What is a pelton turbine

Pelton turbine

The Pelton turbine is a free jet turbine and is mainly used for large heads of 50 to 1500 m. The amount of water is regulated by adjusting the nozzle needle. In the case of very small hydropower plants with a large supply of water, the nozzle is not adjusted. With clean water, the lifespan of Pelton turbines is over 40 years.

In the Pelton turbine, a jet of water sprays out of one jet on a water wheel (Wheel, English runner), which turns through it. The higher the speed of the water jet, the faster the impeller turns. The shovels (cups) of the impeller are shaped so that the water jet is reflected (deflected). The Pelton turbine works best when the water that spurts back out of the cups just falls down. This can be achieved when the speed of the cup is half the speed of the water jet. The best Pelton turbines achieve an efficiency of 92.5%.

To make better use of the turbine, several nozzles can act on one impeller. If there are more than 2 nozzles, the turbine axis is usually arranged vertically so that the falling water does not disturb the water jets. There are also small Pelton turbines which can be operated in a regulated air cushion with counter pressure.

If all of a sudden no more torque is taken from a running Pelton turbine, for example because the power grid fails, the speed increases up to Spin speed. This speed is about 1.9 times higher than the rated speed. Large turbines are not designed for the centrifugal forces of these speeds, which means that you have to be able to interrupt the water supply quickly. But if you close the nozzle too quickly, a high pressure builds up in the pressure line, which can cause the pressure line to burst. In order to quickly interrupt the water supply, a so-called turbine is used with this type of turbine Beam deflector swiveled between nozzle and cup. The water then splashes into the housing and no longer drives the impeller. The nozzle can then be closed slowly.

Design of a Pelton turbine

SizeSymbol, relationshipunitRemarks
Jet speed\ (v = \ sqrt {2gH} \)m / sBecause of the friction, the jet is about 2 to 4% slower
Height of fall\(H\)m 
Acceleration due to gravity\(G\)m / s²9.81 m / s²
Turbine speed\ (\ Omega = \ frac {v} {2R} \)rad / sThe optimal speed is about 2 to 10% lower
Beam circle radius\ (R \)mFrom the center to the middle of the cup
Beam cross-section\ (A = Q / v \)Is adjusted by the position of the nozzle needle
Amount of water\ (Q \)m³ / s 
Beam diameter\ (d = \ sqrt {\ frac {4A} {\ pi}} \)m 
Cup width\ (b \ approx 3 \ cdot d \)mIt is possible to deviate from this figure by +/- 12%

The jet cross-section indicates the cup size, the jet speed and the speed indicate the impeller diameter and thus also the cost of the impeller. A good balance bike with a diameter of 30 cm costs around 10,000 francs. The turbine speed is mostly in fixed ranges around 3000, 1500, 1000 and 750 revolutions per minute. Variable speed generators are rarely used.

Efficiency of a Pelton turbine

The Pelton turbine also has a good degree of efficiency in partial load operation.

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