Page 304 - Fluid Mechanics and Thermodynamics of Turbomachinery

Hydraulic Turbines 285

Surge tank
Reservoir

Penstock head

Penstock
Z R

Pelton wheel

Nozzle
Z N
Datum level
FIG. 9.7. Pelton turbine hydroelectric scheme.

FIG. 9.8. Methods of regulating the speed of a Pelton turbine: (a) with a spear (or needle)
valve; (b) with a deflector plate.

(called “water hammer”). If the spear valve did close quickly, all the kinetic energy
of the water in the penstock would be absorbed by the elasticity of the supply
pipeline (penstock) and the water, creating very large stresses which would reach
their greatest intensity at the turbine inlet where the pipeline is already heavily
stressed. The surge chamber, shown in Figure 9.7, has the function of absorbing
and dissipating some of the pressure and energy fluctuations created by too rapid a
closure of the needle valve.