SURGE TANKS 525
12.10 Surge tanks
12.10.1 General
Surge tanks may be considered essentially as a forebay close to the
machine. Their primary purpose is the protection of the long pressure
tunnel in medium- and high-head plants against high waterhammer pres-
sures caused by sudden rejection or acceptance of load. The surge tank
converts these fast (waterhammer) pressure oscillations into much slower
- and lower – pressure fluctuations due to mass oscillation in the surge
chamber; the detailed treatment of waterhammer analysis is beyond the
scope of this textbook and the reader is referred to Jaeger (1997), Novak
(1983), Chaudry (1987), Fox (1989) and Wylie and Streeter (1993).
The surge chamber (Fig. 12.4) dividing the pressure tunnel into a
short high-pressure penstock downstream and a long low-pressure tunnel
upstream thus functions as a reservoir for the absorption or delivery of
water to meet the requirements of load changes. It quickly establishes the
equilibrium of the flow conditions, which greatly assists the speed regula-
tion of the turbine.
12.10.2 Surges in surge chambers
Sudden changes in load conditions of the turbine cause mass oscillations in
surge tanks which are eventually damped out by the hydraulic friction
losses of the conveyance. The amplitude of these oscillations is inversely
proportional to the area of the surge tank, and if the area provided were
very large dead beats would be set in the tank. Although these conditions
would be favourable to achieving the new equilibrium state very quickly,
the design would be uneconomical. On the other hand, if too small an area
is provided the oscillations become unstable; this is unacceptable. It is
therefore essential to choose a section in which the oscillations become
stable within a short period of time.
The critical section for stability is given by the equation
AscV^20 AtLt/2gP 0 H 0 (Thoma criterion) (12.30)
whereAtandLtare the tunnel cross-sectional area and length respec-
tively, and the suffix ‘0’ defines the steady state conditions prior to the load
variation (Fig. 12.20). A stable tank area is usually chosen with a safety
factor of about 1.5.
Using Manning’s equation with 1/n85 (smooth concrete finish)
gives, from equation (12.30),