reacts quickly, creating accelerating or decelerating heads, and at the
same time the expansion chambers minimize the maximum up- and
downsurge levels, thus limiting the range of surge levels (i.e. easier
governing). In order to reduce the costs of the structure, spilling
arrangements may sometimes be provided either to wastage (if water
is not scarce) or back to the penstock.
- Differential surge tank.This type (also known as Johnson’s differen-
tial tank – Fig. 12.21(d)) consists of an internal narrow riser shaft
with an orifice entry to the larger outer shaft at the bottom. As the
central riser is narrow it responds instantaneously during the upward
phase; at the same time the maximum amplitude is restricted to its
top level, any excess water spilling back into the outer chamber. Simi-
larly, during the downward phase water spills into the narrow riser
528 HYDROELECTRIC POWER DEVELOPMENT
Fig. 12.21 Types of surge tanks
