The fact that silos frequently hold a vast amount of food products means that their cleaning and maintenance must be correctly managed. Best practice typically requires that silos are emptied and thoroughly cleaned at least once or twice a year. Silos containing sugar or other ‘coarse flow’ items are almost effectively self-cleaning in the short and medium term and require only an occasional deep clean. However, most powdered products can be susceptible to infestations from stored product insects, which can enter when product is delivered – live or as eggs – or afterwards, through any gaps, for example around the lid. The lid’s underside is a very likely infestation site, as any product there is not regularly agitated. Infestations can also occur in the ‘dead space’ at the ends of the screw conveyors fitted to many silos. Silos can only be cleaned once they are empty, to avoid wasted or contaminated product, meaning significant cleaning must fit around raw material deliveries and order deadlines.
Left over contents in silos can create various problems. Granules of hydroscopic or water-absorbing products (such as sugar) can clump together and create ‘bridging’ which blocks ready access to the area below and can physically stop powder flow if not removed. The only generally effective removal method is to chip it away from above. However, this can be potentially dangerous as the bridge can give way suddenly and can damage the conveyors and screw feed systems at the silo base.
Silos are normally cleaned dry, although if mould is present, pressure washing can be undertaken. If this is the case, then the silo should be blow-dried afterwards. Most silos have filters on the top, which must be cleaned and examined for soundness and any filter elements must be replaced if necessary. If any signs of infestation exist, a specialist pest control supplier should be called in immediately.
Smaller, liquid ingredient tanks tend to be used for edible oils, yeast and liquid chocolate. These tanks usually have an external water jacket with an agitator to keep contents moving. Problems can occur if the heat supply fails, as this will cause content solidification. Under normal circumstances, cleaning personnel do not enter these vessels, with cleaning carried out from the top access. Where product needs to be physically removed, access is gained using the tripod and winch method and a corresponding safe system of work.