# Water turbine A **water turbine** is a rotating machine that converts the [[Energy|energy]] of water under head into shaft work, usually to drive an electrical generator in a [[Hydroelectricity|hydroelectric]] station. Turbines replaced open [[Water_wheel|water wheels]] in the nineteenth century because an enclosed runner can use far higher heads, turn far faster and deliver far more power from the same stream. They come in two families: impulse turbines, such as the [[Pelton_wheel|Pelton wheel]], turned by free jets at atmospheric pressure; and reaction turbines, such as the [[Francis_turbine|Francis]] and Kaplan types, which run full of water under pressure.[^doe-turbines][^barmeir-6] The head and flow of a site choose between them. <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Water_turbine.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" allow="accelerometer; gyroscope" title="Water turbine — p5.js microsim"></iframe> </div> *Microsim (THY-055): slide head and flow across a log chart; the runner changes from Kaplan to Francis to Pelton, and the diagonal lines mark equal power P = ηρgQH. ILLUSTRATIVE: region boundaries are drawn as soft bands.* Videos: [16:9](https://wikitube-3d-microsims.netlify.app/media/Water_turbine_16x9.mp4) · [9:16](https://wikitube-3d-microsims.netlify.app/media/Water_turbine_9x16.mp4) ## History The turbine grew out of the water wheel as engineers enclosed the runner and learned to guide water onto it. Benoît Fourneyron's outward-flow turbine in France was followed by James B. Francis's inward-flow design, developed from systematic tests at Lowell, Massachusetts, and by Lester Pelton's impulse wheel for the high heads of the Sierra Nevada in the 1870s.[^doe-turbines] Propeller turbines with adjustable blades, the Kaplan type, extended turbines to low heads in the twentieth century. ## Theory of operation Every water turbine works by changing the angular momentum of the water passing through it. The Euler turbine equation states the power as the mass flow times the change in the product of blade speed and the tangential ("whirl") component of the water's velocity, P = ṁ(u₁U_w1 − u₂U_w2).[^barmeir-6] A runner extracts the most energy when the water leaves it with as little whirl and speed as possible. ### Reaction turbines In a reaction turbine the runner is fully submerged and the water's pressure falls as it passes through, so both pressure and velocity do work on the blades. Reaction turbines suit lower heads and higher flows and are the most common type in the United States.[^doe-turbines] ### Impulse turbines In an impulse turbine the whole head is first turned into velocity in a nozzle, and the free jet strikes buckets at atmospheric pressure. Impulse turbines suit high heads and low flows.[^doe-turbines][^barmeir-6] ### Power The power available at a site is P = ηρgQH: efficiency times water [[Density|density]], the acceleration of [[Gravity|gravity]], the volumetric flow and the net head. The same megawatt can come from a small flow falling a long way or a large flow falling a little, which is why head and flow together choose the machine. ### Pumped-storage hydroelectricity Reversible pump-turbines, usually of the Francis type, can run as pumps to lift water into an upper reservoir and as turbines to generate from it, the basis of [[Pumped-storage_hydroelectricity|pumped-storage hydroelectricity]].[^kerlin-9] ### Efficiency Large modern turbines convert a very high share of the water's energy into shaft work; together with the generator, a hydroelectric unit can deliver close to 90% of the water's potential energy as electricity.[^murphy-11] ## Types of water turbines The main types are the Pelton and cross-flow impulse turbines and the Francis and Kaplan (propeller) reaction turbines.[^doe-turbines] ### Reaction turbines Francis turbines, with fixed blades and adjustable guide vanes, are used for medium to high heads, roughly 40 to 600 m (130 to 2,000 ft). Kaplan and propeller turbines, shaped like a ship's propeller in a tube, serve low heads; Kaplan runners have adjustable blades and gates for a wide range of flows.[^doe-turbines] ### Impulse turbine Pelton turbines are used for very high heads and low flows; cross-flow turbines, in which the water passes the blades twice, accept larger flows at lower heads than a Pelton can handle.[^doe-turbines] ## Design and application A designer matches the runner to the site's head and flow and to the speed the generator must turn for the grid frequency — a slow low-head runner needs a generator with many poles, as the [[Electric_generator|electric generator]] microsim shows. ### Specific speed Specific speed combines rotational speed, power and head into one number that characterizes a runner's shape independent of its size: low values correspond to Pelton wheels, intermediate values to Francis runners and high values to propeller turbines. ### Affinity laws The affinity laws relate geometrically similar turbines: at similar operating points, flow scales with speed times diameter cubed and head with the square of speed times diameter, which lets a small model predict a large machine. ### Runaway speed If a turbine loses its electrical load with the gates open, it accelerates to its runaway speed, which the runner and generator must be built to survive. ## Control systems A governor adjusts the flow through the turbine to hold its speed, and therefore the grid frequency, steady as the load changes. ### Wicket gate In reaction turbines the governor acts on the wicket gates (guide vanes), a ring of pivoting vanes that both regulates the flow and gives it the whirl the runner is designed to remove. ## Turbine blade materials Runners are cast or fabricated from steels chosen to resist [[Cavitation|cavitation]] erosion, abrasion by sediment and fatigue; stainless steels are common for exposed blades. ## Maintenance Turbines run for decades; maintenance centres on repairing cavitation and sediment damage, renewing seals and bearings, and refurbishing runners, often with improved blade profiles. ## Environmental impact Turbines injure and kill some of the fish that pass through them, and the dams that feed them change river flows and block migration; fish-friendly runner designs and screens reduce but do not remove these effects. ## See also - [[Pelton_wheel]] - [[Francis_turbine]] - [[Water_wheel]] - [[Hydroelectricity]] - `Kaplan_turbine` · `Cross-flow_turbine` — not yet on Wikitube ## References [^doe-turbines]: U.S. Department of Energy, Water Power Technologies Office. "Types of Hydropower Turbines." https://www.energy.gov/eere/water/types-hydropower-turbines [^barmeir-6]: Bar-Meir, Genick (2025). *Basics of Fluid Mechanics*, version 0.7.5, §6.3 "Machinery Unitizing Momentum" (Euler turbine equation; Pelton wheel), pp. 253–261. https://open.umn.edu/opentextbooks/textbooks/basics-of-fluid-mechanics [^kerlin-9]: Kerlin, Thomas W. (2013). *Future Energy: Opportunities & Challenges*, ch. 9 "Hydroenergy," pp. 203–216 (§9.4 dams and small systems; §9.5 pumped storage; §9.6 river-flow converters). ISA; CC BY 4.0. https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges [^murphy-11]: Murphy, Tom W., Jr. (2021). *Energy and Human Ambitions on a Finite Planet*, ch. 11 "Hydroelectric Energy," pp. 173–181. eScholarship, University of California. https://escholarship.org/uc/item/9js5291m ## Sources - Bar-Meir, Genick (2025). *Basics of Fluid Mechanics*, v0.7.5 — [OTL record](https://open.umn.edu/opentextbooks/textbooks/basics-of-fluid-mechanics); on the [[PORTAL_WT!Thury_Hydrodynamics_Compendium|Compendium]] Fluid core shelf. - Kerlin, Thomas W. (2013). *Future Energy: Opportunities & Challenges* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges); on the [[PORTAL_Thury_Hydrodynamics_Apex_Spine]] and [[PORTAL_Energy]] shelves. - Murphy, Tom W., Jr. (2021). *Energy and Human Ambitions on a Finite Planet* — [eScholarship](https://escholarship.org/uc/item/9js5291m); on the [[PORTAL_Energy]] shelf. ## External links - [Types of Hydropower Turbines](https://www.energy.gov/eere/water/types-hydropower-turbines), U.S. Department of Energy ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Water_turbine) : [Wikitube](https://en.wikitube.io/wiki/Water_turbine) · pinned revision [1368675331](https://en.wikipedia.org/w/index.php?oldid=1368675331) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Energy]]. --- *Thury station wave · 2026-09-10 · drafted · microsim THY-055 (p5.js) · parent [[Hydropower]].*