Section 160: bulk cement storage depots
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Bulk cement storage depots are subject to co-ordination procedures in accordance with Rating Manual - section 6 part 1: Practice Note 1, and the relevant Practice Note to this Section.
The function of depots within this class is to receive both bagged and bulk cement, from cement works, to be stored prior to being delivered to contractors and merchants. Since most supplies come from the extraction and production locations by train, depots on sites usually of around 0.5 to 1 hectare, are often located where rail siding facilities are available and where good road links exist for speedy distribution to customers. In some instances depots to supply concrete batching plants for specific major civil engineering projects are used but these are generally mobile in nature.
3. Survey Requirements
The basis of measurement for this class is Gross Internal Area (GIA). Reference should be made to the VO Code of Measuring Practice for Rating Purposes in England and Wales.
4. Basis of Valuation
In the absence of rack rents, and because the specialised nature of bulk cement storage depots precludes comparison with other rented hereditaments, the preferred method of valuation for this class will be the contractors basis and reference should be made to the appropriate sections of the Rating Cost Guide. In particular, Item Nos 43500A to 43580Z (see Containers – Silos Bunkers Hoppers) of the Guide gives guidance on the costs of silos based on a price related to the volume of the silo. This should be the approach adopted, but in some cases only the capacity of the silo in tonnes will be known. Since bulk cement can be in the range of 1.28 to 1.44 tonnes per m3 the use of a conversion factor should be avoided, but where necessary the following average can be adopted:
Tonnes to m3 divide by 1.36.
5. Valuation Considerations - Plant and Machinery
When the cement is transferred from a truck to a silo, both the piping and the air compressors used in the transfer require frequent refurbishment and an end allowance should be appropriate. The compressors needed for the transfer operations require refurbishing every 14 months or so. The piping, which is at the top of the silos, bends through between 90° and 180°. There is considerable scouring of the inner surface by the powder, which necessitates renewal at six monthly intervals.
Since steel silos should be airtight, welded joints are the most efficient but in some of the older silos riveted construction will be found. Experience has shown that riveted silos tend to open at the joints due to the combined effect of atmospheric changes and pressures in use. If appropriate this should be reflected in the valuation of the silo(s) concerned.
Silos are specifically named in Class 4 Table 4 of The Valuation for Rating (Plant and Machinery) Regulations 1994 and 2000, although exception (d) has had the effect of excluding from rateability a number of silos. Foundations and supporting steelwork will also be rateable under Class 4 Table 3. Air Compressors and relevant Accessories used for blowing cement from rail trucks to the silos are rateable as generators of power under Class 1 Table 1 of the Regulations. Other rateable plant might include the structural part of weighbridges (other than those of Rail Track), elevators, walkways, stairways, platforms and electrical plant.
6. Methods of Cement Transfer
Specially constructed airtight railway wagons can carry 32 or 70 tonnes of cement each, which may increase to 90 tonnes in the future. The cement is transferred from the trucks into a silo by pumping air into the truck to a pressure of about 2 bar and allowing the cement powder to pass via an outlet valve up piping to the top of the silo. Modern silos have up to 4 supply points each.
Modern silos (up to 500 tonnes generally) are sheet steel structures (square box or cylindrical in shape) which load direct into the tanker lorries by gravity feed. This process is speeded by the use of forced aeration enabling the cement to flow. Beneath the silos are weighbridges with electronic transfer of data to the console in the despatch offices. (Modern weighbridges require 15 metre platforms in order to weigh modern tankers.) It is likely that a modern storage depot will also include a despatch office building, a vehicle washdown area and a good circulation area with metalled surface.
However, many variations from the ideal will exist, including pre-war silos of concrete cylindrical construction. For those silos the extraction of cement can be by two methods:
(i) Gravity feed to a channel with a screw to transfer the cement to an elevator.
(ii) By use of an air slide feed to the elevator (in this case the cement slides from the silo to the elevator on a cushion of air).
The first of these methods is very slow and neither is as efficient as modern methods. Many depots will have a single weighbridge by a despatch office. This type of property is becoming more scarce as depots close and new ones open.
The latest trend for new sites is to use a steel portal framed building with storage bays inside to store cement, thus alleviating the need for silos.