Abstract:
The subject invention involves a compressor starting torque converter (CSTC) to enable starting up single-shaft gas turbine driven compressor sets, such as in a liquified natural gas (LNG) plant. The system includes a single-shaft gas turbine with an output shaft coaxially connected to the input shaft of the CSTC&#39;s torque converter, in turn whose output shaft drives the process load centrifugal compressor. The CSTC operates as a fluid coupling whereby the gas turbine and the compressor shafts are coupled via the working fluid (oil). Once the process load compressor is up to speed, a lock up device is engaged to mechanically couple the gas turbine directly to the compressor and then the torque converter is drained. The impeller and turbine wheel of the CSTC are undersized with regard to the maximum shaft power requirement of the compressor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for starting single shaft gas turbine driven or electric motor driven compressor sets, and in a further embodiment relates to methods and apparatus for starting large single shaft gas turbine driven compressor sets of greater than 43,700 horsepower. 
     BACKGROUND OF THE INVENTION 
     Present compressor plants, such as those used in liquified natural gas (LNG) plants, use either the smaller two-shaft gas turbine driver (which has no driver compressor starting problems) or the larger, more cost-effective single-shaft gas turbine driver with its presently-associated complex compressor starting method and apparatus. While two-shaft gas turbine drivers are suitable for starting compressors, they are commercially unavailable in sizes greater than 43,700 horsepower. There is a need for a device which can efficiently start single-shaft gas turbine driven centrifugal compressor sets in processes requiring total power up to 350,000 horsepower and more. 
     Large single-shaft gas turbines have a standard starting system that at most can only start up the gas turbine driver itself and an unloaded, connected electric generator. A specific problem with the single-shaft gas turbine is that everything is connected mechanically on a single common shaft, hence the starting device must start up not only the gas turbine itself, but also the connected load (for example, an electric generator or centrifugal compressor). Everything must be started up and accelerated simultaneously from rest continuously up to full speed. The added load produced by an electric generator in an unloaded state, that is, not connected to the electrical power grid during starting, can be handled by a single-shaft gas turbine&#39;s standard starting system, but not the inertia and aerodynamic loads associated with a large centrifugal compressor. 
     For starting up large, single-shaft gas turbine compressor sets, an additional starting device (an electric motor or steam turbine helper driver or engine) is typically added at the outboard free end of the driven compressor(s). This starting device can either be a steam turbine driver or an electric motor driver, which typically includes a Variable Frequency Electrical Drive system (VFD) to provide variable speed. The electric motor driver requires an external source of electric current, while the steam turbine requires an external source of steam, hence both starting device types are not independent or stand-alone devices. Because both of these systems, in turn, have many sub-systems, they are costly, complex, and are very maintenance intensive. 
     Further, many times because of a remote location, a plant does not have a readily available nearby electrical grid or a steam system. Steam systems require a large source of water. In addition, it is generally more economical to utilize air instead of water for cooling. 
     It is desirable to provide a method and apparatus to start up large loads, such as centrifugal compressors, axial compressors and the like, which is simpler, lower in cost, and requires less maintenance than the systems currently available. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a less costly, less complex, less maintenance-intensive, more reliable and more efficient device and method utilizing proven equipment for starting up compressors and specifically single-shaft, gas turbine-driven or electric motor-driven compressor sets. 
     An additional object of the invention is to provide a device and method that can be used to start up centrifugal compressors, axial compressors, and combinations thereof 
     Another object of the present invention is to allow the more cost-effective single-shaft gas turbine to be used more readily for gas turbine compressor drivers. 
     In carrying out these and other objects of the invention, there is provided, in one form, a power transmission system for driving at least one compressor. The system has a driver, where one end of the shaft is the driver output shaft. A compressor starting torque converter (CSTC) is also present which has a pump impeller on an input shaft and a turbine wheel on an output shaft, where the CSTC input shaft is coaxially connected to the driver output shaft. Further, there is a compressor with an input shaft coaxially connected to the CSTC output shaft. The CSTC further comprises a lock-up device between the pump impeller and the turbine wheel to lock the impeller and the turbine wheel together. The driver may be a single shaft gas turbine or an electric motor. The compressor may be a centrifugal compressor, an axial compressor or a combination thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one example of a typical power transmission system that would be associated with the invention; 
     FIG. 2 is a schematic representation of a power transmission system of the prior art showing a steam turbine or electric motor driver; and 
     FIG. 3 is a cross-sectional detail of the lock-up device in the inventive compressor starting torque converter of the invention. 
     It will be appreciated that the Figures are not to scale or proportion as it is simply a schematic for illustration purposes. Even in the schematic, the compressor starting torque converter (CSTC) is oversized with respect to the other components of the system, to show detail. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention herein employs an improved starting method for starting compressors and specifically compressor trains driven by a single shaft gas turbine machine, such as are used in liquified natural gas (LNG) plants, namely, using a compressor starting torque converter (CSTC) located between the gas turbine output shaft and the compressor input shaft. Preferably the compressor is either a centrifugal compressor, an axial compressor or a combination of the two. Most preferably the compressor is a centrifugal compressor. 
     Referring to FIG. 1 in general terms, the CSTC  20  consists of a torque converter  21  (which can be drained of its working oil  30 ) and a lock-up device  40  that mechanically connects the gas turbine&#39;s output shaft  14  to the driven compressor&#39;s input shaft  26 . Briefly, there are two operating modes for the CSTC namely (i) “Start-Up” and (ii) “Normal Operation”. “Start-Up” involves the filled torque converter  21  being in operation while in “Normal Operation”, the torque converter  21  is drained and out of operation while the lock-up device  40  is engaged mechanically connecting the gas turbine  12  directly to the compressor  18  (and not via the fluid  30  in the torque converter  21  as in “Start-Up”). Briefly, in use, oil  30  is first drained from the CSTC&#39;s torque converter  21  to uncouple the driven centrifugal compressor  18  from the gas turbine  12 , and then the gas turbine  12  is started conventionally. With the gas turbine  12  at minimum governing speed, the CSTC  20  is re-filled and then the centrifugal compressor  18  is started by increasing the power output from the gas turbine  12 . Once the centrifugal compressor  18  is up to a speed which closely matches the gas turbine  12  speed, a lock-up device  40  is activated mechanically connecting the gas turbine to the centrifugal compressor  18 . 
     While the invention is expected to find utility in starting up compressors for LNG plants and other closed refrigeration cycles, particularly propane refrigeration cycles, it is expected that the invention will find utility in starting and driving compressors, preferably centrifugal and/or axial compressors, in other type processes, utilizing single-shaft gas turbine drivers or electric motors. 
     The invention will be described more specifically with reference to FIG.  1 . Shown in FIG. 1 is a power transmission system  10 , having a single shaft gas turbine (GT)  12 , a CSTC  20  and at least one compressor  18  with their respective shafts in coaxial alignment. A conventional gearbox  16  may be present between the output shaft  14  of the single shaft gas turbine  12  and the input shaft  22  of the CSTC  20  only if the speed of turbine  12  and required speed of compressor  18  do not match. 
     The CSTC  20  is an enclosed mechanical device consisting of an input shaft  22  having centrifugal pump impeller  24  thereon and an independent output shaft  26  having an associated turbine wheel  28  thereon. The working medium  30  is oil or other suitable hydraulic fluid, and when the centrifugal pump impeller  24  is rotated by having energy put into it from the gas turbine  12 , the turbine wheel  28  rotates, that is, energy is transmitted out of CSTC  20  via output shaft  26 . In other words, the mechanical energy input to the input shaft  22  and centrifugal pump impeller  24  is first transformed into hydraulic energy by the pump impeller  24 , and then converted from hydraulic energy back into mechanical energy by the turbine wheel  28  and delivered out of CSTC  20  by output shaft  26 , which in FIG. 1 is identical to the input shaft of compressor  18 . The arrows in FIG. 1 show the circulation path of the oil  30 . 
     Two shafts ( 22  and  26 ) hydraulically coupled by a fluid (oil  30 ) is defined as a fluid coupling, while a fluid coupling with guide vanes  32  in the fluid  30  circulation path is defined as a torque converter, a well-known machine perhaps most commonly encountered in the automatic transmission of automobiles. The orientation of the guide vanes  32  help determine the torque amplification produced by the CSTC  20 . 
     A torque converter has a characteristic torque output (or twisting capacity) curve of maximum torque output at zero speed, with decreasing torque output at increasing speed. This is ideal because the driven compressor  18  has the exact opposite torque input requirements, and a characteristic curve of zero torque at zero speed and increasing torque with increasing speed. The excess available output torque of a torque converter at low speeds over the small input torque required by the compressor results in the ability of the gas turbine driver  12  to accelerate the large mass and inertia of the driven compressor(s)  18  from rest up to speed. 
     A unique feature of the CSTC  20  is the ability to drain and refill the unit of its operating fluid  30 , which is the media used for transmitting energy and the transformation of energy. The mechanical shaft energy input into the CSTC  20  is transmitted by and transformed by the fluid  30  in the unit  20  into mechanical shaft output energy, when the lock up device  40  is not activated. By draining the fluid from the CSTC  20 , the input shaft  22  and output shaft  26  are physically decoupled, meaning that there is no connection whatsoever between the input shaft  22  with the output shaft  26 . 
     The basic operating principle behind the CSTC  20  (as used in the power transmission system  10  shown) is to first drain the CSTC  20  of its fluid  30 , and allow the single shaft gas turbine  12 &#39;s standard starting device  34  to start the gas turbine  12  itself Once operational, the starting device  34  is dropped out of operation and the gas turbine  12  brings itself up to minimum governing speed as it normally would if it were connected to an unloaded electrical generator (not shown). Conventional starting device  34  typically drops out and is disconnected once the gas turbine  12  is up to about 40% of operating speed. Such starting devices  34  or motors typically produce approximately 300 to 1,000 hp and greater. However in the case of the invention, the job is easier as there is no connected electrical generator or other load. Once the gas turbine  12  is up to speed and ready to accept load, the CSTC  20  is re-filled and the compressor(s)  18  brought up to speed by the gas turbine driver  12  through the CSTC  20  in a timed manner. 
     In other words, the fundamental principle is a device  20  which allows the driven compressor(s)  18  to be isolated (de-coupled) from the gas turbine driver  12 , allowing the gas turbine&#39;s standard starter system  34  to start and bring the gas turbine  12  itself up to self-sustaining speed, without the driven compressor  18 . When the gas turbine  12  is operational, up to minimum governing speed and ready to be loaded, the gas turbine driver  12 , through the CSTC  20 , starts up the compressor  18 . Although torque converter  21  operation during start-up is inefficient, it is quite short term. Once locked-up, as during normal operation, the torque converter  21  is no longer in operation. An additional advantage of the invention is that start-up time is reduced significantly, as compared with prior art processes. Starting up the compressor  18  using the procedure of this invention is estimated to take from about 0.5 to about 2.0 minutes. 
     Although normally a single-shaft gas turbine  12  cannot start up both itself and a centrifugal compressor  18  from rest, it can start the compressor  18  once the turbine  12  itself is operational and up to speed. The CSTC  20  makes this possible. In essence, there is a two-phase start-up. First, the gas turbine  12  itself is started, then the gas turbine  12  starts the compressor  18  by means of the CSTC  20 . 
     Once the compressor(s)  18  are up to speed, or output shaft  26  (compressor input shaft) speed at least closely matches (to within 2% of its speed) CSTC  20  input shaft  22  speed, lock up device  40  is engaged to physically connect gas turbine  12  to centrifugal compressor  18 . Lock up device  40 , shown in more detail in FIG. 3, has a circular, one piece lock-up ring  52  having teeth on the inside and outside thereof, and is mechanically bolted to a circular, single piece engagement piston  54 . Hydraulic pressure moves the locking ring  52 /engagement piston  54  towards the centrifugal pump impeller  24  (to the left in FIGS. 1 and 3) to physically connect pump impeller  24  with turbine wheel  28 . The hydraulic fluid to move locking ring  52 /engagement piston  54  is admitted through engagement piston hydraulic fluid inlet  56 . The CSTC  20  thus becomes a mechanical connection device or gear coupling with extremely low power transmission loss. In one non-limiting embodiment, the transmission efficiency would be approximately 99.5% in lock-up mode (approximately 0.5% power loss). This is the “Normal Operation” Mode for the gas turbine /CSTC/compressor unit of system  10  and in this mode, the compressor  18  would inherit the speed variation capability of the gas turbine  12 . 
     The use of a CSTC  20  as a single-shaft gas turbine starting device in the manner of this invention would not occur to one of ordinary skill in the art because the horsepower rating of the CSTC at start-up is limited to the horsepower handling capability of the torque converter  21  itself. This torque converter  21  horsepower handling capability is much less than the horsepower transmitting capability of the CSTC  20  in the locked-up mode. That is, when faced with the problem of starting up a machine such as a compressor that may be as large as 50,000 horsepower, one would not normally think of a torque converter, since it is rated much less, and would ordinarily be thought of as unsuitable. However, as has been established, in a CSTC  20  with a lock-up device  40 , the torque converter does not have to be rated at the full power of the complete system but only for the lower gradual start-up power requirement in the method of the invention. In essence, the CSTC in the locked-up “Normal Operation” mode—with the torque converter drained and out of operation handles full power. In the “Start-up” mode, the torque converter—while filled—handles the smaller compressor starting power requirement (typically approximately 25% of full power). 
     Also shown in FIG. 1 is compressor discharge line  42 , high stage flash gas stream  44 , intermediate stage flash gas stream  46 , and low stage flash gas stream  48  returning back to compressor  18 . On existing processes and apparatus, as shown in prior art FIG. 2, if the start-up motor or stream turbine driver  36  (located at the free, non-driven shaft end of compressor  18 ) is not large enough, the gas pressure inside compressor  18  has to be reduced, typically by venting it, sometimes down to almost a vacuum, in order to get compressor  18  started. With the present invention, the compressor  18  does not have to be vented thus avoiding losing a valuable gas like propane to the atmosphere. 
     If the CSTC  20  or other equipment is not powerful enough to start compressor  18 , one option is to add a small auxiliary compressor  50  parallel to and in the compressor discharge line  42  to draw down the pressure somewhat. Lowering the pressure in the process loop lowers compressor  18  start-up power requirements. In one non-limiting embodiment, auxiliary compressor  50  is envisioned to be a screw compressor. 
     An advantage of the CSTC of the invention is that it is a simple, independent, self-sufficient, stand-alone mechanical device that is not a driver or an engine, and thus does not have the complexities and costs associated with a driver or engine nor requires an external power source. While by itself, the CSTC  20  is not a starting device, when skillfully used in conjunction with a single-shaft gas turbine  12 , it allows the single-shaft gas turbine  12  itself to act as the driver to start up the driven compressor(s)  18 , a feat a single-shaft gas turbine normally cannot do. In other words, another complicated driver  36  is not required to start up the total gas turbine  12 /compressor  18  system. 
     The invention permits single-shaft gas turbine drivers to be readily used instead of steam turbine drivers. Steam turbine drivers do not have centrifugal compressor start-up problems because its high pressure motive fluid (steam) is available (from a separate source) before compressor start-up while in a single-shaft gas turbine no hot motive fluid (gas from the gas turbine itself) is available until the gas turbine is started and is up and running. Gas turbines are a less costly alternative to steam turbine drivers because no steam generation equipment is needed and the cooling system is smaller. Gas turbines have a higher thermal efficiency than steam turbines, are more compact, more reliable, and offer a faster start-up. Further, the time taken to manufacture and install a gas turbine is shorter than for a comparable steam turbine system. 
     In another embodiment, the CSTC also can eliminate costly, special and complex motors with variable frequency drives for future motor-driven integrally-geared LNG compressors. The CSTC will allow standard, less costly and more reliable fixed speed electric motors to be used. The main drive electric motor can be started up unloaded by using a small electric motor starter with the CSTC drained mechanically isolating the main drive motor from the compressor. After the main drive electric motor is brought up unloaded to full speed and then connected to the electric grid, the CSTC can be filled and the compressor started. Although little or no speed control is available with the CSTC in this arrangement, compressor capacity control can still be obtained, for example, by adding compressor inlet guide vanes (a standard option on integrally-geared compressors employed in LNG liquefaction processes and on some normal centrifugal compressors). 
     An additional advantage of the invention is that if there is a process trip or problem, the invention permits only the compressor  18  or other process to cease without necessarily tripping the gas turbine  12 . The procedure would be to first unload the compressor until the lock-up device can be disengaged and allow the compressor to coast to a standstill while the gas turbine is kept operational but at minimal speed. Stopping and re-starting a gas turbine is a complicated, lengthy process one tries to avoid. 
     A related advantage of the invention is ease of maintenance. In the present invention, the compressor  18 , as one example, is automatically decoupled from the gas turbine  12  upon draining of the torque converter and the compressor can then be serviced with no fear of physical danger to repair personnel and surrounding equipment. 
     Starting-up a gas turbine is a complex, detailed procedure as it is by nature a minipower plant which runs hot hence rate of temperature rise throughout the machine must be regulated and controlled in a very strict manner. Thus starting-up, shutting down and restarting starting of a gas turbine is to be avoided or at least minimized as much as possible. The absolute best operating condition for a gas turbine is continuous operation as starting and stopping shortens its life and increases the required maintenance. In the present state of the art, if any component causes a problem during starting, the gas turbine most probably has to be stopped and restarted. With the CSTC  20 , the gas turbine is isolated from the compressor that allows, for example, the gas turbine to be started while the compressor and the process is being readied. The gas turbine once started can be idled until the compressor and process are ready to be started. If after normal operation begins, problems occur for example with the compressor and/or the process during starting, the gas turbine does not need to be stopped and instead the CSTC  20  simply has to be drained which isolates the gas turbine. The gas turbine can be kept idling while the problem(s) in the compressor and/or process are worked out. 
     Further, the apparatus and controllability method of the invention is simpler than prior art start up devices and procedures, since the coupling of the compressor  18  to the gas turbine  12  can be made very quickly by draining or filling the CSTC  20  of its working oil  30 . 
     It is also possible using the invention apparatus and method to slow roll the compressor  18  before start-up, which may be desired in some start-up sequences, such as to avoid potential vibration in compressors with long shafts. Slow rolling a single-shaft gas turbine—for example for washing the gas turbine during routine maintenance—by means of its normal starting device  34  will be simpler, quicker and easier with the torque converter  21  drained. This is because only the single-shaft gas turbine itself has to be rotated since the compressor is not connected. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective in providing apparatus and procedures for starting up loads, particularly compressors. However, it will be evident that various modifications and changes can be made thereto without departing from the broader spirit or scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, there may be other ways of configuring and/or operating the equipment differently from those explicitly described and shown herein which nevertheless fall within the scope of the claims. In an additional instance, although the invention is directed towards starting up single-shaft gas turbine driven centrifugal compressors, it will be appreciated that it could also be used for starting up single-shaft gas turbine driven axial compressors, electric motor driven centrifugal compressors and electric motor driven axial compressors.