A guide to silo cleaning in the food production sector

The size of silos - and the fact that they frequently hold several thousand pounds'worth of product - means cleaning and maintenance must be correctly managed.Best practice typically requires that silos are emptied and thoroughly cleaned at least once and usually twice annually.Bob Cannell of Hygiene Group takes a closer look at what this process involves

Silos containing sugar or other 'coarse flow' items are almost effectively self-cleaning in the short and medium term, needing only occasional deep cleaning.However, most powdered product is potentially 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 product there is not regularly agitated. Infestations also commonly occur in 'dead space' at the ends of the screw conveyors fitted to many silos.

Mould can also be a problem, with heat generated by product movement. Its effect can be worsened if the silo is in a windy area.

Silos can only be cleaned when empty, to avoid wasted or contaminated product, meaning significant cleaning must fit around raw material deliveries and order deadlines.

Left-over contents can create other problems.Granules of hydroscopic or water-absorbing products like sugar can clump together, creating 'bridging'which blocks ready access to the area below, and if undetected and left unremoved, can physically stop powder flow.The only generally effective removal method is to chip it away from above - potentially dangerous as the bridge can give way suddenly,damaging conveyors and screw feed systems at the silo base.

The cleaning process A full risk assessment must be undertaken before cleaning commences, with a safe system of work devised, and an escape plan in case any safety issues arise.Good practice also requires preparation of a report on the silo's condition, including 'before' and 'after' images, details of any infestations, and its state of repair.

Most silos have a hatch in the lid allowing access. Before work commences, the 'breathability' of air within must be measured.Work must not begin if the air is unfit. Even if it is of an acceptable standard, cleaners - who must work in teams of at least three - must always wear dust masks.

Access is typically gained via a tripod and winch, although older silos may require specialist rope access methods.All operatives must have received accredited training in working in confined spaces.

The first stage is usually to vacuum down interior surfaces. Some powdered products are potentially explosive if stored or handled incorrectly, so all tools and equipment must be spark-proof, from plastic scrapers through to vacuum cleaners which can be airpowered and must be fitted with anti-static hoses.Any metal product must be phosphor bronze or copper, while lights must be sealed and cleaning operatives must wear disposable paper suits.All equipment is subject to the requirements of ATEX, the name commonly given to two European Directives for controlling explosive atmospheres (further information is available at www.hse.gov.uk).

Silos are normally cleaned dry, although pressure washing can be undertaken if mould is present. If the latter, blow drying is required immediately afterwards.Virtually all silos have filters on top, which must be cleaned and examined for soundness,with filter elements replaced if required. If any signs of infestation exist, a specialist pest control supplier should be immediately called in.

Smaller, liquid ingredient tanks are widely 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, causing 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.

Upon completion of cleaning, vessels are normally sanitised and environmental swabs taken to validate micro-integrity.

Because of the safety issues surrounding silo cleaning and its specialist nature,many companies outsource it to a food facilities cleaning specialist.

This can not only prove more cost-effective, but reduces risk to the company with the job undertaken at the most convenient time.Many contractors have dedicated silo cleaning teams and will more than likely have encountered any common issues before, enabling them to recommend the most efficient and cost-effective solution and to guarantee legal compliance and safe, clean production.