Anti Surge and Load Sharing

© 2002 Compressor Controls Corporation

• 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

Qs, vol

Pd

Machine shutdownno flow, no pressure

• Electro motor is started• Machine accelerates

to nominal speed• Compressor reaches

performance curveNote: Flow goes up faster because pressure is the integral of flow

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

Pd = Compressor discharge pressurePv = Vessel pressureRlosses = Resistance losses over pipe

Developing the surge cycle on the compressor curve

Pd

Pv

Rlosses

B A

C

D


• Rapid flow oscillations • Thrust reversals• Potential damage• Rapid pressure

oscillations with process instability

• Rising temperatures inside compressor

• Operators may fail to recognize surge

FLOW

TIME (sec.)

1 2 3

Major Process Parameters during Surge

PRESSURE

TIME (sec.)1 2 3

TEMPERATURE

TIME (sec.)1 2 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• 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


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


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


Surge parameter based on invariant coordinates Rc and qr

– Flow measured in suction (Po)– Ps and Pd transmitters used to calculate Rc

1UIC

VSDS

Compressor

1FT 1

PsT1

PdT

• The antisurge controller UIC-1 protects the compressor against surge by opening the recycle valve

DischargeSuction

• Opening of the recycle valve lowers the resistance felt by the compressor

• This takes the compressor away from surge

Basic Antisurge Control System

Rc

qr2

Rprocess

Rprocess+valve


A

Rc

B • When the operating point crosses the SCL, PI control will open the recycle valve

• PI control will give adequate protection for small disturbances

SLL = Surge Limit Line

SCL = Surge Control Line

qr2

Antisurge Controller Operation Protection #1 The Surge Control Line (SCL)

• PI control will give stable control during steady state recycle operation• Slow disturbance example


Adaptive Gain Enhancing the Effectiveness of the PI Controller

A

Rc

B

• 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.

• This allows the PID controller to react earlier.

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

• Fast disturbance exampleQ2


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

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

SLL = Surge Limit LineSCL-2 = Open Loop Line

SCL = Surge Control Line

Output to Valve

Time

Open-loop Response

PI Control Response

PI Control Step Change

+

To antisurge valve

Total Response

Rc

Q2

OP


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


d(Ss)dt

C = C1Td

Output to valve

Time

where:• C = Actual step to the valve• C1 = Constant - also defines

maximum step• Td = Scaling constant• d(Ss)/dt = Rate of change of the

operating point

Medium disturbance

PI ControlRecycle Trip®

Total

Large disturbanceOutput to valve

Time

PI Control

Recycle Trip®

Total

Benefits• Maximum protection

– No surge– No compressor damage

• Minimum process disturbance– No process trips

Recycle Trip®

Response calculation:

100%

0%

Recycle Trip® based on derivative of Ss

Antisurge Control


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.

Output to valve

Time

One step response

PI Control

Recycle Trip®

Total

100%

0%

C2

Multiple step responseOutput to valve

Time

PI Control

Recycle Trip®

Total

C2 C2 C2

What if one Recycle Trip® step response is not enough?

- If No: Another step is added to the Recycle Trip® response.


Additional surge margin

Benefits of Safety On® response:Continuous surging is avoidedOperators are alarmed about surge

• Compressor can surge due to:– Transmitter calibration

shift– Sticky antisurge valve

or actuator– Partially blocked

antisurge valve or recycle line

– Unusually large process upset

Antisurge Controller Operation Protection #3The Safety On® Response (SOL)

Rc

qr2

SLL - Surge Limit LineRTL - Recycle Trip® LineSCL - Surge Control Line

New SCL

New RTL

SOL - Safety On® Line


• Also called:– Throughput control– Capacity control– Process control

• Matches the compressor throughput to the load

• Can be based on controlling:– Discharge pressure– Suction pressure– Net flow to the user

Compressor Performance Control


Pd

qr2

Shaft power

qr2

Suction valve open

Suction valve throttled

PIC - SPA

Rprocess

P1

PT1

PIC1

Process

Notes• Common on electric motor

machines• Much more efficient than

discharge throttling• Power consumed changes

proportional to the load• Throttle losses are across

suction valve

Performance Control by suction throttling


Pd

qr2

Shaft power

qr2

min

OP

max

PIC - SPA

Rprocess

P1

PT1

PIC1

Process

Notes:• Improved turndown• More efficient than suction

throttling • Power consumed is

proportional to the load• Power loss on inlet throttling

is eliminated

Performance Control by adjustable guide vanes


Pd

qr2

Shaft power

qr2

Nmin

NOP

Nmax

PIC - SPA

Rprocess

P1

PT1

PIC1

Process

SIC1

Notes• Most efficient: (Power f(N)3) • Steam turbine, gas turbine or

variable speed electric motor• Typically capital investment

higher than with other systems

• No throttle losses

Performance Control by speed variation


• 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.

Limiting control to keep the machine in its stable operating zone

CONTROL BUT DO NOT EXCEED

Discharge Pressure Max. Motor Current

Suction Pressure Max. Discharge Pressure

Net Flow Min. Suction Pressure

Suction Pressure Max. Discharge Temperature


PIC-SP

Power limit

N1

A

R1

Qs, vol

Rc

B

R2

N2

R3

N4

D

N3

C

Benefits:• Maximum protection

– No machinery damage• Maximize production

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

Note: Same approach for othervariables (pressures,temperatures, etc.)

Power limiting with the Performance Controller


• 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

AC

Po

PIC-SP

Rc

Ps

SLL

SC

L

B

Interacting Antisurge & Performance Loops


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 andPerformance 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


Disturbance

• The system is oscillating• Slowing down the

controller tuning would lead to:- Increased risk of surge

• Compressor damage• Process trips

- Bigger surge margins• Energy waste

Interacting Antisurge Control Loops

Rc,2

qr,22

RRc,1

qr,12

R

RR

1

PIC

2UIC

1UIC

VSDS

Section 1 Section 2


• All CCC controllers are connected on a serial network

1

PIC

2UIC

1UIC

VSDS

Section 1 Section 2

Serial network

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

Loop Decoupling between multiple Antisurge Controllers


• 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.

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

Compressor networks


Process

PIC1

1UIC

VSDS

Compressor 1

2UIC

VSDS

Compressor 2

HIC1

Suction header

Swing machine

Base machine

Notes• All controllers act

independently• Transmitters are

not shown

Base Loading Flow Diagram for Control Process


Rc,1

qr,12

Rc,2

qr,22

Compressor 1 Compressor 2

PIC-SP

Swing machine Base machine

QC,2= QP,2QC,1QP,1

where:QP = Flow to processQC= Total compressor flowQC - QP = Recycle flow

QP,1

QP,1 + QP,2 = QP,1 + QP,2

Notes:• 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

Base Loading Parallel Compressor Control

QP,2


Process

PIC1

1UIC

Compressor 1

VSDS

Compressor 2

Suction header

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

1FIC

2FIC

2UIC

outout

out

RSPRSP

RSPRSP

RSPRSP

outout

RSPRSP

Equal Flow Division Loadsharing Flow Diagram for Control Process

VSDS


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

(see next)• Equal flow division is NOT recommended

Rc,1

qr,12

Rc,2

qr,22

PIC-SP

QP,1 QP,2QC,2

Equal flow Equal flowQP,1 = QP,2

Equal Flow Division Loadsharing Parallel Compressor Control


where:QP = Flow to processQC= Total compressor flowQC - QP = Recycle flow


Q2

Rc

N1

N3

N2

A

R1

PIC-SP

FIC-SP

R2

B

C

D

Notes• Causes instability near surge• Poor pressure control due to

positive feedback in control system

PIC1

OUTRSP

FIC1

OUTRSP

Master Slave

SIC

1

Dynamic Response / Pressure To Flow Cascade


Notes• All controllers are

coordinating control responses via a serialnetwork

• Minimizes recycle under all operating conditions

Process

1UIC

VSDS

Compressor 1

VSDS

Compressor 2

Suction header

1

LSIC

2UIC

out

RSP

Serial network

out

RSP

2

LSIC

1

MPIC

Serial network

Serial network

Equidistant LoadsharingFlow Diagram for Control Process


PIC-SP

0.10.2

0.3

DEV = 00.1

0.2

0.3

DEV1 DEV2

SCL = Surge Control LineRc,1

qr,12

Rc,2

qr,22


Dev1 = Dev2

Q1 = Q2

N1 = N2

Notes:• Maximum turndown (energy savings) without recycle or blow-off• Minimizes the risk of surge since all machines absorb part of

the disturbance• Automatically adapts to different size machines• CCC patented algorithm

Equidistant LoadsharingParallel Compressor Control


Process

1AUIC

VSDS

Section 1

VSDS

Section 1

Suction Header

A

LSIC

outout

RSP

Serial network

RSP

B

LSIC

1

MPIC

Serial network

Serial network

Section 2

Section 2

2AUIC

1BUIC

1BUICSerial

network

Serial network

outout

Train B

Train A

• 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

Equidistant Loadsharing for multi-section compressors

• Share the load -- equal DEV’s for both trains -- on the section closest to the SCL


Simplified P&ID for Compressors Operating in Parallel

Anti Surge and Load Sharing

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