Understanding filtration › 6. Operating and purchasing

CHAPTER 6.1

Preparing a shutdown or an inter-campaign shutdown

A planned shutdown is the best opportunity to bring a filter back into good condition. It is also the most frustrating one when the cloths on order arrive after restart. Everything is decided weeks beforehand, on paper.

Nicolas Ract · 8 min read · October 2026

Taking stock

Before costing anything, you need to know what you have. For each filter:

• the number of plates, their type (chamber, membrane, head and tail plates) and their known condition;
• the references of the cloths in service, with their make-up drawing (see chapter 6.4);
• the new cloths in stock, their reference, batch and date of receipt;
• the associated wear parts: seals, membranes, fasteners, necks, backing cloths if the filter uses them.

The service life history (see chapter 4.5) completes this inventory: it shows the positions that fail earlier, the dominant reason for removal and the actual service life in the process unit (cycles, brews, tonnes, campaigns).

Deciding on quantities

There are generally three options:

• Replace the whole pack: the simplest to organise, and the only one that brings the whole filter back to the same level. It is justified when the remaining life of the cloths in service will not cover the next production period.
• Replace the positions at risk: those identified by the history. Cheaper in cloths, but the pack remains uneven.
• Replace nothing and inspect: possible when the history shows a comfortable margin.

Field experience: in a seasonal industry, the question to ask is not "are the cloths still holding up?" but "will they last the whole of the next campaign?". A change mid-campaign almost always costs more than cloths replaced a little early during the inter-campaign shutdown.

Anticipating lead times

A filter press cloth is not a standard item: it is cut and sewn to the plate drawing, often from a fabric woven specially or taken from limited stock. The lead time depends on the supplier, the fabric, the quantity and the season.

Field experience: count in weeks, and longer if the fabric has to be woven or if the order coincides with those from other sites in the same sector (all sugar factories prepare their inter-campaign shutdown at roughly the same time). Ask for the lead time in writing at the enquiry stage, and have it confirmed with the order.

Also allow time upstream: taking or checking the drawing, approving a sample or a first cloth, the internal purchasing process. On a first purchase or a change of reference, these steps often take longer than manufacturing itself.

Safety stock

During production, you need enough to replace a failed cloth without waiting for a delivery. The stock level depends on three things: the number of unplanned replacements observed over the period (the history, again), the cost of a stoppage and the replenishment lead time.

Field experience: store new cloths in their packaging, dry, away from direct light, heat and rodents, and apply "first in, first out". Label the reference and the destination filter clearly: two cloths with similar drawings are easily confused.

The shutdown checklist

A shutdown is also the time to check what wears the cloths. A new cloth fitted on a damaged plate will wear out as quickly as the previous one (see chapter 4.2).

When fitting, inspect both faces of the cloth and the plate surface, clean off product deposits and remove creases: this is a manufacturer's recommendation for cloth replacement [3].

Lockout and safety

A filter press combines several hazardous energies: electricity, closing hydraulics, membrane squeeze air or water, slurry under pressure, and sometimes heat or corrosive substances. The INRS (French national research and safety institute for the prevention of occupational accidents and diseases) describes lockout in four steps: isolate the equipment from its energy and fluid sources, dissipate residual energies (bleeding, draining, blocking), lock and tag the isolating devices, then verify the absence of energy and the identity of the equipment [1].

When an outside contractor works on site (cloth supplier, cleaning contractor, lifting), the regulations require a prior joint inspection and an analysis of interference risks. A written plan de prévention (prevention plan required under French law for work by outside contractors) is mandatory beyond 400 hours of work over twelve months or for work on the list of hazardous operations; the INRS recommends drawing one up even outside these cases [2]. Practical forms for formalising these steps are available in a guide from the Assurance maladie (French national health insurance, which also covers occupational risks) that supplements the INRS brochure [4].

The schedule

Field experience: a simple backward schedule, starting from the restart date, is enough in most cases.

• Several months before: review of the history, decision on quantities, supplier enquiries, drawing checks.
• As soon as the decision is made: order, with lead time confirmed in writing; order seals and membranes at the same time.
• A few weeks before: receipt and inspection of the cloths (reference, dimensions, make-up, quantity), preparation of the prevention plan, booking of lifting equipment and crews.
• During the shutdown: lockout, removal, plate inspection, fitting, entry in the history.
• At restart: close monitoring of the first cycles (leaks, filtrate clarity, cake discharge), and replenishment of the safety stock.