Power and Water on Ascension Island.
Talk by P. Mills (Member! – 10th March. 1993
Ascension Island was discovered on Ascension Day 1501 by Juan da Nova and occupied by the British in 1815 to deny French attempts to spring Napoleon from St. Helena, 700 miles away.
Nelson’s Marines built two catchments on Green Mountain, tunnelled through it and laid a pipeline to Georgetown on the W.coast. This, plus Dampier’s Drip, the only spring on the island, discovered by the Privateer while shipwrecked in 1699, provided the 6 gallon a day water ration for the U.K. expats, a bit dubious with sheep and wild donkeys roaming freely.
Politics decreed that all unskilled labour had to be directly employed by M.P.B.W. with priority for St. Helenians, the balance being from Barbados, Grenada and St. Lucia in the ratio 3:1:1.
Logistics were simple; rock and sea-water were available, everything else had to be brought in, thus chartering ships, planes and encouraging the R.A.F. to include Ascension in their navigation exercises were skills practised. A major restraint was that ships required a minimum cargo of, or payment for 200 tons to induce them to call and being keen to cash in on the S.African fruit trade would not off-load more than 400 tons, the crane limit was 18 tons.
The Brief was to provide for an ADMD of 5MW. voltage and frequency tolerance 4% to avoid causing instability of the BBC transmitters. and 30.000 gpd of potable water, their distribution, a telephone network which included music quality lines for the BBC, Sewage and refuse disposal and a bus service. Apart from the Power Station, other buildings were for the BBC. GCHQ. single and married quarters, shop and school. Midway through the project a groundstation associated with the Appollo project was added.
Initially six E.E. diesel alternators were installed, modified with 2 ton flywheels and GEC static exciters to cope with the BBC’s transmitter load, comprising four, each with a standing load of 450kVA and a further 250 kVA at the voice frequency of the programme.
Each diesel exhausts into a Spanner boiler- which then feeds into one of two steam accumulators. An electrode boiler also feeds into the accumulators, and is used as a stage of load control, the oil fired boiler, originlly provided to power the distillers during the construction period can also supply steam but is primarily used to dispose of waste lube oil.
Two 24,000 gpd ten stage flash distillers are heated from the steam accumulators. A 9,000 gpd submerged tube distiller is operated from heat exchangers extracting heat from the diesel water jackets and lubricating oil.
The sea water intake is believed to be unique, and comprises some 30 boreholes connected to three 70,000 gph about 30 yards inland, thus using the porous rock as a first filter and avoiding the solar gain of a settling pond.
The electricity system has multiple earthing. The underground MV cables have a 5th core for street lighting and split concentric outer, the insulated strands being used to operate a relay in each house to afford water heating control from the P.S. as a stage of load control.
Pyrotenax with sheath return was adopted as a contribution to reducing fire risk and also, in the case of the MV tails of the transformers, to limit fault power.
Apart from 13 amp plugs, HKv fuses for transformers, no fuses are used; protection being by miniature C.B.s.