Surge Tanks
Function and Types of Surge Tanks
Surge
Tanks Definition
Surge tank (or surge chamber) is a device
introduced within a hydropower water conveyance system having a rather long
pressure conduit to absorb the excess pressure rise in case of a sudden valve
closure.
The surge tank is located between the
almost horizontal or slightly inclined conduit and steeply sloping penstock and
is designed as a chamber excavated in the mountain.
There are three main types of surge tanks:
o Simple Surge Tank
o Restricted Surge
Tank
o Differential Surge
Tank
It also acts as a small storage from which
water may be supplied in case of a sudden valve opening of the turbine.
In case of a sudden opening of turbine
valve, there are chances of penstock collapse due to a negative pressure
generation, if there is no surge tank.
Surge
Tank Function
When the valve in a hydroelectric power
plant is suddenly completely closed, because of its small inertia the water in
the penstock stops almost at once.
The water in the pipeline, with large
inertia retards slowly. The difference in flows between pipeline and penstock
causes a rise in the water level in the surge tank.
The water level rises above the static
level of the reservoir water, producing a counter-pressure so that water in the
pipeline flows towards the reservoir and the level of water in the surge tank
drops.
In the absence of damping, oscillation
would continue indefinitely with the same amplitude.
The flow into the surge tank and water
level in the tank at any time during the oscillation depends on the dimension
of the pipeline and tank and on the type of valve movement.
The main functions of a surge tank are:
o It reduces the
amplitude of pressure fluctuations by reflecting the incoming pressure waves
o It improves the
regulation characteristic of a hydraulic turbine.
The surge tank dimensions and location are
based on the following considerations:
o The surge tank
should be located as close to the power or pumping plant as possible;
o The surge tank
should be of sufficient height to prevent overflow for all conditions of
operation;
o The bottom of surge
tank should be low enough that during its operation the tank is drained out and
admit air into the turbine penstock or pumping discharge line; and
o The surge tank must
have sufficient cross sectional area to ensure stability.
Types
of Surge Tanks
There are different types of surge tanks
that are possible to be installed. Some of the most common types of surge tanks
which are as follows:
Simple
Surge Tank:
A simple surge tank is a shaft connected to
pressure tunnel directly or by a short connection of cross-sectional area not
less than the area of the head race tunnel.
Restricted
Orifice Surge Tank:
A type of surge tanks in which the inlet is
throttled to improve damping of oscillations by offering greater resistance and
connected to the head race tunnel with or without a connecting/communicating
shaft
Differential
Surge Tank:
Differential Surge tank is a throttled
surge tank with an addition of a riser pipe may be inside the main shaft,
connected to main shaft by orifice or ports. The riser may also be arranged on
one side of throttled shaft.
In an underground development of hydropower
system, tail race surge tanks are usually provided to protect tail race tunnel
from water hammer effect due to fluctuation in load.
These are located downstream of turbines
which discharge into long tail race tunnels under pressure.
The necessity of tail race surge tank may
be eliminated by ensuring free-flow conditions in the tunnel but in case of
long tunnels this may become uneconomical than a surge tank.
Water
Surface Oscillation
The height of the surge tank is governed by
the highest possible water level that can be expected during operation.
Variations in demand initiated by a rapid
opening or closure of the valve or turbine are followed with a time lag by the
water masses moving in the tunnel.
Upon the rapid and partial closure of the
valve following a sudden load decrease, water masses in the penstock are
suddenly decelerated, and one part of the continuous supply from the tunnel
fills the surge tank.
The water surface in the surge chamber will
be raised to above static level.
In case of rapid opening, the flow in the
tunnel is smaller than the turbine demand to supply water to the turbine.
The water surface in the chamber will start
to drop to below of the steady-state level.
To establish steady-flow conditions, the
water surface will again start to rise from the low point, but owing to the
inertia of moving water, will again rise over the steady-level.
The cycle is repeated all over again with
amplitudes reduced by friction, i.e. the oscillation is damped. The phenomenon
described is the water surface oscillation.
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