Powerhouse layout and design
Once the main dimensions of the generating plant have been finalised, the final
design of the powerhouse itself can commence. Chapter 4 also discusses powerhouse
arrangement and should be read in conjunction with this Chapter.
In our experience, it is better for the mechanical and electrical engineers to do
the initial layout because they have a better understanding of what is needed in the
way of access for normal operation and for maintenance and where the auxiliary plant
should be located relative to the generating plant.
If the flood rise will take the tailwater level well above the generating plant it
is important to minimise the floor area of the powerhouse that will be submerged
during a flood because an unnecessarily large area often leads to high excavation
costs and high costs to resist buoyancy. The external pressures from floodwaters can
have serious structural implications and if the buoyancy effect of the powerhouse is
ignored it is possible for walls and floors to crack and the whole powerhouse to lift
off its foundations. If the powerhouse is founded on rock or clay, rock bolts or piles
are one way of handling the uplift forces.
If the powerhouse is founded in gravel it cannot be held down with rock bolts
so it can be a good idea to extend a horizontal slab outwards from the walls at the
lowest floor level. When the power station is backfilled the weight of the gravel on top
of the slab securely anchors the powerhouse. Figure 83 shows the Wairau station in
New Zealand that was held down in this manner. It also shows how the draft tubes
were extended to avoid the construction of large and expensive retaining walls. If long
draft tubes are adopted it is necessary to check that dangerous negative water hammer
will not occur when the guide vanes close at their maximum rate.
The location of the draft tube stoplogs also needs to be thought about carefully.
In most cases they are located at the end of the draft tube but it is also possible to
install them just outside the powerhouse where the draft tube leaves the building and
use “slot blockers’’ to seal the opening into the draft tube under normal operation.
If the turbine runner is above the normal tail water level another option is to simply
remove the draft tube bend and bolt a steel plate over the inner end of the draft tube
so that if a flood occurs the power station will be sealed off. This is a lot cheaper and
easier than providing stoplogs.
The location of the switchgear, control room and control gear should also be
considered. In many cases, the best idea is to have them in a “lean to’’ against the wall
on the main powerhouse, rather than forming part of the main powerhouse structure.