Understanding how steam turbine blades work and the wonderful world of patents
- Fernando E. Romero, P.E.
- Dec 14, 2024
- 5 min read
In the last post, we saw how we use water and steam to generate power in the Rankine cycle. Today we dive inside the steam turbine itself to discover how exactly we turn the energy stored in the steam into mechanical energy.
As we dive into the theory in today’s post, we will also look at a bit of history and at some of the current practical references or applications of this theory in industry.
The theory
There are two main principles for steam turbine design, based on the way the blades are designed to convert the energy from the steam into mechanical rotational energy.
The principles are commonly called the:
Impulse Principle
Energy is extracted by changing the direction of a high velocity jet of steam as it travels through blades.
First, steam is accelerated through a nozzle (stationary).
The fast jet of steam exiting the nozzle hits the turbine blades.
In an impulse stage pressure drops and accelerates the steam only in the stationary nozzle.
As the steam travels through the bladed section, it changes directions and “bounces” off the blades, transferring momentum to the rotor.
Reaction Principle
Energy is extracted through both a change in direction of the steam and the acceleration of the steam as it travels through the blades.
The pressure of the steam drops through nozzles and the rotating blades, changing direction and accelerating, creating a reaction force that propels or pushes the blades.
In a reaction stage steam accelerates in both stationary nozzle and rotating blades.
This may be a bit hard to visualize, but, luckily for us, we can rely on “prior art”! Prior art is a term used in the world of patents. It represents any public evidence or illustration already accessible when filing a patent.
This means that if you go file for a patent, the patent office will check other patents and publications, to make sure no one else had that idea before you.
A Swedish engineer named Karl Gustaf Patrik de Laval had some pretty innovative ideas back in the 1800s on how to design steam turbines, and he filed some patents!
Delaval’s Impulse Turbine
Patent 522,066 June 26, 1894
The patent contains a stunning illustration of De Laval’s famous impulse turbine. It is reported that this design was able to reach 20,000 to 30,000 RPM by accelerating steam through the nozzle and directing the steam into the blades.
So we could appreciate the design a little better, I created a 3D model of the De Laval turbine.
In gray color, you can see the basic rotor structure, which has a single disk with blades.
In green, I modeled a nozzle like the one in the patent.
In the next figure, I sliced a section from the model so you can appreciate the internal profile of the nozzle and the airfoil of the blade.
High pressure steam will enter the nozzle, and as it travels through the steam it will accelerate. When the steam expands, its pressure will drop, but it will gain speed in return.
When the steam is now travelling really fast it exits the nozzle and hits the blades.
The steam bounces off the surface of the blade and changes direction. This is where the momentum from the steam is transferred onto the blade.
The airfoil design is very symmetrical, sometimes called a bucket.
The deflection of steam is what creates the energy transfer from the steam momentum onto the blade. This generates a force that spins the turbine rotor.
This is what we call an IMPULSE turbine!
Delaval’s Reaction Turbine
Patent: 285,584 September 25, 1883
The turbine in this patent is the “S” shaped wheel that I have filled with yellow (below), or as De Laval describes it in the patent, “the turbine wheel consists of two curved tubes”.
(Note: Do not be misled by the large disk-looking body on the back of the device, that is basically a gearbox.)
De Laval also writes: “the driving fluid passes through the tubes and turns the turbine by reaction”.
This device basically looks a lot like this old school lawn sprinkler!
Why does the sprinkler move you may wonder?
It is because the force of the water exiting the nozzle creates a reaction force in the opposite direction, causing it to rotate.
There are so many fundamental principles of physics and motion in such a simple device, that this blows my mind.
So, in a reaction turbine, the blades are designed to act like line nozzles, meaning that the steam will continue to expand and accelerate as it travels through the rotating blades.
In essence, in a reaction turbine, the steam expands and accelerates as it travels through both the stationary blades (nozzles) and the rotating blades.
The acceleration of the steam, and the fact that the steam has mass, generates a force that exits the blade. And since Newton’s third law of motion of “for every action there is an equal and opposite reaction” is true. That force of the steam exiting the blade generates a reaction that pushes on the blade and makes the rotor turn.
In modern turbines, reaction turbine blades do not look like lawn sprinklers; they have airfoils that guide the steam through the turbine.
Here is a side-by-side comparison of both designs:
We’ve learned about the two fundamental types of turbine designs based on their airfoil designs or energy conversion principles.
Some applications are better suited for one design versus the other; for instance, power generation steam turbines mostly use reaction blades due to the higher efficiency or better suitability of the blades for extracting as much power as possible out of the steam.
We also must consider that this technology and equipment has been around for a very long time, close to 150 years. Impulse style symmetrical buckets were a lot easier to manufacture using the machining techniques available at the end of the 19th century.
Reaction airfoils can get a bit more intricate in manufacturing and require more careful design effort and more modern manufacturing techniques.
In the end, turbines today use both principles. You will likely find airfoils that look like impulse buckets on the high-pressure end of a turbine, where the steam pressure and velocity is the highest; and as you travel downstream the turbine towards the low-pressure end, where there has already been a lot of pressure drop and expansion of the steam and the blades are longer, you will see more reaction airfoils.
There is a lot more that could be said, and the explanations can go much deeper.
To write today’s post I enlisted the help of one of the best engineers I have had the privilege of working with: the exceptional Kirill Grebinnyk.
Kirill happens to be an aerodynamicist and a great friend. I stole an hour of his time, and we doodled all over his white board discussing steam turbine blade design, velocity triangles, and throat areas.
(Note: work with people smarter than you, so you can always learn something new!)
I hope you take away today a basic understanding of these two types of turbine designs.
Learn something new every day ! Thanks