STEELWORK : Power Station Structures.

Industrial steelwork for electrical generating plants varies considerably depending on the size of station and the fuel being used. These variations are most marked in boiler house structures; whereas coal-fired and oil-fired boilers are similar, nuclear power station boilers (reactors) are generally constructed in concrete for biological shielding purposes, steel being used normally in a secondary building envelope role.

Turbine halls are, in principle, largely independent of fuel type, and many of the other plant structures (mechanical annexes, electrical switchgear buildings, coal hoppers, conveyors, pump-houses) are common in style to other industrial uses and so brief descriptions of the salient design features are of general interest. Boiler houses  (coal- or oil-fired) (Fig. 3.1) have to solve one overriding design  criterion and as a result can be considered exercises in pure structural design.

Plan of boiler house framing
Fig. 3.1 Plan of boiler house framing

Modern boilers are huge single pieces of plant with typical dimensions of 20m X 20m X 60m high for a single 500MW coal boiler.Where poor-quality coal is burnt or higher capacities are required, the dimensions can be even larger, up to about  25m X 25m X 80m high for 900MW size sets. As may be anticipated, the weights are equally massive, typically in the range of 7000–10 000 t for the plant sizes noted above.

Boilers are always  top-suspended  from the supporting structures and not built directly from foundation level upwards, nor carried by a combination of top and bottom support. This is because the thermal expansion of the boiler prevents dual support systems; unsurmountable buckling and stability problems  on the thin- walled-tube structure of the casing would arise if the boiler was bottom-supported and hence in compression, rather than top-suspended and hence in tension. For obvious reasons no penetrations of the boiler can be acceptable and therefore no internal columns can be provided. The usual structural system is to provide an extremely deep and stiff system of suspension girders (plate or box) spanning across the boiler with an extensive framework of primary, secondary and (twin) tertiary beams terminating in individual suspension rods or hangers which support the perimeter walls and roof of the boiler itself.

Columns are massively loaded from the highest level and so are  usually constructed from welded box-sections since their loading will be considerably above  the capacity of rolled sections. It is usual practice for a perimeter strip some 5–10m wide to be built around the boiler itself allowing a structural grid to be provided with an adequate bay width for bracing to be installed for lateral stability. It also provides support for ancillary plant and equipment adjacent to specific zones of the boiler, for pipework and valves, for personnel access walkways or floor zones.

The pipework support requirements are often onerous and in particular the pipework designers may require restrictive deflection limitations that are sometimes set as low as 50mm maximum deflection under wind loadings at the top of 90m high structures.

Turbine halls (Fig. 3.2) support and house turbo-generating machines that operate on steam produced by the boiler, converting heat energy into mechanical energy of rotation and then into electrical energy by electromagnetic induction. Turbo- generators are linear in layout, built around a single rotating shaft, typically some 25m long for 500MW units.The function of a steel-framed turbine hall is to protect and allow access to the generator, to support steam supply and condensed water return pipework and numerous other items of ancillary plant and equipment.Heavy crane capacity is usually provided since generators are working machines that require routine servicing as well as major overhaul and repairs. They are probably the largest rotating machines in common use, and dynamic analysis of turbine generator support steelwork is imperative.

Cross section through turbine hall
Fig. 3.2 Cross section through turbine hall

Fortunately they operate at a sensibly constant rotational velocity and so design of the support steelwork is amenable to an analytical examination of dynamic frequencies of motion of the whole support structure with plant loading in each possible mode,with similar consideration of any local frequency effect, such as vibration of individual elements of the structure or its framework.

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