btauon design	Chapter 2
	58.0m
	COAL BUNKERS
	AND MILLS

Fig. 2.21 Layouts showing alternative boiler orientations

to the influence that the choice of fuel and its properties exert on the plant design.

Figures 2.22, 2.23 and 2.24 show how such factors influence both the boiler design and also the choice and layout of auxiliary plant. Such variations can obviously have a major impact on overall station design directly by the addition or omission of equipment, or by the relationship of weight and size of the boiler to the supporting structure and enclosure envelope. Second­ ary areas such as the storage volume of coal bunkers and the type and capacity rating of auxiliary plant such as coal pulverising mills, fans, etc., are also determined by these factors and can influence some of the principal

overall dimensions of the station layout, particularly the minimum boiler centreline spacing.

6.3 Layout conventions

In reviewing the basic layout concepts outlined, the reader should have identified a number of layout practices which are consistent between all the options discussed. These practices have evolved over the years within the CEGB and have been demonstrated to provide economic and practical designs in a UK situa­ tion. Such practices are regularly reviewed in relation

82

Station layout

I-'ig. 2.22 500 MW oil-fired boiler and auxiliary plant

to new plant designs and operational procedures and amended accordingly, but represent the knowledge gained by years of experience within the CEGB on station design, construction and operation of large fossil-fired units.

6.3.1 The unit principle

The unit principle is the association of a single turbine­ generator and boiler, together with its immediate auxiliary services, to form a complete, virtually selfcontained generating unit.

Many power station engineers will be familiar with ‘range’ stations where all boilers steam into a common receiver and from which individual turbine-generators take their supplies. Feedwater is similarly returned to a common receiver from which individual boilers draw their supplies, This arrangement gives the maximum flexibility in relation to overall boiler and turbine availability, but CEGB experience is that the reliability of large modern units makes such a provision unecon­

omic. It has therefore been CEGB practice for many years to build completely integrated units where the boiler and turbine are matched in capacity.

Many services however remain as station-based systems, e.g., cooling water system, town water ser­ vices, fuel handling and storage systems, etc., but this does not invalidate the unit concept.

6.3.2Mirror imaging

‘Mirror imaging’ is the complete or partial handling of plant and systems between pairs of boiler/turbine units to give a symmetrical layout about their common centreline. Such a concept is shown in Fig 2.25.

A mirrored design gives an attractive layout but CEGB experience suggests it has many practical dis­ advantages. It requires the production of two sets of engineering and design details, encourages manufactur­ ing and construction errors and it potentially precipi­ tates operator error.

_____ .........

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Station desigtf and layout

Chapter 2

COLUMN

CENTRE

LINES

KEY
1	STEAM DRUM	24 COAL BUNKER
2	FO FAN SUCTION DUCT	2$ PA FAN
3	SECONDARY REHEATER ELEMENTS	26	PA FAN DISCHARGE DUCT
4	PRIMARY REHEATER ELEMENTS	27	PA FAN SUCTION DUCT
5	REAR WALL BURNER AREA VENTILATION DUCT	28	FURNACE HOPPER
6	FD DISCHARGE ANO CROSS OVER DUCT	20 COLD AIR BUS MAIN
7	SOOTBLOWER AIR CONNECTION	30	HOT AIR TO MILLS DUCT
8	PRIMARY SUPERHEATER ELEMENTS	31	MAIN AIRHEATER AIR BY-PASS DUCT
0 FURNACE GAS EXIT HOPPER		32	HOT AIR RECIRCULATION
10	SECONDARY SUPERHEATER ELEMENTS	33	MAIN AIRHEATER AIR OUTLET DUCT
11	PLATEN SUPERHEATER ELEMENTS	34	GAS TO MAIN AIRHEATER
12	RADIANT SUPERHEATER	35	SECONDARY AIR BUS MAIN
13	DESUPERHEATERS	36	SECONDARY AIR TO PLENUM CHAMBER
14	PULVERISED FUEL PIPES	37	TERTIARY AIR DUCTS
15	HORIZONTAL CYCLONES	38	PLENUM CHAMBER
16	THREE * WAY DISTRIBUTORS	30 BUNKER OUTLET VALVE
17 MAIN AIRHEATER AIR INLET DUCT		40	COAL FEEDERS
18	MAIN AIRHEATER GAS OUTLET DUCT	41	BALL MILL
10 MAIN AIRHEATER		42	MAIN STEAM PIPES
20 MILL AIRHEATER GAS OUTLET DUCT		43	DUST PIPES
21	HOT GAS TAP	44	OUST VALVES
22 MILL AIRHEATER GAS INLET DUCT		45	ECONOMISER
23	MILL AIRHEATER

Fig. 2.23 500 MW coat-fired boiler designed for downshot firing and low volatile fuel

84

Station layout

KEY
1	STEAM DRUM	15 PRIMARY REHEATER		28 VENTURI
2	DOWNCOMER A	15 SECONDARY REHEATER		» FORCED DRAUGHT FAN
3	DOWNCOMERS	17	STEAM PARTITION WALL	30	PRIMARY AIR FAN SUCTION DUCT
4	DOWNCOMER C	18	REAR ENCLOSURE DIVISION WALL	31	PRIMARY AIR FAN
9 DOWNCOMER 0		10 SAFETY VALVE SILENCERS		32	PRIMARY AIR FAN DISCHARGE DUCT
8 FURNACE SIDEWALL		20	COAL BUNKER	33	MILL AIRHEATER
7	WATER SEAL	21	BUNKER OUTLET VALVE	34	HOT AIR BUS MAIN
8	ASH HOPPER	22	COAL FEEDER	35	MAIN AIRHEATER
9	DESUPERHEATER	23	PULVERISING MILL	38	GAS OUTLET TO PRECIPITATORS
10	ECONOMISER	24	FOUR - WAY DISTRIBUTOR	37	COAL CONVEYORS
11	PRIMARY SUPERHEATER	25	BURNER	38	DEAERATOR
12	STEAM DIVISION WALL	28 FORCED DRAUGHT FAN AIR INLET		39	DEAERATOR STORAGE TANK
13	PLATEN SUPERHEATER	27 COMBUSTORS		40 RESERVE FEED WATER TANKS
14	SECONDARY SUPERHEATER

Fig. 2.24 500 MW coal-fired boiler designed for frontwall firing and UK bituminous fuel

85

Station design and layout

Chapter 2

Current station layout practice within the CEGB does not favour mirror imaging plant layout but is based on replication of complete boiler/turbine packages.

6.3.3Turbine island concept

It is conventional practice for turbine-generators to.be supported by foundation blocks which are elevated above the basement or ground floor level of the power station. The height at which the turbine-generator is located is termed the operating floor and the designer has to decide whether to install a complete floor throughout the turbine house at this level or just to elevate the turbine-generator and its immediate walk­ ways and local laydown areas for small turbine com­ ponents. The latter concept is shown in Fig 2.26.

Elevation to operating floor level of the turbine­

generator alone is termed an island	layout	and has
been adopted by the CEGB as its	preferred	option.

This preference is based on the need to provide clear and unhindered access to main plant items. The layout allows direct crane access to all parts of the turbine hall, good maintenance access and efficient lighting and

ventilation to all areas. Defined laydown areas for heavy plant are provided at basement level as are clearways for personnel and equipment movement.

A continuous operating floor level throughout the turbine hall requires that the supporting structure be designed to accommodate plant loads imposed when the unit is dismantled for maintenance. Additionally, separate provisions for plant maintenance, ventilation and lighting arc required for the basement area or any intermediate floor levels.

The benefits claimed for the continuous operating floor are the opportunities for equipment location in intermediate floor levels, better facilities for turbine maintenance, ample laydown space local to the turbine and the generally pleasing and uncluttered appearance of the turbine hall.

Overall, the CEGB considers that the island concept provides the most economic and practical plant layout, and gives the best access and facilities for plant maintenance.

, The appearance of the turbine hall is a question of preference to be exercised by an individual utility, but the CEGB approach is that the station should be seen

86

' v U.5U eotour photograph between pp 66 and pp 67)

layout Station