CCC Compressor Controls Corporation A Presentation by K

CCC Compressor Controls Corporation A Presentation by K. VIVEKANAND & P. . STEEVENSON

Developing the compressor curve Rc process limit adding control margins maximum speed surge limit power limit stonewall or choke limit Actual available operating zone minimum speed stable zone of operation Qs, vol

Developing the surge cycle on the compressor curve • From A to B 20 - 50 ms • From C to D 20 - 120 ms • A-B-C-D-A 0. 3 - 3 seconds Surge cycle • • B • • Drop into surge Jump out of surge Pd Compressor reaches surge point A Compressor looses its ability to make pressure Suddenly Pd drops and thus Pv > Pd Plane goes to stall - Compressor surges Pd A • • Because Pv > Pd the flow reverses Compressor operating point goes to point B C Rlosses Compressor starts to build pressure Compressor “rides” curve towards surge Point A is reached The surge cycle is complete D • • • Result of flow reversal is that pressure goes down Pressure goes down => less negative flow Operating point goes to point C Machine shutdown no flow, no pressure Qs, Pd = Compressor discharge pressure Pv = Vessel pressure Rlosses = Resistance losses over pipe 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 • • Pv • • • 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 vol • • Pressure builds Resistance goes up Compressor “rides” the curve Pd = Pv + Rlosses

Major process parameters during surge FLOW 1 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

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

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

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

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

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: f 1(hr ) Ss = 2 qr , op • The function f 1 returns the value of qr 2 on the SLL for input hr hr hr 2 qr, SLL 2 qr

The surge parameter Ss 2 • The function f 1 returns the value of qr on the SLL for input hr • As a result: Ss = q 2 r, SLL q 2 r, op hr Ss = 1 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 OP = Operating Point 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 hr d <0 d =0 Ss = 1 Benefits: DEV = 0 Ss > 1 DEV < 0 • One standard surge parameter in the plant • No operator confusion: Ss < 1 • DEV > 0 Good d >0 DEV > 0 • DEV = 0 Recycle line • DEV < 0 Bad 2 Surge margin qr

The approach to surge is fast Pd 1 SEC. 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 Qs 0 Pd D

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.

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 Rc Rprocess+valve VSDS Compressor FT 1 Ps. T 1 P d. T 1 2 qr 2 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

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

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

Antisurge controller operation Protection #2: The Recycle Trip® Line (RTL) SLL = Surge Limit Line Rc • Disturbance arrives the Operating Point (OP) moves towards the SCL RTL = Recycle Trip® Line SCL = Surge Control Line • Benefits: – – Energy savings due to smaller surge margin Compressor has more turndown before recycle or blow-off Surge can be prevented for virtually any disturbance – • Operating point Moves back to the safe side of the RTL – – • When OP hits SCL the PI controller opens valve based on proportional and integral action 2 Output to Valve qr • • Total Response PI Control Response • Total response of controller is the sum of the PI control and Recycle Trip action Operating point keeps moving towards surge and hits Recycle Trip Line (RTL) Recycle Trip® Response Time The RT function decays out the step response PI controller integrates to stabilize OP on SCL When the operating point hits the Recycle Trip Line (RTL) the conclusion is: – – – • We are close to surge The PI controller is too slow to catch the disturbance Get out of the dangerous zone An open loop response is triggered

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 (C 1) 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

Recycle Trip® based on derivative of Ss Benefits Recycle Trip® Response calculation C = C 1 T d where: • C 1 • Td • d(Ss)/dt Output to valve • 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% 0% Total PI Control Recycle Trip® Time

What if one Recycle Trip® step response is not enough? • After time delay C 2 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 One step response Multiple step response 100% Total PI Control Recycle Trip® 0% Time C 2 Recycle Trip® C 2 C 2 Time

Antisurge controller operation Protection #3: The Safety On® Line (SOL) SOL = Safety On® Line SLL = Surge Limit Line RTL = Recycle Trip® Line Rc SCL = Surge Control Line • Additional safety or surge margin is added • If Operating Point crosses the Safety On® Line the compressor is in surge • PI control and Recycle Trip® will stabilize the machine on the new SCL 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 • The Safety On® response shifts the SCL and the RTL to the right

Built in surge detector Pressure and Flow Variations During a Typical Surge Cycle 100% Pd 0% 1 TO 2 SECONDS 100% DPo 0% 20 to 50 milli-seconds • 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 • 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

Fall-back strategies for the antisurge and performance controller • • • Antisurge controller – If a pressure transmitter fails, a minimum q 2 r 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 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

Performance control by speed variation SIC 1 Pd Rprocess Process A PIC - SP PT 1 Nmax Nmin Shaft power NOP PIC 1 • 2 qr P 1 2 qr Pressure is controlled by speed of rotation • Changing speed generates a family of curves • Compressor operates in point A for given Rprocess Required power is P 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

Power limiting in the performance controller an example • Rc Power limit • • R 1 • • R 2 R 3 A D B PIC-SP C • N 4 N N 2 3 N 1 • PIC would like to speed machine up to N 4 and operate in point D Primary variable Pd Limiting variable Power Process resistance changes from R 1 to R 2 Compressor operates in point A for R 1 at N 1 • PIC will speed machine up to N 2 in order to control pressure Pd • • Machine hits power limit Compressor operates in point B for R 2 at N 2 • Process resistance decreases further to R 3 However power limiting loop takes control and controls machine at speed N 3 Compressor will operate in point C for R 3 at N 3 Benefits Qs, • Maximum protection – vol • No machinery damage Maximize production – Machine can be pushed to the limits without risk of damage Note: Same approach for other variables (pressures, temperatures, etc. )

Limiting Ps or Pd using the antisurge controller VSDS Compressor FT 1 Suction • • Ps. T 1 UIC 1 P d. T 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

Operator Interface Specs. Alphanumeric Displays DEV – Displays the normalized distance between the operating point and surge limit line. The controller will increase its output when DEV is negative and decreases it when DEV is positive. – Normally Blank. Displays Safety ON surge count when DISPLAY SURGE COUNT key is pressed. – Displays intended valve position – Default display is operating state (RUN, STOP or PURGE). A variety of measured and calculated process variable displays can be selected by pressing MENU and SCROLL. ALT OUT AUX

Operator Interface Specs. Operator Keys AUTO MAN – Toggles controller between its Automatic and Manual operating modes. Auto or Manual LED is lit to indicate currently selected state. – Increments and decrements the valve position (when in manual) Reset Safety On – Pressing this key zeroes the surge count and cancels any increased margin of safety added by the safety on response. – If the surge count is not zero, this key should not be pressed unless the cause of the surges has been corrected. Display Surge Count – Causes ALT window to display the number of surges detected by Safety ON response. Display Limit – Temporarily displays the value and set point controller’s limiting variables in DEV and ALT windows. – If both limiting loops are enabled, their displays will alternate each time this key is pressed.

Operator Interface Specs. Status LEDs AUTO (Green) – Lit when controller is operating in Automatic mode. Manual (Yellow) – Lit when controller is operating in Manual Mode. Pressing the Up and Down arrow keys will then open or close the Antisurge valve. RT (Yellow) – Lit whenever the Recycle trip response is greater than zero. – This response rapidly increases the recycle flow rate to protect against rapid or close approaches to the surge limit. Once the danger is passed, this added flow is allowed to decay. – This response allows the compressor to safely operate close to its surge limit , a Recycle trip should not be taken as a cause for alarm or operator intervention. SO (Red) – Lit whenever the safety on surge count is greater than zero.

Operator Interface Specs. Status LEDs Limit(Yellow) – Lit whenever one or both limiting variables are at or beyond their set points. Tracking(Green) – Flashes when the low output clamp is tracking a designated analog input(i. e. . , when a remote clamp has been assigned and that signal exceeds the internal output clamp. Tran. Fail (Red) – Lit when one or more transmitters are outside the alarm limits. Fallback (Yellow) – Lit when a fallback control strategy is being used. This occurs when a required analog input fails. Com. Err(Red) – Lit when controller is not receiving serially communicated information that is has been configured to use. Fault (Red) – Lit when controller fails to refresh watch-dog timer or detects an internal error which might render its operation unreliable. – 6

How an airplane wing develops lift v 1, p 1 v 2, p 2 Bernoulli’s law • pstatic + 1/2 rv 2 + rg. H = Constant • The term rg. H is negligible for the wing • Then: pstatic + 1/2 rv 2 = Constant • Due to the shape of the wing: v 2 < v 1 thus p 2 > p 1 • As a result there is Dp or lift • And the plane can fly Lift Compressors Previous Rew Fwd Help Menu

How the airplane develops stall 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

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