Aim: - To study the Impulse turbine.
Apparatus Used: - Model of Impulse turbine.
Theory:
Hydraulic machines are defined as those machines which convert either hydraulic energy (energy possessed by water) into mechanical energy or mechanical energy into hydraulic energy. Turbines are defined as hydraulic machines which convert hydraulic energy into mechanical energy. Generally, Hydro turbines are classified into two groups based on how the energy is exchanged between the fluid and the turbine: impulse turbines and reaction turbines.
Impulse
turbine
Impulse turbines operate based on the change of velocity vectors. In general, the potential energy of the water (or another fluid, e.g., steam) based on the height of the waterfall is converted into kinetic energy by one or more nozzles and then water hits the turbine blades at high-speed causing the turbine to spin and consequently generates electricity. These turbines are more suitable for extracting energy from the high head and low flow conditions.
Constructional Details: - The main parts of the Impulse turbine are: -
Nozzle
The nozzle is installed to adjust and jet the fluid flow to strike the buckets. As mentioned earlier, it is the only part of the impulse turbine assembly that the pressure changes and the flow head are converted into kinetic energy. The volume of water jet reaching the buckets is adjusted by a component called a spear, which is a conical needle that moves in and out of the nozzle by a hand wheel or automatically. By moving this needle backward, the water flow increases, and by moving forward, it decreases. The nozzle is typically made from tungsten carbide, which is very hard and can withstand erosive particles.
Runner
The runner consists of a circular disk to which a number of curved blades are attached and a cylindrical shaft in the centre. Shafts and runners are usually made of stainless steel. In cases where the flow head is less, the runner is made of cast iron.
Buckets.
Buckets are a set of spoon-shaped cups that are mounted around the runner to exchange energy between the fluid and the turbine. The fluid jet hits these buckets after leaving the nozzle, making the turbine to rotate and exiting the outer edge of the bucket. The change in the direction of the fluid during the exit compared to the angle of impact varies depending on the design of the turbine. To get the largest momentum, this angle must be 180 degrees. However, this angle is limited to angles of about 170 degrees due to considerations such as that the exit flow from one bucket does not collide with the next bucket and does not cause it to brake. These buckets are made of stainless steel or cast iron.
Casing.
The casing for an impulse turbine is a shield over the turbine to prevent the water from splashing and to guide it to the spillway, which exists for the extra water to protect the structural integrity of the dam. Normally, cast iron is used to manufacture the casing.
Penstock
Penstocks in hydropower plants are pipes and channels that carry water from dams and reservoirs to turbines. In general, they are made of steel. Water flows in these ducts under high pressures.
Working
of Impulse Turbine:
In these turbines, the static pressure inside the runner is constant, and the turbine runner is at atmospheric pressure. The runner spins in the air, and the fluid is sprayed to the blades through the nozzle to exchange energy with the turbine. A jet nozzle or a series of nozzles directs the high-speed flow to the blades, which are usually in the shape of buckets or cups. Therefore, only pressure changes occur in the nozzles. The application of the curved blades is to change the velocity of the flow. This strike causes a change in momentum and based on the law of conversation of energy, a force is applied to the turbine blades.
According to Newton’s second law of motion, the force obtained through the motion of a fluid depends on two factors: the mass of the fluid entering the turbine and the changes in fluid velocity between the turbine inlet and outlet. Since no change in fluid mass occurs, only velocity changes are considered in calculating the force applied to the runner. Thus, in the power generation process in impulse turbines, the following steps are implemented.
·
The stored water flows from a
source upstream through Penstock to be delivered to the nozzle.
·
The potential energy of the
water inside the nozzle is converted into kinetic energy and injected into the
blades or buckets; thus, the runner spins.
·
There is a mechanism to control
the flow of water injected into the runner. The spear usually plays an important
role in this process.
·
The generator attached to the
shaft converts mechanical energy into electrical energy.
Specifications: -
1. Type – Impulse (free jet) turbine.
2. Type of flow – Tangential.
3. Head – more than 250m (high)
4. Mainly Runner shaft is horizontal in pelton turbine.
5. Specific Speed – 8 to 30 for one nozzle (low) Up to 50 for more than one nozzle.
6. Discharge -low.
