CCC负荷分配简介

1、,负荷分配控制,在压缩机网络中，压缩机通常并联运行，有时也有串联运行 形成网络运行的目的包括： 备份 灵活操作 增加额外的能力 通常注重单元机组的运行而忽略网络的优化控制 压缩机制造商通常集中于单元机组的控制。 从“网络”的观点来看，应实现优良的喘振保护和网络的负荷分配优化控制。,压缩机网络,并联机组控制系统的目标是： 保持主性能变量稳定（压力或流量） 将负荷优化分配到网络中的各台机组上，同时： 发生喘振的机率最低。 最低的能耗 在启动或停开单一机组时将所带来的工艺扰动降到最低。,压缩机网络,Process,PIC,1,1,UIC,VSDS,Compressor 1,2,UIC,VSDS,Co

2、mpressor 2,HIC,1,Suction header,用于调节负荷的压缩机,满负荷运转的压缩机,注： 所有控制系统均为独立运行 变送器未标明。,基本负荷法,Rc,1,Rc,2,Compressor 1,Compressor 2,Machines operate at same Rc since suction and discharge of both machines are tied together,Base load one or more compressors and let the other(s) absorb the load swings,Swing machin

3、e,Base machine,Base machine is fully loaded and runs without recycle,Swing machine can be running with recycle,where: QP = Flow to process QC= Total compressor flow QC - QP = Recycle flow,Load could be re-divided to eliminate recycle,QP,1 + QP,2 = QP,1 + QP,2,注： 基本负荷法效率较低。 基本负荷法增加了#1压缩机组发生喘振的危险性，这是由

4、于#1压缩机将独立承担调整任何扰动。 基本负荷法需要操作人员的经常干预。 基本负荷法并不是推荐的方式,基本负荷法控制,Process,PIC,1,1,UIC,Compressor 1,Compressor 2,Suction header,Notes Performance controllers act independent of antisurge control Higher capital cost due to extra Flow Measurement Devices (FMD) Higher energy costs due to permanent pressure los

5、s across FMDs,1,FIC,2,FIC,2,UIC,out,out,RSP,RSP,RSP,out,RSP,Equal Flow Division Loadsharing Flow Diagram for Control Process,Machine 2 operates with recycle while machine 1 still has turn down,Machines operate at same Rc since suction and discharge of both machines are tied together,Equal flow divis

6、ion might work if both machines are identical,Machines are never identical except by coincidence,Bias relay on remote setpoint would only work if curves have same steepness,Notes: Requires additional capital investment in FMDs Requires additional energy due to permanent pressure loss across FMDs Poo

7、r pressure control due to positive feedback in control system (see next) Equal flow division is NOT recommended,Rc,1,Rc,2,QP,1 = QP,2,Equal Flow Division Loadsharing Parallel Compressor Control,Compressor 1,Compressor 2,where: QP = Flow to process QC= Total compressor flow QC - QP = Recycle flow,Q2,

8、Rc,N1,N3,N2,In a typical master-slave control scheme the slave needs to be approx. 5 times faster than the master,A,The machine is operating in point A This is the intersection of 4 lines: Resistance line R1 Performance curve N1 PIC-SP FIC-SP = Output of PIC,Process disturbance causes the resistance

9、 to change from R1 to R2,As a result the machine moves to point B,Since the PIC is slow it does not move its output yet which is the FIC-SP,The FIC reacts fast and will try to maintain its SP The FIC will speed up the machine to point C at speed N3,The disturbance is amplified Positive feedback syst

10、em,Only as the PIC starts to reduce its output to control pressure the FIC-SP comes down and the pressure is restored,Notes Causes instability near surge Poor pressure control due to positive feedback in control system,Dynamic Response / Pressure To Flow Cascade,Notes All controllers are coordinatin

11、g control responses via a serial network Minimizes recycle under all operating conditions,Equidistant LoadsharingFlow Diagram for Control Process,Machines operate at same Rc since suction and discharge of both machines are tied together,The DEV is a dimensionless number representing the distance bet

12、ween the operating point and the Surge Control Line Lines of equal DEV can be plotted on the performance curves as shown,DEV = 0,Machines are kept at the same relative distance to the Surge Control Line (SCL) This means in practice the same DEV for both machines,Recycle will only start when all mach

13、ines are on their SCL,Since DEV is dimensionless all sorts of machines can be mixed: small, big, axials, centrifugals The DEV will be the same for all machines but they will operate at different speeds and flow rates,SCL = Surge Control Line,Rc,1,Rc,2,Compressor 1,Compressor 2,Notes: Maximum turndow

14、n (energy savings) without recycle or blow-off Minimizes the risk of surge since all machines absorb part of the disturbance Automatically adapts to different size machines CCC patented algorithm,Equidistant LoadsharingParallel Compressor Control,Loadsharing Controller,Loop Decoupling,FA Mode,PI,RT,

15、Loop Decoupling,+,Antisurge Controller,Analog Inputs,+,DEV,To antisurge valve,To performance control element,Primary response,Primary response,Compressors in Parallel the primary response,Master Controller,Master controller controls the main Process Variable (PV) via its PID control block,The output

16、 of the master controller PID goes to the primary response block in the loadsharing controller,In the primary response block the controller checks if the machine is close to the SCL: Yes: dont reduce capacity - keep output constant No: reduce capacity as necessary Apply loadsharing gain M0 The outpu

17、t of the master controller goes via the primary response block directly to the performance control element,In order to check if the machine is close to the SCL the primary response block needs the DEV The DEV is reported by the antisurge controller,When the machine is close to the SCL the master con

18、troller will no longer reduce performance to control the primary variable The master controller will start to open the recycle valve to control the primary variable,If DEV = 0 apply loadsharing gain Output goes to antisurge valve,Loop Decoupling,FA Mode,PI,Loop Decoupling,+,Analog Inputs,+,DEV,To an

19、tisurge valve,To performance control element,PID,Load balancing,PV,SP,Primary response,Average,The load balancing response,Loadsharing Controller,Antisurge Controller,Master Controller,The fast master controller controls the primary process variable by directly manipulating the final control element

20、s,In order to balance the machines they need to be kept at the same DEV,The antisurge controller reports the actual DEV to the load balancing block in the loadsharing controller This reported DEV becomes the Process Variable (PV) for the load balancing PID loop,The loadsharing controller reports thi

21、s DEV PV also to the master controller,Other loadsharing controllers also report their DEV PV to the master controller,The master controller calculates the average of all reported DEV PVs,This average DEV is sent out to all loadsharing controllers to become the SP for all load balancing blocks,The l

22、oad balancing block is a slow controller that will equalize all DEVs for all parallel compressors Its output is added to the total output to the performance control element,RT,The Pressure Override Control (POC) response,When a large disturbance occurs it can happen that the performance control elem

23、ent (e.g. speed) is too slow to keep the pressure under control,The operating point rides the curve and the pressure rises sharply,There is a high chance to exceed the relief valve setting and trip the process,The CCC master controller has a Pressure Override Control (POC) mode that will open the an

24、tisurge valve to get the disturbance under control quickly,Opening of the antisurge valve is much faster than a reduction in speed,As soon as the operating point drops under the POC-SP line the antisurge valves start to close again,The primary PID loop will stabilize the operating point on the PIC-S

25、P line,Benefits Fast response during fast upsets Avoid process trips due to lack of response in performance control elements Allows closer operation to process limits without taking risk,PI (One-Sided),Rc,PIC-SP,Relief valve setting,Process,1A,UIC,VSDS,Section 1,VSDS,Section 1,A,LSIC,out,RSP,Serial

26、network,RSP,B,LSIC,1,MPIC,Serial network,Serial network,Section 2,Section 2,2A,UIC,1B,UIC,1B,UIC,Serial network,Serial network,out,Train B,Train A,How to operate equidistant from the Surge Control Line (SCL) when there is more than one section per machine ?,Select per train - in the loadsharing cont

27、roller - the section closest to the SCL,By selecting the section closest to the SCL it is guaranteed that the other section on the same train is not in recycle,Equidistant Loadsharing for multi-section compressors,Share the load - equal DEVs for both trains - on the section closest to the SCL,Loadsh

28、aring Controller,Loop Decoupling,Load balancing,FA Mode,PI,RT,+,Antisurge Controller,Analog Inputs,Average,+,SP,PV,DEV from other loadsharing controllers,DEV1,To antisurge valve-1,To performance control element,PID,PV,Primary response,Both antisurge controllers report their DEV to the loadsharing co

29、ntroller,PI (One-Sided),Primary response,FA Mode,PI,RT,Loop Decoupling,+,Antisurge Controller,DEV2,To antisurge valve-2,Primary response,The lowest DEV is selected: the section closest to the SCL,The selected DEV is reported to: Primary control response blocks,Load balancing block,Master controller

30、averaging block,Selecting the section closest to SCL for parallel operation,Master Controller,Loop Decoupling,Main selection criteria for FMD in antisurge control system: Repeatability Sufficient signal-to-noise ratio Accuracy of the FMD is not critical FMD delays must be absolutely minimal Present

31、state-of-the-art limits the choice of FMD to head flow meters or to other devices that are based on the principle of velocity measurement: Orifice plates Venturis Pitot tubes etc. Recommended flow range for FMD and transmitter is maximum compressor flow Recommended Dp corresponding to Qmax, compress

32、or is 10” WC (250 mmH2O) or more,Flow Measuring Device (FMD) selection criteria,The preferred location of the FMD: Suction of compressor As close to the inlet flange as possible,Compressor,Discharge,Suction,Less preferable location of the FMD: Discharge of compressor As close to the discharge flange

33、 as possible,Selection of the location should be based on: Necessity of surge detection Often more difficult with flow measured in discharge Capital cost of flow measuring device Operating cost of the FMD (permanent pressure loss),Flow Measuring Device (FMD) location,The speed of approaching surge i

34、s high The transmitter type and brand should be selected based on two major factors: Reliability Speed of response Desired rise time for Dp (flow) transmitters is 200 ms or less Pressure step is 100% The first order response (63%) is less than 200 ms Desired rise time for pressure transmitters is 50

35、0 ms or less,Response time of the FMD transmitter,Actual pressure,Transmitter output,63% response 1- (1/e),t1 is less than 200 ms,Knowing the flow is essential to determine the distance between the operating point and the SCL Damping the Dpo (flow) transmitter destroys essential information,Damping

36、the Dpo (flow) transmitter can paralyze the complete antisurge control system!,The effect of damping the Dpo (flow) transmitter,50,0,-50,0,1.25,2.50,3.75,5,Time (seconds),Flow,Criteria for antisurge valve sizing based on CCCs experience Provide adequate antisurge protection for worst possible distur

37、bances Provide adequate antisurge protection in all operating regimes Sized to provide flow peaks greater than what is required in steady state to operate on the Surge Control Line Sized to avoid choke zone Not be oversized from controllability point of view,Take point A at the intersection of the m

38、aximum speed performance curve and the Surge Limit Line (SLL) Calculate Cv,calc (or equivalent) for point A Select standard valve size using the following criteria: 1.8 . Cv, calc Cv,selected 2.2 . Cv, calc,Rc,Qvol,A,Sizing the antisurge control valve,Rc,An alternative method yielding excellent resu

39、lts is:,Take design point of the compressor point A,Draw a horizontal line through the design point,Take point B at intersection of maximum speed performance curve and the horizontal line,Calculate Cv,calc in point B,Select standard valve size using the following criteria: 0.9 . Cv, calc Cv,selected

40、 1.1 . Cv, calc,Sizing the antisurge control valve - alternative method,Qvol,Antisurge valve stroke speed Antisurge valve must have speed of response adequate for antisurge protection for all disturbances Recommended full stroke times: SizeClose to openOpen to close 1” to 4”1 second 3 seconds 6” to

41、12”2 seconds 5 seconds 16” and up3 seconds 10 seconds Closing time needs to be the same order of magnitude to assure the same loop gain in both directions,Antisurge valve characteristic Normally control valves are selected to be open 80% to 90% for design conditions Antisurge valves can operate anyw

42、here between 0% and 100% In order to have an equal loop-gain over the whole operating range a linear valve is required This will allow for the fastest tuning leading to smaller surge margins,Stroke speed and characteristic of the antisurge valve,Most normal control valves can be made to perform as r

43、equired for antisurge control The following steps help improve the performance of the valve Install positioner Minimize tubing length between I/P and valve positioner Install volume booster Minimize volume and resistance between volume booster and actuator Increase air supply line to 3/4” or more In

44、crease size of air connection into the actuator Drill additional holes in actuator - avoids pulling a vacuum,Improving the performance of the antisurge valve,Piping lay-out influences the controllability of the the total system The primary objective of the antisurge controller is to protect the comp

45、ressor against surge This is achieved by lowering the resistance the compressor is feeling The resistance is lowered by opening the antisurge valve Dead-time and time-lag in the system needs to be minimized This is achieved by minimizing the volume between three flanges Discharge flange of the compr

46、essor Recycle valve flange Check valve flange,VSDS,Compressor 1,Piping lay-out consideration when designing an antisurge control system,Section 1,Section 2,In order to protect section 1 the antisurge valve needs to be opened The volume between compressor discharge, check valve and antisurge valve de

47、termines the dead time and lag time in the system,Large volume significantly decreases the effectiveness of the antisurge protection Result Poor surge protection Large surge margins Energy waste Process trips because of surge,Note: This specific piping layout is found on many wet gas compressors in

48、FCCUs,Using a single antisurge valve increases recycle lag time,Section 1,Section 2,The piping lay-out for section 2 is excellent for surge protection Minimum volume between the three flanges,The piping lay-out for section 1 is not ideal Large volume to be de-pressurized decreases ability of the con

49、trol system to protect the machine against surge,Result Poor surge protection Large surge margins Energy waste Process trips because of surge,Sharing recycle coolers degrades surge protection,Compressor 1,Compressor 1 has ideal piping lay-out for surge protection Minimum volume between the three fla

50、nges,Compressor 2,The piping lay-out for compressor 2 is commonly found in the industry The cooler creates additional volume and decreases the effectiveness of the antisurge control system,The piping lay-out for compressor 2 can be acceptable if the additional volume does not create excessive dead t

51、ime and lag in them,Result Increased surge margins Energy waste,Installing recycle valve upstream from cooler improves control response,Compressor has ideal piping lay-out for surge protection Minimum volume between the three flanges for all sections,Recycle lines configured for optimum surge protec

52、tion,Minimum volume,Section 2,Section 3,Section 1,Process,Suction,Lay-out #1 has minimum volume between the flanges and is the best lay-out for antisurge control purposes,Section 2,Section 3,Section 1,Process,Suction,Section 1,Section 2,Section 3,Suction,Process,Lay-out #1: Compressor with recycle l

53、ines optimally configured for antisurge control,Lay-out #2: Compressor with coolers upstream of recycle take-off,When selecting lay-out #2 the residence time of the gas in the “surge” volume should be verified to check acceptable time delays are not exceeded,These two piping lay-outs are most common

54、 for antisurge control,Lay-out #2 requires one cooler less and thus the capital investment is lower,Lay-out #2 will require bigger surge control margins,Which antisurge piping configuration do you choose?,Analog controller,SLL,SCL,100%,0%,Controller output,100%,0%,Leading engineering contractor perf

55、ormed evaluation of execution time influence on ability to protect compressor from surge Dynamic simulation of compressor was built Digital controllers are compared against analog controller on simulation Analog controller has no execution time and is immediate Analog controller tuned for minimum ov

56、ershoot Digital controllers get exact same tuning parameters Digital controllers get exact same disturbance,Operating point,Time,Time,Influence of controller execution time,Analog controller,SLL,SCL,100%,0%,100%,0%,100%,Controller output,Operating point,Digital controller (2 executions per second),Time,Time,Time,Time,Compressor surged Large process upset would have resulted,Analog vs. digital controller at 2 exec