Bevans Cement Works, Northfleet
The Bevans Works of the Associated Portland Cement Manufacturers, Ltd., at Northfleet, Kent, have recently been reconstructed and enlarged, and are now claimed to be the largest and most up-to-date cement works in Europe (Note 1). The capacity is stated to be about half-a-million tons of cement a year.
Historically, the works date back to the dawn of the cement age; in the first half of the last century, when Joseph Aspdin's son erected his first kiln on a portion of the site, and some of the original kilns are still preserved (Note 2). It is not the present intention, however, to review the history of the works, but rather to describe it as it is today.
The geographical advantages of its position are exceptional. Situated on the great Thames Waterway with solid chalk foundations down to deep water, it has shipping facilities of a unique character.
Those who are familiar with the geology of Kent know the almost inexhaustible supplies of chalk and clay which abound in this neighbourhood, and it is not surprising to learn that the Bevans management are free of all worry as to future supplies of the necessary raw materials.
The clay now being used is dug by steam navvy from a deposit of London clay about two miles away (Note 3), and is immediately tipped into washmills and reduced to a thin slurry with about 60 per cent. of water. This fluid clay is then pumped to storage tanks at the (main) washmill, which is in the chalk quarry about half-way to the works. Here the chalk is dug by an electrically-operated navvy weighing 70 tons, which digs 3½ cubic yards of chalk at one bite and deposits it in 10-ton railway wagons, trains of which are hauled by steam locomotives a short distance to a combined electric hoist and tippler which picks them up bodily and tips their contents into the first of a series of heavy washmills.
The required proportion of clay slurry is pumped into the same mill from the storage tanks adjoining, and the two materials are then mechanically mixed while the chalk is being reduced to a very fine state of subdivision by the revolving harrows in the washmill. The resulting mixture of chalk and clay slurry contains about 40 per cent. of water and leaves the first washmill when it is fine enough to pass through the surrounding screens, passing in succession through three other mills, each of which has finer screens than its predecessor. The slurry when it leaves the final mills goes to mixing and storage tanks adjoining where its composition is again checked (Note 4).
It is pumped nearly a mile through two pipe-lines to the final series of storage tanks at the works. These tanks, of which there are six, are situated immediately at the upper end of the rotary kilns, and the slurry gravitates from them to the boots of the four bucket elevators which deliver it at sufficient height to enable a gravity feed to the rotary kilns. The slurry storage and mixing tanks are of standard type throughout, 66 ft. in diameter and 11 ft. deep, and are continuously agitated (Note 5).
The coal used by the works is all sea-borne and is received alongside the eastern extremity of the deep-water jetty, where it is unloaded by electric grab cranes and transported by a belt conveyor system to storage bunkers ashore, situated at the east end of the kiln house. A complete system of extractors beneath the bunkers facilitates the isolation or mixing of fuels as the exigencies of service demand. The pulverised fuel for kiln firing is prepared by six large high-speed vertical mills housed between the coal storage bunkers and the kiln plant.
The powdered fuel is stored in self-trimming hoppers, from which it is continuously circulated along the firing floor, surplus being returned to the storage hoppers. Variable speed extractors tap the coal supply opposite each kiln at the firing end. There are four rotary kilns of the latest design and construction, the loaded weight of each unit being in the region of 800 tons. Three of them are 250 ft. long with separate coolers (Note 6); the fourth is 294 ft. long, and embodies a combined cooler arrangement consisting of a series of small tubes arranged around the periphery of the firing end of the kiln (Note 7). All have enlarged burning zones and are direct driven through gearing from the motor to the girth ring.
The cooled clinker from the kilns is conveyed by band conveyors and bucket elevators to large reinforced concrete storage hoppers, whence it is delivered by gravity to the grinding mills immediately beneath. These mills are of the " combination " type, consisting of a long steel tube 36 ft. long by 7 ft. in diameter, divided into compartments.
The first compartment into which the clinker is delivered is charged with steel balls of 4 in. to 2½ in. diameter. The coarse grit from this compartment passes through a slotted diaphragm into an intermediate compartment, where it is further reduced by balls of 2½ in. to 1½ in. diameter, and then passes to the final stage, where it is reduced to an impalpable powder by smaller grinding media. There are seven of these large mills, five of which are driven by slow speed synchronous motors each taking 750 h.p. and the remaining two being driven through double helical gearing and are rated at 525 h.p. (Note 8)
The cement is then delivered to a Fuller Kinyon pumping installation, which delivers it by means of compressed air into the battery of twelve storage silos, the lift being no less than 102 ft. This method of conveying cement is quite new to this country and a description of the installation may be of interest. The pump itself is a very simple piece of apparatus, consisting of a cylindrical barrel in which a motor-driven screw rotates at fairly high speed. Near one end the cement is admitted into the barrel, and the screw conveys the material forward. It may be noted that the pitch of the screw is diminishing, hence the cement fills the barrel as a piston towards the end of the screw, At the end of the barrel is a ring with an annular space (between the ring and the cylinder walls) through which numerous small holes or ports are drilled at an angle of 45 degrees towards the direction of flow. Compressed air is applied at this point, and maintains the velocity of a stream of air (and cement) towards the open end of the pipe system. The cement is mixed with air, and the transmission of the "fluid" is continuous, but over and above the continuous stream there are more-or-less regular periodic " gusts." The lift is not accomplished by alternate layers or pistons of cement and air, as many conjecture. Perhaps the most accurate picture of the rising main would be to visualise two centrifugal pumps putting two classes of liquid through the same discharge pipe, one (say) heavy oil or tar, and the other water; both would emerge continuously with occasional gusts of the heavier fluid. If the gust were regular the analogy would be complete.
A wharf is provided for the reception of steamers discharging gypsum for slowing the set of the cement at the west end of the river frontage; after unloading, the material is conveyed by an elevated belt conveyor system to unit hoppers constructed in the clinker store immediately over each combination mill.
The twelve cement storage silos are built of reinforced concrete and are capable of holding 22,000 tons of cement. They are subdivided with central stairways to facilitate accurate sampling of the bulk.
The cement handling and packing plant and the wharf facilities at this works are exceptionally interesting, and it is proposed therefore to describe them in some detail (Note 9).
The main scheme consists of a deep-water concrete jetty some 600 ft. long and capable of berthing the largest steamers (up to 15,000 tons) at all states of the tide, flanked by several unit packing plants each independently fed and operated. The jetty itself stands away from' the plant, thus giving outside and inside accommodation for craft, and is equipped with five high-speed heavy duty electric luffing cranes. These cranes travel along the jetty as required and can take slings of cement from the shore plant and deliver it straight into the steamer's hold.
Adjacent plants feed the jetty by means of a circular railway track, thus supplementing the plants immediately facing the jetty. By this means a very high rate of loading can be maintained. When it is considered that some 10,000 tons of cement have to be dispatched each week, and occasionally as much as 12,000 tons, in spite of rain, fog, and other delays incidental to shipping, the necessity for such elaborate plant and organisation will be appreciated. Several complete and independent systems exist for extracting cement from the silos and conveying it to the various packing plants.
Ancillary to the factory proper there is a complete organisation of cooperages, steel-drum factory and sack plant, which supply the many thousands of packages required each day for the large tonnages shipped overside (Note 10).
Special attention has been given to the important question of continuously sampling the product at all stages of manufacture.
Electric power for motive purposes is obtained in bulk from Barking (Note 11), at a pressure of 33,000 volts, 3-phase, alternating current. It is stepped down to 3,000 volts at the main receiving station. As a general rule all motors of 100 h.p. and upwards are on the 3,000-volt circuit, smaller units are fed from a 500-volt distribution. Precautions are taken in the switching on and off of the larger units to preserve an even load factor, as there are some four hundred electric motors in the works ranging up to 750 h.p. each.
A complete organisation exists for " Safety First " welfare, and recreation, including a large club, swimming baths and a sports ground adjacent to the works.
The electrical plant as a whole has one or two notable characteristics; the motors are generally of the squirrel-cage type driving, where the starting conditions are heavy, through centrifugal clutches. All motors, including those for the tube mills, are mounted on roller or ball bearings. The cable system is totally enclosed right up to the points of application, and the smaller wiring (lighting, etc.) is in conduit, with ironclad switches and fuses, reducing risks of failure and shock to a minimum.
The power for the works is transmitted from Barking at 33,000 volts by means of underground cables, and is delivered at the supply company's sub-station at the washmills. The supply is controlled by six " K " type switches, with a breaking capacity of ¾ million kVA each, and passes on to two banks of three single-phase transformers of 2,500 kVA capacity each, i.e. two 7,500-kVA groups, which step the pressure down to 3,000 volts. These transformers are situated between the supply company's sub-station and the works receiving sub-station. In the latter are situated the power company's 3,000-volt control switches, also of " K " class, and on which is accommodated the metering equipment. The bus-bars from these two switches are continued and connected to ten sets of truck-type gear, the switches in which have each a rupturing capacity of 100,000 kVA. This switchboard controls two feeders to the washmill sub-station, six feeders to the main works sub-station, one clay mill feeder, and one metering panel.
Throughout the factory the switchgear and the distribution system have been so arranged that the supply to the different processes of manufacture is controlled from the two sub-stations, one at the washmill and the other at the main works. This also permits of the metering of the unit consumption of each section from these two main points. The factory load is 6,000 kW; as a result the electrical stresses on switch and control gear when operating under short-circuiting conditions are heavy. but these are considerably reduced by the cable system adopted.
For the washmill sub-station the basement of one of the two main motor-houses has been utilised. The position is at the centre of the load dealt with, and has resulted in an economical lay-out of the feeder and distribution cables. The sub-station is equipped with a 3,000-volt switchboard, one 750 kVA 3,000/500 V transformer, a 500 volt switchboard, and a 20 kVA 500/110 V lighting transformer with switchgear. Six truck-type units comprise the H.T. switchboard and control the two incoming feeders from the main sub-station, the H.T. side of the transformer, two feeders to the two main motor houses, and a feeder to the quarry giving a supply to an electric navvy. The 500-volt switch-board is built up of seven pedestal iron-clad units of the draw-out type, one unit controlling the L.T. side of the trans-former, one the lighting transformer, and the remaining five distributors to fuse pillars or boards from which the supplies to motors are taken.
In each of the two main motor houses are installed two 200 h.p. squirrel-cage motors, one driving a roughing mill and the other a line of finishing mills. The machines, fitted with centrifugal clutch pulleys and controlled by heavily-rated auto-transformer starters, totally enclosed and designed for operation by unskilled labour, effectively deal with the heavy starting duty imposed upon them. The roughing-mill motor is also fitted with barring gear driven through a special worm reduction gear by a high-torque motor, which facilitates cleaning out and overhauling the mill.
The chalk, quarried by a 3½ cubic yard electrically-driven navvy fitted with a straight A.C. equipment, taking an average load of 150 kW and peak loads of 230 kW, is conveyed to the washmill in trains made up of standard gauge 10-ton trucks. Here the wagons run on to the electrically-driven tippler hoist, which is controlled by a fully automatic equipment; the operation of the starting button raises the truck about 40 ft. and then empties its contents into a hopper feeding the two roughing mills, afterwards lowering it to rail level. The tippler is fully protected by limit and emergency switches, and in addition a stand-by hand-controlled equipment can be put into operation by the throwing over a dual control change-over switch.
For a given quantity of chalk tipped into the washmill a certain amount of clay slurry is required. The slurry, stored in mixer tanks 30 yards distant from the washmills, is elevated and runs by gravity into the roughing mills. The elevator is driven by a 20 h.p. motor and is controlled by a direct switching contactor starter fitted with an adjustable timing device which can be set so that the elevator on operating the start button runs for a predetermined time corresponding to the amount of clay slurry required. The elevator is controlled at the tippler.
The clay slurry is obtained from the clay plant situated a mile from the main washmill, the power being transmitted to it at 3,000 volts by an overhead line. The transmission line is tapped about a quarter of a mile from the washmill for supplying a 75 kVA transformer giving current at 500 volts to two electrically-driven centrifugal pumps. The line tapping is controlled by a pole-mounted oil-switch fitted with overload protection, whilst the pumps are controlled by automatic starters remote-operated from the washmills. The main units at the clay plant are three washmills and a line of pumps for pumping the slurry to the mixers situated near the washmills; each unit is driven by a 100 h.p. 3,000 volt squirrel-cage motor fitted with a centrifugal clutch and controlled by an auto-transformer starter. The equipment is simple and robust and gives no trouble although operated by very unskilled labour (Note 12).
At the main washmill for pumping the finished slurry through the two pipe lines to the main works six three-throw pumps have been installed, each driven by a 50 h.p. squirrel-cage motor fitted with a centrifugal clutch coupling driving the pumps through reduction gears. The motor equipments are erected in line in the pump house whilst the starters, of the star-delta type, are mounted in a separate control room which also houses eight direct switching starters for 12 h.p. motors each driving a 66 ft. finished-slurry mixer. The separate control house for the control gear is a desirable feature on a washing plant.
Power is transmitted at 3,000 volts from the supply Company's sub-station, a distance of three-quarters of a mile to the main works, by six 0.4 sq. in. feeders drawn through a six-way earthenware duct, inspection and draw pits being provided every 200 yards. The feeders operate in parallel and feed on to the bus-bars of a 15-panel H.T. switchboard in the main works sub-station. The sub-station also contains five 750 kVA 3,000/500 volt transformers, a 17-panel L.T. switchboard, and a 30 kVA 500/110 volt lighting transformer bank with switchgear. The H.T. and L.T. boards are built up of truck type and ironclad pedestal units respectively, similar to those used at the washmill sub-station. The H.T. and L.T. switchboards are at each side of the sub-station facing each other with the transformers between them, the lay giving easy control of the distribution system which has been carried out on similar lines to that described at the washmill. Points of interest in connection with the sub-station are that the 5,000-volt incoming feeders are fitted with parallel protection induction-type relays operating on the definite minimum-time-limit principle, and that the metering panel instruments include an indicating kW meter and power-factor meter showing the value and power factor of the factory load. These instruments are energised through pilot cables from potential and current transformers installed at the Supply Co.'s sub-station. A maximum demand alarm relay which operates a Klaxon horn when the load exceeds a pre-arranged figure is also provided.
In general the motor equipments call for no special comment. The six coal mills are each driven by a 75-h.p. vertical-spindle squirrel-cage motor fitted with a clutch pulley. The grinding mill motor house contains the motor units previously mentioned which total 4,750 h.p., and the building is ventilated by clean washed air under pressure.
Naturally the buildings, silos, washmills, and other structures are practically all constructed of reinforced concrete, and in addition there have been many minor applications of reinforced concrete, such as elevator towers, hot-air flues, etc., described in the following notes. Rapid-hardening Portland cement was used almost exclusively for all the reinforced concrete, and although a large proportion of the work was done in extremely cold weather it was nearly always possible to get a lift on wall work every day.