Flew in from Miami Beach BOAC …
You don't know how lucky you are, boy
Back in the US
Back in the US
Back in the USSR

("Back in the USSR," by the Beatles, 1968)

 

GAS TURBINES GENERATE PROPULSION/THRUST
OR PRIME MOVING/DRIVING POWER

The unmistakable high pitched scream of a Gas Turbine introduces and concludes the Beatles classic song, "Back in the USSR."

The Gas Turbine is a marvel of Humankind genius designed to convert heat energy into jet propulsion or Prime Mover/Driving power.

The Gas Turbine (GT) was originally developed as a self-contained power plant for lightweight jet aircraft.

The first successful application of turbojet technology was used in the Messerschmitt 92 (me 92) WWII German fighter-bomber. On the Allied side, the DH Goblin turbojet gave flight to the Gloucester Meteor.

Tried and tested in the theater of WWII warfare, the peaceful application of turbojets used for propulsion is evident in today's modern commercial aircraft … from dinky Learjets to humongous jumbo jets.

GTs used in aircraft are designed to provide "propulsion" aka "thrust power."

Tweaking the GT propulsion design and technology via adding a shaft to drive a Load transforms the Propulsion Turbo Jet into a "Shaft Power Turbine" … aka Prime Mover/Driver.

When the GT is coupled to an electricity generator, electricity can be generated and distributed throughout the processing plant.

When the GT is coupled to a Compressor Load or a Pump Load, then the PV Pressure can be added to gases and liquids.

As PTOA Readers and Students read this PTOA Segment it may seem hard to believe that the GT is considered the ideal Prime Mover/Driver by virtue of having relatively simple moving parts.

That's because the awe-inspiring complexity of a GT is in the materials of fabrication and technical details of design to 0.001 inch (one one-thousandth of an inch) … aka "a mil."

Even though the GT Shaft and its Rotor Blades can weigh up to 10 tons and rotate at 20,000 rpms, the expertly designed, fabricated, and installed GT will not vibrate more than several thousandths of an inch.

The nearby photo shows a bird's-eye view of one of the world's most efficient power GTs.

 

Minimal vibration is a big deal because it means this Prime Mover/Driver can be relied upon to provide continuous rotational mechanical movement … and thus continuously drive a Load … as long as planned maintenance intervals are adhered to.

Because GTs are designed to be self-cooling, the light weight and reliable GT is the most popular Prime Mover/Driver used in dry, hot climates around the world.

 

OVERVIEW OF GAS TURBINES (aka "GTs")

The "gas" referred to in the descriptor "Gas Turbine" is just hot air.

Otherwise stated, the expansion of compressed, hot air and gaseous combustion products become The Working Fluid of a GT.

The mnemonic device that is used to help students remember the flow path of air as it ultimately generates rotational power is admittedly vulgar.

Albeit lewd, Your Mentor has decided to take the risk of being accused of "not being woke" and perhaps "proliferating toxic masculinity" only because the mnemonic device has proven to be an effective instructional aid.

Fred the Stickman has read ahead and is already blushing.

 

The four steps that describe the flow path of air as it ultimately generates jet propulsion or Prime Mover/Driver power can be summarized:

• Air Intake (SUCK)
• Air Compression (SQUEEZE)
• Ignition of Air/Fuel Mixture and thus Combustion  (BANG),
• Expansion of the Working Fluid and Creation of Rotational Movement (BLOW)

 

The features of each of these steps are mansplained below in detail.

Before launching into the dry, technical details behind the lewd double entendre, PTOA Readers and Students should study the nearby graphic entitled "Jet Engine Cross Section." The flow of air is from left (Intake) to right (Exhaust).

Note that the GT links a Compression Section (left hand side) to a Turbine (right hand side) with a Combustion Section situated in between. The Combustion Section logically houses the Combustor hardware.

Note that Cold Section of the GT houses the Air Intake and Compressor sections of the GT Engine.  The Hot Section of the GT includes the Combustor and Turbine sections of the GT Engine.

 

 

STEP 1: SUCK (aka Air Intake)

The catchy verb "Suck" is intended to describe how filtered … but otherwise common … atmospheric air flows into the GT.

The word "suck" is accurate because the atmospheric air truly must be "sucked" into the machine.

Any PTOA Reader or Student who is reading the PTOA Segments in the intended sequential order can cite chapter and verse why a "lower-than-atmospheric pressure" … aka negative pressure … must exist to encourage atmospheric air to flow in the first place. Any PTOA Reader or Student needing a refresher can access PTOA Segment #151.

The nearby gif shows air (as red gaseous flow) being sucked into the turquoise rotating blades of an Axial Compressor Section of the GT. This gif depicts a Propulsion Turbo Jet GT… not a Power Turbine GT … because the GT is not driving any rotating equipment. The exhausted gas is released to its surroundings thus creating the "thrust" and/or "propulsion" that moves the jet plane.

The rotation of the Compressor's blades creates the lower-than-atmospheric pressure zone which encourages the air to flow into … be "sucked into" .. the GT.

This requirement for the Compressor to create a negative pressure also creates a start up concern for GTs called "Surge."  Surge will be featured in a future PTOA Segment featuring Compressors.

 

STEP 2: SQUEEZE (aka Air Compression)

The catchy verb "Squeeze" refers to cramming the maximum amount of energy-carrying air molecules together so that the maximum amount of energy can eventually be extracted from them. The more the merrier!

The act of "pressing gas molecules together" is called "compressing" … which was defined in PTOA Segment #153.

An Axial Compressor is one type of machine that can be used to compress air in a GT.  Smaller GTs will employ a Centrifugal Compressor. Axial Compressors and Centrifugal Compressors will be featured in future PTOA Segments so do not stress about how they work now.

Just keep in mind that since more molecules of air are crammed together in the process of compression, the Discharge Pressure of the air exiting the Compressor will be greater than the Suction Pressure of the air entering the Compressor.

In a Power Turbine the Suction Pressure is approximately 14.7 psi (1 atm). The Discharge Pressure from an Axial Compressor will be 70 psi to 100 psi (5.8 atm  - 7.8  atm).

Hmmmmm!

Brilliant PTOA Readers and Students all over the world are thinking …

"Aha! The increase of PV Pressure of the flowing air … a fluid … must mean that the velocity of the gas is slowed down because of  the "PV Pressure-Velocity Swap" featured and explained  in PTOA Segment #159. "

Righteeo!

Guess what? The "PV Pressure-Velocity Swap" occurs all over the place in a GT because the marriage of compression (adding PV Pressure) and expansion (relieving the PV Pressure) is capitalized upon to create reliable, rotational movement.

 

Remember how a Steam Turbine converted the thermal energy of super heated steam into rotational mechanical movement as was described in the recent PTOA Segment #193?

The compressed air exiting the Axial Compressor  will have a higher temperature simply because the air has been compressed. Yet this compressed air will still not possess sufficient internal thermal energy to turn the Rotor Blades in the Turbine of the GT.

The compressed air must be heated up … aka "contain more internal energy"... before it will be able to generate rotational movement.

So most of the compressed air discharged from the Axial Compressor flows into the Combustor of the GT as "Primary Air," "Secondary Air," … and even "Tertiary Air" which are explained in the next few paragraphs.

Where does the rest of the compressed air go? The intricate interior architecture of the GT includes ports and passageways that deliver compressed, cooling air to "the Hot Section of the GT. "

The cooling air keeps the first two sets of Turbine Rotor Blades sufficiently cool thus preventing them from growing longer and outward from the Turbine Shaft. This elongation of Rotor Blades is called "Creeping."

Creeping is caused by the special metallurgy of Rotor Blades being impacted by the spinning Turbine's centrifugal forces and the high temperatures generated by combustion.

Remember how GTs are manufactured to the thousandth of an inch? The tight tolerances of GT construction cannot tolerate the Rotor Blades creeping too much. Shutting down the GT to inspect and replace Turbine Rotor Blades is costly.

In other words:

GTs are designed to be self-cooling; some of the compressed air is not sent to the Combustor but rather diverted through ports and passageways and used for cooling the first two Turbine Rotor Blades.

Compressed air can also be used to seal in lube oil and thus keep the lube oil from leaking out of the GT. The function of Seal Oil was defined in PTOA Segment #184.

How air is used to seal in lube oil is explained in a future PTOA Segment.

Wow!  The above paragraphs describe several intricate flow paths of air throughout the Gas Turbine Engine! The attention to detail that is required to manufacture such close internal engine tolerances are yet more examples of what is meant by the umbrella phrase "process technology."

 

 

STEP 3: BANG (aka Ignition of Air/Fuel Mixture and Combustion)

The catchy verb "Bang" is intended to describe ignition of the mixture of compressed air and fuel as they are ignited in the Combustor.

A single "Bang" is not an accurate description.

In reality, compressed  "Primary Air" discharged from the Axial Compressor of the GT mixes with the fuel and is ignited in a controlled, continuous manner as illustrated by the angled Igniters firing synchronously in the nearby gif.

Ergo, The Combustion Reaction takes place in the Combustor. The Combustion Reaction was featured way back in PTOA Segment #73.

PTOA Readers and Students will recall that the Oxygen needed for The Combustion Reaction is 21% of the compressed air volume.

The reaction products evolved from The Combustion Reaction are the Exhaust gas (composed of Carbon Dioxide, Water, and incomplete combustion byproducts) and Heat.

Just a small amount fuel is required to generate sufficient Heat product. That Heat quickly excites non-consumed Oxygen molecules as well as the other gaseous Combustion Reaction products. Now this Working Fluid has sufficient internal thermal energy to do some useful work!

The non-combusted, hot air originates from two sources:

• Some of the "Secondary Air" drawn from the Axial Compressor  is consumed in the Combustor because its job is to insure complete combustion. The remaining "Secondary Air" evens out the distribution of Heat throughout the Combustor.
• "Tertiary Air" drawn from the Axial Compressor mixes with The Working Fluid gas to make sure a safe Turbine Inlet Temperature will be attained.

 

The Combustor could be designed to use hydrocarbon fuels like propane (a gas) or even "jet fuel" or "#1 grade diesel"  (liquids at room temperature and pressure).

Hydrocarbon fuels heavier than natural gas must be atomized into fuel droplets prior to ignition. Any droplets that sneak through the Combustor without being burned will pit the GT's Rotor Blades and Stator Vanes.

Natural gas is the preferred fuel for Power Turbines because it does not require atomizing and just needs to be injected into the Igniter at a controlled rate.

Who amongst the many brilliant PTOA Readers and Students has figured out that a GT falls into the classification of an Engine  because it uses the combustion of  a hydrocarbon to generate a Working Fluid that subsequently creates mechanical/rotational power?  Engines were featured in PTOA Segment #191.

 

STEP 4: BLOW  (aka Expansion of the Working Fluid Creates Rotational Mechanical Movement)

The Working Fluid … aka heated air and Combustion Reaction products... exits the Combustor through Turbine Nozzle Guide Vanes.

The angle of the Turbine Nozzle Guide Vanes can be adjusted to optimize the point of impact of the expanding gas on the first Rotor Blade of the Turbine Section of the GT.

The nearby graphic depicts the Combustor of a GT. The Turbine Nozzle Guide Vanes appear at the exit … on the right side … of the Combustor.

The Turbine Nozzle Guide Vanes play the role of the child's pursed lips in the nearby photo of a child blowing air on a pin wheel toy.

Both the Turbine Nozzle Guide Vanes of the Combustor and the pursed lips of the child cause the PV Pressure of the fluid flowing through their respective nozzles to suddenly drop which causes the velocity of the gases leaving the nozzle to suddenly increase.

The hot, high velocity Working Fluid that exits the Combustor through the Turbine Nozzle Guide Vanes strikes the first Rotor Blade of the Turbine Section of the GT, causing it to turn.

Voila! …

 

Rotational mechanical motion has been created by extracting the thermal energy contained in The Working Fluid while it is expanding!

The Rotor Blades are on discs that are attached to the Turbine's Shaft. As the first Rotor Blade spins, so does the Shaft!

The GT shown in the nearby gif has a "2 Stage Turbine" because there are two sets of rotating discs with Rotor Blades that rotate with the Shaft.

The nearby photo shows the Rotor Blades and Shaft of a Power Turbine/Prime Mover/Driver called "Harriet" that was manufactured by General Electric (GE).

In 2014, Harriet was the most efficient GT in the world.

 

 

 

 

The birds-eye view of Harriet reveals 4 sets of Rotor Blades on the left hand side, each with a successively bigger diameter.

Each stage of rotating Rotor Blades are surrounded by Stator Vanes which do not move because they are attached to the interior casing of the Turbine.

Guess what? Precisely because Stator Vanes are fixed to the internal casing of the Turbine they are really hard to make interesting in a graphic!

The nearby photo shows Stator Vanes on a Ruston GT. PTOA Readers and Students should Imagine discs with their Rotor Blades snugly fitting between the Stators.

Stator Vanes are extremely important!  Neither the Turbine or the Axial Compressor in the GT would work without Stator Vanes.

Stator Vanes direct the flow of the ever-expanding Working Fluid so that it will strike the next set of Rotor Blades in the sweet spot which most efficiently spins the blades and hence … turns the Turbine Shaft.

Each successive set of Rotor Blades and Stator Vanes extracts more and more heat from The Working Fluid while producing more and more rotating power. As The Working Fluid flows and expands through each successive stage in the Turbine, the heat energy of The Working Fluid decreases.

To extract the most energy from The Working Fluid, each stage of the Turbine will have a larger diameter.  This architecture of construction gives a cone shape of increasing diameter to the GT's Turbine, which was recently described in PTOA Segment #193.

The Working Fluid finally exits the Turbine Section of the GT as Turbine Exhaust Gas (TEG).

The TEG exhausted from the Turbine is different than The Working Fluid that flowed into the Turbine from the Combustor's Turbine Nozzle Guide Vanes.

The TEG has:

• Greater Volume
• Greater Velocity
• Less Internal Energy and Temperature
• Less Pressure

 

The rapid expansion of The Working Fluid creates noise that will damage ear drums. Power Turbines … aka Prime Movers/Drivers make the same shrill, high pitched noise as Propulsion Turbo Jets do. Ear Protection is required in the Generator House of every Processing Plant.

The upcoming  PTOA Segments will explain the different configurations of GTs and describe the critical subsystems and operations of GTs.  Keep on reading!

 

 

TAKE HOME MESSAGES: This PTOA Segment used a 'toxically masculine'' mnemonic device to explain the flow path of air while it is compressed, combusted with fuel, and expanded to ultimately create rotational power or propulsion power in a Gas Turbine (GT). PTOA Readers and Students recognized that the GT is an "Engine" because a hydrocarbon is combusted to evolve a Working Fluid that subsequently expands while creating rotational mechanical movement.

Gas Turbines can be used to generate:

• Propulsion/Thrust or jet aircraft.
• Shaft Power (aka Prime Moving/Driving of a Shaft).

 

The Four Steps that cause common air to create rotational, mechanical power are:

• Air Intake (SUCK)
• Air Compression (SQUEEZE)
• Ignition of Air/Fuel Mixture and thus Combustion (BANG)
• Expansion of the Working Fluid and Creation of Rotational Movement (BLOW)

 

A Gas Turbine (GT) has a Compressor Section, a Combustor Section, and a Turbine Section.

The Compressor is on "The Cold Section of the GT." The Combustor and Turbine are on "The Hot Section of the GT."

Key Hardware in a GT featured in the PTOA Segment includes:

• Axial (or Centrifugal) Compressor
• Combustor with Igniters and Turbine Nozzle Guide Vanes
• Turbine with Rotor Blades on a Shaft and stationary Stator Vanes on the inside Turbine casing.

 

Each Turbine Rotor Blade will have a successively larger diameter to extract successively more energy from the Working Fluid. The shape of a Turbine (in real life and ISA symbol) is thus cone-shaped with the smallest diameter near the Inlet and the largest diameter near the Exhaust.  This cone-shaped ISA symbol represents the increasing volume of The Working Fluid as it is expanded through the Turbine.

Interesting phenomena associated with various GT sections include:

• Axial Compressor - Surge during Startup; Draw off of cooling air for heat distribution and oil sealing.
• Combustor - The Combustion Reaction's impact on the Working Fluid; The role of Primary/Secondary/Tertiary Air and heat distribution to protect the Turbine Inlet Temperature.
• Turbine - Creep of Rotor Blades and Stator Vanes; the evolution of Turbine Exhaust Gas (TEG)

 

Creep happens when centrifugal force and hot temperatures cause the metal in Rotor Blades and Stator Vanes to elongate. Cooling air from the Compressor Section of the GT helps cool the first and second Rotor Blades to prevent Creep.

The easiest fuel to use in GTs is natural gas. Heavier hydrocarbons can be used but they must be atomized prior to being injected into the Combustor/Igniters. Liquid fuel droplets that make it into the Turbine would pit Rotor Blades and Stator Vanes.

©2019 PTOA Segment 0195

PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Mover/Drivers - Gas Turbines Focus Study

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