INTRODUCTION

A surge tank 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. 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.

Figure 1: Surge Tank

When water flow is suddenly stopped by quickly closing the valve, water will oscillate in both pipe and surge tank. When water inertia and resistance in surge tanks are ignored, we can write:

LgdVdt±aV 2+z=0

Using the continuity equation,

Q1=¿Q2+QT ¿

(1)

A1V=A2dzdt

+QT

At valve closure, QT=0. Thus,

V=A2A1

dzdt

Where

L = Length of pipe

A1 = Pipe cross section

A2 = Surge tank cross section

V = Water velocity in pipe

z = Water level in surge tank

a = Constant, figure that shows losses in energy (friction + local losses)

Equation (1) and (3) can be written as:

∆V= gL

( z+aV 2 )∆ t

∆ z=A1A2×V ×∆ t

By using equations (4) and (5), values for z and V can be computed.

For case without friction, a = 0. Then equation (1) can be written as:

LgdVdt

+z=0

Substitute equation (3) into above equation, therefore

z= Lgd2 zd t 2

A2A1

Maximum amplitude and swing period can be written as

Zmax=±Q0A1 √ Lg A1A2

Where Q0 = A1V

(2)

(3)

(4)

(5)

(6)

(7)

And T=2π √ Lg A2A1PROCEDURE

1. The supply valve that connects to the fixed level tank was fixed.2. The water level for the fixed tank (water level that is shown by the surge tank) was

recorded, by closing the valve at the bottom.3. The valve was opened and calibrated until the water level in the surge tank remained at

5”.4. The water quantity was measured by using bottom tank (known dimension) and a

stopwatch. The water discharge was measured using:

Q=VolumeTime

5. The valve was closed immediately.6. It was made sure that there was a person on the ladder to observe the water level so

that the best reading could be obtained. The highest and the lowest reading was taken for at least 5 cycles. The others were instructed to record the time at the same time.

7. The stopwatch was started while the valve was closed. The time for each cycle was recorded. (highest and lowest)

8. Steps 5 until 8 were repeated, and was stopped when the water level in the surge tank became still.

9. The water level in the tank was controlled until 27”. Steps 4 until 8 were repeated.

(8)