Abstract:
A control method and apparatus for startup of turbocompressors to avoid overpowering a driver of the turbocompressor. In a first embodiment, the control system monitors input signals from transmitters of various control inputs. When the input signals exceed threshold values, the control system begins to close the antisurge valve. In a second embodiment, the antisurge valve begins to close after a predetermined time measured from the time startup is initiated. In both embodiments, the antisurge valve continues to ramp closed until the compressor has reached its operating zone, or until the compressor&#39;s operating point reaches a surge control line, at which point the antisurge valve is manipulated to keep the compressor from surging.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 12/128,265, filed May 28, 2008, this application is also a Continuation-In-Part of U.S. patent application Ser. No. 13/743,418 filed Jan. 17, 2013, now U.S. Pat. No. 8,540,498 issued Sep. 24, 2013, which is a Divisional of U.S. patent application Ser. No. 12/047,938, filed Mar. 13, 2008, now U.S. Pat. No. 8,360,744, issued Jan. 29, 2013, Priority is claimed from all of the above identified applications and all are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to a control scheme. More particularly the present invention relates to a method and apparatus for reducing a shaft power required when starting up a turbocompressor by manipulating the compressor&#39;s antisurge recycle valve. 
     Background Art 
     As shown in  FIG. 1 , a turbocompressor  100 , whether axial or centrifugal, is driven by a driver such as a variable speed electric motor  110 . A recycle valve  120 , used for antisurge protection, is piped in parallel with the compressor  100 . An inlet throttling valve  130  may be used for compressor capacity or performance control. 
     As all those of ordinary skill in this art know, surge is an unstable operating condition of a turbocompressor encountered at generally low flow rates. The surge region is shown in  FIG. 2  to the left of the surge limit curve  210 . In  FIG. 2 , H p  is the polytropic head and Q is the volumetric flow rate, both associated with the turbo compressor. 
     For the purposes of this document, including the claims, the compressor&#39;s minimum operating speed is hereby defined as the minimum rotational speed, greater than idle speed, at which the compressor may be operated continuously. The minimum operating speed is defined by the compressor manufacturer. It is generally depicted as the lowest performance curve in a compressor performance map such as shown in  FIGS. 2 and 3 . Lower speeds, greater than idle speed, are experienced on startup and shutdown, but the compressor is not operated continuously at these speeds. For turbocompressors operated at a constant speed, such as those driven by constant speed electric motors, the minimum operating speed is simply the constant operating rotational speed. 
     As those of ordinary skill know, the accepted startup procedure for a turbocompressor is to increase the rotational speed of the compressor with the antisurge valve  120  wide open until the compressor reaches the compressor&#39;s minimum operating speed (if the compressor is operated at variable speed) or the compressor&#39;s operating speed (if the compressor is driven by a constant speed driver). At this point in the startup procedure, the antisurge valve  120  is ramped closed and the compressor&#39;s  100  automatic performance control takes control of the compressor&#39;s rotational speed, inlet throttling valve  130 , or variable guide vanes to control the compressor&#39;s  100  capacity. 
     As is recognized by all those of ordinary skill, this startup procedure provides the most safety for the compressor because surge will be avoided, as depicted in  FIG. 3 . The compressor&#39;s  100  operating point trajectory  320  is shown as a dot-dashed line. Curves of constant compressor rotational speed  310   a - 310   e  are shown as solid lines. The curve  310   a  represents the minimum operating rotational speed, while the curve  310   e  represents the maximum operating rotational speed. Because the recycle valve  120  is maintained in its fully open position until minimum rotational speed has been achieved, the compressor operating point trajectory  320  tends to give wide berth to the surge limit curve  210  in the region below the minimum rotational speed curve  310   a.    
     Additional impetuses for startup with the antisurge valve  120  fully open are that the surge limit curve  210  is usually unknown for rotational speeds less than the minimum operating speed, and that pressure and flow sensor signals of reasonable magnitude must be achieved before a valid compressor operating point may be determined. The compressor&#39;s operating point must be calculated to compare its location to the surge limit line  210 , or surge control line  220  to avoid having the compressor&#39;s operating point cross the surge limit line  210 . Antisurge control algorithms are described in the Compressor Controls Series 5 Antisurge Control Application Manual, Publication UM5411 rev. 2.8.0 December 2007, herein incorporated in its entirety by reference. 
     Due to the large flow through the compressor  100  during startup using the above standard procedure, the shaft power required to drive the compressor  100  is large. This results in slower startup and, possibly, tripping of the driver due to power overload. 
     A gas turbine driver may experience high exhaust gas temperatures during the startup of a turbocompressor. An electric motor driver may trip on thermal overload due to a current being too high for too long a duration. 
     There is, therefore, a need for an improved control strategy for the startup of turbocompressors to reduce the loading of the compressor while maintaining the compressor flow out of the unstable, surge region. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and apparatus for safely starting a turbocompressor while minimizing an overall energy required to accomplish the startup. 
     Compressors having gas turbine drivers and variable frequency drive electric motors tend to have long startup times—on the order of several minutes. For this class of compressors, a first embodiment of this invention prescribes that the compressor&#39;s antisurge valve be maintained at its fully open position until predetermined signal strengths are realized from the compressor&#39;s suction and discharge pressure sensors, and the flow sensor. At this point, the antisurge valve is ramped closed at a predetermined rate under control of the antisurge control system to keep the compressor&#39;s operating point from crossing the surge control curve. Startup continues independently of the antisurge controller&#39;s operation. 
     A second class of compressors comprises constant-speed electric motor driven compressors. The startup times for this class of compressors tend to be on the order of less than a minute. In this case, the control system starts the antisurge valve in a fully open position, and begins to ramp the antisurge valve closed at a predetermined rate after a predetermined time has elapsed after the initiation of the startup of the compressor. Because of the rapid startup, the pressure and flow sensor signals become viable very quickly, so antisurge control may be carried out before the compressor&#39;s operating point reaches the surge control curve. 
     The novel features which are believed to be characteristic of this invention, both as to its organization and method of operation together with further objectives and advantages thereto, will be better understood from the following description considered in connection with the accompanying drawings in which a presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood however, that the drawings are for the purpose of illustration and description only and not intended as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic of a compressor, driver, and antisurge recycle valve; 
         FIG. 2  is a first representative compressor performance map; 
         FIG. 3  is a second representative compressor performance map showing a first compressor operating point&#39;s startup trajectory; 
         FIG. 4  is a is a third representative compressor performance map showing lines of constant shaft power; 
         FIG. 5  is a fourth representative compressor performance map showing a second compressor operating point&#39;s startup trajectory; 
         FIG. 6 a    is a schematic of a variable speed motor driven compressor system; 
         FIG. 6 b    is a schematic of a constant speed motor driven compressor system; 
         FIG. 7  is a schematic of a turbine driven compressor system; 
         FIG. 8  is a flow diagram of a first embodiment of the present invention; 
         FIG. 9  is a flow diagram of a second embodiment of the present invention; 
         FIG. 10  is a flow diagram of a third embodiment of the present invention; and 
         FIG. 11  is a detail flow diagram of a startup initiation process 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical compressor performance map in H p -Q coordinates is shown in  FIG. 4 . Here, H p  is polytropic head and Q is volumetric flow rate—usually in the suction. The map of  FIG. 4  comprises solid-line curves of constant rotational speed  310   a - 310   e  and dashed-line curves of constant shaft power  410   a - 410   e . As is clear from the relationship between the curves of constant rotational speed  310   a - 310   e  and the curves of constant shaft power  410   a - 410   e , at a given rotational speed, the required shaft power decreases as the operating point moves toward the surge limit  210 . To avoid overpowering the compressor driver  110 ,  710  (see  FIG. 7 ) an operating point trajectory  520 , shown in  FIG. 5 , running as near the surge limit  210  as possible, should be used. The short-dashed curve  510  represents a surge control line—a line set a predetermined distance from the surge limit line  210  toward the stable operating region, thus providing a safety margin for the antisurge control system. 
     As those of ordinary skill in the art of compressor control know, limit control is applied to the compressor  100  to maintain the operating point at or to the right of the surge control line  510 . To effect this control, an antisurge or recycle valve  120 , as shown in  FIGS. 1, 6   a ,  6   b , and  7 , is manipulated to maintain an adequate flow rate through the compressor  100 . The manipulation of the antisurge valve  120  is carried out via an automatic control algorithm, such as a closed loop control algorithm, in the antisurge controller, A/S PID  610 , of  FIGS. 6 a , 6 b   , and  7 . Typical inputs to the antisurge controller  610  are shown in  FIGS. 6 a , 6 b   , and  7  and comprise a differential pressure signal from a flow transmitter, FT  620 , a suction pressure signal from a suction pressure transmitter, PT1  630 , a discharge pressure signal from a discharge pressure transmitter, PT2  635 , and a rotational speed signal from speed pickup, SE  640  when the driver is variable speed as in  FIGS. 6 a   , and  7 . Often, in applications using a constant speed driver, such as a constant speed electric motor  645 , 
     as shown in  FIG. 6 b   , no speed pickup SE  640  in included. 
     To emulate the operating point trajectory  520  depicted in  FIG. 5 , the antisurge valve  120  is initially fully open, but is ramped closed by the control system as soon as safe operation may be assured. One embodiment of the instant invention is depicted in the flow diagram of  FIG. 8 . This embodiment is particularly useful when the startup process is “slow,” taking on the order of several minutes from its initiation. As mentioned, the antisurge valve  120  is set initially at its full open position as shown in block  800 . The full open position may vary between valve types. Generally, full open in the context of this invention is the greatest opening the antisurge valve  120  will realize in its duty in the specific application. The present invention does not depend on the percent opening value at which the antisurge valve  120  is considered in its full open position. 
     When the antisurge valve  120  is assured fully open, startup can be initiated as shown in block  805 . At startup, the rotational speed of the compressor  100  is increased according to the guidelines and restrictions of the compressor  100  and driver  110 ,  710  manufacturers and the needs of the equipment owner. In particular, critical speeds, if any, are considered and the startup schedule takes these speeds into consideration. Speed increase is depicted in block  810 , and is effected, as shown in  FIG. 11 , by increasing a compressor speed set point used by a Variable Frequency Drive (VFD) controller  650  ( FIG. 6 a   ) or a rotational speed controller  720  ( FIG. 7 ). 
     As the compressor speed increases, the control system  610  repeatedly checks the signals received from the flow transmitter  620 , suction pressure transmitter  630 , and discharge pressure transmitter  635 . The signal values are compared to threshold values, Δp o,min , p s,min , and p d,min , respectively in comparator blocks  815 ,  820 ,  825 . If the signal magnitude of one or more of the input signals, Δp o , p s , and p d , is not at least as great as its respective threshold value, the rotational speed of the compressor  100  continues to be ramped up as indicated in block  810 . 
     Once all three signals, Δp o,min , p s,min , and p d,min , exceed their threshold values Δp o,min , p s,min , and p d,min , two operations are carried out essentially simultaneously and repeatedly. Each of these operations emanates from and returns to the branch block  830 . In one of these operations, the antisurge controller  610  compares the compressor&#39;s operating point to the surge control line  510  to determine how the antisurge valve  120  must be manipulated for antisurge protection. If the compressor&#39;s operating point is to the right of the surge control line  510  as determined in the comparator block  835 , the antisurge valve  120  is ramped toward its closed position according to a predetermined schedule as shown in block  850 . If the operating point is on or to the left of the surge control line  510 , the antisurge controller  610  manipulates the antisurge valve&#39;s  120  position to keep the compressor  100  safe from surge as shown in block  845 . 
     The other essentially simultaneous operation involves continuing to increase the compressor&#39;s rotational speed according to block  855  until the minimum operating speed, N min , or some predetermined value of speed is reached. Continuing to increase the compressor&#39;s rotational speed is effected as explained with regard to block  810 : the rotational speed set point used by the VFD controller  650  or the speed controller  720  is increased with time. Those of ordinary skill in this art are intimate with this aspect of startup control. When the comparator block  840  determines the compressor  100  has reached its minimum operating speed, the control system is shifted from its startup mode to its RUN mode, as shown in block  860 . At that point, the capacity or performance control system takes over varying the compressor speed according to the needs of the process. Note that the minimum operating speed, N min , in comparator block  840  may be the compressor&#39;s operating speed if the compressor  120  is to be operated at a constant speed. 
     An additional embodiment is shown in  FIG. 9 . This embodiment is particularly useful for compressors  120  that may be started rapidly—in less than a minute, for instance. The antisurge valve  120  is set initially at its full open position as shown in block  800 . In block  910 , a timer is reset to zero. 
     When the antisurge valve  120  is assured fully open and the timer has been initialized, startup can be initiated as shown in block  805 . At startup, the rotational speed of the compressor  100  is ramped up according to the guidelines and restrictions of the compressor  100  and driver  110 ,  710  manufacturers and the needs of the equipment owner. Speed rampup is carried out by increasing the VFD controller&#39;s  650  or rotational speed controller&#39;s  720  set point, and is depicted in block  810 . 
     In this embodiment of the invention, the antisurge valve is ramped toward a closed position after a predetermined time elapses. In comparator block  920 , the time as reported by the timer is compared to the time threshold, t PD . If the time does not exceed the threshold time, the speed continues to increase, but no change to the position of the antisurge valve  120  is made. When the threshold time, t PD , has elapsed, two operations are carried out essentially simultaneously and repeatedly. Each of these operations emanates from and returns to the branch block  830 . In one of these operations, the antisurge controller  610  compares the compressor&#39;s operating point to the surge control line  510  to determine how the antisurge valve  120  must be manipulated for antisurge protection. If the compressor&#39;s operating point is to the right of the surge control line  510  as determined in the comparator block  835 , the antisurge valve  120  is ramped toward its closed position according to a predetermined ramp rate as shown in block  850 . If the operating point is on or to the left of the surge control line  510 , the antisurge controller  610  manipulates the antisurge valve&#39;s  120  position to keep the compressor  100  safe from surge as shown in block  845 . 
     The other essentially simultaneous operation involves continuing to increase the compressor&#39;s rotational speed according to block  855  until the minimum operating speed, N min , or some predetermined value of speed is reached. When the comparator block  840  determines the compressor  120  has reached its minimum operating speed, the control system is shifted from its startup mode to its RUN mode, as shown in block  860 . At that point, the capacity or performance control system takes over varying the compressor speed according to the needs of the process. Note that the minimum operating speed, N min , in comparator block  840  may be the compressor&#39;s operating speed if the compressor  120  is to be operated at a constant speed. 
     In  FIG. 10 , a third embodiment is illustrated, differing from the embodiment of  FIG. 9  in that the driver of  FIG. 10  is a constant speed driver, such as a constant speed electric motor  640  ( FIG. 6 b   ). In this embodiment, the process of accelerating the driver up to its operating speed, N op , does not incorporate a decision to continue accelerating the driver inasmuch as the driver will continue to accelerate until its operating speed, N op , is reached or it is tripped. Therefore, block  1055  indicates only that the rotational speed continues to rise. Block  1040  is intended only to indicate the compressor rotational speed will increase until the operating speed, N op , is reached, and not that a decision is being made in this comparator block. Ultimately, when the compressor has reached its operating speed, N op , the control system reverts to a RUN mode  860  wherein performance or capacity control is carried out to satisfy process constraints. Note that, in this case especially, the predetermined time lapse, t PD , in comparator block  920  may be zero so the antisurge valve  120  begins to close immediately as startup begins. 
     The last two embodiments differ from the prior art in that, in the instant invention, time is used to determine when the antisurge valve  120  is ramped toward its closed position, rather than rotational speed. 
     The flow diagrams in  FIGS. 8, 9 and 10  may be considered contents of a logic unit within a compressor control system, such as the antisurge controller  610  depicted in  FIGS. 6 a , 6 b   , and  7 . 
     More detail of the startup initiation block  810  is shown in  FIG. 11 . A check to ascertain the antisurge valve  120  is fully open is first carried out in query block  1110 . If the antisurge valve  120  is not fully open, the flow moves to a valve open function  1120 . Once the antisurge valve  120  is fully open, the turbocompressor rotational speed is increased from an initial, zero value as shown in block  1130 . 
     The above embodiments are the preferred embodiments, but this invention is not limited thereto. It is, therefore, apparent that many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.