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Pelton Hydro Turbine: Structure and Working Principle
An impulse turbine converts the kinetic energy of one or more high-velocity water jets into mechanical rotation. Water from a high-head reservoir travels through a penstock to the turbine. At the nozzle, the pressure energy of the water is converted into jet velocity. The jet then strikes the runner buckets and transfers its momentum to the runner.
The principal impulse-turbine configurations include Pelton hydro turbines, Turgo hydro turbines, and cross-flow turbines. This technical guide focuses on the Pelton turbine, including its runner, buckets, nozzle needle, deflector, casing, and multi-jet arrangement.
| Technical category | Impulse turbine |
|---|---|
| Energy conversion | Pressure energy is converted into a high-speed jet, whose kinetic energy drives the runner. |
| Typical water-head condition | High-head applications, ranging from several hundred metres to more than 1,000 metres in the installations discussed here. |
| Runner element | Double-cup buckets mounted around a wheel disc |
| Flow control | Nozzle needle and needle-moving mechanism |
| Rapid jet diversion | Deflector installed in front of the nozzle |
| Large-unit arrangement | Commonly equipped with 2 to 6 nozzles |
1. Runner and Bucket Structure
The Pelton runner consists of a wheel disc with multiple buckets distributed around its circumference. Because these buckets receive the water jet directly, the machine is also commonly described as a bucket-type impulse turbine.
Each bucket is formed by two spoon-shaped working surfaces positioned side by side. A central splitter, or inlet edge, divides the incoming jet into two approximately equal streams. The water then flows across the curved surfaces and is redirected before leaving the bucket.
2. Working Principle of a Pelton Turbine
The nozzle directs a high-speed water jet toward the runner buckets. When the jet reaches a bucket, the central splitter separates it into two streams. The curved working surfaces then turn and discharge the water on both sides of the runner.
This change in the direction and momentum of the water produces an impulse on the bucket. Repeated jet impact on successive buckets creates continuous runner rotation, allowing the turbine shaft to deliver mechanical power to the connected generator or other driven equipment.
The jet centreline is tangent to the runner pitch circle. The pitch circle passes through the nominal jet-impact points on the runner and is an important reference for arranging the nozzle relative to the buckets. This tangential jet arrangement is a defining feature of the Pelton turbine.
3. Nozzle and Injection Mechanism
The injection mechanism is generally referred to as the nozzle assembly. Its principal components are the nozzle body, the needle, and the mechanism that moves the needle.
Moving the needle changes the effective outlet area of the nozzle. This regulates the water discharge and therefore adjusts the turbine output. When the needle is retracted, the nozzle opening increases and allows more water to pass. When the needle moves forward, the opening becomes smaller and the flow rate decreases.
Needle Actuation
In a manually controlled arrangement, rotating a handwheel moves the nozzle needle. This method is suitable where simple local adjustment is required. Large turbines generally use hydraulic or electric servo mechanisms so that the needle position can be controlled automatically and with greater operating force.
The illustrated arrangement uses an external control mechanism, meaning that the needle-moving components are installed outside the water passage. An internally installed mechanism is also possible. In that configuration, no needle rod extends outside the pipe, and the pipe arrangement does not require the same external elbow layout. This can simplify pipeline installation, although its detailed structure is outside the scope of this guide.
In the normal operating position shown on the left of Figure 6, the needle permits the jet to pass toward the runner. In the position shown on the right, the needle moves forward and blocks the nozzle opening.
4. Deflector and Overspeed Protection
Pelton turbines are used in high-head installations where the penstock may extend from approximately one kilometre to several kilometres. The lower parts of such pipelines are subjected to particularly high water pressure.
If a grid failure or another fault causes the generating unit to trip, the load on the turbine can disappear suddenly. Unless the water acting on the runner is removed quickly, the turbine speed may rise rapidly and endanger the unit.
Closing the nozzle needle immediately is not always safe. A long penstock contains a large moving mass of water, and stopping that flow too quickly can produce a severe pressure rise, commonly associated with water hammer. The protection system must therefore remove the jet from the runner quickly while allowing the needle to reduce the pipeline flow more gradually.
How the Deflector Works
A deflector is installed in front of the nozzle. During normal operation, it remains clear of the jet and does not interfere with water flowing toward the runner. When rapid unloading is required, the deflector moves into the jet and redirects the water away from the buckets and toward the lower discharge area.
In the arrangement described here, the deflector can move to its blocking position within approximately 1 to 2 seconds. The turbine is therefore unloaded quickly without requiring the penstock flow to be stopped at the same rate.
| Component | Primary function | Response role |
|---|---|---|
| Nozzle needle | Changes the nozzle outlet area and regulates discharge. | Controls turbine power and progressively reduces water flow. |
| Deflector | Redirects the high-speed jet away from the runner. | Rapidly removes hydraulic input from the runner during load rejection. |
These components perform different but coordinated functions. The deflector provides fast unloading, while the needle controls and eventually reduces the actual water discharge through the nozzle.
5. Water-Flow Path Through the Runner
In a Pelton turbine principle animation, the reflected water may be shown leaving both the front and rear sides of the runner. The incoming jet follows a centreline tangent to the runner pitch circle, strikes the buckets, separates at the central splitter, and exits along the two curved working surfaces.
The term “Pelton” comes from the name of inventor Lester Allan Pelton. The turbine is also described technically as a tangential-flow impulse turbine because of the relationship between the jet centreline and the runner pitch circle.
6. Main Structure of a Small or Medium Pelton Turbine
A typical small or medium-sized impulse-turbine assembly mainly includes the lower casing, upper casing, runner, inlet pipe, nozzle needle and its driving mechanism, deflector, shaft arrangement, and concrete foundation.
The runner is enclosed by the upper and lower casing sections. Cross-sectional views of the casing reveal the relative positions of the runner, nozzle, and deflector. The foundation section also shows the tailwater trough and water outlet through which discharged water leaves the turbine area.
The deflector is positioned in front of the nozzle. Its shaft passes through the bearing below the nozzle assembly, allowing the deflector to rotate between its normal clear position and its jet-diverting position.
7. Multi-Jet Pelton Turbines
To increase the amount of water acting on a runner and improve turbine capacity for a given runner arrangement, large Pelton turbines often use multiple nozzles. The configurations discussed here generally use between 2 and 6 nozzles.
Each nozzle directs a separate jet toward the runner. The nozzles must be arranged so that the jets strike the correct pitch circle while allowing discharged water to leave without interfering excessively with the incoming jets or adjacent buckets.
8. Functional Summary
| Part | Function |
|---|---|
| Penstock | Conveys high-pressure water from the reservoir to the turbine. |
| Nozzle | Converts water pressure into a high-velocity jet. |
| Needle | Adjusts the effective nozzle opening and water discharge. |
| Runner buckets | Split, turn, and discharge the jet to produce runner torque. |
| Deflector | Rapidly diverts the jet away from the runner during load rejection. |
| Casing | Encloses the runner area and guides discharged water toward the outlet. |
| Foundation and tailwater trough | Support the turbine and provide a path for discharged water. |
Frequently Asked Questions
Why is a Pelton turbine suitable for high-head conditions?
Its nozzle converts the pressure available from a high water head into a concentrated, high-speed jet. The runner then extracts energy from the jet through impulse rather than through a pressure drop inside the runner passages.
What is the difference between the nozzle needle and the deflector?
The needle regulates the nozzle opening and actual water discharge. The deflector moves into the jet to redirect it away from the runner, enabling rapid unloading without immediately stopping the large moving mass of water in the penstock.
Why not close the nozzle immediately after a turbine trip?
Rapid closure can abruptly decelerate water in a long, high-pressure penstock and generate a dangerous pressure rise. The deflector removes the jet from the runner first, while the needle can then reduce flow more gradually.
Why does a Pelton bucket have two spoon-shaped sides?
The central splitter divides the incoming jet into two streams. The two curved working surfaces redirect the water on both sides of the runner, transferring momentum to the bucket while providing a balanced flow path.
Why are multiple nozzles used?
Multiple nozzles allow more jets to act on the runner. Large Pelton turbines commonly use 2 to 6 nozzles, depending on the required hydraulic and mechanical arrangement.