And I'm thinking of you
And all the things that we used to do
Wah-wah, wah-wah

("Wah-Wah," by George Harrison, 1970)

 

SOME GTS ARE SINGLE AND SOME GTS ARE SPLIT 

 

Brilliant PTOA Readers and Students who are reading the PTOA Segments in the intended, sequential order learned that GTs (Gas Turbines) are Prime Movers/Drivers that produce torque power via the controlled heating and subsequent expansion of compressed air.

The nearby gif shows blue air being sucked into The Compressor Section of a GT.

After compression in an Axial Compressor, the compressed air flows into the burners that are located in the inlet of the Combustor. More than likely the fuel supplied to the burners is natural gas.

Upon exiting the Combustor through the Turbine Inlet Guide Vanes, the compressed, hot air and Combustion Reaction Products become a Working Fluid that now contains enough internal energy that can usefully be released/extracted/expanded in the Turbine Section of the GT.

The hot, high velocity Working Fluid spins the Rotor Blades which are attached to the Shaft and cause it to rotate. The intervening Stator Vanes are attached to the internal Turbine casing and redirect the air flow to the next Stage of Rotor Blades.

The process of "expanding the hot gases" in the Turbine simultaneously extracts/decreases the heat contained in the Working Fluid AND  produces rotational power.

"Rotational Power" is also called "Torque," a featured topic in PTOA Segment #186.

Enough review, already! Here is some NEW information:

• The Single-Shaft GT design can efficiently drive an Electricity Generator when the demand for electricity is expected to be constant.
• In the integrated industrial process setting, mechanically driven Loads for Rotating Equipment like Pumps and process Compressors are not constant and the Split-Shaft GT design is most efficient.
• Only one-third (33%) of the GT's  total created rotational torque power is available to rotate the Load that it is coupled to. Most of the power is used to rotate the GT's own Axial Compressor!

 

This PTOA Segment #196 explores the above bulleted statements. At the conclusion of this PTOA Segment brilliant PTOA Readers and Students will understand:

• Why some GTs are Single-Shaft and some are Split-Shaft.
• Be introduced to the concept of "GT operating efficiency."
• Understand the purpose of GT Overspeed Trip Devices.
• Renew the purpose of Gear Reduction which was first featured in PTOA Segment #186.

 

 

THE SINGLE-SHAFT GAS TURBINE

A simplified graphic of a Single-Shaft Gas Turbine is nearby.  The graphic depicts the following bulleted points:

• Cool air … represented as blue arrows …  enters the Compressor on the left side of the graphic. Note once again that the Compressor symbol is "cone-shaped with a decreasing radius." This Compressor symbol infers that the volume of the gas (air) is decreased as the gas flows through the Compressor.
• The Combustor in the middle is shown as a vertical slab. Fuel and cool blue air enter on the left side of the Combustor and red arrows representing hot, compressed air and combustion products exit on the right side.
• The red gases discharged from the Combustor enter the Turbine. The Turbine symbol is "cone-shaped with an increasing radius" because the air volume is expanded as it flows through the Turbine.  A much greater volume of hot air and combustion products exits the Turbine … and this is the big deal …because
• The expansion of the gases has caused the Shaft  to spin and drive some unknown Load that does not appear in the graphic. The spinning of the Shaft is supposed to be inferred by the red ring-shaped arrow.

 

Here is new and interesting information:

Note that the Turbine's Shaft is also the Compressor's Shaft!

 

Gadzooks!

 

This Shaft architecture means that the Turbine is providing rotational power … aka torque … to the unseen Load AND the GT's own Compressor!

 

And yet it is the Compressor that is creating the compressed air which is eventually is expanded in the Turbine, causing the Turbine to  rotate in the first place!

Keeno Neato integrated design, eh?

The PTOA Department of Redundancy Department wishes to reiterate that the Single-Shaft GT design is one in which the Turbine's Shaft is the same Shaft as the Compressor's Shaft. 

In the Single-Shaft GT design, it should be somewhat obvious to PTOA Readers and Students that the Turbine's Output Power End of the Shaft spins at the same speed as Shaft end that is driving the Compressor … because they are the same Shaft!

Gear Reduction would have to take place in a Gear Box situated between the Turbine's Output Power End of the Shaft and the Load so that the Load could spin at a slower speed than both the Compressor and the Turbine.

This direct relationship between the speed of the Compressor, Turbine, and Load suggests that the Single-Shaft GT design would only be beneficial when the GT is coupled to a Load that must be rotated at a constant, efficient, steady speed.

Guess what?

In these United States of America many Electricity Generators rotate at 1800 rpm to produce 60 Hz electricity. Don't stress about that now. All will be explained in the future PTOA Electricity Generation and Distribution Focus Study Area.

For now just recognize that a constant 1800 rpm rotating speed is what's known as a  "predictable Load."

The Single-Shaft Gas Turbine would be a great GT design when the Load is an Electricity Generator in a Power Plant.

The nearby bird's-eye view of GE's Harriet GT reveals that it is a Single-Shaft GT design.

A Reduction Gear would reduce the speed of the Turbine's Output Power End of the Shaft (which is shown on the left side of the picture). The Gear Reduction would make certain that the (4 pole) Generator rotates at 1800 rpm.

The below block diagram of a Single-Shaft GT illustrates the positioning of the Reduction Gear between the Gas Turbine and the (Synchronous) Electricity Generator. The Electricity Generator symbol is a circle with a squiggly line representing the generation of AC electricity.

 

 

LIMITATIONS OF THE SINGLE-SHAFT GT DESIGN

In the integrated processing complex, the Load driven by a GT can vary over a wide range of speed, thus the GT Engine must work more like a mechanical Variable Speed Driver.

Did Your Mentor hear a "HUH?" from Fred?

Yep!

Clarification for Fred, please!

Okay Fred:

Imagine a Single-Shaft GT humming along spinning its Load at peak efficiency …and then the spinning Load is suddenly increased.

For example these very possible situations could occur:

• The Load is an Electricity Generator and an Outside Process Operator suddenly starts a major motor that consumes a lot of electricity.
• The Load is a Pump and the flowrate ... or viscosity... of the liquid being pumped  increases.
• The Load is a Compressor and the flowrate  ... or viscosity ... of the gas being compressed increases.

 

The unexpected, extra Load will cause the rpms of the Single-Shaft GT to slow down … because it is just harder to spin more Load.

The fancy pants  name for this happenstance is "increased torque resistance."

The below bullet points explain what happens when "increased torque resistance" happens to a Single-Shaft GT:

• The Compressor cannot rotate/spin as fast which means→
• Air cannot be compressed as much so →
• The Compression Ratio (aka Discharge Pressure/Suction Pressure) would decrease.

 

Dang it! That means:

• 

  Yep! Another use of the Gas Laws. The oxygen particles in the specimen on the left would take more fuel to burn than the more compressed and dense oxygen particles on the right.

  The compressed air entering the Combustor would be less crammed together … aka becomes "less dense" … aka has "less density" so →

• The air becomes less effective "turbo charging the GT engine" because →

 

• More Combustor fuel would be required to heat up the air so that it would gain sufficient internal heat energy to turn the Rotor Blades upon expansion… which is what causes the Shaft to rotate in the first place!

 

The end result of adding incremental Load to a Single-Shaft GT that had been humming along operating at design speed would be much less efficient operations of the entire GT Engine. Continuous operation at above-design Load condition would cost more $$$ every day 24/7 as well as increase the wear and tear on expensive Engine parts that were designed to operate at a different speed … and thermal conditions.

 

And what about the opposite situation … when the Load suddenly decreases?

Hey this is as good a place as any to bring up the purpose of the Overspeed Trip Device that all GTs have!

If the Load suddenly decreases, the Shaft  of the GT … which is connected to the Load … will start to accelerate, rotating at much greater rpms. And if the Load drops off entirely the speed can increase to the point of destruction. The Overspeed Trip Device prevents rotation above 110% of design speed (which means 10% more than design speed).

Once activated, the Overspeed Trip Device will cut off fuel flow to the Combustor.

 

 

THE SPLIT-SHAFT GT DESIGN

The far more efficient design that accommodates Load changes incorporates two separate Shafts.

What Your Mentor has been calling the "Output Power End of the Shaft" of a Single-Shaft GT is called a "Power Turbine (Load) Shaft" in a Split-Shaft GT. Both extend through the GT's Turbine casing and drive the Load.

The important design difference is: The Split-Shaft GT has a separate  "Gas Producer Shaft" that drives the Compressor. 

The nearby graphic shows a Single-Shaft GT on the left compared to a Split-Shaft GT on the right.

The Single-Shaft GT is driving an Electricity Generator represented as a red ball with a squiggly line inside. PTOA Readers and Students can Imagine the Electricity Generator is being rotated at a constant 1800 rpm.

The Split-Shaft GT has an unspecified box which is with labelled "Load." This "Load" could be a Pump or a process gas Compressor that must operate over a wide range of rotational speeds to supply the pressurized liquids or gases that are demanded as needed.

In a Split-Shaft GT the Gas Producer Shaft spins at the most efficient 100% design speed while the Power Turbine (Load) Shaft speed fluctuates with Load requirements.

Another way to say it is that the Split-Shaft GT design allows the Compressor to maintain optimal Compression Ratio no matter what fluctuations are going on with the Load.

And both of the above two statements infer that the Split-Shaft GT design can more efficiently be used as a mechanical "Variable Speed Driver."

PTOA Readers and Students should note:

• The Gas Producer Shaft of the Turbine section has a common shaft with the Axial Compressor. The Rotor Blades of the Gas Producer Shaft are smaller in diameter. This is the High Pressure (HP) Turbine Section.
• The separate Power Turbine (Load) Shaft has the larger diameter Rotor Blades attached to it and this is the Low Pressure (LP) Turbine Section.

 

The above statements infer that the speed of the Gas Producer Shaft is greater than the Power Turbine (Load) Shaft.

The nearby graphic shows a Titan 250 GT manufactured by Solar, a division of Caterpillar, Inc.

The Axial Compressor is blue.

The Gas Producing Shaft (which drives the Axial Compressor) is pink.

The High Power Turbine Blades are shown in red.

The Low Power Turbine Blades are ...uh mauve … maybe???

And the Power Turbine (Load) Shaft is brown (and not shown coupled to a Load).

The act  of 'gas producing' … aka compressing air … consumes 66% of the total power produced by the GT.

The remaining power produced by the GT … 33%... is what remains to drive the main Load and additional auxiliary rotating equipment..

 

PTOA YOU TUBE AND CHILL TIME!

Hey, once again it's PTOA You Tube and Chill Time!

Many thanks to MAN Diesel & Turbo for their "3D Animation of Industrial Gas Turbine Working Principle" which eventually gets around to illustrating the design differences between Single-Shaft GTs and Split-Shaft GTs. As usual be sure to "Like" the video!

The MAN Diesel & Turbo 3D Animation of Industrial Gas Turbine Working Principle can be directly accessed below or by clicking HERE.

 

Keep on "PTOA You Tube and Chillin!"

Since there is still popcorn leftover, watch this GE commercial for their H-class (Harriet) Gas Turbines.  Thank you to GE for showing PTOA Readers and Students how all the parts and pieces of a Single-Shaft GT fit together. Access the GE Harriet Gas Turbine You Tube directly below or HERE.

 

 

 

AUXILIARY POWER TURBINE LOADS

Besides mechanically spinning the main Load, a portion of the rotational power produced by the Power Turbine (Load) Shaft can be used to drive auxiliary equipment like Lube Oil Pumps, Hydraulic Oil Pumps, Cooling Water Pumps, Fans for Fin-fan Coolers.

Gears were featured in PTOA Segment #186.

So every PTOA Reader and Student who is reading the PTOA Segments in the intended sequential order will completely understand the following:

 

If:

• The Power Turbine (Load) Shaft were also driving a Lube Oil Pump ...
• And combined Gear Ratio between the Power Turbine (Load) Shaft (aka Gear Input Shaft) and the driven shaft of the Pump (aka Gear "Output Shaft") is 5:1.
• And the speed of the Lube Oil Pump shaft (Output Shaft) is 3000 rpm.

 

Then

The Power Turbine (Load) Shaft is rotating at 15,000 rpm!

 

The next PTOA Segment explains why Gas Turbines are called "thermodynamic machines" and why that fancy term is nothing to stress about!

 

TAKE HOME MESSAGES:  In a Single-Shaft GT the Turbine's Output Power Shaft drives a Load and the other end of the same Turbine Shaft drives the GT's Axial Compressor. The Single-Shaft GT design works efficiently when the Load that is driven by the GT requires a constant rotational speed. Many electrical Loads are constant and can be efficiently driven by Single-Shaft GTs. Harriet … an electricity Load driver GT manufactured by GE … is a Single-Shaft GT.

The Single-Shaft GT cannot efficiently operate when the Load is a Pump or a process Compressor, because these Loads vary with process demands. When a Pump or Compressor is being driven at design speed by a GT and additional Load is added, the Axial Compressor will not be able to maintain Compression Ratio and that will cause more fuel to be consumed in the Combustor and wear and tear on the GT internal hardware.

The Split-Shaft GT design allows the Axial Compressor to rotate at design speed while the Load can be mechanically driven at varying speeds. Thus the Split-Shaft GT can be described as a mechanical Variable Speed Driver.

The Turbine in a Split-Shaft GT has a Gas Producing Shaft that drives the Axial Compressor and a separate Power Turbine (Load) Shaft that drives the Load and auxiliary Loads.

The Gas Producing Shaft is attached to the first few Rotating Blades … the ones with the smallest diameters. The Gas Producing part of the GT Turbine is the High Pressure side of the Turbine.

The Power Turbine (Load) Shaft is attached to the Rotating Blades with the largest diameters. The Power Turbine (Load) Shaft part of the GT Turbine is the Low Pressure side of the Turbine.

The HP part of the Turbine generates more torque (aka rotational power) than the LP side of the Turbine. Hey! That means the Axial Compressor is spinning at a faster speed than the Load!

The act of "producing gas" (aka "compressing air") consumes 66% of the overall torque power produced by the GT Engine. Only 33% is available for driving the Load and auxiliary equipment.

In a Single-Shaft GT, Gear Reduction is situated between the Output Power End of the Shaft and the Load.

In a of the Split-Shaft GT, Gear Reduction is situated between a dedicated Output Power (Load) Shaft and the Load. 

The Overspeed Trip Device on all GTs will cut fuel to the Combustor if the speed of the Turbine is sensed to be 110% of design (10% over the design speed).

Thanks  to Man Diesel & Turbo for use of the You Tube "3D Animation of Industrial Gas Turbine Working Principle."

Thanks to General Electric GE for use of the "GE Harriet Gas Turbine You Tube." 

©2019 PTOA Segment 0196

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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