--- title: "Pelton turbine: How it works" description: "How a Pelton turbine works: the nozzle turns head into a fast jet, the buckets take its energy, the housing leads the water away." author: "Erlach & Erlach GmbH" language: en canonical: https://www.erlach-erlach.com/en/pelton-turbine/working-principle/ translation: de: https://www.erlach-erlach.com/pelton-turbine/funktionsweise/ license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) source: Generated at build time from the HTML page; same text, same sources. --- # Pelton turbine: How it works A Pelton turbine turns water with a high head into a fast jet. The jet hits the buckets of a runner and gives up almost all of its energy there. This page holds for **every Pelton turbine**. Where a section is about System Erlach, the horizontal Pelton turbine with up to six nozzles designed by Josef Erlach, its heading says so. The difference is explained in the overview [Pelton turbine](https://www.erlach-erlach.com/en/pelton-turbine/) System Erlach. ## The Pelton principle: head becomes a fast jet Nozzle, jet, runner *[Pelton turbine, Nozzle, jet, runner: A runner with buckets on its rim. From the left, a nozzle with a needle directs a horizontal jet at the lower edge of the runner. An arrow shows the direction of rotation.]* Bucket in section *[Pelton turbine, Bucket in section: A bucket made of two halves, open towards the bottom, with a splitter in the middle. The jet comes from below, hits the splitter, divides and is turned by almost 180 degrees in both halves.]* *The principle. Left, the runner seen from the front: the nozzle turns the pressure of the water into a fast jet that hits the buckets. Right, a bucket in section: the splitter divides the jet, and the two bucket halves turn the water almost fully around. Own schematic drawing.* The water comes down from the mountain through a penstock. At the bottom it is under high pressure. In the nozzle this pressure is converted almost completely into velocity: the water shoots out as a free jet. The runner turns in air and not in water. This is what sets the Pelton turbine apart from Francis and Kaplan turbines, where the water flows through the whole runner. It was invented by Lester Allan Pelton, and his patent dates from 1880. **For engineers:** Impulse turbine. Jet velocity c1 = √(2 · g · H), with H as the head at the reference section of IEC 60193, the inlet of the distributor. For the first estimate the calculation uses the full jet velocity; the losses up to the jet, about 2 per cent of the energy, stand in the loss balance as in Veselý. **Formula as text:** c\_1 = sqrt(2 · g · H). **Quantities:** c\_1 jet velocity in m/s; g gravitational acceleration in m/s², equal to 9.81; H head, referred to the high pressure reference section of IEC 60193, here the inlet of the distributor in m. ## The Pelton nozzle: it forms the jet and controls the discharge A needle sits inside the nozzle. When it moves forward, the opening gets smaller and less water passes through. When it moves back, the jet gets thicker. The velocity of the jet depends mainly on the head and stays almost the same while the nozzle is wide open. At a very small nozzle opening, however, it drops markedly, because the nozzle efficiency falls sharply there (Veselý, Fig. 14). A turbine can have one nozzle or several. What that changes is explained under [Horizontal Pelton turbine](https://www.erlach-erlach.com/en/pelton-turbine/design/horizontal/). How head and discharge determine the jet is explained under [Pelton turbine nozzle](https://www.erlach-erlach.com/en/pelton-turbine/design/nozzle/). ## The Pelton bucket: it splits the jet and turns it around Buckets sit on the rim of the runner. Each bucket has a splitter in the middle. It divides the jet into two halves, and the two halves of the bucket turn the water almost fully around. In doing so, the water passes its momentum to the runner. The runner works best when the buckets move at just under half the velocity of the jet. Relative to the stationary housing, the water then has only a small velocity when it leaves the bucket. Relative to the moving bucket, it continues to flow. The outlet direction must also prevent the water from hitting the next bucket. **For engineers:** Peripheral velocity u1 = ku · √(2 · g · H), usual values of ku are between 0.45 and 0.48. Pitch circle diameter D1 = 60 · u1 / (π · n). **Formulas as text:** u\_1 = k\_u · sqrt(2 · g · H); D\_1 = 60 · u\_1 / (pi · n). **Quantities:** u\_1 peripheral velocity at the pitch circle in m/s; k\_u share of the theoretical jet velocity at which the buckets run, no unit, typically 0.45 to 0.48; g gravitational acceleration in m/s², equal to 9.81; H head, referred to the high pressure reference section of IEC 60193, here the inlet of the distributor in m; D\_1 pitch circle diameter of the runner in m; pi the number pi, no unit, equal to 3.1416; n rotational speed in rpm. ## The Pelton housing: it leads the splash water away from the runner According to the 2012 paper, the water that leaves the buckets still carries about two to four per cent of the energy of the jet. It splashes, spreads out and takes in a lot of air. If it falls back onto the runner, it slows the runner down. The housing therefore has a task that is easily overlooked: it must drain the splash water so that it does not hit the runner again. This task decides how many nozzles a machine can have and whether its shaft can be aligned horizontally. This is where the [Pelton turbine System Erlach](https://www.erlach-erlach.com/en/pelton-turbine/) comes in. **Source:** [J. M. Erlach, New Pelton Concept, Viennahydro 2012, full paper](https://www.erlach-erlach.com/downloads/2012_Viennahydro_Erlach_New-Pelton-Concept_Full-Paper.pdf). ## Water path, Pelton turbine: penstock, distributor, nozzle, bucket, tailwater 1. **Penstock.** It brings the water from the reservoir or the intake to the machine. 2. **Distributor.** It divides the water between the nozzles. More under [Pelton turbine distributor](https://www.erlach-erlach.com/en/pelton-turbine/design/distributor/). 3. **Nozzle.** It turns pressure into velocity. 4. **Buckets.** They take the energy from the jet. 5. **Housing and tailwater.** The water drains off without pressure. ## Use of the Pelton turbine: high head, comparatively little water Pelton turbines are found where the water falls a long way: in the mountains, at reservoirs, on high-pressure pipelines. At low head and with a lot of water, Francis and Kaplan turbines are the better choice. The highest heads are just under 2000 metres, and the largest machines deliver around 400 megawatts. A Pelton turbine can be regulated quickly and also works well with little water. This makes it valuable for plants that cover peak load or whose inflow varies strongly over the year. How to get from head and discharge to a machine is shown in the next branch, which includes a calculator for the number of nozzles: [Pelton turbine design](https://www.erlach-erlach.com/en/pelton-turbine/design/). **Source:** [H. Brekke, Hydraulic Turbines, NTNU 2015](https://www.ntnu.no/documents/381182060/1267681377/HYDRAULIC+TURBINES_Hermod+Brekke+-+2015.pdf), sections [4](https://www.ntnu.no/documents/381182060/1267681377/HYDRAULIC+TURBINES_Hermod+Brekke+-+2015.pdf#page=19) and [5.1](https://www.ntnu.no/documents/381182060/1267681377/HYDRAULIC+TURBINES_Hermod+Brekke+-+2015.pdf#page=27), pp. [15](https://www.ntnu.no/documents/381182060/1267681377/HYDRAULIC+TURBINES_Hermod+Brekke+-+2015.pdf#page=20) and [22](https://www.ntnu.no/documents/381182060/1267681377/HYDRAULIC+TURBINES_Hermod+Brekke+-+2015.pdf#page=27). --- Erlach & Erlach GmbH, Ravensburg. Web page: https://www.erlach-erlach.com/en/pelton-turbine/working-principle/ · License for this text: CC BY 4.0 · Overview for assistants: https://www.erlach-erlach.com/llms.txt