I was too ..
Too hot, baby ...
Too hot to handle.

("Too Hot to Handle," by UFO, 1977)

 

GAS TURBINES ARE "THERMODYNAMIC MACHINES"
(WHICH IS EXPLAINED RIGHT NOW) 

This is a good time to recall what PTOA Readers and Students  learned way back in PTOA Segment #1 about the PV Temperature:

"Temperature" is the indication of how much internal heat energy "something" has acquired. In a GT, that "something" is common air.

The word "dynamic" means "changing" … aka "not staying the same."

In fancy text-book lingo, an increase or decrease in internal heat energy is called a "thermodynamic change."

Process Operators observe thermodynamic changes whenever the PV Temperature that is displayed on a Temperature Indicator (TI) increases or decreases.

Fred wants to know why he needs to known the meaning of the word "'thermodynamic?"

Gas Turbines operate on the basis of "thermodynamics."  Gas Turbines are "thermodynamic machines."

Otherwise stated in plain English:

The internal heat energy of air and The Working Fluid are intentionally changed while flowing through the GT so that mechanical rotational power can be created for the purpose of driving an electricity generator, pump, or compressor.

Controlling the build-up and extraction of internal heat energy is fundamental to operating a Gas Turbine safely and efficiently.

 

Process Operators observe changes in internal heat energy as changes in the PV Temperature.

By the time PTOA Readers and Students finish this PTOA Segment they will have a competent understanding of how the PV Temperature is used to gauge the amount of internal energy gained by both the air and The Working Fluid as these gases flow through the GT's Compressor and Combustor. PTOA Readers and Students will also understand how the PV Temperature helps gauge how much internal heat energy is extracted from The Working Fluid in the GT's Power Turbine for the purpose of producing mechanical rotational power.

 

"THE WORKING FLUID OF A GT" IS DEFINED ONE FINAL TIME

Brilliant PTOA Readers and Students … meaning those who read the PTOA Segments in the intended sequential order … do not need to be reminded that the compressed air that is drawn into the GT's Air Compressor evolves into a Working Fluid  composed of hot, compressed air and Combustion Reaction products.

As shown in the nearby graphic, one of the byproducts of The Combustion Reaction is "Heat." This "Heat" infuses the compressed air and the other Combustion Reaction products with internal heat energy.

The hot, compressed gases that make up The Working Fluid exit the GT's Combustor and enter the GT's Turbine via Turbine Inlet Guide Vanes which are shown in the nearby graphic.

Heck all of the above is old news that was covered in PTOA Segment #195.

Movin' on to new stuff right now!

 

THE "TURBINE INLET GAS TEMPERATURE" IS DEFINED

Brilliant PTOA Readers and Students already learned in PTOA Segment #196 that the Compression Ratio (CR) of the GT's Air Compressor significantly impacts the fuel efficiency of the entire Gas Turbine Engine. The higher CR the better!

On the Turbine side of the GT, efficiency is determined by the amount of internal heat energy The Working Fluid acquires in the Combustor.

The hotter the better!

 

The Turbine Inlet Gas Temperature is the temperature of The Working Fluid that exits the Combustor's Turbine Inlet Guide Vanes and is destined to strike the first Rotor Blade of the Turbine.

The Turbine Inlet Gas Temperature determines how much rotational mechanical power the Turbine will be able to produce.

The above boxed statement should not surprise any card-carrying PTOA Reader or Student!

Except maybe Fred because he is looking confused.

At this juncture every PTOA Reader and Student … including Fred … should have gleaned that the only job of the GT's Turbine is to extract internal heat energy … aka thermal energy … from The Working Fluid and convert that thermal energy into rotational mechanical power ... aka "horsepower (hp)."

(PTOA Segment #186 already clarified that the mathematical definition of hp = (Torque)X(Shaft Speed in rpms) ÷ 5252).

Well …

The  hottest The Working Fluid is ever going to be is exactly where The Working Fluid exits the Turbine Inlet Guide Vanes and impacts the first stage of Rotor Blades and Stator Blade!

Yes Indeedo!

Once the extraction of heat from The Working Fluid has begun… it's all downhill with respect to the PV Temperature of The Working Fluid!

The PV Temperature of The Working Fluid decreases as heat is extracted and mechanical rotational power is produced!

 

The above statement means that

• IF you were a Plant Manager interested in buying a GT that would reliably operate safely and efficiently …
• THEN you would select the GT model that could produce the highest Turbine Inlet Gas Temperature because that model will be able to extract the most internal heat energy from The Working Fluid and therefore create the most mechanical rotational power (aka hp).

 

How hot does it get in a GT, anyway?

The typical industrial process industry GT in an industrial processing plant power house would have a Turbine Inlet Gas Temperature of 2000+ °F.

A GT like GE's Harriet is specifically selected by a utility company to generate reliable electricity to the masses 24/7. Harriet can have a Turbine Inlet Gas Temperature of 2700 °F. An  aircraft engine propulsion/thrust GT can have a Turbine Inlet Gas Temperature of 3000 °F!

 

Wow! The Turbine Inlet Gas that exits the Combustor's Turbine Inlet Guide Vanes sure has a high PV Temperature and high PV Pressure before heat extraction and expansion starts happening in the Turbine!

But uh-ho!

These high temperatures are too hot for the 1500 °F-1700 °F temperature limit of hardware components that will come in contact with the hot gas. And no doubt the constant exposure to hot temperatures will cause metal fatigue and creep of the Rotor Blades and Stator Blades.

Alrighty, then!

 

In that case the above decision-making statements must be amended:

• 

  The hot air that impacts GE's Harriet Turbine can be 2700°F.

  IF you were a Plant Manager interested in buying a GT that would reliably operate safely and efficiently …

• THEN you would select the GT model that had the highest Turbine Inlet Gas Temperature yet still could reliably operate 24/7 without melting down the Turbine's Rotor Blades and Stator Blades … or causing these blades to elongate and creep.

 

For this reason GT manufacturers incorporate the below advanced technologies in GT designs:

• The first and second stage Rotor Blades and Stator Vanes … those closest to the Combustor and thus exposed to the Turbine Inlet Gas Temperature …  are fabricated with a top-secret super alloy metal and coated with heat resistant materials.
• The "wheels" or "hubs" of the first and second stage Rotor Blades are ingeniously cooled with much cooler compressed air which has been drawn from the Axial Compressor (as was explained in the "Squeeze" part of PTOA Segment #195). Remember! In a Split-Shaft GT, the 1st and 2nd stage Rotor Blades are attached to the Gas Producer Shaft … not the Power Turbine Shaft … as was explained in PTOA Segment #196.

 

THE TURBINE EXHAUST GAS (TEG)  TEMPERATURE DEFINED

The gas that exits the Turbine of a GT is logically called Turbine Exhaust Gas (TEG).

The top part of the nearby graphic features a Single-Shaft GT shown as separate components: the Compressor, the Combustor, and the Turbine.

The PV Temperature and PV Pressure of the Turbine Exhaust Gas (TEG) are measured at the location below the Turbine, at Point "4".

TEG is compositionally different than the Turbine Inlet Gas that flowed into the Turbine from the Turbine Inlet Guide Vanes at Point "3" (located above the Turbine). TEG has been diluted with cooling air.

TEG is physically different than Turbine Inlet Gas, too.

How? Read on!

 

 

THE THERMODYNAMIC AND PRESSURE CHANGES THAT OCCUR IN A GT

The nearby schematic offers an excellent visual opportunity to study the "thermodynamics" that occur in a Single-Stage GT.

Find the "Air Inlet" arrow at the bottom of the GT's Compressor  (near the label "1").

• The air is sucked into the Compressor with a PV Pressure of P₁ = 1 bar (aka almost 1 Atm or 14.5 psi). The PV Temperature T₁  is 300 °K (aka 27 °C or 80 °F).

 

• The purpose of the Compressor is to increase the PV Pressure of the air. The air is discharged from the Compressor at Point "2." The "Discharge Pressure " of the air is  P₂ = 13.6 bar (aka 3.4 Atm or 197 psi). As would be predicted by the Gas Laws featured in PTOA Segment #152, compressing the air has also caused the PV Temperature to increase.  The "Compressor Discharge"  temperature T₂ = 600 °K (aka 326 °C or 620 °F). The newly compressed and warmer air flows into the Combustor.

 

• The Combustor outlet pressure P₃ is 13.0 bar (aka 12.8 Atm or 188.5 psi). Note that the Combustor's outlet temperature is also the Turbine Gas Inlet Temperature (T₃). The Turbine Inlet Gas Temperature T₃=1580 °K (aka 1306 °C or 2876 °F).

 

Wow! That's hot!

Now the very hot, compressed air will be intentionally expanded in the Turbine to  create rotational mechanical power (aka hp)!

• The process of expansion/heat extraction reduces the PV Pressure of the Turbine Exhaust Gas (P₄) to 1 bar (almost 1 Atm or 14.5 psi). The Turbine Exhaust Gas (TEG) Temperature is 900 °K (aka 626 °C or 1160 °F).

 

• The Turbine Exhaust Gas Temperature is measured with thermocouples. These thermocouples are  placed on the Turbine Exhaust Duct (where T₄ is shown on the schematic).

 

• The Turbine Exhaust Duct is exactly what it sounds like … it is a large diameter exhaust duct through which the expanded Working Fluid and cooling air exit the GT after having most of the internal heat energy extracted.

 

The TEG Temperature must always be above 400 °F otherwise corrosive gasses will form and attack the exhaust duct.

Note that the process of gas expansion/heat extraction produces the equivalent of 147 Mega Watts of electricity via an unseen electricity generator coupled to the Turbine's Power Shaft!

 

"YOU-TUBE AND CHILL" WITH AGENT JZ OF JET CITY AGAIN!

Hey, doesn't this "Turbine Exhaust Temperature"  jargon ring a distant bell?

This is a great time to revisit Agent JZ's "Power Turbines and Thermocouples" You Tube that was featured way back in PTOA #112. Back then PTOA Readers and Students were instructed to focus on just the thermocouple application.

PTOA Readers and Students are so much more GT savvy now! Agent JZ's entire You Tube will make more sense.

Access the video HERE or directly below after reading all the explanatory notes.

Definitely take the time to "Like" Agent JZ's video because he came in on a Sunday to film it.

Be alert for the following events in the You Tube:

The Air Compressor:

Agent JZ has just finished refurbishing a GE LT 2500 GT.

Agent JZ shows the inlet air duct, which looks an awful lot like an aircraft part. The inlet air valves are closed of course because the GT is disengaged from the process.

Compressors will be featured in upcoming PTOA Segments so no  PTOA Reader or Student should stress about Compressors now. However, Agent JZ shows the Axial Compressor's "linkage bar" … like the disengaged one shown in the nearby photo. Linkage Bars are used to vary the pitch of the Compressor's guide vanes.

 

Fuel Injection

Agent JZ points out the natural gas and liquid fuel lines that supply fuel to fuel injectors. Each combustion canister in the GT's Combustor has its own fuel injector.

Agent JZ shows the placement of water lines that inject water into each of the fuel injectors.

Agent JZ mistakenly states that the purpose of water injection is to "augment efficiency." The injection of water will cool the equipment and lower efficiency. However water injection is necessary to allow the GT to meet green house gas emission limits, in particular nitrous oxides. The water injected is not common water but highly treated, ultra-low conductivity water.

 

Agent JZ defines "The Engine Part of the Turbine" (aka "Engine Core")

Agent JZ defines the "Compressor-Combustor- and Gas Producer Turbine/Shaft"  components of the Turbine as  'the engine part of the LM2500." The term typically used in industry is "the GT engine core." The Gas Producer Turbine drives the GT's Axial Compressor.

Agent JZ refers to the Power Turbines as "PTs."  Recall that Power Turbines drive external Rotating Equipment and are not part of "the GT engine core." 

Agent JZ  explains that the LM2500 engine core is very versatile because it can be mated with PTs that have a varied amount of stages. According to Agent JZ, the LM2500 core can be mated with 2 stage, 5 stage (used on an aircraft), or 6 stage PTs. The 6 stage PT that is being refurbished in the You Tube is not clearly visible through its protective housing.

Agent JZ mentions that the Gas Producer Turbine Shaft of the LM2500 core engine rotates at 9200 rpm. The 6 stage Power Turbine Shaft rotates at 3000-3200 rpm.

Agent JZ highlights the Output Power Shaft of the 6 stage PT. This Output Power Shaft has a grooved spline that will be mated with the shaft of the driven piece of rotating equipment. Later on in the You Tube Agent JZ compares the 6 stage PT's Output Power Shaft to the Output Power Shaft a single stage PT that is sitting in the yard on a slab.

Agent JZ discusses the Rotor of the single stage Power Turbine … which is sitting on a cement slab near the 5 stage aircraft PT. The "disc" of the single stage PT is clearly shown in the below title photo of Agent JZ's "Power Turbines and Thermocouples" You Tube.

Note that the Rotor Blades are attached to each "disc" (which is sometimes called a "hub"). Ports are typically incorporated into the discs/hubs of the 1st (and 2nd stage) Rotor Blades to allow flow of cool, compressed air. In a Split-Shaft GT design, the 1st (and maybe 2nd stage) Rotor Blades and Stator Blades are on the Gas Producer Shaft .

 

Agent JZ compares the 5 stage aircraft PT to the single stage PT. The single stage PT is heavier than the 5 stage PT yet can achieve the same rotational speed of 5500 rpm. Industrial GTs are larger and heavier than propulsion GTs because industrial GTs are much less concerned about weight.

This part should sound very familiar: Agent JZ shows how a ring of thermocouples are used to detect and measure the Turbine Exhaust Gas (TEG)Temperature. Sometimes he mistakenly infers that these thermocouples are detecting the Turbine Inlet Gas Temperature, a temperature that would be 2000+ °F. As drawn, the 900°F temperature inferred represents the temperature of the Turbine Exhaust Gas. However, PTOA Readers and Students will learn in the next PTOA Segment that the Turbine Exhaust Gas Temperature can be used to infer the Turbine Inlet Gas Temperature!

Agent JZ laments that the 5 stage PT and the single stage PT will never be used and are rusting on cement slabs. And yet these dismembered GT parts have an after life clarifying the structure of GTs to PTOA Readers and Students! Thank you Agent JZ of Jet City!

 

 

COGENERATION OF ELECTRICITY AND STEAM
REDUCES GLOBAL WARMING AND SAVES PLANT OPERATIONS $$$

In the jet propulsion application of GTs, the Turbine Exhaust Gas (TEG) is released into the atmosphere while creating "thrust" aka "propulsion."

At the time this PTOA Segment was written, the contribution to global warming caused by air travel is a growing concern.

The TEG in the nearby gif of a jet propulsion GT is shown as emitted white light.

Venting the Turbine Exhaust Gas (TEG) from an industrial power GT situated in the power house of an industrial plant would:

• Add significantly to the problem of global warming.
• Be extremely wasteful …  the typical 900 °F TEG Temperature indicates TEG still contains a significant amount of useful thermal energy.
• Split the ear drums of everyone nearby … just like standing next to a jet engine would do.

 

Many GTs are configured into systems that exchange the internal heat energy of the Turbine Exhaust Gas with Boiler Feed Water (BFW) in a Heat Recovery Steam Generator (HRSG)  which creates steam.

The steam is then sent to a Steam Turbine (featured in PTOA Segment #193) which can drive a generator, a compressor, or a pump!

The familiar schematic shown nearby depicts a GT that is in a Cogeneration System. Recall that Point "4" describes the PV Temperature and PV Pressure of the Turbine Exhaust Gas.

After extracting internal heat energy to create rotational mechanical power, the PV Temperature of the TEG that exits the GT (T₄) is 900 °K (aka 626 °C or 1160 °F).

Wow! There is still  plenty of  internal heat energy that should be used and not wasted! Here's how:

• The TEG  exchanges heat with Boiler Feed Water in a Heat Exchanger (aka HSRG … which will include a  Package Boiler).
• The steam that is generated at Point "7" is fed to a Steam Turbine that drives a supplemental power generator (shown as W_sup).

• The TEG that exits the Heat Exchanger at Point "5" has a PV Pressure (P₅) of 1 Atm (aka 14.7 psi) and a PV Temperature (T₅) of 400 °K (aka 126 °C or 260 °F).

 

To recap:

In a Cogeneration System, the fuel that is used to add internal heat energy to The Working Fluid is also used to heat up BFW to make steam that can be used in a Steam Turbine Driver … typically to make more electricity! The cogeneration of electricity and steam significantly increases the overall thermal efficiency of the GT.

For example … some models of GE's Harriet in a Cogeneration System achieves 63+% efficiency!

The best any coal-fired power plant can achieve on a good day is 42% … AND coal generation still has nasty global warming gas emissions.

For this reason woke Power Plant Managers are replacing coal-fired GTs with natural gas GTs in a Cogeneration System. And that is precisely why PTOA Readers and Students should learn everything in this PTOA Segment!

The next PTOA Segment shows how the Turbine Inlet Gas Temperature and the Turbine Exhaust Gas (TEG) are used to gauge whether or not the GT is operating as expected.

 

TAKE HOME MESSAGES: Gas Turbines are "thermodynamic machines" which are dependent upon (and designed for) the creation and rapid extraction of heat energy to produce rotational mechanical power (aka horsepower, hp). 

Controlling the generation and extraction of internal heat energy (aka "thermal energy") is fundamental to understanding how a Gas Turbine operates safely and efficiently. Process Operators observe changes in internal heat energy as changes in the PV Temperature.

The Turbine Inlet Gas Temperature determines how much power the GT will be able to generate. The Turbine Inlet Gas Temperature is so hot that the 1st and 2nd stage Rotor Blades are fabricated with proprietary alloyed metals and coated for heat resistance. The 1st and 2nd stage Rotor Blades are also cooled with compressed air. In a Split-Shaft GT design, the 1st (and maybe 2nd) stage Rotor Blades are in the Gas Producer Turbine, not the Power Producer Turbine. 

The Turbine Exhaust Gas (TEG) is the gas that is discharged from the GT through a Turbine Exhaust Gas Duct. The TEG must never be lower than 400 °F otherwise the TEG Duct will corrode. 

Thank you Agent JZ of Jet City for:

• Clarifying that the Compressor-Combustor-Gas Producer Turbine are the "engine core" of the GT that can be mated to a variety of Power Turbines.
• Clarifying the operational differences between single, 5-stage, and 6 stage Power Turbines.
• Demonstrating the use of thermocouples in the measurement of the TEG temperature.

 

PTOA Readers and Students followed the gas path of air through a Single-Stage GT. They gained an understanding of the thermodynamics (aka "changes in internal heat energy") that must occur to the air that is sucked into a GT as it becomes part of The Working Fluid and thence Turbine Exhaust Gas (TEG) vented from the GT. These thermodynamics result in the creation of rotational mechanical energy, aka horsepower.

Cogenerating electricity and steam with the same investment in Combustor fuel is economically wise and ecologically prudent.GTs that are part of a Cogeneration System exchange the internal heat of TEG with BFW in a Heat Exchanger known as a Heat Recovery Steam Generator (HSRG). This steam is often used to drive an supplemental electricity generator but might also be used to drive a pump or compressor. 

©2019 PTOA Segment 0197

PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Movers/Drivers - Gas Turbine Focus Study

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