If you are seeing downstream contamination with normal pressure readings, describe the housing to us – bypass is the first thing we would check.