For high head turbines there is often a
choice between Francis and Pelton. The main advantages of Francis
turbines are that the dimensions will be smaller and the speed higher.
Due to this the cost will be lower, both for the electro-mechanical
equipment and for the powerhouse civil works. By further utilizing the
most efficient solutions available, the construction of modern high-head
Francis turbines give a more easy to use and less maintenance demanding
result than previously.
This presented technology has been used
in a several of the world’s high-head Francis turbines. It traces its
roots back to the 1950s, when already Francis turbines with heads above
400 m were routinely delivered. One example is Holen 3 in Norway with a
rated net head of 610 m and a maximum gross head of 660 m. In Asia, it
has recently been used in more than a dozen high head Francis turbine
projects in China. Recently, it was also being used for the Nam Theun 2
project in the Republic of Laos, which will have 4 of 247 MW Francis
turbines at a rated head of 350 m. This paper gives an introduction to
several important solutions of the high head Francis turbine design. It
includes some of the important design details on runners, guide
bearings, shaft seal, shaft couplings, guide vane friction device and
the principle of equalized loads utilized on stationary part embedment.
Implementation of these design features will have positive impact on
construction cost, maintenance program and the general operation of the
turbine. Following are descriptions of each design solution with its
listed advantages for making the turbine more reliable, stabile and
flexible to operate.
Most recent, several features are being
incorporated in the Nam Theun 2, High-head Francis turbines. It uses a
X-Blade runner but without splitter vanes, vertical shaft guide bearing
with fixed pads, non-contact seal and guide vane friction device. And
the bottom cover is together with the draft tube cone not embedded,
making them available to be dismantled downwards and sideways with good
access for runner inspection and 
