Abstract:
A control method and apparatus for simultaneously protecting a compression system from driver overpowering and turbocompressor surge. When overpowering is detected, flow rate through the each compressor in the turbocompressor train is reduced by closing an inlet throttling valve at the inlet of each respective compressor stage unless a compressor operating point is sufficiently near surge. In this latter case, the inlet throttling valve is not closed. In this way, overall flow rate through the compressor train is reduced while maintaining adequate flow through compromised stages to avoid surge.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. 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 to drive a multistage turbocompressor by selectively manipulating throttle valves at the compressor stages&#39; inlets while simultaneously protecting the compressor stages from surge. 
     2. Background Art 
     During some modes of operation a load imposed by the process on a single- or multistage compressor may exceed a maximum power available from the driver or drivers. Compressor shutdown may be required to avoid damage to the driver. Shutdown is to be avoided due to its inherent production loss. 
     A known method to avoid shutdown while still protecting the driver from damage reduces the load on the train by throttling the inlet flow using an inlet throttle valve on each stage of compression. 
     The present-day scheme of protection calls for reducing the opening of the inlet throttle valves, when present. The anticipated result is a reduction of flow through each of the compressor stages, and a consequent reduction in power consumed by compressor train. 
     Compressor surge is an unstable operating condition that is to be avoided. Modern control systems provide antisurge protection by calculating an operating point of the compressor and determining a proximity of the operating point to the compressor&#39;s surge limit. Antisurge control is explained in the Compressor Controls Series 5 Antisurge Control Application Manual, Publication UM5411 rev. 2.8.0 Dec. 2007, herein incorporated in its entirety by reference. 
     A surge control line is defined by providing a safety margin to the surge limit. When the compressor&#39;s operating point approaches the surge control line, a recycle, or antisurge, valve plumbed in parallel with the compressor is opened to provide sufficient flow to the compressor to keep it safe from surge. 
     Throttling the inlet flow of a turbocompressor stage operating at or near its surge control line causes that stage&#39;s operating point to be driven nearer to surge. When the antisurge control system is actively manipulating the antisurge valve to protect its compressor stage from surge, closing the inlet throttling valve will cause the control system to increase the opening of the antisurge valve to compensate for the reduction of the inlet flow rate. Thus no reduction of shaft power is realized. 
     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 effectively reducing the shaft power required to drive a multistage turbocompressor. It is a further object of the present invention to provide this reduction in shaft power while maintaining the compressor train in a stable operating condition. 
     The instant invention uses compressor driver power limiting to simultaneously close inlet throttling valves in the train to reduce the overall driver power consumption by the compressor train. All inlet valves are closed in this manner except those valves on compressor stages operating nearer surge than a predetermined distance. Therefore, inlet throttling valves are not closed past the point where the compressor&#39;s operating point is at that predetermined distance from surge. 
     The instant invention can be used for to control any compressor train with one or more stages of compression, where the shaft load must be limited to avoid shutdown, and where suction throttling valves are available. For the purposes of this document, including the claims, the term compressor train is hereby defined as one or more turbocompressors or turbocompressor stages on a single shaft. Shaft power may be provided by one or more drivers such as gas or steam turbines, or electric motors. 
     The novel features 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 and examples are for the purpose of illustration and description only, and not intended in any way 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 train wherein each stage of compression is outfitted with an inlet throttling valve; 
         FIG. 2  is a schematic of a compressor train and a control system for the same; 
         FIG. 3  is a representative compressor performance map in (Q,H p ) coordinates; 
         FIG. 4  is a representative compressor performance map in (Q,{dot over (W)}) coordinates; 
         FIG. 5  is a flow diagram illustrating a logic of the control scheme of the instant invention; 
         FIG. 6  is a schematic of a compressor train driven by a gas turbine driver; 
         FIG. 7  is a detail of an overpower query using electric motor current or power as the criterion for detecting overpowering; 
         FIG. 8  is a detail of an overpower query using steam turbine steam flow rate as the criterion for detecting overpowering; 
         FIG. 9  is a detail of an overpower query using gas turbine exhaust gas temperature as the criterion for detecting overpowering; and 
         FIG. 10  is a detail of an overpower query using shaft torque as the criterion for detecting overpowering. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A three-stage compressor train is shown, schematically, in  FIG. 1 . The present invention is useful on compressor trains of any number of compressor stages  115   a - 115   c , and is, therefore, not limited to the three-stage train shown in  FIG. 1 . Shaft power to drive the compressors  115   a - 115   c  is, in this case, provided by a steam turbine  110  and an electric motor  120 . 
     Instrumentation for monitoring and control comprises flow meter transmitters  125   a - 125   c , suction pressure transmitters  130   a - 130   c , and discharge pressure transmitters  135   a - 135   c  for each stage of compression  115   a - 115   c.    
     The drivers are also instrumented: the electric motor  120  is fitted with an electric current or power transmitter  155  while the steam flow rate into the steam turbine  110  is measured by the steam flow transmitter  160 . 
     In  FIG. 6 , a gas turbine  610  is shown as the driver of the compressor train. Instrumentation on the gas turbine might include an Exhaust Gas Temperature (EGT) transmitter  620  and a shaft torque meter  630 . 
     Each compressor stage  115   a - 115   c  is fitted with an inlet throttling valve  140   a - 140   c  by which performance or capacity control is effected and load balancing between the individual compressor stages  115   a - 115   c  is carried out. 
     Adequate flow through the compressor stages  115   a - 115   c  is provided for antisurge control by manipulating the antisurge valves  145   a - 145   c.    
     As with many refrigeration compressors, sidestreams  150   a - 150   b  are integral to the compression system. 
     In  FIG. 2 , the same compressor train as illustrated in  FIG. 1  is shown with a control system. Some of the reference numbers shown in  FIG. 1  are not shown in  FIG. 2  for clarity. A typical control system comprises antisurge controllers  210   a - 210   c  and performance controllers  220   a - 220   c  for each stage of compression  115   a - 115   c , and a load sharing controller  230 . 
     Into each antisurge controller  210   a - 210   c  is inputted signals representing: a flow rate from the flow meter transmitter  125   a - 125   c , a suction pressure from the suction pressure transmitter  130   a - 130   c , and a discharge pressure from the suction pressure transmitter  135   a - 135   c . Other signals may also be provided and the present invention is not limited to any particular set of input signals to the antisurge controllers. The output signal from each of the antisurge controllers  210   a - 210   c  is a signal to manipulate the antisurge valve  145   a - 145   c.    
     The performance controllers  220   a - 220   c  manipulate the inlet throttling valves  140   a - 140   c  based on a load sharing control scheme such as those disclosed in U.S. Pat. No. 5,743,715, hereby incorporated by reference. The load sharing controller  230  communicates with the performance controllers  220   a - 220   c , causing them to manipulate their respective inlet throttling valves  140   a - 140   c  to maintain a process variable at a predetermined set point. 
     Note that all individual controllers  210   a - 210   c ,  220   a - 220   c ,  230  are able to communication one with another over a hardwired or wireless network represented by dash-dot-dot lines in  FIG. 2 . Therefore, when a driver is overpowered—for instance: the electric motor current (or power) exceeds a predetermined upper threshold—the load sharing controller  230  is able to detect that event by comparing the signal from the current (or power) transmitter  155  to the predetermined threshold, and is then able to signal the performance controllers  220   a - 220   c  to cause their respective inlet throttling valves  140   a - 140   c  to close. Additionally, the performance controllers  220   a - 220   c  can receive information from the antisurge controllers  210   a - 210   c  regarding the position of their respective compressor&#39;s operating points. With this information, each performance controller  220   a - 220   c  will determine if and how much to close the inlet throttling valve  140   a - 140   c  to simultaneously reduce the electric motor&#39;s load and safeguard the compressors  115   a - 115   c  from surge. 
     A typical compressor performance map in polytropic head vs. Q coordinates is shown in  FIG. 3 . Here, Q is volumetric flow rate—usually measured at the inlet. The map of  FIG. 3  comprises curves of constant rotational speed  310   a - 310   d , a surge limit  320 , a surge control line  330 , and a power limiting curve  340 . The surge limit  320  is the boundary between the surge region and the stable operating region, usually simply referred to as the operating region. The surge control line  330  is a curve set apart from the surge limit  320  by a safety margin, sometimes referred to as the surge margin. The power limiting curve  340  is a curve set apart from the surge control line  330  by a predetermined distance. When the driver is overpowered, the inlet throttling valve  140   a - 140   c  of each turbocompressor  115   a - 115   c  is ramped closed to the point where the compressor&#39;s operating point reaches the power limiting curve  340 . In this fashion, the antisurge valve  145   a - 145   c  of that particular turbocompressor stage  115   a - 115   c  is not forced to open to protect the compressor  115   a - 115   c  from surge. 
     In  FIG. 4 , another compressor performance map is shown. Here, the performance curves are in shaft power vs. Q coordinates. Each curve  410   a - 410   d  is, again, a line of constant rotational speed. It is clear from the curves of shaft power  410   a - 410   d , at a given rotational speed, the required shaft power decreases as the compressor&#39;s operating point moves toward the surge limit  320 . 
     In  FIG. 5 , the control algorithm of the present invention is illustrated in a flow diagram. This diagram may be considered the programmed algorithm in the control system  210   a - 210   c ,  220   a - 220   c ,  230  shown in  FIG. 2 . Because the individual controllers  210   a - 210   c ,  220   a - 220   c ,  230  are able to communicate with one another, any part of the algorithm shown in  FIG. 5  may be executed in any particular controller  210   a - 210   c ,  220   a - 220   c ,  230 . Necessary inputs and outputs to each controller function are communicated via the inter-controller communication links. 
     As is well known in the art, in the usual course of operation, some aspect of performance or capacity control is carried out on the compressors  115   a - 115   c  via the manipulation of the inlet throttling valves  140   a - 140   c . This usual mode of operation is indicated in the top block  510  of  FIG. 5 . The control system  210   a - 210   c ,  220   a - 220   c ,  230  monitors some aspect or aspects of the driver  110 ,  120 ,  610  to determine if the driver  110 ,  120 ,  610  is overpowered. Aspects that may be monitored include, but are not limited to: electric motor current, electric motor power, gas turbine exhaust gas temperature, shaft torque, and steam turbine steam flow rate. 
     When the monitored aspect, or one of the monitored aspects, exceeds a threshold (see  FIGS. 7-10 ), the driver  110 ,  120 ,  610  is deemed overpowered, as indicated in the first query block  520 . When the query proves true, that is, the driver  110 ,  120 ,  610  is overpowered, the algorithm calls for a query of the control system  210   a - 210   c ,  220   a - 220   c ,  230 , in the second query block  530 , to determine if each compressor&#39;s operating point is to the right of the power limiting curve  340 —that is, if it is safe to close the inlet throttling valve  140   a - 140   c . If the result of this query  530  is false, control of the inlet throttling valve  140   a - 140   c  remains with the performance controller in block  510 . 
     Whenever the query  530  is true, the opening of the respective throttling valve  140   a - 140   c  is reduced in block  540  while continuously or periodically checking if the driver  110 ,  120 ,  610  remains overpowered and, if so, if it remains safe to close the inlet throttling valve  140   a - 140   c  further. Note that the function illustrated in  FIG. 5  is carried out for each of the turbocompressors  115   a - 115   c  in the compressor train that has an inlet throttling valve. 
       FIGS. 7-10  clarify the first query block  520  in  FIG. 5 . In  FIG. 7 , the criterion used for determining if the electric motor  120  is overpowered is motor current or motor power, according to the signal received from the current or power transmitter  155 . The signal received from the transmitter  155  is compared to a threshold value for that signal in a query block  710  to make the determination as to whether or not the driver is overpowered. 
     In  FIG. 8 , the criterion used for determining if the steam turbine  110  is overpowered is steam flow rate, according to the signal received from the steam flow rate transmitter  160 . The signal received from the transmitter  160  is compared to a threshold value for that signal in a query block  710  to make the determination as to whether or not the driver is overpowered. 
     In  FIG. 9 , the criterion used for determining if the gas turbine  610  is overpowered is the exhaust gas temperature, according to the signal received from the exhaust gas temperature transmitter  620 . The signal received from the transmitter  620  is compared to a threshold value for that signal in a query block  710  to make the determination as to whether or not the driver is overpowered. 
     In  FIG. 10 , the criterion used for determining if the driver  110 ,  120 ,  610  is overpowered is the shaft torque, according to the signal received from the torque transmitter  630 . The signal received from the transmitter  630  is compared to a threshold value for that signal in a query block  710  to make the determination as to whether or not the driver is overpowered. 
     The above embodiment is the preferred embodiment, but this invention is not limited thereto, nor to the figures and examples given above. 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.