compressor.ppt

Compressor control menuCompressor control menu

Main MenuHelpFwdPrevious Rew

Cost of operating turbo compressors

Compressor operation

Maintenance cost

Operating cost

Commissioning cost

Putting it in perspective

Centrifugal compressors

Axial compressors

Compressor system classifications

Developing the compressor curve

The surge phenomena

Compressor control

Acrobat

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Repairs are expensiveRepairs are expensive

$50,000

$25,000

$750,000

Costs of repairs - materials and laborCosts of repairs - materials and labor

3,000 hpProcess gas compressor

20,000 hpAxial air blower

Seals

Bearings

Rotor Assembly

$20,000

$10,000

$200,000

FwdPrevious RewCompressors Help Menu

Operating costs are largeOperating 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 FwdPrevious RewEnergy Savings Predictions Compressors Help Menu

Energy savings examplesresulting from reduced recycle or blow-off

Energy savings examplesresulting from reduced recycle or blow-off

Actual Energy Savings Result From Actual Energy Savings Result From Improved Antisurge Protection and Capacity ControlImproved Antisurge Protection and Capacity Control$1,200,000

$155,000

$78,000

Compressor shaft power

Actual achieved savings

Propylene refrigeration

FCCU air blower

Centac air compressor

40,000 hp ( 29 MW)

15,000 hp (11.2 MW)

1,500 hp (1.1 MW)

Compressor application

FwdPrevious RewEnergy Savings Predictions Compressors MenuHelp

Available energy savings can be predictedAvailable energy savings can be predicted

Less than one year pay-backs* typical by reducing recycle of blow-Off

Pay-backPeriod

(Months)

Reduced Recycle (Per Cent of Maximum Compressor Flow)

*Assumes electro motor power At $0.05 US per kilowatt hour or turbine power at $327per horsepower per year. Tax consequences are not considered in pay-back period

due to varying tax policies around the world.

12

11

10

9

8

7

6

5

4

3

2

1

0 5 10 15 20 25 30 35

Pay-back approximately 1Month with 15% Reduction

1000 HP

3,500 HP

20,000 HP

Energy Saving Examples FwdPrevious Rew

Pay-back less than 10Months with 15% Reduction

Pay-back less than 6Months with 15% Reduction

Compressors MenuHelp

Downtime costs can be enormous!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.

FwdPrevious RewCompressors MenuHelp

• 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.

Commissioning costs are largeCommissioning costs are large

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$2,300,000

100 MMSCFDNatural Gas Plant

60,000 BPDCat Cracker

Start-up Cost Per Day $375,000

Process


Putting it in perspective30-year life cycle costs for a 20,000 hp compressor

Putting it in perspective30-year life cycle costs for a 20,000 hp compressor

Energy Costs$180 Million

Initial Cost $1.5 Million

Maintenance Costs$4.5 Million

97% of total costs

Source: Experiences in Analysis and Monitoring Compressor PerformanceBen Duggan & Steve LockeE.I. du Pont, Old Hickory, Tennessee24th Turbomachinery Symposium

Costs in constant dollars


Putting it in perspective 30-year cost per 1,000 hpPutting it in perspective 30-year cost per 1,000 hp

?

What can we control?

0.0

5.0

10.0

15.0

Initial Cost Maintenance Energy LostProduction

$ Millions

ControllableControllable

Uncontrollable Uncontrollable

Source: Experiences in Analysis and Monitoring Compressor PerformanceBen Duggan & Steve Locke, E.I. du Pont, Old Hickory, Tennessee24th Turbomachinery Symposium

Costs in constant dollars


Centrifugal compressorsCentrifugal 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

Single Case Compressor Centrifugal Impeller

DIFFUSERS

IMPELLERS

Cross Section of Horizontal Split

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Picture of Horizontal Split

Cross Section of Barrel Type

Picture of Barrel Type

Cross Section of Integrally Geared

Picture of Gear and Impellers

Picture of Integrally Geared


Cross section of horizontal splitCross section of horizontal split

Picture of Horizontal Split FwdPrevious RewCentrifugals

Compressor inlet nozzle

Thrust bearing

Journal bearing

Shaft and labyrinth seal

Impeller inlet labyrinth sealsDischarge volutes

Impellers

Drive coupling

Casing (horizontally split flange)

Compressor discharge nozzle

MenuHelp

Picture of horizontal splitPicture of horizontal split

FwdPrevious RewCentrifugalsCross Section of Horizontal Split MenuHelp

Cross section of barrel type compressorCross section of barrel type compressor

FwdPrevious RewCentrifugalsPicture of Barrel Type MenuHelp

Picture of barrel type compressorPicture of barrel type compressor

FwdPrevious RewCentrifugalsCross Section of Barrel Type MenuHelp

Cross section of bull gear compressorCross section of bull gear compressor

FwdPrevious RewCentrifugalsPicture of Integrally GearedPicture of Gear and Impellers

Compressor volutes

Gear casing

Pinion shafts

Journal bearing

Impellers

Drive coupling

Labyrinth seals

Main gear

Inlet guide vanes

MenuHelp

Picture of bull gear compressorPicture of bull gear compressor

FwdPrevious RewCentrifugalsCross Section of Integrally GearedPicture of Gear and Impellers MenuHelp

Picture of (bull) gear and impellersPicture of (bull) gear and impellers

FwdPrevious RewCentrifugalsCross Section of Integrally GearedPicture of Integrally Geared MenuHelp

• 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

Axial compressorsAxial compressors

Stator Blades

RotorBlades

Casing

Rotor Blades

StatorBlades

Casing

Shaft

FwdPrevious RewCross Section of Axial Picture of Axial Compressors MenuHelp

Cross section of axial compressorCross section of axial compressor

FwdPrevious RewAxialsPicture of Axial

Compressor outlet nozzle

Rotor blades

Labyrinth sealsGuide-vane actuator linkage

Compressor rotor

Compressor inlet nozzle

Thrust bearing

Adjustable guide vanes

MenuHelp

Picture of axial compressorPicture of axial compressor

FwdPrevious RewAxialsCross Section of Axial MenuHelp

Compressor system classificationsCompressor system classifications

Single-Section, Three-Stage Single-Case, Two-Section, Six-Stage

Two-Case, Two-Section, Six-Stage

Series Network

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Parallel Network


Developing the compressor curveDeveloping the compressor curve

Pd Discharge Pressure (P2)Pc Differential Pressure (Pd - Ps) or (P2 - P1) Rc Pressure Ratio (Pd/Ps) or (P2/P1) Hp Polytropic HeadRc

Qs, normalQs, massQs, vol

Compressor curve for a specific

speed N1

Rprocess,1

Q1

Rc1

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Rprocess,2

Q2

Rc2


Qs, vol

Rc

minimum speed

maximum speedsurge limit

stonewall orchoke limit

power limit

process limit

Developing the compressor curveDeveloping the compressor curve

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stable zonestable zoneof operationof operation

adding control margins

Actual availableoperating zone


How an airplane wing develops liftHow an airplane wing develops lift

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Bernoulli’s law

• pstatic + 1/2v2 + gH = Constant

• The term gH is negligible for the wing

• Then: pstatic + 1/2v2 = Constant

• Due to the shape of the wing:

v2 < v1 thus p2 > p1

v1, p1

v2, p2

• As a result there is p or lift

LiftLift

• And the plane can fly


How the airplane develops stallHow the airplane develops stall

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Lift

• As the wing tilts back the v changes and thus the p

• This leads to more lift

LiftLift

• When the wing is tilted too much the streaming profile

suddenly changes from laminar to turbulent

Lift

• The air no longer “sticks” to the wing and the lift is

lost• The plane starts to fall down


Developing the surge cycle on the compressor curveDeveloping the surge cycle on the compressor curve

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Qs, vol

Pd

Machine shutdownno flow, no pressure

• Electro motor is started• Machine accelerates to nominal

speed• Compressor reaches performance

curve• Note: Flow goes up faster because

pressure is the integral of flow

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

A

• 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

B

• Because Pv > Pd the flow reverses• Compressor operating point goes to point B

C

• Result of flow reversal is that pressure goes down

• Pressure goes down => less negative flow• Operating point goes to point C

D• System pressure is going down• Compressor is again able to overcome Pv

• Compressor “jumps” back to performance curve and goes to point D

• Forward flow is re-established

Pd

Pv

Rlosses

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

• Compressor starts to build pressure• Compressor “rides” curve towards surge• Point A is reached• The surge cycle is complete

• 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


• Rapid flow oscillations

• Thrust reversals

• Potential damage

• Rapid pressure oscillations with process instability

• Rising temperatures inside compressor

Major process parameters during surgeMajor process parameters during surge

FLOW

PRESSURE

TIME (sec.)

1 2 3

TEMPERATURE

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TIME (sec.)

1 2 3

TIME (sec.)

1 2 3


Surge descriptionSurge 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 consequencesSome 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 surgeFactors 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


Compressor controlCompressor control

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Objectives

Antisurge control

Performance control

Other topics

Loadsharing for parallel compressorsMajor challenges of compressor control

Location of the surge limit

High speed of approaching surge

Control loop interactions

Loadsharing of multiple compressors

Protection #1: PI control

Protection #2: Recycle Trip

Protection #3: Safety On

Output linearization

The tight shut-off line

Fall-back strategies

Compressor networks

Base loading parallel compressors

Equal flow division system

CCC’s equidistant Loadsharing system

Limiting control

Pressure Override Control (POC)

Flow Measuring Devices (FMD’s)

Antisurge control valve

Piping lay-out considerations

Dynamic simulation single compressor

Dynamic simulation parallel compressors

Basic antisurge control system

Influence of controller execution time

MenuHelp

Major control system objectives(user benefits)

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

• 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

Energy Saving Examples

FwdPrevious RewCompressor control MenuHelp

Calculating the distance between the SurgeLimit Line and the compressor operating point

Calculating the distance between the SurgeLimit 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


• Typical compressor maps include: (Qs, Hp), (Qs, Rc), or (Qs, pd) coordinates, where:

Commonly used (OEM provided) coordinatesystems of the compressor map

Commonly used (OEM provided) coordinatesystems of the compressor map

Qs = Suction flow and can be expressed as actual or standard volumetric flowHp = Polytropic HeadRc = Compressor Ratio (pd / ps)pd = Discharge pressure of the compressorps = Suction pressure of the compressorks = Exponent for isentropic compression

• These maps are defined for (1) specific set of inlet conditions:

ps, Ts, MW and ks


The problem with commonly used (OEM provided)coordinate systems of the compressor map

The problem with commonly used (OEM provided)coordinate systems of the compressor map

• These coordinates are NOT invariant to suction conditions as shown

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• For control purposes we want the SLL to be presented by a single curve for a fixed geometry compressor

Compressor control MenuHelp

Developing invariant coordinatesDeveloping invariant coordinates• The following variables are used to design and to characterize

compressors

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Fundamental variables characterizing compressor

operation

Hp = f0(Q, , , , a, d, )

J = f1(Q, , , , a, d, )

where:

• Hp = Polytropic head

• J = Power• Q = Volumetric flow rate = Rotational speed = Viscosity = Density• a = Local acoustic velocity

• d = Characteristic length

= Inlet guide vane angle

• Through dimensional analysis (or similitude) we can derive two sets of invariant coordinates

Dimensional analysisor Similitude

Set 1hr

qr

Ne

jr

Re

Invariant coordinates

Set 2Rc

qr

Ne

jr

Re

where:

• hr = Reduced head

• qr = Reduced flow

• Ne = Equivalent speed = Guide vane angle

• jr = Reduced power

• Re = Reynolds number

• Rc = Pressure Ratio


Coordinates (Hp, Qs) and (hr, qr2)Coordinates (Hp, Qs) and (hr, qr2)

(Hp, Qs) NOT invariant coordinates

(hr, qr2)


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where:

• Hp = Polytropic head

• Qs = Volumetric suction flow

• hr = Reduced head

• qr2 = Reduced flow squared


Coordinates (Rc, Qs) and (Rc, qr2)Coordinates (Rc, Qs) and (Rc, qr2)

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(Rc, Qs) NOT invariant coordinates

where:

• Rc = Pressure ratio

• Qs = Volumetric suction flow

• qr2 = Reduced flow squared

Compressor control Menu

(Rc, qr2)


qr2

Help

Coordinates (Rc, jr) and (Rc, Ne2)Coordinates (Rc, jr) and (Rc, Ne2)

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(Rc, jr)


(Rc, Ne2)


where:

• Rc = Pressure ratio

• jr = Reduced power

• Ne2 = Equivalent speed squared


Representing the SLL as a single curveusing reduced coordinates

Representing the SLL as a single curveusing reduced coordinates

• A coordinate system that is invariant to suction conditions is:

hH

(ZRT)rp

s

and qQ

ZRTrs

s

( )

• Squaring the flow will still keep coordinates invariant:

hH

(ZRT)rp

s

and qQ

ZRTrs

s

22

( )

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qr2

hr

increasing MW, N

decreasing T s

Design Nitrogen Off-gas

MW MW MW

Ps Ps Ps

Ts Ts Ts

ks ks ks


Calculating qr2 (reduced flow squared)Calculating qr2 (reduced flow squared)

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qr2 =

Qs2

(ZRT)s

=

K . Zs . Ru . Ts

MW

po,s. ps

• where:

• R = Ru / MW

• Ru = Universal gas constant

• R = Specific gas constant• MW = Molecular Weight of the gas

• ps = Suction pressure

• K = Orifice plate constant po,s = Differential pressure across orifice plate

• Ts = Temperature of the gas in suction

• Zs = Compressibility of gas in suction of compressor

(ZRT)s

= po,s

ps

• The antisurge controller calculates qr2 using ps and po,s transmitters


Calculating hr (reduced head)Calculating hr (reduced head)

• For polytropic compression Rt = Rc thus =

• 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 = Exponent for polytropic compression

log(Rt)

log(Rc)

• The antisurge controller calculates hr using pd, ps, Td and Ts transmitters

(ZRT)s

hr =

Zs . Ru . Ts

MW

Rc-1

Hp

=

.

(ZRT)s

= Rc

-1


Building the Surge Limit LineBuilding the Surge Limit Line

• Any curvature of the Surge Limit Line can be characterized as a function of the ordinate hr

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qr2

hr

• The surge parameter is defined as: Sfqs

r

r op

12

(h )

,

• The function f1 returns the value of qr2 on the SLL for input hr

hr

qr,SLL2


The surge parameter SsThe surge parameter Ss

• The function f1 returns the value of qr on the SLL for input hr

2

qr2

hr

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hr

qr,SLL2

• As a result: Ss = qr,op2

qr,SLL2

qr,op2

OPOP

OP = Operating Point

• Ss < 1 : stable operating zoneSSs s < 1< 1

• Ss = 1 : surge limit line (SLL)

S s = 1

• Ss > 1 : surge region

SSss > 1 > 1


Introducing the distance between the operating point and the Surge Control Line

Introducing the distance between the operating point and the Surge Control Line

• Step 1: Introduce parameter d = 1 - Ss

qr2

hr

Ss < 1

Ss > 1

Ss = 1

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d = 0

d > 0

d < 0

• Step 2: Introduce parameter DEV = d - surge margin

DEV = 0

Surge margin

DEV > 0

DEV < 0

• The parameter DEV is independent of the size of the compressor and will be the same for each compressor in the plant

Benefits:

• One standard surge parameter in the plant

• No operator confusion:

• DEV > 0 Good

• DEV = 0 Recycle line

• DEV < 0 Bad


Simplifying the surge parameterby replacing hr with Rc

Simplifying the surge parameterby replacing hr with Rc

• Reduced Head hr can be replaced by Rc while keeping the coordinate system invariant to suction conditions

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• The surge parameter Ss now becomes Ss =f1(Rc)

qr,op2

where the function f1( ) returns the value of qr,SLL

on the SLL for the input Rc

2

• 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


The simplest CCC surge parameterThe simplest CCC surge parameter

• An antisurge algorithm can be designed around two transmitters: po and pc

• The parameter

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Ss =f1(Rc)

qr2

is invariant to inlet conditions and speed

• For two transmitters choose the function f1 to be (Rc - 1)

Ss =f1(Rc)

qr2

=Rc - 1

po

ps

pd

ps

- 1( ) . ps

po

= =pd - ps

po

= po

pc

• Selecting the specific function for f1(Rc) to be (Rc - 1) keeps the map invariant to inlet conditions and speed


Disadvantage of the pc /po surge parameterDisadvantage of the pc /po surge parameter

• The SLL is rarely a straight line in the coordinates qr2 and Rc

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qr2

Rc

Actual Surge Limit Line (SLL)

• The parameter pc /po represents a straight line in the invariant

coordinates qr2 and Rc

SLL calculated by antisurge controller using

pc /po = constant

• The pc /po approach results in loss of turn down and

unnecessary recycle

loss of operating envelope


Actual field data showing disadvantage of pc /po surge parameter

Actual field data showing disadvantage of pc /po surge parameter


Surge parameter for compressor with sidestreamProblem definition

Surge parameter for compressor with sidestreamProblem definition

po,1

P1

T1

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po,2

P2

T2

1 2 3

q3 and T3 are internal to the compressor and cannot be measured


Derive a new surge parameter for compressor with sidestream: qrNe

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 qrm

pZRT

.

.where:• m = mass flow• Z = Compressibility• R = Gas constant• Ne = Equivalent speed

• qr = Reduced flow

• N = Rotational speed• p = Pressure• T = Temperature

Step 2: Equivalent speed NeN

ZRT

Step 3: Combine qr and Ne

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qrNem

pZRT N

ZRT.

.m N

=p

.


Calculating the invariant parameter qrNeCalculating the invariant parameter qrNe

q3

po,1

p1

T1

po,2

p2

T2

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m1

.m2

.

(m1 + ) . N.

m2

.

p2

=m3

. N.

p3

=N

A ppT

A ppT

N

pe,3

1 o,11

12 o,2

2

2

2

1 2 3


rr

Developing invariant surge patameterRc vs. qrNe

Developing invariant surge patameterRc vs. qrNe


The approach to surge is fastThe approach to surge is fast

• 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, PO dropped by 14%, with a 2% change in Pc

Pd

Qs

AD

100%

0

100%

0Pc

100%

Pd

0

1 SEC.

Po

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A C

D

B

A CB


The approach to surge is fast - another exampleThe approach to surge is fast - another example

For a 2% increase in pressure differential (Pc), flow rate Po dropped 9% in 0.3 sec.

100%

0

0

Po

Pc

100%

1 sec.


• Surge parameter based on invariant coordinates Rc and qr

– Flow measured in suction (Po)

– Ps and Pd transmitters used to calculate Rc

Basic antisurge control systemBasic antisurge control system

FwdPrevious RewCompressor control

1UIC

VSDS

Compressor

1FT 1

PsT1

PdT

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

DischargeSuction

Rc

qqrr22

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

Rprocess

Rprocess+valve

• This takes the compressor away from surge

MenuHelp

Antisurge controller operationProtection #1: The Surge Control Line (SCL)

Antisurge controller operationProtection #1: The Surge Control Line (SCL)

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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

• PI control will give stable control during steady state recycle operation

• Slow disturbance example

SLL = Surge Limit Line

SCL = Surge Control Line

qr2


Adaptive GainEnhancing the effectiveness of the PI controller

Adaptive GainEnhancing the effectiveness of the PI controller

A

Rc

B

• 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

FwdPrevious Rew

qr2


Antisurge controller operationProtection #2: The Recycle Trip® Line (RTL)

Antisurge controller operationProtection #2: The Recycle Trip® Line (RTL)

• Disturbance arrives the Operating Point (OP) moves towards the SCL

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Rc

qr2

SLL = Surge Limit Line

RTL = Recycle Trip® Line


Output to Valve

Time

• When OP hits SCL the PI controller opens valve based on proportional and integral action

• Operating point keeps moving towards surge and hits Recycle Trip Line (RTL)

• 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

• Operating point Moves back to the safe side of the RTL

– The RT function decays out the step response

– PI controller integrates to stabilize OP on SCL

Recycle Trip® Response

PI Control Response

• Total response of controller is the sum of the PI control and Recycle Trip action

PI Control Recycle Trip®

Action

+

To antisurge valve

Total Response

• 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


Improving the accuracy of Recycle Trip® open loop control

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


Recycle Trip® based on derivative of SsRecycle Trip® based on derivative of Ss

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

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Recycle Trip®

Response calculation

100%

0%


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

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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


Antisurge controller operationProtection #3: The Safety On® Line (SOL)

Antisurge controller operationProtection #3: The Safety On® Line (SOL)

• If Operating Point crosses the Safety

On® Line the compressor is in surge

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Rc

qqrr22

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

• The Safety On® response shifts the SCL and the RTL to the right

New SCL

New RTL

• Additional safety or surge margin is added

Additional surge margin

• PI control and Recycle Trip® will stabilize the machine on the new SCL

SOL = Safety On® Line

Compressor can surge due to:

• Transmitter calibration shift

• Sticky antisurge valve or actuator

• Partially blocked antisurge valve or recycle line

• Unusual large process upset

Benefits of Safety On® response:

• Continuous surging is avoided

• Operators are alarmed about surge


Pressure and Flow Variations During a Typical Surge Cycle

Built in surge detectorBuilt in surge detector

100%

100%

0%

0%

Pd

Po

20 to 50 milli-seconds

1 TO 2 SECONDS

FwdPrevious Rew

• 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


Increase compressor system reliability and availability with fall-back strategies

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


Fall-back strategies for the antisurge and performance controller

Fall-back strategies for the antisurge and performance controller

• Antisurge controller– If a pressure transmitter fails, a minimum q2

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


Output linearizationOutput linearization

Controller

output

Flow rate

through

valve

• For antisurge control a linear valve is preferred

• Linear valve gives the same dynamic flow response over its complete stroke

• Existing valve has equal percentage trim

Valve trimequal percentage

• Controller output is characterized as mirror image in the linear valve line

Controller output

• Dynamic flow response becomes linear

• Existing valve has quick opening trim

Valve trimquick opening

• Controller output is characterized as mirror image in the linear valve line

Controller output

• Dynamic flow response becomes linear

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


The Tight Shut-off Line (TSL)The Tight Shut-off Line (TSL)

Controlleroutput

Flow ratethrough

valve

• Many antisurge valves have the following characteristic:

• from 0% to low clamp value the flow rate through the valve is (almost) zero and does not change

• Once the low clamp is reached the characteristic is linear

• Typical low clamp value can be 5% - we will use the 5% as the value throughout in this example

0% to the valve

Low clamp on controller output• For dynamic control we want to use

the range 5% - 100% on the valve

Dynamic control range

• The 5% or low clamp value represents the closed position for control purposes

• At the low clamp value the valve

• Usually still leaks which results in energy waste

• Makes an annoying noise

• Typical for worn valves and valves with Teflon seat

• CCC antisurge controller has a Tight Shut-off Line (TSL) that eliminates the disadvantages

TSL = Tight Shut-off Line

• When the operating point is to the right of the TSL the controller closes the valve at 0% - point A

• This is below the low clamp value

A

A

• When the controller crosses the TSL the output of the controller jumps to the low clamp value - point B

B

B

• The controller is now “ready to go” when the operating points hits the SCL - point C

C

FwdPrevious Rew

Rc

qr2

SLL RTLSCL

SOL

C

PI Control

Benefits• No leakage and noise when controller

is far away from surge - point A

• Eliminates noise and energy waste

• Eliminates dead time in the response of the antisurge valve when the operating point is close to the SCL

Time

Controlleroutput


Compressor performance controlCompressor 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 blow-off or recyclePerformance control by blow-off or recycle

• Compressor operates in point A

Pd

qr2

Shaft power

FwdPrevious Rew

qr2

Curve 1

ARprocess + Rvalve

• Required power in point A is P1

Curve 1

P1

• Pressure is controlled by blow-off

PIC - SP

• Point B represents the point that would deliver the pressure for Rprocess

Curve 2

Rprocess

B

• Required power in point B is P2

Curve 2

P2

PT1

PIC1

Process

• Power loss is P1 - P2• Qloss represents energy waste

Qloss

Notes

• Most inefficient control method

• Regularly found in plant air systems

• Rare in other systems

• Not recommended

Compressor control

Notes

• Curve 2 represents:

• Lower speed on variable speed systems

• IGVs closed on variable geometry compressors

• Inlet throttle valve closed on fixed speed compressors

MenuHelp

Performance control by discharge throttlingPerformance control by discharge throttling

• Compressor operates in point A

Pd

qr2

Shaft power

FwdPrevious Rew

qr2

Curve 1

ARprocess + Rvalve

• Required power is P1

Curve 1

P1

• Pressure is controlled by pressure drop over valve

PIC - SP

Pressure loss across valve

• Opening of valve would reduce resistance to Rprocess

Rprocess

• Lower resistance would require less speed and power

Curve 2

Curve 2

P2

PT1

PIC1

Process

• Power loss is P1 - P2

Notes

• Extremely inefficient (consumes approx. the same power for every load)

• Rarely used

• Not recommended

Compressor control

Notes

• Curve 2 represents:

• Lower speed on variable speed systems

• IGVs closed on variable geometry compressors

• Inlet throttle valve closed on fixed speed compressors

MenuHelp

Performance control by suction throttlingPerformance control by suction throttling

• Inlet valve manipulates suction pressure

Pd

qr2

Shaft power

qr2

• Changing suction pressure generates a family of curves

Suction valve open

Suction valve throttled

• Pressure is controlled by inlet valve position

PIC - SP

• Compressor operates in point A for given Rprocess

A

Rprocess


P1

PT1

PIC1

Process

FwdPrevious Rew

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 adjustable guide vanesPerformance control by adjustable guide vanes

• Change of guide vanes angle results in different compressor geometry

Pd

qr2

Shaft power

qr2

• Different geometry means different performance curve

min

OP

max

• Pressure is controlled by inlet guide vane position

PIC - SP


A

Rprocess


P1

PT1

PIC1

Process

FwdPrevious Rew

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 speed variationPerformance control by speed variation

• Changing speed generates a family of curves

Pd

qr2

Shaft power

qr2

Nmin

NOP

Nmax

• Pressure is controlled by speed of rotation

PIC - SP


A

Rprocess


P1

PT1

PIC1

Process

FwdPrevious Rew

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


Limiting control to keep the machine in its stable operating zone

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.


Power limiting in the performance controlleran example

Power limiting in the performance controlleran example

• Primary variable Pd

PIC-SP

• Limiting variable PowerPower limit • Compressor operates in point A for R1 at N1

N1

A

R1

Qs, vol

Rc

FwdPrevious Rew

• Process resistance changes from R1 to R2

B

R2

• PIC will speed machine up to N2 in order to control pressure Pd

N2

• Machine hits power limit

• Compressor operates in point B for R2 at N2

• Process resistance decreases further to R3

R3

• PIC would like to speed machine up to N4 and operate in point D

N4

D

• However power limiting loop takes control and controls machine at speed N3

• Compressor will operate in point C for R3 at N3

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 other variables (pressures, temperatures, etc.)


Limiting Ps or Pd using the antisurge controllerLimiting Ps or Pd using the antisurge controller

1UIC

VSDS

Compressor

1FT 1

PsT1

PdT

• 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

FwdPrevious Rew

DischargeSuction


• 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

Interacting antisurge and performance loopsInteracting antisurge and performance loops

AC

Po

PIC-SP

Rc

Ps

SLL

SC

L

FwdPrevious Rew

B


The performance controller interacts withthe antisurge controller

The performance controller interacts withthe 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


Ways to cope with antisurge andperformance loop interactions

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


Interacting antisurge control loopsInteracting antisurge control loops


1

PIC

2UIC

Rc,2

qr,22

RRc,1

qr,12

R

• Disturbance comes from the discharge side

• Pd,2 increases• Ps,2 remains constant• Rc,2 increases• Section 2 moves towards surge

Disturbance

R

• Antisurge controller UIC-2 will open the recycle valve to protect section 2 against surge

• Pd,2 decreases• Ps,2 increases• Rc,2 decreases• Section 2 moves away from surge

• Opening of recycle valve on section 2 caused Ps,2 = Pd,1 to increase

• Result:• Pd,1 increases• Ps,1 remains constant• Rc,1 increases• Section 1 moves towards surge

1UIC

VSDS

Section 1 Section 2

R

• Antisurge controller UIC-1 will open the recycle valve to protect section 1 against surge

• Pd,1 decreases• Ps,1 increases• Rc,1 decreases• Section 1 moves away from surge

• Opening of recycle valve on section 1 caused Pd,1 = Ps,2 to decrease

• Result:• Ps,2 decreases• Pd,2 remains constant• Rc,2 increases• Section 2 moves towards surge

• 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

MenuHelp

Loop decoupling between multiple antisurge controllersLoop decoupling between multiple antisurge controllers

1

PIC

2UIC

1UIC

VSDS

Section 1 Section 2


• All CCC controllers are connected on a serial network

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 MenuHelp

Loop decoupling simplified block diagramLoop decoupling simplified block diagram

FAMode

PI RT

Loop DecouplingAntisurge

Controller 1

Analog Inputs

DEV1


FAMode

PI RT

Antisurge Controller 2

Analog Inputs

DEV2

2

UIC

1

UIC

VSDS

Section 1 Section 2

Serial network

• Antisurge controller UIC-2 opens its valve to protect section 2 against surge

To antisurge valve 2

+

• UIC-1 is protecting section 1 against surge using PI and Recycle Trip®

+

To antisurge valve 1

• UIC-2 reports PI and Recycle Trip® output to UIC-1• Loop decoupling block multiplies reported PI and Recycle Trip® values with decoupling gain M2

PI2 . M2

+

RT2 . M2

• Loop decoupling value is added to output to antisurge valve 1

+

• Loop decoupling values of other controllers (performance and antisurge) are added to output to antisurge valve 1

From other controllers

PIn . Mn

+

RTn . Mn

• Each controller has its own decoupling gain Mn to allow for tuning of relative loop gains between different controllers

• UIC-1 reports its PI and Recycle Trip values to UIC-2• Same decoupling takes place

Loop Decoupling

PI1 . M1

+

RT1 . M1

PIn . Mn

+

RTn . Mn

+

From other controllers

Benefits• Avoids control system oscillations• Allows faster tuning of control system• Reduced risk of surge• Allows closer operation to surge limits

without taking risk

MenuHelp

Compressor networksCompressor 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


Control system objectivesfor compressors in parallelControl system objectivesfor 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


Process Flow Diagram for base load controlProcess Flow Diagram for base load control

Process

PIC1

1UIC

VSDS

Compressor 1

2UIC

VSDS

Compressor 2

HIC1

Suction header

FwdPrevious Rew

Swing machine

Base machine

Notes• All controllers act independently• Transmitters are not shown


Parallel compressor control by base loadingParallel compressor control by base loading

Rc,1

qr,12

Rc,2

qr,22

FwdPrevious Rew

Compressor 1 Compressor 2

• Machines operate at same Rc since suction and discharge of both machines are tied together

PIC-SP

• Base load one or more compressors and let the other(s) absorb the load swings

Swing machine Base machine

• Base machine is fully loaded and runs without recycle

QC,2= QP,2

• Swing machine can be running with recycle

QC,1QP,1

where:

• QP = Flow to process

• QC = Total compressor flow

• QC - QP = Recycle flow

• Load could be re-divided to eliminate recycle

QP,1 QP,2

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


Process Flow Diagram for equal flow divisionProcess Flow Diagram for equal flow division

Process

PIC1

1UIC

VSDS

Compressor 1

VSDS

Compressor 2

Suction header

FwdPrevious Rew

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

out

out

RSP

RSP

RSP

out

RSP


Parallel compressor control by equal flow divisionParallel compressor control by equal flow division

Rc,1

qr,12

Rc,2

qr,22

FwdPrevious Rew



PIC-SP

• Machine 2 operates with recycle while machine 1 still has turn down

QP,1 QP,2QC,2

Equal flow Equal flowQP,1 = QP,2

where:

• QP = Flow to process

• QC = Total compressor flow

• QC - QP = Recycle flow

• Equal flow division might work if both machines are identical• Machines are never identical except by coincidence - different resistance due to piping arrangments

• Bias relay on remote setpoint would only work if curves have same steepness

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


Dynamic control problem with pressure to flow cascade system

Dynamic control problem with pressure to flow cascade system

FwdPrevious Rew

qr2

Rc

N1

N3

N2

• Pressure controller (PIC) provides Remote SetPoint (RSP) for Flow controller (FIC)

PIC1

OUTRSP

FIC1

• The FIC provides the RSP for the speed controller, suction throttle valve or guide vanes

OUTRSP

SIC1

• The PIC is the master and the FIC is the slave

Master Slave

• In a typical master-slave control scheme the slave needs to be approx. 5 times faster than the master

A

• The machine is operating in point A

• This is the intersection of 4 lines:– Resistance line R1

– Performance curve N1

– PIC-SP– FIC-SP = Output of PIC

R1

PIC-SP

FIC-SP

• Process disturbance causes the resistance to change from R1 to R2

R2

• As a result the machine moves to point B

B

• Since the PIC is slow it does not move its output yet which is the FIC-SP

• The FIC reacts fast and will try to maintain its SP

• The FIC will speed up the machine to point C at speed N3

C

• The disturbance is amplified

• Positive feedback system

• Only as the PIC starts to reduce its output to control pressure the FIC-SP comes down and the pressure is restored

D

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


Process Flow Diagram for equidistant control for parallel compressors

Process Flow Diagram for equidistant control for parallel compressors

Process

1UIC

VSDS

Compressor 1

VSDS

Compressor 2

Suction header

FwdPrevious Rew

Notes• All controllers are coordinating

control responses via a serial network

• Minimizes recycle under all operating conditions

1

LSIC

2UIC

out

RSP

Serial network

out

RSP

2

LSIC

1

MPIC

Serial network

Serial network


Parallel compressor control by equidistant operationParallel compressor control by equidistant operation

Rc,1

qr,12

Rc,2

qr,22

FwdPrevious Rew



PIC-SP

• The DEV is a dimensionless number representing the distance between the operating point and the Surge Control Line

• Lines of equal DEV can be plotted on the performance curves as shown

.1.2

.3

DEV = 0.1

.2

.3

• Machines are kept at the same relative distance to the Surge Control Line (SCL)

• This means in practice the same DEV for both machines

DEV1 DEV2

• Recycle will only start when all machines are on their SCL

• Since DEV is dimensionless all sorts of machines can be mixed: small, big, axials, centrifugals

• The DEV will be the same for all machines but they will operate at different speeds and flow rates


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


Compressors in parallel - the primary responseCompressors in parallel - the primary response

Master ControllerMaster Controller

Loadsharing Controller

Loop Decoupling

FAMode

PI RT

Loop Decoupling

+

Antisurge Controller

FwdPrevious Rew

Analog Inputs

+

DEV

To antisurge valve To performance control element

• Master controller controls the main Process Variable (PV) via its PID control block

PID

PV

SP

• The output of the master controller PID goes to the primary response block in the loadsharing controller

Primaryresponse

• In the primary response block the controller checks if the machine is close to the SCL:

– Yes: don’t reduce capacity - keep output constant

– No: reduce capacity as necessary

• Apply loadsharing gain M0

• The output of the master controller goes via the primary response block directly to the performance control element

DEV > 0Don’t change

output

x

Yes

No

Apply loadsharing gain

To performance control element

Primary response

• In order to check if the machine is close to the SCL the primary response block needs the DEV

• The DEV is reported by the antisurge controller

DEV DEV

• When the machine is close to the SCL the master controller will no longer reduce performance to control the primary variable

• The master controller will start to open the recycle valve to control the primary variable

Primaryresponse

• If DEV <= 0 apply loadsharing gain

• Output goes to antisurge valve

DEV < 0Don’t change

output

x

Yes

No

Apply loadsharing gain

To antisurge valve

Primary response


The load balancing responseThe load balancing response



Loop Decoupling

FAMode

PI RT

Loop Decoupling

+


FwdPrevious Rew

Analog Inputs

+

DEV

To antisurge valve To performance control element

• The fast master controller controls the primary process variable by directly manipulating the final control elements PID

• In order to balance the machines they need to be kept at the same DEV

• The antisurge controller reports the actual DEV to the load balancing block in the loadsharing controller

• This reported DEV becomes the Process Variable (PV) for the load balancing PID loop

Loadbalancing

PV

PV

SP

Primaryresponse

• The loadsharing controller reports this DEV PV also to the master controller

DEV DEV

DE

V

• Other loadsharing controllers also report their DEV PV to the master controller

DEV from other loadsharing controllers

Primaryresponse

• The master controller calculates the average of all reported DEV PV’s

Average

• This average DEV is sent out to all loadsharing controllers to become the SP for all load balancing blocks

SP

• The load balancing block is a slow controller that will equalize all DEV’s for all parallel compressors

• Its output is added to the total output to the performance control element


The Pressure Override Control (POC) responseThe Pressure Override Control (POC) response



Loop Decoupling

Loadbalancing

FAMode

PI RT

Loop Decoupling

+


FwdPrevious Rew

Analog Inputs

Average

+

SPPV


DEV

DE

VTo antisurge valve To performance

control element

• When a large disturbance occurs it can happen that the performance control element (e.g. speed) is too slow to keep the pressure under control

PID

PV

SP

• The operating point rides the curve and the pressure rises sharply

Primaryresponse

• There is a high chance to exceed the relief valve setting and trip the process

• The CCC master controller has a Pressure Override Control (POC) mode that will open the antisurge valve to get the disturbance under control quickly

DEV DEVPOC-SP

PI(One-Sided)

SP

PV

• Opening of the antisurge valve is much faster than a reduction in speed

• As soon as the operating point drops under the POC-SP line the antisurge valves start to close again

Primaryresponse

• The primary PID loop will stabilize the operating point on the PIC-SP line

Rc

qqrr22

PIC-SP

Relief valve setting

Benefits• Fast response during fast upsets• Avoid process trips due to lack of response

in performance control elements• Allows closer operation to process limits

without taking risk


Loadsharing for multi-section compressorsLoadsharing for multi-section compressors

Process

1AUIC

VSDS

Section 1

VSDS

Section 1

Suction Header

FwdPrevious Rew

A

LSIC

out

RSP

Serial network

RSP

B

LSIC

1

MPIC

Serial network

Serial network

Section 2

Section 2

2AUIC

1BUIC

1BUICSerial

network

Serial network

out

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

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


Selecting the section closest to SCL for parallel operationSelecting the section closest to SCL for parallel operationSelecting the section closest to SCL for parallel operationSelecting the section closest to SCL for parallel operation



Loop Decoupling

Loadbalancing

FAMode

PI RT

Loop Decoupling

+


FwdPrevious Rew

Analog Inputs

Average

+

SPPV


DEV1

To antisurge valve-1 To performance control element

PID

PV

SP

Primaryresponse

DEV1 DEV2

• Both antisurge controllers report their DEV to the loadsharing controller

PI(One-Sided)

SP

PV

Primaryresponse

FAMode

PI RT

Loop Decoupling

+


DEV2

To antisurge valve-2

Primaryresponse

• The lowest DEV is selected: the section closest to the SCL

<

• The selected DEV is reported to:

• Primary control response blocks

• Load balancing block

• Master controller averaging block


Flow Measuring Device (FMD) selection criteriaFlow 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 p corresponding to Qmax, compressor is 10” WC (250 mmH2O) or more


Flow Measuring Device (FMD) locationFlow Measuring Device (FMD) location

FwdPrevious Rew

• The preferred location of the FMD:• Suction of compressor • As close to the inlet flange as

possible

VSDS

Compressor

DischargeSuction

minimum possible

• Less preferable location of the FMD:• Discharge of compressor • As close to the discharge flange as

possible

minimum 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)


Response time of the FMD transmitterResponse time of the FMD transmitter

• The speed of approaching surge is high

FwdPrevious Rew

100%

0

100%

0Pc

100%

Ps0

1 SEC.

Po

A C

D

B

A CB

• In only 400 ms, PO dropped by 14%, with a 2% change in Pc

• The transmitter type and brand should be selected based on two major factors:

– Reliability– Speed of response

• Desired rise time for p (flow) transmitters is 200 ms or less

– Pressure step is 100%– The first order response (63%) is less

than 200 ms

Time

Actualpressure

Transmitteroutput

63% response1- (1/e)

1 is less than 200 ms

• Desired rise time for pressure transmitters is 500 ms or less


The effect of damping the po (flow) transmitterThe effect of damping the po (flow) transmitter

• Knowing the flow is essential to determine the distance between the operating point and the SCL

• Damping the po (flow) transmitter destroys essential information

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

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FlowStart of Surge

Actual Flow

= 16.0 s

= 1.70 s

= 0.20 s = 0.03 s

Damping the po (flow) transmitter can paralyze the complete antisurge control system!!!


Sizing the antisurge control valveSizing 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

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• 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

Rc

Qvol

A


Sizing the antisurge control valve - alternative methodSizing the antisurge control valve - alternative method

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Rc

Qvol

• An alternative method yielding excellent results is:

• Take design point of the compressor point A

A • Draw a horizontal line through the design point

• Take point B at intersection of maximum speed performance curve and the horizontal line

B

• Calculate Cv,calc in point B

• Select standard valve size using the following criteria:

0.9 . Cv, calc < Cv,selected < 1.1 . Cv, calc


Stroke speed and characteristic of the antisurge valveStroke speed and characteristic of the antisurge valve

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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 Open to close– 1” to 4” 1 second < 3 seconds– 6” to 12” 2 seconds < 5 seconds– 16” and up 3 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


Improving the performance of the antisurge valveImproving the performance of the antisurge valve

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• 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


Piping lay-out consideration when designing an antisurge control system

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

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VSDS

Compressor 1volume to be

minimized


Using a single antisurge valve increases recycle lag timeUsing a single antisurge valve increases recycle lag time

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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

• Large volume significantly decreases the effectiveness of the antisurge protection

• Result– Poor surge protection– Large surge margins– Energy waste– Process trips because of surge

Note• This specific piping layout is found on

many wet gas compressors in FCCU’s


Sharing recycle coolers degrades surge protectionSharing recycle coolers degrades surge protection

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Section 1 Section 2

• The piping lay-out for section 2 is excellent for surge protection• Minimum volume between the three flanges

Small volume

• 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


Installing recycle valve upstream from cooler improves control response

Installing recycle valve upstream from cooler improves control response

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Compressor 1

• Compressor 1 has ideal piping lay-out for surge protection• Minimum volume between the three flanges

Compressor 2Minimum volume

• 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

Increased volume due

to cooler

• 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


Recycle lines configured for optimum surge protectionRecycle lines configured for optimum surge protection

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• Compressor has ideal piping lay-out for surge protection

• Minimum volume between the three flanges for all sections

Section 2 Section 3Section 1

Minimum volume

ProcessSuction


Which antisurge piping configuration do you choose???Which antisurge piping configuration do you choose???

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• These two piping lay-outs are most common for antisurge control• Lay-out #1 has minimum volume between the flanges and is the best lay-out for antisurge control purposes

Section 2 Section 3Section 1

ProcessSuction

Section 1 Section 2 Section 3

Suction Process

Lay-out #1: Compressor with recycle lines optimally configured for antisurge control

Lay-out #2: Compressor with coolers upstream of recycle take-off

• Lay-out #2 requires one cooler less and thus the capital investment is lower• When selecting lay-out #2 the residence time of the gas in the “surge” volume should be verified to check acceptable time delays are not exceeded

• Lay-out #2 will require bigger surge control margins


Influence of controller execution timeInfluence of controller execution time


Analog controller

SLLSCL

100%

0%

Controlleroutput

100%

0%

• 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

Operatingpoint

Time

Time

MenuHelp

Analog vs digital controller at 2 executions per second Analog vs digital controller at 2 executions per second

Analog controller


SLLSCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint SLL

SCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint

Digital controller(2 executions per second)

Time

Time

Time

Time

• Compressor surged

• Large process upset would have resulted

Tuning same as analog controller

MenuHelp

Analog vs digital controller at 3 executions per secondAnalog vs digital controller at 3 executions per second

Analog controller


SLLSCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint SLL

SCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint


Time

Time

Time

TimeTuning same as analog controller

• Compressor surged

• Large process upset would have resulted

MenuHelp

Analog vs digital controller at 10 executions per secondAnalog vs digital controller at 10 executions per second

Analog controller


SLLSCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint SLL

SCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint


Time

Time

Time


• Compressor almost surged

• Control system would have to be set up with bigger surge margins

MenuHelp

Analog vs CCC controller at 25 executions per secondAnalog vs CCC controller at 25 executions per second

Analog controller


SLLSCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint SLL

SCL

100%

0%

Controlleroutput

100%

0%

Operatingpoint

CCC antisurge controller(25 executions per second)

Time

Time

Time


• Response of CCC controller nearly indentical to analog controller

• Adding Recycle Trip® to PI control will allow even smaller surge margins

MenuHelp

Dynamic simulation single compressorDynamic simulation single compressor


1UIC

VSDS

Compressor

1FT 1

PsT1

TsT

ProcessSuction

1PdT

1TdT

1ST

1PIC

1HIC

LoadNote: Speed transmitter for indicating purposes only

Start simulation

• 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

Serial network

MODBUS MenuHelp

Dynamic simulation parallel compressorsDynamic simulation parallel compressors


Process

HIC1

Load

1UIC

VSDS

Compressor 1

VSDS

Compressor 2

1

LSIC

2UIC

out

RSPRSP

Serial network

out

RSP

2

LSIC

1

MPIC

Serial network

Serial network

MODBUS Start simulation MenuHelp

Documents

compressor.ppt