Abstract:
An improved system and methodology for starting up a gas-turbine driven multi-stage compressor. The improvement involves isolating individual compression stages and creating positive pressure in each stage prior to initiating rotation of the compressor/driver system. The isolation of individual compression stages allows the turbine to reach normal operating speeds with substantially no supplemental power from an auxiliary source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to turbine-driven multi-stage compressors. In another aspect, the invention concerns an improved methodology for starting up a multi-stage compressor driven by a single-shaft gas turbine. 
     2. Description of the Prior Art 
     Gas turbines are commonly used to drive large, industrial compressors, such as those employed in the refrigeration cycles of liquefied natural gas (LNG) facilities. Gas turbines used to drive large compressors generally have a single-shaft or a split-shaft configuration. Compressor systems driven by split-shaft gas turbines are typically easier to start-up, but single-shaft gas turbines are available in higher power ratings. Generally, split-shaft gas turbines either are not commercially available or are not economically viable for use in very high load applications, such as for driving the multi-stage compressors of an LNG facility. Therefore, single-shaft gas turbines are usually selected to drive very large multi-stage compressors in industrial applications. 
     One disadvantage associated with employing a single-shaft gas turbine to drive a large, multi-stage compressor is the requirement for auxiliary power to help start-up the compressor/turbine system. In the past, such auxiliary start-up power has typically been provided by electric motors. These auxiliary motors run at or near full capacity during start-up to help overcome the inertial and aerodynamic forces of the system. After start-up, the auxiliary motor is shut off or scaled back, as the gas turbine takes over primary responsibility for powering the system. Obviously, the requirement for an auxiliary source of rotational power during start-up adds to the overall capital expense of the system. 
     Another disadvantage of using a single-shaft gas turbine to drive a large, multi-stage compressor is the potential for creating a vacuum in the system upon start-up, which creates a mechanism for air ingress into the system. While manageable, air-contamination of the working fluid is highly undesirable and can present additional operational and/or safety problems. 
     Thus, a need exists for an improved system and methodology to efficiently start-up large, industrial multi-stage compressors. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, there is provided a method of operating a multi-stage compressor. The method comprises: (a) isolating at least two compression stages of the multi-stage compressor from fluid flow communication with one another; and (b) simultaneously with step (a), initiating rotation of the multi-stage compressor. 
     In another embodiment of the present invention, there is provided a system for operating a multi-stage compressor having a plurality of compression stages with each compression stage having an inlet and an outlet. The system comprises a driver for rotating the multi-stage compressor, a plurality of flow loops, and an isolation valve fluidly disposed between two of the flow loops. Each of the flow loops is associated with a compression stage and is configured to provide fluid flow communication from the outlet to the inlet of the compression stage with which it is associated. The system is shiftable between a start-up mode and an operating mode. During the start-up mode, the isolation valve is closed to thereby prevent fluid flow between two of the flow loops. During the normal mode of operation, the isolation valve is open to thereby permit fluid flow between two of the flow loops. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Certain embodiments of the present invention are described in detail below with reference to the enclosed figures, wherein: 
         FIG. 1  is a schematic view of a compressor/driver system that includes a three-stage compressor driven by a single-shaft gas turbine; and 
         FIG. 2  is a flowchart of steps involved in the start-up of the compressor/driver system illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIG. 1 , a simplified compressor/driver system  10  is illustrated as generally comprising a gas turbine  12 , a multi-stage compressor  14 , and a compressor flow control system  16 . In general, gas turbine  12  powers multi-stage compressor  14 , while flow control system  16  directs the flow of gas through the stages of multi-stage compressor  14 . 
     Gas turbine  12  can be any suitable commercially available industrial gas turbine. In one embodiment, gas turbine  12  is a single-shaft gas turbine having a power rating greater than about 35,000 hp, greater than about 45,000 hp, or greater than 55,000 hp. For example, gas turbine  12  can be a single-shaft GE Frame-5, Frame-6, Frame-7, or Frame-9 gas turbine available from GE Power Systems, Atlanta, Ga. or the equivalent thereof. Gas turbine  12  receives a stream of filtered air from conduit  13  and fuel via conduit  15  as controlled by valve  19 . The combustion of the air and fuel provides energy to rotate gas turbine  12 . According to one embodiment, gas turbine  12  additionally comprises a built-in starting device (not shown) coupled to the air compressor side (i.e., the “cold end”) of gas turbine  12 . 
     Gas turbine  12  is operably coupled to multi-stage compressor  14  by a single common output drive shaft  18 . Multi-stage compressor  14  comprises a plurality of compression stages operable to sequentially compress a gas stream to successively higher pressures. Compressor  14  of  FIG. 1  is illustrated as having three compression stages: a low compression stage  20 , an intermediate compression stage  22 , and a high compression stage  24 . Multi-stage compressor  14  can be a centrifugal compressor, an axial compressor, or any combination thereof. In the embodiment shown in  FIG. 1 , compressor  14  is a three-stage centrifugal compressor. 
     As previously mentioned, the compressor/driver system  10  includes compressor flow control system  16  that is operable to direct the flow of gas associated with multi-stage compressor  14 . As illustrated in  FIG. 1 , flow control system  16  includes a plurality of flow loops  26 ,  28 ,  30 , each associated with a respective compressor stage  20 ,  22 ,  24  of multi-stage compressor  14 . Each flow loop is operable to provide a path of fluid flow from the outlet of its associated compression stage to the inlet of the same compression stage. For example, low-stage flow loop  26  is operable to route compressed gas from the discharge of low compression stage  20  to its suction via discharge conduit  32 , intercooler  34 , recycle conduit  36 , anti-surge valve  38 , and suction conduit  40 . Intermediate-stage flow loop  28  is operable to route compressed gas from the discharge of intermediate compression stage  22  to its suction via discharge conduit  42 , intercooler  44 , recycle conduit  46 , anti-surge valve  48 , and suction conduit  50 . High-stage flow loop  30  is operable to route compressed gas from the discharge to the suction of high compression stage  24  via discharge conduit  52 , intercooler  54 , recycle conduit  56 , anti-surge valve  58 , and suction conduit  60 . 
     Compressor/driver system  10  of the present invention can be operated in two distinct modes: a start-up mode and a normal mode. During the normal mode of operation, flow loops  26 ,  28 ,  30  are in fluid flow communication with each other. As discussed in detail below, the start-up mode of operation is characterized by the isolation of flow loops  26 ,  28 ,  30  from fluid flow communication with each other. In one embodiment, fluid flow communication between flow loops  26 ,  28 ,  30  is controlled with a first isolation system  62  and a second isolation system  64 . First isolation system  62  generally includes a first conduit  66 , a first isolation valve  68 , and a first bypass valve  70 . Similarly, second isolation system  64  generally includes a second conduit  72 , a second isolation valve  74 , and a second bypass valve  76 . To allow fluid flow communication between flow loops  26 ,  28 ,  30 , isolation valves  68 ,  74  and/or bypass valves  70 ,  76  are open to thereby allow compressed gas to flow between the low, intermediate, and high compression stages  20 ,  22 ,  94 . When fluid flow communication is allowed between the compression stages  20 ,  22 ,  24  of multi-stage compressor  14 , flow loops  26 ,  28 ,  30  are said to be “de-isolated.” When flow loops  26 ,  28 ,  30  are de-isolated (i.e., during normal mode of operation), compressed gas flows from the outlet of low compression stage  20  into the suction of intermediate compression stage  22  and from the discharge of intermediate compression stage  22  to the suction of high compression stage  24 . To isolate flow loops  26 ,  28 ,  30  by preventing fluid flow communication between low, intermediate, and high compression stages  20 ,  22 ,  24 , isolation valves  68 ,  74  and bypass valves  70 ,  76  are closed. The methodology of starting up compressor/driver system  10  will be discussed in further detail in a subsequent section. 
     According to the embodiment illustrated in  FIG. 1 , compressor flow control system  16  can additionally comprise a start-up gas system  78 , which is operable to control the flow of start-up gas to and from compression stages  20 ,  22 ,  24  and flow loops  26 ,  28 ,  30 . Start-up gas system  78  generally includes a start-up gas source  80  in fluid communication with low-, intermediate-, and high-stage flow loops  26 ,  28 ,  30  by respective start-up gas injection conduits  82 ,  84 ,  86 . Each start-up gas conduit includes a respective start-up gas injection valve  90 ,  92 ,  94  to control the flow of the start-up gas from start-up gas source  80  to flow loops  26 ,  28 ,  30 . In addition, each flow loop  26 ,  28 ,  30  can additionally include a respective purge valve  96 ,  98 ,  100  to vent gas from the system as needed. During normal operation mode, start-up gas injection valves  90 ,  92 ,  94  and purge valves  96 ,  98 ,  100  are typically closed. As detailed in a subsequent section, these valves can either be open or closed during start-up to establish positive pressure in flow loops  26 ,  28 ,  30  and compression stages  20 ,  22 ,  24 . 
     As illustrated in  FIG. 1 , compressor flow control system  16  also includes a working fluid inlet conduit  102  having disposed therein an inlet control valve  104  and a working fluid outlet conduit  106  in fluid communication with an outlet control valve  108 . During normal operation, control valves  102 ,  108  are generally open to allow flow of the working fluid into and out of multi-stage compressor  14  and its associated flow loops  26 ,  28 ,  30 . As discussed below in further detail, control valves  104 ,  108  can be closed during start-up mode of operation in order to isolate low compression stage  20  and high compression stage  24  from the inlet and outlet  102 ,  106  working fluid conduits and other respective upstream and downstream processing equipment. 
     In another embodiment, compressor flow control system  16  can also include one or more intermediate-stage and/or high-stage feed streams (not shown). If present, these additional feed streams combines with the discharged gas from the upstream compression stage prior to entering the compression stage with which it is associated. 
     The start-up mode of operation of the compressor/driver system  10  illustrated in  FIG. 1  will now be described in detail with reference to the flow chart provided in  FIG. 2  and the valve position summary represented in Table 1 below. 
                                                                                                                       TABLE 1                   Valve Position Summary During Start-Up Mode            Valve       Block (FIG. 2)            (FIG. 1)   Function   200   204   206   208   212   214   216   218                    19   Fuel to Gas Turbine   C   C   C   O   O   O   O   O       38   Low-stage Anti-Surge   O   O   O   O   O   O   O   OAC       48   Intermediate-stage Anti-Surge   O   O   O   O   O   O   O   OAC       58   High-stage Anti-Surge   O   O   O   O   O   O   O   OAC       68   First Isolation   C   C   C   C   C   C   O   O       70   First Bypass   C   C   C   C   C   C   O   C       74   Second Isolation   C   C   C   C   C   C   O   O       76   Second Bypass   C   C   C   C   C   C   O   C       90   Low-stage Start-up Gas   C   C   O   C   O   C   C   C       92   Intermediate-stage Start-up Gas   C   C   O   C   O   C   C   C       94   High-stage Start-up Gas   C   C   O   C   O   C   C   C       96   Low-stage Purge   C   O   C   C   C   C   C   C       98   Intermediate-stage Purge   C   O   C   C   C   C   C   C       100   High-stage Purge   C   O   C   C   C   C   C   C       104   Working Fluid Inlet   C   C   C   C   C   C   C   O       108   Working Fluid Outlet   C   C   C   C   C   C   C   O               Valve Positions:       Open (O),       Closed (C), or       Open, Automatic Control (OAC)            
In particular,  FIG. 2  outlines the major steps involved in starting up the compressor/driver system  10  and Table 1 summarizes the positions of each valve shown in  FIG. 1  as described above during the start-up and normal modes of operation.
 
     As previously discussed, the start-up mode of compressor/driver system  10  in  FIG. 1  is characterized by the isolation of flow loops  26 ,  28 ,  30  from fluid flow communication with each other as regulated by first and second isolation systems  62 ,  64 . Thus, the first step to start-up compressor/driver system  10  is to isolate each flow loop, as depicted by block  200  in  FIG. 2 . As shown in Table 1, this requires that first and second isolation valves  68 ,  74 ; first and second bypass valves  70 ,  76 ; working fluid inlet valve  104 ; and working fluid outlet valve  108  be closed to thereby prevent fluid flow between flow loops  26 ,  28 ,  30 , compression stages  20 ,  22 ,  24 , and the working fluid entering and discharged from multi-stage compressor  14  via conduits  102  and  108 , respectively, as illustrated in  FIG. 1 . In addition, during this step, purge valves  96 ,  98 ,  100  and start-up gas valves  90 ,  92 ,  94  are also closed. Anti-surge valves  38 ,  48 ,  58  are opened in order to create a pathway for compressed gas to ultimately flow in a closed isolated flow loop during a subsequent stage of the start-up mode, as described in more detail shortly. At this point, gas turbine  12  may not be rotating, and fuel valve  19  may be closed. As used herein, the term “closed” refers to a valve that is greater than 75 percent, greater than 85 percent, greater than 95 percent, or greater than 99 percent closed. 
     Once flow loops  26 ,  28 ,  30  have been isolated, a positive pressure can be established in each flow loop as represented in block  202  of  FIG. 2 . In one embodiment, the positive pressure of flow loops  26 ,  28 ,  30  can be in the range of from about 0.5 to about 50 pounds-per-square-inch, gauge (psig), about 0.75 to about 25 psig, or 1 to about 20 psig. To adjust the positive pressure in one or more flow loops, gas may be added or removed from the isolated loops as needed. If the pressure in a flow loop is too high, excess gas may be purged from the system by a purge valve. For example, if the positive pressure in intermediate compression stage  22  is too high, excess vapor can be vented, as shown by block  204  in  FIG. 2 , to a hydrocarbon flare system or routed to the low-stage suction of another compressor by opening purge valve  98 , as illustrated in Table 1. Similarly, opening purge valves  96 ,  100 , as shown in Table 1, can reduce the positive pressure in the low and high compression stages  20  and  24 , respectively. 
     If the positive pressure in a flow loop is too low, additional gas may be introduced into the system, as shown in block  206  in  FIG. 2 , by start-up gas system  78  illustrated in  FIG. 1 . Start-up gas source  80  may be any internal or external source capable of delivering gas into flow loops  26 ,  28 ,  30  while maintaining their respective positive pressures. In one embodiment, start-tip gas can be a hydrocarbon-containing gas. Generally, start-up gas is introduced into low, intermediate, and/or high compression stage  20 ,  22 ,  24  as needed by opening respective start-up gas injection valves  90 ,  92 ,  94 , as shown in Table 1. In one embodiment, start-up gas may be used as a purge gas to remove existing material from one or more flow loops prior to establishing positive pressure. 
     Because flow loops  26 ,  28 ,  30  remain isolated (as shown in Table 1) during the steps depicted in blocks  200 ,  204 , and  206  in  FIG. 2 , it is possible to alter the positive pressure in one or more individual flow loops without affecting the pressure in other flow loops. In one embodiment, the positive pressure in one or more flow loops may be within about 50 percent, about 75 percent, about 90 percent, or 95 percent of the positive pressure in another flow loop. In another embodiment, the positive pressures in each flow loop are substantially equal. 
     The next step in the start-up mode of compressor/driver system  10  is to initiate compressor/driver system rotation as outlined in block  208  in  FIG. 2 . In one embodiment, compressor/driver system  10  illustrated in  FIG. 1  additionally comprises an optional auxiliary motor  21  coupled to the output drive shaft  18  on the outboard end of low compression stage  20  to provide supplemental power to rotate gas turbine  12  during this phase of the start-up method. In accordance with one embodiment of the present invention, the optional auxiliary motor provides less than about 50 percent, less than about 30 percent, less than about 20 percent, less than about 10 percent, or less than 5 percent of the total power required to initiate rotation of compressor/driver system  10 . 
     In another embodiment, the rotation of compressor/driver system  10  is initiated solely under the power of gas turbine  12  and its built-in starting device (not shown). As illustrated in Table 1, fuel valve  19  can be opened during this step and gas turbine  12  may be started. 
     Once rotation has been initiated, the system can be checked to ensure a minimum positive pressure has been maintained, as illustrated in block  210  in  FIG. 2 . If the positive pressure is too low, additional start-up gas may be introduced into the system, as represented by block  212 , by means of start-up gas system  78  illustrated in  FIG. 1 , as previously described. As shown in Table 1, start-up gas may be introduced into low, intermediate, and/or high compression stage  20 ,  22 ,  24  by opening start-up gas injection valves  90 ,  92 ,  94  respectively. 
     Once an adequate positive pressure has been reestablished, the compressor/driver system  10  can then be allowed to achieve minimum rotational speed, as shown in block  214  of  FIG. 2 . As illustrated by the valve positions in shown in Table 1, the flow loop  26 ,  28 ,  30  remain isolated and, as the rotational speed of compressor/driver system  10  is increased to a minimum rotational speed, compressed gas discharged from each compression stage can be circulated back to its suction via its recycle conduit and anti-surge valve, as described previously. The minimum rotational speed of the compressor/driver system  10  depends on several factors, including the turbine size, compressor size and configuration, and the like. In one embodiment, the minimum rotational speed is at least about 500 revolutions per minute (rpm), at least about 1,500 rpm, or at least 3,000 rpm. In one embodiment, each flow loop maintains a desired minimum positive pressure. In accordance with one embodiment, maintaining positive pressure during the rotation of compressor/driver system  10  prevents the pressure in each flow loop from dropping below atmospheric pressure (i.e., a vacuum). 
     After compressor/driver system  10  achieves the minimum rotational speed, the flow loops can be de-isolated, as depicted by block  216  in  FIG. 2 . As discussed previously, when the flow loops are de-isolated, gas flow is permitted between two or more the stages of multi-stage compressor  14 . As shown in Table 1, flow loops  26 ,  28 ,  30  can be de-isolated by opening isolation valves  68 ,  74  while the compressor/driver system  10  continues to rotate at or above its minimum speed. 
     In one embodiment immediately prior to opening isolation valves  68 ,  74 , bypass valves  70 ,  76  can be opened to reduce the pressure differential across the isolation valves and equalize the positive pressure between two adjacent loops. For example, according to the embodiment illustrated in  FIG. 1 , opening bypass valve  70  immediately prior to opening isolation valve  68  can equalize the pressure between isolated low compression stage  20  and intermediate compression stage  22 . Similarly, reducing the pressure differential between intermediate compression stage  22  and high compression stage  24  can include opening bypass valve  76  prior to opening isolation valve  74 . In one embodiment, bypass valves can have smaller port sizes than their corresponding isolation valves. In another embodiment, a bypass valve can be positioned parallel to its corresponding isolation valve. Positions of each valve shown in  FIG. 1  during the step of flow loop de-isolation are shown in Table 1. 
     At this point, the working fluid can now be introduced into the compressor, as depicted in block  218  of  FIG. 2 . As shown in Table 1, working fluid inlet control valve  104  and working fluid outlet control valve  108  can be opened to introduce the working fluid into low compression stage  20  and thereby transition the compressor/driver system  10  into its normal mode of operation. In one embodiment, anti-surge valves  38 ,  48 ,  58  may be placed on automatic control during the normal mode of operation. 
     In one embodiment of the present invention, the compressor system described and illustrated herein can be employed to compress one or more refrigerant streams. For example, the turbine-driven compressor systems described herein can be used to compress hydrocarbon-containing refrigerants employed as part of a mechanical refrigeration cycle used to cool natural gas in a liquefied natural gas (LNG) plant. In one embodiment, the compressor system can be utilized in a mixed-refrigerant LNG process, such as the process described by U.S. Pat. No. 4,445,917, which is incorporated herein by reference. In another embodiment, the inventive compressor system can be employed in a cascade-type LNG refrigeration process, such as the one disclosed in U.S. Pat. No. 6,925,387, which is incorporated herein by reference. 
     Numeric Ranges 
     The present description uses numeric ranges to quantify certain parameters relating to the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claims limitation that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds). 
     DEFINITIONS 
     As used herein, the terms “a,” “an,” “the,” and “said” means one or more. 
     As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination. 
     As used herein, the term “anti-surge valve” refers to a valve used to regulate flow from the discharge of a compression stage to the suction of the same compression stage. 
     As used herein, the term “auxiliary motor” refers to an electric motor or other driver coupled to the outboard end of a gas turbine used to provide additional power to help rotate the gas turbine during the start-up mode. 
     As used herein, the term “cascade refrigeration process” refers to a refrigeration process that employs a plurality of refrigeration cycles, each employing a different pure component refrigerant to successively cool natural gas. 
     As used herein, the term “compression stage” refers to one element of a compressor wherein the pressure of an incoming gas in increased. 
     As used herein, the terms “containing,” “contains,” and “contain” have the same open-ended meaning as “comprising,” “comprises,” and “comprise.” 
     As used herein, the terms “comprising,” “comprises,” and “comprise” are open-ended transition terms used to transition from a subject recited before the term to one or elements recited after the term, where the element or elements listed after the transition term are not necessarily the only elements that make up of the subject. 
     As used herein, the term “de-isolate” refers to the act of establishing fluid flow communication between two or more previously-isolated flow loops. As used herein, the term “flow loop” refers to the flow path between a compressor stage&#39;s discharge and suction, piece 
     As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise.” 
     As used herein, the term “hydrocarbon-containing” refers to material that contains at least 5 mole percent of one or more hydrocarbon compounds. 
     As used herein, the terms “including,” “includes,” and “include” have the same open-ended meaning as “comprising,” “comprises,” and “comprise.” 
     As used herein, the term “intercooler” refers to any device used to cool fluid between compression stages. 
     As used herein, the term “multi-stage compressor” refers to a compressor that utilizes two or more compression stages to successively increase the pressure of an incoming gas. 
     As used herein, the term “mixed refrigerant” means a refrigerant containing a plurality of different components, where no single component makes up more than 75 mole percent of the refrigerant. 
     As used herein, the term “positive pressure” refers to a pressure above atmospheric pressure. 
     As used herein, the term “pure component refrigerant” means a refrigerant that is not a mixed refrigerant. 
     As used herein, the term “start-up gas” refers to a stream of internal or external gas supplied to the system in during the start-up mode to purge existing material and/or establish adequate positive pressure in one or more flow loops. 
     As used herein, the term “working fluid” refers to the gas being compressed during normal operation of a compressor. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.