Understanding filtration › 4. Diagnosing a cloth

CHAPTER 4.1

Reading a used cloth

A cloth that has just been removed is the best incident report you have. Before throwing it away, take ten minutes to read it: it often tells you where the problem lies, and sometimes that the problem is not in the cloth.

Nicolas Ract · 7 min read · October 2026

Before dismantling: what you will not be able to recover afterwards

Once the cloth has been removed and rolled up, some of the information is lost. First note down:

• the plate number and its position in the pack (head, middle, tail);
• the fitting date or, failing that, the approximate number of cycles;
• the appearance of the last filter cakes: thickness, moisture, discharge. A cake that is noticeably thicker in one chamber than in the next indicates a pressure imbalance [1];
• what was observed in service: leakage, cloudy filtrate, longer cycles, sudden failure.

Take a photo of the cloth in place, then once removed, laid flat, on the cake side and the plate side.

Step 1: look at it flat, then against the light

Spread the cloth out on a clean surface and look at it first under grazing light: holes, cuts, sharp creases and glazed areas show up. Then hold it up to a light source.

• Areas lighter than the rest: the fabric has thinned or opened up (wear, abrasion, overly aggressive washing).
• Darker or opaque areas: the fabric is loaded with particles or deposits (blinding, see chapter 4.4).

Field experience: comparing with a new cloth of the same reference, held alongside in front of the same light, makes these differences much more visible.

Step 2: locate each defect relative to the plate

Mark each defect on a sketch of the plate: neck and feed eye, sealing edge, drainage surface, seams and fastenings. A defect that follows the pattern of the plate (pips, ribs, edge) comes from contact with it; a defect unrelated to the geometry points more towards the process, the washing or the chemistry. The causes associated with each area are detailed in chapter 4.2.

Step 3: touch

• Supple like a new cloth: the fibre has not been attacked; look for a mechanical cause or blinding.
• Stiff, board-like, brittle: think first of temperature or chemistry (product, washing). Each fibre has its temperature limits [2].
• Hard crust on the surface or grit that grates under the fingers: deposit or embedded particles.
• Yarns unravelling, pilling: abrasion.

Step 4: two simple comparisons with a new cloth

Field experience, with no value as measurements but very telling:

• Dry weight: a blinded cloth, once dried, weighs noticeably more than a new cloth of the same reference. The difference gives an order of magnitude of what has remained in the fabric.
• Water flow-through: pour the same volume of water onto the same area of both cloths and compare the time it takes to pass through. Only the comparison counts, never the absolute value.

Step 5: the laboratory, when confirmation is needed

To decide between two hypotheses or compare references, a textile laboratory can measure:

• air permeability, using a standardised method (for example ISO 9237) [3], compared with that of the new cloth;
• tensile strength, to find out whether the fibre has lost its properties;
• the nature of a deposit, by chemical analysis, to choose a suitable wash.

The inspection sheet

What not to conclude too quickly

• One cloth does not make a diagnosis: if possible, examine two or three cloths from different positions.
• Changing cloth reference is not a default remedy: if the cause is a damaged plate or a setting, the new cloth will fail in the same place.
• A new cloth that fails quickly almost always points to the installation or the process, rarely to a manufacturing defect.