Pelton Wheel

Part of Hydro Generator

A high-efficiency impulse turbine ideal for high-head, low-flow sites — where water falls a great height through a small pipe and drives a wheel by jet impact.

Why This Matters

The Pelton wheel is the right choice when you have significant head — at least 10 meters, ideally 30-100+ meters — but limited flow. This describes mountain streams, steep hillsides with springs, and elevated water sources in general. A Pelton wheel on 50 meters of head with only 5 liters per second of flow can generate nearly 2 kW of useful power. An undershot wheel trying to generate the same power from that trickle of high-altitude water would be impossible to build at any reasonable size.

The Pelton wheel was invented by Lester Pelton in California during the 1870s gold rush, when miners needed to power hydraulic mining equipment from mountain streams. It quickly became the standard for high-head hydroelectric installations and remains so today — the largest Pelton wheels in the world (at installations like the Grand Coulee Dam) still use the same fundamental principle as the smallest micro-hydro units.

For rebuilding communities in mountain regions or anywhere with a significant elevation advantage over a water source, the Pelton wheel is the most efficient and practically buildable high-head turbine option.

Operating Principle

Unlike water wheels and breastshot/overshot wheels that work by the weight of water, the Pelton wheel is an impulse turbine — it works by the velocity of a water jet.

Water falls through a penstock (high-pressure pipe) from an elevated source. At the bottom, a precision nozzle converts the pressure into a high-velocity jet. The jet strikes cup-shaped buckets arranged around the rim of a wheel. Each bucket has a central splitter ridge that divides the jet and deflects the water to each side at nearly 180°. This deflection transfers almost all of the jet’s momentum to the wheel.

Efficiency: A well-made Pelton wheel achieves 85-92% efficiency — converting nearly all the water’s potential energy into shaft rotation. This is because the impulse principle avoids the hydraulic losses inherent in reaction turbines (Francis, Kaplan) where water must be continuously flowing through the runner under pressure.

Jet velocity: V_jet = Cv × √(2gH), where Cv ≈ 0.97 (velocity coefficient for a good nozzle). For 50m head: V_jet = 0.97 × √(2 × 9.81 × 50) = 0.97 × 31.3 = 30.4 m/s — about 110 km/h. This jet has significant erosive power; bucket material and surface finish matter.

Optimal wheel speed: Maximum efficiency occurs when the bucket velocity equals approximately half the jet velocity. For 50m head: U_optimal = 30.4/2 = 15.2 m/s at the bucket pitch circle. This determines the wheel diameter for any target rpm.

Bucket Design and Fabrication

The Pelton bucket is the most critical and most difficult component to fabricate. It must:

• Deflect the jet through nearly 180° (slightly less to avoid the returning jet hitting the next bucket)
• Have a smooth interior to minimize turbulence
• Be hard enough to resist erosion from the high-velocity water jet
• Be accurately positioned and balanced

Classical shape: A double-cup shape with a central ridge (splitter). The inner surface is a smooth curved depression. The bucket is narrower at the jet entry point (nozzle side) and wider at the exit, with turned-up edges to redirect water cleanly.

Fabrication approaches:

Casting: Sand-cast bronze or cast iron, then machine the interior surfaces. This is the traditional method and gives good results with adequate casting equipment. Pattern-making and sand casting skills required.

Welded steel: Cut two “half-bucket” profiles from 6-8mm steel plate. Weld together with the central ridge. Grind interior smooth. Harder to get the correct curvature but achievable with careful template use. Harden the inner surface by case hardening (pack in charcoal, heat to bright red, quench in oil).

Machined from solid: If you have a milling machine and 3D template patterns, machine buckets from solid steel bar. Time-consuming but gives the best surface finish and accuracy.

Simpler approximation: For a first-generation low-power Pelton wheel, the bucket can be approximated by two spoons welded back-to-back with a ridge. Lower efficiency (maybe 70% versus 85%) but much simpler fabrication. Serviceable for power generation even if not optimal.

Wheel and Shaft Assembly

Number of buckets: 14-20 buckets for most wheel sizes. The next bucket must be entering the jet before the current bucket has fully exited, to minimize unsteady loading. Spacing is calculated so jet strike angle remains within the bucket width.

Wheel disc: Heavy steel disc (or spoked steel frame for larger wheels). Buckets are bolted to the rim, allowing individual replacement when worn or damaged.

Shaft: Horizontal shaft in two radial bearings. The shaft must handle both the bending moment (jet force acting at the bucket pitch circle creates a large side force) and the torque. Size conservatively.

Nozzle: A converging steel tube ending in a replaceable hardened steel or bronze tip. The nozzle tip should be easily replaceable — it erodes over time, especially with gritty water. A needle valve inside the nozzle (a sharp-pointed rod that can be advanced into the orifice) allows fine flow adjustment without slamming the water column (which causes water hammer in the penstock).

Penstock and Civil Requirements

The penstock is central to a Pelton installation — it must withstand full water pressure (plus surge pressure during valve closure), carry flow without excessive friction losses, and be protected from freezing and physical damage.

Pressure rating: For 50m head, static pressure is 5 bar (70 psi). Add 1.5x safety factor: penstock must be rated to 7.5 bar minimum. Steel pipe schedule 40 or 80, HDPE DR11 or heavier. All joints must be pressure-rated.

Water hammer: When the nozzle is closed quickly, the momentum of the water column in the penstock creates a pressure surge that can be several times the static head. Surge pressure = ρ × V × wave speed / g — can be enormous for fast valve closure. Always close turbine nozzle slowly (at least 5-10 seconds). A surge relief valve or surge tank near the turbine also protects against this.

Penstock sizing: Diameter should be chosen so friction losses are under 5-10% of available head. For a 50m, 100mm penstock carrying 10 liters/second: friction loss = approximately 0.8m per 100m of pipe length. Check with Darcy-Weisbach formula for your exact pipe size and flow.

Intake: Screen the penstock intake finely (2-3mm mesh maximum) — grit in the water is the primary cause of Pelton bucket erosion. Clean the screen regularly.

A well-built Pelton wheel installation in a mountain community can run for decades with minimal maintenance, providing reliable power from a resource that requires no fuel and no regular inputs beyond keeping the intake clean.