Pelton hydro turbine

Impulse turbines are water turbines that utilize the kinetic energy of high-pressure jet streams to do work. Water from high-head reservoirs is guided to the turbine through penstocks. The high-pressure water is converted into high-speed jet streams via the turbine's nozzles, which then strike the turbine's buckets, causing the turbine to rotate and perform work.
There are three main types of impulse turbines: Pelton hydro turbines, Turgo hydro turbines, and cross-flow turbines. This section introduces the more commonly used Pelton turbines and Turgo turbines.

Runner and Working Principle

Figure 1 shows the runner of a Pelton turbine, with the front view on the left and the side view on the right. The runner consists of a wheel disc and multiple buckets, so it is also called a bucket turbine.

Figure-1 Pelton turbine runner

Figure 2 is a cross-sectional view of a bucket. It can be seen from the cross-section of a bucket that the bucket is composed of two spoon-shaped bodies arranged side by side. The water flow is jetted into the two spoon-shaped bodies, driving the runner to rotate.

Figure-2 cross-sectional view of a bucket

Figure 3 is a working principle diagram of a Pelton turbine. High-speed water flow is sprayed toward the buckets through the nozzle, reflected and discharged by the buckets. The kinetic energy of the water pushes the buckets, enabling the runner to rotate. The blue lines indicate the water flow sprayed by the nozzle and the water flow reflected by the runner.

Figure 3 -- Working Principle of Pelton Turbine

Figure 4 is a diagram showing the flow direction of water jetting onto the buckets. The high-speed water flow ejected from the nozzle shoots towards the buckets, is split by the inlet edge to the working surfaces on both sides, and is then reflected out of the buckets by the working surfaces. After being reflected by the buckets, the high-speed jet flow transfers its kinetic energy to the buckets, pushing them forward.

Figure-4 Flow jet of Pelton turbine runner

Injection Mechanism

The injection mechanism, referred to as the nozzle for short, is mainly composed of a nozzle, a needle, and a needle moving mechanism. The size of the nozzle outlet is changed by moving the needle inside the nozzle, thereby altering the water flow rate from the nozzle to adjust the power of the turbine. Figure 5 is a schematic diagram of the structure of the injection mechanism, in which the needle is retracted into the pipe and the nozzle is in an open state.

Figure 5 -- the structure of the pipe inlet and injection mechanism

The movement of the needle is accomplished by the needle moving mechanism. In the diagram, the needle is moved by manual control—rotating the handwheel allows the needle to move, thereby changing the water flow rate of the nozzle. For large-scale water turbines, hydraulic or electric servo mechanisms are used to move the needle. The aforementioned moving mechanisms are installed outside the pipe and belong to the externally controlled injection mechanism. There is another type of injection mechanism installed inside the nozzle, which has no needle rod extending outside the pipe and does not require an elbow, bringing great convenience to pipeline layout. However, it will not be introduced here.

On the left of Figure 6, the needle is in the normal working position, and the water flow is directed towards the bucket. On the right of Figure 6, the needle moves forward to block the nozzle opening, and the nozzle is in a closed state.

Figure 6—Controlling Water Flow by Moving the Needle

Deflector

Now let's introduce the deflector. Pelton turbines are high-head turbines with a head range from several hundred meters to over one thousand meters. The pipelines from the reservoir to the turbine can be as long as one kilometer to several kilometers, and these pipelines have to withstand enormous water pressure, especially at the lower sections. In the event of a power grid failure causing a trip, the water source must be shut down immediately to stop the turbine; otherwise, the turbine will lose its load, leading to a rapid increase in rotational speed and damage to the unit. Due to the long length of the pipelines, the large amount of moving water inside cannot stop flowing quickly. If the pipelines are shut down rapidly, extremely high water pressure will be generated, seriously endangering the safety of the penstocks. The only solution is to redirect the water sprayed towards the turbine so that it does not hit the turbine, rather than shutting off the water flow.
Installing a deflector in front of the nozzle is the simplest method. During normal operation, the deflector is lifted, not affecting the water flow 喷出 from the nozzle, and the turbine runs normally (left of Figure 7). When the deflector is lowered, the water flow from the nozzle is blocked by the deflector and redirected to the lower outlet (right of Figure 7), and the turbine stops operating. The deflector can be turned to the blocking position within 1 to 2 seconds.

Figure 7 -- Working Principle of the Deflector

Figure 8 is the principle animation of a Pelton turbine. The small green beads indicate the water flow reflected from the front side of the runner, and the small orange beads indicate the water flow reflected from the back side of the runner. The center line of the water flow ejected from the nozzle is tangent to the pitch circle of the runner. The pitch circle is the circle passing through the jet impact points on the runner, hence the name "Pelton turbine" (literally meaning "tangential impact turbine").