19 Turbine hall and boiler house construction

19.1 General

The walls in older power stations built before 1939 were usually of brickwork, built with a solid plinth course and with brick piers to carry the loads from floors, roofs, etc. Roofs were pitched and consisted of wrought iron and, later, steel roof trusses carrying timber or angle iron purlins with slated roofs. Floors were supported on cast iron joists the ends of which were supported on padstones built into the brickwork or supported on circular cast iron columns. Floors were of timber joist and board construction except for those required to carry heavy loads where cast iron plates or continuous brick arches spanning between cast iron beams were used. The introduction of rolled-steel members and later reinforced concrete enabled the main buildings to be constructed using a structural frame, in which the walls ceased to form structural elements but served mainly to insulate and exclude the weather. Flat roofs of reinforced concrete or precast concrete units covered with asphalt became common anil floors were of reinforced concrete, or where heavy loads were to be carried, concrete reinforced by small section rolled-steel joists. The latter is termed filled joist construction.

Compound sections using one or two rolled sections with additional plates riveted to the flanges, and riveted plate girders made up from flat plates jointed by angles were introduced to carry the increased loads on columns and beams. The cladding became a plinth of brickwork to withstand mechanical damage with bitumen-coated, galvanised corrugated steel or asbestos sheeting to the wall$ and roofs. Lighting and ventilation were obtained by incorporating steel windows or areas of patent glazing in walls and roofs. The offices and other ancillary buildings were usually combined on the main elevation of the station, and the facing of brickwork and artificial stone on these buildings was usually extended to the whole of the main buildings on this elevation in order that a good external appearance was presented.

In the period since 1946, the standard walling con­ struction of brickwork gave way to a number of turbine halls being built with reinforced concrete frames and roofs. Precast concrete, in-situ concrete and composite construction were used, and the use of concrete shell roofs with concrete columns and beams resulted in many excellent buildings.

Until recently it was common practice to plan the construction of a power station in two sections, the first section often being commissioned before the second was commenced. Problems presented by this form of construction included difficulties due to differential scttlemerfi of foundations owing to the settlement in the second section lagging behind that of the first. Re­ levelling of crane rails was necessary and provision had

Turbine hall and boiler house construction

to be made for movement between all interconnected parts of the building and plant. A temporary gable end had to be provided to the completed first section and this usually took the form of a steel frame covered with cladding.

Several power stations have been built in the past using a semi-outdoor construction for the boilers. A structure was not provided to house the boilers with this arrangement, although hutments and covered stair­ cases, gantries and walkways were provided to protect operators, when working on the boilers. When these power stations were built, brick walls and concrete roofs were the accepted methods of construction for main buildings and hence considerable capital savings were effected. These savings are not so apparent when outdoor boiler construction is compared with the more modern method of boiler house construction using lightweight cladding. Outdoor construction also has the disadvantage that many repairs have to be carried out in the open and extensive scaffolding may be necessary for work at the higher levels. There are also obvious objections to this type of construction on aesthetic grounds and it is unlikely to be used in this country in the future.

Although brick and steel chimneys have been used in the past, concrete is used for chimneys on projects now under construction. For chimneys in excess of 90 m high concrete construction becomes cheaper than brick.

The first cooling towers consisted of a timber frame built over concrete cooling ponds, the outside of.the frames being covered with wood boarding. Timber stacks were provided inside to ensure adequate cooling of the water. Steel was also used but corrosive condi­ tions were so severe that in spite of maintenance paint­ ing the average life of a steel tower was only 8 years. Concrete was then adopted for cooling tower construc­ tion and the first concrete towers were similar in shape to the timber towers. These towers were superseded by the hyperbolic concrete towers, which although smaller were of the same pattern as those now being constructed.

The main buildings superstructure for a modem 2000 MW power station is unique in size and associated loads when compared with other projects carried out by building and civil engineering contractors. The design of the buildings is the result of co-ordinated efforts by civil engineers, architects, quantity surveyors, land­ scape architects, steelwork and reinforced concrete designers, services engineers and other specialists. Similarly the execution of the work on the site results from the combined efforts of steelwork, civil engineer­ ing and building contractors and also the many other specialist contractors and suppliers of materials.

All work on the superstructure of the main building is usually carried out under three m :in contracts:

•The structural steelwork contract which includes cladding and roofing.

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Civil engineering and building works

•The painting contract.

•The building and civil engineering work eontracl

which	includes	floors,	brickwork,	finishings,
services, etc.

The programme requirements for many types of build­ ings are simple from the client’s viewpoint, starting and finishing dates being the only limitations placed on the contractor. This type of programming is especially applicable where buildings are to be occupied by personnel who move in on completion, and can ;ils<> be used for structures such as chimneys and cooling towers. The programming of contracts for the boiler house and turbine hall is, however, much more com­ plex. Although the commencement and completion dates for the various contracts can be established from the overall project programme, many intermediate completion dates must be established to ensure that foundations and cover are available for a multitude of plant erection contractors and sub-contractors and electrical contractors to proceed.

19.2 Structural considerations

The total weight of structural steelwork required for the frame of a power station depends upon the plant layout and other factors which include the provision or omission of a basement and the use of reinforced concrete columns to the turbine hall. Hence the weight of steelwork for a 2000 MW station expressed in terms

Chapter 3

of plant capacity varies considerably from 12 t io 19.5 t per MW.

The basic dimensions of the boiler house and turbine hall are decided by the plant layout, and work cannot proceed on the design of the structures until this is established. When this stage is reached, development of the plant details and finalisation of the building design can develop together.

Because of the long beam spans and the heavy loads on columns and beams, structural steel frames arc

.normally used, although composite constitution in which reinforced concrete may be employed for turbine hall columns and for the frames of associated struc­ tures. Figures 3.51 and 3.52 show cross-sections through the steel frames for unit 1 and unit 2 respec­ tively at Thorpe Marsh power station, which were com­ pleted in the early 1960s. The longitudinal arrangement of the turbines resulted in the turbine hall crane having a span of 24.5 m.

In stations completed before 1960, boilers were slung from the main steelwork, the top beams of the large portals of the boiler house providing support for the boilers and roof whilst the columns formed the outer wall of the boiler house. The weight of a boiler in the newer 2000 MW power stations often exceeds 13 000 tonnes, and the span of the boiler house is so large that intermediate columns are introduced to reduce the span of the steelwork carrying the boilers. The boilers are carried by slings from the overhead sling deck, supported on columns which transfer the load to the foundation. For unit 1 at Thorpe Marsh the columns on

1 Al IK'T G1HDLHS

FORMING UOILLH

SUPPORTING STEELWORK

268

Turbine hall and boiler house construction

Fig. 3.52 Section through Thorpe Marsh power station unit 2

lines D and E, as shown in Fig 3.51, are used to support the boilers and for unit 2 the weight of the boilers is carried on the columns in lines C, H and J, as shown in Fig 3.52.

It is only after consultation with the steelwork contractor that decisions on the type of members to be used can be made. Certain contractors asso­ ciated with plate manufacturers favour box members, whereas others can tender more economically on the basis of rolled sections. Compound members were used throughout in the frame for unit 1 at Thorpe Marsh and although similar construction methods were used for the turbine hall of unit 2, box members have been used in the boiler house construction. Lattice girders have been used to support the roof of the turbine hall for both units.

Figure 3.53 shows the cross-section through the structural steelwork for West Burton power station on the centre line of the boilers. This is a 2000 MW station with a longitudinal arrangement of the four 500 MW sets which gives a crane span in the turbine hall of 38.5 m. The boilers are supported from the columns on lines D, F and G, and all the columns in the boiler house and turbine hall, together with the main beams in the boiler house, are box members. A longitudinal arrangement of sets usually results in large spaces between boilers which in the ease of West Burton have been used to accommodate the bunkers. Figure 3.54 shows the cross-section through the steelwork on the centre line of the bunkers. The roof beams used for the

turbine hall at West Burton are of triangular section and lattice construction. The compression members of the roof beams are at the apex of the triangle and the tension members at the bottom, which is level with the roof, the beams thus forming a triangular shaped pent­ house in which glazing and ventilation is provided.

Figure 3.55 shows the cross-section through the 2(KX) MW power station at Fawlcy. This station also has a longitudinal arrangement of sets and the span of the turbine hall crane in the station is 48.75 m. The main roof beams to the turbine hall are box girders and the roof deck is level with the underside of the main beams.

Even longer spans, up to 60 m, result from the trans­ verse arrangement of sets on other stations. This arrangement presents problems when the heavier loads associated with larger sets are considered and the use of two crane tracks involving a further line of columns carrying two crane rails has been developed. The posi­ tion of this extra line of columns is not necessarily central, its actual location depends upon the lifting arrangements for the turbine and generator. At Ferry­ bridge C power station, where this arrangement has been used, the smaller span crane is located over the generators which is advantageous owing to the greater loads on this crane. On this station, the central columns extend to the full height of the turbine hall and are used to support the roof in addition to the crane rails, thus reducing the spans of the main roof beams.

Wind pressures of 1400 N/m2 can result in total horizontal loads up to 15001 on the face of a boiler

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Civil engineering and building works

Chapter 3

Fig. 3.53 Section through West Burton power station

house and wind girders and bracing i^ necessary to

19.3

Erection of steelwork

resist these forces. The main columns are often located

The design

and

fabrication

of the steel frame is

at centres in excess of 12 m. and large section sheeting

governed

largely

by available

fabrication

techniques,

rails are necessary to

carry the weight

of the cladding

transport

facilities

and lilting

equipment.

Hence box

and resist the wind loading on such a span. It is usual

columns are manufactured in lengths not usually exceed­

therefore to introduce

intermediate

columns to

give a

ing 13 m. Sections of members such as roof trusses may

sheeting rail span usually between 2.75 m and 4.5 m.

be assembled into a complete member before it is lifted

The main roof beams in the boiler house and turbine

at the site. Columns of a cross-section 2 m by 1 m are

hall support the

roof

structures which normally

consist

often used and beams are up to 4 m deep. The weight

of lightweight

decking

supported

on

rolled

section

of sections

delivered to site is

normally limited by the

purlins. Some of the

more recent

stations have been

capacity of the works and site cranes.

provided with a

roof

of

precast

concrete units

to the

Although the use of large tower type cranes is being

boiler as these have been found

to

be

less susceptible

tried

out,

the normal crane arrangements

for erecting

to mechanical damage.

steelwork are shown in Fig 3.56. Derricks up to 50 t

Beams and columns are used at intermediate levels

capacity are used for steelwork erection, although

to support floors, pipework and items of plant. Open

special erection methods involving the use of catheads

grid flooring is normally used for floors, gantries and

on the top of columns arc sometimes used for lifting

staircases above operating floor levels as this permits

special beams in excess of the crane capacity. It will be

easy flow of air. The boiler house operating floor and

apparent from the arrangement of the cranes shown,

also the floors around the sets in the turbine hall are

that in many cases two cranes can be employed for a

normally of reinforced concrete construction. Solid

single lift as in the case of the turbine hall main beams.

floors are often constructed from precast concrete units The derricks are usually supported on trestles known as

or a combination of precast concrete units and in-situ	gabbards, and the whole structure mounted on bogies
concrete to facilitate speed of erection and avoid	travelling longitudinally along the boiler and turbine
difficulties in supporting shuttering at such a height.	buildings. As erection for one boiler is completed, the
Permanent shutterings of rigid galvanised sheet units	derricks move away and are finally dismantled outside
are sometimes used.	the buildings on completion of the steelwork erection.

270

Turbine hall and boiler house construction

l;Ki. 3.54 Suction through West Burton bunkers

198 12 m

55 47 m

Fig. 3.55 Section through Fawley power station

271