~ South African Construction World ~

     Following the failure of the tailrace screens as a result of galvanic action between steel bolts and stainless steel nuts at the Palmiet Pumped Storage Scheme in mid 1992, Eskom implemented a program to inspect the screens at all its remaining hydroelectric stations. Initial inspections by divers at the 1,000 MW in KwaZulu-Natal showed that 13 of the 64 screens had fallen out in the tailrace and were lying in the rock trap below the intake. The screens measure 4.5 m by 0.5 m and weigh 1.2 tons. No screens had fallen out in the headrace but a major percentage of the bolts were loose at both headrace and tailrace screens.

     The missing screens posed a high risk to the four reversible 250 MW Francis turbines as they were exposed to debris which could cause irreparable damage to the runners. Remedial measures were initiated by the engineering arm of Peaking Generation, the group responsible for maintenance on Eskom's hydro plant. The remedial measures had to be carefully planned to ensure maximum availability of the turbines to the Eskom system.

     To facilitate planning, a partial dewatering exercise was carried out to determine the actual extent of the damage and to remove the tailrace screens that had fallen out for closer inspection and repair.

     The partial dewatering exercise showed that it was possible to draw down the tailrace Kilburn Reservoir by 29.6 million ml in approximately 65 hours by using the three turbines in pump mode. However, to draw down the reservoir an additional 0.8 million ml to the control level in the tailrace canal would take a further 162 hours using one throttled turbine (provided, of course, that the turbine experienced no problems). This was not acceptable and therefore a coffer dam was needed at the control point in the tailrace tunnel (to retain a 500 mm head) to limit the draw down time to 65 hours.

     The proposed sandbag solution would have required 2,900 sandbags and the estimated time of installation was 12 hours. One drawback was that the plastic covers used on the sandbags provide a fairly low resistance to sliding, and would thus have posed a possible threat to the stability of the coffer dam.

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Article: South African Construction World, Vol14/Iss2 (Page 01)