LM6000 Mechanical Drive Gas Turbine

LM6000 Mechanical Drive Gas Turbine:
Operations with a direct-coupled low pressure turbine
in naval mechanical drive applications
J. Stephen Maynard, CAPT, U.S. Navy (ret)
GE Marine, Cincinnati, Ohio, USA
SYNOPSIS
Historically, gas turbine mechanical drive propulsion plants aboard warships have employed engines that feature
aerodynamically coupled low pressure (power) turbines. Naval operators have accepted this design approach
because it facilitates a flexible range of turbine shaft output speeds, even during low power operations. The
original equipment manufacturer of the LM6000 has recently initiated a program that will package this highly
successful aero-derivative gas turbine for marine mechanical drive applications. The LM6000 engine architecture
features a low pressure turbine that is directly coupled to a low pressure compressor and the driven load. This
design differs from the traditional marine gas turbine approach that has an aerodynamically coupled power turbine
connected to the driven load. This paper compares the differences in these two architectures and highlights the
LM6000’s ability to function properly during starting, stopping, low power conditions and other transitory
operations in warship applications.
INTRODUCTION
LM6000 gas turbines are in operation across the commercial industrial sector with more than 27
million hours of service. While demonstrating operation in a wide range of environments,
including marine installations, these turbines have principally been employed in constant speed
electrical power generation applications. General Electric’s Marine Division is currently
developing a 50MW (ISO conditions) mechanical drive package that adapts the variable speed
capabilities of the LM6000 gas turbine for future marine applications.
The LM6000 engine architecture features a low pressure turbine (LPT) that is directly coupled to
both a low pressure compressor (LPC) and the driven load. Figure 1 illustrates the main
components of the LM6000 engine design. This direct-coupled design is a change from the low
pressure (or power turbine) design typically employed in LM2500-powered ships, where the
LM2500 LPT is aerodynamically connected to the high pressure side of the engine—the gas
generator. The LM2500’s aerodynamically coupled design approach enables the turbine that
drives the load (i.e. power turbine) to remain independent of the turbine that drives the
compressor (i.e. gas generator).
Given the alternative design approach of the LM6000, naval operators may question if having
the LPT drive both the load and the LPC restricts the engine’s speed range and the operating
envelope of the load. This paper will address the technical feasibility and operational suitability
of the LM6000 mechanical drive design for warship applications in terms of both system
integration and propulsion plant operations. It will show that propulsion plant operations with
Author’s Biography
J. Stephen Maynard, CAPT, USN (ret) has served as a marine applications engineer at GE Marine in Cincinnati, Ohio since 2008.
Prior to his employment with General Electric, he served almost 30 years in the US Navy as a Surface Warfare Officer with two
command tours at sea. He has extensive operational experience with gas turbine propulsion plants.
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the LM6000 will be essentially identical to those of the LM2500, and that the engine will be able
to function properly across a full operating envelope, to include starting, stopping, low power
conditions and other transitory evolutions.
Figure 1: Basic Components of the LM6000
COMPARISON OF LM2500 AND LM6000 ENGINE ARCHITECTURES
The LM2500 aero-derivative marine gas turbine has a rich history of reliable at-sea operations,
serving in 33 international navies and installed aboard more than 75 classes of naval vessels.
The engine is highly adaptable to various marine propulsion architectures as demonstrated by its
expansive use in naval applications that feature Combined Gas turbine And Gas turbine
(COGAG), COmbined Diesel Or Gas turbine (CODOG) or COmbined Diesel And Gas turbine
(CODAG), Integrated Electric Drive, and Hybrid Electric Drive plant arrangements.
LM2500
The LM2500 is a simple cycle, two-shaft high performance turbine with a single compressor.
The first shaft links the high pressure compressor (HPC) with the high pressure turbine (HPT).
The second shaft is integral to the LPT which, in mechanical drive propulsion plants, is
connected to the ship’s load through a coupling shaft and main reduction gearbox. In this
manner, the turbine that drives the load is independent of the turbine that drives the compressor.
This shaft arrangement is often referred to as aerodynamically coupled, which allows the HPT to
rotate at a different speed than the LPT. Figure 2 illustrates the basic component sections of the
LM2500. For clarity, the combined HPC and HPT assemblies are commonly referred to as the
gas generator, and the LPT that drives the load is commonly referred to as the power turbine.
Figure 2: LM2500 Basic Architecture
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The two-stage HPT drives the high-speed shaft at speeds ranging from approximately 510010,050 rpm. The LPT drives the load with a full speed range capability of 800-3600 rpm for
mechanical drive propulsion plants.
LM6000
Derived from the CF6-80C2 aircraft engine, the LM6000 has been widely accepted for
commercial electrical power generation applications due to its high output power and impressive
simple cycle thermal efficiency of 42 percent. The success of other LM series gas turbines,
particularly the LM2500, was factored into the development of the LM6000. The engine has
performed with superior reliability in numerous commercial marine and offshore applications.
The LM6000 is also a simple cycle, two-shaft gas turbine that features both low and high speed
shafts with low and high pressure compressors. Like the LM2500, the LM6000 has a gas
generator, in which the HPC and HPT are linked by the first shaft assembly. The low pressure
core consists of a 5-stage LPC (sometimes referred to as a booster) and a 5-stage LPT directly
connected by the second shaft subassembly. Because of the direct-coupled design on the low
pressure side, the LPC rotates at the same speed as the LPT. The high pressure and low pressure
cores are also, in fact, aerodynamically coupled. This is because the low speed shaft is
concentrically positioned inside the high-speed shaft, enabling the high-speed and low speed
shafts to rotate at different speeds depending upon the gas stream energy level produced by the
gas generator. The principle difference is that the load in the LM6000 is driven by the LPT in
the rear of the engine, which also drives the LPC at the front of the engine. Hence, the LPT
essentially functions as the power turbine, although it is not completely aerodynamically coupled
(or free) given that it is directly connected to the LPC. This arrangement is illustrated in Figure
3.
Figure 3: LM6000 Basic Architecture
The LM6000’s two-stage HPT drives the high-speed shaft at speeds up to 10,810 rpm. The LPT,
which is driven by the high-energy exhaust gas flow from the high pressure core, drives the LPC
and the load with a full speed range capability of 800-3850 rpm for mechanical drive propulsion
plants. This LPT speed range is slightly expanded from the LM2500. A comparison of the
LM6000 HPT and LPT rotor speeds against the LM2500 is shown in Figure 4. The illustrated
rotor speed relationships are representative of typical steady state cubic load applications, where
the engine’s design point is established at maximum turbine speed.
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Figure 4: Comparison of LM2500 and LM6000 Rotor Speeds
The combined pumping action of the LPC and HPC produces a high overall compression ratio
that approaches 33:1, thus helping to increase thermal efficiency of the engine. To match the
airflow of the two compressors and achieve the desired performance characteristics across a wide
speed range, the gas turbine is equipped with three systems that control airflow. Variable inlet
guide vanes are used at the front of the LPC to modulate airflow entering the flow path. Variable
bleed valves are incorporated between the LPC and HPC, allowing some airflow to be dumped at
low power conditions to maintain an excellent HPC stall margin. Lastly, six stages of variable
stator vanes are incorporated on the HPC to properly match the airflow between the two
compressors over the power range.
INTEGRATION OF LM6000 INTO MECHANICAL DRIVE PROPULSION PLANTS
Self-Synchronizing Clutch
As with a free power turbine installation, a GT self-synchronizing overrunning clutch, such as
that from the SSS (Synchro-Self-Shifting) Clutch Company, will be situated between the output
side of the LM6000 high speed coupling shaft and the high speed input (quill) shaft of the main
reduction gear. For CODOG or CODAG gearboxes, the clutches may be mounted in the
intermediate speed shaft position to isolate both gas turbine and high-speed gearing. The
clutches can be either in-line or quill shaft-mounted as above.
The SSS Clutch is a freewheel-type, overrunning clutch which transmits torque through
concentric surface-hardened gear teeth. Unlike a servo-actuated tooth coupling that is difficult to
shift into mesh at rest or at speed, phasing and engagement of the SSS Clutch teeth at
synchronous speed is accomplished automatically without any external controls and without
possibility of error. Also, unlike a tooth coupling, disengagement of the clutch will occur
whenever the input slows down relative to the output without the need to maintain an unloaded
turbine condition for disengagement. The clutch will automatically shift to “engaged” if the gas
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turbine shaft starts to rotate at a higher speed than the output side of the clutch, as would be the
case during an engine start. Conversely, if the output side of the clutch is driven from the gear
unit side faster than the overrunning clutch input side, the clutch automatically transitions to the
disengaged position as during an engine stop. The clutch will feature a manual selection that
will enable the clutch to go from “locked out” to “active” (ratcheting) with a special device when
the propulsion plant is secured and when automatic engagement is not desired. The locked-out
position is used for gas turbine test and maintenance, water washing or motoring. The positions
of the synchronizing clutch will be monitored via the main reduction gear programmable logic
controller (PLC) to the machinery control system.
High Speed Coupling Shaft
A high speed coupling shaft and adapter, connected to the LPT rotor, will transmit the low speed
shaft’s power to the connected load. The shaft has many of the same attributes as those used in
the LM2500 family of engines. Flexible couplings on each end of the drive shaft will have more
transient torsional margin than the gearbox, and will be able to accommodate the axial and radial
deflections anticipated in soft mount configurations. While the engine is capable of driving the
connected load from either the “hot end” or the “cold end,” the mechanical drive package
initially fielded for naval applications will use the hot end drive option. As such, the initial
LM6000 mechanical drive package will use a coupling shaft that traverses an exhaust collector
tunnel, with the output flange extending beyond the aft enclosure wall approximately 15
centimeters.
Low Pressure Turbine Brake
The LM6000 will employ the use of a power turbine brake similar in design to that found aboard
other LM2500 warship installations. The brake will protect engine bearings of a non-operating
gas turbine from damage due to vibratory and impact loads introduced by the gear train. The
brake is typically a spring release, pneumatically-actuated disc brake that is positioned at the
turbine end of each gear input shaft. The GT brake will be designed to activate upon command
only when the gas generator reaches idle speed.
Propeller Shaft Line Brake
A shaft brake may be optionally installed on the gear unit output shaft with a brake disc mounted
on the output flange. The brake is needed to hold the secondary gear parts and propeller
shaft. The brake will be actuated on command by electrically-controlled valves through the main
reduction gear PLC, which counter-checks the actual conditions and actuates the valves. When
the brake is engaged in propulsion plant casualty conditions or to prevent propeller fouling, for
instance, the LM6000 gas generator may continue to run normally; however, the SSS engine
clutch will remain engaged thus holding the LPC and LPT stationary until such time that the
shaft line brake may be disengaged.
Engine Control
A digital engine controller will regulate fuel flow to the combustion section of the gas generator
to control gas generator speed. The speed of the LM6000 LPT, for any given setting of gas
generator speed, will vary as a function of the mechanical load characteristics. For this reason,
the LPT speed is not directly controlled, but is established by the gas stream energy level
produced by the gas generator. Controlling the gas generator speed, and its resultant gas
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horsepower, provides a predictable LPT speed that yields a given propeller speed and ship speed.
With a controllable pitch propeller system, engine power and speed is sequenced with
modulation of the propeller oil distribution box, which actuates and controls the angle of the
propeller blades.
Similar to the LM2500, the LM6000 will feature an integrated electronic fuel control, using an
electric actuator for the liquid fuel metering valve for accurate and timely throttle response. The
gas turbine controller will sense LPC inlet temperature and pressure, gas generator (core) speed,
HPC discharge pressure, and fuel metering valve position. Using this data, the engine control
will schedule steady state and transient fuel flow to maintain variable geometry positions and
corresponding gas generator speeds. This will prevent over-temperature or compressor stall
during accelerations or decelerations. The engine controller will also protect the LPT from
overspeed conditions by use of a topping governor to limit rotational speed to 110 percent of
3850 rpm. Of note, under certain conditions, it may be desirable to control power turbine speed
directly via a power turbine speed reference, such as during heavy sea states. The LM6000
engine governor is capable of controlling the engine in this mode, if necessary.
LM6000 OPERATIONS IN MECHANICAL DRIVE PROPULSION PLANTS
Starting
When in port, preparing for underway operations with a CODOG or CODAG propulsion system,
operators routinely start and engage a main propulsion diesel (or cruising engine(s)) to begin
rotating the propeller shafts. Thus, the gas turbine(s) is at rest with the propeller shaft already
rotating in the ahead direction. The engineers then start and engage the gas turbine(s). With a
SSS clutch, propulsion train engagement automatically occurs once the LPT brake is released,
the engine is started, and the clutch input shaft reaches synchronism with the output shaft.
However, in non-standard situations, rotation of the propulsion train could also be initiated using
the gas turbine, if necessary. The normal starting time of the LM6000 is around 90 seconds, with
a maximum time of two minutes. The engine is started by rolling the high-pressure core in the
same manner as the LM2500. The LPT will be free to rotate with the turbine brake off. As the
LPT begins to rotate, a SSS clutch will automatically engage with the power train at the instant
the speed of the turbine shaft commences to overtake that of the reduction gear quill shaft. At
this point, the LPT may stop rotating because of drive train inertia or propeller shaft sag [this is a
common occurrence even with LM2500 power turbines]. To overcome the resistance to rotation,
the operator “bumps” the integrated throttle to accelerate the HP core (i.e. gas generator) to break
the gear train free—in much the same manner as with the LM2500.
From mechanical drive test demonstrations, the LM6000 HP core has proven capable of
developing over 25,000 N-m of breakaway torque at gas generator speeds in excess of 8,100 rpm
with the LPT locked under engine start-up conditions. As such, even though the LP shaft is
directly connected to the load, the engine is fully capable of developing significant low-speed
breakaway torques that will be necessary to roll propeller shafting aboard warships, without
detriment to the engine. See Figure 5 for breakaway torque demonstration results.
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Figure 5: LM6000 Breakaway Torque Demonstration Results
One operational consideration with any mechanical drive gas turbine is the duration in which the
low pressure (or power) turbine may remain stationary or “locked” while the gas generator is
rotating. From demonstration testing, we know the LM6000 LPT is able to remain stationary for
at least five minutes with the gas generator at idle. Thus, the LM6000 LPT will be able to
remain stationary without concern for undesirable conditions, such as high bearing sump
temperatures, for the reasonably short period of time it takes to roll the propeller shaft after
engine start.
Stopping
With a SSS clutch integrated into a reduction gear arrangement, if the LM6000 is commanded to
stop for any reason, the clutch will commence to disengage as soon as the gas turbine slows
down relative to the propeller shaft. The clutch simply functions in the same overrunning
manner as with a free power turbine design. If, in the unusual event, a propeller shaft came to
rest with the gas turbine’s SSS clutch still engaged, starting of another offline engine and its
subsequent clutch engagement would disengage the clutch of the stopped gas turbine and the
LPT would remain stationary.
Once stopped, synchronizing clutches include a means to lock-out the clutch so that the clutch
output is free to rotate in either direction without clutch engagement or so the clutch input could
rotate forward without engaging and rotating the clutch output. This feature will enable engine
tests, engine water wash, or other maintenance procedures to be conducted safely.
Trail Shaft Operations
The gas turbine brake is principally designed to stop the low pressure output shaft and keep it at
a stand-still if the propeller shaft is trailed while the gas generator is stopped. As trail shaft
operations are a common fuel savings measure, this arrangement will protect engine bearings of
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a non-operating gas turbine from damage. Protocol for use of the power turbine brake will be
similar to that of any other gas turbine engine.
Low speed propeller shaft operations
It is important to note that the LM6000 is designed to operate over a wide operating range in
mechanical drive applications. From idle to full power, the high pressure rotor speed varies from
5000-10,810 rpm. The capability of the LM6000 to function properly in a mechanical drive
naval propulsion plant during partial power conditions is especially important. The engine has
demonstrated this capability through extensive customer load tests conducted for industrial
mechanical drive projects and for American Bureau of Shipping (ABS) testing. These tests have
validated that the HPT is capable of operating at speeds below 5000 rpm and the LPT is capable
of operating at speeds as low as 800 rpm without impinging upon engine operability (or stall)
margins. This attribute is consistent with LM2500 LPT speed profiles on surface combatants
such as the U.S. Navy DDG-51 class, where power turbine idle speeds range from 1000-1200
rpm. Of course, LPT speeds for an engine connected to the load, while at idle, are dependent
upon engine power and the mass/inertia characteristics of the propulsion drive train. In plant
alignment modes where only the gas turbine clutch is engaged, resultant LM6000 engine powers
at low pressure turbine speeds nominally range from 100-300kw. This performance attribute
will provide design authorities with inherent flexibility to consider a range of propeller shaft
speeds and pitches during low power operations, such as when operating alongside the pier or
harbor maneuvering at ship speeds less than 12 knots. With typical warship gear ratios of 1924:1, the projected LM6000 LPT speeds will result in propeller shaft speeds ranging from 40-55
rpm, which is typical of warship designs today.
The performance diagram below (Figure 6) depicts the projected minimum and maximum power
curves of the LM6000 in the low speed/low power quadrant of the engine operating
envelope. The minimum power (lower) curve is largely determined by the engine’s gas
generator idle speed, which is driven by operability margin. The maximum power (upper) curve
is defined by mass flow capability, which is a function of airfoil geometry and the engine’s
speed, torque, and firing temperature limits. Superimposed is a brake power versus shaft speed
load curve, typical of a conventional hull, which assumes the ship’s brake power varies as the
cube of the speed (i.e. a cubic load curve). The area surrounding the notional cubic load curve
(between the engine’s upper and lower performance curves) represents the degree of operating
margin for the controllable pitch propeller. The range of available power between 900-1200
LPT speeds is 1.3MW to 3.0MW. This range of power offers a wide window from which to
establish a propeller design curve that will account for propeller pitch variations due to changes
in hull resistance, sea states, or service life of the vessel. As such, the LM6000 engine power
curves predict that there is more than sufficient operating margin to accommodate standard
controllable pitch propeller design curves, allowing the propeller manufacturer and naval
architect to consider a range of propeller shaft speeds and propeller pitch/diameter ratios that fall
within the bounds of the engine’s power and speed capabilities. In short, the LM6000 LPT speed
can be tailored to comfortably accommodate a wide range of main reduction gear ratios and
propeller speeds for both ahead and astern operations.
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Figure 6: Projected LM6000 Low Speed Performance Map
Transient Response
Acceleration capability, sometimes referred to as acceleration reserve, is given by the difference
between the steady state cubic load line and the gas turbine speed/power envelope. A robust
acceleration reserve allows the engine to rapidly increase power without necessitating a high rate
of change in propeller shaft speed, thus improving maneuvering responsiveness. Accordingly, in
a shipboard mechanical drive propulsion plant, the LM6000 will not limit vessel acceleration or
deceleration. Figure 7 illustrates the acceleration reserve for the engine under two ambient air
conditions. Across a good portion of the LPT speed range, a margin of greater than 10MW
between the nominal cubic load curve and the maximum power curve exists. This will provide
superior responsiveness for the most demanding throttle commands including crash back, which
entails rapid speed changes from full ahead to full astern. Under these throttle commands, the
LM6000 is predicted to respond from low power to maximum power in 30 to 45 seconds.
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Figure 7: Projected LM6000 Maximum Load Performance Map
CONCLUSIONS
The use of a gas turbine architecture that incorporates an alternative design to the well-known
aerodynamically-coupled power turbine may evoke questions regarding the technical and
operational suitability of such a design. This is an understandable perspective since most naval
operators are accustomed to enjoying the operational flexibility provided by aerodynamicallycoupled power turbines, such as the LM2500. GE has thoroughly investigated the capability of
the LM6000 in variable speed mechanical drive applications. The engine’s design, featuring
advanced airfoil geometry and bleed valve controls, is capable of efficiently matching variable
airflows between the low and high pressure compressors across a full range of LPT speeds and
loads. Developmental testing has validated that the LM6000 will function in warship mechanical
drive propulsion plants in a manner that is essentially identical to the LM2500. The use of
advanced controllable pitch propellers, SSC overrunning clutches, and automated gear and
engine control features will allow the engine and ship to function safely during transitory
operating modes without constraining the operation of the propulsion plant. And, the engine’s
operating envelope will accommodate the design flexibility that naval architects require for
integrating controllable pitch propeller designs with resistance curves typical of a wide range of
naval vessels.
ACKNOWLEDGEMENTS
The technical support of Mr. Morgan Hendry, President of SSS Clutch Company, is gratefully
acknowledged.
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REFERENCES
Casper, Russell, Application of the LM6000 for Power Generation and Cogeneration,
EuroPower Quarterly, Summer 1993
Clements, H. A., Operational Experience of the S.S.S. (Synchro-Self-Shifting) Clutch
Particularly in Naval Propulsion Machinery, American Society of Mechanical Engineers, March
1972
Dupuy, Paul, Gas Turbines, Modern Marine Engineer’s Manual, Vol. I, Third Edition, 1993
Ellington, Louis & McAndrews, Glenn, Gas Turbine Propulsion for LNG Transports, ASME
Turbo Expo, May 2006
GE Aero Energy, LM6000 (PG Model) Installation Design Manual, March 2010
Ham, A. A., Comparative Study of Propulsion Configurations for a Naval Replenishment Ship,
Report 1673, March 1990
Hendry, Morgan, U. S. Navy Experience with SSS (Synchro-Self-Shifting) Clutches, Mechanical
Engineering Magazine, August 2010
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