Impulse Turbine is defined as the turbine in which high-velocity jets of water or steam hits the blades of the turbine to rotate the turbine and produce electricity using this rotation.
The impulse turbine is named so because it acts on the impulsive force created during the striking of the water jet to the turbine blades.
In the impulse turbine, the water strikes the blades tangentially so it is also known as a tangential flow turbine.
Impulse turbines are suited for high head and low discharge of water. It means it is used when the volume of water flow is low and there is high pressure due to the high location of the water head.
The turbines are mainly classified into two categories, they are:
i) Impulse Turbine
ii) Reaction Turbine
In this article, we will discuss the first type that is an impulse turbine.
In the impulse turbine, the kinetic energy high-velocity water jets coming from nozzles are converted into turbine shaft work.
The blades of the impulse turbines are usually of bucket shape.
In 1672 an impulse steam turbine-driven car was designed by Ferdinand Verbiest.
In 1827 a compound impulse turbine was patented by Frenchmen Real and Pichon.
Components Of an Impulse Turbine:
The penstock is a channel or pipe for conveying water to the Impulse turbine. The water at the high head is brought to the turbine using this penstock.
This penstock is connected to the water reservoir. The water reservoir is normally located several meters high.
The nozzle is used to increase the kinetic energy of water and spray the water to the blades of the turbine.
This nozzle creates a jet with high velocity. It points the flow of water towards the blade in a particular direction.
In an impulse turbine, single or multiple nozzles can be used.
The runner is a circular disk is mounted on a rotating shaft. This rotating shaft is known as a rotor.
On the runner, cup-shaped blades also which are evenly spaced are mounted.
Cup-shaped blades also known as buckets are mounted on the runner. These buckets are mounted in such a way that these buckets are evenly spaced.
Buckets are cup or spoon-shaped blades of the turbine. The buckets are mounted around the periphery of the runner so that when pressurized fluid hits the bucket, the bucket will receive the momentum from the fluid and helps the runner to rotate using the fluid momentum.
In an impulse turbine, a casing is used to stop the splashing of water and guide it to the spillway so that there is no wastage of water.
This casing is also used to protect the components from the external environment.
The casing is generally made up of cast iron.
6) Braking Jet:
A braking Jet is used to stop the turbine blades after the supply of water from the nozzle is stopped. Turbine blades continue to rotate even after the water supply from the nozzle is stopped due to inertia. Hence, the braking jet is hit on the opposite side of the turbine blades to stop the blades from rotating instantly.
Working Of Impulse Turbine:
The water at the reservoir has potential energy due to its high location with respect to the nozzle.
This potential energy is converted into kinetic energy by letting the water flow through the penstock and then come out at high pressure through the nozzle.
Inside the nozzle, there is a spear-like arrangement that can be loosened and tighten to control the flow rate of water.
The jet hits the bucket:
When the water jet from the nozzle strikes the curved surface of the bucket, it imparts its energy to the bucket which results in the rotation of the bucket.
Due to the curvature of the direction of the water jet is almost reversed to absorb the maximum force from the water jet. The reversal is about 165 degrees.
The flow rate of water controlled at the nozzle helps in determining the speed and power generated at the rotor.
The rotor is connected to a shaft and this shaft is connected to a gearbox that modulates and transmits the power to a generator at a specified speed. This generator produces the electricity at the required frequency.
The water after getting deflected from the bucket flows through a path called tailrace and exits the turbine.
Types Of Impulse Turbine:
1) Pelton Turbine:
>>Pelton Turbine is the most famous type of impulse turbine.
>> Each bucket used in this type of turbine have double cups with a splitter between the cups.
>> The water jet hits the splitter between the cups in the Pelton turbine and hence the efficiency of the turbine is increased.
>> Pelton turbines can achieve efficiency up to 95% and up to 90% efficiency is achievable in micro-scale hydropower plants.
2) Turgo Turbine:
>> Turgo turbine is the impulse turbine that is most suitable for medium head operation.
>> The buckets used in the Turgo turbines have single cups and the cups are more shallow.
>> Unlike the Pelton turbine, the water jet hits the bucket obliquely (about 20 degrees) in the Turgo turbine.
>> Turgo turbines can be used at a very high rotational speed.
>> They can handle high flow rates than the Pelton turbine of the same size.
3) Cross-Flow Turbine:
>> Cross Flow Turbines are modified forms of impulse turbines mainly used in small hydropower plants.
>> Cross-Flow Turbine was developed by Anthony Michell, Donat Banki, and Fritz Ossberg in 1903.
>> They are easy to fabricate and need very little maintenance.
>> In cross-flow turbines, the water passes through the turbine transversely or across the turbine blades, unlike the other turbines where the water flows axially or radially.
Advantages Of Impulse Turbine:
1) This turbine has high efficiency.
2) This turbine can work easily at low discharge.
3) It is very easy to assemble.
4) This turbine can be adjusted according to the flow rate condition. At high flow rates multiple nozzles are used and at very low flow rates single nozzle is used.
Disadvantages Of Impulse Turbine:
1) It is very costly to install.
2) Its efficiency decreases with time.
3) The size of this turbine is large as compared to other types.
4) This turbine works only in high heads which is difficult to control.