now totally within the control of the designer. Veloci­ ties are in the order of 2 m/s in the channels approach­ ing the forebay (often called the suction dock). Levels are set by the locations out of the cooling towers. These will normally be designed so that the maximum still water level in the tower, pond is at or about site level. Theoretically the water in the tower pond only needs to be deep enough to provide the necessary gradient to make it flow towards the pumps, but in this case the system is quite highly tuned. With continuous loss of water through purge and evaporation a failure in the make-up system would rapidly lead to loss of pump suction. For this reason, most modern stations have a substantial reserve capacity in the cooling tower ponds (up to 2 m working range) and this reflects on the lowest operating level in the pump forebay, which also has to account for the hydraulic gradient losses in the tower flumes. Figure 2.65 (b) shows the hydraulic gradient for Drax power station.

The requirements for screening are much reduced on the tower cooled design but coarse-raked screens are now being considered to deal with the substantial problem of wind blown debris, such as plastic bags, which otherwise may end up on the condenser tube plates.

Isolation arrangements are required to allow removal of a pumpset without draining down the forebay, which is always common to all units.

The pumphouse may also house auxiliary pumps for duties such as compressor cooling, hydrant systems and

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auxiliary cooling water. Figures 2.71 and 2.72 show one of the two similar pumphouses at Drax power station.

18.3 Main cooling water pumps

The CEGB has standardised on vertical spindle, con­ crete volute, mixed flow pumps for the largest CW applications, and these have proven tc be extremely reliable over many years of operation. The CEGB has a policy of site testing equipment to ensure that it meets its specification. This is especially important for con­ crete volute pumps because it is impossible for the pump manufacturer to works test the pumps and so a scale model is made and tested as part of the pump contract. During commissioning, the prototype is care­ fully tested for conformity to the model characteristics of torque, head and flow. Brief details of the testing methods are given here because they have layout impli­ cations in the pumphouse.

The concrete volute pumps are driven at low speed (often less then 200 r/min) through reduction gearboxes by 11 kV synchronous motors. The CEGB is examining the possibility of direct drive, multiple pole motors to dispense with the gearboxes, but so far, the slight gains in efficiency and reliability have not warranted the substantial increase in motor cost.

Other pqwer generation authorities have had con­ siderable success with the vertical-spindle bowl pump design. This design allows a more straightforward

suction arrangement but has the disadvantage of very long pump shafts in the flow. These are subject to vibration, leading to gland and bearing wear. The metal casings of the pump also suffer from corrosion unless special measures are taken to avoid it. Corrosion is completely avoided by the concrete volute design. CEGB is therefore unlikely to use bowl pumps for main CW applications, although they are under considera­ tion for lower flow rate applications such as cooling tower make-up systems on indirect cooled stations. Metal casing volute pumps arc also used when the head and How are low enough to give an acceptably small thickness for the pump casing; the changeover from

metal casing to concrete volute occurs at approximately 7 m3/s. The changeover is not abrupt, but pumps below 6 m3/s would certainly be metal casing, and pumps above 10 m3/s would certainly be concrete volute type.

18.4 Screening plant

The CEGB uses three types of screening plant at CW pumphouses, coarse screens, fine mesh screens and pressure strainers. Coarse screens of bars are provided at the inlet to the system, which may or may not be at the pumphouse, in order to prevent the ingress of large

baulks of timber which could damage the finer screens. If the system has an offshore intake, this coarse screen is likely to be made of 50 mm bars on a 200 mm pitch. Where the coarse screen is at the pumphouse, the bar pitch is much less, typically 50 mm, and in this case it is necessary to examine the need for permanent raking of the bars. Current practice is to provide room in the pumphouse civil works for the provision of a raking screen. This is usually achieved by enlarging one of the bulkhead gate slots. However, the screens are not fitted until a need is proven, or the station is sited in an area of known debris ingress.

I •inc mesh screens arc provided to stop the passage of weed and fish into the CW system where they could cause a blockage of the condenser tube plates. The

majority of the fine screens are of the moving, self­ cleaning, open type such as band or drum screens, although pressure strainers have been installed, down­ stream of the pumps, at a number of stations. Drum screens arc the preferred type because they are sub­ stantial steel structures which can be designed to withstand the differential water pressure which could occur if the screen became completely blocked by debris (see Fig 2.67). They are relatively cheap, reliable and the only recurring area where particular attention is required is in the repair and reinstatement of protec­ tive coatings. They have lower head losses than other types ol screen ol similar duty. The only disdavantage of the drum screen is that it needs to extend both above the highest tide level and approximately 2 m below the