How can we talk about rotors in turbomachinery without a proper introduction!

(I had some help from the non-registered and unlicensed engineers in the family, coming up with these Pokémon inspired cards for my heroes of thermodynamics. Thanks Gingui!)

I feel really silly today. We started this journey talking about API 687 and turbomachinery, and I did not take the time to properly introduce the equipment we were talking about.

I hope all the turbines, expanders, and compressors out there forgive me for not being a good host!

We are going to make a brief parenthesis on API RP 687, and before we continue talking about rotating equipment repairs allow me to properly introduce the subjects of our study!

API RP 687 presents recommendations for:

• Special Purpose Steam Turbines (API 612)

• Special Purpose Gears (API 613)

• Axial and Centrifugal Compressors and Expander-Compressors (API 617)

• Rotary-Type Positive Displacement Compressors (API 619)

• Hot Gas Expanders used in Fluid Catalytic Cracking Units (FCCU) Power Recovery and Nitric Acid Service

This equipment can be immediately separated into two groups:

• Machines that convert energy from a fluid into mechanical power, like Steam Turbines and Hot Gas Expanders.

  Since these machines take energy from a fluid and convert it to mechanical energy, they are used to drive other machines like compressors and generators.

  
  

• Machines that convert mechanical energy from a rotor and increase the pressure of a fluid, like Axial Flow Compressors, Centrifugal Compressors, and Rotary-Type Positive Displacement Compressors.

Let’s talk about machines that make power first.

We need to move things!

The world we live in today requires things to move, to do work for us.

Before we used steam to move machines or create electricity, we had limited sources of mechanical power.

We could try to move things by hand; for example, if you watch the awesome original Ben-Hur movie, you will see how we used men to row boats.

Or we could use animals to pull wagons or use a horse/donkey mill to move a large milling stone around.

These first two sources of motion, use humans or animals as engines that convert food into mechanical energy.

Then we got clever and harnessed the energy in the wind or water, through windmills and waterwheels.

These two machines, convert energy freely available from nature, which is great, but what happens when the wind does not blow or the rivers freeze or have no flow through them? What would happen to people that did not live in a windy place or lived far from a river?

We need a way to make machines that could be used anywhere, in any place, at any time. This is where we turned to water! It turns out that water, specifically steam, is an excellent medium for power conversion!

It takes a lot of heat to convert liquid water into steam. This also means that when water vaporizes into steam, it also holds large amounts of energy stored in it. This energy can be released when steam expands! This almost magical property in water is called “latent heat of vaporization”.

Latent heat of vaporization is the amount of energy absorbed by a liquid when it turns into gas. It so happens that water has the highest latent heat than any other liquid. This means there is a lot of energy stored in steam.

That steam is then expanded through a steam turbine, as the steam travels through the turbine energy gets transferred from the steam to the turbine rotor.

A lot more can be said of exactly how this energy transfer occurs, but I promise to do that at another time. Right now, I want to get back to our story.

Today we use steam turbines in several industries. We use them to generate electricity, and we use them in petrochemical complexes to drive compressors.

One could ask, why not use electrical motors? The answer is that for the type of work required in a petrochemical plant, the use of steam is versatile and can be more cost effective than using electricity.

Today less than 20% of the electricity we use comes from renewable sources. We are still some efforts away from finding ways to make electricity with truly carbon neutral effects. For now, in large scale industrial applications that require a lot of power, steam turbines are better than motors.

We come now almost full circle, the fact steam turbine are used in petrochemical applications is the reason why API 611, API 612, and API RP 687 exist.

API 611 is the document that describes the design and manufacturing requirements for General Purpose Steam turbines.

API 612 is the document that describes the requirements for Special Purpose Steam Turbines.

Now, how do we tell whether a steam turbine is a general purpose or a special purpose steam turbine?

This table summarizes the two categories of steam turbines per API.

Regardless of the purpose, all steam turbines use steam.

Let’s talk a little about how steam is made and used in a turbine.

From the equipment description above, we understand that steam turbines require steam at high temperature and pressure.

Another interesting fact is that in a power plant or industrial plant, steam is used in a closed cycle, meaning that the water used, in theory, should not get lost or consumed.

The cycle starts with water, water is pressurized, and then turned to steam. Steam will travel through the turbine converting/transferring power to the rotor, and then it will condense back to water so it can start the cycle all over again.

We call this the steam cycle, and it was made famous by several bearded men, one with the last name Rankine. In modern engineering textbooks and thermodynamics books, the steam cycle is called the Rankine cycle.

The Three Amigos of Thermodynamics

A Scotsman, an Englishman, and a German walk into 100,000 Pascals… Get it?!

100,000 Pascals! A Bar!!

There are a set of rules or laws that we’ve created to describe how thermodynamic systems work.

A thermodynamic system is a way scientists and engineers study how things interact with their surroundings in terms of transferring or exchanging heat and energy.

Just like we have figured out other laws or rules in nature, like Sir Issac Newton’s F=ma or Ohm’s V=IR or Einsteins E=mc2, we have useful rules for thermodynamics.

Meet these three amigos:

The Scotsman

William John Macquorn Rankine

Besides having an enviable beard, Rankine was a Scottish mathematician, physicist, and engineer.

Along with his other two friends that you will soon meet, he developed the theory of the steam engine.

So involved this man was in thermodynamics that he also created a scale that defines the absolute zero temperature. Which is the lowest possible temperature achievable in the thermodynamic scale. Basically, so cold that atoms almost stop moving.

Rankine died at 52 years of age.

The Englishman

Lord Kelvin was originally born William Thomson.

He was such a rock star in science that he was knighted and was given the title “Baron Kelvin of Largs”. Apparently, he did not like his title and would most probably say, “My friends call me Lord… Lord Kelvin”.

Kelvin was also playing around with temperatures a lot, just like Rankine, and created another famous temperature scale that defines an absolute zero temperature.

The German

Rudolf Clausius, another prominent bearded man.

He contributed by formulating the statement for the 1st and 2nd Laws of Thermodynamics.

So, although he was an engineer, you could technically call him the Lawyer or first Judge of Thermodynamics. He is also the father of Entropy.

He strikes me as the quiet one of the bunch, but indeed made unmeasurable contributions.

We owe so much to these men and their magnificent beards.

Lord Kelvin, Mr. Rankine, Mr. Clausius, we thank you and we salute you!

Together they laid out the foundation for how steam is used in steam turbines, which is what we will learn about in my next post.