Anti Surge and Load Sharing

Developing the surge cycle on the compressor curve Pd • • •

From A to B…….20 - 50 ms…………….. Drop into surge From C to D…….20 - 120 ms…………… Jump out of surge A-B-C-D-A……….0.3 - 3 seconds……… Surge cycle

Rlosses

Pd = Compressor discharge pressure Pv = Vessel pressure Rlosses = Resistance losses over pipe

Pd B

Pv

A D C

• • • •

Pressure builds Resistance goes up Compressor “rides” the curve Pd = Pv + Rlosses

• •

Electro motor is started Machine accelerates to nominal speed Compressor reaches performance curve

•

Machine shutdown no flow, no pressure © 2002 Compressor Controls Corporation

Qs,

Note: Flow goes up faster because pressure is the integral of flow vol

Major Process Parameters during Surge FLOW

1

2

3

TIME (sec.)

PRESSURE

1

2 TIME (sec.)

3

TEMPERATURE

1

2 TIME (sec.)

3

© 2002 Compressor Controls Corporation

• • • •

Rapid flow oscillations Thrust reversals Potential damage Rapid pressure oscillations with process instability • Rising temperatures inside compressor • Operators may fail to recognize surge

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


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 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


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 • • • •

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 © 2002 Compressor Controls Corporation

Calculating the distance between the SLL and the compressor operating point The Ground Rule – The better we can measure the distance to surge, the closer we can operate to it without taking risk

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 © 2002 Compressor Controls Corporation

Coping with the high speed of approaching surge • Increase overall system speed of response wherever feasible – – – –

Transmitters Valves Controllers System Volumes

• Specialized Control Responses – – – –

Automated open loop (Recycle Trip) Control loop decoupling Adaptive surge control line Adaptive gain


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

VSDS

Rprocess Rprocess+valve

Compressor

FT 1

PsT 1

PdT 1

Discharge

Suction UIC 1

2

qr 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

B A

2

qr

• PI control will give adequate protection for small disturbances

• PI control will give stable control during steady state recycle operation • Slow disturbance example © 2002 Compressor Controls Corporation

Adaptive Gain Enhancing the Effectiveness of the PI Controller • When the operating point moves quickly towards the SCL, the rate of change (dS/dT) can be used to dynamically increase the surge control margin.

Rc

B

• This allows the PID controller to react earlier.

A

• Smaller steady state surge control margins can be used w/o sacrificing reliability. • Fast disturbance example 2

Q


Antisurge Controller Operation Protection #2 The Recycle Trip® Line (RTL) Rc

SLL = Surge Limit Line SCL-2 = Open Loop Line SCL = Surge Control Line

OP

Benefits:

– Reliably breaks the surge cycle – 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

2

Q

Output to Valve

Total Response

PI Control

Step Change

PI Control Response

Open-loop Response Time © 2002 Compressor Controls Corporation

+ To antisurge valve

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


Antisurge Control

Recycle Trip® based on derivative of Ss Recycle Trip® Response calculation:

Benefits

• Maximum protection

d(Ss) C = C1Td dt where: • C • C1 • Td • d(Ss)/dt

Output to valve

– No surge – No compressor damage

• Minimum process disturbance

= Actual step to the valve = Constant - also defines maximum step = Scaling constant = Rate of change of the operating point

– No process trips

Medium disturbance

Output to valve

Large disturbance

100%

Total PI Control Total PI Control Recycle Trip®

Recycle Trip®

0% Time © 2002 Compressor Controls Corporation

Time

Antisurge Control

What if one Recycle Trip® step response is not enough? After time delay C2 controller checks if Operating Point is back to safe side of Recycle Trip® Line - If Yes: Exponential decay of Recycle Trip® response. - If No: Another step is added to the Recycle Trip® response. Output to valve

Multiple step response Total

Output to valve

One step response PI Control

100% Recycle Trip®

Total PI Control Recycle Trip®

0%

Time C2 © 2002 Compressor Controls Corporation

C2 C2 C2

Time

Antisurge Controller Operation Protection #3 The Safety On® Response (SOL) SOL - Safety On® Line SLL - Surge Limit Line RTL - Recycle Trip® Line SCL - Surge Control Line

Rc

New SCL New RTL Additional surge margin

2

qr

• Compressor can surge due to:

– Transmitter calibration shift – Sticky antisurge valve or actuator – Partially blocked antisurge valve or recycle line – Unusually large process upset

Benefits of Safety On® response: Continuous surging is avoided Operators are alarmed about surge © 2002 Compressor Controls Corporation

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 suction throttling Pd Process

Rprocess A

P T1

PIC - SP

Suction valve open Suction valve throttled

PI C1

Notes 2

Shaft power

qr

• • •

P1

• 2

qr © 2002 Compressor Controls Corporation

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 adjustable guide vanes Pd Rprocess

Process

A

P T1

PIC - SP

amin

aOP

PI C1

amax

Notes: 2

Shaft power

qr

• • •

P1

•

2


Improved turndown More efficient than suction throttling Power consumed is proportional to the load Power loss on inlet throttling is eliminated

Performance Control by speed variation Pd

SI C1

Rprocess

Process

A

P T1

PIC - SP

Nmax

PI C1

NOP Nmin

Notes 2

Shaft power

qr

• • •

P1

• 2


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

Limiting control to keep the machine in its stable operating zone •

While controlling one 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.

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


Power limiting with the Performance Controller Rc

Power limit R1 R2 R3

A

D

B

PIC-SP

C

N4 N N2 3 N1

Benefits:

• Maximum protection

– No machinery damage

Qs,

Note: Same approach for other variables (pressures, temperatures, etc.) © 2002 Compressor Controls Corporation

vol

• Maximize production

– Machine can be pushed to the limits without risk of damage

Interacting Antisurge & Performance Loops Rc

B C

A

PIC-SP

DPo Ps


• 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 – Large pressure deviations during disturbances – Increased risk of surge

Performance & Antisurge Controller’s interaction • Both controllers manipulate the same variable - the operating point of the compressor • 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


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 © 2002 Compressor Controls Corporation

Interacting Antisurge Control Loops VSDS

Section 1

Section 2

UIC 1

UIC 2

PIC

1

Disturbance

R

Rc,1

R

Rc,2

R

2 qr,1 •

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


R

2

qr,2

Loop Decoupling between multiple Antisurge Controllers VSDS

Section 1 UIC 1

• • • • • •

Section 2 Serial network

UIC 2

Serial network

PIC

1

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

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 © 2002 Compressor Controls Corporation

Compressor networks • Compressors are often operated in parallel and sometimes in series • The purposes 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. © 2002 Compressor Controls Corporation

Compressor networks 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


Base Loading

Flow Diagram for Control Process 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

Base Loading

Parallel Compressor Control 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 QP,1 QC,1 QP,1

Notes: • •

• •

qr,2 QP,2 QC,2= QP,2 where: QP = Flow to process QC= Total compressor flow QC - QP = Recycle flow

Base loading is inefficient Base loading increases the risk of surge since compressor #1 will take the worst of any disturbance Base loading requires frequent operator intervention Base loading is NOT recommended


Equal Flow Division Loadsharing

Flow Diagram for Control Process

VSDS

RSP Compressor 1

out UIC 1

FIC 1

RSP

out 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

Equal Flow Division Loadsharing

Parallel Compressor Control

Rc,1

Compressor 1

Rc,2

Compressor 2

PIC-SP

QP,1 = QP,2

Equal flow

Equal flow

2

2

qr,1 QP,1

Notes: • • • •

where: QP = Flow to process QC= Total compressor flow QC - QP = Recycle flow

qr,2 QP,2QC,2

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 (see next) Equal flow division is NOT recommended


Dynamic Response / Pressure To Flow Cascade Rc

R2 C

R1

B D

PIC-SP

A

N3

N1 N2

FIC-SP PIC 1

OUT

Master

RSP

FIC 1

OUT

RSP SIC 1

Slave


Q2

Notes • Causes instability near surge • Poor pressure control due to positive feedback in control system

Equidistant Loadsharing

Flow Diagram for Control Process 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


Parallel Compressor Control

Rc,1

Compressor 1

Rc,2

Compressor 2 DEV = 0 0.1 0.2 0.3

SCL = Surge Control Line 0.1 0.2 0.3

PIC-SP

Dev1 = Dev2 Q1 = Q2 N1 = N2 2

2

qr,1 DEV1

qr,2 DEV2

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



for multi-section compressors RSP

VSDS

Train A Section 1

Section 2 Serial network

UIC 1A

Suction Header

UIC 2A

out LSIC Serial Serial A network network

MPIC

RSP

VSDS

1

Train B Section 1

Section 2

Process

out UIC 1B

• • • •

Serial network

UIC 1B

Serial network

LSIC

B

How to operate equidistant from the Surge Control Line (SCL) when there is more than one section per machine ??? Select per train -- in the loadsharing controller -- the section closest to the SCL By selecting the section closest to the SCL it is guaranteed that the other section on the same train is not in recycle Share the load -- equal DEV’s for both trains -- on the section closest to the SCL © 2002 Compressor Controls Corporation

Simplified P&ID for Compressors Operating in Parallel


Anti Surge and Load Sharing

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