OTHER COMPONENTS OF HYDROPOWER PLANTS 519
The forebay functions as a small balancing reservoir (with spilling
arrangements) upstream of the power house which accommodates
the sudden rejections and increased demands of the load. This may
be developed by enlarging the channel just upstream of the intake for
the penstocks. The usual components of an intake structure, such as
trash racks, gates, etc., must be included. The provision of air inlets at the
back of the gates to the penstocks is essential in order to prevent the devel-
opment of excessive negative pressures in the penstocks as a result of their
sudden draining, i.e. when the head gates are closed and the turbine gates
opened.
(See also Section 9.2 for details of intake structures).
12.9.2 PenstocksThe penstocks are pipes of large diameter, usually of steel or concrete,
used for conveying water from the source (reservoir or forebay) to the
power house. They are usually high-pressure pipelines designed to with-
stand stresses developed because of static and waterhammer pressures
created by sudden changes in power demands (i.e. valve closures and
openings according to power rejection and demand). The provision of
such a high-pressure line is very uneconomical if it is too long, in which
case it can be divided into two parts, a long low-pressure conveyance
(tunnel) followed by short high-pressure pipeline (penstock) close to the
turbine unit, separated by a surge chamber which absorbs the waterham-
mer pressure rises and converts them into mass oscillations.
(a) Design criteria
The hoop stress, pt, can be obtained as
ptpD/2e (12.25)whereDis the internal diameter, eis the wall thickness of the penstock and
pis the internal pressure including all the waterhammer effects. Equation
(12.25) gives the wall thickness as
epD/2ptj (12.26)wherejis the joint efficiency (0.9–1.0 for welded joints).
For a chosen penstock pipeline, the wall thickness can be calculated
by equation (12.26) assuming an allowable tensile stress, pt, of the material
(for steel, pt150 N mm^2 ). Further, the pipe thickness should be such as
to withstand additional stresses due to free spanning or cantilevering