Compressors
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Compressors

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Compressors Compressors Presentation Transcript

  • © 2004 Compressor Controls Corporation Compressors 1
  • Types of Compressors © 2004 Compressor Controls Corporation • Positive Displacement • Rotating 2
  • Positive Displacement © 2004 Compressor Controls Corporation • Reciprocating (Piston) • Screw 3
  • © 2004 Compressor Controls Corporation Rotating • Centrifugal • Axial 4
  • Centrifugal compressors © 2004 Compressor Controls Corporation • Widespread use, many applications • Gas is accelerated outwards by rotating impeller • Can be built for operation as low as 5 psi, or operation as high as 8,000 psi (35 kPa or 55,000 kPa) • Sizes range from 300 hp to 50,000 hp DIFFUSERS Cross section of horizontal split Picture of horizontal split Cross section of barrel type compressor Picture of barrel type compressor Cross section of bull gear compressor Picture of bull gear compressor IMPELLERS Single Case Compressor 5 Centrifugal Impeller Picture of (bull) gear and impellers
  • Cross section of horizontal split Discharge volutes Impellers Impeller inlet labyrinth seals Shaft and labyrinth seal Drive coupling © 2004 Compressor Controls Corporation Journal bearing Casing (horizontally split flange) Thrust bearing Compressor discharge nozzle Compressor inlet nozzle 6
  • © 2004 Compressor Controls Corporation Picture of horizontal split 7
  • © 2004 Compressor Controls Corporation Cross section of barrel type compressor 8
  • © 2004 Compressor Controls Corporation Picture of barrel type compressor 9
  • Cross section of bull gear compressor Labyrinth seals Drive coupling Impellers © 2004 Compressor Controls Corporation Main gear Journal bearing Inlet guide vanes Pinion shafts Gear casing Compressor volutes 10
  • © 2004 Compressor Controls Corporation Picture of bull gear compressor 11
  • © 2004 Compressor Controls Corporation Picture of (bull) gear and impellers 12
  • Axial compressors • Gas flows in direction of rotating shaft • Can be built for lower pressures only 10 to 100 psi (0.7 to 6.8 Bar) © 2004 Compressor Controls Corporation • High flow rate • Efficient • Not as common as centrifugals Stator Blades Shaft Rotor Blades Casing Rotor Blades Stator Blades Casing 13
  • Cross section of axial compressor Guide-vane actuator linkage Labyrinth seals Compressor rotor © 2004 Compressor Controls Corporation Rotor blades Adjustable guide vanes Compressor inlet nozzle 14 Thrust bearing Compressor outlet nozzle
  • © 2004 Compressor Controls Corporation Picture of axial compressor 15
  • Compressor system classifications © 2004 Compressor Controls Corporation Single-Section, Three-Stage Parallel Network Single-Case, Two-Section, Six-Stage Two-Case, Two-Section, Six-Stage Series Network 16
  • Developing the compressor curve Rcc c H Rp ∆P Pd Polytropic Pressure ) 2) Pressure Ratio (P DifferentialHead d/P(Por P2) process,2 Discharge Pressure s(Pd -(PsRor (P2 - P1) /P1) Rprocess,1 © 2004 Compressor Controls Corporation Rc2 Rc1 O.P. Compressor curve for a specific speed N1 Q2 17 Q1 Qs, vol s, mass normal Compressors
  • Developing the compressor curve Rc Adding control margins Process limit Maximum speed Surge limit © 2004 Compressor Controls Corporation Power limit Stonewall or choke limit Actual available operating zone Minimum speed Stable zone of operation Qs, 18 vol
  • © 2004 Compressor Controls Corporation Antisurge Control . . . 20
  • Basic Antisurge Control System • The antisurge controller UIC-1 protects the compressor against surge by opening the recycle valve • Opening of the recycle valve lowers the resistance felt by the compressor , This takes the compressor away from surge • The essence of the surge protection is to determine when and how much to open or close the recycle valve R Rc © 2004 Compressor Controls Corporation VSDS process Rprocess+valve Compressor F T 1 Ps T 1 PdT 1 Discharge Suction UIC 1 2 qr Surge parameter based on invariant coordinates Rc and qr – Flow measured in suction (∆Po) – Ps and Pd transmitters used to calculate Rc 21
  • Surge description • Flow reverses in 20 to 50 milliseconds • Surge cycles at a rate of 0.3 s to 3 s per cycle © 2004 Compressor Controls Corporation • Compressor vibrates • Temperature rises • “Whooshing” noise • Trips may occur 22 • Conventional instruments and human operators may
  • Some surge consequences • Unstable flow and pressure © 2004 Compressor Controls Corporation • Damage in sequence with increasing severity to seals, bearings, impellers, shaft • Increased seal clearances and leakage • Lower energy efficiency 23 • Reduced compressor life
  • Major Process Parameters during Surge FLOW 2 TIME (sec.) PRESSURE © 2004 Compressor Controls Corporation 1 1 2 TIME (sec.) 3 • Rapid pressure oscillations with process instability 3 • Rising temperatures inside compressor which can be seen at the Discgarge TEMPERATURE 1 24 2 TIME (sec.) • Rapid flow oscillations • Reversal flow leads to reversal thrust • Potential damage 3 Operators may fail to recognize surge
  • Developing the surge cycle on the compressor curve Pd • • • From A to B…….20 - 50 ms…………….. Drop into surge From C to D…….20 - 120 ms…………… Jump out of surge A-B-C-D-A……….0.3 - 3 seconds……… Surge cycle © 2004 Compressor Controls Corporation B Pd Pv DPT FT UIC Pd = Compressor discharge pressure Pv = Vessel pressure Rlosses = Resistance losses over pipe A • • • C Pressure builds Resistance goes up Compressor “rides” the curve • • D Electro motor is started Machine accelerates to nominal speed Compressor reaches performance curve • Machine shutdown no flow, no pressure 25 Qs, Note: Flow goes up faster because pressure is the integral of flow vol
  • © 2004 Compressor Controls Corporation How far away are we from Surge? 26
  • Calculating the distance between the SLL and the compressor operating point The Ground Rule – The better we can measure the distance to surge, the closer we can operate to it without taking risk © 2004 Compressor Controls Corporation The Challenge – The Surge Limit Line (SLL) is not a fixed line in the most commonly used coordinates. The SLL changes depending on the compressor inlet conditions: Ts, Ps, MW, ks Conclusion – The antisurge controller must provide a distance to surge calculation that is invariant of any change in inlet conditions – This will lead to :• Safer control • Reducing the surge Safety control margin 27
  • Commonly used (OEM provided) coordinate systems of the compressor map © 2004 Compressor Controls Corporation • Typical compressor maps include: (Qs, Hp), (Qs, Rc), or (Qs, pd) coordinates, where: Qs = Suction flow and can be expressed as actual or standard volumetric flow Hp = Polytropic Head Rc = Compressor Ratio (pd / ps) pd = Discharge pressure of the compressor ps = Suction pressure of the compressor ks = Exponent for isentropic compression • These maps are defined for (1) specific set of inlet conditions: ps, Ts, MW and ks 28
  • The problem with OEM provided coordinate systems of the compressor map © 2004 Compressor Controls Corporation • These coordinates are NOT invariant to suction conditions as shown :- Ts = 37 oC Ts = 29 oC Ts = 21 oC • For control purposes we want the SLL to be presented by a single curve for a fixed geometry compressor 29
  • Understand the limitations of maps © 2004 Compressor Controls Corporation NOT invariant coordinates (Hp, Qs) 30 where: Hp = Polytropic head Qs = Volumetric suction flow hr = Reduced head qr2 = Reduced flow squared Invariant coordinates (hr, qr2) •Choose the right coordinates for the antisurge control system
  • Coordinates (Rc ;Qs) and (Rc ;qr2) © 2004 Compressor Controls Corporation NOT invariant coordinates (Rc, Qs) Invariant coordinates (Rc, qr2) Ts = 37 oC Ts = 37 oC Ts = 29 oC Ts = 29 oC Ts = 21 oC Ts = 21 oC qr2 where: Rc = Pressure ratio Qs = Volumetric suction flow qr2 = Reduced flow squared 31
  • Coordinates (Rc ;jr) and (Rc ;Ne2) © 2004 Compressor Controls Corporation Invariant coordinates (Rc, jr) where: Rc = Pressure ratio jr = Reduced power Ne2 = Equivalent speed squared 32 Invariant coordinates (Rc, Ne2)
  • Representing the SLL as a single curve using reduced coordinates • A coordinate system that is invariant to suction conditions is: Q H hr = p (ZRT)s and qr = s ( ZRT)s • Squaring the flow will still keep coordinates invariant: Hp Q2 s 2 © 2004 Compressor Controls Corporation hr = hr N W, M ng g T s si n rea asi inc cre de 2 qr 33 (ZRT)s and qr = ( ZRT)s
  • Calculating qr2 (reduced flow squared) K . Zs . Ru . Ts q © 2004 Compressor Controls Corporation 2 r = Qs2 (ZRT)s where: R Ru R MW ps K ∆po,s Ts Zs = . ∆po,s ps MW (ZRT)s = ∆po,s ps = Ru / MW = Universal gas constant = Specific gas constant = Molecular Weight of the gas = Suction pressure = Orifice plate constant = Differential pressure across orifice plate = Temperature of the gas in suction = Compressibility of gas in suction of compressor The antisurge controller calculates qr2 using ps and ∆po,s transmitters 34
  • Calculating hr (reduced head) Zs . Ru . Ts hr = Hp (ZRT)s = © 2004 Compressor Controls Corporation For polytropic compression Rcσ -1 . σ MW (ZRT)s σ = Rcσ -1 = σ log(Td/Ts) log(Rt) = log(Pd/Ps) log(Rc) where: Rt = Td / Ts Temperature ratio Rc = pd / ps Pressure ratio σ = Exponent for polytropic compression The antisurge controller calculates hr using pd, ps, Td and Ts transmitters Calculating σ improves accuracy when: - Gas composition varies - Compressor efficiency changes 35
  • Building the Surge Limit Line © 2004 Compressor Controls Corporation • Non-linearity in the SLL can be accommodated using a coordinate CHARACTERIZER f1 • The function f1 returns the value of qr2 on the SLL for input hr • The surge parameter is defined as: 36 Ss = f1(hr ) qr2,op hr SLL hr O.P. q2r,SLL f1(hr ) q2 r,0p 2 qr
  • The surge parameter Ss • The function f1 returns the value of q2 on the SLL r for input hr q2 r,SLL • As a result: Ss = 2 qr,op • Ss < 1 1 hr S = stable operating zone S >1 © 2004 Compressor Controls Corporation s s • Ss = 1 surge limit line (SLL) • Ss > 1 surge region OP hr Ss < 1 q2 r,SLL q2 r,op OP = Operating Point 37 2 qr
  • Introducing the distance between the operating point and the Surge Control Line • • Step 1 Introduce parameter: d = 1 - Ss Step 2 Introduce parameter: DEV = d - surge margin • The parameter DEV is independent of the size of the compressor and will be the same for each compressor in the plant © 2004 Compressor Controls Corporation d =0 Ss = 1 hr d <0 Ss > 1 DEV < 0 DEV = 0 d >0 Ss < 1 DEV > 0 2 qr 38 • One standard surge parameter (DEV) DEV > 0 Good DEV = 0 - Operating Point Surge margin (b1) Benefits: On Control Line DEV < 0 Bad
  • Disadvantage of the ∆ pc /∆ po surge parameter • The SLL is rarely a straight line in the coordinates qr2 and Rc • The parameter ∆pc /∆po represents a straight line in the invariant coordinates qr2 and Rc © 2004 Compressor Controls Corporation • The ∆pc /∆po approach results in loss of turn down and unnecessary recycle Rc Actual Surge Limit Line (SLL) Loss of operating envelope SLL calculated by antisurge controller using ∆pc /∆po = constant 2 qr 39
  • Antisurge Controller Operation Protection #1 The Surge Control Line (SCL) Slow Disturbance Example Rc SLL = Surge Limit Line SCL = Surge Control Line • When the operating point crosses the SCL, PI control will open the recycle valve © 2004 Compressor Controls Corporation B A • PI control will give adequate protection for small disturbances 2 qr • PI control will give stable control during steady state recycle operation 40
  • Adaptive Gain Enhancing the Effectiveness of the PI Controller FAST disturbance example Rc • When the operating point moves quickly towards the SCL, the rate of change (dSs/dT) can be used to dynamically increase the surge control margin. • This allows the PI controller to react earlier. B © 2004 Compressor Controls Corporation A 2 qr Smaller safety control margins can be used without sacrificing reliability. 41
  • Antisurge Controller Operation Protection #2 The Recycle Trip® Line (RTL) SLL = Surge Limit Line RTL = Recycle Trip Line (open Loop) Rc SCL = Surge Control Line – Reliably breaks the surge cycle – Energy savings due to smaller surge margins needed OP © 2004 Compressor Controls Corporation Benefits: – Surge can be prevented for virtually any disturbance 2 qr Output to Valve Total Response PI Control Response C Open-loop Response 42 C2 Time Recycle Trip® Action PI Control + To antisurge valve
  • “Improving the accuracy of Recycle Trip® open loop control © 2004 Compressor Controls Corporation • Recycle Trip® is the most powerful method known for antisurge protection • The magnitude required from the Recycle Trip® response (C) is a function of the rate of change of the operating point or d(Ss)/dt C = C1Td where: • C • C1 • Td • d(Ss)/dt 43 d(Ss) dt = Actual step to the valve = Constant - also defines maximum step = Scaling constant = Rate of change of the operating point
  • Recycle Trip® based on derivative of Ss Benefits • Maximum protection – No surge – No compressor damage Recycle Trip® Response calculation: • Minimum process disturbance © 2004 Compressor Controls Corporation d(Ss) C = C1Td dt Output to valve – No process trips Output to valve Medium disturbance Large disturbance 100% Total PI Control C C Recycle Trip® 0% Tim e 44 Tim e
  • What if one Recycle Trip® step response is not enough? After time delay C2 controller checks if Operating Point is back to safe side of Recycle Trip® Line - If Yes: Exponential decay of Recycle Trip® response. - If No: Another step is added to the Recycle Trip® response. © 2004 Compressor Controls Corporation Output to valve Output to valve Multiple step response Total One step response PI Control 100% Recycle Trip® Total PI Control Recycle Trip® 0% C2 45 Tim e C2 C2 C2 Tim e
  • Antisurge Controller Operation Protection #3 The Safety On® Response (SOL) © 2004 Compressor Controls Corporation Rc SOL - Safety On® Line SLL - Surge Limit Line RTL - Recycle Trip® Line SCL - Surge Control Line New SCL New RTL Additional surge margin 2 qr • Compressor can surge due to: – Transmitter calibration shift – Sticky antisurge valve or actuator – Partially blocked antisurge valve or recycle line – Unusually large process upset Benefits of Safety On® response: Continuous surging is avoided Operators are alarmed about surge 46
  • Built-in surge detector Pressure and Flow Variations During a Typical Surge Cycle 100% • Thresholds should be configured slightly more conservative than the actual rates of change during surge. © 2004 Compressor Controls Corporation Pd 0% 1 TO 2 SECONDS 100% • Surge is detected when the actual rates of change exceed the configured thresholds • The following methods have been used: – – – – ∆Po 0% 20 to 50 milli-seconds 47 • Surge signature should be recorded during commissioning. • Rates of change for flow and pressure transmitters should be calculated. Rapid drops in flow and pressure Rapid drop in flow or pressure Rapid drop in flow only Rapid drop in pressure only • When surge is detected a Safety On® response is triggered • A digital output can be triggered upon a configurable number of surge cycles
  • Limiting Ps or Pd using the Antisurge Controller VSDS Compressor F T 1 Suction Ps T 1 Pd T 1 UIC Discharge © 2004 Compressor Controls Corporation 1 • The antisurge controller can be configured to limit: - Maximum discharge pressure (Pd) - Minimum suction pressure (Ps) - Both maximum Pd and minimum Ps • This does NOT conflict with antisurge protection 48
  • © 2004 Compressor Controls Corporation Increase compressor system reliability and availability with fall-back strategies • Over 75% of the problems are in the field and not in the controller • The CCC control system has fall-back strategies to handle these field problems • The controller continuously monitors the validity of its inputs • If an input problem is detected the controller ignores this input and automatically switches to a fall-back mode Benefits – Avoids nuisance trips – Alarms operator of latent failures – Increases machine and process availability 49
  • Fall-back strategies for the antisurge and performance controller • Antisurge controller – If a pressure transmitter fails, a minimum q2r algorithm is used © 2004 Compressor Controls Corporation – If a temperature transmitter fails, hr is characterized as a function of compression ratio – If the speed transmitter fails, a conservative speed setting is used – If the flow transmitter fails • Redundant transmitter is used • Output is driven to: – Last value OR – Last Value selected: If Last Value >Pre-selected fixed value OR Pre-selected fixed value selected: If Pre-selected fixed value>Last Value • All transmitter failures are alarmed 50
  • © 2004 Compressor Controls Corporation Performance Control . . . 51
  • Compressor Performance Control © 2004 Compressor Controls Corporation • Also called: – Throughput control – Capacity control – Process control • Can be based on controlling: – Discharge pressure – Suction pressure – Net flow to the user 52
  • Limiting control to keep the machine in its stable operating zone • While controlling one primary variable, we might have to limit another one or two variable © 2004 Compressor Controls Corporation • Exceeding limits might cause machine or process damage CONTROL BUT DO NOT EXCEED Discharge Pressure Max. Motor Current Suction Pressure Max. Discharge Pressure Max. Discharge Temperature Net Flow 53 Min. Suction Pressure
  • Power limiting with the Performance Controller Rc Power limit SI C 1 R1 Process R2 A A’ PIC-SP C D B © 2004 Compressor Controls Corporation P T 1 R3 PI C 1 B’ N4 N3 N2 N1 Benefits: • 2 Note: Same approach for other variables (pressures, temperatures, etc.) 54 qr Maximum protection • Maximize production – No machinery damage – Machine can be pushed to the limits without risk of damage
  • Interacting Antisurge & Performance Loops VSDS SC L Section 1 SL L Rc PIC 1 UIC 1 B © 2004 Compressor Controls Corporation C A PIC-SP • Interaction starts at B • Results of interaction 2 qr 55 – Degrades press. Control accuracy – Large pressure deviations during disturbances – Increased risk of surge
  • Performance & Antisurge Controller’s interaction • Both controllers manipulate the same variable - the operating point of the compressor © 2004 Compressor Controls Corporation • The controllers have different and sometimes conflicting objectives • The control action of each controller affects the other • This interaction starts at the surge control line - near surge - and can cause surge 56
  • Ways to cope with Antisurge and Performance Loop interactions © 2004 Compressor Controls Corporation • De-tune the loops to minimize interaction. Result is poor pressure control, large surge control margins and poor surge protection • Put one loop on manual, so interaction is not possible. Operators will usually put the Antisurge Controller on manual. Result - no surge protection and often partially open antisurge valve • Decouple the interactions. Result - good performance control accuracy, good surge protection and no energy wasted on recycle or blow off 57
  • Decoupling Antisurge and Performance Loop interactions VSDS Section 1 PIC © 2004 Compressor Controls Corporation 1 Serial network UIC 1 Decouple the interactions. • Result - good performance control accuracy • Good surge protection • No energy wasted on recycle or blow off 58
  • Loadsharing Compressor Networks Base Loading Parallel Compressors © 2004 Compressor Controls Corporation Equal Flow Division System Equidistant Loadsharing System Compressor Efficiency 59
  • Compressor networks Control system objectives for compressors in parallel: © 2004 Compressor Controls Corporation • Maintain the primary performance variable (pressure or flow) • Optimally divide the load between the compressors in the network, while: – Minimizing risk of surge – Minimizing energy consumption – Minimizing disturbance of starting and stopping individual compressors 60
  • Base Loading VSDS Compressor 1 Swing machine © 2004 Compressor Controls Corporation UIC 1 PIC 1 Suction header HIC 1 Proces s VSDS Compressor 2 UIC 2 61 Base machine
  • Base Loading Parallel Compressor Control Rc,1 Compressor 1 Rc,2 Swing machine Compressor 2 Base machine © 2004 Compressor Controls Corporation PIC-SP 2 2 qr,1 QP,1 Notes: • • • • 62 qr,2 QP,2 Base loading is inefficient Base loading increases the risk of surge since compressor #1 will take the worst of any disturbance Base loading requires frequent operator intervention Base loading is NOT recommended
  • Equal Flow Division Loadsharing Flow Diagram for Control Process VSDS RSP Compressor 1 out UIC 1 FIC 1 RSP out © 2004 Compressor Controls Corporation PIC 1 Suction header VSDS Proces s RSP Compressor 2 out UIC 2 63 FIC 2 P RS 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 loss across FMD’s
  • Equal Flow Division Loadsharing Parallel Compressor Control Rc,1 Compressor 1 Rc,2 Compressor 2 © 2004 Compressor Controls Corporation PIC-SP Equal flow Equal flow 2 2 qr,1 QP,1 QP,2 QP,1 = QP,2 64 qr,2
  • Equidistant Loadsharing Flow Diagram for Control Process VSDS RSP Compressor 1 out UIC 1 Serial network LSIC 1 Serial network MPIC © 2004 Compressor Controls Corporation 1 Suction header VSDS Proces s RSP Compressor 2 out UIC 2 65 Serial network LSIC 2 Notes • All controllers are coordinating control responses via a serial network • Minimizes recycle under all operating conditions
  • Equidistant Loadsharing Parallel Compressor Control Rc,1 Compressor 1 Rc,2 SCL = Surge Control Line 0.1 0.2 Compressor 2 0.3 DEV = 0 0.1 0.2 0.3 © 2004 Compressor Controls Corporation PIC-SP Dev1 = Dev2 Q1 = Q2 N1 = N2 2 2 qr,1 Notes: DEV1 qr,2 DEV2 • Maximum willa dimensionless all relative distance • Recycleturndown (energysame Rwithout recycle orare Since DEVoperate at savings) call sorts of Machinesis is kept at the same machines blowsince suction and The DEV are only start when number off machines of bothmixed: (SCL) tiedaxials, on the Surge Control since small, big,absorb part of to their SCL representing be machines are discharge riskthe distance between together • Minimizes thecan of surge Lineall machines the • the disturbance in practice the same DEV for both centrifugals This means operating point and the Surge Control Line • Automatically adapts to different size machines the • Lines of equal DEV can befor all machines The DEV will be the same plotted on machines • CCC patented algorithm but they will curves at shown performanceoperate as different speeds and flow rates 66
  • © 2004 Compressor Controls Corporation Engineering Panel Mounts Behind the Front Pane 67
  • © 2004 Compressor Controls Corporation Series 3 Plus Engineering Keyboard 68
  • TrainView® Operator Interface Controller Overview Compressor Map Screen © 2004 Compressor Controls Corporation Control System • • Standard and Customized Screens • On-Line Operation and Control • 69 Design Screens Alarm and Event Management • Critical Event Archiving Remote OnlookTM Diagnostics
  • CCC Antisurge and Performance Control Selling Points • Distance to surge calculation © 2004 Compressor Controls Corporation • A/S adjusts it’s own set point automatically based on process conditions (DEV) • Recycle Trip (R.T.) adjusts step change automatically based on process conditions • Safety On (S.O) gets compressor out of surge and does not allow it to go back into surge • Surge Detection and Emergency Shutdown 70
  • CCC Antisurge and Performance Control Selling Points • Fallback strategies © 2004 Compressor Controls Corporation • Limiting control • Decoupling prevents control loop interaction • Loadsharing 71
  • Customers keep coming back © 2004 Compressor Controls Corporation 80% of projects are from repeat customers 72
  • © 2004 Compressor Controls Corporation 73