6 Foundations for main and secondary structures

6.1 Boiler house foundations

Boilers are the largest items of plant, and in modern power stations they are suspended from a framework of heavy steel beams just below roof level. The boilers, with their ancillary plant and associated coal bunkers, comprise a very high proportion of the total load. The foundations provided are largely required to carry loads arising from the plant, the loads due to the build­ ings being only a relatively small proportion of the total.

Since it has been CEGB practice for many years to build completely integrated boiler/turbine-generator units it follows that the foundations layout is repeated about successive centre lines of the boiler and turbine layout, say at 100 m centres for the current coal-fired reference design.

Pile caps or other types of separate foundations are provided for structural steelwork and specific heavy plant items such as the fans, mill foundations, air­ heaters and the ash removal system. In particular the heavy inertial loads generated by the mill operation require careful detailing to provide isolation of the other foundations and plant from dynamic loading.

In general, the floor plant area will be predominantly occupied by the discrete foundations described. As infill will be required to facilitate construction and subsequent operation, this can take the form of a rein­ forced concrete slab, say 300 mm thick, designed to withstand heavy wheeled traffic and laid on well com­ pacted fill. A typical layout of a power block found­ ation for a 2 x 900 MW station is shown in Fig 3.15.

6.2 Turbine hall foundations

Whilst substantial loadings will result from the main structural steelwork forming the turbine hall frame­ work and supporting the main overhead cranes, other substantial loadings arise from the turbine-generator blocks. The layout adopted for the turbines whether transverse, longitudinal or angled, will affect the loadings arising from the building itself in several ways. Hence layout needs to be established before serious foundation design can start.

Other turbine hall plant items can be supported economically on a simple combined pile cap of uniform thickness surrounding, but separate from, the massive discrete foundation blocks supporting the heavier plant and from the pile caps holding the building’s structural

of a pile-reinforced concrete slab 750 mm thick, sur­ mounted by a 200 mm mass concrete topping. Site drill­

Foundations for main and secondary structures

ing of plant fixings into this upper layer then can be undertaken without hazarding the structural slab.

In contrast the turbine-generator block sub-founda­ tion and main steel column bases would be separately supported on pile caps some 2.5 m thick.

The turbine hall foundation layout is considerably simplified by minimising CW culverts and cable tunnels below the building, hence avoiding the complicated forms which these items necessitated oh previous stations.

Figure 3.15 shows the turbine hall layout for a 2 X 900 MW station and its relationship to the boiler house.

6.3 Turbine-generator blocks

A turbine-generator block provides support for the machine in its static and cold condition and in its hot and rotating condition. That support extends the full length of the shaft at its base level but is normally separated to support particular shaft bearings indivi­ dually at machine operating level.

The height of a turbine-generator block is dependent on the type and disposition of the condensers, the requirements of the operators and the costs or savings involved in constructing a basement in relation to the capitalised cost of pumping cooling water to a greater height.

The block has openings within it to accommodate plant and pipework and is itself carried on a sub­ foundation.

Turbine-generator blocks are made in reinforced concrete or steel with the condensers often placed under or alongside on adjacent plinths. This arrange­ ment gives a maximum basement height of about 12 m. Some units arc constructed with condensers not located under the machine. This allows the height of the block to be less, consequently reducing both the height and capital cost of the turbine hall. One such arrangement is for the condensers to be placed on each side of the machine, these being known as pannier condensers.

Most turbine-generator blocks used to be built in reinforced concrete but an alterpative is to construct in steel. This reduces the foundation load, and being more

water pipes and other plant. In the case of a concrete block it is an advantage if it is built ahead of the machine erection in order to allow hydration thermal shrinkage to occur, whereas with a steel block this problem does not exist. As the material properties of steel are more consistent and more accurately known, the analysis of differential settlement problems is less difficult. The compatibility and better control of the properties of the construction material enables the dynamic design of the block to be done as part of the overall machine design.

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MILL FOUNDATION

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TYPICAL ANNEXE FOUNDATIONS

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KEY TO PILE DIAMETERS

600mm

750mm

900mm

1200mm

1500mm

works building and engineering Civil

A static loading diagram for a 500 MW unit is illustrated in Fig 3.16. This shows a concrete block with axial condensers beneath the machine.

A design of a 500 MW unit on a steel block and with pannier condensers is illustrated in Fig 3.17. The reduction in height of the unit made possible by the choice of condensers should also be noted.

Whichever type of turbine-generator block is ulti­ mately adopted the requirements and basic principles of the design are similar. The plan of the block is determined by the machine designer who provides the civil engineer with the position of bearings and the loads and tolerances cold and hot. Specified differential movements subsequent to alignment are remarkably low, being in the region of 0.01 mm reducing to 0.005 mm for points close together, though actual values may vary with different manufacturers. In this respect it is an advantage if all bearing supports can be mounted over piers or columns rather than on beams. Certain geometrical modifications may be permitted by the manufacturer in order to facilitate construction of the block, to assist the designer to keep deflections within the prescribed limits or, to ensure that resonance will not occur at or close to the machine’s normal running speed, thereby minimising operational vibration levels.

With concrete blocks, homogeniety and lack of

finish or strength characteristics.

There are numerous theoretical advantages in adopt­ ing special concrete mix designs for turbine generator blocks, but insufficient numbers are built to justify firm conclusions. One option more suited to concrete blocks than steel is to pre-compensate the levels of the bear­ ing pads so that when the block is heat-soaked the greater expansion of the steam end of the block brings the main shaft into alignment at operating speed and temperature.

At the steam end of the turbine foundation care must be taken to ensure that the concrete is shielded from the high temperature parts.

In some cases it is necessary to provide a reflecting or insulating shield. Special care should be taken to provide reinforcement to take care of the temperature stresses.

Although a good quality concrete is required, a very high strength is not necessary as the concrete stresses arc comparatively low owing to the considerable crosssectional area of most members. The average quantity of reinforcement required is likely to be about 1% of the appropriate concrete cross-section. This should be placed vertically, longitudinally and transversely in all structural members, to prevent any possible cracking due to vibration, even though it may not be theoreti­ cally required in all planes.

Some blocks have been constructed from pre-stressed concrete, considerably reducing the weight of steel used, and taking fuller advantage of high strength concrete by obtaining a better balance between effec­

Foundations for main and secondary structures

sections in about four vertical ‘lifts’ in a sequence which gives an approximately balanced load on the founda­ tion. If the programme time permits it is an advantage to allow a period of one month between concrete placing of ‘lifts’, to allow temperatures in the concrete to return towards ambient levels.

The design of the block’s structural form and com­ ponents has to be such that their natural frequency (or its harmonics) are at least ±20% different from those of the machine at normal operating speed (e.g., 50 Hz for a 3000 r/min machine) in order to avoid resonance. For a given material the frequency of vibration of any member may be changed by altering the dimensions or sections but not by prestressing. Slender cantilevers and thin diaphragm walls are particularly liable to vibration and attention should be paid to their natural frequency. If necessary, the section of these members should be increased.

In the case of the vertical columns, and other sub­ stantial members, there may be a choice between raising or lowering the natural frequency. The latter choice may be the more suitable as it would represent a considerable saving in material, providing it leaves the structure sufficiently strong. However, this means that these frequencies must be passed through every time the machine is run up or down.

A massive monolithic foundation is essential in order to provide a stable base for the turbine-generator block and to absorb vibration. The thickness of the founda­ tion should not be less than one-tenth of its length, and a foundation of this type is shown in Fig 3.18. It is not piled in this instance, as it is founded on a firm stratum at this extra depth. Discontinuity between the block foundation and the basement floor is attempted to provide some vibrational isolation throughout the turbine hall. However, cooling water culverts passing

Cooling water and the surrounding ground contribute to vibration damping.

Relative measurements are made to determine any change in the level of the block which could cause rough running of the machine as a result of movement of the foundation or of the block itself. Steel levelling plates are cast into the block around both the basement and operating floor level. A separate reference point is provided in the turbine hall, which may have to be carried on an isolated pile driven through a hole in the basement floor and retained completely independent of the floor. Measurements from the reference point and. around the levelling plates are carried out using an optical micrometer level or a micrometer water level. An, Invar rod is installed to measure down from operating floor level.

208

GOVERNOR

BEARING

CENTRELINE

RELAY CHEST 10t

PLAN AT BASEMENT LEVEL

CALCULATION OF LOADS

GENERAL NOTES

THE LOADS ARE THOSE DUE TO WEIGHT OF PLANT.

MINOR LOADS HAYE BEEN OMITTED AND ALSO THOSE DUE TO

BLOCK STRUCTURAL STEELWORK, ERECTION AND SUPER­

IMPOSED LOADS AT FLOOR LEVEL.

TOTAL LOAD TO BE CARRIED ON FOUNDATION IS THAT DUE

TO WORST CONDITION FROM PLANT PLUS WEIGHT OF

REINFORCED CONCRETE BLOCK. ASSUMED TO BE 2400kg.'nv

works building and engineering Civil

Fig. 3.16 Simplified loading plan for 5(X) MW turbine-generator

3 Chapter

Fig. 3.17 Arrangement of 500 MW turbine-generator on steel block with side mounted condensers

209

structures secondary and main for Foundations