CA1256835A - Method and apparatus for controlling a multicompressor station - Google Patents
Method and apparatus for controlling a multicompressor stationInfo
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- CA1256835A CA1256835A CA000475723A CA475723A CA1256835A CA 1256835 A CA1256835 A CA 1256835A CA 000475723 A CA000475723 A CA 000475723A CA 475723 A CA475723 A CA 475723A CA 1256835 A CA1256835 A CA 1256835A
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- compressor
- surge
- control line
- station
- criterion
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Abstract
ABSTRACT OF THE DISCLOSURE
A method of control and a control apparatus are provided herein for load sharing between multiple compressors working in parallel and/or in series which enables all of the load-sharing compressors to carry their optimum share of the load. One embodiment of the method comprises transforming the predetermined equation of each respective surge control line into a variable criterion representing a relative distance between each respective compressor's operating point and its surge control line, the variable criterion reaching a predetermined value when the relative distance reaches zero value; simultaneously changing the set points for all of the respective criteria representing the relative distance between each respective compressor's operating point and its surge control line by changing an output of the station control means to control the performance of the compressor station; and keeping the set points equal and maintaining the criterion substantially equal at substantially all times for each of the respective compressors. An embodiment of the improved apparatus comprises means for maintaining one of said gas parameters constant for each respective compressor;
means for computing a criterion representing a relative distance between each respective compressor's operating point from the respective surge control line; set point control means for controlling one of the gas parameters by simultaneously changing the set points of each respective compressor for the criterion, Abstract of the Disclosure continued (page 2) representing the relative distance between each respective compressor's operating point and its surge control line; and means for maintaining the criterion substantially equal at substantially all times for each of the compressors.
A method of control and a control apparatus are provided herein for load sharing between multiple compressors working in parallel and/or in series which enables all of the load-sharing compressors to carry their optimum share of the load. One embodiment of the method comprises transforming the predetermined equation of each respective surge control line into a variable criterion representing a relative distance between each respective compressor's operating point and its surge control line, the variable criterion reaching a predetermined value when the relative distance reaches zero value; simultaneously changing the set points for all of the respective criteria representing the relative distance between each respective compressor's operating point and its surge control line by changing an output of the station control means to control the performance of the compressor station; and keeping the set points equal and maintaining the criterion substantially equal at substantially all times for each of the respective compressors. An embodiment of the improved apparatus comprises means for maintaining one of said gas parameters constant for each respective compressor;
means for computing a criterion representing a relative distance between each respective compressor's operating point from the respective surge control line; set point control means for controlling one of the gas parameters by simultaneously changing the set points of each respective compressor for the criterion, Abstract of the Disclosure continued (page 2) representing the relative distance between each respective compressor's operating point and its surge control line; and means for maintaining the criterion substantially equal at substantially all times for each of the compressors.
Description
The present inYention relate~ gPn~rally to a method and apparatU~ ~or compressors, and more partlcul~rly to a method and apparatus for con~rolling compre~ors in parallel or in ~eries which en~bles all o~ the load-sharing comp~e~sor ~o carry kheir optimum share of the load.
Conventional control sy~tems of compressor 3tation con-si~tlng of centrifugal and/or axial co~prea~ors do not cope with two major problems:
Those associated with load sharing of multiple com-lo pressors workiny in parallel and/or in series and those associated with controller loop interaction especially betw~en process con-trol loops and an~isurge pro~ect~ve loops.
Conventional load-sharing techniques or multiple com-pressors allow one compressor to operate closer to its surge limi~ than other compressors. For instance for parallel opera-tion~, one compressor may even be overloaded to the risk o~
tripping off and shutting down, while another compressor is less loaded on the verge o~ going in~o surge. This creates not only the dangers of surge-and overload-created compressor damage, but 20 also of wasted energy through recycling or blowing o~f flow and the possibility of process interruption.
Since conventional load-sharing control ætrate~ies are unable to load com~ressors properly, compressors frequently wrestle loads bac~ and forth hetween them. This load oscillation degrade~ ~y~kem perfor~ance and e~iaiency, and lt may lead to damage or proces 8 ~hutdown.
il3~
Th$ r~ormal function of a station process control is to maintain.its controlled variable ~qu~l to 80me 8~t poi~. For instanc~ may be reguired to main~ain ~he ~i~charg~ pr~u~e or flow o~ ~he ~tation. When called upon to pr~tect one of the compre880r8 ~rom ~urge, an antisurgQ controller wlll haYel as its ~unction, to reduc~ the COmpre3~0rB~ pra~8ur~ dlfferential and to increase ~low at ~he same time. Oper~ting independently and at croS8 purpo~eBt then, conventional procegg ~ontroller~ and anti~
SUrge controllers may create 03cillation. of pressure and flow lo that degradP process control and reduce antisurge protection.
To prevent such oscillations, conventional process and antisurge controllers are made to react slowly. l~his results in degraded process control, reduced antisurge pro~ection and surye damage becau~ 0~ the 810W con~rol re~ponue and because o~ intar-action between the two controllers. It may also shorten com-pressor life due to surges resulting from less than optimum con~
trol. Conventional controllers cannot achieve fast control and ~ stability at the same time. They, therefore, are detuned to pro-vide ~luggi~h, but stable, control.
The control of multicompressors may be significantly improved by using a cascade control including a process control loop, load-sharing control loops and antisurge protective loops.
The load~~haring loops enable all of the load-sharinq compressors to carry their optimum share of total flo~ ~for parallel opera-tions) or ~otal pre~sure differential (~or series operation), pro-viding ~or e~uidistan~ ~peration from surge aontrol lines. In addition, each load sharing loop provide~ for effective decoupling between process control loop and control member of compressor while the compres~or~' operating point crosse~ the surge control line. I
3L;256~35 An object of one aspect of this invention is decoupling between the process control and compressor by using the cascade system including a primary process control loop and a secondary load control loop. This secondary loop controLs the criterion representing the relative distance between operating point of compressor and its surge control line. This criterion can be increased only by the limited value which is not enough to drive a compressor to surge.
An object of another aspect of this invention is optimization of load-sharing between multiple compressors by controlling the criterion representing the relative distance between operating point of compressor and its surge control line on the equa], level for all load-sharing compressors in operatlon.
~ n obJect of still anothcr aspe~t Oe the Lnvention Is to provide $or control apparatus of a type described above, which is characterized by its simplicity, great transient and steady state precision and high reliability.
An obJect of a principal aspect of the present invention is to enable all of the load-sharing multiple compressors to carry their optimum share of total flow or total pressure differential (equidistant from a preselected surge control line) without risking surge in any of the compressors. Some of the advantages of this invention are the expansion of the safe operating zone of the compressor station without re-circulation or blow-off, and the increase of safety of operation of compressors and the process using the compressed gas.
s.~ :
~56al3$
By a broad aspect of this invention, an improved me~hod is provided for controlling a compressor station receiving ~as from an upstream process~ compressing the gas and delivering the compressed gas to a process downstream thereof, the compressor station having a station control means to ad3ust the station performance to the demand of a process located upstream or downs-tream, -the compressor station including also a plurality of dynamic compressors driven by a plurality of respective prime movers, each of the dynamic compressors having a variable performance, a main control means associated with each of the compressors for changing its performance and a surge control means assoc.iated with each respective compressor for ma:intaining the respectlve compressor alon~ a surge control :line havln~ a pre-determined equation and located a predetermilled distance orm the respective surge limit of each respective compressor. The improved method comprises: transforming the predetermined equa-tion of each respective surge control line into a variable criterion representing relative distance between each respective compressor's operating point and its surge control line, the variable criterion reaching a predetermined value when the relative distance reaches zero value; simultaneously changing the set points for all of the respective criteria representing the relative distance between each respective compressor's operatin~
point and its sur~e control line by changing an output of the station control means to control the performance of the A, ~
6~l35 compressor station; and keeping the set points equal and maintaining the criterion substantially equal at substantially all times for each of the respective compressors.
The method may further include limiting the respective set points and providing for a decouplin~ between each respective compressor and the station control means to avoid a dan~erous approaching of the respective surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from the control line.
In a preferred embodiment, the equation is Kf(Z)Y + b = X, where Kf(Z) is the variable slope of the surge control line and b is the bias representLng the respective d:Lstance between the sllr~e contrul ;I:i.n~ and thc ~urge ;limlt line, the criterion equaling Kf(Z)Y ~ b where X, Y and Z are the compressor variables representing the respective compressor performance and are used to build the respective compressor maps.
X may be a parameter related to 10w through each respective compressor, Y may be a parameter related to the pressure ditferential across each respective compressor, and Z may be a parameter related tD the speed of rotation of each respective compressor. Alternatively, X may be a parameter related to flow through each respective compressor, Y may be a parameter related L ~
~2S6~3~
to the pressure differential across eacll respective compressor, and Z may represent the position of guide vanes of each respective compressor. Still further alternatively, X may be a parameter related to flow through each respective compressor, and Z may represent a parameter related to the inlPt density of the fluid entering each respective compressor.
By another aspect of this invention, an improved method is provided for controlling a compressor s-ta~ion receiving gas from an upstream process, compressing the gas and delivering the compressed gas to a process downstream thereof, the compressor station having a station control means to ad~ust the station performance to the demand o~ a process located upstream or downs~ream, the compressor statLon including also one dynam:ic compressor driverl by a pr:ime mover, the dyrlclmic cumpresscr havirlg a variable performance, a main control means associated with the compressor for changing its performance, and a surge control means associated with the compressor for maintaining the compressor along a surge control line having a predetermined equation and located a predetermined distance from its surge limit. The improved method comprises: transforming the predetermined equation of the surge control line into a variable criterion representing a relative distance between the compressor's operating point and its surge control line, the variable criterion reaching some predetermined value when the relative distance reashes zero value; and changing set points for .. . .
. . ` ~ ~ !
~2~i6~i the criterion representing the relative distance between the compressor's operating point and its surge control line by chan~ing an output of the station control means to control the performance of the compressor station and limiting the set poin-t and providing for a decoupling between the compressor and the station control means to avoid a dangerous approaching of the surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from the control line.
By still another aspect of this invention, an improved method is provided for controlling a compressor station receiving a gas from upstream of a process, compressing ~.he gas and del:lver:Ln~ the compresse(l ~as to tht! process dowrlstream thereo~, the compressor station including at least one dynamic compressor driven by at least one respective prime mover, each of the dynamic compressors having a variable performance defined by three variable parameters, a main control means associated with each of at least one compressor for changing ~he performance of each respective compressor, a surge control means associated with each respective compressor for maintaining each respective compressor along a predetermined surge control line and located a predetermined distance from the respective surge limit of each respective compressor. The method comprises: maintaining one of the gas parameters constant for each respective compressor;
computing a criterion representing a relative distance between ~2~ 35i each respective compressor's operating point from the respec~ive surge control line; con-trolling one of the gas parameters by simultaneously changing ~he set points of each respec~ive compressor Eor the criterion, representing the relative distance between each respective compressor's operating point and its surge control line and maintaining the criterion substantially equal at substantially all times for each of the compressors.
By yet another aspect of this invention, improved apparatus is provided for controlling a compressor station receiving a gas from upstream of a process, compressing the gas and delivering the compressed gas to the process downstream thereof, the compressor station including a plurality of dynamic compressors driven by a plurality o~ respective prime movers, each the ~ynamic compressors having a var:Lable pcr~ormance defined by three variable parameters, a main control means associated with each of the compressors for changing the performance of each respective compressor, a surge control means associated with each respective compressor for maintaining each respective compressor along a predetermined surge control line and located a predetermined distance from the respective surge limit of each respective compressor. The improved apparatus comprises: means for maintaining one of the gas parameters constant for each respective compressor; means for computinV a criterion representing a relative distance between each respective compressor's operating point from the respective surge control ;
,. ..
ji6~35 g line; set point control means for controlling one of the gas parameters by simultaneously changing the set points of each respective compressor for the criterion, representing the relative distance between each respective compressor's operating point and its surge control line; and means for maintaining said criterion substantially equal at substantially all times for each of the cDmpressors.
The means for maintaining one of the gas parameters ronstant may include a primary closed loop controller having an output.
The set point control means may comprise a secondary closed loop controller means for each of the compressors for receiving the output of the primary controller as its set point and changing the perormance oE each respective compressor by openlng its main control means, each Oe th~ secondary closed loop controller means further maintaining the criteriorl representing the relative distance between each compressor's operating point and each respective surge control line at the required level corresponding to the set point developed by said primary controller means.
According to one preferred embodiment of this invention, each compressor of a multicompressor station is operated by two interconnected control loops. The first of them is the antisurge control loop preventillg from crossing the preestablished safe operating surge control line and computing a criterion representing the distance between compressors' operating point and said surge control line. This control loop operates the antisurge recycle or blow-off valve. The secondary loop is the load-sharing control loop which is secondary loop in an automatic cascade system including also a primary master station controller.
In the accompanying drawings, Fig. 1 is a schematic diagram of a control system for a multiple compressor station cons~ructed in accordance with an embodiment of the present invention; and Fig. 2 shows a compressor map of a typical single one of the compressors of Fig. 1 with plotted lines representing operating conditions.
`' " !
~ .'-'`' a.;i~56~35 Referring now to the dxawlng~, Fig. 1 shows a compressor statlon wlth a control system including a load control loop constructed i~ accordance with the present invention~ This installa~ion includeR dynamic compressors 101 and 201 working lin paral~el for compressing a gas. Turbine drives 102 and 202 ¦are provided for driving compressors 101 and 201 respectively~
Steam distributing means 103 and 203 having actuators 104 and 20 are also connected to compressors 101 and 201 respectively. A
pipeline 105 connects the compressors 101 and 20S with a user 106 of compressed gas.
Each compressor 101 and 201 i8 ~upplied by A relief means 107 or 207 with actua~ors 108 or 208 respectivel~. Each com-pressor 101 a~d 201 also has a check valve 109 or 209 associated therewith and being located downstxeam from the respective xelief means 107 and 207 respectively and upstream from the common pipeline 105.
The control system of Fig. 1 multiple compressor station consists of nine control modules. The first suah control module is a module 110 including four transmitters: a pressure differen-tial transmitter 111, measuring a pressure differential acrossthe out orifice 112; a pressure differential transmitter 113, measuring the pressure differential across compressor 101; a speed transmitter 114 and a station pressure tra~ mitter llSr The second control module 210 includes three transmitters:
a pressure differential transmitter 211, measuring the pressure .
differential across the outle~ orifice 212; a pressure differen-tial transmitter 213 across compre~sor 201 and a speed trans-mitter 214.
~ third and a fourth moduie~ are speed governor 116 and $
216 controlling the speed of ~urbine~ 102 and 202 respectively.
A fifth and a sixth control modules are the antisurge modules controlling 117 and 217 operating the relief valves 107 and 207 respectively.
. The ~quat~on of th3 surge control line o~ each controller 117 and 217 is, for instance:
Ki PCi ~ bi = P i ( 1 ) where: ~ci is the pressure differential across compressor;
o P . is the pressure differential across Ol the inlet orifice.
K1 is the slope of surge control line;
bi is the distance between surge control and surge limit line7`
i i5 the identification index of the compressor.
If some other equations of surge control line are used, depending on selected compressor map, their general equation of the surge control line may be presented as:
Kf~z) Y + b = X . (2) 0 where: Y and X are chosen compressor map coordinates;
K X f(Z) is the variable slope of surge control line;
Z is a con~rol variable influencing the slope of surge limit, such as molecular weight, po ition of guide vanes, etc.
On th~ known Compressor map, the distance between the : ~ .
compxessors operating point and selected Surge Control Line 1l .. ,. ~
6~
may be measured using Criterion "S" as ~ollows:
S = kf(Z)Y + b S varies from values close to zero to 1. The value of 1 i8 reached w~ile the compre~sor~' operating point meet~ the Surge ~ontrol Line. The antisurge controllers 117 and 21'7 com-pute criterion "S" for compr~ssors 101 and 201 respectivelyO
A seventh and an eighth control module comprise the load controllers 118 and 2180 These controllers control the criterion "S" for compressors 101 and 201 respectively. Computed by each lo controller 117 or 217, the Criterion "S" i9 normalized accord-ing to the ~ollowing equation:
Sni = Bli (Si 1) B2 j I ) where: Sni i~ normalized value o~ cri~erion "S";
Bli and B are constant coefficients.
This normalization is provided to transform the cri~erion SL with the minimum value variation from 0.1, for instance, to 0.8 to the Sui value with the range from zero to 1.
In addition, if B2 = 1, then the process variable o~ load Controller 118 or 218 Sni reaches its highest value 1, while the Critarion "Si" also reaches the value 1, which means that the compressors' operating point meets its Surge Control Line.
In order to prevent the pressure increase up to the level C, the "S" Criterion goes through he first order lag filter .
located in the Controller 118. Thi~ pr~vents interaction ~2~6~
between loops controlling "S" criterion of load controllers 118 and 218 and discharge pressure through pressure controller 119.
The transient process which starts at the point A has the transient path A~D without the above-identified first order lag filter and it has the transient path ABD with the first order lag filter disposed within load controllers 118 and 218 and in both cases ends while the compressors' operating point reaches the location D on the compressor map (See Fig. 2.). Further increase of the process resistance finally moves the compressors' operating point to the location where it meets with the surge control line antisurge controller 117.
Ie a normalization coefficient B2 is equal to 1, then the above-identiied point ~ corre~ponds to the maximum olltpnt of ~he master controLler 119. If, however B2 is slightly less than I
(e.g. if B2 = 0.98.), then the master pressure controller 119 still has room to drive the operating point toward surge, crossing the surge control line of the controller 117. ~fter such crossing, the antisurge controller 117 starts to open the relief valve 107, until criteria S is now restored to the value 1, corresponding to t~e point D on the compressor map~ The main advantage of the above scheme is the decoupling provided by load controller 118 preventing the master pressure controller 119 from driving compressor 101 toward surge after the compressor crosses its surge control line.
~C :
~2~
If both compressors 101 and 201 are in operation, then the master pressure controller 119 changes the set points for the load controllers 118 and 218, controlling normalized criterion Sn~ The same settings of coefficients s21 for both compressors will cause then the simultaneous reaching of the surge control lines. This is also true for compressors operating in series or a combination of machines operated in parallel and in series.
The above-identified scheme is very flexible and can be used for different size compressors.
It is noted that the compressor map of Fig. 2 shows how controllers move the operating point on the compressor map. The first load sharing controller (e,g. l18) moves the operatirlg point of the compressor toward the surge control line (S~l). As soon as the operating poin~ meets with the surge control line, the antisurge controller (e.g. 117) will start to open the relief valve (e.g. 107), keeping the operating point on the surge control line. At the same time, the master pressure controller (119) prevents a changing of the pressure. As a result the operating point will stay at the point of intersection of the line of constant pressure (ADE) with the surge control line ( S--l ) .
, ~
Conventional control sy~tems of compressor 3tation con-si~tlng of centrifugal and/or axial co~prea~ors do not cope with two major problems:
Those associated with load sharing of multiple com-lo pressors workiny in parallel and/or in series and those associated with controller loop interaction especially betw~en process con-trol loops and an~isurge pro~ect~ve loops.
Conventional load-sharing techniques or multiple com-pressors allow one compressor to operate closer to its surge limi~ than other compressors. For instance for parallel opera-tion~, one compressor may even be overloaded to the risk o~
tripping off and shutting down, while another compressor is less loaded on the verge o~ going in~o surge. This creates not only the dangers of surge-and overload-created compressor damage, but 20 also of wasted energy through recycling or blowing o~f flow and the possibility of process interruption.
Since conventional load-sharing control ætrate~ies are unable to load com~ressors properly, compressors frequently wrestle loads bac~ and forth hetween them. This load oscillation degrade~ ~y~kem perfor~ance and e~iaiency, and lt may lead to damage or proces 8 ~hutdown.
il3~
Th$ r~ormal function of a station process control is to maintain.its controlled variable ~qu~l to 80me 8~t poi~. For instanc~ may be reguired to main~ain ~he ~i~charg~ pr~u~e or flow o~ ~he ~tation. When called upon to pr~tect one of the compre880r8 ~rom ~urge, an antisurgQ controller wlll haYel as its ~unction, to reduc~ the COmpre3~0rB~ pra~8ur~ dlfferential and to increase ~low at ~he same time. Oper~ting independently and at croS8 purpo~eBt then, conventional procegg ~ontroller~ and anti~
SUrge controllers may create 03cillation. of pressure and flow lo that degradP process control and reduce antisurge protection.
To prevent such oscillations, conventional process and antisurge controllers are made to react slowly. l~his results in degraded process control, reduced antisurge pro~ection and surye damage becau~ 0~ the 810W con~rol re~ponue and because o~ intar-action between the two controllers. It may also shorten com-pressor life due to surges resulting from less than optimum con~
trol. Conventional controllers cannot achieve fast control and ~ stability at the same time. They, therefore, are detuned to pro-vide ~luggi~h, but stable, control.
The control of multicompressors may be significantly improved by using a cascade control including a process control loop, load-sharing control loops and antisurge protective loops.
The load~~haring loops enable all of the load-sharinq compressors to carry their optimum share of total flo~ ~for parallel opera-tions) or ~otal pre~sure differential (~or series operation), pro-viding ~or e~uidistan~ ~peration from surge aontrol lines. In addition, each load sharing loop provide~ for effective decoupling between process control loop and control member of compressor while the compres~or~' operating point crosse~ the surge control line. I
3L;256~35 An object of one aspect of this invention is decoupling between the process control and compressor by using the cascade system including a primary process control loop and a secondary load control loop. This secondary loop controLs the criterion representing the relative distance between operating point of compressor and its surge control line. This criterion can be increased only by the limited value which is not enough to drive a compressor to surge.
An object of another aspect of this invention is optimization of load-sharing between multiple compressors by controlling the criterion representing the relative distance between operating point of compressor and its surge control line on the equa], level for all load-sharing compressors in operatlon.
~ n obJect of still anothcr aspe~t Oe the Lnvention Is to provide $or control apparatus of a type described above, which is characterized by its simplicity, great transient and steady state precision and high reliability.
An obJect of a principal aspect of the present invention is to enable all of the load-sharing multiple compressors to carry their optimum share of total flow or total pressure differential (equidistant from a preselected surge control line) without risking surge in any of the compressors. Some of the advantages of this invention are the expansion of the safe operating zone of the compressor station without re-circulation or blow-off, and the increase of safety of operation of compressors and the process using the compressed gas.
s.~ :
~56al3$
By a broad aspect of this invention, an improved me~hod is provided for controlling a compressor station receiving ~as from an upstream process~ compressing the gas and delivering the compressed gas to a process downstream thereof, the compressor station having a station control means to ad3ust the station performance to the demand of a process located upstream or downs-tream, -the compressor station including also a plurality of dynamic compressors driven by a plurality of respective prime movers, each of the dynamic compressors having a variable performance, a main control means associated with each of the compressors for changing its performance and a surge control means assoc.iated with each respective compressor for ma:intaining the respectlve compressor alon~ a surge control :line havln~ a pre-determined equation and located a predetermilled distance orm the respective surge limit of each respective compressor. The improved method comprises: transforming the predetermined equa-tion of each respective surge control line into a variable criterion representing relative distance between each respective compressor's operating point and its surge control line, the variable criterion reaching a predetermined value when the relative distance reaches zero value; simultaneously changing the set points for all of the respective criteria representing the relative distance between each respective compressor's operatin~
point and its sur~e control line by changing an output of the station control means to control the performance of the A, ~
6~l35 compressor station; and keeping the set points equal and maintaining the criterion substantially equal at substantially all times for each of the respective compressors.
The method may further include limiting the respective set points and providing for a decouplin~ between each respective compressor and the station control means to avoid a dan~erous approaching of the respective surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from the control line.
In a preferred embodiment, the equation is Kf(Z)Y + b = X, where Kf(Z) is the variable slope of the surge control line and b is the bias representLng the respective d:Lstance between the sllr~e contrul ;I:i.n~ and thc ~urge ;limlt line, the criterion equaling Kf(Z)Y ~ b where X, Y and Z are the compressor variables representing the respective compressor performance and are used to build the respective compressor maps.
X may be a parameter related to 10w through each respective compressor, Y may be a parameter related to the pressure ditferential across each respective compressor, and Z may be a parameter related tD the speed of rotation of each respective compressor. Alternatively, X may be a parameter related to flow through each respective compressor, Y may be a parameter related L ~
~2S6~3~
to the pressure differential across eacll respective compressor, and Z may represent the position of guide vanes of each respective compressor. Still further alternatively, X may be a parameter related to flow through each respective compressor, and Z may represent a parameter related to the inlPt density of the fluid entering each respective compressor.
By another aspect of this invention, an improved method is provided for controlling a compressor s-ta~ion receiving gas from an upstream process, compressing the gas and delivering the compressed gas to a process downstream thereof, the compressor station having a station control means to ad~ust the station performance to the demand o~ a process located upstream or downs~ream, the compressor statLon including also one dynam:ic compressor driverl by a pr:ime mover, the dyrlclmic cumpresscr havirlg a variable performance, a main control means associated with the compressor for changing its performance, and a surge control means associated with the compressor for maintaining the compressor along a surge control line having a predetermined equation and located a predetermined distance from its surge limit. The improved method comprises: transforming the predetermined equation of the surge control line into a variable criterion representing a relative distance between the compressor's operating point and its surge control line, the variable criterion reaching some predetermined value when the relative distance reashes zero value; and changing set points for .. . .
. . ` ~ ~ !
~2~i6~i the criterion representing the relative distance between the compressor's operating point and its surge control line by chan~ing an output of the station control means to control the performance of the compressor station and limiting the set poin-t and providing for a decoupling between the compressor and the station control means to avoid a dangerous approaching of the surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from the control line.
By still another aspect of this invention, an improved method is provided for controlling a compressor station receiving a gas from upstream of a process, compressing ~.he gas and del:lver:Ln~ the compresse(l ~as to tht! process dowrlstream thereo~, the compressor station including at least one dynamic compressor driven by at least one respective prime mover, each of the dynamic compressors having a variable performance defined by three variable parameters, a main control means associated with each of at least one compressor for changing ~he performance of each respective compressor, a surge control means associated with each respective compressor for maintaining each respective compressor along a predetermined surge control line and located a predetermined distance from the respective surge limit of each respective compressor. The method comprises: maintaining one of the gas parameters constant for each respective compressor;
computing a criterion representing a relative distance between ~2~ 35i each respective compressor's operating point from the respec~ive surge control line; con-trolling one of the gas parameters by simultaneously changing ~he set points of each respec~ive compressor Eor the criterion, representing the relative distance between each respective compressor's operating point and its surge control line and maintaining the criterion substantially equal at substantially all times for each of the compressors.
By yet another aspect of this invention, improved apparatus is provided for controlling a compressor station receiving a gas from upstream of a process, compressing the gas and delivering the compressed gas to the process downstream thereof, the compressor station including a plurality of dynamic compressors driven by a plurality o~ respective prime movers, each the ~ynamic compressors having a var:Lable pcr~ormance defined by three variable parameters, a main control means associated with each of the compressors for changing the performance of each respective compressor, a surge control means associated with each respective compressor for maintaining each respective compressor along a predetermined surge control line and located a predetermined distance from the respective surge limit of each respective compressor. The improved apparatus comprises: means for maintaining one of the gas parameters constant for each respective compressor; means for computinV a criterion representing a relative distance between each respective compressor's operating point from the respective surge control ;
,. ..
ji6~35 g line; set point control means for controlling one of the gas parameters by simultaneously changing the set points of each respective compressor for the criterion, representing the relative distance between each respective compressor's operating point and its surge control line; and means for maintaining said criterion substantially equal at substantially all times for each of the cDmpressors.
The means for maintaining one of the gas parameters ronstant may include a primary closed loop controller having an output.
The set point control means may comprise a secondary closed loop controller means for each of the compressors for receiving the output of the primary controller as its set point and changing the perormance oE each respective compressor by openlng its main control means, each Oe th~ secondary closed loop controller means further maintaining the criteriorl representing the relative distance between each compressor's operating point and each respective surge control line at the required level corresponding to the set point developed by said primary controller means.
According to one preferred embodiment of this invention, each compressor of a multicompressor station is operated by two interconnected control loops. The first of them is the antisurge control loop preventillg from crossing the preestablished safe operating surge control line and computing a criterion representing the distance between compressors' operating point and said surge control line. This control loop operates the antisurge recycle or blow-off valve. The secondary loop is the load-sharing control loop which is secondary loop in an automatic cascade system including also a primary master station controller.
In the accompanying drawings, Fig. 1 is a schematic diagram of a control system for a multiple compressor station cons~ructed in accordance with an embodiment of the present invention; and Fig. 2 shows a compressor map of a typical single one of the compressors of Fig. 1 with plotted lines representing operating conditions.
`' " !
~ .'-'`' a.;i~56~35 Referring now to the dxawlng~, Fig. 1 shows a compressor statlon wlth a control system including a load control loop constructed i~ accordance with the present invention~ This installa~ion includeR dynamic compressors 101 and 201 working lin paral~el for compressing a gas. Turbine drives 102 and 202 ¦are provided for driving compressors 101 and 201 respectively~
Steam distributing means 103 and 203 having actuators 104 and 20 are also connected to compressors 101 and 201 respectively. A
pipeline 105 connects the compressors 101 and 20S with a user 106 of compressed gas.
Each compressor 101 and 201 i8 ~upplied by A relief means 107 or 207 with actua~ors 108 or 208 respectivel~. Each com-pressor 101 a~d 201 also has a check valve 109 or 209 associated therewith and being located downstxeam from the respective xelief means 107 and 207 respectively and upstream from the common pipeline 105.
The control system of Fig. 1 multiple compressor station consists of nine control modules. The first suah control module is a module 110 including four transmitters: a pressure differen-tial transmitter 111, measuring a pressure differential acrossthe out orifice 112; a pressure differential transmitter 113, measuring the pressure differential across compressor 101; a speed transmitter 114 and a station pressure tra~ mitter llSr The second control module 210 includes three transmitters:
a pressure differential transmitter 211, measuring the pressure .
differential across the outle~ orifice 212; a pressure differen-tial transmitter 213 across compre~sor 201 and a speed trans-mitter 214.
~ third and a fourth moduie~ are speed governor 116 and $
216 controlling the speed of ~urbine~ 102 and 202 respectively.
A fifth and a sixth control modules are the antisurge modules controlling 117 and 217 operating the relief valves 107 and 207 respectively.
. The ~quat~on of th3 surge control line o~ each controller 117 and 217 is, for instance:
Ki PCi ~ bi = P i ( 1 ) where: ~ci is the pressure differential across compressor;
o P . is the pressure differential across Ol the inlet orifice.
K1 is the slope of surge control line;
bi is the distance between surge control and surge limit line7`
i i5 the identification index of the compressor.
If some other equations of surge control line are used, depending on selected compressor map, their general equation of the surge control line may be presented as:
Kf~z) Y + b = X . (2) 0 where: Y and X are chosen compressor map coordinates;
K X f(Z) is the variable slope of surge control line;
Z is a con~rol variable influencing the slope of surge limit, such as molecular weight, po ition of guide vanes, etc.
On th~ known Compressor map, the distance between the : ~ .
compxessors operating point and selected Surge Control Line 1l .. ,. ~
6~
may be measured using Criterion "S" as ~ollows:
S = kf(Z)Y + b S varies from values close to zero to 1. The value of 1 i8 reached w~ile the compre~sor~' operating point meet~ the Surge ~ontrol Line. The antisurge controllers 117 and 21'7 com-pute criterion "S" for compr~ssors 101 and 201 respectivelyO
A seventh and an eighth control module comprise the load controllers 118 and 2180 These controllers control the criterion "S" for compressors 101 and 201 respectively. Computed by each lo controller 117 or 217, the Criterion "S" i9 normalized accord-ing to the ~ollowing equation:
Sni = Bli (Si 1) B2 j I ) where: Sni i~ normalized value o~ cri~erion "S";
Bli and B are constant coefficients.
This normalization is provided to transform the cri~erion SL with the minimum value variation from 0.1, for instance, to 0.8 to the Sui value with the range from zero to 1.
In addition, if B2 = 1, then the process variable o~ load Controller 118 or 218 Sni reaches its highest value 1, while the Critarion "Si" also reaches the value 1, which means that the compressors' operating point meets its Surge Control Line.
In order to prevent the pressure increase up to the level C, the "S" Criterion goes through he first order lag filter .
located in the Controller 118. Thi~ pr~vents interaction ~2~6~
between loops controlling "S" criterion of load controllers 118 and 218 and discharge pressure through pressure controller 119.
The transient process which starts at the point A has the transient path A~D without the above-identified first order lag filter and it has the transient path ABD with the first order lag filter disposed within load controllers 118 and 218 and in both cases ends while the compressors' operating point reaches the location D on the compressor map (See Fig. 2.). Further increase of the process resistance finally moves the compressors' operating point to the location where it meets with the surge control line antisurge controller 117.
Ie a normalization coefficient B2 is equal to 1, then the above-identiied point ~ corre~ponds to the maximum olltpnt of ~he master controLler 119. If, however B2 is slightly less than I
(e.g. if B2 = 0.98.), then the master pressure controller 119 still has room to drive the operating point toward surge, crossing the surge control line of the controller 117. ~fter such crossing, the antisurge controller 117 starts to open the relief valve 107, until criteria S is now restored to the value 1, corresponding to t~e point D on the compressor map~ The main advantage of the above scheme is the decoupling provided by load controller 118 preventing the master pressure controller 119 from driving compressor 101 toward surge after the compressor crosses its surge control line.
~C :
~2~
If both compressors 101 and 201 are in operation, then the master pressure controller 119 changes the set points for the load controllers 118 and 218, controlling normalized criterion Sn~ The same settings of coefficients s21 for both compressors will cause then the simultaneous reaching of the surge control lines. This is also true for compressors operating in series or a combination of machines operated in parallel and in series.
The above-identified scheme is very flexible and can be used for different size compressors.
It is noted that the compressor map of Fig. 2 shows how controllers move the operating point on the compressor map. The first load sharing controller (e,g. l18) moves the operatirlg point of the compressor toward the surge control line (S~l). As soon as the operating poin~ meets with the surge control line, the antisurge controller (e.g. 117) will start to open the relief valve (e.g. 107), keeping the operating point on the surge control line. At the same time, the master pressure controller (119) prevents a changing of the pressure. As a result the operating point will stay at the point of intersection of the line of constant pressure (ADE) with the surge control line ( S--l ) .
, ~
Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of controlling a compressor station receiving gas from an upstream process, compressing said gas and delivering said compressed gas to a process downstream thereof, said compressor station having a station control means to adjust the station performance to the demand of a process located upstream or downstream, said compressor station including also a plurality of dynamic compressors driven by a plurality of respective prime movers, each of said dynamic compressors having a variable performance, a main control means associated with each of said compressors for changing its performance and a surge control means associated with each respective compressor for maintaining said respective compressor along a surge control line having a predetermined equation and located a predetermined distance from the respective surge limit of each respective compressor; said method comprising:
transforming said predetermined equation of each respective surge control line into a variable criterion representing a relative distance between each respective compressor's operating point and its surge control line, said variable criterion reaching a predetermined value when said relative distance reaches zero value;
simultaneously changing the set points for all of said respective criteria representing the relative distance between each respective compressor's operating point and its surge control line by changing an output of said station control means to control the performance of said compressor station; and keeping said set points equal and maintaining said criterion substantially equal at substantially all times for each of said respective compressors.
transforming said predetermined equation of each respective surge control line into a variable criterion representing a relative distance between each respective compressor's operating point and its surge control line, said variable criterion reaching a predetermined value when said relative distance reaches zero value;
simultaneously changing the set points for all of said respective criteria representing the relative distance between each respective compressor's operating point and its surge control line by changing an output of said station control means to control the performance of said compressor station; and keeping said set points equal and maintaining said criterion substantially equal at substantially all times for each of said respective compressors.
2. The method of claim 1 further comprising limiting said respective set points and providing for a decoupling between each respective compressor and said station control means to avoid a dangerous approaching of said respective surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from said control line.
3. The method of claim I wherein said equation is Kf(Z)Y + b = X, where Kf(Z) is the variable slope of the surge control line and b is the bias representing the respective distance between the surge control line and the surge limit line, said criterion equaling where X, Y and Z are the compressor variables representing the respective compressor performance and are used to build the respective compressor maps.
4. The method of claim 3 wherein X is a parameter related to flow through each respective compressor, Y is a parameter related to the pressure differential across each respective compressor, and Z is a parameter related to the speed of rotation of each respective compressor.
5. The method of claim 3 wherein X is a parameter related to flow through each respective compressor, Y is a parameter related to the pressure differential across each respective compressor, and Z represents the position of guide vanes of each respective compressor.
6. The method of claim 3 wherein X is a parameter related to flow through each respective compressor, Y is a parameter related to the pressure differential across each respective compressor, and Z represents a parameter related to the inlet density of the fluid entering each respective compressor.
7. An improved apparatus for controlling a compressor station receiving a gas from upstream of a process, compressing said gas and delivering said compressed gas to the process downstream thereof, said compressor station including a plurality of dynamic compressors driven by a plurality of respective prime movers, each of said dynamic compressors having a variable performance defined by three variable parameters, a main control means associated with each of said compressors for changing the performance of each respective compressor, a surge control means associated with each respective compressor for maintaining each respective compressor along a predetermined surge control line and located a predetermined distance from the respective surge limit of each respective compressor; the improvement comprising:
means for maintaining one of said gas parameters constant for each respective compressor;
means for computing a criterion representing a relative distance between each respective compressor's operating point from the respective surge control line;
set point control means for controlling one of said gas parameters by simultaneously changing the set points of each respective compressor for said criterion, representing the relative distance between each respective compressor's operating point and its surge control line; and means for maintaining said criterion substantially equal at substantially all times for each of said compressors.
means for maintaining one of said gas parameters constant for each respective compressor;
means for computing a criterion representing a relative distance between each respective compressor's operating point from the respective surge control line;
set point control means for controlling one of said gas parameters by simultaneously changing the set points of each respective compressor for said criterion, representing the relative distance between each respective compressor's operating point and its surge control line; and means for maintaining said criterion substantially equal at substantially all times for each of said compressors.
8. The improved apparatus of claim 7 wherein said means for maintaining one of said gas parameters constant includes a primary closed loop controller having an output.
9. The improved apparatus of claim 8 wherein said set point control means comprises a secondary closed loop controller means for each of said compressors for receiving the output of said primary controller as its set point and for changing the performance of each respective compressor by opening its main control means, each of said secondary closed loop controller means further maintaining said criterion representing the relative distance between each compressor's operating point and each respective surge control line at the required level corresponding to the set point developed by said primary controller means.
10. A method of controlling a compressor station receiving gas from an upstream process, compressing said gas and delivering said gas to a process donwnstream thereof, said compressor station having a station control means to adjust the station performance to the demand of a process located upstream or downstream, said compressor station including also one dynamic compressor driven by a prime mover, said dynamic compressor having a variable performance, a main control means associated with said compressor for changing its performance, and a surge control means associated with said compressor for maintaining said compressor along a surge control line having a predetermined equation and located a predetermined distance from its surge limit; said method comprising:
transforming said predetermined equation of said surge control line into a variable criterion representing a relative distance between said compressor's operating point and its surge control line, said variable criterion reaching some predetermined value when said relative distance reaches zero value; and changing set points for said criterion representing the relative distance between said compressor's operating point and its surge control line by changing an output of said station control means to control the performance of said compressor station and limiting said set point and providing for a decoupling between said compressor and said station control means to avoid a dangerous approaching of surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from said control line.
transforming said predetermined equation of said surge control line into a variable criterion representing a relative distance between said compressor's operating point and its surge control line, said variable criterion reaching some predetermined value when said relative distance reaches zero value; and changing set points for said criterion representing the relative distance between said compressor's operating point and its surge control line by changing an output of said station control means to control the performance of said compressor station and limiting said set point and providing for a decoupling between said compressor and said station control means to avoid a dangerous approaching of surge limit line after the compressor's operating point crosses its surge control line and reaches some predetermined deviation from said control line.
11. A method of controlling a compressor station receiving a gas from upstream of a process, compressing said gas and delivering said compressed gas to the process downstream thereof, said compressor station including at least one dynamic compressor driven by at least one respective prime mover, each of said dynamic compressors having a variable performance defined by three variable parameters, a main control means associated with each of said at least one compressor for changing the performance of each respective compressor, a surge control means associated with each respective compressor for maintaining each respective compressor along a predetermined surge control line and located a predetermined distance from the respective surge limit of each respective compressor; said method comprising:
maintaining one of the gas parameters constant for each respective compressor;
computing a criterion representing a relative distance between each respective compressor's operating point from the respective surge control line;
controlling one of said gas parameters by simultaneously changing the set points of each respective compressor for said criterion, representing the relative distance between each respective compressor's operating point and its surge control line; and maintaining said criterion substantially equal at substantially all times for each of said compressors.
maintaining one of the gas parameters constant for each respective compressor;
computing a criterion representing a relative distance between each respective compressor's operating point from the respective surge control line;
controlling one of said gas parameters by simultaneously changing the set points of each respective compressor for said criterion, representing the relative distance between each respective compressor's operating point and its surge control line; and maintaining said criterion substantially equal at substantially all times for each of said compressors.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000475723A CA1256835A (en) | 1985-01-14 | 1985-01-14 | Method and apparatus for controlling a multicompressor station |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000475723A CA1256835A (en) | 1985-01-14 | 1985-01-14 | Method and apparatus for controlling a multicompressor station |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1256835A true CA1256835A (en) | 1989-07-04 |
Family
ID=4129958
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000475723A Expired CA1256835A (en) | 1985-01-14 | 1985-01-14 | Method and apparatus for controlling a multicompressor station |
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| Country | Link |
|---|---|
| CA (1) | CA1256835A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10789657B2 (en) | 2017-09-18 | 2020-09-29 | Innio Jenbacher Gmbh & Co Og | System and method for compressor scheduling |
-
1985
- 1985-01-14 CA CA000475723A patent/CA1256835A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10789657B2 (en) | 2017-09-18 | 2020-09-29 | Innio Jenbacher Gmbh & Co Og | System and method for compressor scheduling |
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