Understanding filtration › 4. Diagnosing a filter cloth

CHAPTER 4.3

Leaks at filter press joints

A filter press that "weeps" between its plates dirties the workshop, wastes product and wears the cloths along their edges. Before tightening harder, you need to know what is leaking, and why.

Nicolas Ract · 7 min read · October 2026

What is leaking: filtrate or slurry

A cloth is porous: through capillary action, some liquid migrates through the fabric to the edge of the plate, and the filtration pressure accentuates this seepage [1]. This liquid is generally clear, since it has passed through the cloth.

A cloudy liquid, loaded with solids, means that the slurry is passing between the plates without being filtered: this is a genuine sealing defect that needs to be dealt with.

Field experience: collect a little of each leak in a transparent container. The difference is immediately visible, and the photo will be useful for follow-up.

Cause 1: filter cake on the sealing face

This is the most common cause. Pieces of filter cake left on the sealing surface prevent the plates from closing properly and cause slurry leaks during filling [2]. Filter cake that stays stuck to the edges can also distort the plates and misalign the plate pack, leading to leaks in subsequent cycles [1].

Remedy: clean the edges at every opening, by hand or by rinsing, and ensure complete cake discharge (see chapter 4.2, cause 4).

Cause 2: hydraulic conditions

One technical source cites several causes related to the installation [1]:

• excessive back-pressure on the filtrate side;
• poorly designed filtrate discharge piping;
• pumped flow rates beyond what the filter is designed to accept.

Field experience: if leaks appear mainly at the end of filling, when the pressure rises, look first at the pump and the filtrate outlet before calling the cloths into question.

Cause 3: plates and closure

A chipped, scratched or encrusted sealing face, a warped plate or insufficient closing force lets the slurry through, always at the same spot. These defects are identified at shutdown, plate by plate.

Field experience: a leak that always returns between the same two plates, whatever the cloth, almost always points to the plate.

Cause 4: the cloth itself

• a cloth that is off-centre, creased or too small does not properly cover the sealing face;
• a worn or frayed edge no longer does its job;
• a cloth poorly suited to the process can encourage leaks [1].

For cloths without a gasket, a sealing coating (latex or synthetic rubber) can be applied to the edges: it limits capillary seepage and protects the edges from premature wear [1]. There are also plates and cloths with integrated gaskets, designed for a tighter seal; the choice depends on the filter and the process.

From symptom to cause

What about closing pressure?

Raising the closing pressure beyond the manufacturer's values may mask a leak for a while, but it loads the plates and crushes the cloths. Stick to the manufacturer's settings and deal with the cause.