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9. What is surge-1.ppt Surging protection techniques
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- 2. © 2008 Compressor Controls Corporation 2 Stable zone of operation Developingthe compressor curve Minimum speed Power limit Maximum speed Process limit Qs, vol Adding control margins Stonewall or choke limit Surge limit Rc Actual available operating zone
- 3. © 2008 Compressor Controls Corporation 3 • Flow reversesin 20 to 50 milliseconds • Surge cycles at a rate of 0.3 s to 3 s per cycle • Compressor vibrates • Temperature rises • “Whooshing” noise • Trips may occur • Conventional instruments and human operators may fail to recognize surge Surge description
- 4. © 2008 Compressor Controls Corporation 4 Some surge consequences •Unstable flow and pressure • Damage in sequence with increasing severity to seals, bearings, impellers, shaft • Increased seal clearances and leakage • Lower energy efficiency • Reduced compressor life
- 5. © 2008 Compressor Controls Corporation 5 • From Ato 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 • Compressor reaches surge point A • Compressor loses its ability to make pressure • Suddenly Pd drops and thus Pv > Pd • Compressor surges -“Plane goes to stall” Qs, vol Pd Machine shutdown no flow, no pressure • Electro motor is started • Machine accelerates to nominal speed • Compressor reaches performance curve Note: Flow goes up faster because pressure is the integral of flow • Pressure builds • Resistance goes up • Compressor “rides” the curve • Pd = Pv + Rlosses • Because Pv > Pd the flow reverses • Compressor operating point goes to point B • Result of flow reversal is that pressure goes down • Pressure goes down => less negative flow • Operating point goes to point C • System pressure is going down • Compressor is again able to overcome Pv • Compressor “jumps” back to performance curve and goes to point D • Forward flow is re-established Pd = Compressor discharge pressure Pv = Vessel pressure Rlosses = Resistance losses over pipe • Compressor starts to build pressure • Compressor “rides” curve towards surge • Point A is reached • The surge cycle is complete Developing the surge cycle on the compressor curve Pd Pv Rlosses B A C D
- 6. © 2008 Compressor Controls Corporation 6 Factors leading toonset of surge • Startup • Shutdown • Operation at reduced throughput • Operation at heavy throughput with: – Trips – Power loss – Operator errors – Process upsets – Load changes – Gas composition changes – Cooler problems – Filter or strainer problems – Driver problems • Surge is not limited to times of reduced throughput. • Surge can occur at full operation
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- 8. © 2008 Compressor Controls Corporation 8 Surge parameter basedon invariant coordinates Rc and qr – Flow measured in suction (DPo) – Ps and Pd transmitters used to calculate Rc • The antisurge controller UIC-1 protects the compressor against surge by opening the recycle valve 1 UIC VSDS Compressor 1 FT 1 PsT 1 PdT Discharge Suction • Opening of the recycle valve lowers the resistance felt by the compressor • This takes the compressor away from surge Basic Antisurge Control System 2 Rc qr Rprocess Rprocess+valve
- 9. © 2008 Compressor Controls Corporation 9 A Rc B • When theoperating point crosses the SCL, PI control will open the recycle valve • PI control will give adequate protection for small disturbances SLL = Surge Limit Line SCL = Surge Control Line qr 2 Antisurge Controller Operation Protection #1 The Surge Control Line (SCL) • PI control will give stable control during steady state recycle operation • Slow disturbance example
- 10. © 2008 Compressor Controls Corporation 10 Antisurge Controller OperationProtection #2 The Recycle Trip® Line (RTL) Benefits: – Energy savings due to smaller surge margins needed – Compressor has more turndown before recycle or blow-off – Surge can be prevented for virtually any disturbance • Disturbance arrives - the operating point moves towards the SCL SLL = Surge Limit Line RTL= Recycle Trip® Line SCL = Surge Control Line Output to Valve Time • When the operating point reaches the SCL, the PI controller opens the a/s valve based on its proportional and integral action. • The operating point overshoots the SCL until it reaches RTL • When the operating point hits RTL the conclusion is: – We are close to surge – The PI controller is too slow to catch the disturbance – Move the valve now! • An open loop response is triggered • Operating point moves back to the safe side of RTL – The Open-loop function should be ramped out – PI controller integrates to stabilize the operating point on the SCL Recycle Trip® Response PI Control Response • Total response of the controller is the sum of the PI control and the Recycle Trip® action PI Control Recycle Trip® Action + To antisurge valve Total Response Rc Q 2 OP
- 11. © 2008 Compressor Controls Corporation 11 Additional surge margin •Benefits of Safety On® response: - Continuous surging is avoided - Operators are alarmed about surge • Compressor can surge due to: – Transmitter calibration shift – Sticky antisurge valve or actuator – Partially blocked antisurge valve or recycle line – Unusually large process upset Antisurge Controller Operation Protection #3 The Safety On® Response (SOL) • If Operating Point crosses the Safety On® Line the compressor is in surge Rc qr 2 SLL - Surge Limit Line RTL - Recycle Trip® Line SCL - Surge Control Line • The Safety On® response shifts the SCL and the RTL to the right New SCL New RTL • Additional safety or surge margin is added • PI control and Recycle Trip® will stabilize the machine on the new SCL SOL - Safety On® Line
- 12. © 2008 Compressor Controls Corporation 12 1 UIC VSDS Compressor 1 FT 1 PsT 1 PdT • The antisurgecontroller 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 Discharge Suction Limiting Ps or Pd using the Antisurge Controller
- 13. © 2008 Compressor Controls Corporation 13 Disturbance comes fromthe discharge side • Pd,2 increases • Ps,2 remains constant • Rc,2 increases • Section 2 moves towards surge Disturbance • The system is oscillating • Slowing down the controller tuning would lead to: - Increased risk of surge • Compressor damage • Process trips - Bigger surge margins • Energy waste Interacting Antisurge Control Loops Rc,2 qr,2 2 R Rc,1 qr,1 2 R R R Antisurge controller UIC-1 will open the recycle valve to protect section 1 against surge • Pd,1 decreases • Ps,1 increases • Rc,1 decreases • Section 1 moves away from surge Opening of recycle valve on section 1 caused Pd,1 = Ps,2 to decrease Result: • Ps,2 decreases • Pd,2 remains constant • Rc,2 increases • Section 2 moves towards surge Antisurge controller UIC-2 will open the recycle valve to protect section 2 against surge • Pd,2 decreases • Ps,2 increases • Rc,2 decreases • Section 2 moves away from surge Opening of recycle valve on section 2 caused Ps,2 = Pd,1 to increase Result: • Pd,1 increases • Ps,1 remains constant • Rc,1 increases • Section 1 moves towards surge 1 PIC 2 UIC 1 UIC VSDS Section 1 Section 2
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- 15. © 2008 Compressor Controls Corporation 15 • Also called: –Throughput control – Capacity control – Process control • Matches the compressor throughput to the load • Can be based on controlling: – Discharge pressure – Suction pressure – Net flow to the user Compressor Performance Control
- 16. © 2008 Compressor Controls Corporation 16 PIC - SP •Compressor operates in point A Pd qr 2 Shaft power qr 2 Curve 1 A Rprocess + Rvalve • Required power in point A is P1 Curve 1 P1 • Pressure is controlled by blow-off • Point B represents the point that would deliver the pressure for Rprocess Curve 2 Rprocess B • Required power in point B is P2 Curve 2 P2 • Power loss is P1 - P2 • Qloss represents energy waste Qloss Notes: • Most inefficient control method • Regularly found in plant air systems • Rare in other systems • Not recommended • Curve 2 represents: • Lower speed on variable speed systems • IGVs closed on variable geometry compressors • Inlet throttle valve closed on fixed speed compressors Performance Control by blow-off or recycle PT 1 PIC 1 Process
- 17. © 2008 Compressor Controls Corporation 17 • Compressor operates inpoint A Pd qr 2 Shaft power qr 2 Curve 1 A Rprocess + Rvalve • Required power is P1 Curve 1 P1 • Pressure is controlled by pressure drop over valve PIC - SP Pressure loss across valve • Opening of valve would reduce resistance to Rprocess Rprocess • Lower resistance would require less speed and power Curve 2 Curve 2 P2 • Power loss is P1 - P2 Notes: • Extremely inefficient (consumes approximately the same power for every load) • Rarely used • Not recommended • Curve 2 represents: • Lower speed on variable speed systems • IGVs closed on variable geometry compressors • Inlet throttle valve closed on fixed speed compressors Performance Control by discharge throttling PT 1 PIC 1 Process
- 18. © 2008 Compressor Controls Corporation 18 • Inlet valve manipulatessuction pressure Pd qr 2 Shaft power qr 2 • Changing suction pressure generates a family of curves Suction valve open Suction valve throttled • Pressure is controlled by inlet valve position PIC - SP • Compressor operates in point A for given Rprocess A Rprocess • Required power is P1 P1 Notes • Common on electric motor machines • Much more efficient than discharge throttling • Power consumed changes proportional to the load • Throttle losses are across suction valve Performance Control by suction throttling PT 1 PIC 1 Process
- 19. © 2008 Compressor Controls Corporation 19 • Change ofguide vanes angle a results in different compressor geometry Pd qr 2 Shaft power qr 2 • Different geometry means different performance curve amin aOP amax • Pressure is controlled by inlet guide vane position PIC - SP • Compressor operates in point A for given Rprocess A Rprocess • Required power is P1 P1 P T 1 PI C1 Process Notes: • Improved turndown • More efficient than suction throttling • Power consumed is proportional to the load • Power loss on inlet throttling is eliminated Performance Control by adjustable guide vanes
- 20. © 2008 Compressor Controls Corporation 20 • Changing speed generatesa family of curves Pd qr 2 Shaft power qr 2 Nmin NOP Nmax • Pressure is controlled by speed of rotation PIC - SP • Compressor operates in point A for given Rprocess A Rprocess • Required power is P1 P1 P T 1 PI C 1 Process SI C 1 Notes • Most efficient: (Power f(N)3) • Steam turbine, gas turbine or variable speed electric motor • Typically capital investment higher than with other systems • No throttle losses Performance Control by speed variation
- 21. © 2008 Compressor Controls Corporation 21 • While controllingone primary variable, constrain the performance control on another variable • Exceeding limits will lead to machine or process damage • Performance controller controls one variable and can limit two other variables. Limiting control to keep the machine in its stable operating zone CONTROL BUT DO NOT EXCEED Discharge Pressure Max. Motor Current Suction Pressure Max. Discharge Pressure Net Flow Min. Suction Pressure Suction Pressure Max. Discharge Temperature
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- 23. © 2008 Compressor Controls Corporation 23 Compressor networks Control systemobjectives for compressors in parallel: • 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 • Compressors are often operated in parallel and sometimes in series
- 24. © 2008 Compressor Controls Corporation 24 Compressor networks • Thepurposes of networks include: – Redundancy – Flexibility – Incremental capacity additions • Often each compressor is controlled, but the network is ignored • Compressor manufacturers often focus on individual machines. • A “network view” of the application is essential to achieve good surge protection and good performance control of the network.
- 25. © 2008 Compressor Controls Corporation 25 Notes • All controllersare coordinating control responses via a serial network • Minimizes recycle under all operating conditions Process 1 UIC VSDS Compressor 1 VSDS Compressor 2 Suction header 1 LSIC 2 UIC out RSP Serial network out RSP 2 LSIC 1 MPIC Serial network Serial network Equidistant Loadsharing Flow Diagram for Control Process
- 26. © 2008 Compressor Controls Corporation 26 • Machines operateat same Rc since suction and discharge of both machines are tied together PIC-SP • The DEV is a dimensionless number representing the distance between the operating point and the Surge Control Line • Lines of equal DEV can be plotted on the performance curves as shown 0.1 0.2 0.3 DEV = 0 0.1 0.2 0.3 • Machines are kept at the same relative distance to the Surge Control Line (SCL) • This means in practice the same DEV for both machines DEV1 DEV2 • Recycle will only start when all machines 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 qr,1 2 Rc,2 qr,2 2 Compressor 1 Compressor 2 Dev1 = Dev2 Q1 = Q2 N1 = N2 Notes: • Maximum turndown (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 Loadsharing Parallel Compressor Control
- 27. © 2008 Compressor Controls Corporation 27 Simplified P&ID for CompressorsOperating in Series