FIELD OF THE INVENTION
The present disclosure relates to gas turbines in mechanical drive
applications. More specifically, the subject matter disclosed herein concerns
multiple-shaft gas turbines, such as aeroderivative twin-shaft gas turbines for
mechanical drive applications.
DESCRIPTION OF THE RELATED ART
Gas turbines have found a wide use in several applications, such as power
generation, as well as mechanical drive, where the gas turbines are commonly used
as first mover for one or a plurality of driven machines, such as compressors, in
particular centrifugal compressors. Typical mechanical drive applications are in the
field of natural gas liquefaction, carbon dioxide recovery and the like.
A gas turbine includes one or more sequentially arranged air compressors for
compressing ambient air, a combustor burning fuel together with the compressed air
and one or more turbines for driving the compressor(s) and generate useful
mechanical power. The power generated by the turbine(s) exceeding that required to
drive the compressor(s) is used for driving the load.
^~ Gas turbines ingest a large amount of air. Particles in the form of aerosols
^ ^ present in the air sucked by the gas turbine partly exit the gas turbine with the
exhaust gases. However, there are particles which contaminate the turbomachinery,
by sticking to the stationary vanes and to the rotary blades thereof. This
contamination, also called fouling, particularly negatively affects the initial part of
the flow path within the gas turbine, i.e. the compressor or compressors.
Contaminants forming deposits on the stationary blades (vanes) and rotary blades of
the compressor alter the geometry of the blades and increase gas friction, thus
reducing the overall compressor efficiency. In particular, the particles stuck to the
surfaces of the vanes and the rotary blades of the compressor alter the aerodynamic
properties of the flow passages defined by the blades and the vanes. The alteration of
the aerodynamic properties causes loss of mass flow and therefore reduction of the compressor efficiency. Typically the compressor of a gas turbine consumes the major part of the
power generated by the turbine or turbines, i.e. approximately 60% of said power. Areduction in the compressor efficiency thus negatively affects the overall efficiency of the gas turbine, reducing the power available for driving the load. One of the ways to reduce fouling of the compressor in a gas turbine is to wash the gas paths in the gas turbine. Washing is typically practiced by injecting a washing liquid in the gas path upstream of the compressor inlet. The turbomachinery is allowed to rotate during washing such that the liquid is forced through the
4fc compressor and exits at the rear of the gas turbine. The wash liquid can contain water
and chemical additives and is fed in the form of a fine spray which will distribute the washing liquid over the entire compressor inlet face. Atomization is provided by suitable nozzles which are fed with pressurized washing liquid.
An effective way of washing the gas turbine is the so-called offline washing. In this case washing is performed while the gas turbine is not fired, but is turning at a rotary speed which is a fraction of the rated rotary speed during normal operation,i.e. when running at load. An additional mover is required, to keep the gas turbine rolling at offline washing speed.
Online washing is also possible. In this case the gas turbine is washed while running under load conditions. Such washing process is, however, less effective, due Jik to the speed and temperature conditions in the compressor, which result in inefficient washing of the blades, centrifugation of the washing liquid towards the casing of the compressor and evaporation of the washing liquid due to the temperature increase
provoked by the high compression ratio. When online washing is used, fouling of the
compressor can only be reduced but not avoided. Therefore, offline washing capability must anyhow be available. The gas turbine will in fact require offline washing when the amount of particle deposits on the vanes and rotary blades of the compressor becomes unacceptable, in spite of online washing. Aeroderivative gas turbines are increasingly used for machine drive and
power generation applications. Some eroderivative gas turbines comprise a multishaft arrangement. A multi-shaft arrangement is one in which more than one shaft is provided, to drivingly connect turbines and compressors to one another. In some
multi-shaft gas turbines, the power turbine, i.e. the turbine which provides the mechanical power to drive the load, is mechanically connected through one of the gas turbine shafts with one of the compressors.
Fig. 1 shows a schematic diagram of a twin-shaft aeroderivative gas turbine
used in a typical mechanical drive application. Reference number 1 globally
indicates an apparatus comprising a gas turbine and a load. The gas turbine 3
comprises a low pressure compressor 5, a high pressure compressor 7, a high
pressure turbine 9 and a low pressure turbine, or power turbine 11. The high pressure
compressor 7 is drivingly connected to the high pressure turbine 9 by means of a first
^ P gas turbine shaft 13. The low pressure turbine or power turbine 11 is drivingly
connected to the low pressure compressor 5 by means of a second gas turbine shaft
15, arranged coaxial with the first gas turbine shaft 13 as well as coaxial with the
high pressure turbine 9 and the high pressure compressor 7.
Ambient air is compressed by the low pressure compressor 5 and by the high
pressure compressor 7 and enters a combustor 17 where gaseous or liquid fuel is
added to the compressed air stream and burned to generate a flow of high-pressure,
high-temperature combustion gases. The combustion gases are sequentially
expanded in the high pressure turbine 9 and in the low pressure turbine 11 before
being discharged.
The power generated by the expansion of the combustion gases in the high-
^m pressure turbine 9 is entirely exploited to drive the high pressure compressor 7.
Conversely, the mechanical power generated by expanding the combustion gases in
the low pressure turbine 11 is only partly used to drive the low pressure compressor
5. A large amount of the mechanical power available on the low pressure turbine 11
output shaft 21 is used to drive the load.
The output shaft 21 of the power turbine or low pressure turbine 11 forms
part of a load coupling 23, which transmits the mechanical power from the gas
turbine 3 to a load. In the example of Fig. 1 the load is represented as a centrifugal
compressor 25. A gear box 27 is provided in this exemplary embodiment between
the gas turbine output shaft 21 and the centrifugal compressor 25. A gear box is
4
usually provided when the rotary speed of the low pressure turbine on the one hand
and of the load on the other is not identical in terms of rpm or if it has to be reversed.
In some embodiments the power turbine 11 can be directly connected to the load
shaft, i.e. to the shaft of an electric generator or a turbomachinery, such as a
centrifugal compressor.
In a twin shaft aeroderivative gas turbine as the one illustrated in Fig.l,
washing of the compressors 5 and 7 requires both the first (high pressure) and the
second (low pressure) gas turbine shafts to be rotated. The former is rotated with the
onboard starting motor of the gas turbine itself, the latter requires an external mover.
Moreover, rotating the second gas turbine shaft requires high power input because
^ * the second gas turbine shaft is permanently mechanically connected to the load.
Gas turbines are also used as prime movers in power generation applications,
wherein mechanical power available on the gas turbine output shaft is used to drive
an electric generator. The electric generator converts mechanical power from the gas
turbine into electric power. Single-shaft gas turbines are often used in power
generation applications of this kind. The gas turbine comprises a compressor and a
turbine, mechanically connected to one another by a shaft. Compressed air provided
by the compressor is delivered to a combustor and mixed with fuel therein. The airfuel
mixture is ignited to produce compressed hot combustion gases. The combustion
gases are expanded in the gas turbine to generate mechanical power. Part of the
mechanical power produced by the turbine is used to drive the compressor. Excess
J^ mechanical power is available on the single gas turbine shaft for driving the electric
^ ^ generator.
In order to start the single-shaft gas turbine and the electric generator
mechanically linked thereto, the use of two combined movers is known. A first
mover comprises a low speed electric motor. A second mover comprises a high
speed, internal combustion engine. To start rotation of the gas turbine and generator
train, the low speed electric motor is energized first. Once a pre-set rotary speed of
the shaft line has been achieved, further acceleration of the system is performed by
the high speed, internal combustion engine. The slow speed electric motor is also
used for slow turning the power plant following shut down, to prevent bowing of the
rotor aggregate of the gas turbine.
5
SUMMARY OF THE INVENTION
A dual-speed turning gear is combined to a multiple shaft gas turbine
arrangement, wherein the multiple shaft gas turbine drives a load, wherein the load is
drivingly connected to the power turbine and the latter is in turn connected to at least
one compressor of the gas turbine, said compressor requiring offline washing. The
dual-turning gear is comprised of a motor arrangement which can drive the output
shaft of the dual-speed turning gear at at least two different stationary rotary speeds.
In some embodiments two motors are used, e.g. electric motors, such as for example
AC motors. In some embodiments three phase motors can be used, but other movers
can be envisaged, such as one-phase AC motors or DC motors. When two motors are
used, said motors can also be of different kind, e.g a DC and an AC motor,
respectively. The motors can be equal to one another and an arrangement of gear
reducers be provided, to drive the output shaft of the dual-speed turning gear at
different speeds, as required for performing various actions on the turbomachinery
connected thereto, as will become clearer from the description here below. In some
embodiments, however, motors of different rated power and/or different rpm are
used. Moreover, the two motors can provide different torques. A low speed motor
and a high speed motor can be used. The term "high speed" and "low speed" are
referred to relative rotary speeds of the output shaft. The dual-speed turning gear is
therefore designed such that the low speed motor will drive the output shaft of the
dual-speed turning gear at a first rotary speed and the high speed motor will drive the
output shaft of the dual-speed turning gear at a second rotary speed; the second
^ B rotary speed being higher than the first rotary speed.
The multi-shaft gas turbine can comprise only two co-axially arranged shafts.
Other embodiments can comprise more than two shafts, additional further
compressor(s) and/or power turbines, combined with the high and low pressure
compressor and the high pressure turbine and power turbine.
According to one embodiment, an apparatus for driving a load is provided,
comprising a multiple-shaft gas turbine, comprised of: a high pressure compressor
and a high pressure turbine drivingly connected to one another by a first gas turbine
shaft; and a low pressure compressor and a power turbine drivingly connected to one
another by a second gas turbine shaft, extending coaxial with said first gas turbine
6
shaft, as well as coaxial with said high pressure compressor and said high pressure
turbine. The apparatus further comprises a load coupling which drivingly connects
the power turbine to the load. The apparatus further comprises a dual-speed turning
gear with an output shaft drivingly engageable to and disengageable from said load.
The dual-speed turning gear comprises a low speed turning motor and a high speed
turning motor, said low speed turning motor and said high speed turning motor being
arranged and controlled to selectively drive said load at a first rotary speed and at a
second rotary speed, the first rotary speed being lower than the second rotary speed.
Engagement of the dual-speed turning gear to the load and disengagement therefrom
can be obtained by means of an engageable and disengageable joint, such as a clutch.
^ ^ In some embodiments, a synchro self-shifting clutch can be used for that purpose.
This kind of clutch makes engagement and disengagement particularly swift and
reliable. Other clutches or clutch systems can be used.
In some embodiments, as will be disclosed later on with respect to an
exemplary embodiment, the output shaft of the dual-speed turning gear is engageable
frontally to the load. For example the output shaft of the dual-speed turning gear can
be engageable to the shaft of the last one of a series of compressors forming a
compressor train or of just one compressor, forming the load driven by the gas
turbine. Different arrangements are not excluded, such as one using a gearbox
positioned between the last compressor of a compressor train or the compressor
forming the load, and the output shaft of the dual-speed turning gear.
g^ In some embodiments the load can comprise at least one centrifugal
^ ^ compressor, or else a plurality of centrifugal compressors. In other embodiments,
other rotary machines, different from centrifugal compressors can be connected to
the gas turbine, possibly in combination with centrifugal compressors.
In some embodiments, an overrunning clutch can be provided to disengage
the slow speed turning motor from the output shaft of the dual-speed turning gear
when the high speed turning motor accelerates the output shaft of the dual-speed
turning gear beyond the maximum speed provided for by the low speed turning
motor.
An exemplary embodiment of the dual-speed turning gear comprises a clutch
7
arrangement which is designed and arranged such that the output shaft of the dualspeed
turning gear will be rotated by the slow speed turning motor until a first rotary
speed and will be rotated by the high speed turning motor when the rotary speed of
the output shaft exceeds said first rotary speed. An overrunning clutch can be used
for that purpose, arranged between the slow speed turning motor and the output shaft
of the dual-speed turning gear, such that the high speed turning motor will take over
control of the output shaft once the speed thereof exceeds the speed imposed by the
low speed turning motor.
In some embodiments a first clutch can be provided, selectively engaging and
disengaging the output shaft of said dual-speed turning gear to said load and a
^ ^ second clutch selectively engaging the low speed turning motor to the output shaft
and disengaging it there from.
The dual-speed turning gear can comprise gear arrangements to provide the
proper ratio between the rotary speed of the slow speed turning motor and the output
shaft of the dual-speed turning gear as well as between the rotary speed of the high
speed turning motor and said output shaft. The gear arrangement can comprise at
least one worm gear. In some embodiments, two worm gears are arranged in a
cascade fashion, i.e. in sequence. The rotary speed of the slow speed turning motor
will then be reduced twice by two sequentially arranged worm gears, to drive the
output shaft at a first slow speed rotary motion. The rotary speed of the high speed
turning motor can be reduced through only one of said worm gears to rotate the
ML output shaft of the dual-speed turning gear at a higher rotary speed.
According to some embodiments, during offline washing of the gas turbine
the dual-speed turning gear is arranged and controlled such as to initially accelerate
the load, the low pressure compressor and the power turbine from a stationary
condition to a first rotary speed by means of the low speed turning motor and
subsequently to further accelerate the load, the low pressure compressor and the
power turbine from the first rotary speed to a second rotary speed, higher than said
first rotary speed. The second rotary speed can be an offline washing speed which is
maintained during offline washing of the gas turbine.
In some embodiments the slow speed turning motor and the high speed
8
turning motor are arranged and controlled such that during slow-rolling of the load,
rotation of said load is controlled by said low speed turning motor.
An auxiliary turning motor, e.g. is provided for driving into rotation said
high pressure compressor and said high pressure turbine. Such auxiliary turning
motor can be the onboard starting motor of the gas turbine itself. The auxiliary
turning motor can be used for slow turning the core of the gas turbine, i.e. the high
pressure compressor and the high pressure turbine after shut down of the gas turbine,
to prevent bowing of the rotor aggregate of said core. The same auxiliary turning
motor can be used as a starter, to start rotation of the high pressure compressor and
high pressure turbine of the gas turbine. The low pressure compressor and the low
i ^ pressure turbine are not mechanically connected to the turbine core and therefore the
auxiliary turning motor acting as a starter does not require driving into rotation the
low pressure compressor and low pressure turbine as well as the load connected
thereto.
According to a further aspect, the disclosure relates to a method for offline
washing a multiple shaft gas turbine, such as a twin-shaft gas turbine, connected to a
load. A dual-speed turning gear is selectively drivingly connected to the load driven
by the multiple shaft gas turbine and is used to gradually accelerate the load and the
power turbine, along with the low pressure compressor of the gas turbine to an
offline washing rotary speed. A slow speed turning motor and a high speed turning
motor can be used in sequence to accelerate the apparatus as required.
^ff According to some embodiments of the method disclosed therein, offline
washing is performed on a gas turbine comprising: a high pressure compressor and a
high pressure turbine drivingly connected to one another by a first gas turbine shaft;
a low pressure compressor and a power turbine drivingly connected to one another
by a second gas turbine shaft, extending coaxial with said first gas turbine shaft, said
high pressure compressor and said high pressure turbine. In some embodiments, the
method comprises the steps of:
providing a dual-speed turning gear with an output shaft drivingly engageable
to and disengageable from the load, wherein the dual-speed turning gear comprises a
low speed turning motor and a high speed turning motor;
9
initially accelerating the low pressure compressor, the power turbine and the
load with the low speed turning motor up to a first rotary speed;
when the first rotary speed is reached, continuing accelerating the low
pressure compressor, the power turbine and the load with the high speed turning
motor until an offline washing rotary speed is reached;
maintaining the offline washing rotary speed while washing said gas turbine.
According to some embodiments, a further shaft of the gas turbine, on which
turbomachinery other than the power turbine and the low pressure compressor are
^ ^ supported, can be turned at the offline washing speed by an auxiliary turning motor,
^ e.g. an onboard starting motor of the gas turbine itself.
According to further embodiments, the dual-speed turning gear is used
alternatively to rotate the gas turbine components during offline washing and during
slow-roll following turbine shutdown, for example, in order e.g. to prevent bowing of
the centrifugal compressors connected to the power turbine via the load coupling.
The method can comprise the steps of: shutting down the gas turbine; slow-rolling
the load, the low pressure compressor and the power turbine by means of the slow
turning motor of said dual-speed turning gear, until a required temperature profile of
said load is achieved.
The disclosure herein also concerns a method of slow-rolling a multi-shaft
^_ gas turbine connected to a load, said gas turbine comprising: a high pressure
^ ^ compressor and a high pressure turbine drivingly connected to one another by a first
gas turbine shaft; a low pressure compressor and a power turbine drivingly
connected to one another by a second gas turbine shaft, extending coaxial with said
first gas turbine shaft, said high pressure compressor and said high-pressure turbine.
In some embodiments, the method comprises the steps of:
providing a dual-speed turning gear with an output shaft drivingly engageable to
and disengageable from the load, wherein the dual-speed turning gear
comprises a low speed turning motor and a high speed turning motor;
selectively slow-rolling the load and the second gas turbine shaft at a slow-rolling
speed with the slow speed turning motor during cooling of the gas turbine and
10
the load following shut-down of the gas turbine; or
rolling the load and the second gas turbine shaft at an offline washing speed, with
the high speed turning motor during offline washing of the gas turbine, the
offline washing speed being higher than said slow-rolling speed;
Features and embodiments are disclosed here below and are further set forth
in the appended claims, which form an integral part of the present description. The
above brief description sets forth features of the various embodiments of the present
invention in order that the detailed description that follows may be better understood
and in order that the present contributions to the art may be better appreciated. There
^fc are, of course, other features of the invention that will be described hereinafter and
which will be set forth in the appended claims. In this respect, before explaining
several embodiments of the invention in details, it is understood that the various
embodiments of the invention are not limited in their application to the details of the
construction and to the arrangements of the components set forth in the following
description or illustrated in the drawings. The invention is capable of other
embodiments and of being practiced and carried out in various ways. Also, it is to be
understood that the phraseology and terminology employed herein are for the
purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception, upon
which the disclosure is based, may readily be utilized as a basis for designing other
structures, methods, and/or systems for carrying out the several purposes of the
^ ^ present invention. It is important, therefore, that the claims be regarded as including
such equivalent constructions insofar as they do not depart from the spirit and scope
of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the disclosed embodiments of the invention
and many of the attendant advantages thereof will be readily obtained as the same
becomes better understood by reference to the following detailed description when
considered in connection with the accompanying drawings, wherein
11
Fig. 1 illustrates a compressor driven by gas turbine according to the state of
the art;
Fig.2 illustrates a gas turbine driving a compressor train provided with a dualspeed
turning gear according to the subject matter disclosed herein;
Fig.3 illustrates a front view of the dual-speed turning gear; and
Fig.4 illustrates a side view according to line IV-IV of the dual-speed turning
gear of Fig.3.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
The following detailed description of the exemplary embodiments refers to
the accompanying drawings. The same reference numbers in different drawings
identify the same or similar elements. Additionally, the drawings are not necessarily
drawn to scale. Also, the following detailed description does not limit the invention.
Instead, the scope of the invention is defined by the appended claims.
Reference throughout the specification to "one embodiment" or "an
embodiment" or "some embodiments" means that the particular feature, structure or
characteristic described in connection with an embodiment is included in at least one
embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in
one embodiment" or "in an embodiment" or "in some embodiments" in various
places throughout the specification is not necessarily referring to the same
4fc embodiment(s). Further, the particular features, structures or characteristics may be
combined in any suitable manner in one or more embodiments.
Fig. 2 illustrates a schematic diagram of a twin-shaft aeroderivative gas
turbine used in a typical mechanical drive application. Reference number 101
globally indicates the apparatus comprising a gas turbine 103 and a load 104. The
gas turbine 103 comprises a low pressure compressor 105, a high pressure
compressor 107, a high pressure turbine 109 and a low pressure turbine or power
turbine 111.
The high pressure compressor 107 is drivingly connected to the high pressure
turbine 109 by means of a first gas turbine shaft 113. The power turbine 111 is
12
drivingly connected to the low pressure compressor 105 by means of a second gas
turbine shaft 115, arranged coaxial inside the first gas turbine shaft 113 as well as
coaxial with the high pressure turbine 109 and the high pressure compressor 107.
Ambient air enters the low pressure compressor 105, is compressed at first
pressure level and subsequently enters the high pressure compressor 107 to be
compressed at a final pressure level. The compressed air enters a combustor 117
where a gaseous or liquid fuel is added to the compressed air stream and burned to
generate a flow of high-pressure, high-temperature combustion gases. The
combustion gases are sequentially expanded in the high pressure turbine 109 and in
^ ^ the power turbine 111 before being discharged.
The mechanical power generated by the expansion of the combustion gases in
the high pressure turbine 109 drives the high pressure compressor 107. The
mechanical power generated by gas expansion in the power turbine 111 is partly
used to drive the low pressure compressor 105. The exceeding mechanical power
available on the shaft of the power turbine 111 is transferred to an output shaft 121 of
the power turbine 111 to drive the load 104.
In the embodiment illustrated in Fig.2 the output shaft 121 is connected to the
load 104 through a gearbox 125. In other embodiments, not shown, the gearbox 125
can be dispensed with. An exit shaft 126 from the gearbox 125 transmits the power
to the load 104. Therefore, the load, the power turbine 111 and the low pressure
compressor 105 are permanently connected mechanically to one another.
In the embodiment illustrated in Fig.2 the load 104 comprises a compressor
train. The compressor train comprises in turn a first compressor 127 and a second
compressor 129. By way of example only, as shown in Fig.2 die first compressor 127
is a double compressor, having a double casing. The two compressors 127, 129 are
driven by the same shaft 126 and rotate therefore at the same rotary speed. It shall be
understood that the shaft 126 can be actually made of more than one shaft portion,
connected to one another by suitable joints. In other embodiments,'' not shown, a
further gearbox can be arranged between the first compressor 127 and the second
compressor 129, for example if the two compressors require to be driven at different
rotary speeds. In a possible embodiment, the gear box 125 can be dispensed with and
13
a gearbox can be arranged between the first compressor 127 and the second
compressor 129, for example if the first compressor 127 rotates at the same rotary
speed as the power turbine 111 (direct drive) and the second compressor 129 is
required to rotate at different speed.
In other embodiments, not shown, the load 104 can comprise more than just
two compressors 127, 129, with or without gearboxes interposed therebetween.
A centrifugal compressor train 104 as schematically shown in Fig. 2 is
typically used in natural gas liquefaction plants, wherein each centrifugal compressor
is used to process a refrigerant gas or a mixture of refrigerant gases used to chill and
^ p finally liquefy a natural gas for storage or transportation.
In the embodiment shown in Fig.2 the core of the gas turbine, comprising the
high pressure compressor 107 and the high pressure turbine 109 connected to one
another by the first gas turbine shaft 113, is provided with an auxiliary turning motor
or starter 131, which can be used during offline washing of the gas turbine or for
slow turning following shut down of the gas turbine, if required. Since the core of the
gas turbine is not mechanically connected to the second gas turbine shaft 115, the
low pressure compressor 105 and the power turbine 111 are not driven by the
auxiliary motor 131. The auxiliary turning motor or starter 131 can be an electric
motor or a hydraulic motor or another mover, having relatively low rated power.
For slow rolling or rotating the low pressure compressor 105 and the power
4 h turbine 111 as well as the load 104 during offline washing, the apparatus comprises a
dual-speed turning gear 141 which can be selectively engaged with a shaft 128 of the
last compressor 129 of the load 104. The dual-speed turning gear 141 is designed and
controlled to slow-rolling the load 104, the power turbine 111 and the low pressure
compressor 105 when the gas turbine is shut down, in order to prevent bowing of the
centrifugal compressors 127, 129. The same dual-speed turning gear 141 is also
designed and controlled to rotate the power turbine 111 and the low pressure
compressor 105, as well as the load which is stably connected thereto, during offline
washing of the gas turbine.
One embodiment of the dual-speed turning gear 141 is shown in Figs. 3 and
4. The dual speed-turning gear 141 comprises a support 143 for connection to the
14
foundation. Attached to the support 143 is a first turning motor 145, e.g. an electric
motor, e.g. a three-phase electric motor. The first turning motor 145 drives into
rotation a first shaft 147. The first shaft 147 drives a first worm gear 149 comprising
a gear 148 connected to an output shaft 151 via an overrunning clutch 152. The shaft
151 is the input shaft of a second worm gear 153. The gear 155 of the worm gear 153
is keyed on an output shaft 156 of the dual-speed turning gear. Said output shaft 156
is frontally engageable the load shaft 128 and disengageable therefrom via a synchro
self-shifting clutch 157.
The dual-speed turning gear 141 further comprises a second turning motor,
for example an electric motor, such as a three phase electric motor 161. The output
^ ^ shaft of the second turning motor 161 is coaxial to the shaft 151.
The first turning motor 145 and the second turning motor 161, as well as the
transmission ratios of the worm gears 147 and 153 are chosen such that the
appropriate rotary speeds are obtained for both slow-rolling of the centrifugal
compressors 127, 129 as well as offline washing rotation of the power turbine 111
and the low pressure compressor 105. If one or more gearboxes such as gearbox 125
are provided along the load coupling which connects the power turbine 111 to the
centrifugal compressors 127, 129 of the compressor train, the transmission ratios of
the worm gears 147, 153 of the dual-speed turning gear 141 will be designed taking
into account the transmission ratios of said gearboxes.
The first turning motor 145 is a slow speed turning motor and the second
^ P turning motor 161 is the high speed turning motor of the dual-speed turning gear
141. The terms "slow speed" and "high speed" are intended as relative terms and are
referred to the speed of the output shaft 156 of the dual-speed turning gear 141. The
speeds and the transmission ratios mentioned above are chosen such that the slow
speed turning motor 145 can rotate the output shaft 156 and therefore the centrifugal
compressors at a slow speed of around 0,5-10 rpm, and more particularly for
example between 1 and 7 rpm. This rotary speed is suitable for slow rolling of the
turbomachinery following shut down of the gas turbine, such as to prevent bowing of
the centrifugal compressors during cooling thereof. The slow-roll speed ranges
mentioned above are given by way of example only, and shall not be construed as
limiting the scope of the present disclosure.
15
The second turning motor 161 and the speed ratio of the second worm gear
153 are chosen such that the second turning motor 161 can rotate the low pressure
compressor 105 and the power turbine 111 at an offline washing rotary speed. Said
offline washing rotary speed can range between 50 and 500 rpm, preferably between
100 and 400 rpm, for example between 200 and 300 rpm. The ranges of the offline
washing speed mentioned herein are given by way of example only, and shall not be
construed as limiting the scope of the present disclosure.
The low speed turning motor is preferably a low-power motor, having e.g. a
rated power of between 1 and 20 kW, preferably between 2 and 15 kW, and more
preferably between 5 and 12 kW. These values are given by way of example only,
^ ^ and can vary, e.g. depending upon the kind and number of centrifugal compressors
forming the load, as well as upon other parameters, such as the dimension of the gas
turbine, the number of compression and expansion stages and other design
parameters.
The high speed turning motor has preferably a higher rated power of e.g.
between 20 and 100 kW, preferably between 30 and 80 kW and more preferably
between 40 and 50 kW. These values are given by way of example only, and can
vary e.g. depending upon the kind and number of centrifugal compressors forming
the load as well as upon other parameters, such as the dimension of the gas turbine,
the number of compression and expansion stages and other design parameters.
The dual-speed turning gear can be controlled as follows. When the gas
^ P turbine 103 requires offline washing, the high pressure compressor 107 and the high
pressure turbine 109 can be driven into rotation by the auxiliary motor 131 having a
relatively limited rated power. The remaining turbomachinery, including the low
pressure compressor 105, the power turbine 111 and the load 104 require a higher
torque to start rotating and a higher power to be maintained in continuous rotation at
the offline washing rotary speed required to achieve efficient washing of the axial
compressor 105. The slow speed turning motor 145 is used to overcome the
breakaway torque of the turbomachinery, which requires high torque to be applied to
the shaft 128. The torque required to start rotation of the turbomachinery is achieved
by properly selecting the slow speed turning motor 145 and due to the very high
reduction ratio achieved by the combined cascade arrangement of the first worm gear
16
149 and the second worm gear 153. The synchro self-shifting clutch 157 connects the
output shaft 156 of the dual-speed turning gear 141 with the shaft 128 of the load and
the torque generated by the slow speed turning motor 145 is transmitted to the load
104 and to the gas turbine 101, starting rotation of the turbomachines forming the
load 104 as well as the power turbine 111 and the low pressure compressor 105.
Once the output shaft 156 of the dual-speed turning gear 141 has achieved a
sufficient rotary speed, e.g. in the range of 1-10 rpm, further angular acceleration can
be imparted by the high speed turning motor 161. When the speed of the shaft 151,
under the control of the high speed turning motor 161 exceeds the maximum output
^ speed of the first worm gear 149, the overrunning clutch 152 disengages shaft 151
^ ^ from the first, low speed turning motor 145, such that the second, high speed turning
motor 161 can continue to angularly accelerate the shaft 128 until the final offline
washing rotary speed is achieved.
When slow-rolling is required, e.g. following a gas turbine shutdown, to
prevent bowing of the centrifugal compressors 127, 129, the dual-speed turning gear
141 is controlled such that the shaft 126, 128 is maintained into a slow-rolling
condition, not exceeding the speed, which can be reached by the slow speed turning
motor 145. The latter has sufficient power to maintain the turbomachinery in the
slow-rolling condition and the second, high speed turning motor 161 does not require
to be activated. If the gas turbine 103 is fe-started while the load is in the slowrolling
condition, the synchro self-shifting clutch 157 will disengage the dual-speed
j ^ turning gear 141 from the shaft 128 automatically.
While the disclosed embodiments of the subject matter described herein have
been shown in the drawings and fully described above with particularity and detail in
connection with several exemplary embodiments, it will be apparent to those of
ordinary skill in the art that many modifications, changes, and omissions are possible
without materially departing from the novel teachings, the principles and concepts
set forth herein, and advantages of the subject matter recited in the appended claims.
Hence, the proper scope of the disclosed innovations should be determined only by
the broadest interpretation of the appended claims so as to encompass all such
modifications, changes, and omissions. In addition, the order or sequence of any
process or method steps may be varied or re-sequenced according to alternative
17
embodiments.

WE CLAIM :
1. An apparatus for driving a load, comprising:
a multiple-shaft gas turbine comprising:
- a high pressure compressor and a high pressure turbine drivingly
connected to one another by a first gas turbine shaft;
- a low pressure compressor and a power turbine drivingly coimected to
one another by a second gas turbine shaft, extending coaxial with said
first gas turbine shaft, said high pressure compressor and said high
pressure turbine;
a load coupling drivingly coimecting said power turbine to said load;
^m a dual-speed turning gear with an output shaft drivingly engageable to and
disengageable fi'om said load;
wherein said dual-speed turning gear comprises a low speed turning motor and a
high speed turning motor, said low speed turning motor and said high speed turning
motor being arranged and controlled to selectively drive said load.
2. Apparatus according to claim 1, fiarther comprising an auxiliary
turning motor arranged and controlled for starting rotation of said high pressure
compressor and said high pressure turbine and/or for slow-turning the high pressure
compressor and the high pressure turbine.
3. Apparatus according to claim 1 or 2, wherein said load comprises at
least one centrifugal compressor.
^ ^ 4. Apparatus according to claim 1 or 2, wherein said load comprises a
plurality of centrifugal compressors.
5. Apparatus according to one or more of the preceding claims, wherein
said dual-speed turning gear comprises an overrurming clutch to disengage said slow
speed turning motor fi-om the output shaft of the dual-speed turning gear when the
high speed turning motor accelerates said output shaft beyond the maximum speed
provided for by the low speed turning motor.
6. Apparatus according to claim 5, wherein said dual-speed turning gear
comprises a clutch arrangement designed and arranged so that the output shaft of the
dual-speed turning gear is rotated by the slow speed turning motor until a first rotary
19
speed and is rotated by the high speed turning motor when the rotary speed of the
output shaft exceeds said first rotary speed.
7. Apparatus according to claim 6, wherein said dual-speed turning gear
comprises an overrurming clutch arranged between the slow speed turning motor and
the output shaft, such that the high speed turning motor takes over control of the
output shaft once the speed thereof exceeds the speed imposed by the low speed
turning motor.
8. Apparatus according to one or more of the preceding claims, wherein
said dual-speed turning gear comprises a first clutch selectively engaging and
^ ^ disengaging the output shaft of said dual-speed turning gear to and firom said load
and a second clutch selectively engaging and disengaging the high speed turning
motor to and fi-om the output shaft.
9. Apparatus according to one or more of the preceding claims, wherein
said dual-speed turning gear comprises at least one worm gear
10. Apparatus according to one or more of the preceding claims, wherein
said dual-speed turning gear comprises a first worm gear and a second worm gear.
11. Apparatus according to claim 10, wherein the rotary speed of said low
speed turning motor is transmitted to the output shaft of said dual-speed turning gear
through said first worm gear and said second worm gear in series.
^ p 12. Apparatus according to claim 10 or 11, wherein the rotary speed of
said high speed turning motor is transmitted to the output shaft of said dual-speed
turning gear through said second worm gear.
13. Apparatus according to claim 10, 11 or 12, wherein a shaft of said
second worm gear is connected on a first end to said slow speed turning motor and at
a second end to said high speed turning motor, and wherein a clutch is arranged
between said second end of said shaft and an output shaft of said high speed turning
motor, such that when the rotary speed of the output shaft of said high speed turning
motor exceeds the rotary speed of said shaft of said second gear worm, the high
speed turning motor takes control over the second worm gear.
20
14. Apparatus according to one or more of the preceding claims, wherein
said dual-speed turning gear is arranged and controlled to drive the apparatus during
offline washing of the gas turbine by initially accelerating said load, said low
pressure compressor and said power turbine from a stationary condition to a first
rotary speed with said low speed turning motor, and subsequently by further
accelerating said load, said low pressure compressor and said power turbine from
said first rotary speed to a second rotary speed, higher than said first rotary speed.
15. Apparatus according to one or more of the preceding claims, wherein
said slow speed turning motor and said high speed turning motor are arranged and
^ ^ controlled so that during slow-rolling of the load, rotation of said load is controlled
^ ^ by said low speed turning motor.
16. Apparatus according to one or more of the preceding claims, wherein
an auxiliary turning motor is provided for driving into rotation said high pressure
compressor and said high pressure turbine, said auxiliary turning motor being
arranged and controlled to operate as a slow-rolling motor for slow-turning said high
pressure compressor and said high pressure turbine following shut down of the gas
turbine, and/or to operate as a starter for starting rotation of said high pressure
compressor and said high pressure turbine when the gas turbine is started.
17. A method of offline washing a multiple shaft gas turbine connected to
a load, said gas turbine comprising: a high pressure compressor and a high pressure
turbine drivingly connected to one another by a first gas turbine shaft; a low pressure
^m compressor and a power turbine drivingly connected to one another by a second gas
turbine shaft, extending coaxial with said first gas turbine shaft, said high pressure
compressor and said high pressure turbine; said method comprising the steps of:
providing a dual-speed turning gear with an output shaft drivingly engageable to
and disengageable from said load, wherein said dual-speed turning gear
comprises a low speed turning motor and a high speed turning motor;
initially accelerating said low pressure compressor, said power turbine and said
load with said low speed turning motor up to a first rotary speed;
when said first rotary speed is reached, continuing accelerating said low pressure
compressor, said power turbine and said load with said high speed turning
motor until an offline washing rotary speed is reached;
21
maintaining said offline washing rotary speed while washing said gas turbine.
18. Method according to claim 16, further comprising the step of
accelerating said high pressure compressor and said high pressure turbine by means
of an auxiliary turning motor.
19. Method according to claim 16 or 18, further comprising the steps of:
shutting down said gas turbine;
slow-rolling said load, said low pressure compressor and said power turbine by
means of said slow turning motor, until a required temperature profile of said
load is achieved.
20. Method according to one or more of claims 17 to 19, wherein said
load comprises at least one compressor.
21. Method according to one or more of claims 17 to 20, wherein said
load comprises a train of compressors.
22. Method according to any one of claims 17 to 21, comprising the step
of slow-rolling the high pressure turbine and the high pressure compressor of said
gas turbine by means of an auxiliary turning motor after shut-down of the gas
turbine.
23. A method of slow-rolling a multi-shaft gas turbine and a load
coimected thereto, said gas turbine comprising: a high pressure compressor and a
^ ^ high pressure turbine drivingly connected to one another by a first gas turbine shaft;
a low pressure compressor and a power turbine drivingly connected to one another
by a second gas turbine shaft, extending coaxial with said first gas turbine shaft, said
high pressure compressor and said high-pressure turbine; said method comprising the
steps of:
providing a dual-speed turning gear with an output shaft drivingly engageable to
and disengageable fi-om said load, wherein said dual-speed turning gear
comprises a low speed turning motor and a high speed turning motor;
selectively slow-rolling said load and said second gas turbine shaft at a slowrolling
speed with said slow speed turning motor during cooling of said gas
turbine and said load following shut-down of said gas turbine; or
22
rolling said load and said second gas turbine shaft at an offline washing speed,
with said high speed turning motor during offline washing of said gas turbine,
said offline washing speed being higher than said slow-rolling speed.
24. Method according to claim 23, comprising slow-rolling said high
pressure compressor and said high pressure turbine with an auxiliary turning motor
following shut-down of said gas turbine.