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Case

Storie

s

Case

Storie

s Conjet Robot boosts up

 power plant capacity 

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A Swedish Conjet Robot 361hydrodemolition machine has played a major role in boosting the electricity generating capacity of three hydro-electric power plants 

on the Drava River in the Republic of Slovenia. Contractor Gradis is using its Conjet Robot for removing concrete from nine vertical turbine draft tubes. This operation will make way for new reshaped steel linings and larger diameter, more efficient, greater capacity turbine generator sets.

Gradis worked on its DM 2.9 million contractfor client and power station owner DravskeElektrarne Maribor as part of the approximateDM 200 million second phase of refurbishing

and upgrading three of the Slovenia’s chainof eight hydro-electric power plants. Thesepower stations are strategically located alongan approximate 100 km stretch of the Dravariver, which falls about 120 m between theupper and lower power stations. The powerplants, which operate on the run-of-riverstorage principle, were built during a 60 yearperiod from 1918 to 1978 and provide about23% of Slovenia’s electricity. But the ageingpower plants’ electrical and mechanicalequipment were worn out and were graduallyreplaced over a period of several years.

The hydro power plant at Fala, the first tobe commissioned in 1918, was also the firstto be upgraded. This plant’s original seventurbine generators were replaced with two

new more efficient ones in 1991. The secondphase of refurbishment, which started in 1996,was centred on the three hydro-electric stationsat Dravograd, Vuzenica and Mariborski otok,which were all built during the 1940s and early

1950s. Dravograd’s renovation was completewith three new turbine generator sets whichincreased the power station’s total installed out-put by 31% from 22.8 MW to 29.8 MW. One ofMariborski otok’s three turbines was alsoreplaced, but Gradis focused on removingconcrete from the three turbine draft tubes atVuzenica, before returning to complete itscontract on the two remaining turbines atMariborski otok.

One of Vuzenica’s three turbine generatorsets had already been replaced when Gradiswas removing concrete to increase the diame-ter of the power station’s second turbine drafttube. But before Gradis could put its ConjetRobot to work and make a start onhydrodemolition, the power station’s overheadgantry crane had to lift out the old worn outgenerator and turbine. This operation wasfollowed by another demolition contractorcutting out an approximate 5.25 m high sectionof the turbine’s cylindrical to rectangularstainless steel draft tube casing liner, which wasembedded in concrete.

65 m3 of concrete had tobe removed from the draft tube by

hydrodemolitionGradis, which adopted the same hydrodemo-lition technique for all nine turbines, placed itsConjet Robot on a moveable platform inside

Conjet Robot 361 in the process of increasing the chamber diameter in the turbine draft tube.3312800326

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Equipment used

1 Conjet Robot 361 MPA1 Conjet Powerpack 535-550 kW

the confined 4.65 m diameter vertical turbinehousing. About a 840 mm thickness ofconcrete had to be cut out round the entireperiphery to increase the diameter to 6.33 m.This was done using the Robot 361 makingthree separate circum-ferential cuts from theworking platform set at two different levels. Thefirst two passes removed about 780 mmfollowed by a third and final cleaning cut of

about 60 mm. About 65 m3

of concrete had tobe removed from the draft tube byhydrodemolition, which the Robot 361 wasable to achieve at about 1m3 /hour. Theconcrete spoil was allowed to fall into the drafttube sump were it was loaded by a miniexcavator into a skip and lifted to the surfacefor disposal.

Conjet proved to be moreefficient than expected

”Hydrodemolition was specified for thiscontract,” said Gradis technical director IvanKosi. ”We were aware of Conjet and knew their

hydrodemolitionequipment wouldwork. Even sowe’ve been verysurprised with theConjet Robot 361as it has been moreproductive than weexpected and alsoproved to be veryrobust and reliableworking in a veryconfined and ho-

stile environment.”All the Robot361’s functionswere remotelycontrolled by theRobot operatorworking with abird’s eye view highabove the cuttingarea. High press-ure water

for the Conjet Robot was supplied bya Hammelmann power pack. Thisconsisted of a heat and sound

insulated ISO container housing a 750hp (550 kW) Caterpillar diesel enginedriving a pump operating at 1400 barpressure and flow of 194 litres/minute.

Hydrodemolition, theonly approved method

”We have been very pleased withGradis and their Conjet Robot, whichhas performed to our expectations,”said Dravske Elektrarne Maribor engineerDuvìan Rajh, who spent a long time evaluatinghydrodemolition prior to starting on the

refurbishment programme. ”I read an articleabout Conjet and went to Stockholm to seetheir equipment working for myself and wasvery impressed,” adds Mr Rajh. ”Our consultingengineer IBE was also in favour ofhydrodemolition so the technique was written

into the contract specification as the onlyapproved method which would be allowedto remove the concrete in the turbinehousings. We would not allow pneumaticbreakers to be used as they would havecaused cracking and secondary damage tothe concrete left behind.”

Turbine up-grade boosts the

power output by 43 per centOnce Gradis had completed the concreteremoval a new and larger 4.85 m diameterstainless steel liner casing, reshaped toimprove hydraulic efficiency, was positionedin the void, which was backfilled with freshconcrete. This was followed by installing anew four bladed, variable pitch turbine, builtby Litostroj in Slovenia, coupled to a Sie-mens generator. The new runner, just 200mm larger than the old one and operatingat 125 rev/min with the same maximumgross head of 13.65 m, is able toaccommodate an approximate 15%

increase in water flow to just over 183 m3 / second, raising rated power output of thereplacement turbine generator set to 18.6MW. This second turbine generator at theVuzenica hydro-electric power station isscheduled to go on stream in May 1999.Completion of the identical third unit, aboutone year later, will boost the refurbished anduprated plant’s total installed power outputby nearly 43% to 64 MW.

After finishing the hydrodemolition workat Vuzenica, Gradis moved back to com-plete its outstanding work at Mariborski otok

and used the Conjet Robot 361 in an iden-tical manner to remove concrete from thestation’s two remaining draft tubes.Completion of the second phase of the re-novation and upgrading of the upper DravaRiver hydro power plants provided not onlyfurther electricity generation in those plantsbut also additional power capacity of roughly40 MW.

The operator monitors and controls the work progress,from a position above the working area.

The new four blade, variable pitch, turbine.

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