Pelton turbine System Erlach: Horizontal up to six nozzles

System Erlach with six nozzles: the same discharge, a smaller machine, a horizontal shaft

For every Pelton turbine: a single jet must not become too thick in relation to the width of the runner bucket. As the discharge grows, it is therefore split over several nozzles. Each additional nozzle allows a higher speed at the same head and the same discharge. Whether the next speed step fits the pole number of the generator has to be checked in each case. If this is used, runner and generator become smaller. The characteristic number behind this is the specific speed. More under Pelton turbine specific speed.

Conventionally a choice has to be made: many nozzles on a vertical shaft or few on a horizontal shaft. System Erlach has up to six nozzles and a horizontal shaft.

Pelton turbine, conventional vertical: usual from three nozzles upwards, because of the splash water

The water that leaves the buckets still carries about two to four per cent of the energy of the jet. It splashes and takes in a lot of air. With one or two nozzles on a horizontal shaft it drains downwards. With more nozzles, part of it is thrown upwards, bounces off the housing and falls back onto the runner. That costs power.

At the beginning there were only horizontal machines with one or two nozzles. To get more power into one machine, the shaft was set vertical, which allowed up to six nozzles. In Norway, for example, a horizontal machine with two nozzles still went into operation at Skjåk in 1965, and a vertical machine with six at Evanger in 1968.

Source: J. M. Erlach, New Pelton Concept, Viennahydro 2012, Full Paper, sections 1 and 2.2; H. Brekke, Hydraulic Turbines, NTNU 2015, p. 3; Josef Erlach, 11 October 2026 (spacing of the two nozzles, 60 to 70 degrees).

Pelton turbine, conventional horizontal: three nozzles only with narrow spacing

On the usual horizontal machine with two nozzles both sit in the lower part of the runner, 60 to 70 degrees apart; the water falls away downwards, and the buckets have enough time to empty. With three nozzles on a horizontal shaft the nozzles have to be set just as close, so that no water gets to the top. This shortens the time for emptying and lowers the power. In any case the spacing must be at least 60 degrees and depends on the arrangement and the space available.

If the nozzles are distributed evenly around the circumference, three nozzles stand 120 degrees apart. The model test shows a higher efficiency for this than for 60 degrees. This only works if the splash water is guided.

Source: Viennahydro 2012, Full Paper, sections 2.2 and 2.5, Fig. 7. The paper shows curves that are related to the best value and states no efficiency in per cent; Josef Erlach, 3 and 11 October 2026 (spacing at least 60 degrees, two nozzles 60 to 70 degrees).

System Erlach, horizontal shaft: simpler to build, simpler bearings

System Erlach keeps the horizontal shaft. For every Pelton turbine: a horizontal shaft makes the machine simpler. The shaft can be arranged in a more straightforward way, and the bearings and the support of the generator become less elaborate. This applies especially at lower heads.

Source: F. J. J. Hahn, Numerical investigation and optimisation of a new Pelton turbine distributor system, doctoral thesis, TU Wien 2024, introduction.

System Erlach, six nozzles horizontal: possible because two guide rings guide the splash water

In the Pelton turbine System Erlach, invented by Josef Erlach, two guide rings in the housing catch the splash water and lead it to the outlet without it falling back onto the runner. The nozzles can then sit all around the runner, as the runner requires. The housing has the same design for all arrangements from two to six nozzles.

The horizontal Pelton turbine with six nozzles based on the concept of Josef Erlach ran as a model machine on the test rig of Lucerne University of Applied Sciences and Arts from 2008. The first machine of this type in a power plant went into operation at Gerlos in 2022, built by Voith. The full sequence is set out under Pelton turbine history.

System Erlach, benefit of six nozzles: power density, speed, part load

• More power per machine. Up to six jets instead of two or three.
• Higher speed. This lowers the cost of the generator above all.
• Smaller machine. The building or the cavern can become smaller.
• Good operation with little water. More under Pelton turbine efficiency.

Source: Viennahydro 2012, Full Paper, section 3.1; leaflet for Viennahydro, 22 November 2012.

System Erlach, distributor: six nozzles need the same inflow six times

Six nozzles need the same amount of water six times, and the water should arrive steadily. That is the job of the distributor. This holds for every Pelton turbine with several nozzles. The two designs that System Erlach has for this are described on the next page: Pelton turbine distributor.