("Start Me Up," M. Jagger/K. Richards of The Rolling Stones, 1981)

 

HOW THE HECK IS THE HIGHLY INTEGRATED GT MACHINE STARTED UP?

The most brilliant PTOA Readers and Students have deduced that Gas Turbines (aka GTs) are highly integrated machines and thus they are wondering:

"How does this thing start up?"

The GT's HP Turbine Shaft must be rotating so that  …

The common GT Air Compressor Shaft is enabled to rotate …

which then draws air into the GT and compresses that air …

which is thence heated to very high PV Temperatures in the GT's Combustor ...

which makes it possible to extract that heat energy via expansion in the GT's Turbine ...

which then causes the GT's HP Turbine Shaft  to rotate!

 

Furthermore, in PTOA Segment #197 PTOA Readers and Students learned that Gas Turbines are "thermodynamic machines" whose purpose is to convert heat energy into rotational power (aka hp).

So that means a rushed GT Startup or sloppy GT Shutdown can result in:

• Thermal shock.
• "Creep" of Turbine Blades (defined in PTOA Segment #195).
• Bowing of the Turbine Shaft (discussed in the next PTOA Segment).
• Degradation of the lube oil due to improper cooling.

 

So how exactly is a Process Operator supposed to Startup, operate, and Shutdown such an integrated machine?

This PTOA Segment  #199 features:

• The GT Advanced Control Systems that greatly enhance the control and operations of GTs.
• GT Startup phenomena (GT Shutdown phenomena are featured in the next PTOA Segment).

 

The PTOA greatly appreciates the fine examples of GE GT Advanced Control System HMI  slides which have been borrowed from the shared work of Muddasir Shariff.

 

GAS TURBINE ADVANCED CONTROL SYSTEMS ARE HOUSED IN GAS TURBINE CONTROL CONSOLES

At the time that this PTOA Segment was written there was great anxiety over the extent to which human worker bees have been replaced by artificial intelligence and automatic control.

Well …

A picture of a modern Gas Turbine could serve as a poster child for the benefits of using advanced instrument controls to assist human beings with the safe creation of mechanical rotational power from heat energy.

Each Gas Turbine purchased by the Plant Manager will be supplied with an accompanying Gas Turbine Control Console which will enclose that GT's Advanced Control System software and hardware.

In other words, each GT will be supplied with its own Distributed Control System (DCS) and Human Machine Interface screens (HMI) … just like those featured in PTOA Segment #13.

The GT's Advanced Control System software is capable of sequencing multiple complex functions automatically …

and Yes Indeedo ...

These complex functions can be performed real-time by the GT's Advanced Control System much faster and accurately than any human being can do.

For example …

The software assists in bringing the GT from a standstill position through a series of steps that methodically completes the GT Startup procedure.

The GT Startup ends when the GT is lined out at the very high  PV Temperatures and PV Pressures needed to produce consistently reliable mechanical rotational power via thermodynamics.

Likewise the Advance Control System software follows a stepwise protocol to return the GT to the standstill status known as a planned GT Shutdown.

And of course the Advanced Control System assists in an abbreviated Emergency GT Shutdown protocol when the need arises.

As stated many times, the PTOA does not advocate nor advertise any particular type of Gas Turbine.

Gas Turbine selection is specific to the process application. For just one example, the primary fuel and back up fuel for a GT will be selected based upon which fuels are most economical for the location.

With that understanding, some examples of Gas Turbines and their Advanced Control Systems and consoles are listed below:

• A Solar GT (manufactured by Caterpillar) will have a "Turbotronics" Advanced Control System inside a Turbotronics Console.
• A GE GT will have a "Speedtronics (Mark V or Mark VI or...)"  Advanced Control System inside the Speedtronics Console.
• A Siemens GT will have an "SPPA-(Model Number)" Advanced Control System inside the SPPA Console.
• A Mitsubishi GT will have a "Mitsubishi-Hitachi Power System … (MHPS)" Advanced Control System inside a MHPS Console.

 

The below picture gives PTOA Readers and Students an idea of the instrumentation strapped to the GT core engine that is controlled by the GT's Advanced Control System.

 

Brilliant PTOA Readers and Students …  meaning those who are reading the PTOA Segments in the intended sequential order …  have already learned about two crucial control schemes managed by the GT Advanced Control System which involve the main fuel valve:

• The Overspeed Trip Device will cut the fuel to the Combustor when the Turbine's speed is sensed to be above 110% of design (refer to PTOA Segment #196). The nearby picture depicts a catastrophic GT failure due to failure of  overspeed control.
• The  Gas Turbine Inlet Temperature is "topped out" when the Turbine Exhaust Gas Temperature exceeds a predetermined limit. "Temperature Topping" of the Gas Turbine Inlet Temperature happens when the GT Advanced Control System sends a signal to the main fuel valve to hold the fuel rate to the Combustor steady (refer to PTOA Segment #198).

 

 

GTs  HAVE 3 LUBE OIL PUMPS:  MAIN LUBE OIL PUMP, AUXILIARY LUBE OIL PUMP, AND EMERGENCY LUBE OIL PUMP

The Gas Turbine Lubrication Loop

The extremely important purpose of circulating lubrication oil  is to minimize friction-generated heat which will be created by functioning journal bearings, axial bearings, thrust bearings, and gear assemblies.

The circulating lubrication oil carries away the heat as was thoroughly described in the 4 part PTOA Tribology Focus Study .

In the nearby simple flow diagram, lubrication oil flows from the light green, horizontal "Coupling Oil distribution manifold" and the blue, horizontal "Bearing Oil distribution manifold" to their respective users.

The light blue boxes labelled  Accessory Coupling, Active Thrust (Bearing), Inactive Thrust (Bearing), #1 Journal Bearing, #2 Journal Bearing, Load Coupling, and Load Equipment represent GT hardware that must be lubricated while the GT is operating.

The block flow diagram shows that the spent lubrication oil is drained and circulated back to the (Lube Oil) Reservoir/Tank.

 

The Main Lube Oil Pump and Auxiliary Lube Oil Pump Interactions

During normal operation, the Main Lube Oil Pump infuses the circulating lubrication oil with the PV Pressure so that  the oil can continue circulation around the Lube Oil loop.

More than likely, the Main Lube Oil Pump is an "Auxiliary Load" driven by the shaft of the LP or Output Power Turbine via a reduction-gear assembly as was described in PTOA Segment #196.

During GT Startup, the LP or Output Power Turbine Shaft is not rotating at a high enough speed to drive the Main Lube Oil Pump.

For that reason, the GT Process Operator may "trick on" the Auxiliary Lube Oil Pump by activating the GT's dedicated START button …and then immediately deactivating that same GT START button!

Why? Because the Auxiliary Lube Oil Pump will kick on if the Advance Control System senses the GT is being shutdown.

 

Think about it!

The Auxiliary Lube Oil Pump must have a separate driver because it would be useless if it were also driven by the GT's LP or Output Power Turbine!

More than likely the Auxiliary Lube Oil Pump is driven by an independent  A/C Motor (featured in PTOA Segment #187).

During the GT Startup, the Auxiliary Lube Oil Pump infuses the PV Pressure into the circulating lubrication oil until the GT achieves "idling speed" (approximately 62% to 65% of design speed). The Auxiliary Lube Oil Pump in HRSG GTs may not stop until 95% of Turbine Speed.

Whichever the case … eventually the GT's LP or Output Power Turbine will be rotating at a sufficient speed to drive the GT's Load and Auxiliary Loads … including the Main Lube Oil Pump.

In summary, the Auxiliary Lube Oil Pump is used to maintain the PV Pressure of the circulating lubrication oil when the Main Lube Oil Pump cannot maintain the lubrication oil pressure by itself.

The Main Lube Oil Pump and Auxiliary Lube Oil Pump are typically similar in design flow rates and discharge pressure. The flowrate at pump discharge of 180 gallons per minute (gpm) and a design pressure of 100 psi is common.

And obviously …

If the Main Lube Oil Pump is driven by an independent electric or gas or air motor, then an Auxiliary Lube Oil Pump would not be necessary. However the overall daily operating expenses for the GT would significantly increase.

 

The Emergency Lube Oil Pump (ELOP)

The Emergency Lube Oil Pump (ELOP) is exactly what it sounds like, an emergency source of lube oil supply in the event the Main Lube Oil Pump and Auxiliary Lube Oil Pump fail.

The primary objective of the ELOP is to provide just enough lubrication to the bearings while seeing the GT through an emergency shutdown while both the Main Lube Oil Pump and Auxiliary Lube Oil Pump are not operationally available.

This situation means a simultaneous loss of supplied electrical power AND loss of the GT's LP or Output Power Shaft rpms has occurred … truly the definition of an "emergency situation."

The ELOP is designed with much less supply capacity and discharge PV Pressure. For example the ELOP might only circulate 70 to 100 gpm lube oil at a discharge pressure of 25 psi.

Brilliant PTOA Readers and Students have already figured out that the ELOP must be independently driven by a driver that can function in the absence of supplied electrical power supply AND an operating GT.

Ergo, ELOPs might be driven by gas or diesel engines. They can also be driven by DC power … which all PTOA Readers and Students  instantly know means "battery supplied power" (most recently defined in PTOA Segment 187).

 

SO … HOW IS A GT STARTED UP? 

The GT Startup Device

Each Gas Turbine is started up with a independently-driven "Startup Device."

The "Startup Device" could be a diesel engine.

More than likely the "GT Startup Device" is an AC Motor.  For this reason the PTOA will default to the term "Startup Motor."

The GT Startup Motor operates through a Torque Converter that is coupled to  "Accessory Drive Gear."

The Accessory Drive Gear generates sufficient torque to rotate the common Compressor-HP Turbine Shaft from a standstill position.

The duties of the GT Startup Motor:

• Initiates rolling of the common Compressor-HP Turbine Shaft.
• "Cranks" the Shaft at a slow speed so that water and other debris can be purged. (Cranking and purging are defined later in this PTOA Segment).
• Increases the speed of the Shaft to a "firing speed" at which point the ignitors are fired and The Working Fluid starts being generated.

 

Once the HP Turbine attains a pre-determined self-sustaining speed the mechanical clutch disengages the Start Up Motor.

The Startup Motor and Accessory Drive Gear are physically located near the Compressor … aka "The Cold Side of the GT" of the GT (refer to PTOA Segment # 195).

 

A GENERAL GT STARTUP  LIST OF EVENTS

Each GT has its own GT Startup and GT Shutdown  procedures which must be strictly adhered to. In each of the procedures the general actions described in the following paragraphs will occur.

Pre-Startup Procedure

Each Startup Motor has a dedicate, separate ON switch in the GT's Advanced Control System Console.

Egads! Every alert PTOA Reader and Student knows:

Turning the Startup Motor ON means the Turbine Rotor will be rotating!

That means precautions must be made in advance to mitigate the heat of friction that will be generated between moving metal parts!

Prior to activating the Startup Motor's dedicated ON Switch, the Auxiliary Lube Oil Pump must be engaged. Older GT Startup procedures may include a formal step which describes "tricking the Auxiliary Lube Oil Pump on" by activating-then deactivating the dedicated GT ON switch as was described earlier in this PTOA Segment.  More modern GTs will have more direct means to insure the Auxiliary Lube Oil Pump is activated prior to activating the Startup Motor.

Don't stress, Fred!

A HMI GT Startup screen like the one shown in the nearby graphic will inquire if all systems are ready prior to allowing a GT Startup to commence. One of the conditions that must be satisfied is sensing oil pressure in the Lube Oil Loop!

Pre-check/approval HMI screens are just another way Advanced Control System assist in the GT Startup!

In the event the "Startup Device" is a diesel engine, then GT Process Operator/Technician must anticipate starting up the diesel engine via its own Startup procedure.

Whichever GT "Startup Device" is used, the 'jaw-clutch' in the Accessory Drive Gear will self-engage when the Startup Motor/Device's dedicated ON switch is activated by the GT Operator/Technician.

The clutch "breaks away" and the Startup Motor begins rotating the common Compressor-HP Turbine Shaft.

 

"Cranking the Turbine Shaft" for the Purpose of "Purging the GT"

The Startup Motor will bring the Turbine Shaft to 20% design speed and holds the speed steady for several minutes.

Maintaining the Turbine Shaft at a specified speed which is significantly below design is called "Cranking."

At this point in the GT Startup, air is being drawn through the Compressor and Turbine which purges debris from GT internals.

"Purging the GT" means that the ambient air that has been drawn into the GT is displacing/flushing out debris from GT internals.

 

Examples of debris in GT internals are:

• Stale fuel from the ignitors and Combustor (especially if liquid fuel is used).
• Accumulated water left behind in the Air Compressor after the Water Wash Step (featured in the next PTOA Segment).

 

The length of the GT purge step is controlled by a timer within the GT Advanced Control System. The duration of the timer is predetermined and based upon the type of fuel used in the GT and whether or not the GT is experiencing a "Normal Startup" or a "Fast Startup."

 

Firing the Ignitors followed by GT Warmup

Albeit dependent upon GT design, Firing of the Ignitors typically begins at the conclusion of purging at 20% Turbine speed and continues to approximately 50% of Turbine speed. During this time interval, the GT internals thermally equilibrate while heating up.

The  below graph entitled "Start up curve of a gas turbine" labels the GT Warmup interval. The duration of Warmup is shown to be 1 minute. During Warmup the PV Temperature increases from 190 °F to 780 °F.

Albeit brief, the GT Warmup interval mitigates thermal fatigue of all the metal surfaces that are exposed to hot GT temperatures.

Heating up the GT is greatly aided by the Advanced Control System as is explained below.

 

Balancing the Fuel Feedrate, Combustor Pressure and Temperature, and Turbine Shaft Speed During Startup

Like Steam Turbines, the first Gas Turbines had Governors to coordinate the fuel injection/rotational speed interface.

Nowadays the algorithms in the GT Advanced Control System coordinate the fuel feed rate with increasing PV Temperature sensed as Turbine Exhaust Gas, PV Pressure sensed in the Combustor, and the increasing speed of the HP Shaft better than any human being can do.  That's just a fact.

Take a moment to ponder the change in the PV Pressure inside the GT's Combustor as it increases from a standstill Atmospheric Pressure (14.7 psi) to an assumed natural gas supply pressure of 140 psi.

During GT Startup … before ignitors have been fired …  the Axial Compressor discharge PV Pressure may be just 5 to 7 psi. The resulting 132 psi "Pressure Drop" (aka ΔP ) at the fuel injectors would create a high velocity of natural gas shooting through the Combustor.

Even worse … once ignited, the flame shard would be thrown through the Combustor and into the HP Turbine!

For this reason the GT's fuel system architecture and Advance Control System work together to maintain a relatively constant ΔP across each fuel injector.

Startup Motor Disengages

Once the expansion of The Working Fluid results in spinning the HP Turbine Shaft at a speed in excess of "idling speed" (approximately 62-65% of design speed) …

the turning gear drive speed in the Accessory Drive Assembly will be exceeded and the mechanical clutch in the Startup Motor/Device will automatically disengage.

The HP Turbine Shaft is now driving the Air Compressor without help from the Startup Motor.

External Loads may now be added to the LP Power Turbine Shaft.

The Main Lube Oil Pump will be loaded, and the Auxiliary Lube Oil Pump will shutdown.

 

Final GT Start Up Stages include Surge Prevention

The speed of the Turbine will increase to 95% of design.

Surge Prevention 

At a certain point in the GT Startup, alert GT Process Operators and Technicians will hear the Gas Turbine's Axial Compressor as it works through Surge prevention.

Surge is an Centrifugal and Axial Compressor phenomena that will be thoroughly reviewed in upcoming PTOA Segments.

Fred, don't stress about it now!

At this point just be aware that one design method to avert Surge is to design in bleed valves.

At 95% design speed the bleed valves will close.

Eventually the GT will attain 100% of design speed, the PV Pressure of the fuel will be at supply pressure (100 psi-140 psi), the Gas Turbine Inlet Temperature will exceed 2000+ °F, and the GT will be loaded up and driving external Rotating Equipment or perhaps the Load is an electrical generator!

Whew! That was almost TOO MUCH INFORMATION!

The next PTOA Segment features GT Shutdown phenomena!

 

 

 

TAKE HOME MESSAGES:  The Distributed Control System (DCS) of proprietary GT Advanced Control Systems will accompany the purchase of a Gas Turbine. The hardware and software of a GT Advanced Control Systems is enclosed in a Gas Turbine Control Console with proprietary Human-Machine Interface (HMI).

The Startup procedure of the highly integrated Gas Turbine engine relies upon the existence of a Startup Device/Motor that operates through a  torque converter that is coupled to  "Accessory Drive Gear."  The Accessory Drive Gear generates sufficient torque to rotate the common Compressor-HP Turbine Shaft from a standstill position.

Whichever GT "Startup Device" is used, the 'jaw-clutch' in the Accessory Drive will self-engage when the Startup Motor/Device's dedicated ON switch is activated by the GT Operator/Technician (and will disengage at a specified Turbine rotor speed).

GT Advanced Control Systems perform the sequencing of multiple complex functions during GT Startup and GT Shutdown.

During GT Startup, the GT Advanced Control System will:

• Sense if Lube Oil Pressure is sufficient in the Lube Oil Loop.
• Coordinate the Fuel Feedrate, Combustor PV Temperature and Pressure, and Turbine Shaft Speed during Startup.
• Coordinate the sequencing of the Startup Motor/Device duties which include:

1. 1. Commencing rolling of the common Compressor- HP Turbine/Power Turbine Shaft.
   2. Cranking and Purging the GT engine.
   3. Firing the Ignitors in the Combustor
   4. Warmup of GT internals.

Once the HP Turbine speed is in excess of 'idling speed' a mechanical clutch in the Auxiliary Drive Assembly will disengage the Startup Motor.

Gas Turbines typically have 3 Lube Oil Pumps:

• The Main Lube Oil Pump is an Auxiliary Load driven by the LP or Output Turbine.
• The Auxiliary Lube Oil Pump is similar in design capacity to the Main Lube Oil Pump and is typically driven by an independent AC Motor.
• The Emergency Lube  Oil Pump (ELOP) has a smaller design capacity and is driven by a DC Motor or other means that do not rely upon electrical utility or an operating Gas Turbine.

 

The Auxiliary  Lubrication Pump must be operating before the Startup Motor is engaged. Nowadays the Advanced Control System does not allow a Startup to proceed unless sufficient Lube Oil Pressure is sensed in the Lube Oil Loop. Older GT Startup procedures may include a step wherein the Operator/Technician may have to "trick on" the Auxiliary Lube Pump.

Once the GT Startup has advanced past loading the Main Lube Oil Pump the Auxiliary Lube Oil Pump will be disengaged.

The Emergency Lube Oil Pump is only used when electrical power utility has been lost AND the GT is not operating. The purpose of the ELOP is to provide bearing lubrication during an emergency GT shutdown.

©2019 PTOA Segment 0199

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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