Discussion. Maintenance of Hydro-Electric Schemes and the Development of Sidestream Intakes.

Abstract

DiscussionMr C. M. Roberts (Partner, Sir Wm Halcrow & Partners) said that he had been associated with the works mentioned in the Paper for a number of years.101. His remarks concerned the conveyance of water from a sidestream intake to a tunnel beneath it through a considerable drop in level. This question was dealt with by the Authors in 0 80 onwards. As they indicated, the main problem in effecting this conveyance was to avoid entraining air, not only because its intermittent explosive release through the intake itself might cause damage but also because it might be objectionable at the turbines to which the water was ultimately conveyed.102. At the Neaty Bum intake, which was being constructed for the North of Scotland Hydro-Electric Board as part of the Strathfarrar and Kilmorack project, the choice had been between the type of arrangement which prevented the mixing of water and air during descent (siphon, sloping shaft, or spiral shaft) and that which accepted this mixing during the descent of the water (vertical shaft) and relied on a device at the bottom to extract the air in an orderly way.103. For reasons of cost and maintenance facility a system had been adopted which was essentially the latter one, namely, a vertical shaft, but with waterways at the head of the shaft designed to make the nappe cling to the walls of the shaft and thus to minimize air entrainment on entry. It had of course been realized that, however successful the design might be in making the nappe cling to the walls, it would break up into foaming water after a certain drop and the problem still remained of designing an effective airextracting device at the foot of the shaft. The straight-flow type of de-aerating chamber, mentioned in 0 89, had been adopted and an investigation had been carried out to ascertain the shape of this chamber necessary to give the desired results. This investigation had been made for the North of Scotland Hydro-Electric Board by model tests at the laboratory of George Wimpey & Co. Ltd and he proposed to show a film illustrating stages of the investigation. He thanked the Board for permission to show the film, and Messrs Wimpey for lending it to him. 104. The model was to a scale of about 1 to 7. It included the de-aerating chamber itself and a portion of the 5-ft-6-h-dia. shaft and 12-in.-dia. vent pipe about 68 ft high, or roughly one quarter of the prototype height. The discharge was equivalent to 80 cusecs. The arrangement for introducing the water into the shaft had been deliberately designed to entrain a lot of air, about 80 cusecs, equal to the water discharm, much more than would occur in the prototype structure, in order to magnify the relative merits of the various arrangements. Some of the successive designs evolved were shown. 105. The model, with the de-aerating chamber as originally designed, allowed 0.38% of the entrained air to pass into the tunnel. The final design (Fig. 8), evolved as a result of the model tests, reduced this -re to 0.0012%. t