You might need somethin' to hold on to
When all the answers, they don't amount to much
Somebody that you could just to talk to
And a little of that Human Touch

("Human Touch," by the Boss … Bruce Springsteen, 1992)

 

THE INTERFACE BETWEEN THE GAS TURBINE
AND THE GT PROCESS OPERATOR/TECHNICIAN

In PTOA Segment #199 PTOA Readers and Students learned how the GT's Advance Control System greatly assists during the Gas Turbine Startup.

In PTOA Segment #200 PTOA Readers and Students learned how the GT's Advanced Control System greatly assists during the Gas Turbine Shutdown.

So no PTOA Reader or Student would be blamed for wondering …

"Does the GT Process Operator/Technician impact the operations of a Gas Turbine at all or is the GT's Advanced Control System in charge of everything?"

 

The actual factual situation is that the secret to achieving a successful run length between the GT's scheduled maintenance intervals and scheduled overhauls requires critically important human interaction.

Just a little of that human touch is crucially needed in Gas Turbine operations because "the devil hides in the details" when it comes to keeping a GT operating until the next planned maintenance interval, thus avoiding an Emergency Shutdown.

Here's a few examples of cause-and-effect "details" a GT Process Operator/Technician takes care of:

 

IF the Axial Compressor sucks in CLEAN air …

THEN the debris that clogs the Compressor's Rotor and Stator Blades will be minimized.

IF the Lubrication Oil is clean and within the correct PV Temperature range to have the required viscosity …

THEN the lubrication oil will be able to remove the optimal amount of friction-generated heat from the GT Engine.

IF the fuel to the Combustor is not contaminated …

THEN the corrosion of metals at high PV Temperature will not occur.

In summary …

The GT's Advanced Control System cannot provide "the crucial human touch" that is necessary to keep the GT's operating environment and auxiliary systems well maintained on a daily basis.

And although the GT's Advanced Control System can relay information collected from the multitude of PV Temperature and PV Pressure instrumentation which adorns all GT Engines …

Only the human GT Process Operator/Technician can interpret the condition of the GT from the displayed indications and take appropriate action to insure that the GT has the expected run length between planned maintenance intervals.

This PTOA Segment #201 emphasizes the "why" and the "how" behind the crucial human touch that is needed to avoid an Emergency Shutdown.

There are actually so many "devils hiding in the details" with respect to the operation and maintenance of GTs that the Plant Manager may chose to invest in a maintenance service contract with the GT supplier.

Typically whichever company manufactured the GT that was installed at the Plant will also be under contract to provide both technical service and maintenance service for the Gas Turbine.

PTOA Readers and Students should take note that both the "contracted GT Technician/Maintenance Tech" and the "in-house on-the-payroll GT Process Operator/Technician/Mechanical Tech" are both examples of Process Technology careers.

 

THE INTAKE AIR SYSTEM AND FLOWPATH … AND THE PROCESS OPERATOR

Review of Intake Air Uses

By now every PTOA Reader and Student knows that the "gas" in a "gas turbine" is none other than atmospheric air because they learned that back in PTOA Segment #195.

To recap:

• "Primary Air" is used for the Combustion Reaction with a hydrocarbon fuel.
• "Secondary Air" helps to complete combustion and distributes heat evenly within the Combustor.
• "Tertiary Air" makes certain the PV Temperature of the compressed, hot air does not melt down the first Rotor Blade.
• Some air is diverted from the Axial Compressor prior to being discharged and instead flows to the Turbine's Rotor Discs to help mitigate "Creeping" aka "Elongation" of the Turbine's Rotor Blades.

 

Here's another use of Intake Air:

Air is also used to control the amount of air leaking out of the Gas Turbine Engine!

Uh-oh ... Fred is confused again.

Hey, Fred! Don't stress out! Just keep reading!

 

 

GTs Need Air Leakage Prevention

By this point in the PTOA GT Driver/Prime Mover Focus Study, PTOA Readers and Students should be aware that the Axial Compressor's job is to increase the PV Pressure of Intake Air from atmospheric pressure (14.7 psia) to approximately 70 psi -100 psi at discharge.

So, PTOA Readers and Students also realize that leakage of the air through the nooks and crannies along the free spinning Shaft would decrease the Discharge Pressure of the Axial Compressor.

What exactly is preventing the higher pressure air from escaping? After all, the natural tendency of the higher PV Pressure compressed air would be to flow to the lower PV Pressure area … which is the much lower surrounding atmospheric air pressure where the Gas Turbine is situated.

 

Labyrinth Seals Revisited

High speed GTs use Labyrinth Seals to control the leakage of air from the free spinning Shaft.

 PTOA Segment #184  introduced Labyrinth Seals. Labyrinth Seals are made of metal which is so soft that the incidental contact with the Shaft will not score the Shaft.

There are two facts to remember about Labyrinth Seals:

Labyrinth Seals do not totally stop leakage but rather allow a minimal amount of controlled leakage from the seal.

 

Labyrinth Seals work by creating Pressure Drop (Δ P) in the fluid that is flowing through them.

In the nearby Labyrinth Seal diagram, assume that the red arrow and flow illustrates random Air trying to escape down the Shaft of the Axial Compressor. The green arrow is high PV Pressure Air that has been diverted from the Axial Compressor before otherwise being discharged into the Combustor.

As the red, escaping Air enters the Labyrinth Seal from the right it is forced to flow up and down through the pathway formed by the successive grooves of the seal (aka "the labyrinth"). This air turbulently flows in eddys between each groove of the Labyrinth Seal.

The Shaft might even be fabricated with "teeth" that fit into the Labyrinth Seal grooves to further restrict airflow through the Labyrinth Seal.

Each directional change of the air flow extracts more and more PV Pressure from the gas that is trying to escape. Otherwise stated, the flow of high pressure gas experiences A LOT of PV Pressure Drop (ΔP) in a relatively short flow path.

Eventually the red escaping air is so pooped out that the PV Pressure from the blocking, green diverted Air forces a controlled, minimize flow of leaking Air to exit via a well placed vent.

 

How Gas Turbine Process Operators/Technicians
Directly Impact Intake Air Quality

All of the above uses of Intake Air reiterate the need to make certain that only CLEAN AIR is sucked into the Axial Compressor.

Pollen, leaves, dust … virtually any air contaminant will be sucked into the Axial Compressor and will adhere to the Axial Compressor's Rotor Blades.

Guess what???

 

 

Mother Nature has a sense of humor when it comes to sources of air contamination!

For example, the Rotor Blades of both propulsion and power GTs will rapidly be destroyed by volcanic ash spewed by erupting volcanoes.  And volcanoes are not known for giving much notice when they are going to erupt.

Even when not threatened by a suddenly erupting volcano, the outcome of a merely dirty Axial Compressor will be a reduction in the maximum Discharge Pressure that the Axial Compressor can attain.

PTOA Readers and Students already learned that a mere 5% decrease in the Axial Compressor's Discharge Pressure will significantly decrease the Axial Compressor's Compression Ratio (Discharge Pressure in psia ÷ Suction Pressure in psia).

Sure, the dirty GT Engine will still work … but it will never be able to attain full speed and produce maximum hp. The lost efficiency of the GT will result in continuously increased operating costs and the run length of the GT will be shortened to regain optimal efficiency.

The GT Process Operator/Technician must be vigilant regarding cleaning and/or changing out dirty air filters for both the GT and any Auxiliary Equipment. GT Process Operators must be ready for rapid response when the unusual atmospheric condition occurs.

Changing air filters is a cumbersome task; so much so that the Plant Manager might very well invest in a self-cleaning filtration device.

Be that the case the GT Process Operator/Technician's job duties morph from changing the filters into making certain the self-cleaning filtration device is effectively working.

 

 

PTOA TRIBOLOGY FOCUS STUDY TOPICS
APPLIED TO THE GAS TURBINE ENGINE

Why should a Plant Owner invest in a Gas Turbine to drive Rotating Equipment or Electricity Generators instead of Motors or other types of Engines?

One of the many reasons is because only a GT can have a Shaft and Rotor they weigh 10 tons and still be able to rotate at 20,000+ rpm speed ...

and yet not vibrate more than several thousandths of one inch! 

The nearby graphic shows a tape measure. The distance between the "1" and the "2" is one inch and a mil is that distance divided by 1000!

How can it be that a Gas Turbine Shaft can spin so fast and yet move so little?

Why doesn't the rotating Shaft just propel itself and zoom out of the Gas Turbine casing?

Any PTOA Reader and Student who slogged through the 4 part PTOA Tribology Focus Study could take a good guess that Bearings minimize the Shaft's Axial Movement  (aka horizontal forward and backward movement) and Radial Movement (aka vertical upward and downward movement).

The considerable natural tendency for the Shaft to experience Axial Movement requires Bearings with "shoes"… to create some buttressing action!

Whaaaahhh??

The Shaft in the nearby graphic is surrounded by a labelled  "(Axial) Thrust Collar" which is  held in place by bright green and vertical standing "Thrust Shoes." In real life the "Thrust Shoes" are Babbitt-lined and low friction.

The pressure of circulating Lubrication Oil lubricates the touching metal parts and helps the "Thrust Collar" stay properly positioned, generating an opposing force to the otherwise natural Axial Movement of the Shaft.

The nearby graphic and photo of a real Thrust Bearing shows how circulating Lubrication Oil flows into the bottom of the Thrust Bearing and exits from the top.

 

Both of the diagrams of Axial Thrust Bearings with "Throat Collars" stabilized with "Throat Shoes" also show the Journal Bearing that supports and surrounds The Shaft. Hey! The Thrust Bearing graphic above hints at a green, horizontal Sleeve Bearing.

PTOA Readers and Students already know that Journal and Sleeve Bearings are Radial Thrust Bearings that diminish Radial Movement of the Shaft. Typically, these Radial Bearings are also lined with a low-friction metal (usually Babbitt). Alternatively, the Shaft might be surrounded by a Roller Bearing to reduce Radial Movement.

Whichever type of Radial Bearing is used, the viscosity of the Lubrication Oil is critical to forming the film and wedge of oil that will lift the rotating Shaft away from the supporting Radial Thrust Bearing. The below graphics need no explanatory captions because the subject matter is review material from the  PTOA 4 Part Tribology Focus Study!

 

 

THE GT PROCESS OPERATOR AND THE GT LUBRICATION OIL LOOP

Features of the GT's Lubrication Oil Loop

Hey!

It was just mentioned that the important functions of the Gas Turbine's Lubrication Oil Loop include:

• Lubrication between moving metal parts.
• Pressurizing and Sealing.
• Keeps the GT Engine internals cool via removing the heat of friction.

 

Just recently in PTOA Segments #200 and #201 PTOA Readers and Students learned the critical need for constant lubrication during a GT Shutdown, during the cleaning of the Axial Compressor cleaning, and during GT Startup.

Typically, the circulating GT Lubrication Oil is distributed through a Supply Lubrication Oil Manifold to three separate areas of the GT Engine.

Lubrication Oil is delivered  to the Forward Lubrication Oil Compartment where the oil lubricates the Roller Bearings in the fans that are situated within the Air Inlet Housing.

After lubricating these Bearings, the lube oil drains by gravity from the Air Inlet Housing to the Accessory Drive Housing where the lube oil is sprayed onto the Accessory Drive Gear Train and the Axial Compressor's forward Bearing.

A separate oil jet delivered to the Forward  Lubrication Oil Compartment from the Supply Lubrication Oil Manifold lubricates the Axial Compressor's rotor drive and Bearings.

Lubrication Oil is delivered to the Mid Lubrication Oil Compartment which is located in the vicinity of the Diffuser (between the Axial Compressor Discharge and the Combustor Inlet). This Lubrication Oil lubricates the Gas Producing Turbine's forward Bearing. The spent oil from this region drains by gravity to the bottom of the Diffuser casing.

The "Aft" or "Rear" Lubrication Oil Compartment is located toward the rear end of the Power Turbine rotor and includes a system of reduction gears that are lubricated via oil jets or oil splashes or oil mists.  The purpose of this Lubrication Oil is to  lubricate the rotating mating metal surfaces of the reduction gears while they reduce the rotary speed of the Power Turbine Shaft (for example, 22,000 rpm) to a more useable speed (for example, 1600 rpm). The spent oil drains by gravity.

The "Scavenged Oil" (shown in teal green in the above GT Lubrication Oil Loop schematic) is the spent Lubrication Oil that drains from the GT Engine. The Scavenged Oil is collected in a common Return Lubrication Oil Header and flows to the Lubrication Oil Tank.

 

The Interface between GT Process Operators/Technicians and the GT's Lubrication Oil Loop

The purpose of describing the GT's Lubrication Oil Loop in detail is to impress the following upon PTOA Readers and Students:

The GT Process Operator/Technician must vigilantly monitor the operating condition of each component within the crucially important Lubrication Oil Loop.

The condition of the Lube Oil must be determined by analyzing samples. Thermally degraded oil cannot perform the various Lubrication Oil duties adequately.

The Pressure Drop (aka ΔP) measured between the Lubrication Oil Filter Inlet and Lubrication Oil Filter Outlet  notifies the alert GT Process Operator/Technician regarding when it is time to change the elements in the Lubrication Oil Filter. 

WARNING! GT Process Operators/Technicians must also be aware that an increasing ΔP across the Lube Oil Filters  will not be sensed if the PV Pressure upstream and downstream of the Lube Oil Filter have both been elevated!

The Lubrication Oil must also be circulating within the designed PV Temperature range so that the oil has the required viscosity to perform its lubrication duties.

What is the source of the heat that must be constantly removed from the circulating Lubrication Oil?

The heat of friction is generated by the rotating parts within the GT Engine internals and is thence transferred into the circulating Lubrication Oil via conduction and convection.

Typically an Air/Lube Oil Fin Fan Heat Exchanger will be used to remove heat from the circulating Lubrication Oil. This heat exchanger is labelled "Air/Oil Cooler" in the nearby GT Lubrication Oil Loop schematic.

GT Process Operators/Technicians must make certain that the same kinds of atmospheric air debris that collects on the GT's Intake Air Filters does not clog up the external surfaces of the tubes that radiate heat from the Air/Lube Oil Fin Fan Heat Exchanger.

Likewise, the internal surfaces of the tubes in the Air/Lube Oil Fin Fan Heat Exchanger must not be fouled with debris collected by the circulating Lubrication Oil. For this reason the Lubrication Oil filters are situated upstream of the Air/Lube Oil Fin Fan Heat Exchanger!

Depending upon what type of fuel is used in the GT Combustor, a Fuel/Lube Oil Cooler may be in service to transfer more heat into the Fuel. The effectiveness of this Heat Exchanger must likewise be monitored.

Furthermore  …  Given the below characteristics of a GT Engine …

• All GT Engines use compressed Intake Air for sealing in Lubrication Oil to prevent it from escaping and
• There are three separate Lubrication Oil Compartments delivering Lubrication Oil to rotating hardware within the GT Engine ...

 

Who amongst  the brilliant PTOA Readers and Students would be surprised to learn that the spent Lubrication Oil drained from the Gas Turbine Engine has a large volume of entrained air?

Lube Oil with entrained air is called "Sour Lube Oil."

The GT Process Operator/Technician must make certain that the Lubrication Oil is effectively degassed and that condensed water from the air is constantly drained from water traps.

 

The nearby graphic shows Sour Lube Oil with entrained air (labelled "A").

"Sweet Lube Oil" without entrained air is shown on the bottom (labelled "B").

 

 

SHAFT MOVEMENT MONITORING AND THE GT PROCESS OPERATOR/TECHNICIAN

Vibration Probes and Vibration Monitors detect and record the distance of Shaft movement in mils. Recall that a mil is just a mere 0.001 inch.

Due to the close internal tolerances that undergo thermodynamic changes at high speeds, Vibration Monitoring is a mega big deal with Gas Turbine Engines.

To monitor Axial Movement, magnetic Vibration Probes are typically positioned close to the Shaft in the Axial Compressor's Bearing support housing and in the Power Turbine's Bearing support housing.

Assume that the Vibration Probe in the nearby photo generates 1 Volt when the Shaft moves left or right just 3 mils.

Hey, that's just .003 inch!

Using the factor labeling method for conversions learned in PTOA Segment #148 , PTOA Readers and Students can figure out that:

If the GT Process Operator observes a Vibration measurement indicated as 12 volts, the Shaft must have moved 0.036 inch … because ...

 

12 Volts *( .003 Inch/1 Volt) = 0.036 Inches

Radial Movement of the Shaft is detected by mounting two Vibration Probes at 90° angles. The tips of the Vibration Probes are positioned near the Shaft.

 

SUMMARIZING THE GT ENGINE/GT OPERATOR RELATIONSHIP

The GT Process Operator/Technician must not ever be lulled into a false sense of security provided by the GT's Advanced Control System.

 

The "devil hides in the details" that the GT Process Operator/ Technician is ultimately accountable for.

The GT Process Operator/Technician is must be constantly vigilant with respect to:

• Providing consistently stellar housekeeping within the powerhouse structure that surrounds the GT.
• Making certain that effective filtration of Intake air, Combustor fuel, and circulating Lubrication Oil constantly occurs.
• Constantly monitoring each component in the GT's Lubrication Oil Loop to insure:
  • Expected circulating PV Discharge Pressure from the Lube Oil Pump.
  • Expected (and genuine) ΔP across the Lube Oil filters.
  • Expected PV Temperature of the Lube Oil to insure the oil will have the required viscosity.
  • Expected cleanliness of Lube Oil (free of contaminants, entrained air and water).
• Constantly monitoring the Axial and Radial Movement of the Shaft via Vibration Probes.
• Applying everything learned in this PTOA Segment to the other Auxiliary Equipment and Loads associated with the Gas Turbine Engine.

 

Hey! One more human touch!

HIGH FIVE YOUR MENTOR!

PTOA Readers and Students who have comprehended the content of the PTOA GT-Driver Focus Study possess an introductory level of GT core competency that Plant Managers will appreciate.

 

 

 

TAKE HOME MESSAGES: The Gas Turbine Process Operator/Technician provides the crucial human touch that keeps a Gas Turbine Engine running until the next schedules shutdown or overhaul. The secret to avoiding an Emergency GT Shutdown is the GT Process Operator's/Technician's efforts to keep the Intake Air, Combustion Fuel, and Lubrication Oil continuously clean.  

The important uses of Intake Air and Lubrication Oil are featured in this PTOA Segment to illustrate in detail why the GT Process Operator/Technician must keep these media effectively filtered and thus clean. This PTOA Segment also illustrated how the topics in the PTOA Tribology Focus Study specifically apply to a Gas Turbine Engine.

The GT Process Operator must also understand how each component in the important GT Auxiliary System known as the GT's Lubrication Oil Loop work together to supply clean Lubrication Oil with the desired viscosity to the Lubrication Oil Supply header and Manifold. The Process Operator must understand the difference between the PV Pressure and Pressure Differential (ΔP) to properly interpret the condition of the Lube Oil filter.

After the Lubrication Oil provides sealing and lubrication duties, the spent oil is collected as "Scavenged Oil" and returned to the Lube Oil Tank and ultimately recirculated to the Gas Turbine Engine.

Lubrication Oil that has entrained air is "Sour Oil." The GT Process Operator/Technician must make certain air and condensed water are removed from the oil ("Sweet Lubrication Oil").  The Lubrication Oil must also be analyzed by human beings to monitor its thermal degradation.

Vibration Probes and Vibration Monitors are used by GT Process Operators/Technicians to monitor the Radial and Axial Movement of the Gas Turbine. Magnetic Vibration Probes are used to convert volts to mils of movement. A mil is one inch divided by a thousand.

©2019 PTOA Segment 0201

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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