Load sharing method and apparatus for controlling a main gas parameter of a compressor station with multiple dynamic compressors

Abstract: A method and apparatus for maintaining a main process gas parameter such as suction pressure discharge pressure, discharge flow, etc. of a compressor station with multiple dynamic compressors, which enables a station controller controlling the main process gas parameter to increase or decrease the total station performance to restore the main process gas parameter to a required level, first by simultaneous change of performances of all individual compressors, for example, by decreasing their speeds, and then after operating points of all machines reach their respective surge control lines, by simultaneous opening of individual antisurge valves. In the proposed load-sharing scheme, one compressor is automatically selected as a leading machine. For parallel operation, the compressor which is selected as the leader is the one having the largest distance to its surge control line. For series operation, the leader has the lowest criterion "R" value representing both the distance to its surge control line and the equivalent mass flow rate through the compressor. The leader is followed by the rest of the compressors, which equalize their distances to the respective surge control lines or criterions "R" with respect to that of the leader.

Description: The present invention relates to a compressor station comprising a station control means for producing a station control signal in dependence on a detected main gas parameter, a plurality of compressors, antisurge control means for each compressor, for producing respective surge control variable signals and protecting each compressor from surge, and a respective unit final control means for controlling the performance of each compressor. Also, the present invention relates to a method of operating a compressor station comprising a plurality of compressors, the method comprising the steps of: producing a station control signal in dependence on a detected main gas parameter; producing respective surge control variable signals for each compressor for protecting each compressor from surge; and controlling'each compressor in dependence on the station control signal. All known control systems for parallel working compressors and for compressors working in series can be divided into two categories. In the first category, the antisurge protective devices and the control device for controlling the station gas parameter are independent and not connected at all to each other. The station control device changes the performances of individual compressors by establishing the preset gains and biases which remain constant during station operation. For some compressors, the gains are equal to zero and the biases are set to provide for a base-load operation with a constant and often maximum speed. This category of control system cannot cope with two major problems. The first problem is associated with the necessity to vary the gains and biases for load-sharing device set points, for optimum load sharing under changes of station operating conditions, such as inlet conditions or deterioration of some machines. The second problem is associated with possible interactions between the station control device and the antisurge control devices of individual compressors under conditions when the process flow demand is continuously decreasing. It is very usual for this category of control system to operate one compressor far from surge while keeping one or more compressors dangerously close to surge, including premature recycle flow to prevent surge. In the second control system category, there is a cascade combination of the station control device and the load-sharing devices of individual machines. In this category, the station control device manipulates the set points for the distances between the individual operating points and the respective surge limits. If, for the parallel operation some stabilization means is effective to make such cascade approach workable, then for series operation, it will not work at all. But even for parallel operation, the above identified stabilization means degrades the dynamic precision of control.In the proposed load-sharing scheme, one compressor is automatically selected as a leading machine. For parallel operation, the compressor which is selected as the leader is the one having the largest distance between its operating point and its surge control line. For the series operation, the leader has the lowest criterion "B" value representing both the distance to its surge control line and the equivalent mass flow through the compressor. The leader is followed by the rest of the compressors, which equalize their distances to their respective surge control lines or criterions "B" with respect to that of the leader. To overcome the aforementioned problems, the dynamic control of compressors may be significantly improved for both parallel and series operated machines by eliminating cascading, but still providing for equalization of relative distances to the respective surge control lines. It can be even further improved by providing special interconnection between all control loops to eliminate dangerous interactions in the vicinity of surge. A main purpose of this invention is to enable operating points of all compressors working simultaneously to reach their respective surge control lines before control of the main process gas parameter is provided by wasteful recycle flow, such as recirculation. Another purpose of this invention is to enable the control system to provide for stable and precise control of the main process gas parameter while providing for effective antisurge protection and optimum load sharing between simultaneously working compressors. Examples of prior art systems can be found in EP-A-0 431 287 and US-A-4 494 006. According to a first aspect of the present invention, a compressor station is characterized by a selection means for identifying one compressor as the leader compressor on the basis of the operation of each compressor relative to a respective surge control line and producing a further control signal on the basis of the operation of the identified leader compressor, and the unit control means, associated with non-leader compressors, being configured to use the further control signal as a reference so as to balance the performances of the compressors. According to a second aspect of the present invention, a method of operating a compressor station is characterized by identifying one compressor as the leader compressor on the basis of the operation of each compressor relative to a respective surge control line; producing a further control signal on the basis of the operation of the identified leader compressor; and using the further control signal as a reference for controlling non-leader compressors so as to balance the performance of the compressors. In the proposed scheme, the station control system can decrease the performance of each compressor only until the compressor is in danger of surge. After such danger appears, the main process gas parameter is controlled by controlling the antisurge valves to change the flow through the process. The main advantages of this invention are: an expansion of safe operating zone without recirculation for each individual compressor and for the compressor station as a whole; a minimization or decoupling of loop interaction; and an increase of the system stability and speed of response. In an embodiment of the present invention, each dynamic compressor of the compressor station is controlled by three interconnected control loops. The first loop controls the main process gas parameter common for all compressors operating in the station. This control loop is implemented in a station controller which is common for all compressors. The station controller device is capable of manipulating sequentially: first, a unit final control for each individual compressor, such as a speed governor, an inlet (suction) valve, guide vanes, etc. And then each individual antisurge final control device, such as a recycle valve, blow-off valve, etc. The second control loop provides for optimum load sharing. This loop is implemented in a unit controller, one for each compressor. The unit controller enables the compressor operating point to reach the respective surge control line simultaneously with the operating points of the other compressors and before any antisurge flow, such as recirculation, starts. The output of the unit controller for each individual compressor is interconnected with the output of the station controller common to all compressors, to provide a set point for the position of the unit final control device. A third control loop is implemented in an antisurge controller which computes the relative distance to the surge control line; prevents this distance from decreasing below a zero level; and transmits this distance to the companion unit controller. The output of the antisurge controller is interconnected with the output of the station controller to manipulate the position of the antisurge final control device. The interconnection between all three control loops, which contribute to the operation of each individual machine, is provided as follows: The set point for the unit final control device of the i th individual compressor is manipulated by both the station controller and the respective unit controller, however, the output of the station controller can increase or decrease said set point only when the relative distance to the respective surge control line d ci is higher than or equal to the preset value "r i ." It can only increase said set point when d ci < r i . The set point for the position of each respective antisurge final control device can be manipulated either by respective antisurge controllers or by the station controller. The antisurge final control device can be closed only by the antisurge controller. It can, in one implementation, be opened by either one, whichever requires the higher opening when d ci < r i . Alternatively, in a second implementation, the corrective actions of both the antisurge controller and the station controller can be added together when both require the antisurge final control device to be opened, and the result used to open the antisurge final control device when d ci < r i . The optimum load sharing between parallel working compressors is provided in the present invention as follows: Each unit controller receives the relative distance to the respective surge control line computed by its companion antisurge controller, and compares said distance with the largest relative distance selected by the station controller between all compressors being in parallel operation. The compressor with the largest relative distance to its respective surge control line is automatically selected as a leader. The set point for the leader's unit final control device is manipulated only by the station controller. The set points for the unit final control device of the remainder of the compressors in the parallel system are manipulated to equalize their relative distances to the respective surge control lines with that of the leader, in addition to being manipulated by said station controller to control the main process gas parameter common for all compressors. For the series operation of the compressors, the unit controller for the i th compressor computes a special criterion "B i " value which represents both the relative distance to the surge control line for the i th compressor and the equivalent mass flow rate through the i th compressor. The unit controller controls the load sharing for the associated compressor by equalizing its own criterion B i value with the minimum criterion B sp value of the leader compressor, which was selected by the station controller. Similarly, as with parallel operating compressors, a leader compressor is selected and the rest of the compressors follow the leader. For series compressors, however, they do so by equalizing their criterion B i values with that of the leader. An object of the present invention is to prevent the wasteful gas flow through the antisurge final control device, such as recirculation, for purposes of controlling the main process gas parameter until all load-sharing compressors have reached their respective surge control lines. This is done by equalizing the relative distances to the respective surge control lines for parallel operating compressors; and by equalizing the criterion "B" values representing both the relative distance to the respective surge control line and the equivalent mass flow rate through the compressor for compressors operated in series. The equivalent mass flow rate compensates for flow extraction or flow admission between the series operated machines. Another object of the present invention is to prevent interaction among control loops controlling the main process gas parameter of the compressor station with the antisurge protection of each individual compressor. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 and Fig. 2 respectively, present the schematic diagrams of control systems for compressor stations with dynamic compressors operating in parallel, and for compressor stations with dynamic compressors operating in series. Fig. 1 is comprised of Fig. 1(a) and 1(b); Fig. 2 is comprised of Fig. 2(a) and 2(b). Referring now to the drawings wherein like reference numerals designate identical or corresponding parts throughout the several views: Fig. 1(a) shows two parallel working dynamic compressors 101 and 201 driven each by a steam turbine 102 and 202 respectively, and pumping the compressed gas to a common discharge manifold 104 through the respective nonreturn valves 105 and 205. Each compressor is supplied by a recycle valve 106 for compressor 101 and a recycle valve 206 for compressor 201, with respective actuators with positioners 107 and 207. The steam turbines have speed governors 103 and 203 respectively, controlling the speed of respective dynamic compressors. Each compressor is supplied by a flow measuring device: device 108 for compressor 101 and device 208 for compressor 201. Transmitters 111, 112, 113, 114, 115, and 116 are provided for measuring differential pressure across a flow element in suction 108, suction pressure, suction temperature, discharge pressure, discharge temperature, and rotational speed respectively, for compressor 101. Transmitters 211, 212, 213, 214, 215, and 216 are provided for measuring differential pressure across a flow element in suction 208, suction pressure, suction temperature, discharge pressure, discharge temperature, and rotational speed respectively, for compressor 201. Both recirculation lines 150 and 250 feed into a common suction manifold 199, which receives gas from the upstream process and passes the gas through a common cooler 198 and a common knockout drum 197, to a common manifold 196. Both compressors 101 and 201 are supplied by a station control system providing for pressure control in the common manifold 104, and also for optimum load sharing and antisurge protection of individual compressors. The control system consists of the following: one common station controller 129 controlling the main process gas parameter (discharge pressure in this example) measured by a pressure transmitter 195, using calculated corrective signal ΔS out ; two unit controllers 123 and 223 for compressors 101 and 201 respectively, which control the performance of each compressor by controlling the set points u out1 and u out2 to speed governors 103 and 203 respectively; and two antisurge controllers 109 and 209 for compressors 101 and 201 respectively, which manipulate the set points A out1 and A out2 of positioners 107 and 207 for recycle valves 106 and 206 respectively. Fig. 1(b) shows a control system for one of the parallel compressors 101; the second compressor 201 incorporates a duplicate control system, but it is not illustrated herein. An antisurge controller 109 comprises seven control modules: a measurement module 110 which receives signals from six transmitters 111, 112, 113, 114, 115, and 116; computational module 117; comparator module 118; P+I module 119; output processing module 120; nonlinear functional module 121; and multiplier module 122. A unit controller 123 comprises five control modules: normalizing module 124, P+I module 125, summation module 126, nonlinear functional module 127, and multiplier module 128. A station controller 129 is common for both compressors and comprises three control modules: measurement module 130 receiving a signal from a pressure transmitter 195, P+I+D module 131, and selection module 132. Because the antisurge controllers 109 and 209 and the unit controllers 123 and 223 are absolutely identical, an interconnection between their elements may be described in the following example by one compressor 101. The computational module 117 of the antisurge controller 109 receives data collected from six transmitters by way of a measurement module 110: pressure differential 111 across the flow measuring device 108, suction pressure 112, temperature 113, discharge pressure 114, temperature 115, and speed 116. Based on data collected, the computational module 117 computes a relative distance d r1 of the operating point of compressor 101 to its respective surge limit line; said relative distance may be computed as: d r1 = 1 - K . ( R σ c - 1 ) (σ) . f ( N ) Δ P 0 P s (1) where: f(N) represents the variation of the slope of the respective surge limit with variation of speed (N) of compressor 101; R c is the compression ratio produced by compressor 101; ΔP o is the pressure differential across the flow measuring device in suction; P s is the suction pressure; σ is the polytropic exponent for compressor 101; and K is a constant. The compression ratio R c , in turn, is computed as: R c = P d P s (2) where P d and P s are in absolute units, and exponent σ is computed using the law of polytropic compression: yielding σ = logR T logR c (4) where R T is the temperature ratio: R T = T d T s (5) with T d and T s being the discharge and suction temperatures respectively, in absolute units. Based on computed said relative distance d r1 to the surge limit line, the comparator module 118 calculates the relative distance d c1 to the respective surge control line: d c1 = d r1 -b 1 where b 1 is the safety margin between respective surge limit and surge control lines. The P+I module 119 has a set point equal to 0. It prevents the distance d c1 from dropping below zero by opening the recycle valve 106. The valve 106 is manipulated with an actuator by positioner 107 which is operated by output processing module 120 of antisurge controller 109. The output processing module 120 can be optionally configured as a selection module or a summation module. As a selection module, module 120 selects either the incremental change of P+I module 119 or the incremental change of multiplier 122, whichever requires the larger opening of valve 106. As a summation module, the incremental changes of both the P+I module 119 and the multiplier module 122 are summed. The multiplier module 122 multiplies the incremental change ΔS out of the P+I+D module 131 of the station controller 129 by a nonlinear function 121 of the relative distance d c1 and station controller corrective signal ΔS out . The value of this nonlinear function can be equal to values M 11 , M 12 , or zero. This value is always equal to zero, except when d c1 < r 1 and ΔS out ≥0, in which case it is equal to value M 11 ; or when d c1 < r 1 and ΔS out <0, in which case it is equal to M 12 . The unit controllers 123 and 223 are also absolutely identical, and the operation of both can be sufficiently described by one controller 123, using the following example. The relative distance d c1 is directed to unit controller 123 where the normalizing module 124 multiplies the relative distance d c1 computed by antisurge controller 109 by a coefficient β 1 . The purpose of such normalization is to either position the operating point of compressor 101 under its maximum speed and required discharge pressure in such a way that B 1 = β 1 . d c1 = 1 at its maximum; or to position each operating point at its maximum efficiency zone under the most frequent operational conditions. The coefficient β 1 may also be dynamically defined by a higher level optimization system. The output of normalizing module 124 is directed to selection module 132 of station controller 129 and to P+I module 125. Selection module 132 selects B sp as the highest value between B 1 and B 2 for compressors 101 and 201 respectively, and sends this highest value as the set points to P+I module 125 of unit controller 123 and its counterpart P+I module of unit controller 223. If the B sp value selected by module 132 is B 1 , compressor 101 automatically becomes the leader. Its P+I module 125 then produces an incremental change of output equal to 0. As a result, the summation module 126 is operated only by the incremental changes of the output ΔS out of the P+I+D module 131, provided the nonlinear function 127 is not equal to zero. If module 132 selects the normalized distance B 2 , then the P+I module 125 of unit controller 123 equalizes its own normalized distance B 1, to that of compressor 201 which automatically becomes the leader. In this case, the summation unit 126 changes its output based on the incremental changes of two control modules: P+I module 125 of unit controller 123 and P+I+D module 131 of station controller 129. Because of the nonlinear function produced by functional control module 127, the incremental change ΔS out of the P+I+D module 131 is multiplied by module 128 by a value equal to either M 13 , M 14 , or zero. When relative distance d c1 is higher than or equal to value "r 1 ," the multiplication factor is always equal to M 13 . It is equal to M 14 when d c1 < r 1 , and the incremental change ΔS out of the output of the P+I+D module 131 is greater than zero. However, when d c1 < r 1 and the incremental change ΔS out of the output of the P+I+D module is less than or equal to zero, the multiplication factor is equal to zero. This means that while controlling the discharge pressure in the common manifold 104, the station controller cannot decrease the relative distance d c1 to its respective surge control line for compressor 101 below some preset level "r 1 ." The output of summation moduie 126 of unit controller 123 manipulates the set point u out1 for speed governor 103. Station controller 129 changes the incremental output ΔS out of its P+I+D module to maintain the pressure measured by transmitter 195 in the common discharge manifold 104. The operation of the control system presented by Fig. 1 may be illustrated by the following example. Let us assume that initially both compressors 101 and 201 are operated under the required discharge pressure in common manifold 104, and with completely closed recycle valves 106 and 206. The normalized relative distances B 1 and B 2 of their operating points to the respective surge control lines are equal to the same value, say "2." Assume further that process demand for flow decreases in common manifold 104. As a result, the pressure in manifold 104 starts to increase. The normalized distance B 1 of compressor 101 to its surge control line decreases to the value D 1 . And for compressor 201 the value of its normalized relative distance B 2 decreases from the value 2 to the value D 2 . Also, assume that D 1 >D 2 and both relative distances B 1 and B 2 are greater than their respective preset values "r 1 " and "r 2 ." Selection module 132 selects the value of B 1 as the set point B sp for P+I module 125 of unit controller 123 and its counterpart P+I module of unit controller 223. The compressor 101 has, therefore, been automatically selected as the leader. Since B 1 >r 1 , the nonlinear function 127 is equal to M 13 , and the summation module 126 of unit controller 123 receives, through the multiplier 128, the incremental decreases ΔS out of the output of P+I+D module 131 multiplied by M 13 , which is required to restore the pressure in the manifold 104 to the required level. Said incremental decreases of the output of the P+I+D module decrease the set point of speed governor 103 for the turbine 102, decreasing the flow through compressor 101. Simultaneously, the summation moduie of unit controller 223 of compressor 201 changes the set point of speed governor 203 for compressor 201 under the influence of both the incremental changes of the output of P+I+D module 131 of station controller 129, and changes of the output of the P+I module of unit controller 223 of compressor 201. The transient process continues until both distances B 1 and B 2 are equalized and the pressure in discharge manifold 104 is restored to the required level. Assume again that the process flow demand decreases further and the speed of each individual compressor is decreased until B 1 =B 2 =0. Any further decrease of flow demand will cause the beginning of the opening of both recycle valves 106 and 206 by the P+I modules of antisurge controllers 109 and 209, to keep the operating points on their respective surge control lines. Further decrease of flow demand will increase the discharge pressure again, and the distances B 1 and B 2 will decrease below levels r 1 and r 2 respectively; and station controller 129 will lose its ability to decrease the speeds of compressors 101 and 201. Instead it will start to send the incremental changes ΔS out of the output of its P+I+D module 131 to the output processing module 120 and its equivalent of antisurge controllers 109 and 209. If these output processing modules perform a selection function, and if the incremental changes ΔS out require more opening of recycle valves 106 and 206 than required by both P+I modules, then the recycle valves will be opened by the ΔS out incremental changes to restore pressure to the required level. If the output processing module 120 and its counterpart perform a summation function, the incremental changes of both modules will combine to open the recycle valves 106 and 206 to restore pressure to the required level. As soon as distances B 1 and B 2 become higher than preset levels r 1 and r 2 respectively, the P+I+D module 131 of station controller 129 will function through unit controllers 123 and 223 to decrease the speeds of both compressors. This process will continue until the pressure in the common discharge manifold 104 will be restored to its required level. Assume further that the flow demand increases. As a result, pressure in manifold 104 drops, and distances B 1 and B 2 become positive. Station controller 129, through its P+I+D module 131, will start to immediately increase the speed of both compressors. At the same time, the antisurge controllers, through their respective P+I modules, will start to close the recycle valves 106 and 206. Assume also that distance B 2 becomes higher than B 1 . As a result, compressor 201 will automatically become the leader. P+I module 125 will speed up compressor 101, adding to the incremental increase of the output of the P+I+D module of station controller 129. As a result, both compressors will equalize their distances B 1 and B 2 . If, as a result of reaching its maximum speed, compressor 201 will not be capable of decreasing its respective distance B 2 , this limited compressor 201 will be eliminated from the selection process. As a result, compressor 101 will be automatically selected as the leader, giving the possibility for station controller 129 to increase the speed of compressor 101, and to restore the station discharge pressure to the required level. Referring now to Fig. 2(a), the compressor station is shown with two centrifugal compressors 301 and 401 working in series. Compressors 301 and 401 are driven by turbines 302 and 402 with speed governors 303 and 403 respectively. Low-pressure compressor 301 receives gas from station suction drum 304 which is fed from inlet station manifold 305. Before entering drum 304, the gas is cooled by cooler 306. High-pressure compressor 401 receives gas from suction drum 404 which is fed from suction manifold 405. Before entering suction drum 404, the gas is cooled by cooler 406. There is also a sidestream 412 entering manifold 405. As a result, the mass flow rate through high-pressure compressor 401 is higher than the mass flow rate through low-pressure compressor 301. Each compressor is equipped with the following: suction flow measuring device 307 for compressor 301 and device 407 for compressor 401; discharge flow measuring device 308 for compressor 301 and device 408 for compressor 401; nonreturn valves 311 and 411 located downstream of flow measurement devices 308 and 408 respectively; and recycle valve 309 for compressor 301 and valve 409 for compressor 401. The recycle valves are manipulated by actuators with positioners: positioner 310 for compressor 301 and positioner 410 for compressor 401. Generally, the minimum mass flow rate w m passing through all compressors in series, from suction manifold 305 to discharge manifold 413, is the minimum of all mass flow rates measured by the discharge flow measuring devices. Let w d1 and w d2 be the mass flow rates measured by discharge flow measuring devices 308 and 408 for compressors 301 and 401 respectively. Let the sidestream mass flow in sidestream manifold 412, admitted into manifold 405, be w s2 . If said sidestream mass flow rate w s2 is positive, then mass flow is being added to manifold 405. Therefore, mass flow rate w d2 will be greater than mass flow rate w d1 by the amount of mass flow w s2 being added at manifold 405; and this minimum mass flow rate w m will be equal to discharge mass flow rate w d1 for compressor 301. If sidestream mass flow rate w s2 is negative, then mass flow is being extracted from manifold 405. In this case, mass flow rate w d2 will be less than mass flow rate w d1 by the amount of mass flow w s2 being extracted at manifold 405; and minimum mass flow rate w m will be equal to discharge mass flow rate w d2 for compressor 401. The difference Δ i between the minimum mass flow rate w m and the discharge mass flow rate w di for the i th compressor, is added downstream or extracted upstream of the minimum flow compressor. Each compressor is further supplied by a set of transmitters; for example, the low-pressure compressor: differential pressure 314 across a flow element 307 in suction, suction pressure 315, suction temperature 316, discharge pressure 317, discharge temperature 318, differential pressure 319 across a flow element in discharge 308, and rotational speed 320. The high-pressure compressor 401 incorporates a similar array of transmitters: differential pressure 414 across flow element 407 in suction, suction pressure 415, suction temperature 416, discharge pressure 417, discharge temperature 418, differential pressure 419 across a flow element in discharge 408, and rotational speed 420. Both compressors 301 and 401 are supplied by a station control system maintaining the pressure in suction drum 304, while sharing the overall pressure ratio between the compressors in an optimum way, and protecting both compressors from surge. The station control system consists of: one common station controller 336 controlling the main process gas parameter [suction drum 304 pressure in this example] measured by pressure transmitter 341 using calculated corrective signal ΔS out ; two unit controllers 329 and 429 for compressors 301 and 401 respectively, which control the performance of each compressor by controlling set points u out1 and u out2 to speed governors 303 and 403 respectively; and two antisurge controllers 328 and 428 for compressors 301 and 401 respectively, which manipulate the set points A out1 and A out2 of positioners 310 and 410 for recycle valves 309 and 409 respectively. Fig. 2(b) shows a control system for the low-pressure compressor 301; the high-pressure compressor 401 incorporates an identical control system, but it is not illustrated herein. An antisurge controller 328 comprises seven control modules: a measurement control module 326 which receives signals from seven transmitters 314, 315, 316, 317, 318, 319, and 320; computational module 327; proportional plus integral (P+I) module 322; comparator module 321; output processing module 323; multiplier module 324; and a nonlinear functional module 325. A unit controller 329 comprises six control modules: a computational module 330, normalizing module 331, nonlinear functional module 332, multiplier module 333, summation module 334, and a proportional plus integral (P+I) module 335. A station controller 336 is common for both compressors and comprises four control modules: a measurement module 339 reading a signal from a pressure transmitter 341, minimum criterion B selection module 338, minimum mass flow selection module 337, and a proportional plus integral plus derivative (P+I+D) module 340. Because the antisurge controllers 328 and 428 are absolutely identical, their operation may be explained in the following example by referring to one controller 328. Measurement control module 326 of the antisurge controller 328 collects data from seven transmitters: differential pressure 314 measuring the pressure differential across a flow measuring device 307, suction pressure 315, suction temperature 316, discharge pressure 317, discharge temperature 318, differential pressure 319 measuring the pressure differential across a flow measuring device 308, and speed 320. Identical with parallel operation [see equations (1) through (5)] and based on data collected from the transmitters, the computational module 327 computes the relative distance d r1 of the operating point of compressor 301 from its respective surge limit line. It also computes the mass flow rate w c1 through flow measuring device 307: where ΔP os , P s , and T s are read by transmitters 314, 315, and 316 respectively; and computes the mass flow rate w d1 through the flow measuring device 308: Where ΔP od , P d , and T d are read by transmitters 319, 317, and 318 respectively. Both computed mass flow rates w c1 and w d1 are directed to the computational module 330 of companion unit controller 329. Mass flow rate w d1 is also directed to minimum flow selective module 337 of station controller 336 to select minimum mass flow rate w m , which passes through both compressors. The computed relative distance to the respective surge limit line, d r1 , is directed to the comparator module 321 which produces the relative distance d c1 of the operating point for compressor 301 to its surge control line, by subtracting the safety margin b 1 from the relative distance d r1 : d c1 = d r1 -b 1 This relative distance to the surge control line is directed to normalizing module 331 and the nonlinear control module 332 of unit controller 329, and to both the nonlinear control module 325 and the P+I module 322. The P+I module has a set point equal to zero, and it prevents distance d c1 from dropping below a zero level by opening recycle valve 309. The recycle valve 309 is manipulated with an actuator by positioner 310, which is operated by output processing module 323. The module 323 can be optionally configured as a selection module or a summation module. As a selection module 323, it selects either the incremental change received from P+I module 322 or the incremental change of multiplier 324, whichever requires the larger opening of valve 309. As a summation module, the incremental changes of both the P+I module and the multiplier module are summed. Multiplier module 324 multiplies incremental change ΔS out of P+I+D module 340 of station controller 336 by the nonlinear function 325 of the relative distance d c1 and incremental change ΔS out . This function can be equal to value M 11 or M 12 or zero. This value is equal to zero when d c1 ≥ r 1 ; or it is equal to M 11 when d c1 < r 1 and ΔS out ≥0; or it is equal to M 12 when d c1 <r 1 and ΔS out < 0. The unit controllers 329 and 429 are also absolutely identical and the operation of both can be sufficiently described by one controller 329, using the following example. The normalizing module 331 normaiizes the relative distance d c1 to the surge control line of the compressor 301 by multiplying d c1 by a coefficient, β 1 . The purpose of such normalization is to either position the operating point of the compressor under its maximum speed and required discharge pressure in such a way that: d cn1 = β 1 ·d c1 = 1 at its maximum, or to position each operating point at its maximum efficiency zone under the most frequent operating conditions. The coefficient β 1 may also be dynamically defined by a higher level optimization system. The output of normalizing module 331 together with the computed mass flows w c1 and w d1 received from computational module 327, and with the minimum discharge flow w m selected by selection control module 337, enters the computational module 330. For stable operation, we must actively share the load between series operated compressors. However, it is not enough to equalize the relative distances d ci of compressor operating points to their respective surge control lines; rather, load sharing is especially important when compressors operate on their surge control lines, and the relative distances d c1 and d c2 are equal to zero. The control system then becomes neutral and load sharing based solely on the relative distances d c1 and d c2 becomes impossible. The most convenient criterion for optimum series load sharing should consist of both the relative distance to the surge control line and the equivalent mass flow rate, which is equal to the minimum flow passing all series working compressors from the suction manifold 305 to its discharge manifold 413. The criterion used should provide for equivalent mass flow rates through all compressors and equal distances to the respective surge control lines. The computational control module 330 computes, as such criterion, the criterion B which is defined as follows: B 1 = w e1 (1-d cn1 ) where w e1 = w c1 -Δ 1 and Δ 1 = w m -w d1 The minimum discharge mass flow rate w m is selected by flow selection module 337 from mass flow rates w d1 and w d2 computed for compressors 301 and 401 respectively. In the system shown in Fig. 2(a), with sidestream mass flow rate w s2 positive, w d1 = w m and for compressor 301 Δ 1 = 0. But for compressor 401, value Δ 2 is positive and B 2 = (1-d cn2 )(w c2 -Δ 2 ) The output B 1 of computational module 330 is directed to the P+I module 335 as the process variable, and to the selection module 338. Selection module 338 selects B sp , the lowest criterion B value from the outputs of computational control module 330 and its counterpart of compressors 301 and 401 respectively. The selected lowest criterion B sp is used as a set point for both P+I modules of the respective unit controllers. For one of the two P+I modules, the criterion B i process variable is equal to the set point B sp . The output of this P+I module is, therefore, not changing. If B 1 ≠B 2 , the output of the other P+I module will, however, be changing to equalize the criterion B values. If, as in this example, the low-pressure compressor 301 is selected as the leader, changes of the output of the summation module 334 will be based only on the incremental changes of the output of P+I+D module 340. The station controller 336, by means of nonlinear control function 332, exactly as it was described for the parallel operation, can decrease or increase the output of the summation module 334 only if the relative distance d c1 of the operating point of the compressor to its surge control line is greater than or equal to the preset level "r 1 ." When d c1 < r 1 , the P+I+D module 340 can only increase the output of the summation module 334. In the case when criterion B 2 is lower than criterion B 1 , the high-pressure compressor 401 is selected as the leader. As such, the changes of the output of the summation module 334 are based on changes of the output of the P+I module 335, and on incremental changes of the output of the P+I+D module 340. As a result, the speed of compressor 301 is corrected to equalize the computed criterion B 1 value with the selected minimum criterion B sp =B 2 . Equalizing criterion B values in the case when the recycle valves 309 and 409 are closed, provides automatically for equalizing the relative distances d c1 and d c2 , because the equivalent mass flows through both compressors are equal by the nature of series operation. When the operating points of both compressors are on the respective surge control lines, and normalized relative distances d cn1 and d cn2 are kept equal to zero by the antisurge controllers 328 and 428 respectively, equalizing criterion B; automatically provides for equalizing the equivalent mass flow rates through both compressors, which, in turn, provides for optimum load sharing, including the recycle load. The operation of the system shown in Fig. 2 is described using the following example. Let us assume initially that compressors 301 and 401 work with speeds N 1 and N 2 respectively. Their recycle valves 309 and 409 are completely closed, and the compressors are operating on equal normalized relative distances to their respective surge control lines: d cn1 = d cn2 = a 1 >0 Therefore, both criterion values B 1 and B 2 are also equal: B 1 = B 2 = a 2 Also, the pressure in suction drum 304 of the compressor station is equal to the required set point, therefore, ΔS out =0. Assume further that the amount of flow entering suction drum 304 decreases. As a result, the suction pressure in the suction drum will also decrease. Since station controller 336, through incremental changes ΔS out of the output of its P+I+D module 340, will start to decrease the outputs of both multipliers of the unit controllers 329 and 429, decreasing also the outputs of both summation modules of unit controllers 329 and 429, thereby decreasing the set points of the speed governors 303 and 403 to decrease the speed of both compressors. Assume also that as soon as the speeds of both compressors start to decrease, the criterion B 2 becomes greater than criterion B 1 . Then selection control module 338 of the station controller 336 selects B 1 as a set point B sp for both P+I modules of the unit controllers 329 and 429. The output of P+I module 335 of the unit controller for compressor 301 will not be changing, and the summation control module 334 will decrease its output only under the influence of the output of P+I+D module 340. On the contrary, the output of the P+I module of the high-pressure compressor, decreases in order to equalize criterion B 2 with criterion B 1 . This process continues until the pressure on suction drum 304 is restored to the required level, and both criterion B 1 and criterion B 2 are equalized. Assume further that there is a continuous decrease of the flow supply to the suction drum 304, and that the operation of the control system shown in Fig. 2 brings the operating points of both compressors to their respective surge control lines, which means that d c1 =d c2 =0. If, under the above circumstances, the pressure in the suction drum 304 is still lower than required, then the station controller 336 through its P+I+D module 340, further decreases the distances d c1 and d c2 until both are equal to the preset levels "r 1 " and "r 2 ," respectively. Simultaneously, both antisurge controllers 328 and 428 will start to open the recycle valves 309 and 409. If the suction pressure continues to drop, the P+I+D module 340 will override the antisurge controllers 328 and 428 to open the recycle valves even more to restore the suction pressure to the required level. As soon as the distances d c1 and d c2 become higher than their respective preset levels "r 1 " and "r 2 ," station controller 336, through the summation units of the respective unit controllers, will decrease the compressor speeds. This process will continue until the suction pressure is at the required level and the respective criterion B values for both compressors are equal, thereby optimally sharing the compression load.

Claims: A compressor station comprising a station control means (129;336) for producing a station control signal (ΔS out ) in dependence on a detected main gas parameter, a plurality of compressors (101,201;301,401), antisurge control means (109,209;328,428), for each compressor, for producing respective surge control variable signals (d c1 ,d c2 ) and protecting each compressor from surge, and a respective unit final control means (123,223;329,429) for controlling the performance of each compressor (101,201;301,401), characterized by a selection means (132;338) for identifying one compressor (101,201;301,401) as the leader compressor on the basis of the operation of each compressor (101,201;301,401) relative to a respective surge control line and producing a further control signal (B sp ) on the basis of the operation of the identified leader compressor, and the unit control means (123,223;329,429), associated with non-leader compressors, being configured to use the further control signal as a reference so as to balance the performances of the compressors (101,201;301,401). A compressor station according to claim 1, wherein each unit control means (123,223;329,429) and antisurge control means (109,209;328,428) uses the surge control variable signal (d c1 ,d c2 ) of its respective compressor (101,201;301,401) to discriminate for and against control of that compressor (101,201;301,401) on the basis of the station control means signal (ΔS out ). A compressor station according to claim 1 or 2, wherein the compressors (101,201) operate in parallel and said further control signal (B sp ) is derived from the difference between the current operating point of the leader compressor (101,201) and its surge control line. A compressor station according to claim 3, wherein the antisurge control means (109,209) are operative to produce the respective surge control variable signals (d c1 ,d c2 ) by calculating the distance between the respective compressor's (101,201) operating point and its surge control line or a value directly related thereto, the surge control line representing a boundary of a region in which a respective recycle valve (106,206) remains closed. A compressor station according to claim 4, wherein each antisurge control means (109,209) comprises means (117,118) for producing a surge control variable signal (d c1 ,d c2 ), a P + I controller (119) receiving said surge control variable signal (d c1 ,d c2 ) and a zero set-point value signal as its inputs, calculation means (121) for multiplying a change in the station control signal (ΔS out ) by a non-linear function of said surge control variable signal (d c1 ,d c2 ), a multiplier means (122) for multiplying said change in the station control signal (ΔS out ) by the output of said calculation means (121), and an output processing means (120) responsive to the output of said P + I controller (119) and the output of said multiplying means (122) for producing a recycle valve control signal. A compressor station according to claim 5, wherein the output processing means (120) comprises a summing means for summing the output of said P + I controller (119) and the output of said multiplying means (122). A compressor station according to claim 5, wherein the output processing means (120) comprises selection means arranged to select either the output of said P + I controller (119) or the output of said multiplying means (122) whichever is indicative of the greater required recycle valve opening. A compressor station according to any one of claims 3 to 7, wherein each unit control means (123,223) of each compressor is operative to equalize a function of the respective surge control variable signal (d c1 ,d c2 ) with said further control signal (B sp ). A compressor according to claim 8, wherein each unit control means (123,223) comprises a normalizing means (124) for multiplying the respective surge control variable signal (d c1 ,d c2 ) with a coefficient (β 1 ) for ensuring operation of the respective compressor within at least one predetermined condition, a P + I controller (125) arranged to receive the output of the normalizing means as input and the further control signal (B sp ) as set-point, calculation means (127) for multiplying a change in the station control signal (ΔS out ) by a non-linear function of said surge control variable signal (d c1 ,d c2 ), a multiplier means (128) for multiplying said change in the station control signal (ΔS out ) by the output of said calculation means (127), and a summing means (126) for summing the output of said P + I controller (125) and said multiplier means (128) to produce a compressor speed control signal (u out1 ,u out2 ). A compressor station according to any one of claims 3 to 9, wherein the station control means (129) comprises selection means (132) for selecting the largest of the surge control variable signals (d c1 ,d c2 ) on which to base the further control signal (B sp ). A compressor station according to claim 1 or 2, wherein the compressors (301,401) operate in series, and said further control signal (B sp ) is derived from the difference between the current operating point of the leader compressor and its surge control line and the equivalent mass flow rate through the leader compressor. A compressor station according to claim 11, wherein the antisurge control means (328,428) are operative to produce the respective surge control variable signals (d c1 ,d c2 ) by calculating the difference between the respective compressor's (301,401) operating point and its surge control line or a value directly related thereto and to produce input and output gas mass flow rate signals (w c1 ,w d1 ), the surge control line representing a boundary of a region in which a respective recycle valve (310,410) remains closed. A compressor station according to claim 12, wherein each antisurge control means (328,428) comprises means (321,327) for producing a surge control variable signal (d c1 ,d c2 ), a P + I controller (322) receiving said surge control variable signal (d c1 ,d c2 ) and a zero set-point value signal as its inputs, calculation means (325) for multiplying a change in the station control signal (ΔS out ) by a non-linear function of said surge control variable signal (d c1 ,d c2 ), a multiplier means (324) for multiplying said change in the station control signal (ΔS out ) by the output of said calculation means (325), and an output processing means (323) responsive to the output of said P + I controller (322) and the output of said multiplying means (324) for producing an recycle valve control signal. A compressor station according to claim 13, wherein the output processing means (323) comprises a summing means for summing the output of said P + I controller (322) and the output of said multiplying means (324). A compressor station according to claim 13, wherein the output processing means (323) comprises selection means arranged to select either the output of said P + I controller (322) or the output of said multiplying means (324) whichever is indicative of the greater required recycle valve opening. A compressor station according to any one of claims 12 to 15, wherein each unit control means (329,429) comprises a normalizing means (331) for multiplying a respective surge control variable signal (d c1 ,d c2 ) with a coefficient (β 1 ) for ensuring operation of the respective compressor within at least one predetermined condition, a computational means (330) for producing a control criterion signal (B 1 ,B 2 ) from the output of the normalizing means, the input and output gas mass flow rate signals (w c1 ,w d1 ) for the respective compressor and a signal (w m ) representing the smallest output gas mass flow rate of all those for the compressors, a P + I controller (335) arranged to receive the output of the computational means as input and the further control signal (B sp ) as set-point, calculation means (332) for multiplying a change in the station control signal (ΔS out ) by a non-linear function of said surge control variable signals (d c1 ,d c2 ), a multiplier means (333) for multiplying said change in the station control signal (ΔS out ) by the output of said calculation means (332), and a summing means (334) for summing the output of said P + I controller (335) and said multiplier means (333) to produce a compressor speed control signal (u out1 ,u out2 ). A compressor station according to any one of claims 12 to 16, wherein the station control means (336) comprises first selection means (338) for selecting the smallest of said control criterion signals (B 1 ,B 2 ) as the further control signal (B sp ) and second selection means (337) for selecting the smallest output gas mass flow rate signal (w m ) of all those of the compressors. A compressor station according to any one of claims 12 to 17, wherein each unit control means (323,423) of a non-leader compressor is operative to equalize the respective control criterion signals (B 1 ,B 2 ) with said further control signal (B sp ). A method of operating a compressor station comprising a plurality of compressors (102,201;301,401), the method comprising the steps of: producing a station control signal (ΔS out ) in dependence on a detected main gas parameter; producing respective surge control variable signals (d c1 ,d c2 ) for each compressor (101,201;301,401) for protecting each compressor from surge; and controlling each compressor (101,201;301,401) in dependence on the station control signal (ΔS out ), characterized by identifying one compressor (101,201;301,401) as the leader compressor (101,201;301,401) on the basis of the operation of each compressor (101,201;301,401) relative to a respective surge control line; producing a further control signal (B sp ) on the basis of the operation of the identified leader compressor (101,201;301,401); and using the further control signal (B sp ) as a reference for controlling non-leader compressors (101,201;301,401) so as to balance the performance of the compressors (101,201;301,401). A method according to claim 19, wherein the surge control variable signal (d c1 ,d c2 ) of a compressor (101,201;301,401) is used to discriminate for and against control of that compressor (101,201;301,401) on the basis of the station control signal (ΔS out ). A method according to claim 19 or 20, wherein the compressors (101,201) are operated in parallel and said further control signal (B sp ) is derived from the difference between the current operating point of the leader compressor (101,201) and its surge control line. A method according to claim 21, wherein the surge control variable signals (d c1 ,d c2 ) are produced by calculating the distance between the respective compressor's (101,201) operating point and its surge control line or a value directly related thereto, the surge control line representing a boundary of a region in which a respective recycle valve (106,206) remains closed. A method according to claim 22, including production of a recycle valve control signal by the following steps: applying the surge control variable signal (d c1 ,d c2 ) to a P + I control process with a zero set-point value; multiplying changes in the station control signal (ΔS out ) by a non-linear function of the surge control variable signal (d c1 ,d c2 ); multiplying the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ); and outputting the recycle valve control signal in dependence on the output of the P + I control process and the result of the multiplication of the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ). A method according to claim 23, wherein the step of outputting the recycle valve control signal comprises summing the output of the P + I control process and the result of the multiplication of the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ). A method according to claim 23, wherein the step of outputting the recycle valve control signal comprises selecting either the output of the P + I control process or the result of the multiplication of the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ). A method according to any one of claims 21 to 25, comprising the step of controlling each compressor (101,201) so as to equalize a function of its surge control variable signal (d c1 ,d c2 ) with said further control signal (B sp ). A method according to claim 26, wherein said equalizing step for each non-leader compressor (101,201) comprises multiplying its surge control variable signal (d c1 ,d c2 ) by a coefficient (β 1 ) for ensuring operation of the compressor within at least one predetermined condition, performing a P + I control process on the result of said multiplication with the further control signal (B sp ) as the set-point, producing the product of a change in the station control signal (ΔS out ) and a non-linear function of the respective surge control variable signal (d c1 ,d c2 ), multiplying said change in the station control signal (ΔS out ) by said product, and summing the output of the P + I control process and result of the multiplication of said change in the station control signal (ΔS out ) and said product. A method according to any one of claims 21 to 27, comprising the step of selecting the largest of the functions of the surge control variable signals (d c1 ,d c2 ) to be the further control signal (B sp ). A method according to claim 19 or 20, wherein the compressors (301,401) are operated in series, and said further control signal (B sp ) is derived from the difference between the current operating point of the leader compressor and its surge control line and the equivalent mass flow rate through the leader compressor. A method according to claim 29, comprising the steps of: producing a surge control variable signal (d c1 ,d c2 ) for each compressor (301,401) by calculating the difference between the operating point of each compressor (301,401) and its surge control line or a value directly related thereto; and producing input and output gas mass flow rate signals (w c1 ,w c2 ) for each compressor (301,401), the surge control line representing a boundary of a region in which a respective recycle valve (310,410) remains closed. A method according to claim 30, including production of a recycle valve control signal by the following steps: applying the surge control variable signal (d c1 ,d c2 ) to a P + I control process with a zero set-point value; multiplying changes in the station control signal (ΔS out ) by a non-linear function of the surge control variable signal (d c1 ,d c2 ); multiplying the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ); and outputting the recycle valve control signal in dependence on the output of the P + I control process and the result of the multiplication of the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ). A method according to claim 31, wherein the step of outputting the recycle valve control signal comprises summing the output of the P + I control process and the result of the multiplication of the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ). A method according to claim 31, wherein the step of outputting the recycle valve control signal comprises selecting either the output of the P + I control process or the result of the multiplication of the changes in the station control signal (ΔS out ) by the product of the changes in the station control signal (ΔS out ) and said non-linear function of the surge control variable signal (d c1 ,d c2 ). A method according to any one of claims 30 to 33, comprising equalizing the performance of the compressors (301,401) by applying to the non-leader compressors (301,401) the steps of: multiplying its surge control variable signal (d c1 ,d c2 ) by a coefficient (β 1 ) for ensuring operation of the compressor within at least one predetermined condition; computing a control criterion signal (B 1 ,B 2 ) from the result of said multiplication, the input and output gas mass flow rate signals (w c1 ,w d1 ) for the respective compressor and a signal (w m ) representing the smallest output gas mass flow rate of all those for the compressors (301,401); performing a P + I control process on said control criterion signal (B 1 ,B 2 ) with the further control signal (B sp ) as the set-point; producing the product of a change in the station control signal (ΔS out ) and a non-linear function of the respective surge control variable signal (d c1 ,d c2 ); multiplying said change in the station control signal (ΔS out ) by said product; and summing the output of the P + I control process and result of the multiplication of said change in the station control signal (ΔS out ) and said product. A method according to any one of claims 30 to 34, comprising the step of selecting the smallest of said control criterion signals (B 1 ,B 2 ) to be the further control signal (B sp ) and selecting the smallest output gas flow rate signal (W m ) of all those of the compressors (301,401). A method according to any one of claims 30 to 35, comprising operating each non-leader compressor (301,401) so as to equalize its control criterion signal (B 1 ,B 2 ) with the further control signal (B sp ).

Inventors: MIRSKY SAUL, NEGLEY PAUL M, REINKE PAUL A, SIBTHORP ROBERT J DR, STAROSELSKY NAUM DR, MIRSKY, SAUL, NEGLEY, PAUL M., REINKE, PAUL A., SIBTHORP, ROBERT J., DR., STAROSELSKY, NAUM, DR.
Applicants: COMPRESSOR CONTROLS CORP, COMPRESSOR CONTROLS CORPORATION
Application year: 1993
Main classification: F04D27
IPC codes: F04B49, G05D16, F04D27