the total enclosed floor area and overall volume of the turbine hall needs to be determined by consider ing the plant layout in relation to the erection, operation and routine maintenance requirements.

The majority of modern turbine-generator plant, is assembled from modular packages and their adoption necessitates consideration being given at the design stage to the means of access and the handling arrange­ ments to be used for installation and maintenance. The designer is required to assess whether the loads to be handled are ‘one off’ installation packages which are dismantled for normal maintenance, or components requiring routine handling facilities during such periods.

Current design practice on CEGB stations is to utilise specialist lifting equipment for the ‘one off’ installation lifts and install permanent cranage facilities for those components requiring routine maintenance attention. Typically the generator stator would be installed utilising specialist jacking equipment, but permanent lifting facilities would typically be provided to hli the generator rotor, steam turbine top covers and ■'team tmbine rotors.

On multi-unit stations, where up to six large units have been installed, the opportunity was taken to provide a permanent lifting capability for the generator stator by operating two overhead cranes in tandem with a load spreader beam. The cranes would normally be provided with a low speed selector on the main hoist gearbox to give this facility.

The need to size the overhead cranage facilities to handle all major plant components requiring attention during an overhaul period, and the adoption of the island layout concept, means that the majority of remaining equipment and auxiliary systems within the turbine hall can be adequately serviced by the overhead cranes without recourse to local handling equipment. To ensure that adequate handling for such plant is provided, a ‘hook approach and plant laydown dia­ gram’ is produced to illustrate the area of coverage and the accommodation provided for the various plant items and components dismantled during the main­ tenance periods. The extent of laydown space provided will depend on both the number of installed turbine­ generator units and the maintenance programmes, but for a 2-unit station it would be CEGB practice to provide sufficient laydown area for one complete unit, as shown in Fig 2.30.

When nominating suitable plant laydown areas’, consideration needs to be given to access and personnel safety issues, and to avoid potentially hazardous situa­ tions being created, irrespective of whether a unit is being overhauled, clear and safe access is required for normal operational procedures, and it is necessary to make provision for dedicated access clearways through the turbine hall. These restricted zones should be clearly marked to preserve the integrity of3the areas and operator familiarisation.

Boiler systems

The provision of such laydown areas, access clear­ ways and the layout of the plant itself determines the overall plan area needed for the turbine hall, bearing in mind on both the transverse and longitudinal layouts the need for generator rotor withdrawal. The overall turbine hall height is determined by the lifting height required for dismantling and movement to laydown areas of the longest vertical plant item, together with the necessary clearance room for the crane facilities themselves.

8Boiler systems

8.1Pulverised fuel system

Coal pulverising mills in CEGB stations are usually located on individual foundation blocks which are isolated from the main station foundations by the use of an insulating material which absorbs and reduces trans­ mission of the vibrations generated by this equipment. The minimum spacing of the mills is determined by the overall size of this foundation system and the need to provide maintenance access to the equipment.

Pulverising mills are usually a high-level maintenance item and such frequent attention, involving heavy lifts, necessitates the provision of a suitable component handling system. Normal CEGB practice is to provide a dedicated facility and a continuous access clearway through the mill bay for equipment movement. The mills need to be located in close proximity to the boiler firing face to minimise the length of pulverised fue[ (PF) pipework. PF pipework should be arranged so that for any one coal mill, the burners are uniformally distributed to ensure that for whichever combination of coal mills is in operation, the heat input to the furnace is evenly distributed.

The relationship of the mill bay with respect to the firing arrangement of the boiler has been discussed in Section 6.2.2 of this chapter.

8.2 Draught system

Figure 2.31 shows how the basic boiler layout options influence the layout of the air and gas ducts.

A rear bunker and mill bay, results in the airheaters being centrally located within the station and requires the routing of large flue gas ducts along each side of the boiler, which must pass through or around the bunker/ mill bay to reach the precipitators. This often results in an increase in the boiler centreline spacing, all other factors being equal. The central bunker and mill bay eliminates this disadvantage and allows short and direct gas duct routes to the precipitator inlets.

The adoption of enclosed boiler houses has allowed advantage to be taken of recovering heat losses which invariably occur from the boiler plant. Reference to Fig 2.22 shows that the forced draught fan suction ducts are routed from the upper areas of the boiler house in •

(a) COAL MILLS BETWEEN BOILER AND TURBINE

FIRING FROM TURBINE SIDE

(C) COAL MILLS ON PRECIPITATOR SIDE OF BOILER

FIRING FROM TURBINE SIDE

(b) COAL MILLS ON PRECIPITATOR SIDE OF BOILER FIRING ON SIDE REMOTE FROM TURBINE

(d)COAL MILLS BETWEEN BOILERS FIRING FROM TURBINE SIDE

order to recover the hot air which rises to the top levels of the building by convective action. The arrangement of the ducting is comparatively simplej but a route reservation down each side of the boiler which allows straight runs without offsets is necessary.

The gas and air ducting which connects the boiler unit to the airheaters, fans and precipitators occupies a considerable space within the boiler house. The duct cross-sections must be large to reduce gas and air How resistance to an economic minimum and be designed to avoid sharp bends and changes in section which, besides inducing pressure loss and turbulence, can, in the case of flue gas ducting, precipitate fall-out of pulverised fuel ash particles.

Typical CEGB practice on larger fossil-fired units is to locate the airheater as close as possible to the economiser gas outlet flues at about operating floor level, with the flue gas ducts being run below this floor level to the precipitator inlets. The combustion air

ducting from the forced draught fans is also normally run below operating floor level to the airheater inlets.

8.3 Oil firing system

The majority of conventional stations have an oil firing capability, although the extent varies from a fairly minimal requirement for boiler lighting-up on some coal-fired stations through to a full load capability on totally oil-fired stations.

A fully oil-fired boiler is attractive from a layout viewpoint in that the boiler house design is simplified by the omission of the coal pulverising mills, bunkers, ash hoppers, etc., and the boiler orientation can be chosen to provide the<best arrangement of air and gas ducting.

Equipment provision for oil firing is reasonably simple. A station-based ring main is usually adopted

which continuously circulates heated oil from the main storage tanks. Individual boilers take oil from this ring main via additional pumping and heating units which control the oil condition to suit the boiler burner characteristics. The oil passes from these unitised pumping and heating units to another ring main which circulates the boiler firing face and allows individual burners to tap off their oil supply through a control valve. The ring main spills back to the pumping and heating units.

On coal-fired boilers, a supply of oil is used for lighting-up and initial operation of the boiler until a sufficient load is established to maintain a coal mill in service. The design requirements for such a lighting-up system are similar to full oil firing, but are generally sized to carry only a small percentage of full boiler load. On some coal-fired stations provision for a larger oil firing capability is made. Such ‘overburn’ facilities can be used to support unit output in the unlikely event of an excessive number of coal mills being out of service at the same time.

8.4 Boiler fittings

The boiler has many externally-mounted items of equipment to which pipework and electrical services are required together with personnel and maintenance access provisions. The most important are:

•Main steam stop valves and integral pipework.

•Reheater inlet and outlet connections.

•Feedwater inlet connections to economiser.

•Blowdown and drain valves.

•Sootblowers.

•Burners.

•’ Boiler safety valves and silencers.

•Steam and water sampling equipment.

•Chemical injection'equipment.

•Air release valves.

•Drum level gauges and alarms — local and remote.

•Control and instrumentation sensors and transmitters.

•Various access and observation doors.

•Ash and dust removal points.

•Gas ducting dampers.

The frequency and extent of access for operation and maintenance activities will vary for each of these activities, and it is necessary to review the full scope of activities required at the design stage to ensure that adequate and safe provisions are provided. This is primarily the task of the layout engineers.

Many operational activities on the boiler will con­ centrate on the burner area, including on-load main­

Boiler systems

tenance and adequate attention to the provision of artificial Lighting to workshop standards, fire protection and means of escape for personnel.

The layout of platforms and galleries at the firing floors should allow for easy withdrawal of oil burners and for filters to work comfortably and safely. Pro­ vision for accommodating oil spillage should be pro­ vided with adequate drainage and collection arrange­ ments to ensure the minimum fire risk.

Generally the equipment identified here is located over the whole boiler area and demands a comprehen­ sive series of galleries around the boiler casing at suit­ able levels, al) the way from the ground up to drum level (see Fig 2.32).

8.5 Dust extraction plant

Electrostatic precipitators are now routinely used for the removal of pulverised fuel ash from the boiler flue gases. They need to be located between the boiler and the stack in a position which allows the gas duct routing to be as short and straight as possible. The most suit­ able location is therefore immediately to the rear of the boiler house.

It is common practice to divide the flue gases into streams and utilise a number of precipitator units in parallel with typically three, but sometimes four, passes being adopted for each boiler unit. The overall size of the precipitator plant is influenced by the fuel charac­ teristics and in some instances the overall width of the plant may affect the minimum boiler spacing. *

8.6 Flue gas desulphurisation plant

Flue gas desulphurisation (FGD) equipment is likely to be installed at all major fossil-fired stations in the future, and in fact retrofit installations are also being carried out at a number of existing stations.

A number of treatment systems are already commer­ cially available with several more in the development stage, and the CEGB has selected both the limestone­ gypsum and a regenerative system for its initial installa­ tions of this plant. In terms of layout, both systems have similar basic requirements in that the plant is divided into unit and common equipment.

Figures 2.33 and 2.34 illustrate the studies which have been carried out for the two systems at the Drax site for units 4. 5 and 6, the most recently installed plant.

In each case the unit plant, consisting mainly of absorbers, fans, reheaters and ducting is located in the area immediately behind the unit precipitators between the induced fan discharge and the dust bunker access road.

The common plant area containing either the lime­ stone and gypsum treatment or the regeneration equip­ ment is located to the north-east of the station area and