'Hie two gases are combined in a mixing facility before being introduced into the catalyser section of the methanation plant. The catalyst reacts with the hydrogen/carbon dioxide gas mixture to produce methane gas, which is passed on via a cooler to a compressor. The compressed methane gas flows through drying equipment before being stored in cylinders.

Some power stations do not operate an on-site methane generator plant, but prefer to hold methane gas in cylinder stores. In these cases, delivery of the gas is by road trailer and the storage facility is provided with the necessary trailer access and means of unload­ ing. From the storage cylinders, the gas is passed through pressure control equipment before being distri­ buted through a pipework system to the point of usage. The storage area is located in an open environment and care must be taken to ensure separation from sources of oxidants, for example, oxygen stores.

There are other gases which are used in relatively small quantities (e.g.. for laboratory purposes) which also require assessment ol storage and safety require­ ments, some of these are:

These gases arc normally stored in cylinders and may be portable.

24 Pumped storage plant

The plant required for pumped storage power stations is in principle much simpler than on more conventional power stations because there is no prime requirement for boiler and fuel handling plant. However, because of the potential for providing system reserve and the need to maintain sufficient suction head during the pumping mode, other features and design methods are required which are not found on conventional stations. The principal features affecting the design and layout of the station plant are:

•Station rating.

•Operating response on start-up.

•Duty cycle (daily or weekly).

•Plant operating regime.

•Cost.

Pumped storage plant

•Environmental impact.

•High availability and reliability.

•Flooding hazard.

•Fire hazard.

The last two items in the list arc more important if the main plant is to be located underground. Flooding could be avoided by either complex isolation arrange­ ments such as bulkhead doors, or by qualifying the integrity of the hydraulic system. Combating the fire

the plant is located underground. In the case of plant located at ground level, the plant items required will be the same butthe layout task is eased.

In designing the main plant layout some contingency must be made to cover the possibility of encountering severe faulting in the rock. For this reason the initial design will try to use the minimum possible excavation which has an impact on arrangements for initial con­ struction and subsequent maintenance of the plant. A balance is required which aims to minimise the con­ struction programme.

One strategy for minimising the risk of the project if poor ground is encountered is to use a number of

will be given to the most economical arrangement of multiple caverns to give flexibility of construction, so that if for any reason work is held up on one excava­ tion. work can continue on the others. This can be achieved by arranging multiple access routes to the main excavations. This strategy was adopted with great success on the Dinorwig project; Fig 2.91 shows the arrangement of tunnels and chambers in more detail. The underground complex consists of the machine hall, main inlet valve (MIV) gallery, draft tube valve (DTV) gallery and the transformer hall. The 400 kV transmission substation is located on the upper floor of

hall with the transformer hall, and accommodate the generator-motor busbars and 11 kV electrical switch­ gear (see Fig 2.92). Within the machine hall there is a 9-storcy building accommodating control equipment, electrical plant and welfare facilities. At each end of the DTV gallery there is a chamber housing starting equipment for the generator-motors. Above the main complex a system of high level tunnels provides ventilation.

On discharge from the turbines, the water flows through the DTVs which isolate the pump-turbines when dewatering. The conduit between the DTV and the pump-turbine is the lowest part of the system and the arrangements for dewatering and drainage sumps are located at this point.

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24.1 Hydraulic machines

Pumped storage requires the installation of two hy­ draulic machines, a pump and a turbine. The two functions can be either separate, as at Ffestiniog, or combined into a single reversible machine called a pump-turbine as has been used at Dinorwig. Figures 2.93 and 2.94 show the two arrangements. For reasons of economy the highest practicable running speed is necessary. Large hydro-electric plant is limited in running speed by the ability of the generator to withstand the maximum runaway speed of the turbine. For reversible pump-turbines the ratio of runaway speed to normal running speed is about 1:4 whereas for turbines the normal speed selected can be higher than for a turbine of the same output and head. However this higher speed requires a deeper submergence when operating in the pumping mode to avoid cavitation. In the case of underground power stations the economic penalty for this greater submergence is not very great since, apart from the small extra length of access , tunnels and penstocks, the total excavation is un­

changed. Penalty in the civil works is more than compensated for by the lower machine costs. Design considerations for the generator-motor also influence

the speed selected. At Dinorwig a speed of 5<K) r/min was selected with a minimum submergence of 60 m at the runner centreline.

The pump-turbine structure is subjected to very large hydraulic forces and partial or complete embedment in concrete is sensible to physically restrain movement and to keep noise levels to acceptable levels.

24.2 Generator-motors

The generator-motors can cither be reversible as at Dinorwig, or have a single operating direction as at Ffestiniog. The latter is only possible when the hydrau­ lic machine modes of pump and turbine are realised in separate components. The factors affecting the choice of generator-motor are:

•Unit size and speed.

•Cost.

•Cooling and reliability.

•Physical size.

The specific speed of the pump-turbines dictates the generator-motor speed and many combinations are

TURBINE

INLET PIPE

TURBINE

INLET VALVE

TURBINE SHAFT

PUMP COUPLING

PUMP SHAFT

STORAGE PUMP

DISCHARGE VALVE

GENERATOR < MOTOR

RESERVOIR

WATER LEVEL

£3

TURBINE

RELIEF

OUTLET

TURBINE

RELIEF VALVE

TURBINE

DRAFT TUBE

OUTLETS

possible. The choice is usually made on economic grounds without resorting to large extrapolation from existing practice.

Costs usually favour a small number of large machines. This is because the size of the caverns increases greatly with many small machines, whereas the plant costs stay reasonably constant because the optimum speed of operation decreases with unit size, leading to more expensive electrical machines.

Generator-motors may be cooled either by air or water. Air cooling is currently just feasible up to 450 MW unit size but this requires the extrapolation of all the critical design features of the cooling system. Water cooling would offer a more secure engineering design but the reliability of the water-cooled machine may not be high enough. At Dinorwig, air cooling was used based on the following reservations about water cooling:

•CEGB experience of water-cooled stators.

•No international experience with reversible watercooled sets.

•Existing and proposed designs for water-cooled sets

did not include high air pressures within the machine; water leaks would be critical and cause damage to the stator windings.

•The large number of load cycles and stress cycles

•There was no experience of reversible pump­ turbines at outputs as great as 450 MW with a high head of 500 m.

•There was no point in reducing the size of the rotor

by employing water cooling if additional inertia had to be built-in to maintain the required inertia constant.

• The 300 MW and 225 MW machines were within acceptable parameters for air cooling.

• The lack of experience of any set running at 600 r/min above 135 MW made the 500 r/min machine preferable.