Compressor

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Compressor control menu Cost of operating turbo compressors Maintenance cost Operating cost Commissioning cost Putting it in perspective Compressor operation Centrifugal compressors Axial compressors Compressor system classifications Developing the compressor curve The surge phenomena Compressor control Acrobat Previous Rew Fwd Help Main Menu Repairs are expensive Costs of repairs - materials and labor 3,000 hp Process gas compressor 20,000 hp Axial air blower Seals $20,000 $50,000 Bearings $10,000 $25,000 $200,000 $750,000 Rotor Assembly Compressors Previous Rew Fwd Help Menu Operating costs are large Cost to operate one turbo compressor per year: Plant air compressor 1,000 HP (746 kW) $457,000 Wet gas compressor 4,000 HP(2,984 kW) $1,830,000 Propylene refrigeration comp. 40,000 HP(29,480 kW) $18,300,000 Assumes power at $.07 per kilowatt hour or $457 per horsepower per year. Energy costs vary due to local conditions. Energy Saving Examples Energy Savings Predictions Compressors Previous Rew Fwd Help Menu Energy savings examples resulting from reduced recycle or blow-off Compressor application Compressor shaft power Actual achieved savings Actual Energy Savings Result From Propylene refrigeration 40,000 hp ( 29 MW) $1,200,000 Improved Antisurge Protection and Capacity Control FCCU air blower Centac air compressor 15,000 hp (11.2 MW) $155,000 1,500 hp (1.1 MW) $78,000 Energy Savings Predictions Compressors Previous Rew Fwd Help Menu Available energy savings can be predicted Less than one year pay-backs* typical by reducing recycle of blow-Off Pay-back 12 Period 11 (Months) 10 Pay-back less than 10 Months with 15% Reduction 9 8 7 Pay-back less than 6 Months with 15% Reduction 6 5 1000 HP 4 3 3,500 HP Pay-back approximately 1 Month with 15% Reduction 2 1 20,000 HP 0 5 10 15 20 25 30 35 Reduced Recycle (Per Cent of Maximum Compressor Flow) *Assumes electro motor power At $0.05 US per kilowatt hour or turbine power at $327 per horsepower per year. Tax consequences are not considered in pay-back period due to varying tax policies around the world. Energy Saving Examples Compressors Previous Rew Fwd Help Menu Downtime costs can be enormous! • 60,000 BPD Cat Cracker: $90,000 per hour, lost sales plus fixed expenses. The biggest units are twice this size! • Natural Gas Production, 100 MMSCFD: $12,500 per hour lost sales plus fixed expenses. • Consequences of downtime: Lost profit, lost customer goodwill, repair costs, attention of top management. Compressors Previous Rew Fwd Help Menu Commissioning costs are large Process Start-up Cost Per Day 100 MMSCFD Natural Gas Plant $375,000 60,000 BPD Cat Cracker $2,300,000 • Includes lost sales plus fixed operating expenses. • Most turbo compressor control system design problems are discovered during commissioning. • Delays due to turbomachinery control problems are not unusual. • Startups are faster with a properly designed turbomachinery control system. Compressors Previous Rew Fwd Help Menu Putting it in perspective 30-year life cycle costs for a 20,000 hp compressor Maintenance Costs $4.5 Million Initial Cost $1.5 Million 97% of total costs Energy Costs $180 Million Costs in constant dollars Source: Experiences in Analysis and Monitoring Compressor Performance Ben Duggan & Steve Locke E.I. du Pont, Old Hickory, Tennessee 24th Turbomachinery Symposium Compressors Previous Rew Fwd Help Menu Putting it in perspective 30-year cost per 1,000 hp $ Millions 15.0 What can we control? 10.0 Uncontrollable Controllable ? 5.0 0.0 Initial Cost Maintenance Energy Costs in constant dollars Lost Production Source: Experiences in Analysis and Monitoring Compressor Performance Ben Duggan & Steve Locke, E.I. du Pont, Old Hickory, Tennessee 24th Turbomachinery Symposium Compressors Previous Rew Fwd Help Menu Centrifugal compressors • 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 Picture of Barrel Type Cross Section of Integrally Geared Picture of Integrally Geared IMPELLERS Single Case Compressor Centrifugal Impeller Compressors Picture of Gear and Impellers Previous Rew Fwd Help Menu Cross section of horizontal split Discharge volutes Impeller inlet labyrinth seals Impellers Shaft and labyrinth seal Drive coupling Journal bearing Casing (horizontally split flange) Thrust bearing Compressor discharge nozzle Compressor inlet nozzle Picture of Horizontal Split Centrifugals Previous Rew Fwd Help Menu Picture of horizontal split Cross Section of Horizontal Split Centrifugals Previous Rew Fwd Help Menu Cross section of barrel type compressor Picture of Barrel Type Centrifugals Previous Rew Fwd Help Menu Picture of barrel type compressor Cross Section of Barrel Type Centrifugals Previous Rew Fwd Help Menu Cross section of bull gear compressor Labyrinth seals Drive coupling Impellers Main gear Journal bearing Inlet guide vanes Pinion shafts Gear casing Compressor volutes Picture of Gear and Impellers Picture of Integrally Geared Centrifugals Previous Rew Fwd Help Menu Picture of bull gear compressor Picture of Gear and Impellers Cross Section of Integrally Geared Centrifugals Previous Rew Fwd Help Menu Picture of (bull) gear and impellers Picture of Integrally Geared Cross Section of Integrally Geared Centrifugals Previous Rew Fwd Help Menu 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) • High flow rate • Efficient • Not as common as centrifugals Stator Blades Shaft Rotor Blades Casing Rotor Blades Stator Blades Casing Cross Section of Axial Picture of Axial Compressors Previous Rew Fwd Help Menu Cross section of axial compressor Guide-vane actuator linkage Labyrinth seals Compressor rotor Rotor blades Adjustable guide vanes Thrust bearing Compressor inlet nozzle Compressor outlet nozzle Picture of Axial Axials Previous Rew Fwd Help Menu Picture of axial compressor Cross Section of Axial Axials Previous Rew Fwd Help Menu Compressor system classifications Single-Section, Three-Stage Parallel Network Single-Case, Two-Section, Six-Stage Two-Case, Two-Section, Six-Stage Series Network Compressors Previous Rew Fwd Help Menu Developing the compressor curve R Pcdpc c DP R H DischargeRatio Differential Pressure Polytropic Pressure Head Pressure (Pd/P(P ) 2or ) -(P PR )process,2 or s(P d s/P 2 1) (P2 - P1) Rprocess,1 Rc2 Rc1 Compressor curve for a specific speed N1 Q2 Q1 Compressors Qs, Previous normal mass vol Rew Fwd Help Menu Developing the compressor curve process limit Rc adding control margins maximum speed surge limit power limit stonewall or choke limit Actual available operating zone stable zone of operation minimum speed Qs, Compressors Previous vol Rew Fwd Help Menu How an airplane wing develops lift v1, p1 v2, p2 Bernoulli’s law • pstatic + 1/2rv2 + rgH = Constant • The term rgH is negligible for the wing • Then: pstatic + 1/2rv2 = Constant • • • Lift Due to the shape of the wing: v2 < v1 thus p2 > p1 As a result there is Dp or lift And the plane can fly Compressors Previous Rew Fwd Help Menu How the airplane develops stall Lift Lift Lift Lift • As the wing tilts back the Dv changes and thus the Dp • This leads to more lift • When the wing is tilted too much the streaming profile suddenly changes from laminar to turbulent • The air no longer “sticks” to the wing and the lift is lost • The plane starts to fall down Compressors Previous Rew Fwd Help Menu Developing the surge cycle on the compressor curve Pd • Compressor reaches surge point A • • From A to B 20 - 50 ms Drop into surge Compressor looses itsstarts abilitytotobuild make • Compressor pressure pressure • From C•to DCompressor 20 - 120“rides” ms Jumptowards out of surge curve surge • Suddenly P drops and thus P > P d v d • Point A is -reached • • A-B-C-D-A 0.3 3 seconds Surge cycle Plane• goes tosurge stall -cycle Compressor surges The is complete B Pv Rlosses Pd Pd = Compressor discharge pressure Pv = Vessel pressure Rlosses = Resistance losses over pipe A • • • • Pressure builds • System pressure is going down D Resistance goes up • Compressor is again able to overcome Pv Compressor “rides” the curve “jumps” back to C • Compressor performance curve and goes to point D Pd = Pv + Rlosses Electroflow motor is started • • Forward is re-established • Because Pv > Pd the flow reverses • Result of flow reversal is that pressure goes • Machine accelerates to nominal • Compressor operating point goes to point B down speed • Pressure goes down => less negative flow • Compressor reaches performance curve • Operating point goes to point C • Machine shutdown no flow, no pressure Qs, Note: Flow goes up faster because pressure is the integral of flow vol Compressors Previous Rew Fwd Help Menu Major process parameters during surge FLOW 1 2 TIME (sec.) 2 TIME (sec.) 2 TIME (sec.) • Rapid pressure oscillations with process instability • Rising temperatures inside compressor 3 TEMPERATURE 1 Rapid flow oscillations Thrust reversals Potential damage 3 PRESSURE 1 • • • 3 Compressors Previous Rew Fwd Help Menu Surge description • Flow reverses in 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 Compressors Previous Rew Fwd Help Menu 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 Compressors Previous Rew Fwd Help Menu Factors leading to onset of surge • Startup • Shutdown • Operation at reduced throughput • Operation at heavy throughput with: - • Trips Operator errors Load changes Cooler problems Driver problems - Power loss Process upsets Gas composition changes Filter or strainer problems Surge is not limited to times of reduced throughput. Surge can occur at full operation Compressors Previous Rew Fwd Help Menu Compressor control Objectives Performance control Major challenges of compressor control Loadsharing for parallel compressors Location of the surge limit Compressor networks High speed of approaching surge Base loading parallel compressors Control loop interactions Equal flow division system Loadsharing of multiple compressors CCC’s equidistant Loadsharing system Antisurge control Other topics Basic antisurge control system Fall-back strategies Protection #1: PI control Limiting control Protection #2: Recycle Trip Pressure Override Control (POC) Protection #3: Safety On Flow Measuring Devices (FMD’s) Output linearization Antisurge control valve The tight shut-off line Piping lay-out considerations Influence of controller execution time Dynamic simulation single compressor Dynamic simulation parallel compressors Previous Rew Fwd Help Menu Major control system objectives (user benefits) 1. Increase reliability of machinery and process • Prevent unnecessary process trips and downtime • Minimize process disturbances • Prevent surge and surge damage • Simplify and automate startup and shutdown 2. Increase efficiency of machinery and process Energy Saving Examples • • • • Operate at lowest possible energy levels Minimize antisurge recycle or blow-off Minimize setpoint deviation Maximize throughput using all available horsepower • Optimize loadsharing of multiple units Compressor control Previous Rew Fwd Help Menu Calculating the distance between the Surge Limit Line 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 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 yet reducing the surge control margin which means: • Bigger turndown range on the compressor • Reduced energy consumption during low load conditions Compressor control Previous Rew Fwd Help Menu Commonly used (OEM provided) coordinate systems of the compressor map • 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 Compressor control Previous Rew Fwd Help Menu The problem with commonly used (OEM provided) coordinate systems of the compressor map • These coordinates are NOT invariant to suction conditions as shown • For control purposes we want the SLL to be presented by a single curve for a fixed geometry compressor Compressor control Previous Rew Fwd Help Menu Developing invariant coordinates • • The following variables are used to design and to characterize compressors Through dimensional analysis (or similitude) we can derive two sets of invariant coordinates Fundamental variables characterizing compressor operation Hp = f0(Q, w, m, r, a, d, a) Invariant coordinates Set 1 Set 2 hr qr Ne a jr Re Rc qr Ne a jr Re Dimensional analysis or Similitude J = f1(Q, w, m, r, a, d, a) where: • Hp • J • Q  w  m  r • a • d  a = Polytropic head = Power = Volumetric flow rate = Rotational speed = Viscosity = Density = Local acoustic velocity = Characteristic length = Inlet guide vane angle where: • hr • qr • Ne  a • jr • Re • Rc Compressor control Previous = Reduced head = Reduced flow = Equivalent speed = Guide vane angle = Reduced power = Reynolds number = Pressure Ratio Rew Fwd Help Menu Coordinates (Hp, Qs) and (hr, qr2) (Hp, Qs) (hr, qr2) NOT invariant coordinates Invariant coordinates where: • Hp • Qs • hr • qr2 = Polytropic head = Volumetric suction flow = Reduced head = Reduced flow squared Compressor control Previous Rew Fwd Help Menu Coordinates (Rc, Qs) and (Rc, qr2) (Rc, Qs) (Rc, qr2) NOT invariant coordinates Invariant coordinates qr2 where: • Rc • Qs • qr2 = Pressure ratio = Volumetric suction flow = Reduced flow squared Compressor control Previous Rew Fwd Help Menu Coordinates (Rc, jr) and (Rc, Ne2) where: • Rc • jr • Ne2 (Rc, jr) (Rc, Ne2) Invariant coordinates Invariant coordinates = Pressure ratio = Reduced power = Equivalent speed squared Compressor control Previous Rew Fwd Help Menu Representing the SLL as a single curve using reduced coordinates • A coordinate system that is invariant to suction conditions is: Hp hr  (ZRT)s • and qr  Qs ( ZRT)s Squaring the flow will still keep coordinates invariant: Hp hr  (ZRT)s and 2 Q s qr2  ( ZRT)s hr 2 qr Design Nitrogen Off-gas MW MW MW Ps Ps Ps Ts Ts Ts ks ks ks Compressor control Previous Rew Fwd Help Menu Calculating qr2 (reduced flow squared) qr 2= Qs 2 (ZRT)s • • • • • • • where: R Ru R MW ps K  Dpo,s • Ts • Zs K . Zs . Ru . Ts Dpo,s . MW ps = (ZRT)s Dpo,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 Dpo,s transmitters Compressor control Previous Rew Fwd Help Menu Calculating hr (reduced head) hr = Hp (ZRT)s Zs . Ru . Ts Rcs-1 . MW s = (ZRT)s Rcs-1 = s log(Rt) • For polytropic compression Rt = Rcs thus s = log(Rc) • R = Ru / MW • Rt = Td / Ts Temperature ratio • Rc = pd / ps Pressure ratio where: • Ru = Universal gas constant • R = Specific gas constant • MW = Molecular Weight of the gas • pd = Discharge pressure • ps = Suction pressure • Zs = Suction compressibility  s = Exponent for polytropic compression • The antisurge controller calculates hr using pd, ps, Td and Ts transmitters Compressor control Previous Rew Fwd Help Menu Building the Surge Limit Line • Any curvature of the Surge Limit Line can be characterized as a function of the ordinate hr • The surge parameter is defined as: f1(hr ) Ss  2 qr ,op • The function f1 returns the value of qr2 on the SLL for input hr hr hr 2 2 qr,SLL qr Compressor control Previous Rew Fwd Help Menu The surge parameter Ss 2 • The function f1 returns the value of qr on the SLL for input hr • As a result: Ss = q2r,SLL q2r,op hr Ss > 1 • Ss < 1 : stable operating zone • Ss = 1 : surge limit line (SLL) • Ss > 1 : surge region OP hr Ss < 1 2 2 qr,SLL 2 qr,op qr OP = Operating Point Compressor control Previous Rew Fwd Help Menu 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 hr d =0 Ss = 1 Benefits: d <0 DEV = 0 • One standard surge parameter in the plant • No operator confusion: Ss > 1 DEV < 0 Ss < 1 • DEV > 0 Good d >0 • DEV = 0 Recycle line DEV > 0 • DEV < 0 Bad 2 Surge margin qr Compressor control Previous Rew Fwd Help Menu Simplifying the surge parameter by replacing hr with Rc • Reduced Head hr can be replaced by Rc while keeping the coordinate system invariant to suction conditions • The surge parameter Ss now becomes Ss = f1(Rc) 2 qr,op 2 where the function f1( ) returns the value of qr,SLL on the SLL for the input Rc • This eliminates the need for Td and Ts transmitters for control Important Note: CCC still strongly recommends Td and Ts transmitters as well as rotational speed N for compressor monitoring purposes Compressor control Previous Rew Fwd Help Menu The simplest CCC surge parameter • An antisurge algorithm can be designed around two transmitters: Dpo and Dpc • The parameter Ss = f1(Rc) 2 qr is invariant to inlet conditions and speed • For two transmitters choose the function f1 to be (Rc - 1) f1(Rc) Ss = 2 qr Rc - 1 = Dp o ps ( = pd -1 ps Dpo ) . ps pd - ps = Dpo = Dpc Dpo • Selecting the specific function for f1(Rc) to be (Rc - 1) keeps the map invariant to inlet conditions and speed Compressor control Previous Rew Fwd Help Menu Disadvantage of the Dpc /Dpo surge parameter • The SLL is rarely a straight line in the coordinates qr2 and Rc • The parameter Dpc /Dpo represents a straight line in the invariant coordinates qr2 and Rc • The Dpc /Dpo approach results in loss of turn down and unnecessary recycle Rc loss of operating envelope Actual Surge Limit Line (SLL) SLL calculated by antisurge controller using Dpc /Dpo = constant 2 qr Compressor control Previous Rew Fwd Help Menu Actual field data showing disadvantage of Dpc /Dpo surge parameter Compressor control Previous Rew Fwd Help Menu Surge parameter for compressor with sidestream Problem definition 1 Dpo,1 P1 T1 2 3 Dpo,2 P2 T2 q3 and T3 are internal to the compressor and cannot be measured Compressor control Previous Rew Fwd Help Menu Derive a new surge parameter for compressor with sidestream: qrNe • Any combination of invariant parameters results in another invariant parameter • Derive equation for surge parameter that does not require measurement of T and qr at point 3 Step 1: Reduced flow Step 2: Equivalent speed . m ZRT qr  p N Ne  ZRT . Step 3: Combine qr and Ne where: . • m • Z • R • Ne • qr • N • p • T = mass flow = Compressibility = Gas constant = Equivalent speed = Reduced flow = Rotational speed = Pressure = Temperature . qrNe  m ZRT . p Compressor control mN N = ZRT p Previous Rew Fwd Help Menu Calculating the invariant parameter qrNe 1 Dpo,1 p1 . T1 m1 q3 Ne,3 2 3 Dpo,2 p2 . m2 T2      p p 1 2 N   A1 Dpo,1 . . .  A D p   2 o,2   . T1 m3 . N  T2    (m1 + m2 ) N   = = p2   p2 p3     Compressor control Previous Rew Fwd Help Menu Developing invariant surge patameter Rc vs. qrNe r Compressor control Previous Rew Fwd Help Menu The approach to surge is fast Pd 1 SEC. 100% D Qs 0 Pd 100% • Typically, performance curves are extremely flat near surge • Even small changes in compressor pressure differential cause large flow changes. • The speed of approaching surge is high. In only 0.4 seconds, DPO dropped by 14%, with a 2% change in DPc ABC D DPo 0 100% AB C DPc 0 A Compressor control Previous Rew Fwd Help Menu The approach to surge is fast - another example 100% DPo 0 1 sec. 100% DPc 0 For a 2% increase in pressure differential (DPc), flow rate DPo dropped 9% in 0.3 sec. Compressor control Previous Rew Fwd Help Menu 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 Rprocess Rc Rprocess+valve VSDS Compressor FT 1 PsT 1 PdT 1 2 qr Discharge Suction UIC 1 • Surge parameter based on invariant coordinates Rc and qr – Flow measured in suction (DPo) – Ps and Pd transmitters used to calculate Rc Compressor control Previous Rew Fwd Help Menu Antisurge controller operation Protection #1: The Surge Control Line (SCL) Rc SLL = Surge Limit Line SCL = Surge Control Line • When the operating point crosses the SCL, PI control will open the recycle valve • PI control will give adequate protection for small disturbances • PI control will give stable control during steady state recycle operation • Slow disturbance example B A 2 qr Compressor control Previous Rew Fwd Help Menu Adaptive Gain Enhancing the effectiveness of the PI controller Rc • When the operating point moves fast towards the SCL, adaptive gain moves the SCL towards the operating point. • This allows the PI controller to react earlier • As a result a smaller steady state surge control margin can be achieved without sacrificing reliability • Fast disturbance example B A 2 qr Compressor control Previous Rew Fwd Help Menu Antisurge controller operation Protection #2: The Recycle Trip® Line (RTL) SLL = Surge Limit Line Rc RTL = Recycle Trip® Line SCL = Surge Control Line • • • Total response Disturbance When Operating the OP hits operating point arrives SCL of keeps Moves controller the the point PI moving back Operating controller hits is tothe the the Pointside opens towards Recycle safe sum of(OP) valve the Trip surge ofmoves PIthe based Line control and RTL (RTL) towards on hits and proportional Recycle the Recycle the SCL and conclusion Trip integral LineRT(RTL) action is: actiondecays out the – The function – – step response We are close to surge PI controller integrates to stabilize The PI controller is too slow to Benefits: OP on SCL catch the disturbance – Get Energy due to smaller – out savings of the dangerous zone surge margin An open loop response is – Compressor has more turndown triggered before recycle or blow-off – Surge can be prevented for virtually any disturbance 2 Output to Valve qr Recycle Trip® Action PI Control Total Response PI Control Response + Recycle Trip® Response To antisurge valve Time Compressor control Previous Rew Fwd Help Menu Improving the accuracy of Recycle Trip® open loop control • Recycle Trip® is the most powerful method known for antisurge protection • But, open loop control lacks the accuracy needed to precisely position the antisurge valve • Open loop corrections of a fixed magnitude (C1) are often either too big or too small for a specific disturbance • The rate of change (derivative) of the compressor operating point has been proven to be an excellent predictor of the strength of the disturbance and the magnitude required from the Recycle Trip® response • Therefore, the magnitude of actual step (C) of the Recycle Trip response is a function of the rate of change of the operating point or d(Ss)/dt Compressor control Previous Rew Fwd Help Menu Recycle Trip® based on derivative of Ss Recycle Trip® Response calculation C = C1Td where: • C • C1 • Td • d(Ss)/dt Output to valve Benefits • Maximum protection – No surge – No compressor damage d(Ss) dt • Minimum process disturbance – No process trips = Actual step to the valve = Constant - also defines maximum step = Scaling constant = Rate of change of the operating point Medium disturbance Output to valve Large disturbance 100% Total PI Control Total PI Control Recycle Trip® Recycle Trip® 0% Time Time Compressor control Previous Rew Fwd Help Menu 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 (RTL) – If Yes: Exponential decay of Recycle Trip® response – If No: Another step is added to the Recycle Trip® response Output to valve Output to valve One step response Multiple step response 100% Total PI Control Total PI Control Recycle Trip® 0% Time C2 Recycle Trip® Time C2 C2 C2 Compressor control Previous Rew Fwd Help Menu Antisurge controller operation Protection #3: The Safety On® Line (SOL) Rc SOL = Safety On® Line SLL = Surge Limit Line RTL = Recycle Trip® Line SCL = Surge Control Line ®Recycle ® will •• Additional PI The If Operating control Safetyand safety On Point response or crosses surge Trip shifts margin the Safety theis SCL added stabilize RTL to the onright the new SCL On® and Linethe themachine compressor is in surge Compressor can surge due to: • Transmitter calibration shift • Sticky antisurge valve or actuator • Partially blocked antisurge valve or recycle line • Unusual large process upset New SCL New RTL Additional surge margin 2 qr Benefits of Safety On® response: • Continuous surging is avoided • Operators are alarmed about surge Compressor control Previous Rew Fwd Help Menu Built in surge detector Pressure and Flow Variations During a Typical Surge Cycle 100% • Surge signature is recorded during commissioning • Rates of change for flow and pressure during surge are determined • Thresholds are configured slightly more conservative than the actual rates of change during surge Pd 0% 1 TO 2 SECONDS 100% DPo 0% 20 to 50 milli-seconds • Surge is detected when the actual rates of change exceed the configured thresholds • The following methods can be used: • 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 Compressor control Previous Rew Fwd Help Menu 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 Compressor control Previous Rew Fwd Help Menu Fall-back strategies for the antisurge and performance controller • Antisurge controller – If a pressure transmitter fails, a minimum q2r algorithm is used – 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 • • Performance controller – Switches to redundant transmitter upon primary transmitter failure – Output goes to pre-selected value if all transmitters fail or is frozen All transmitter failures are alarmed Compressor control Previous Rew Fwd Help Menu Output linearization • Controller Dynamic Existing Linear For antisurge valve valve flow output gives control response hasisthe equal quick characterized asame linear becomes percentage opening valveas as linear trim dynamic is preferred mirror mirror image flow image in response the in the linear linear over valve valve its line line complete stroke Flow rate through valve Controller output Valve trim quick opening Valve trim Controller output equal percentage Controller output Notes • Used to improve controllers operation when non-linear valves are used • Used on retrofits to avoid additional investment in new valve • Works well with equal percentage characteristics • Works less satisfactory with quick opening characteristics Compressor control Previous Rew Fwd Help Menu The Tight Shut-off Line (TSL) Dynamic control range Flow rate through valve Controller output PI Control Controller output Low clamp on controller output 0% to the valve Rc SOL SLL RTL C SCL TSL = Tight Shut-off Line B A A B Time C • At CCC The For When Many the dynamic 5% controller antisurge antisurge the low oroperating controller low clamp control is clamp controller valves now value point crosses we value “ready have the want has isvalve represents to the the to to a the Tight go” use TSL Shut-off the right following when closed range output of the the Line operating characteristic: 5% of position TSL the -(TSL) 100% the controller for controller points that on control the eliminates hits jumps valve closes theto • Usually still leaks which results the purposes SCL disadvantages valve low - energy point clamp at C 0% value - point - point A value B the • in from 0% towaste low clamp • This flow is below rate the low the clamp valve value is • Makes anthrough annoying noise Benefits (almost) zero and does not •• Typical for worn valveswhen and valves No leakage and noise controller change with Teflon seat is• far away from point A Once the lowsurge clamp- is reached • Eliminates noise and energy the characteristic is linearwaste • Eliminates time in the response • Typical dead low clamp value can be of the valve when thethe 5%antisurge - we will use the 5% as operating point is close to the SCL value throughout in this example 2 qr Compressor control Previous Rew Fwd Help Menu Compressor performance control • 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 control Previous Rew Fwd Help Menu Performance control by blow-off or recycle Pd Rprocess B A Process Rprocess + Rvalve PIC - SP PT 1 Curve 1 PIC 1 Curve 2 Shaft power P1 P2 Notes 2 • •Q Compressor Pressure Point Required Power represents B2 loss represents is power is controlled operates P1 energy in -P point the by point waste A B point blow-off is that P1A loss 2 in 2 Most inefficient control method qr Curve represents: would deliver the pressure for Rprocess Qloss • Regularly foundon in plant air speed systems • Lower speed variable systems • Rare in other systems Curve 1 • IGVs closed on variable geometry • Not recommended compressors Curve 2 • Inlet throttle valve closed on fixed speed compressors 2 qr Compressor control Previous Rew Fwd Help Menu Performance control by discharge throttling Pd A Pressure loss across valve Rprocess + Rvalve Rprocess Process PIC - SP PT 1 Curve 1 Curve 2 PIC 1 2 Shaft power qr P1 P2 Curve 1 Curve 2 2 Notes • Power Compressor Required Pressure Opening Lower resistance loss of is power is valve controlled operates P1 is -would would PP21 in byreduce require point A • Curve 2 represents: pressure resistance less speed drop to and Rover power valve process • Lower speed on variable speed Notes systems • Extremely inefficient (consumes • IGVs closed on variable geometry approx. the same power for every load) compressors • Rarely used • Inlet throttle valve closed on fixed • Not recommended speed compressors qr Compressor control Previous Rew Fwd Help Menu Performance control by suction throttling Pd Rprocess Process A PIC - SP PT 1 Suction valve open Suction valve throttled PIC 1 • 2 Shaft power qr P1 2 qr Required Inlet Changing Pressure Compressor valveis power suction manipulates controlled operates is pressure P1 in by suction point inlet A pressure generates valve for given position Raprocess family of curves 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 Compressor control Previous Rew Fwd Help Menu Performance control by adjustable guide vanes Pd Rprocess Process A PIC - SP amin aOP PT 1 amax PIC 1 Required Different Pressure Compressor geometry is power controlled operates is Pmeans by point inletaA Change of guide vanes angle 1 in different guide for given vane Rdifferent positioncompressor curve results inperformance process Notes geometry • Improved turndown • More efficient than suction throttling • Power consumed is proportional to the load • Power loss on inlet throttling is eliminated • 2 Shaft power qr P1 2 qr Compressor control Previous Rew Fwd Help Menu Performance control by speed variation SIC 1 Pd Rprocess Process A PIC - SP PT 1 Nmax NOP PIC 1 Nmin • 2 Shaft power qr P1 2 qr Required is Changing Pressure Compressor power speed controlled operates is generates P1 in bypoint speed a A of family rotation for given of curves Rprocess 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 Compressor control Previous Rew Fwd Help Menu Limiting control to keep the machine in its stable operating zone • While controlling one primary variable, constrain the performance control on another variable 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 Exceeding limits will lead to machine or process damage Performance controller controls one variable and can limit two other variables. Compressor control Previous Rew Fwd Help Menu Power limiting in the performance controller an example Rc • • Power limit R1 R2 • R3 A D B PIC-SP However Primary Limiting Compressor Machine Process PIC will speed would variable variable resistance hits power like operates power machine tolimiting PPower speed changes limit inup loop point to d decreases A from N further machine takes in R order control to atup RR Nto to and control N4 and controls pressure operate Compressor operates in point 2for 11to 32 1 P in machine point atNspeed N3 Bdfor R2 D at 2 Compressor will operate in point C for R3 at N3 C N4 N N2 3 N1 Benefits • Maximum protection – No machinery damage • Qs, vol Maximize production – Machine can be pushed to the limits without risk of damage Note: Same approach for other variables (pressures, temperatures, etc.) Compressor control Previous Rew Fwd Help Menu Limiting Ps or Pd using the antisurge controller VSDS Compressor FT 1 Suction • • PsT 1 PdT 1 UIC 1 Discharge 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 Compressor control Previous Rew Fwd Help Menu Interacting antisurge and performance loops • Interaction starts at B • Performance controller on discharge pressure reduces performance to bring pressure back to setpoint • Unless prevented, PIC can drive compressor to surge • Antisurge controller starts to operate at B • Even if surge is avoided, interaction degrades pressure control accuracy • Results of interaction Rc B C A DPo Ps PIC-SP – Large pressure deviations during disturbances – Increased risk of surge Compressor control Previous Rew Fwd Help Menu The performance controller interacts with the antisurge controller • Both controllers manipulate the same variable - the operating point of the compressor • 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 Compressor control Previous Rew Fwd Help Menu Ways to cope with antisurge and performance loop interactions • 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 Compressor control Previous Rew Fwd Help Menu Interacting antisurge control loops VSDS Section 1 Section 2 UIC 1 UIC 2 PIC 1 Disturbance Rc,1 R R R Rc,2 R 2 2 qr,1 • • qr,2 • The system is oscillating of controller recycle on section Antisurgeofcontroller Opening recycle valve UIC-1 on will section open 2 •• Opening Disturbance Antisurge comesvalve from UIC-2 the will open1 caused Pd,1side = Ps,2 lead to decrease caused the recycle Ps,2 valve = P•d,1to to protect increase section 1 controller discharge the recycle valve protect Slowing down the tuning would to: section 2 • Pd,2 increases • Increased risk of surge against against surge • Result: Result: surge •• •• •• •• • Compressor damage P decreases • •• Pd,1 d,1 increases • Process trips P increases • •• Ps,1 s,1 remains constant • Bigger surge margins • •• R decreases Rc,1 c,1 increases • surge Energy Section from surge waste •• Section 1 1 moves moves away towards remains constant Ps,2s,2 decreases PP d,2decreases increases c,2remains Pd,2 increases PR constant s,2 Section 2 moves towards surge R decreases Rc,2c,2increases Section22moves movestowards away from surge Section surge Compressor control Previous Rew Fwd Help Menu Loop decoupling between multiple antisurge controllers VSDS Section 1 UIC 1 Section 2 Serial network UIC 2 Serial network • All CCC controllers are connected on a serial network • This allows them to coordinate their control actions • When UIC-2 opens the recycle valve: • Section 2 will be protected against surge • Section 1 will be driven towards surge PIC 1 • How much section 1 is driven towards surge depends on how much the recycle valve on section 2 is opened • The output of UIC-2 is send to UIC-1 to inform UIC-1 about the disturbance that is arriving • UIC-1 anticipates the disturbance by immediately opening its valve Note: The same applies when the antisurge valve on section 1 is opened first Compressor control Previous Rew Fwd Help Menu Loop decoupling simplified block diagram Benefits VSDS Section 1 • • Section 2 Serial network UIC 1 Analog Inputs UIC 2 ® Each controller itshas PI and its own Recycle values gain to M toPI Antisurge UIC-2 UIC-1 Loop decoupling reports is protecting controller PI and block value values section UIC-2 Recycle is multiplies ofadded opens other 1decoupling against TripTrip to controllers its reported output output valve surge to to PI using UIC-1 protect and nUIC-2 • Avoids control system oscillations ® ® allow for tuning relative loop between section and Recycle antisurge (performance Recycle 2Trip against valve Trip and values 1ofsurge antisurge) with aregains added gain to output M2 to Same decoupling takes place • Allows faster tuning ofdecoupling control system different controllers antisurge valve 1 • Reduced risk of surge • Allows closer operation to surge limits without taking risk From other controllers FA Mode FA Mode DEV1 PI From other controllers Analog Inputs RT PIn . Mn + RTn . Mn + Antisurge Controller 1 To antisurge valve 1 PI2 . M2 + RT2 . M2 DEV2 PI RT + Loop Decoupling PI1 . M1 + RT1 . M1 + PIn . Mn + RTn . Mn + Loop Decoupling Antisurge Controller 2 To antisurge valve 2 Compressor control Previous Rew Fwd Help Menu Compressor networks • Compressors are often operated in parallel and, less frequently, in series • The purposes of networks include: – Redundancy – Flexibility – Incremental capacity additions • Usually, each compressor is controlled, but the network is ignored • Compressor manufacturers often focus on individual machines • Control of the network is essential to achieve good surge protection and good performance control of the network Compressor control Previous Rew Fwd Help Menu Control system objectives 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 Compressor control Previous Rew Fwd Help Menu Process Flow Diagram for base load control VSDS Compressor 1 Swing machine UIC 1 PIC 1 Suction header HIC 1 Process VSDS Compressor 2 UIC 2 Base machine Notes • All controllers act independently • Transmitters are not shown Compressor control Previous Rew Fwd Help Menu Parallel compressor control by base loading Rc,1 Compressor 1 Rc,2 Compressor 2 Swing machine Base machine PIC-SP QP,1 + QP,2 = QP,1 + QP,2 2 2 qr,1 qr,2 QP,1 QC,1 QP,1 QP,2 QC,2= QP,2 Notes • Load Machines Base Swing load machine could machine operate one be is or re-divided can fully more atbe same loaded running compressors toReliminate since with runs suction recycle and recycle without letand therecycle discharge other(s) absorb of both the c and • Base loading is inefficient machines load swings are tied together • Base loading increases the risk of surge since compressor #1 will take the worst of any disturbance where: • QP = Flow to process • Base loading requires frequent operator intervention • QC = Total compressor flow • Base loading is NOT recommended • Q -Q = Recycle flow C Compressor control Previous P Rew Fwd Help Menu Process Flow Diagram for equal flow division VSDS RSP Compressor 1 out UIC 1 FIC 1 RSP out PIC 1 Suction header Process VSDS RSP Compressor 2 out UIC 2 FIC 2 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 Compressor control Previous Rew Fwd Help Menu Parallel compressor control by equal flow division Rc,1 Compressor 1 Rc,2 Compressor 2 PIC-SP QP,1 = QP,2 Equal flow Equal flow 2 2 qr,1 QP,1 qr,2 QP,2QC,2 Notes Machine Equal Machines Bias relay flow 2are operate on operates division remote never atmight with identical same setpoint recycle work Rc except since would ifin while both suction by only machines machine coincidence work andif1discharge are curves still-identical different hashave turn of both • •Requires additional capital investment FMD’s machines down resistance same steepness are duetied to piping together arrangments • Requires additional energy due to permanent pressure loss across FMD’s where: • Poor pressure control due to positive feedback in • QP = Flow to process • QC = Total compressor flow control system (see next) • QC - QP = Recycle flow • Equal flow division is NOT recommended Compressor control Previous Rew Fwd Help Menu Dynamic control problem with pressure to flow cascade system • Rc • • R2 C R1 B D PIC-SP A disturbance isthe amplified Pressure The In Process As Since Only aatypical FIC PIC machine result as the the provides reacts is disturbance controller PIC the the master-slave PIC is is master fast machine operating starts slow and (PIC) causes RSP itand todoes will moves control reduce in provides for the try point the not the FIC to toitsis A Remote speed the scheme resistance point move maintain output slave B its controller, toSetPoint the output its control toslave SP change yet suction pressure needs (RSP) which from for to throttle is R the be Flow the toFICR FICPositive feedback system This is the intersection of 4 1lines:2 controller valve approx. SP comes or 5guide times (FIC) down vanes faster and the than pressure the is The FIC will speed the machine to – Resistance line Rup 1 master restored point C at speedcurve N 3 N1 – Performance – – PIC-SP FIC-SP = Output of PIC N3 N1 N2 2 FIC-SP PIC OUT 1 Master RSP FIC 1 OUT Slave RSP SIC 1 qr Notes • Requires additional capital investment in FMD’s • Requires additional energy due to permanent pressure loss across FMD’s • Poor pressure control due to positive feedback in control system • Equal flow division is NOT recommended Compressor control Previous Rew Fwd Help Menu Process Flow Diagram for equidistant control for parallel compressors VSDS RSP Compressor 1 out UIC 1 Serial network LSIC 1 Serial network MPIC 1 Suction header Process VSDS RSP Compressor 2 out UIC 2 Serial network LSIC 2 Notes • All controllers are coordinating control responses via a serial network • Minimizes recycle under all operating conditions Compressor control Previous Rew Fwd Help Menu Parallel compressor control by equidistant operation Rc,1 Compressor 1 Rc,2 SCL = Surge Control Line .1 .2 Compressor 2 DEV = 0 .1 .2 .3 .3 PIC-SP  Dev2 Q1  Q2 N1  N2 2 Dev1 2 qr,1 qr,2 DEV1 • • • DEV2 Notes Since The Machines DEV DEV isare operate is a dimensionless kept at at same the same Rcnumber all since relative sorts suction representing ofdistance machines and discharge to the can the distance be Surge mixed: of Control both between small, Line the • Maximum turndown (energy savings) without recycle or blow-off machines operating (SCL) big, axials, point are centrifugals tied and together thewhen Surge Line Recycle will only start allControl machines are on their SCL • Minimizes the risk of surge since all machines absorb part of the Lines This The DEV means of equal willinbe practice DEV the can same the befor plotted same all machines DEV on the for both performance but machines they willcurves operateasatshown disturbance different speeds and flow rates • Automatically adapts to different size machines • CCC patented algorithm Compressor control Previous Rew Fwd Help Menu Compressors in parallel - the primary response • • • • • • • If DEV Master The In When the output primary the <= controller 0machine of apply the response loadsharing controls master is close block controller the to the gain main the SCL controller PID Process the goes Variable to checks master the primary controller if(PV) the machine via response its will PID no iscontrol block close longerin toblock reduce the the SCL: Output goes to antisurge valve loadsharing performance controller to control the primary variable Yes: to don’t reduce capacity - keepisoutput In– order check if the machine close to constant the the controller primary response needs TheSCL master will startblock to open the – No: reduce capacity as necessary the DEV recycle valve to control the primary variable Apply loadsharing gain M0 The DEV is reported by the antisurge The output of the master controller goes via controller the primary response block directly to the performance control element Master Controller PV PID SP Analog Inputs Don’t change output FA Mode No DEV < >0 DEV DEV DEV Yes PI Apply loadsharing gain Primary response RT Primary response x + + Antisurge Controller Primary response To Toantisurge performance valve control element Loop Decoupling Loop Decoupling To antisurge valve Compressor control Loadsharing Controller To performance control element Previous Rew Fwd Help Menu The load balancing response • • This In Other The order fast antisurge loadsharing master average loadsharing to master balance controller DEV controller controller controller is controllers the sent calculates machines reports out controls reports to also all the the they this report the actual need The load balancing block is a slow primary to DEV their average loadsharing bePV to DEV kept the also process ofPV at all load controllers the to to reported the the balancing same variable master master DEV DEV toby block become controller controller PV’s directly in the SP controller that will equalize all DEV’s for all manipulating loadsharing for all load balancing controller the finalblocks control elements parallel compressors This reported DEV becomes the Process Its output(PV) is added the balancing total output to Variable for thetoload PID the loopperformance control element DEV from other loadsharing controllers Master Controller PV PID SP Average Analog Inputs DEV • • FA Mode DEV DEV DEV PV PI Primary response RT SP Load balancing Primary response + + Antisurge Controller Loop Decoupling Loop Decoupling To antisurge valve Compressor control Loadsharing Controller To performance control element Previous Rew Fwd Help Menu The Pressure Override Control (POC) response Benefits • The When There Opening As soon operating CCC primary a islarge aas of master high the PID disturbance point antisurge chance operating loop controller rides will to valve point stabilize exceed the occurs has curve is drops a Pressure the it much the can and relief under the • DEV from other loadsharing controllers Rc PV PID PV SP PI (One-Sided) SP Average Analog Inputs Relief valve setting POC-SP DEV • • Master Controller operating happen pressure valve Override faster the POC-SP setting than that Control rises point aline the reduction and sharply on performance the (POC) trip the antisurge the PIC-SP in mode process speed control that line valves willstart open Fast response during fast upsets element the to close antisurge again (e.g. valve speed) toisget toothe slow disturbance to keep the Avoid process trips due to lack of pressure under control under quickly control response in performance control elements Allows closer operation to process limits without taking risk FA Mode DEV DEV DEV PIC-SP PV PI Primary response RT SP Load balancing Primary response + + Antisurge Controller Loop Decoupling Loop Decoupling Loadsharing Controller 2 qr To antisurge valve Compressor control To performance control element Previous Rew Fwd Help Menu Loadsharing for multi-section compressors VSDS RSP Train A Section 1 Section 2 out UIC 1A Serial network UIC 2A Serial network LSIC A Serial network MPIC 1 Suction Header VSDS Process RSP Train B Section 1 Section 2 out UIC 1B • Serial network UIC 1B Serial network LSIC B How Select By Share selecting tothe per operate load train the--equidistant --section equal in theDEV’s loadsharing closest from forto the both the controller Surge trains SCL Control it--ison --guaranteed the the Line section section (SCL) that closest closest when the other there totothe theis more section SCL than on one the same section train perismachine not in recycle ??? Compressor control Previous Rew Fwd Help Menu Selecting the section closest to SCL for parallel operation Master Controller PV PID PV SP PI • Both The selected lowest antisurge DEV DEV controllers is is selected: report the section reported their to:closest DEV to the to the loadsharing SCL • Primarycontroller control response blocks (One-Sided) SP DEV from other loadsharing controllers • Load balancing block • Master controller averaging block Average Analog Inputs FA Mode FA Mode DEV1 DEV1 PI PV RT Primary response To antisurge valve-1 DEV2 DEV2 SP Load balancing Primary response + Antisurge Controller < PI + Loop Decoupling Loop Decoupling Primary response RT + Loadsharing Controller To performance control element Compressor control Antisurge Controller Loop Decoupling To antisurge valve-2 Previous Rew Fwd Help Menu Flow Measuring Device (FMD) selection criteria • 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 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 Venturi’s Pitot tubes etc. • Recommended flow range for FMD and transmitter is maximum compressor flow • Recommended Dp corresponding to Qmax, compressor is 10” WC (250 mmH2O) or more Compressor control Previous Rew Fwd Help Menu Flow Measuring Device (FMD) location VSDS Compressor minimum possible minimum possible Suction • The preferred location of the FMD: • Suction of compressor • As close to the inlet flange as possible • Discharge • Less preferable location of the FMD: • Discharge of compressor • As close to the discharge flange 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) Compressor control Previous Rew Fwd Help Menu Response time of the FMD transmitter • The speed of approaching surge is 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 • 1 SEC. 100% 0 Ps 100% ABC D Desired rise time for pressure transmitters is 500 ms or less 0 DPo Actual pressure Transmitter output 63% response 100% AB C 1- (1/e) DPc • 0 In only 400 ms, DPO dropped by 14%, with a 2% change in DPc Time t1 is less than 200 ms Compressor control Previous Rew Fwd Help Menu The effect of damping the Dpo (flow) transmitter • Knowing the flow is essential to determine the distance between the operating point and the SCL • Damping the Dpo (flow) transmitter destroys essential information Start of Surge Flow 50 t = 16.0 s t = 1.70 s t = 0.03 s t = 0.20 s 0 Actual Flow -50 0 1.25 2.50 3.75 Time (seconds) 5 Damping the Dpo (flow) transmitter can paralyze the complete antisurge control system!!! Compressor control Previous Rew Fwd Help Menu Sizing the antisurge control valve • Criteria for antisurge valve sizing based on CCC’s experience – Provide adequate antisurge protection for worst possible disturbances – 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 Rc A • Take point A at the intersection of the maximum 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 Qvol Compressor control Previous Rew Fwd Help Menu Sizing the antisurge control valve - alternative method • Rc A B Qvol An alternative method yielding excellent results 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 < 1.1 . Cv, calc Compressor control Previous Rew Fwd Help Menu Stroke speed and characteristic of the antisurge valve Antisurge valve stroke speed • Antisurge valve must have speed of response adequate for antisurge protection for all disturbances • Recommended full stroke times: – Size Close to open – 1” to 4” 1 second – 6” to 12” 2 seconds – 16” and up 3 seconds • Open to close < 3 seconds < 5 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 anywhere 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 Compressor control Previous Rew Fwd Help Menu Improving the performance of the antisurge valve • Most normal control valves can be made to perform as required 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 Increase size of air connection into the actuator Drill additional holes in actuator - avoids pulling a vacuum Compressor control Previous Rew Fwd Help Menu Piping lay-out consideration when designing an antisurge control system • Piping lay-out influences the controllability of the the total system • The primary objective of the antisurge controller is to protect the compressor 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 compressor – Recycle valve flange – Check valve flange VSDS Compressor 1 volume to be minimized Compressor control Previous Rew Fwd Help Menu Using a single antisurge valve increases recycle lag time Large volume 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 determines the dead time and lag time in the system Large volume significantly decreases the effectiveness of the antisurge protection Result Note – – – – Poor surge protection Large surge margins Energy waste Process trips because of surge • This specific piping layout is found on many wet gas compressors in FCCU’s Compressor control Previous Rew Fwd Help Menu Sharing recycle coolers degrades surge protection Section 1 Section 2 Small volume • • 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 control system to protect the machine against surge • Result • • • • Poor surge protection Large surge margins Energy waste Process trips because of surge Compressor control Previous Rew Fwd Help Menu Installing recycle valve upstream from cooler improves control response Compressor 1 Increased volume due to cooler Compressor 2 Minimum volume • • Compressor 1 has ideal piping lay-out for surge protection Minimum volume between the three flanges • • 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 time and lag in the system • Result • • Increased surge margins Energy waste Compressor control Previous Rew Fwd Help Menu Recycle lines configured for optimum surge protection Section 1 Section 2 Section 3 Suction Process Minimum volume • Compressor has ideal piping lay-out for surge protection • Minimum volume between the three flanges for all sections Compressor control Previous Rew Fwd Help Menu Which antisurge piping configuration do you choose??? Lay-out #1: Compressor with recycle lines optimally configured for antisurge control Section 1 Section 2 Section 3 Suction Process Lay-out #2: Compressor with coolers upstream of recycle take-off Section 1 Section 2 Section 3 Suction Process • When selecting lay-out the most residence time of the gas in the “surge” volume Lay-out These two #1 #2 piping has requires minimum lay-outs one#2 cooler volume are less between common and thus thefor the flanges antisurge capital and investment is control the bestislay-out lower should be verified to purposes check acceptable time delays are not exceeded for antisurge control • Lay-out #2 will require bigger surge control margins Compressor control Previous Rew Fwd Help Menu Influence of controller execution time • Analog controller 100% SCL SLL Operating point • • 0% Time • 100% • Controller output 0% Time • • Leading engineering contractor performed 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 overshoot Digital controllers get exact same tuning parameters Digital controllers get exact same disturbance Compressor control Previous Rew Fwd Help Menu Analog vs digital controller at 2 executions per second Analog controller Digital controller (2 executions per second) 100% 100% SCL SLL Operating point 0% SCL SLL Operating point 0% Time 100% Time 100% Controller output 0% Time Controller output 0% Tuning same as analog controller • Compressor surged • Large process upset would have resulted Compressor control Previous Rew Time Fwd Help Menu Analog vs digital controller at 3 executions per second Analog controller Digital controller (3 executions per second) 100% 100% SCL SLL Operating point 0% SCL SLL Operating point 0% Time 100% Time 100% Controller output 0% Time Controller output 0% Tuning same as analog controller • Compressor surged • Large process upset would have resulted Compressor control Previous Rew Time Fwd Help Menu Analog vs digital controller at 10 executions per second Analog controller Digital controller (10 executions per second) 100% 100% SCL SLL Operating point 0% SCL SLL Operating point 0% Time 100% Time 100% Controller output 0% Time 0% Tuning same as analog controller Time Controller output • Compressor almost surged • Control system would have to be set up with bigger surge margins Compressor control Previous Rew Fwd Help Menu Analog vs CCC controller at 25 executions per second Analog controller CCC antisurge controller (25 executions per second) 100% 100% SCL SLL Operating point 0% SCL SLL Operating point 0% Time 100% Time 100% Controller output 0% Time 0% Tuning same as analog controller Time Controller output • Response of CCC controller nearly indentical to analog controller • Adding Recycle Trip® to PI control will allow even smaller surge margins Compressor control Previous Rew Fwd Help Menu Dynamic simulation single compressor VSDS Compressor Note: Speed transmitter for indicating purposes only ST 1 Load FT 1 PsT 1 TsT 1 PdT 1 Td T 1 PIC 1 HIC 1 UIC 1 Suction Process Serial network • • • Compressor is controlled on Pd by PIC-1 HIC-1 controls the process load and can be used to create process disturbances Controllers communicate via serial communication to computer running the simulation MODBUS Start simulation Compressor control Previous Rew Fwd Help Menu Dynamic simulation parallel compressors VSDS RSP Compressor 1 out UIC 1 Serial network LSIC 1 Serial network Load MPIC 1 HIC 1 Process VSDS RSP Compressor 2 out UIC 2 MODBUS Serial network Start simulation LSIC 2 Compressor control Previous Rew Fwd Help Menu