High Head Plants. Fig. 11.6 shows the elevation of a high head plant. The water is taken from
the reservoir through tunnels which distribute the water to penstock through which the water is con-veyed to the turbines. Alternately, the water from the reservoir can be taken to a smaller storage known
as a forebay, by mans of tunnels. From the forebay, the water is then distributed to the penstocks. The
function of the forebay is to distribute the water to penstocks leading to turbines. The inflow to the
forebay is so regulated that the level in the forebay remains nearly constant. The turbines will thus be fed
with under a constant static head. Thus, the forebays help to regulate the demand for water according to
the load on the turbines. Trash racks are fitted at the inlets of the tunnels to prevent the foreign matter
from going into the tunnels. In places; where it in not possible to construct forebays, vertical construc-tions known as `surge tanks' are built. The surge tanks are provided before the valve house and after the
tunnel from the head works. The function of the surge tank is to prevent a sudden pressure rise in the
penstock when the load on the turbines decreases and the inlet valves to the turbines are suddenly
closed. In the valve house, the butterfly valves or the sluice type valves control the water flow in the
penstocks and these valves are electrically driven. Gate valves are also there in the power house to
control the water flow through the turbines. after flowing through tile turbines . The water is discharged