CIRCULATING WATER SYSTEM
TIR JOHN POWER STATION
Under an agreement with the
Great Western Railway Co., the circulating water for
cooling Tir John Power Station was drawn from the King's
Dock and, after passing through the power station’s
condensers, it was returned to the Queen's Dock. The
quantity of water required for the first section of the
station, i.e. 120,000 kw., was of the order of six
million gallons per hour. As the Kings Dock and Queens
Dock are inter-connected, the total cooling surface area
was in excess of 200 acres.
The circulating water was
conveyed from King's Dock to the power station and back
to Queen's Dock in two deep underground tunnels
terminating in vertical shafts at either end of each
tunnel. Sections of the tunnels and shafts are shown on
the drawings below. The tunnels, which remain in place
to this day, are of horseshoe cross-section about 9 feet
in diameter, and are lined with 9 inches of concrete.
They were driven in rock at a depth of 300 feet below
ground level at the power station end, and 250 feet and
240 feet at the King's Dock and Queen's Dock ends
respectively. The difference in levels is due to the
gradients given to the tunnels for the purpose of
drainage during construction. The length of the intake
tunnel from King's Dock to the former Tir John site is
about half a mile, and the length of the discharge
tunnel from the site to Queen's Dock is about three
quarters of a mile.
vertical shafts, today filled and capped, were 14 feet
in diameter, and the two at the power station end were
sunk through solid rock and lined with concrete. At the
docks end, the upper sections of the two shafts had to
be lined with cast iron where they passed through the
water-bearing strata to the rock below, and from that
level down they too were concrete lined. The sinking of
the shafts at the Tir John end and the boring of the
tunnels to the docks offered no great difficulties, but
the sinking of the intake and discharge shafts at the
docks end called for special treatment. The
water-bearing strata had to be passed before solid rock
could be reached, and the overall depth of the strata
between the surface and the rock was around 130 feet.
This was too great a depth for the use of compressed
air, so a freezing process was adopted. This process
consisted of a ring of borings of the required depth
into which pipes were inserted. Then a freezing mixture
was circulated through the pipes until a circular ice
wall of the necessary thickness was formed. The shafts
were sunk and permanently lined inside this wall, after
which the ground was allowed to thaw out.
The Turbine Room was 105ft. wide and 200ft. long,
including an unloading bay at the eastern end with a connection
to the power station’s sidings. It contained two 30,000 kw.
turbo-alternators and one 750 kw. diesel engine-driven
alternator for auxiliary purposes. Space for a future 60,000 kw.
turbo-alternator was also provided.
The turbo-alternators were designed
to operate with a steam pressure of 600 lbs. per square inch,
having a total temperature of 825 degrees Fahrenheit. Each unit
consisted of a two cylinder tandem turbine driving a three-phase
alternator, with mam and auxiliary exciters mounted on a common
shaft driven from the rotor shaft through a flexible coupling.
The turbines were of the pure reaction type, with blading of
stainless steel, and the speed of the turbo-alternators was
3,000 revs per minute.