I'm looking through you,
Where did you go? 
I thought I knew you,
What did I know?
You don't look different, but you have changed. 
I'm looking through you, you're not the same.

("I'm Looking Through You," by The Beatles, 1965)

 

POWER TURBINE OUTPUT AND CRUCIAL TURBINE TEMPERATURES 

Brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended sequential order … are already knowed-up on how thermodynamics rule Gas Turbines.  They also know the definition, importance, and difference between:

• The Turbine Inlet Gas Temperature and
• The Turbine Exhaust Gas (TEG) Temperature.

 

This PTOA Segment again features the Turbine Inlet Gas Temperature and how it can be correlated with a Gas Turbine Performance Curve to determine the expected Power Turbine Output.

 

The Temperature Profile of a GT Engine

In the lower right corner of the nearby graphic there is a GT with numbers that correspond to the Temperature profile graphed in red on the left.

The Y axis can be presumed to have an increasing upward scale, albeit the magnitude is not detailed.

Point "1" is the temperature of the ambient air sucked into the GT's Compressor. Point "2" is the temperature of the air at the Compressor discharge. Aha! The process of compressing the air has increased its PV Temperature somewhat.

The temperature profile peaks at Point "3" which is where the Turbine Inlet Gas Temperature would be located.

The temperature profile then steadily declines as the internal heat energy of The Working Fluid is extracted by the GT's Turbine. At Point "7" the TEG Temperature is measured with thermocouples.

 

Gas Turbine Performance Curves and Power Turbine Performance Charts

Gas Turbine Performance Curves and PowerTurbine Performance Charts are generated by the Turbine manufacturer during GT Engine testing.

Real Power Turbine Performance Charts are overlaid with multiple Turbine performance relationships which would be too intimidating for the novice GT operator to interpret. This PTOA Segment uses the simple Power Turbine Performance Chart shown nearby for instruction. 

This Power  Turbine Performance Chart  is from Turbine research studies which were dedicated to isolating cause-and-effect relationships of Turbine modifications on Turbine performance.

The structure of an actual operating industrial Power Turbine Performance Curve is similar; The Power Turbine Output is graphed on the Y axis as a function of Turbine Inlet Gas Temperature on the X axis.

By the time PTOA Readers and Students are done reading this PTOA Segment they will have observed:

• How the measured Turbine Exhaust Gas (TEG) Temperature is used to infer the much hotter Turbine Inlet Gas Temperature.
• How the Turbine Inlet Gas Temperature is used to gauge the expected Power Turbine Output  (which can be expressed in MW or Hp).
• Why the "real world" observed Power Turbine Output may be different than the expected Turbine Output Power that the manufacturer's Power Turbine Performance Chart describes.

 

 

THE DIRECTLY LINEAR RELATIONSHIP
BETWEEN THE TEG TEMPERATURE AND THE TURBINE INLET GAS TEMPERATURE

In the most recent PTOA Segment #197 PTOA Readers and Students learned that the Turbine Inlet Gas Temperature determines how much Power Turbine Output the GT will be capable of producing.

The hotter the Turbine Inlet Gas Temperature, the more Power Turbine Output from the GT engine!

The familiar nearby Gas Turbine component graphic indicates a Turbine Inlet Gas Temperature  of 1580 °K at Point "3."

The thermowells that would protect the thermocouples that could measure this hot Turbine Inlet Gas Temperature would melt!

Hey! The TEG Temperature at Point "4" is not too hot to measure! So ..

The TEG Temperature is used to infer/monitor the Turbine Inlet Gas Temperature!

 

The solution to infer the Turbine Inlet Gas Temperature from the TEG Temperature could only work if a directly linear relationship existed between the two temperatures.

The below graph is a set of Gas Turbine Performance Curves. Each "curve" depicts the typical relationship between "The Exhaust Gas (TEG) Temperature" on the Y axis and the Turbine Inlet Gas Temperature" on the X axis.

The graph reveals that the relationship between the TEG Temperature and the Turbine Inlet Gas Temperature is directly linear … not very curvy!

That means when the undetectable Turbine Inlet Gas Temperature has increased … the measured TEG Temperature has increased … and vice versa.

The direct linear relationship between the TEG Temperature and the Turbine Inlet Gas Temperature is so rock solid that the TEG Temperature is used as a safety control set point that can limit or even stop fuel flow to the Combustor!

Otherwise stated:

If the TEG Temperature starts increasing and is hotter than expected, that means the Turbine Inlet Gas Temperature must likewise be excessively hotter than desired and approaching unsafe temperatures!

When the TEG Temperature exceeds a specified amount, the typical GT advanced controls send a signal to the Combustor fuel valves to tell them "hold the fuel flowrate steady!"

 

The Gas Turbine Inlet Temperature will thus be "topped out" and will not  increase.

 

THE PROCESS OF INFERRING THE TURBINE INLET GAS TEMPERATURE
FROM THE TEG TEMPERATURE GRAPHED ON A GAS TURBINE PERFORMANCE CURVES

The nearby chart of Gas Turbine Performance Curves includes a blue line with black addition signs (labelled ITGT in the legend).

Just FYI but do not stress about it: The blue line with black addition signs is the Gas Turbine Performance Curve in a Gas Turbine Engine study that had two Compressors in series with a fin fan intercooler.

 

 

As stated above and repeated here, the Gas Turbine Performance Curve is generated by the manufacturer of the GT engine during post-fabrication testing.

The procedure to predict the Turbine Inlet Gas Temperature from the TEG Temperature plotted on a Gas Turbine Performance Curve is as follows:

 

• Find the familiar "component diagram" of a Cogeneration System a few paragraphs above.Recall that the TEG Temperature (T₄) is shown to be 900 °K (626 °C = 1160 °F) at Point "4."
• On the nearby graph, find 900°K on the vertical axis labelled Exhaust Temperature. 900 is midway between the 800 and 1000 tick marks.
• Draw a straight horizontal line to the right until it intersects with the dark blue Performance Curve (the line that corresponds to the ITGT label). This dark blue line with black addition signs on it is the second line from the top, below the green line with triangles.
• From the point of intersection, drop straight vertically down to the horizontal X-axis, which should be labelled Turbine Inlet Gas Temperature ( °K). A Turbine Inlet Gas Temperature of 1580 °K is determined … which happens to match the T₃ Temperature shown at Point "3" on the GT schematic.

 

Voila! The Turbine Inlet Gas Temperature has been inferred by correlating the measured TEG Temperature.

 

THE TURBINE INLET GAS TEMPERATURE
PREDICTS THE POWER TURBINE OUTPUT

Once the Turbine Inlet Gas Temperature is known, the Power Turbine Output in MegaWatts (MW) or Horsepower (Hp)  can be determined using a correlating methodology similar to the one demonstrated above but this time using the Power Turbine Performance Chart.

This Power Turbine Performance Chart shown nearby clearly shows the solid linear relationship between the newly found "Turbine Inlet Gas Temperature (°K) on the X axis and the Power Turbine Output (in MegaWatts, MW) on the Y axis.

When the driven equipment is a Pump or Compressor the Power Turbine Output might be expressed as "Output Shaft Horsepower (Hp)" instead of MW.

Whichever way the Power Turbine Output is expressed …

The Turbine Gas Inlet Temperature versus Power Turbine Output is another  directly linear relationship!

 

The hotter the Turbine Inlet Gas Temperature (on the X axis) the more Turbine Power Output in MegaWatts (MW) will be produced … and vice versa.

The procedure to determine the Power Turbine Output in MW from the Turbine Inlet Gas Temperature (°K) is as follows:

• Find the Turbine Inlet Gas Temperature of 1580 °K on the X-axis. 1580 °K is mid way between the last hash mark before "1600" and the number "1600" on the X axis.
• Draw an imaginary vertical line upwards until it intersects the Power Turbine Performance Curve shown as dark blue line with black addition signs.
• From the point of intersection, draw an imaginary horizontal line to the left until it intersects the Power (Turbine)Output in MW shown on the Y Axis.
• Voila! The Power (Turbine) Output from the Power Turbine of the GT that has the Turbine Performance Curve drawn as  dark blue with black addition signs is 250 MW.

Hey! The Turbine Output Power shown in the nearby familiar component graphic of a Gas Turbine is 147 MW … much lower than the graphed 250 MW Power Turbine Output!

What a pity the graphic does not state how much supplemental power is produced by the generator driven by the Steam Turbine (drawn as W_sup)!

According to the Power Turbine Performance Chart, a total of 250 MW could be produced from the specified Power Turbine so maybe W_sup produces approximately 100 MW.

 

GAS TURBINE EFFICIENCY LOSSES

The Power Turbine Output can decline between the scheduled GT maintenance intervals.

The best Process Operators notice when the "observed power output" of the Gas Turbine is different from the "expected power output" described by the Power Turbine Performance Curve plotted on the Power Turbine Performance Chart.

One common change in Power Turbine Performance is typically beyond the Process Operator to control:

 

The GT Engine performance varies with the ambient air temperature. 

 

Colder air is more dense when it is sucked into the GT, so it is already more compressed than warmer air would be. The colder the ambient air, the more power will be produced by the Power Turbine.

A 30 °F difference in inlet air temperature can easily increase/decrease Turbine Power Output by 565 hp!

Other sources of efficiency losses in Power Turbine are described below:

 

Efficiency Losses Attributed to Air Flow Friction

Air flowing through the Inlet Air Duct and Turbine Exhaust Duct creates friction losses …  just exactly like the  friction losses that were described in PTOA Segment #165 which were attributed to flowing liquid contacting internal pipe walls.

Additionally, air filtration is serious business for Gas Turbines! The air that is sucked into the GT via the Inlet Air Duct must be filtered prior to entering the GT's Air Compressor.  What PTOA Reader or Student would be surprised to learn that the process of shoving air particles through a filter media creates friction losses?

As usual, the PTOA does not advocate the use of any particular air filter over another. Whichever filter system is used, more than likely in the USA a Donaldson "Huff N Puff" self-cleaning air filtration apparatus will be incorporated into the air intake system.

Yet even the best air filtration system cannot prevent debris from coating the Stator and Rotor Blades of the GT's Air Compressor. A gradual reduction in the Air Compressor's Compression Ratio (CR) and simultaneous increase in fuel consumption will be noticed by the alert Process Operator. PTOA Segment #196 already explained how a decrease in the Compression Ratio reduces the operating efficiency of the entire GT Engine.

A "dirty Compressor" section in a Gas Turbine can easily reduce the Compressor Discharge Pressure by 5% or more  which will prevent the GT Engine from attaining full rotational speed.

 

 

Efficiency Losses Due to Mechanical Friction

Friction losses will also occur in the bearings that support the GT's Turbine and Compressor. Brilliant PTOA Readers and Students who have experienced the PTOA Tribology Focus Study (PTOA Segments #177 through #180) could have predicted that!

The GT Lubrication Oil System will be featured the next PTOA Segment.

Spoiler Alert! A GT has THREE lube pumps that kick on and off depending upon if the GT is starting up, shutting down, up and running, or in an emergency situation.

Otherwise the attention the GT Lubrication System demands from the Outside Process Operator is standard to that of any piece of Rotating Equipment:

An  increase or decrease in the lube oil PV Temperature and an increase in the ΔP between the inlet and outlet of the Lube Oil filter are red flags to the vigilant Outside Process Operator as was described in the PTOA Tribology Focus Study.

• If the Lube Oil Temp is too low, the oil will become more viscous and coat the bearings instead of lubricating them.
• If the Lube Oil Temp is too high, the viscosity of the oil will not be able to create the "regime change" wedge of oil that is needed to lubricate and support journal bearings.
• An increasing ΔP across the Oil Filter indicates debris is accumulating in the Lube Oil and the filter must be replaced.

 

 

Efficiency Losses due to Auxiliary Loads

In PTOA Segment #196, PTOA Readers and Students learned that the Power Turbine can drive its Load AND auxiliary equipment. Auxiliary Loads can create efficiency losses due to increased power consumption over time.  The Outside Process Operator must vigilantly monitor the operating condition of Auxiliary Loads as well as the operating condition of the GT Engine. A list of Auxiliary Loads includes:

• Lube Oil Pumps.
• Hydraulic Oil Pump.
• Cooling water pump.
• Electrical power for controls, motors, etc.
• Radiator fans/induced draft fans/forces draft fans.
• The excitation source for the generator.
• The heating/cooling/ventilating for the power house building wherein the GT is installed.
• Air compressors used for plant air/instrument air/atomization (of fuel) air.

 

Efficiency Losses Due to Environmental Controls

Aha!

This category is like the Earth telling the Plant Manager "pay me a little now for limiting air pollution… otherwise pay me a whole bunch more later in global warming."

The auxiliary equipment needed for noise suppression reduces GT efficiency. So does the auxiliary equipment needed to reduce the content of nitrous oxides (NOx) and sulfur oxides (SOx) in the TEG.

Not to worry! All Plant Managers still make mucho dinero because they pass the investment cost of cleaner air technologies onto their customers!

FYI:

PTOA Readers and Students who prefer an environmentally focused career could easily apply their knowledge of Process Technology while working for the environmental technology companies that manufacture the auxiliary equipment that is required to meet environmental compliance.

 

 

Efficiency Losses Due to GT Operation Limits

Recall that the TEG Temperature must be above 400 °F … otherwise corrosive gases will form and attack the materials used to fabricate the GT's Exhaust Duct.

Note that those corrosive gases would be the same corrosive gases that would be shot up into the air and blown into nearby neighborhoods were the environmental controls mentioned in the previous paragraph not in place!

 

GAS TURBINE UPGRADES TO PERFORMANCE

Brilliant PTOA Readers and Students are already aware that Gas Turbines fall into the category of "Engines" … drivers that create rotational power by expanding a Working Fluid that is created when an air/hydrocarbon fuel mixture is ignited (refer to PTOA Segment #191).

Ergo ...

The performance of the GT Engine can be enhanced by similar techniques used by any motor-head to increase the power of their automobile.

GT performance enhancements include:

• Increasing the mass flow of air through the Turbine by installing a larger compressor. This modification would be the same approach to changing the cylinder size in a Chevy engine from a 317 cid "small block" with a  454 cid "big block."
• Increasing the Compression Ratio (CR) of the Compressor by adding more stages. This would be equivalent to replacing the 8:1 pistons in the auto engine with a 12:1 pistons.
• The attainable Turbine Inlet Gas Temperature could be increased by replacing the Turbine Blades and Turbine Nozzles with hardware that is fabricated with improved metallurgy, better coatings, and even better air cooling. This modification strategy is the same as using forged aluminum alloy pistons to replace cast aluminum pistons.
• Fuel nozzles with that are capable of discharging higher fuel flow rates can be installed in the Combustion Section of the GT. In older cars this modification was made by having two 4 barrel carburetors instead of one.

 

Note that all of these modifications would result in higher emissions from the GT.  Today's environmental controls would mandate the use of Programmable Logic Controllers to make certain the NOx and Carbon Monoxide emissions are in compliance with the Industrial Plant's air pollution permit limits.

 

TAKE HOME MESSAGES: Performance Curves plotted on Performance Charts can be used by Process Operators (and Instrument Technicians) to determine if the Power Turbine is generating as much power as it should be. Performance Curves and Charts are supplied by the Turbine manufacturer that fabricated the GT Engine.

The Turbine Inlet Gas Temperature determines the amount of power that a GT can produced. The relationship between Turbine Power Output and the Turbine Inlet Gas Temperature is directly linear. The greater the Turbine Inlet Gas Temperature the more Turbine Power Output … and vice versa.

The Turbine inlet Gas Temperature is inferred from the TEG Temperature. The relationship between the Turbine Inlet Gas Temperature and the TEG Temperature is linear … the hotter the TEG Temperature, the hotter the Turbine Inlet Gas Temperature.

The TEG Temperature is used to control the Turbine Inlet Gas Temperature. If the TEG Temperature exceeds a desired setpoint, a signal is sent to hold the amount of fuel to the Combustor steady. The Turbine Inlet Gas Temperature (which is inferred from the TEG Temperature)  is then described as being "topped out."

The ambient air temperature significantly impacts GT Engine performance. The colder the ambient air temperature the more Power Turbine Output … and vice versa.

The Power Turbine Output and thus Turbine efficiency can also be impacted by the following phenomena. An asterisk denotes which phenomena the alert Outside Process Operator can impact:

• Friction losses due to air filtration*,  air flow resistance with Air Inlet Duct and Turbine Exhaust Duct.
• Friction losses due to hardware interactions and faulty Lubrication Oil System.*
• Auxiliary Loads.*
• Permitted GT Operation Limits and Safety Controls.

 

A "dirty Axial Air Compressor" can prevent the GT Engine from attaining full rotational speed. When the Air Compressor Stator Blades and Rotor Blades are clogged with dirt or salt, the Discharge Pressure can easily be reduced by 5%. … or more!

Gas Turbines must be operated very close to design levels to mitigate air pollution that contributes to global warming.

Just like a car engine, the GT Engine can be modified to generate more power. Design modifications are not the responsibility of the Process Operator. Modifications that produce more power will require additional emission control auxiliary equipment to mitigate the addition to global warming.

©2019 PTOA Segment 0198

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