ability of materials and the relative costs based on the capital and running costs. The connections from the pumphouse to the turbine hall should then be kept as short and as straight as possible to minimise costs and pressure losses in the system.

Care should be taken in routing the culverts or pipes so as to avoid the crossing of culverts or obstacles that could create air pockets in the system.

The syphonic weir and seal pit should be located as close as possible to the turbine hall to limit the extent of the more expensive pressurised cooling water culverts. The outlet culverts from the seal pit can be designed to a lower pressure. Care should be taken when routing culverts to ensure that satisfactory clearances exist to other services and structures to enable their satisfactory construction.

3.8.2 Closed cooling tower water system

A closed cooling tower water system, or indirect system, is used when the water supply available is inadequate for direct cooling, and the condensers operate on a closed circuit. A typical arrangement is shown in Fig 1.2S. The essential elements of the system include a cooling water pumphouse and forebay, intake and outlet culverts, cooling towers and return culverts to the lorebay. A make-up and purge system is provided to control the salts concentrated in the system due to continuous recirculation and to replace the amount lost by evaporation from the cooling towers.

The location of the cooling water pumphouse (see Fig 1.29) is again dictated by construction aspects and the need to minimise the pressurised cooling water culvert lengths.

The principles previously explained in Section 3.8.1 relevant to culvert routing equally apply to indirectcooled systems. The major impact on layout of a closed cooling tower water system is without doubt the cooling towers. At Drax power station (41XX) MW), which was the most recent to be built by the CEGB using cooling towers, there are a total of twelve cooling towers, each being approximately 115 m high and 93 m diameter at the pond cill levels. It may be appreciated therefore that the towers, together with the open return culverts to the pumphouse forebay, require a considerable area of land.

Cooling towers are usually grouped and sited so that the CW system as a whole is as compact as possible, maintaining an adequate clearance between adjacent towers, and between the towers and any object which might obstruct the air flow into them. Towers should be so positioned that any spray at the base of the tower does not blow on to public or station roads in frequent use, or onto the coal delivery rail sidings, and it is desirable to reduce the risk of coal dust and ash dust blowing into the tower ponds.

The location of the towers is also influenced by the preferences of the Consultant Architects commissioned by the CEGB for layout studies.

Site layout — thermal power stations

The layout of the make-up and purge system is dictated by the relative locations of the nearest river source and the cooling towers.

3.9 Fuel supplies and storage

3.9.1 Coal plant

The costs of coal deliveries by British Rail (BR) from British Coal arc dependent not only on the distances involved, but also on demurrage rates for locos and rolling stock and the efficient use of rail capacity by high­ speed permanently-coupled wagons. These costs can be minimised by providing the most rapid and efficient turnround at the power station unloading point. For this reason, the favoured arrangement for coal unloading at any power station site is the merry-go-round system, whereby bottom-opening hopper wagons unload the coal into underground hoppers, with the train runningon to leave the site without stopping. For the train to turn round and return to the loading colliery, a loop is required, with a 250 m minimum radius of track, and having the appropriate standing room for signal delays, means that a considerable area is required for such a loop arrangement. The land within the loop provides a convenient coal stock-out area.

Where insufficient area is available, or where access

. problems exist, a compromise solution can be adopted with sidings before and after the unloading track hopper and with provision for the loco to run round the train prior to exit from the site.

Figure 1.30 shows typical siding layout schematics. The track hoppers are situated as close to the boiler

house as possible to minimise conveyor lengths, but still providing sufficient distance for the rise from under­ ground hoppers to boiler house bunker tops to be achieved at a suitable inclination angle, allowing for junction lowers as required.

Another factor in the coal plant layout derives from the British Coal working arrangements requiring a week’s coal burn to be delivered in five working days. Thus, on average, two-sevenths of each day’s delivery must be stocked out for reclamation at the weekend. Consequently, stock-out and reclaim on a regular basis must be facilitated, and large travelling bucket wheel machines on rail tracks are often used for stocking out and reclaiming from the appropriate parts of the total fuel stocks. Longer term strategic stocks <ire held as part of the total stock, but transport to and from these more remote areas of the coal stock area is more economically achieved by bowl scraper mobile equip­

ment.

The total area required for coal store, rail arrange­ ments and handling equipment can be up to 20 hectares for a 2000 MW station.

Similar layout considerations may apply to coastal stations with sea-borne coal deliveries and short-term stocks as a ‘buffer’ between ship arrivals and longer term strategic stocks.

TO STATION

DRAIN

MAIN ANO S’i-T UP

,AIR PUMPS 300 gal min 130 gal m.n 5 _

ACW DW . 2C0C <;.« '•

FEED PUL'i

Oil COOLE.

120 9*1! rr.:-

FEED LIQUID Rt 3-20 gat -n

LUB OIL COOLERS 1800 f gal mtn <

s

layout site and siting ton

25-TOV.S :•••

PUMPHC-ii *.-ANE

10-TONNE AUXILIARY PUMPHOUSE crane

Fig. 1^29 Typical CW forebay and pumphouse for a closed cooling lower water system

power thermal — layout Site

3.9.2Fuel oil plant

To date the CEGB has not located an oil-fired station where its supply of heavy fuel oil would be dependent

fuel is available, or on coasts and estuaries where deliveries from sea-going tankers can be received. Quantities stored depend on a judgement of the security of supplies according to the proximity or otherwise of the source, and factors such as whether import and export to other nearby consumers is required. At least two and possibly up to five, large storage tanks are required. Ideal situations would be close to the main buildings on the ‘fuel delivery side’, but leaving adequate distances to minimise fire hazard to the station, and from other plant and equipment to the tanks themselves. Also adequately firm ground conditions are required and a suitable area large enough for a bund to contain the contents of one tank in case of a tank fracture.

Figure 1.31 shows the fuel oil delivery and storage arrangements at the 3 x 660 MW Littlebrook D power station.

On coal-fired stations, the need for boiler lighting-up oil requires delivery and storage arrangements. The

quantities of the lighter grade of oil needed arc relatively small and so delivery is normally by road tanker. Storage is in tanks within a bund located as close to the main boiler house as other layout con­ siderations allow.

3.10 Ash and dust disposal

The site layout must provide means of disposal for furnace bottom ash and for the large quantities of pulverised fuel dust produced as waste products. Although purchasers can be found at times for certain quantities of these waste products in the construction industry, in concretes or simply as landfill, long term dumping provisions are required. These can be close to the site or some distance away involving the pumping of dust as slurry, for example, to local natural or artificial lagoons, or transport by rail or sea in a dry condition, or by road in a wet condition. Market opportunities vary over the life of the station; some dumping grounds may become full or otherwise unavailable and disposal economics vary. Consequently the layout is likely to require several disposal options to be kept open in the longer term, whatever the immediate or initial short­ term disposal may be (see Fig 1.12).