is connected to the unit plant by means of a pipe bridge and cable trench.

The plant is serviced by many external facilities such as rail sidings, water services, electrical services, etc.

The location of FGD plant on a greenfield site would need to be considered as part of the overall site layout exercise.

See Volume B, Chapter 4 for detailed descriptions of FGD processes.

9 Main steam pipework

The main concern in developing the layout for main steam pipework is to provide for flexibility and to

minimise loadings at the terminal points of the boiler and turbine. In practice, because of the high cost of this pipework, a minimum total piping run is preferred and often the layout adopted will be a compromise based on economic as well as technical factors.

The thermal expansion and contraction of the pipe­ work needs to be accommodated by the inherent flexi­ bility designed into the system, and by the use of cold pull-ups and constraints which are applied to the pipe­ work where needed. Cold pull-up is the term used to describe the prestressing of a pipework system in the cold condition, such that it is in a neutral state after expanding to the hot operating condition.

Flexibility is a major concern during the layout process, and it is advisable for continuous reviews of

Fig. 2.35 Main steam pipework arrangement for transverse turbine

The longitudinal turbine arrangement often results in a longer asymmetrical piping layout as shown in Fig 2.36, unless the ideal layout can be adopted by off-setting the turbine-generator unit from the boiler centreline. In an overall station layout context, whilst off-setting of the turbine hall relative to the boiler house may be viable in a technical sense, the overall effect on the station’s visual appearance may need to be considered. Additionally, if an asymmetrical piping layout is adopted than a review of steam pressure and temperature variations should be undertaken to ensure that the steam conditions at the turbine inlets are within acceptable tolerance levels.

To permit the monitoring and recording of pipework expansions and creep, a measurement system has been introduced on more recent stations within the CEGB. These measurements are taken following completion of construction at regular intervals in the life of the station. Suitable access facilities should therefore be provided in the layout for this activity.

Fig. 2.36 Main steam pipework arrangement for longitudinal turbine

10 Low pressure pipework and valves

Low pressure pipework and valves are usually asso­ ciated with the following services systems within the power station:

•Town water.

•River water.

•Treated (demineralised) water.

•Chlorine solution dosing.

•Reserve feedwater.

•Blowdown water recovery.

•Fire hydrant system.

•Fixed water spray protection.

•Ash sluicing supplies.

•Nitrogen supplies.

•Potable water supplies (fit for drinking).

•Compressed air supplies.

Many of these services lend themselves to trunk main routing throughout the length of the station with branch supplies out to each unit. Ample space for the routing of the pipework must be reserved, for example in the heater bay between the sets and the boilers, or below the operating floor walkways.

In the early stages of design it is essential that diagrams arc prepared for all low pressure services and a pattern established of valve and pipework require­ ments. The diagrams are then constantly developed as

Station design and layout

the requirements of the main plant items services become known. Low pressure pipework system design and layout must be responsive to the osciall main plant layout. As with all pipework design, the route and pressure drop must be optimised to give best overall efficiency.

Pipework should not be run in trenches or small ducts out of sight. It should be visible and accessible for maintenance together with its associated valves. There, must be correct drainage falls with air release valves fitted where necessary. Pipework should be wellsupported and secured. Pipe joints are usually the weakest links in a pipework system and demand the highest standards of materials and workmanship. The number of joints should be minimised by the use of welding.

11 Water storage tanks

Water storage represents a large space requirement and could, for instance, be in excess of 9090 nr’ of town water to cover station requirements for 24 hours. Reserve feedwater should be available for at least

Chapter 2

24 hours maximum continuous rating (MCR) supply lor each boiler.

Storage facilities lor town water are normally located externally to the main building, perhaps adjacent to the water treatment plant, but certainly with consideration being given to other uses and needs on this particular commodity, i.e., specific reserve for fire fighting pur­ pose, FGD requirements, etc.

The design of the storage facility is dependent on site space availability and various forms can be considered, e.g., low profile pressed steel tanks, concrete reservoirs of vertical cylindrical tanks. The vertical cylindrical type of tank is most common for reserve feedwater

is uneconomical in terms of steelwork to support the load. In addition, a disaster situation would arise if a tank weld or interconnecting pipe connection failed. Figure 2.37 shows a typical town water and raw waler system for an oil-fired station.

PUMPS SHUTDOWN

FIRST TRANSFER PUMP STARTS SECOND TRANSFER PUMP STARTS THIRD TRANSFER PUMP STARTS LOW LEVEL ALARM

EXTREME LOW LEVEL ALARM

STATION HIGH LEVEL BREAK TANKS

PLANT