Filters are necessary to remove cement dust from the excess air leaving the silo during the filling operation. The air filter is often located on top of the silo with extracted dust allowed to drop into the silo below. Easier access to the filter can be provided by placing the filter closer to ground level and connecting it to the top of the silo with a feed pipe of 200mm diameter. Extracted cement dust can then be conveyed to the weighing hopper at ground level. Most modern filters are self-cleaning pressure filters. While a minimum of attention is required with these filters, it is recommended that a weekly check of the filter efficiency be made using the electronic control unit located at ground level. A visual inspection of the filter bags should also be undertaken every 2-3 months, depending on the frequency of use. Older static type filters (i.e. non-fan-assisted) require a greater degree of maintenance if they are to perform properly. The filter bag should be inspected and cleaned (as necessary) at least once per week. This avoids blockages which would cause excessive dust and dangerous internal pressures during filling. An air filter should be large enough to handle the maximum air flow experienced through it. For normal road tankers, operating at pressures of up to 200kN/m2, an air flow of up to 20m3/min is expected. An extra allowance of 10m3/min should be added for filtration of air from the silo discharge point. A total filter capacity of 30m3/min is therefore recommended as a minimum.   The cement filling pipe attachment is best located 1 metre above ground level for easy access by the truck operator. The cement weigh hopper requires accuracy to within 3% at all times. Hardened cement in the hopper can affect the weighing accuracy and so the hopper should be regularly cleaned and calibrated. Filling pipes should be arranged such that excessive horizontal runs and/or tight bends of less than 1 metre radius are avoided. The filling pipe should enter the top of the silo at a tangent. If horizontal runs are necessary they should be kept at ground level where air pressures are highest. The total length of the delivery pipe should be kept to a minimum. Overfilling can cause severe dust emissions from the silo. The structural soundness of the silo may also be put at serious risk posing a threat to personal safety. The inclusion of a high level alarm is recommended to avoid the consequences of overfilling. A pressure relief valve is necessary should excessive air pressures build up in the silo. A valve of 100mm diameter and responding to a pressure of 5kN/m2 is sufficient.

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Irish Cement Ltd., Platin, Drogheda, Co. Louth, Ireland. Tel: ++ 353 41 987 6000 Fax: ++ 353 41 987 6400 email: info@irishcement.ie