Reference: www.rockmass.net March 2008

AIR CUSHION SURGE CHAMBER a cost-effective solution in hydropower design

by Arild Palmstrom, Ph.D, RockMass as

The purpose of the air cushion surge chamber is to damp surges in the headrace pressure tunnel of the

underground hydropower plant. It replaces the conventional, open surge shaft, see Figure 1.

The surge chamber is hydraulically connected to the headrace tunnel via a short connection tunnel (Figure 3).

Six of the chambers have pressure above 4 MPa, one as high as 7.7 MPa. The volume varies from 2,000 to

110,000m3. The air cushion itself occupies from 25% to 85% of the cavern volume with typically 2 to 5m

depth of the cavern water bed, see Figure 2.

An important provision in a successful design and construction of air cushion surge chambers is the layout

and relevant measures regarding the rock stresses acting and the permeability of the surrounding rock

masses. The application of a special designed water curtain has proven an effective means of reducing

possible air losses during plant operation, as is shown in Figures 2 and 3.

HEADRACE TUNNEL

HEADRACE TUNNEL

PRESSU

RE S

HAFT

PENSTOCK

POWER STATION

POWER STATION

POWER STATION

TAILRACE TUNNEL

TAILRACE TUNNEL

ACCESS TUNNEL

ACCESS TUNNEL

RESERVOIR

RESERVOIR

RESERVOIR

HWL

HWL

HWL

PRESSURE TUNNEL

OLD SOLUTION WITH PENSTOCK

SOLUTION WITH PRESSURE SHAFT

SOLUTION WITH PRESSURE TUNNEL AND AIR CUSHION SURGE CHAMBER

SURGE SHAFT

SURGE SHAFT

AIR CUSHIONSURGE CHAMBER

LAKE TAP

LAKE TAP

LAKE TAP

FIGURE 1. DEVELOPMENT IN DESIGN OF UNLINED PRESSURE CONDUITS INTO PRESSURE TUNNEL WITH AIR

CUSHION SURGE CHAMBER

Reference: www.rockmass.net March 2008

Headra

ce tu

nnel

Con

nect

ion

tunn

el

ChamberWater curtain holes

0 50m

Air pressure4.44 MPa

Water curtain borehole

Water curtain gallery

Plug

Water bed

Connection tunnel

PLAN

SECTION A - A

A A

Headrace tunnel

FIGURE 2. LAYOUT OF THE AIR CUSHION SURGE CHAMBER AT TORPA HYDROPOWER PLANT, DESIGNED BY NORCONSULT

In addition to the layout of

the chamber, the calculation

of air loss and the design of

mechanical equipment as

well as the surge calculations

are vital features in the air

cushion surge chambers.

A total number of 10 air

cushion surge chambers have

been constructed in Norway,

the first one has been in

operation since 1973. All

together they provide more

than 200 years of experience.

FIGURE 3. AIR LOSS IN NORWEGIAN AIR CUSHION SURGE CHAMBERS. AS SEEN, WATER SEALING BY GROUTING AND

ESPECIALLY INSTALLATION OF A WATER CURTAIN HAS REDUCED THE AIR LEAKAGE.

Norm

alize

d A

ir L

os

s

[Nm

/ (h

m

MP

a)]

3

2x

x

Mean Operating Pressure Ratio

no grouting or water curtain

with initial water curtain

initial

after grouting

initial

after installation of water curtain

0 0.5 1 1.5 2 2 .5 3.0

1

0.1

0.01

0.001

0.0001