Pelton wheel: components and working

The Pelton wheel is a tangential-flow impulse turbine for high heads (above about 250 m) and small discharge. All the available head is converted into kinetic energy in the nozzle, and the jet at atmospheric pressure strikes buckets on the wheel rim.

        Penstock
           |
        [Spear valve]---> nozzle
              \  jet
               \             _
   Casing       \     ()  ()   ) buckets
                 ===>(  WHEEL  )
                      ()  ()
                    ______________
                    Tailrace (open)
   Deflector plate hinged at nozzle exit

Main components

1. Penstock: brings water from the reservoir to the nozzle.
2. Nozzle with spear (needle) valve: the nozzle forms a high-velocity jet. A conical spear moved axially by the governor changes the annular exit area, hence the discharge, while the jet velocity stays almost the same.
3. Runner and buckets: a disc carries double-hemispherical (twin cup) buckets on its rim. A splitter ridge in the middle divides the jet into two halves that leave from the sides. Each bucket has a notch (cut-out) so that the jet enters it freely and the following bucket does not cut off the jet too early. Buckets are of cast iron, bronze or stainless steel.
4. Casing: a cover that prevents splashing and guides the water down to the tailrace. It is open to the atmosphere (the wheel works at atmospheric pressure) and does not run full of water.
5. Brake nozzle: a small jet applied to the back of the buckets to stop the wheel quickly after shutdown.
6. Deflector (jet deflector): when the load drops suddenly, it deflects the jet away from the buckets without closing the spear quickly, which avoids water hammer in the penstock.

Working

1. Water of net head $H$ flows through the penstock to the nozzle; the pressure head turns into jet velocity $V_1 = C_v\sqrt{2gH}$, $C_v\approx0.97$ to $0.99$.
2. The jet strikes the splitter at the centre of the bucket and is divided into two streams, which turn through about $165^\circ$ (not $180^\circ$, so that the water leaving a bucket does not hit the next bucket).
3. The change of momentum of the jet produces a tangential force on the buckets; the wheel rotates and drives the generator shaft.
4. Water leaves with low velocity and falls into the tailrace. The wheel is placed above the tailrace level, so a draft tube is not used.

Maximum efficiency occurs when the bucket speed is about 0.45 to 0.46 of the jet speed ($u\approx\tfrac12 V_1$ for a frictionless 180° bucket). Overall efficiency is 85 to 90 %.

Governing is by moving the spear (change of discharge) and, for sudden load rejection, the deflector.