1.5   Component levels

 

Consideration must also be given to the elevations at which the various feed system components are placed. Their relative elevations are of prime importance as they influence the ability of components to meet the functional needs of the system.

The levels at which the various components are installed for a feed system using surface type heaters are shown in Fig 3.9.

Pictorial representation of the relative levels of heaters and feet pumps for a power station using horizontal HP and LP tubular surface type feedheaters

Taking the LP turbine horizontal centre line as the datum, the vertical height of the condenser and its transition piece, between condenser and turbine, combined with the NPSH needs of the condensate extraction pump, determine the basement depth needed below the machine. The water level in the condenser hotwell is fixed by the individual condenser design. Assuming a transverse condenser for each turbine LP cylinder, the water level is set at a convenient height above the condenser bottom. As the traditional split-casing design of extraction pump usually stands on the basement floor, the basement depth has to be set to provide the head of condensate needed to satisfy the extraction pumps NPSH. However, current practice is to use a vertical multistage condensate extraction pump of the 'caisson' type, which is sunk into a local pit in the basement floor. Use of this pump design allows the basement floor to be set just below condenser bottom-plate level as the pump impellers can be placed as far below floor level as needed. The principles involved are illustrated in Fig 3.10. It is evident that the use of a caisson type of extraction pump reduces the basement depth and thereby reduces station cost. A detailed description of the various types of extraction pumps used in CEGB power stations is given in Chapter 4.

Determination of basement depth

On the latest stations with all surface-type heaters (excluding the de-aerator), a horizontal heater attitude is adopted as opposed to the traditional vertical position. This allows the heaters to be placed below the bottom of the turbine cylinders but above the condenser water level. The heater levels are set with sufficient static head for them to cascade by gravity from the highest pressure HP heater to the lowest pressure, and then to the condenser. Even when set at these levels, by using horizontal heaters it is possible to arrange the bled-steam pipework to have a continuous slope from the bleed point on the turbine down to the heater. The same arrangement is also possible with horizontal LP heaters. This has the important advantage that the bled-steam lines can drain towards the heaters and the heater drains can cascade by gravity to the condenser, as depicted in Fig 3.11.

Component level diagram for tubular surface type HP and LP heaters

Gravity drainage allows operation at low unit load and unit start-up without alternative drainage arrangements, using pumps or special operational procedures.

On those stations which use direct contact LP heaters, the method of fixing the heights of the heaters is more complex. A typical DC heater level diagram is shown in Fig 3.12.

Component level diagram for direct contact LP heaters

The elevation between two adjacent DC heaters is arranged to accommodate a column of condensate which forms a seal between adjacent heaters. The seal is provided in the form of a loop. The condensate column counter balances heater differential steam pressures, pipework and waterbox/spray losses.

The height of the condensate column at full-load is about half the vertical separation between heaters. The loop seal must be of sufficient depth to withstand the most severe turbine load changes envisaged; for if it were ruptured by a temporary reversal of condensate flow, two turbine stages would become directly connected and there would be the possibility of steam and water being transferred to the lower stage.

Further information on the detailed dynamic behaviour of DC feed systems is to be found in the paper by Dartnell [1] and the hydrodynamic aspects are examined by Kubie, Rowe and Jones [2].

The height of the de-aerator relative to the boiler feed pump is important as this provides the static head on the BFP suction to satisfy its NPSH needs under all conditions of operation. The other considerations which can influence de-aerator tank height are discussed in detail in Section 3 of this chapter.

 

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