12 Cranes

Cranage is required within the turbine hall to cater for the erection of plant during the station constructional phase and maintenance during the subsequent station operational phase for the turbine-generator units and their respective auxiliary plant. The cranage is needed to cover the maximum possible floor area with due regard to the disposition of the plant, laydown areas and loading bays.

There have been three different configurations of steam generating units to turbine layouts within the current programme of 660 MW sets, i.c., longitudinal, transverse and diagonal. The selection of the earlier stations’ turbine hall layout was constrained by the then available design of turbine hall cranes which had limits of span and capacity. Ultimately the design of cranes permitted spans and capacities large enough to accom­ modate the full length of the turbine, i.e., transverse designs. The span for a 660 MW set is about 65 metres. The transverse design was generally adopted because optimisations of turbine hall span, high pressure and reheat pipework runs, the type of low pressure and high pressure feedheaters when used, the laydown require­ ments and particularly in the case of nuclear stations the consequences of turbine disintegration, showed the most economic design. However, with larger units, it is possible that economics may change the disposition of the turbines from the preferred transverse design.

Figure 2.38 shows an arrangement of turbine hall cranes.

The cranage hook approaches at the end of and at the sides of the building will then determine the travel ami span required for the cranage. The height of the crane is determined by the clearances that are required for erection and dismantling of plant, although it should be noted that the heaviest load is not neces­ sarily, and probably unlikely to be, that which deter­ mines the cranage rail height. From the foregoing information it is then possible to define the major building dimensions with the exception of the turbine roof height which is dependent on the cranage lifting philosophy.

There are two basic lifting philosophies that can be adopted, firstly to lift the heaviest indivisible load by cranage, or secondly to use a synchronous jacking tech­ nique to lift the heaviest loads. There are a number of different ways of providing the former, viz., by a single crane or by two cranes lifting in tandem. The method that is generally chosen is lifting in tandem because of the lighter wheel loads spread over a longer track length which results in smaller column sizes. The depth of bridge beam is smaller, thus decreasing the height of the turbine hall roof. More importantly perhaps is the improved flexibility of operation of cranes during the construction and maintenance periods.

The latter, the synchronous jacking technique, per­ mits a much lower capacity crane to be used and has the

Fire protection

benefits of even lighter wheel loads and small bridge beam depths, and can be accommodated into the over­ all station programme without increase in programme time. This technique however has some detrimental effects in that floor areas have to be strengthened locally, it is cumbersome in operation during the construction of the station and interferes with other contractors working in the vicinity.

On an economic basis there is little difference in the cost of the two options when account is taken of the relative crane, building and specialist equipment costs. However, there must be a cost advantage to the improved flexibility offered with a two crane system lifting the heaviest indivisible load which is difficult to quantify over the life of the station, but which will be of major benefit if the heavy indivisible loads, such as the generator stator, need to be replaced within the station life.

The majority of modern stations thus contain two electric overhead travelling cranes in the turbine hall which will lift on their main hooks, either the heaviest indivisible load in tandem or the heaviest maintenance load individually. Where a station has more than four units a third crane can be added. The turbine hall cranes also usually have an auxiliary hook which is capable of lifting smaller loads at much faster speeds than the main hook, thus making the crane a versatile lifting machine.

13 Fire protection

13.1 Introduction

The primary objective of any fire protection philosophy must be to minimise the probability and the conse­ quences of any postulated fire. In spite of steps taken to reduce the probability of a fire, it is possible that fires may occur from time to time. The fire protection phil­ osophy for a modern power station should therefore be tirranged to achieve a balance in:

•Preventing fires from starting.

•Limiting the consequences of a fire.

•Reducing the severity of any fires that do start.

It is not anticipated that any one of the above items can be totally effective, but the strengthening of one area can compensate for weaknesses in other areas.

13.2 Prevention of fires

The prevention of fire involves systematic consider­ ation of the use, nature and physical arrangement of combustible materials, the potential of plant to become ignition sources, and the relative location of combust­ ible material with respect to ignition sources. A careful check is therefore kept on the use of combustible

layout and design Station

2 Chapter

materials throughout the design stage of a station, and materials which have beneficial characteristics such as limited ignition and reduced fire propagation are speci­ fied. In addition, plant is designed, selected and laid out to limit the risk of ignition and the spread of fire from one area to another. Maintenance activities such as welding and cutting are tightly controlled as is the movement and storage of large quantities of combust­ ible material such as fuel oil or cleaning fluid.

13.3 Limiting the consequences of a fire

It is obviously desirable, and in the case of nuclear stations it is often essential, to limit the amount of equipment affected by any potential fire so that station availability can be preserved or plant can be safely shut down. In addition, there is often a requirement to provide a safe-means-of-escape route for personnel in the event of fire. The primary method of achieving this is by the sub-division of the station into separate areas so that the extent of any fire is limited to that area. This sub-division is normally achieved by separation of buildings or plant by an intervening space and is of particular importance at the site layout stage. Further physical segregation of plant into fire compartments and fire zones takes place within the buildings in order to reduce the average combustible loading, to segregate sources of ignition from combustible material, to segre­ gate redundant plant or to create escape routes. Very often natural fire barriers are created within the building due to the structural requirements of the building.

There may however be a requirement to further segregate the building into smaller compartments, and in this case additional fire barriers are erected. These fire barriers would be rated and certified as having been tested against a recognised standard such as BS476 Part 22 [1],

The need for segregation imposes a significant burden on space requirements during the layout stage, especially in the design of cable routes. It is therefore important that they are identified and established at an early stage in the development of the project.

13.4 Reducing the severity of fires

The severity of a fire can be reduced if it is detected early enough and measures are taken to extinguish the fire or to prevent it from spreading to adjacent areas. Water is the most common form of extinguishant used on a power station although foam systems are used on some fuel storage areas, and gases such as Halon or carbon dioxide are used in electrical equipment areas. The water extinguishing systems take two forms, an

Fire protection

external hydrant system serving each building, and a permanently installed water spray system covering selected plant. Each system is separated from the other and has its own water supply and dedicated pumping plant.

In the case of the hydrant system, water is pumped around the site via underground pipes fed from an inexhaustible supply such as the CW forebay. In general no building is greater than 75 m from any one hydrant and most buildings are served by several. Hydrants conform to basic Fire Brigade patterns and are used as a source of water for fire tenders or dry rising mains in the buildings. Figure 2.39 shows an external hydrant system layout.

The fixed fire protection system employs nozzles which spray extinguishing water over the envelope of the risk; the following areas are usually covered:

•Turbine-generator.

•Transformers greater than 100 kVA.

•Boiler fronts.

•Cable tunnels and cable flats.

•Standby diesel generators.

•Coal conveyors.

•Gas circulators.

•Oil or gas storage vessels.

Figure 2.40 shows a typical spray water system for h 3-unit oil-fired station. The water is pumped from a central pumping station which includes standby pump­ ing plant, a dedicated water supply with enough water stored to supply the largest risk, and a pressurised water tank to allow for instantaneous response.

Detection systems are selected from a range of commercially available equipment to suit the charac­ teristics of the working environment and the expected fire. With the exception of the turbine, systems are arranged to be automatic in operation and great care is taken to ensure that spurious operation is avoided. Typical areas of coverage are as follows:

• Turbine plant — frangible bulb heat detec­

tion and rate-of-tempera- ture-risc detection.