SURGE TANKS 527
Instantaneous 100% demand conditions result in too large a
maximum downsurge, as the normal practice is to allow for 0–10% of full
load demand quickly but, thereafter, the unit is brought to full load only
gradually. Maximum downsurges are normally calculated against 75–100%
of full load and, once again, the bottom level of the chamber is controlled
by the reservoir at its lowest drawdown level condition. This condition is
invariably more critical than the one governed by the maximum down-
surge after a load rejection.
Excessive surges may occur if several quick load variations are
imposed on the unit (overlapping surges). These may create additional
governing difficulties and the top and bottom levels of the surge chamber
may have to be modified to accommodate these excessive surges.
In order to achieve conservative designs of the surge chamber it is
usual to assume a lower conduit friction factor than average for calculating
the maximum upsurge and a higher friction factor for the maximum down-
surge. In all cases due consideration of the effect of ageing of the tunnel
must be given. Head losses of all types are assumed to be proportional to
V^2. Turbine efficiency is assumed to be constant throughout its operational
range; it may be recalled that the Kaplan-type turbine satisfies this
assumption over a very wide range of its loading conditions.
12.10.3 Types of surge tanks- Simple surge tanks. The simple surge tank (Fig. 12.21(a)) is of
uniform cross-section and is open to the atmosphere, acting as a
reservoir. It is directly connected to the penstock so that water flows
in and out with small head losses when load variations occur. It is
usually large in size with expensive proportions and sluggish in
responding to damping surges. These are very rarely used in modern
practice except in installations where load changes are either small or
very gradual. - Throttled tank.In the throttled tank (restricted orifice type tank) the
restricted entry (Fig. 12.21(b)) to the surge tank creates retardation
and acceleration conditions of flow in the tunnel upstream of it, thus
reducing the storage requirement and minimizing the maximum up-
and downsurges. Although this type of surge tank is economical
(because of its smaller size) compared with the simple tank section,
the rapid creation of retarding and accelerating heads complicates
the governing mechanism, requiring additional inertia in the turbo-
generator units. - Surge tank with expansion chambers. This type of surge tank
(Fig. 12.21(c)) consists of a narrow riser (main surge shaft); attached
to it at either end are large expansion chambers. The narrow riser