4.1 Reaction and Impulse turbines
In reaction turbines, the steam expands in both stationary and moving blades. In this case stationary blades act as nozzle for the next row of moving blades. Moving blades are also acts as the nozzle for the next row of stationary blades. Since the moving blades are nozzles, the pressure will drop across them. This pressure drop will cause a reaction force. Another force is generated by steam jet. Combinations of these two forces cause the rotor to rotate. In order to reaction turbines operate properly, the designs of moving blades are important. In an optimum design, the leakage around the moving blades shall be minimized. The leakage happens at the clearance between the tip of blade and the casing, thus this clearance shall be minimized. Steam flows either through the blades or leaks through the clearance between casing and blade tip. By minimizing tip clearance, the pressure difference between both sides of blade will raise, same principal as labyrinths. To avoid large trust forces, a balancing piston shall be used in reaction turbines.
The impulse turbine is the simplest type of turbine. It consists of a row of nozzles followed by a row of blades. The gas is expanded in the nozzle, converting the high thermal energy into kinetic energy Impulse turbines consisting of buckets on the runner use the kinetic energy of the water stream. The flowing water after hitting the buckets and rotating the runner leaves the housing of the turbine. The impulse turbines find their best applications in high head and low flow rate sites. Pelton, Turgo, and cross-flow turbines are types of impulse turbines. Pelton turbines are best for the high head water drops and low flow rate. These turbines consist of a circular disk mounted on the shaft coupled to the rotor of the generator. The buckets are mounted along the circumference of the circular disk in such a way that when water hits the buckets they move along the tangent of the disk. The efficiency of the Pelton turbine is restricted to 90% by the friction losses, nozzle losses, and aerodynamic drag. To maintain the frequency and the speed of the turbine, the flow rate through the turbine is controlled by the valve mounted on the jet. Reaction and impulse turbines have different principal of operation. This means they will act differently in a same situation. The comparison can be done in few topics.
1. Blading: Generally, reaction turbines require 75% to 80% more staging for the same work output.
2. Rotor structure: For the impulse turbines, rotors are built from a number of disks connected to each other, while in the reaction turbines rotors are formed from a single drum and then machined. Disks have some cons mostly around the heat shocks. If the heat shock is great disks have a tendency for distortion, while in the drum, resisting thermal shocks is much more important.
3. Axial thrust: as mentioned in the previous chapter, axial thrust for reaction turbines is higher and these turbines may require balancing pistons in order to overcome the thrust.
4. Maintenance: Impulse turbines have longer maintenance periods due to less rotating parts and heavier diaphragms that fixed blades are mounted on.