What is Impulse Turbine : Working & Its Applications

A turbine is a rotating mechanical device, used to generate electricity. The classification of these turbines can be done based on the requirement like water turbine or hydro turbine, gas turbine, steam turbine, and wind turbine. Among them, Hydro turbines are classified into two groups mainly depending on how the force is exchanged between the water and the turbine-like impulse turbine & reaction turbine. These turbines are mainly used to change the potential & kinetic energy of the water supply into mechanical work. So this article discusses one of the types of hydro turbines like impulse turbines and its working with applications.

What is Impulse Turbine?

The most accessible type of turbine is an impulse turbine where a water jet from the penstock drives the rotor and colloids directly through the rotor blades. As the name suggests, this turbine works on the impulse force that is created to strike the blade of the water jet. This turbine includes major components like a set of blades & nozzles. The impulse turbine diagram is shown below.

This turbine includes several stationary nozzles that change the jet water pressure into kinetic energy. Once the water flows throughout the nozzle, then it hits the impeller blades which will get the kinetic energy from the jet water to change it into water speed. So, these types of turbines utilize for high head functions & less water flow rates.

The main difference between an impulse turbine and a reaction turbine is; in an impulse type all hydraulic power of the water can be changed into kinetic energy using nozzles and there are no force changes throughout the process whereas, in a reaction turbine, just some amount of accessible energy is changed into kinetic energy.

Impulse Turbine Construction

The construction of an impulse turbine can be done by using different components. So impulse turbine components mainly include;

Penstock

The penstock of this turbine is a pipe or a channel, used to supply water to the impulse turbine. This penstock is very helpful in providing water from the reservoir to the turbine because the turbine is arranged at the downside whereas the reservoir is at the high end.

Nozzle

The main function of the nozzle is to provide the water supply to the impeller blades. Here, the water from the reservoir is supplied within a nozzle so that the water pressure energy can be changed into kinetic energy. Once this conversion is done, it supplies enhanced kinetic energy of water to the impeller blades where the water hits these blades.

Runner/Impeller

The shape of the runner in the turbine is a circular disk that is arranged on a revolving shaft which is called a rotor. The cup-shaped blades (buckets) are also arranged on the runner which is evenly curved.

Bucket

The Buckets are spoon or cup-shaped blades in a wind turbine which are arranged approximately at the edge of the runner so that energy can be exchanged in between the turbine & the water. The materials used to make these buckets are cast iron or stainless steel. The water jet strikes the buckets in the turbine once exiting from the nozzle to make the impulse turbine turn & leave the external bucket edge. Based on the turbine design, the fluid direction will change during the exit compared to the impact angle.

To acquire huge energy, this angle should be 180 degrees. But, here the angle is restricted to 170 degrees because the exit flow from one bucket does not crash through the next bucket & does not cause it to break.

Casing

The casing of the turbine is a protection shield, made with cast iron. This shield is used to protect from water splatter & also directs the spillway channel so that water does not disperse. This shield also protects all the components of the turbine from the outside environment.

Braking Jet

Braking jets play a key role in avoiding the blades of the turbine after the water flow is turned off from the nozzle. The blades of the turbine will rotate even after the water supply is turned off. So, the blade of the turbine will strike from the reverse side of the blade to avoid the blade of the turbine from turning instantly.

Spear

The structure of the spear is in the conical form in the turbine & it is connected to the nozzle to control the water supply in & out of the nozzle & strike the bucket.

Impulse Turbine Working

The impulse turbine works on the principle of Newton’s second law of motion. For these types of turbines, the water can be stored in reservoirs at high places & supplies throughout penstock to the turbine which is located at the ground. This turbine works in the following steps.

• A water jet moves from a dam or reservoir to the connected nozzles in the turbine.
• Once the water is supplied into the nozzle of the turbine, then it changes the energy of the pressure water into kinetic energy.
• After discharging throughout the nozzle, the water jet strikes the impeller blades & turns the impeller through its axis.
• These blades mainly change the kinetic energy (K.E) of the water jet into speed & enhance the water speed.
• The water with high speed hits the turbine so that shaft of the turbine starts turning.
• The shaft of the turbine is connected through the generator coil and the rotating motion of the shaft will rotate the generator coil.
• Once the generator coil starts turning, then it generates electricity and it delivers to different houses & industries.

Impulse Turbine Types

Impulse turbines are available in three types like Pelton, turgo, and crossflow.

Pelton Turbine

This is the most frequently used type of impulse turbine. In this type of turbine, every bucket includes two cups with splitters which are arranged in between the double cups. This turbine divides the water jet in between the two cups to enhance the performance of the turbine. The efficiency of Pelton turbines is up to 95% whereas, in micro-level hydroelectric plants, the efficiency can be achieved up to 90%.
The main features of the Pelton turbine include the following.

• The water discharge capacity of these turbines ranges from 5 liters to 1000 liters per second
• Installation is easy as compared to reaction turbines because the flow rate of these turbines is low & small pipes are used.
• These turbines work through high water head because it requires complex & costly Penstock.

Turgo Turbine

The Turgo turbine is suitable for medium-head operation. In this turbine, the buckets include single cups which are thinner. Not like the Pelton turbine, the water jet in this turbine strikes the bucket exclusively at 20 degrees. These turbines are used at extremely high rotational speeds because they can handle maximum flow rates as compared to Pelton turbines.

The main features of a turgo turbine include the following.

• High flow rates
• Suitable for high speed
• This turbine handles with a high water flow rate
• Simple to assemble

Cross-Flow Turbine

These turbines were developed by Donnet Banki, Fritz Osberg & Anthony Michel in the year 1903. These are customized types of impulse turbines, used in tiny hydropower plants. The designing of these turbines is very easy but needs very less maintenance. In these types of turbines, water supplies across the turbine blade or through the turbine but in other turbines, water supplies radially or axially.
The main features of cross-flow turbine include the following.

• Its design & maintenance is simple
• These turbines deliver water with 20 to 2000 liters per second flow rate from 2m to 200m
• These are used in hydropower plants which have 5 kW to 100 kW power rating & also in large 3 MW power plants.

Impulse Turbine Example Problems

Example1: A Pelton turbine includes semi-circular buckets which work below 140m ahead & use 0.05m^3/sec of water. If a turbine with 740mm rotates at 700rpm then measure the available power at the nozzle in the turbine and hydraulic efficiency of the turbine. Here the velocity coefficient can be assumed as unity.

According to the given data;

Head (H) = 140m

Discharge of water (Q) = 0.05m^3/sec

Diameter of turbine (D) = 740mm = 0.74m

Turbine speed (N) = 700rpm

Velocity coefficient (Cv) = 1

The available power for nozzle (P) = wxQxH = 9.81x 0.05×140 = 68.67kW

Jet velocityV1 = Cv √2gH = 1* √2X9.81X140 = 1*√2747 = 52.41m/s

Turbines tangential velocity µ = π*D*N/60 = (3.14X0.74X700)/60 => 1626.52/60 = 27.10m/s

When buckets are in semi-circular, then the gets simply reflected through 180 degrees theoretically &

the blade angle will be Φ = 0 degrees

Hydraulic efficiency (ηh) = 2µ (v1- µ)(1+cos Φ)/V1^2

= 2×27.10 (52.41-27.10)(1+cos0)/( 52.41)^2

= 54.2 (25.31)(1+1)/2746.80= 741445.16/2746.80 = 0.998 = 99%

Example2: A pelton turbine works under a 300m of head at 800rpm speed and it develops 4000kW of power. Find out the least jet diameter & the diameter of pitch cycle of turbine. Assume the overall turbine efficiency is 80% and Cv=0.90 & speed ration is 0.35, so find the number of buckets.

From the above given data,

Head(H) = 300m

Speed (N) = 800rpm

Power (P)= 4000kW

Efficiency overall (ηo) =80%

CV = 0.90

Speed ration Ku = 0.35

Jet velocity V1 = Cv * √2gH

= 0.90√2×9.81×300 = 0.90√5886 = 0.90×76.72 = 69.04m/sec

Turbine velocity µ = Ku x V1 = Ku * √2gH = 0.35×76.72 = 26.85m/sec

Turbine velocity µ = π*D*N/60

26.85 = 3.14xDx800/60 = 26.85 = 2512D/60

1611 = 2512D = >D = 0.641m

Overall efficiency ηo = P/wQH => 0.80 = 4000/9.81xQx300 => 0.80 = 4000/2943Q

2354.4Q = 4000=> Q = 1.69m^3/sec

Q = jet area x jet velocity

Q = π/4 *d^2*v1 => 1.69 = 3.14xd^2×69.04

1.69 = 216.78 d^2 => d^2 = 1.69/216.78 = 0.007

d= √0.007 = 0.0836

No.of buckets Z = 15+D/2d

Z = 15 + (0.641/2×0.0836) = 18.833

Advantages

The advantages of an impulse turbine include the following.

• These turbines have simple construction and maintenance.
• High efficiency
• These turbines have an easy assembly.
• It functions at atmospheric force.
• The rotational speed is high

Disadvantages

The disadvantages of impulse turbines include the following.

• Large size
• It needs a high head which is not easy to handle.
• The efficiency will be decreased over time.
• Installation cost is high
• It is suitable for low discharge.
• For high flow rates, it is not suitable

Applications

The impulse turbine uses include the following.

• Impulse turbines are used in drinking water supply systems.
• Used in hydropower plants.
• This type of turbine is used to generate electrical energy
• It is used in Hydro Power Plant.

Thus, this is all about an overview of an impulse turbine and its working with applications. The degree of reaction of the impulse turbine is equivalent to ‘0’ which means that the whole enthalpy drop can be used within the nozzle & the exit velocity is very high from the nozzle. Here is a question for you, what are the Impulse turbine characteristics?