Bulb / Pit / S — bulb, pit and S-type hydro turbines up to 80 MW

Description

Bulb, pit and S-type turbines are horizontal axial machines for low-head hydro plants with a head of up to 30 m and large water flows. For decades the manufacturer has refined the hydraulics, design and manufacturing technology of such turbines. By its assessment, the higher efficiency at full load and the greater throughput of bulb and pit turbines compared with vertical Kaplan turbines give a higher annual energy yield and a lower relative construction cost at low-head sites. The design offers good access for inspecting components and is built for a long service life.

The bulb turbine is the most widespread option for high outputs at low head: the generator sits in a bulb directly in the flow, and the runner is mounted on the shaft as a cantilever. Pit turbines with a gearbox between the runner and the generator are used at heads below 10 m. Pit and S-type machines, where the generator is located above or below the flow, are chosen for economical solutions at small hydro plants of up to about 10 MW. Runners have 3, 4 or 5 blades. The hydraulics are developed using CFD calculations of pressure and velocity distribution and model tests.

Since 1955, more than 282 machines with outputs from 19 kW to 76.55 MW have been installed, with runner diameters from 500 to 8400 mm. Technical capabilities extend to 80 MW, 30 m head and a runner diameter of 8.5 m. References include Murray Lock and Dam (USA), where the world's largest pit units operate with 8.4 m runners, 20.5 MW / 22.8 MVA and a two-stage planetary gearbox. Ybbs-Persenbeug has 48 MW with a 7.5 m runner, Altenwörth 44 MW. At Santo Antonio and Jirau there are 23 units of about 76 MW with 7.5 m runners, on the Ohio River 11 units with 7.7 m runners, at Rheinfelden 4 × 25 MW. Toyomi in Japan has the world's largest vertical bulb unit of 32 MW. At Oxec II an S-upstream unit of 19.5 MW operates at a head of 31 m, and at Lower Olt the reversible bulb pump-turbines of 14.5 MW have been modernized.

The turbines are supplied together with bulb generators from the same manufacturer. The voltage of such generators has risen from the initial 3.3 and 6.6 kV to 11.0 and even 13.8 kV on more powerful units, and the outputs exceed 80 MVA. Cooling has evolved from separate heat exchangers to maintenance-free closed circuits that release heat directly into the river water. For very high outputs and high-speed units, compressed air can additionally be used.

Technical specifications

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