Hydraulic Turbines: What You Need to Know
Design of Hydraulic Turbines: The Role of 3D Designing in the Creation Process
A hydraulic turbine is a machine that converts the pressure and kinetic energy of water, called hydraulic energy, into mechanical energy. These are also called water turbines.
The mechanical energy of the turbine is further converted into electric energy by an electric generator which is directly coupled to the shaft of the hydraulic turbine. The electrical power generated is known as hydroelectric power.
Hydraulic turbine. Image source: Directindustry
Hydraulic turbines are efficient. They have low wear and tear and ease of maintenance. However, their capital cost is high with a long gestation period because it requires constructing a dam across the river and laying the long pipelines.
Classification of Turbines:
Turbines are classified according to the type of energy available at the inlet of the turbine head, direction of flow through the vanes and specific speed of the turbines.
The following are the important classification of turbines:
(a) According to the type of energy at the inlet:
Impulse turbine: The available energy at the inlet is only kinetic energy (K. E.) e.g. Pelton turbine.
Reaction turbine: Energy at the inlet is kinetic energy (K. E.) e.g. Francis turbine.
(b) According to the head at the inlet of the turbine:
Low head turbine: If the head available is less than 60 meters e.g. Kaplan turbine.
Medium head turbine: If the head available is in between 60 to 250 meter e.g. Francis turbine.
High head turbine: If the head available is above 250 meters e.g. Pelton turbine.
(c) Based on direction flow through runner.
Tangential flow turbine: When water flows along the tangent of the runner wheel.
Radial flow turbine: When water flows in the radial direction through the runner.
Axial flow turbine: When water flows through the runner along the direction parallel to the runner’s axis of rotation.
(d) According to specific speed of the turbine:
The specific speed of the turbine is similar to a geometrical turbine, in that the turbine develops one unit power under one unit head.
According to a specific speed, turbines are classified as:
- Low specific speed turbines: Having a specific speed less than 60 e.g. Pelton wheel turbine.
- Medium specific speed turbines: Having specific speed that is between 60 to 400 e.g. Francis turbine.
- High specific speed turbine: Having a specific speed greater than 400 e.g. Kaplan turbine.
Selection of Turbine:
Image source of turbine
The various considerations in the selection of the type of turbine are as follows:
- Site condition
- Specific speed
- Head and specific speed
- Efficiency Load
- Pressure rise and speed rise
- Turbine setting excavation required
Turbine Selection for Various Discharge are as follows:
- High speed and minimum discharge
- Minimum discharge and high head
- Moderate discharge and head
- Maximum discharge and low head
Pelton Wheel or Pelton Turbine:
Peloton Turbine image source: The Constructor
The Pelton wheel is a tangential flow turbine. In 1880, L.A. Pelton designed this Pelton wheel, an impulse type of turbine which is suitable for the high head (greater than 150 m) and low discharges. It consists of a wheel mounted on a shaft that is supported on rigid surfaces.
A wheel consists of a series of buckets of special shape mounted on it, with a central ridge and cups on both sides. The outlet tip angle lies between 10° and 20° and the number varies between 6 to 50. The incoming velocity varies between 5 to 40 m/s, and the working speed of the turbine varies between 60 - 1000 RPM.
1. Nozzle with flow control system:
A nozzle is used to transform the pressure energy of water into kinetic energy, hence it is fixed at the end of the penstock.
For a small unit, it is operated manually by a rotating wheel fitted at the end of a threaded spear rod. Conical needle is used to increase or decrease the cross-sectional area of the nozzle available for the flow of water.
2. Runner and Buckets:
The Pelton wheel runner refers to a circular disc within which the arrays of buckets are placed evenly on the periphery. The buckets are available in a double hemispherical shape. Each bucket is divided into two symmetrical parts by a dividing wall which is connected to the runner, and known as a splitter or a ridge. A jet of water coming out of the nozzle strikes on the splitter and divides the jet into two equal parts without shocks, and flows over both portions of the bucket.
The bucket is made up of cast steel, cast iron, bronze or other types of steel. The surface of the bucket is very smooth. The main benefit of a double cup-shaped bucket is that axial thrust is neutralised.
At the lowest tip of the bucket, a notch is cut which prevents the jet from striking the presiding bucket which would be intercepted by the next bucket very soon.
A casing does not have any hydraulic function to perform therefore it is not needed in the case of impulse turbines because the runner runs under atmospheric pressure. A casing is used to prevent the splashing of water and safeguard people against accidents. It also prevents the lead from the water from getting to the tail race. The casing is made up of cast iron split into two halves.
4. Braking jet:
Whenever the turbine is brought to rest, the nozzle is completely closed by pushing the spear forward. However, the runner continues to rotate due to its inertia for a considerable period due to its inertia till it comes to rest. To bring the runner to a stop in the shortest time, a small nozzle is provided which releases the water jet and falls on the back of buckets. It acts as a hydraulic brake for reducing the speed of the runner.
Reaction turbine image source: Baker Hughes
In the reaction turbine, part of the total head of water is transformed into a velocity head before it comes to the runner. It does not require a nozzle at the end of the penstock, but the penstock is directly connected to the casing and the water fills all the passages of the runner. The two main reaction turbines are Francis turbine, Radial flow reaction turbine, Kaplan turbine, and Axial flow reaction turbine.
Francis Turbine (Reaction Turbine - Head 30-60 m):
It consists of scroll casing, guide vanes, runner and draft tube. It has fixed blades i.e. a runner where water under pressure enters. The runner blade converts hydraulic energy into mechanical energy of rotation through a reaction shaft made of steel and connects the turbine to the generator. The water at the exit point has some velocity and is recovered by the draft tube beyond the exit of the runner. The hydraulic efficiency varies between 85-90 %.
Main Parts of Reaction Turbine (Francis Turbine) (30 to 60 m Head)
A reaction turbine has the following main parts:
- Scroll casing
- Guide Vanes
- Runner and
- Draft tube
1. Scroll casing:
The scroll casing facilitates the smooth flow of water over the guide vanes and also ensures that all the guide vanes receive a full quantity of water. The design of the casing depends on the cross-sectional area. The cross-sectional area is decreased continuously around the circumference. The cross-sectional area is increased at the entrance and decreases at the tip. The casing will be spiral, hence it is called a scroll casing or spiral casing.
For small heads, the casing is made of concrete but for high heads, it is usually made up of steel plates or cast steels.
2. Guide mechanism:
The function of the guide mechanism is to direct water at the correct angle to the runner vanes. This guide mechanism consists of two rings in which several guide blades are pivoted around the periphery of this ring. Each guide blade can move on its pivot centre which changes the area of flow. The guide blades are for the aerofoil section to have smooth and edgeless flow. Generally, the guide blades are made of cast steel.
The runner is a circular wheel in which a series of radially curved vans are mounted. The shape of the radial curved vanes is in such a way that water enters and exits the runner without shock.
In the case of the Francis turbine, the number of vanes are between 16 to 24 and they are fixed, whereas in the case of the Kaplan turbine, the number of blades are 3 to 6 and they can rotate along their axis.
4. Draft tube:
In a reaction turbine, the pressure at the end of the runner is generally less than atmospheric pressure. Hence the water at the end does not directly discharge to the tailrace. A draft tube is a tube or pipe of a gradually increasing area. It is used to discharge water from the exit of the turbine to the tailrace. In the Francis turbine, water is directed to the wheel through an external guide passage. Water, after flowing radially inside the wheel, leaves it in a direction parallel to the axis.
It consists of a runner mounted on the shaft and is surrounded by a ring of guide vanes which are of aerofoil shape. The flow of water is controlled by guide vanes operated by servo motor mechanism to keep the speed of the turbine constant. The guide vane ring is surrounded by a steel scroll casing that is connected to the penstock pipe.
Advantages and Disadvantages of Francis Turbine
Following are the advantages of the Francis turbine:
- Head variation is easily controlled
- When there is a large variation in tailwater level as compared to total head, the operating head can be utilised.
- Economical, as the size of the runner generator and power plant required is small.
- Mechanical efficiency due to wear does not decrease like in the case of Pelton wheel.
- The maximum and minimum number of operating heads may be two.
The following are the disadvantages of the Francis turbine:
- Sediment may wear high head turbines rapidly.
- Inspection and repair are tedious.
- More susceptible to water hammer effect.
- Cavitation problem is more likely.
- Efficiency is reduced if run below 50% of the load.
For the expression of physical law in the generation of electrical energy using hydropower plants, a 3D printed hydroelectric power plant model with Pelton turbine is used. By changing the height and water flow, the user can instantly affect the production of electricity. Hence, the BLDC motor is used as a generator and the LED strip as an indicator of produced power. An investigational system is a good tool for showcasing the basics of converting energy from potential energy to kinetic energy to mechanical energy and to electric energy.
In a hydroelectric power plant, the kinetic energy of flowing water is converted into electricity. The current power of the power plant and the amount of energy it produces is dependent on the drop and flow of water. The flow and drop vary in an experimental model. A Pelton turbine which is fit for the hydroelectric power plant is installed in the model for the low flow and large drop.
Simulation and Testing of Hydraulic Turbines
Before production, many industries make good use of simulation and modeling. Simulation and modeling is important in understanding the performance of the system.
Simulation is also important when it comes to providing insight data of the functionality as well as any flaws in the proposed design. Hence it’s of importance for one to take the data generated seriously through simulation seriously.
There are various tools available. The most popular ones includes Solidworks, Navisworks, Teamcenter, and many others.
Having looked at the various classifications of turbines and simulation and testing, we would like now to learn on how to use 3D modeling to prepare prototypes of the turbines.
How to Use 3D Modeling to Prepare Prototypes of Hydraulic Turbine
Prototype of an aeroplane being designed in SelfCAD
You can design the Pelton wheel, Pelton turbine and Francis turbine using a 3D modeling software. There are several 3D design software available that you can use to prepare prototypes of the hydraulic turbines, but we recommend using SelfCAD as it’s easier to learn and you don’t need to have previous experience to be able to create impressive prototypes.
With SelfCAD, you can draw and develop the plans, and include all the functionalities of the hydraulic turbine to make it easier later during the development process.
SelfCAD is an all-in-one software. This means that you don’t need another software when you have SelfCAD. You can model, sculpt, and even 3D print all in a single software package. So after you are done with the design process, you can use the in-built slicer of SelfCAD to prepare it for 3D printing without having to switch to another software or install any extension.
Importance of Using 3D Modeling to Create Prototypes of Hydraulic Turbine
- Making changes to the design is much easier as making adjustments in the 3D design software is easy
- It’s a faster way of creating prototypes unlike when done manually.
- No technical experience is required. You can self-learn 3D modeling with much ease especially now there are easy-to-learn 3D modeling software like SelfCAD.
- 3D design and 3D printing make it easier to understand how the hydraulic turbine works and presenting to the customer or stakeholders are easier.
- It’s economical. There are no wastes in the production stage as errors can be detected earlier before the production.
Enjoy powerful modeling, rendering, and 3D printing tools without the steep learning curve with SelfCAD.
Need to learn 3D modeling? Get started with interactive tutorials.