Load Sharing for Different Performance Curves

© 2002 Compressor Controls Corporation FwdPrev Rew Menu

CompressorsCompressors


Cost of Operation

Compressor Operation

Compressor Control

Compressors Compressors

Maintenance costsOperating costsEnergy savings

Downtime costsCommissioning costs

Lifecycle costs

Centrifugal compressorsAxial compressors

Compressor system classificationsDeveloping the compressor curve

The surge phenomena


Costs of repairs - materials and labor

Repairs are expensive

$20,000 $50,000

$10,000 $25,000

$200,000 $750,000

Axial air blower20,000 hp

Seals

Bearings

Rotor Assembly

Process gas compressor 3,000 hp

Compressors


Operating costs are large

Assumes power at $.07 per kilowatt hour or $457 per horsepower per year

1,000 HP (746 kW) $457,000

4,000 HP(2,984 kW) $1,830,000

40,000 HP(29,480 kW) $18,300,000

Cost to operate one turbocompressor

per year

Plant air compressor

Wet gas compressor

Propylene refrigeration compressor

Compressor shaft power

Compressors


Energy savings from reduced recycle or blow-off

40,000 HP (29 МW) $1,200,000

15,000 HP (11.2 МW) $155,000

1,500 HP (1.1 МW) $78,000

Actual energy savings result from improved Antisurge and Capacity Control

Compressor shaft power

Actual achieved savings

Propylene refrigeration

FCCU air blower

Centac air compressor

Compressors


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

Pa

y-b

ac

k P

eri

od

(M

on

ths

)

Reduced Recycle (Per Cent of Maximum Compressor Flow)

* Assumes electro motor power At $0.05 US per kilowatt hour or turbine power at $327 per horsepower per year.

12

11

10

9

8

7

6

5

4

3

2

1

0 5 10 15 20 25 30 35

Pay-back approximately 1

Month with 15% Reduction 1000 HP

3,500 HP

20,000 HP

Pay-back less than 10

Months with 15% Reduction

Pay-back less than 6

Months with 15% Reduction

Available energy savings can be predicted

Compressors


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,– Phone calls in the middle of the night.

Compressors


$2,300,000$375,000

100 MMSCFDNatural Gas Plant

60,000 BPDCat Cracker

Start-up Cost Per Day

Process

Commissioning costs are large

• Includes lost sales plus fixed operating expenses.• Most turbo compressor control system design

problems are discovered during commissioning.• Delays due to turbomachinery control problems

are not unusual.• Startups are faster with a properly designed

turbomachinery control system.

Compressors


Lifecycle costs

30-year life cycle costs for a 20,000 hp compressor

Costs in constant dollars

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

Maintenance Cost$4.5 Million

Initial Cost $1.5 Million

Energy Cost$180 Million

97% of total costs

Compressors


Lifecycle costsLifecycle costs

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

30-year costs per a 1,000 hp

What can we control?

0.0

5.0

10.0

15.0

Initial Cost Maintenance Energy Lost Production

$ Millions

?

Controllable

UncontrollableCosts in constant dollars

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

Centrifugal compressors

Single Case Compressor Centrifugal Impeller

IMPELLERS

Picture of horizontal split

Cross section of barrel type compressor

Picture of barrel type compressor

Cross section of bull gear compressor

Picture of (bull) gear and impellers

Picture of bull gear compressor

Cross section of horizontal split

DIFFUSERS

Compressors


Compressor inlet nozzle

Thrust bearing

Journal bearing

Shaft and labyrinth seal

Impeller inlet labyrinth seals

Discharge volutes

Impellers

Drive coupling

Casing (horizontally split

flange) Compressor discharge nozzle

Cross section of horizontal split

Picture of Horizontal Split Compressors


Picture of horizontal split

CompressorsCross Section of Horizontal Split


Cross section of barrel type compressor

CompressorsPicture of Barrel Type


Picture of barrel type compressor

CompressorsCross Section of Barrel Type


Compressor volutes

Gear casing

Pinion shafts

Journal bearing

Impellers

Drive coupling

Labyrinth seals

Main gear

Inlet guide vanes

Cross section of bull gear compressor

CompressorsIntegrally GearedGear and Impellers


Picture of bull gear compressor

CompressorsGear and ImpellersCross section


Picture of (bull) gear and impellers

CompressorsIntegrally GearedCross Section


Axial compressors

• Gas flows in direction of rotating shaft• Can be built for lower pressures only 10 to

100 psi (0.7 to 6.8 Bar)• High flow rate • Efficient• Not as common as centrifugals

RotorBlades

Casing

StatorBlades

Stator BladesRotor Blades

Casing

Shaft

CompressorsPicture of AxialCross Section


Compressor outlet nozzle

Rotor blades

Labyrinth sealsGuide-vane

actuator linkage

Compressor rotor

Compressor inlet nozzle Thrust bearing

Adjustable guide vanes

Cross section of axial compressor

AxialsPicture of Axial


Picture of axial compressor

AxialsCross Section


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

Two-Case, Two-Section, Six-Stage

Series Network

Parallel Network

Compressor system classifications

Compressors


Developing 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

Rprocess,2

Q2

Rc2

Compressors


Stable zoneof operation

Developing the compressor curve

Minimum speed

Power limit

Maximum speed

Process limit

Qs, vol

Adding control margins

Stonewall orchoke limit

Surge limit

Rc

Actual availableoperating zone

Compressors


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”

Lift

• And the plane can fly

How an airplane wing develops lift

Compressors


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

• This leads to more lift

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

How the airplane develops stall

Compressors


• 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

• Compressor reaches surge point A• Compressor loses its ability to make

pressure• Suddenly Pd drops and thus Pv > Pd• Compressor surges -“Plane goes to stall”

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

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

point B

• Result of flow reversal is that pressure goes down

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

• 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 = 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

Developing the surge cycle on the compressor curve

Pd

Pv

Rlosses

B A

C

D

Compressors


• Rapid flow oscillations • Thrust reversals• Potential damage

FLOW

PRESSURE

TEMPERATURE

TIME (sec.)

1 2 3

TIME (sec.)

1 2 3

TIME (sec.)

1 2 3

Major Process Parameters during Surge

• Rapid pressure oscillations with process instability

• Rising temperatures inside compressor

Operators may fail to recognize surge

Compressors


Surge description

• Flow reverses in 20 to 50 milliseconds• Surge cycles at a rate of 0.3 s to 3 s

per cycle• Compressor vibrates• Temperature rises• “Whooshing” noise• Trips may occur• Conventional instruments and human

operators may fail to recognize surge

Compressors


Some surge consequences

• Unstable flow and pressure• Damage in sequence with increasing

severity to seals, bearings, impellers, shaft

• Increased seal clearances and leakage

• Lower energy efficiency• Reduced compressor life

Compressors


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

Compressors


Protection #2: Recycle Trip

Protection #3: Safety On

Compressor ControlCompressor ControlObjectives

Antisurge Control

Performance Control

Other topics

Loadsharing

Major challenges of Compressor ControlLocation of the

surge limit

High speed ofapproaching surge

Control loop interactions

Loadsharing of multiple

compressors

Protection #1: PI Control

Basic antisurge Control System

Compressor Networks

Base Loading Parallel

Compressors

Equal Flow Division System

Equidistant Loadsharing System

Compressor Efficiency

Fall-back strategiesPressure Override Control (POC)Flow Measuring Devices (FMD’s)

Antisurge Control Valve

Piping lay-out considerations

Dynamic simulation single compressorDynamic simulation parallel compressors

Influence of controller execution time

Output linearization

The tight shut-off line


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

Compressor Control


Major challenges of Compressor ControlMajor challenges of Compressor Control

Location of the surge limitHigh speed of

approaching surgeControl loop interactions

Loadsharing of multiple compressors

Compressor Control


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

Major Challenges


Commonly used (OEM provided) coordinate systems of the compressor map

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

Qs = Suction flow and can be expressed as actual or standard volumetric 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

Major Challenges


• These coordinates are NOT invariant to suction conditions as shown

The problem with OEM providedcoordinate systems of the compressor map

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

Major Challenges


Developing invariant coordinates

where:Hp= Polytropic headJ = PowerQ = Volumetric flow rate = Rotational speed = Viscosity = Densitya = Local acoustic velocityd = Characteristic length = Inlet guide vane angle

Dimensional analysisor Similitude

where:hr = Reduced headqr = Reduced flowNe = Equivalent speed = Guide vane anglejr = Reduced powerRe= Reynolds numberRc = Pressure Ratio

• The following variables are used to design and to characterize compressors

Fundamental variables characterizing

compressor operation

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

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

Set 1hr

qr

Nejr

Re

Set 2Rc

qr

Nejr

Re

Invariant coordinates

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

Major Challenges


NOT invariant coordinates (Hp, Qs) Invariant coordinates (hr, qr2)

where:Hp = Polytropic headQs = Volumetric suction flowhr = Reduced headQr

2 = Reduced flow squared

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

Major Challenges


Coordinates (Rc ;Qs) and (Rc ;qr2)

NOT invariant coordinates (Rc, Qs)

qr2

Invariant coordinates (Rc, qr2)

where:Rc = Pressure ratioQs = Volumetric suction flowQr

2 = Reduced flow squared

Major Challenges


Coordinates (Rc ;jr) and (Rc ;Ne2)

Invariant coordinates (Rc, jr) Invariant coordinates (Rc, Ne2)

where:Rc = Pressure ratiojr = Reduced powerNe

2 = Equivalent speed squared

Major Challenges


• 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

( )

increasing MW, N

decreasing T s

qr2

hr

Representing the SLL as a single curve using reduced coordinates

Design Nitrogen Off-gas MW MW MWPs Ps Ps

Ts Ts Ts

ks ks ks

Major Challenges


qr2 =

Qs2

(ZRT)s

where:R = Ru / MWRu = Universal gas constantR = Specific gas constantMW = Molecular Weight of the gasps = Suction pressureK = Orifice plate constantpo,s = Differential pressure across orifice plateTs = Temperature of the gas in suctionZs = Compressibility of gas in suction of

compressor

=

K . Zs . Ru . Ts

MW

po,s. ps

(ZRT)s

= po,s

ps

The antisurge controller calculates qr2 using ps and po,s

transmitters

Calculating qr2 (reduced flow squared)

Major Challenges


hr = Hp

(ZRT)s

where:R = Ru / MWRt = Td / Ts Temperature ratioRc = pd / ps Pressure ratio Ru = Universal gas constantR = Specific gas constantMW = Molecular Weight of the gasPd = Discharge pressureps = Suction pressureZs = Suction compressibility = Exponent for polytropic compression

=

Zs . Ru . Ts

MW

Rc-1.

(ZRT)s

= Rc

-1

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

log(Rt)

log(Rc)For polytropic compression Rt = Rc

thus =

Calculating hr (reduced head)

Major Challenges


The relationship between pressure and temperature for polytropic compression is:

Calculating improves accuracy when:- Gas composition varies- Compressor efficiency changes

Solving for:

can be calculated from Pressure and Temperature Measurements

Ts

Td = =

Ps

Pd

Rc

log

log

Ts

Td

Ps

Pd

Major Challenges


• The surge parameter is defined as: S

fq

sr

r op

12

(h )

,

• The function f1 returns the value of qr2 on

the SLL for input hr

hr

qr,SLL2

• Non-linearity in the Surge Limit Line can be accommodated using a function based on a piecewise characterization of either map axis

qr2

hr

Building the Surge Limit Line

Major Challenges


qr2

hr

qr,op2

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

2

hr

qr,SLL2

• As a result: Ss = qr,op2

qr,SLL2

OP

OP = Operating Point

• Ss < 1stable operating zone

Ss < 1• Ss = 1

surge limit line (SLL)

S s = 1

• Ss > 1surge region

Ss > 1

The surge parameter Ss

Major Challenges


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 = 1d = 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

- Operating Point

Benefits:One standard surge parameter in the plantNo operator confusion:

DEV > 0 GoodDEV = 0 Recycle LineDEV < 0 Bad

Major Challenges


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

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

qr,op2

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

Simplifying the surge parameterby replacing hr with Rc

Major Challenges


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

• The parameter 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

The simplest CCC surge parameter

Major Challenges


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

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

invariant coordinates qr2 and Rc

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

qr2

Rc

Actual Surge Limit Line (SLL)

SLL calculated by antisurge controller using pc /po = constant

Loss of operating envelope

Disadvantage of the pc /po surge parameter

Major Challenges


SLL from map

pc /po,c =constant

Actual field data showing disadvantage of pc /po surge parameter

Major Challenges


Calculating equivalent flows when suction flow is measured

Major Challenges


Surge Protection of multi-section compressors

Discharge Flow measured

• A multi-section compressor has one or more ports between the suction and discharge where sidestreams can be withdrawn or injected.

• This generally requires the calculation of Equivalent Flow Measurements for all but the first or last section or the calculation of proximity to surge from Combined Mass Flow rates.

• If a single recycle valve is used to protect more than one section of the compressor, Valve Sharing should be employed to ensure adequate protection under changing process conditions.

Suction Flow measured

Major Challenges


Equivalent Flow Measurements

• The equations used to calculate equivalent flow measurements for combined or split streams are derived from mass and energy balances over those streams.

• Each section’s Antisurge Controller measures the flow in its sidestream and adds it to OR subtracts it from the flow reported by the adjacent section’s controller.

• The mass flow through an orifice plate (or similar flow-measuring element) can be calculated as:

• Given equal compositions and assuming constant specific heats, the energy balance across the junction can be written as:

• If all three streams have the same composition, any change in their molecular weight would have a proportional effect on all three flow measurements that would cancel out. Thus, the composition of the three streams must be equal but not necessarily constant. If we also assume equal compressibility, we can write the mass balance as:

Major Challenges


• If Controller’s Mode fA 64 is selected, the Controller uses its standard formula and compensating inputs to calculate the squared, normalized side-stream mass flow:

Combined Mass Flow

• Mode fA 64 should be used only if the molecular weights and compressibilities of the streams are constant (but not necessarily equal), or at least nearly enough so to achieve good results using a worst-case composition.

• The compensating pressure and temperature measurements should correspond to the sidestream conditions.

• That pressure will usually be either the suction or discharge pressure, unless there is a valve between the orifice and compressor.

• This sidestream flow is added to the mass flow (Wc) reported by

a specified Adjacent Section Controller:

Major Challenges


• Each section of a compressor should be equipped with its own Antisurge Controllers even if there is only one recycle path and control valve for the entire machine.

Valve Sharing

• The Master Controller protects the entire compressor from surge by selecting the highest of several PI and RT responses. The response of a valve-sharing is used to manipulate the recycle valve.

Major Challenges


Calculating equivalent flows when discharge flow is measured

Major Challenges


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

Major Challenges


• Performance curves are usually very 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%

0

100%

0

1 SEC.

Po A CDB

The approach to surge is fast

Pd A C D

B

Pc A C D

B

Major Challenges


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.

The approach to surge is fast another example

Major Challenges


Antisurge ControlAntisurge Control

Protection #1: PI ControlProtection #2: Recycle Trip

Protection #3: Safety OnOutput linearization

The tight shut-off line

Basic antisurge Control System

Compressor Control


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

Antisurge Control


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

Antisurge Control


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 Control


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

Benefits:– 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 LineRTL= Recycle Trip® Line

SCL = Surge Control Line

Output to Valve

Time

Recycle Trip® Response

PI Control Response

PI Control Recycle Trip®

Action

+

To antisurge valve

Total Response

Rc

Q2

OP

Antisurge 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

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.

Antisurge Control


Additional surge margin

• Benefits of Safety On® response:- Continuous surging is avoided- Operators 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 #3

The Safety On® Response (SOL)

Rc

qr2

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

New SCL

New RTL

SOL - Safety On® Line

Antisurge Control


Built-in surge detector

Pressure and Flow Variations During a Typical Surge Cycle

100%

100%

0%

0%

Pd

Po

20 to 50 milli-seconds

1 TO 2 SECONDS

• Surge signature should be recorded during commissioning.

• Rates of change for flow and pressure transmitters should be calculated.

• Thresholds should be 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 have been 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

Antisurge Control


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

Antisurge Control


Fall-back strategies for the antisurge and performance controller

• Antisurge controller– If a pressure transmitter fails, a minimum q2r algorithm is

used– If a temperature transmitter fails, hr is characterized as a

function of compression ratio– If the speed transmitter fails, a conservative speed

setting is used– If the flow transmitter fails

• Redundant transmitter is used• Output is driven to:

– Last value OR– Last Value selected: If Last Value >Pre-selected fixed value OR

Pre-selected fixed value selected: If Pre-selected fixed value>Last Value

• Performance controller– Switches to redundant transmitter upon primary

transmitter failure– Output goes to pre-selected value if all transmitters fail or

is frozen• All transmitter failures are alarmed

Antisurge Control


Controller output

Flow ratethrough

valve

Valve trim

quick opening

Controller output

Notes• Used to improve controllers operation

when non-linear valves are used• Used on retrofits to avoid additional investment in new valve• Works well with equal percentage characteristics• Works less satisfactory with quick opening characteristics

• 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

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

• Dynamic flow response becomes linear

• Existing valve has quick opening trim

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

• Dynamic flow response becomes linear

Valve trimequal percentage

Controller output

Output linearization

Antisurge Control


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

Dynamic control range

TSL = Tight Shut-off Line

B

Rc

qr2

SLL RTLSCL

SOL

C

PI Control

Controlleroutput

The Tight Shut-off Line (TSL)

A

A Time

BC

Antisurge Control


Performance ControlPerformance Control

Discharge throttlingSuction throttling

Adjustable guide vanesSpeed variationLimiting Control

Blow-off or recycle

Interacting Antisurge &Performance Loops

Loop Decoupling

Compressor Control


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

• Matches the compressor throughput to the load

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

Compressor Performance Control

Performance Control


PIC - SP

• Compressor operates in point A

Pd

qr2

Shaft power

qr2

Curve 1

ARprocess + Rvalve

• Required power in point A is P1

Curve 1

P1

• Pressure is controlled by blow-off

• 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

• 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

Notes• Curve 2 represents:

- Lower speed on variable speed systems

- IGVs closed on variable geometry compressors

- Inlet throttle valve closed on fixed speed compressors

Performance Control by blow-off or recycle

PT

1

PIC1

Process

Performance Control


• Compressor operates in point A

Pd

qr2

Shaft power

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

• Power loss is P1 - P2Notes:• Extremely inefficient

(consumes approx. the same power for every load)

• Rarely used• Not recommended

Notes:• Curve 2 represents:

- Lower speed on variable speed systems

- IGVs closed on variable geometry compressors

- Inlet throttle valve closed on fixed speed compressors

Performance Control by discharge throttling

PT

1

PIC1

Process

Performance Control


• 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

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

PT

1

PIC1

Process

Performance Control


• 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

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

Performance Control


• 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

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

Performance Control


• 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

Performance Control


N4

D

• Primary variable Pd

PIC-SP

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

N1

A

R1

Qs, vol

Rc

• 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

• 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 othervariables (pressures,temperatures, etc.)

Power limiting with the Performance Controller

Performance Control


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

DischargeSuction

Limiting Ps or Pd using the Antisurge Controller

Performance Control


• 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 Control


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

Performance Control


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

Performance Control


Disturbance comes from the discharge side

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

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

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 decreaseResult:

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

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 increaseResult:

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

1

PIC

2UIC

1UIC

VSDS

Section 1 Section 2

Performance Control


• 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

Performance Control


FAMode

PI RT

Loop Decoupling

Antisurge Controller UIC-1

Analog Inputs

DEV1

FAMode

PI RT

Antisurge Controller UIC-2

Analog Inputs

DEV2

• 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

Loop Decoupling simplified Block Diagram

VSDS

Section 1 Section 2

Serial network 2

UIC

1

UIC

Performance Control


LoadsharingLoadsharing

Compressor Networks

Base Loading Parallel Compressors

Equal Flow Division System

Equidistant Loadsharing System


Compressor Control


• 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

Loadsharing


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

Loadsharing


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

Loadsharing


Rc,1

qr,12

Rc,2

qr,22

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 processQC= Total compressor flowQC - QP = Recycle flow

• Load could be re-divided to eliminate recycle

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

Loadsharing


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

Loadsharing


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


• Equal flow division might work if both machines are identical

• Machines are never identical except by coincidence

• 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

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

Loadsharing


Q2

Rc

N1

N3

N2

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

positive feedback in control system

PIC1

OUTRSP

FIC1

OUTRSP

Master Slave

SIC1

Dynamic Response / Pressure To Flow Cascade

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

Loadsharing

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

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

Master Slave


Notes• All controllers are

coordinating control responses via a serial network

• 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

Loadsharing



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

0.10.2

0.3

DEV = 00.1

0.2

0.3

• 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

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

Loadsharing


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

Loadsharing


Loadsharing Controller

Loop Decoupling

FAMode

PI RT

Loop Decoupling

+

Antisurge Controller

Analog Inputs

+

DEV

To antisurge valve To performance control element

Primaryresponse

DEV DEV

Primaryresponse

To performance control element

DEV > 0

Don’t change output

x

Yes

No

Primary response

Primary response

Don’t change output

No

x

YesApply

loadsharing gain

To antisurge valve

DEV 0

Compressors in Parallel the primary response

Master Controller

PV

SP

PID

Loadsharing

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

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

• 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

• 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

• 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

• If DEV <= 0 apply loadsharing gain• Output goes to antisurge valve


Loadsharing Controller

Loop Decoupling

Loadbalancing

FAMode

PI RT

+


Analog Inputs

Average

+

SPPV

DEV from other loadsharing controllers

DEV1

To antisurge valve-1 To performance control element

PID

PV

SP

Primaryresponse

• Both antisurge controllers report their DEV to the loadsharing controller

DEV1

PI(One-Sided)

SP

PV

Primaryresponse

FAMode

PI RT

Loop Decoupling

+


DEV2

To antisurge valve-2

DEV2

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

Selecting the section closest to SCL for parallel operation

Master Controller

Loop Decoupling

Loadsharing


( E

ffic

ien

cy )

Q

HP

user

0% 100%Compressor Capacity

Recycle or Blow-off point


Loadsharing


Other topicsOther topics

Load Balancing ResponcePressure Override Control (POC)Flow Measuring Devices (FMD’s)

Antisurge Control ValvePiping lay-out considerations

Dynamic simulation single compressorDynamic simulation parallel compressors


Compressor Control

Fall-back strategies


Increase compressor system reliabilityand 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

FB Strategies


• 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

Fall-back strategies for the antisurge and performance controller

FB Strategies


Loop Decoupling

FAMode

PI

Loop Decoupling

+

Analog Inputs

+

DEV


PID

Loadbalancing

PV

PV

SP

Primaryresponse

DEV DEV

DE

V


Primaryresponse

Average

SP

The load balancing response

Loadsharing Loadsharing ControllerController

Antisurge Antisurge ControllerController

Master Controller

FB Strategies

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

• 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

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

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

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

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

• 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

RT


The Pressure Override Control (POC) response

FB Strategies

• 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

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

• 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

• 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

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

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

Loop Decoupling

FAMode

PI

Loop Decoupling

+

Analog Inputs

+

DEV


PID

Loadbalancing

PV

PV

SP

Primaryresponse

DEV DEV

DE

V


Primaryresponse

Average

SP

Loadsharing Loadsharing ControllerController

Antisurge Antisurge ControllerController

Master Controller

RT

PI(One-Sided)

SP

PV

POC-SP

Rc

qqrr22

PIC-SP

Relief valve setting


• 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) selection criteria

FB Strategies


• 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)

Flow Measuring Device (FMD) location

FB Strategies


REMOVE

Remove Blow-OffValve and installa Blank Flange to

block old Blow-Off Line

Remove Check Valve

IR CENTAC

Install Flow

Element

Install Check Valve

To Header

Install Blow-off Valve and Positioner Here

VentSilencer

10 Dia. 5 Dia.

Adding Flow MeasurementAdding Flow Measurement

FB Strategies


• The speed of approaching surge is high• The transmitter type and brand should

be selected based on two major factors:– Reliability– Speed of response

• Desired rise time for p (flow) transmitters is 200 ms or less– Pressure step is 100%– The first order response (63%) is less

than 200 ms• Desired rise time for pressure

transmitters is 500 ms or less

Response time of the FMD transmitter

FB Strategies

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

100%

0

100%

0

100%

0

1 SEC.

Po A CDB

Pd A C D

B

Pc A C D

B

Time

Actual pressure

Transmitter output

63% response1- (1/e)

1 is less than 200 ms


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

• Damping the po (flow) transmitter destroys essential information

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

The effect of damping the po (flow) transmitter

FB Strategies

50

0-50

0

1.25 2.50 3.75 5Time (seconds)

FlowStart of Surge

Actual Flow

= 16.0 s

= 1.70 s

= 0.20 s = 0.03 s


• 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

• 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

QQvolvol

A

Sizing the antisurge control valve

FB Strategies


A

Rc

An alternative method yielding excellent results is:

• Take design point of the compressor point A

• Draw a horizontal line through the design point

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

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

Sizing the antisurge control valve - alternative method

FB Strategies

QQvolvol


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

Stroke speed and characteristic of the antisurge valve

FB Strategies


• 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

Improving the performance of the antisurge valve

FB Strategies


• Piping lay-out influences the controllability of the the total system

• The primary objective of the antisurge controller is to protect the compressor against surge

• This is achieved by lowering the resistance the compressor is feeling

• The resistance is lowered by opening the antisurge valve

• Dead-time and time-lag in the system needs to be minimized

• This is achieved by minimizing the volume between three flanges– Discharge flange of the

compressor– Recycle valve flange– Check valve flange

VSDS

Compressor 1volume to be

minimized

Piping lay-out consideration when designing an antisurge control system

FB Strategies


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

Using a single antisurge valve increases recycle lag time

FB Strategies


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

Sharing recycle coolers degrades surge protection

FB Strategies


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 them

• Result- Increased surge margins- Energy waste

Installing recycle valve upstream from

cooler improves control response

FB Strategies


• Compressor has ideal piping lay-out for surge protection

• Minimum volume between the three flanges for all sections

Recycle lines configured for optimum surge protection

Minimum volume

Section 2 Section 3Section 1

Process

Suction

FB Strategies


• Lay-out #1 has minimum volume between the flanges and is the best lay-out for antisurge control purposes

Section 2 Section 3Section 1

Process

Suction

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

• 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

• These two piping lay-outs are most common for antisurge control

• Lay-out #2 requires one cooler less and thus the capital investment is lower• Lay-out #2 will require bigger surge control margins

Which antisurge piping configuration do you choose???

FB Strategies


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


FB Strategies


Analog controller

SLLSCL

100%

0%

100%

0%

100%

Controlleroutput

Operatingpoint

Digital controller(2 executions per second)

Time

Time

Time

Time

• Compressor surged• Large process upset would have resulted

Analog vs. digital controller at 2 executions per second

FB Strategies

SLLSCL

0%

100%

0%Tuning same as analog controller


Controlleroutput

Operatingpoint

100%

0%

100%

0%

Time

Time

• Compressor surged• Large process upset would have resulted



FB Strategies

SLLSCL

Analog controller

SLLSCL

0%

100%



100%

100%

0%

100%

0%

Time

Time

• Compressor almost surged• Control system would have to be set up with

bigger surge control margin


Controlleroutput

Operatingpoint

FB Strategies

SLLSCL

Analog controller

SLLSCL

0%

100%




100%

0%

100%

0%

Time

Time

Controlleroutput

Operatingpoint

• Response of CCC digital controller nearly identical to the analog controller

• Adding specialized response algorithm to PI control will allow even smaller surge margins



FB Strategies

SLLSCL

Analog controller

SLLSCL

0%

100%



Process

HIC1

Load

1UIC

VSDS

Compressor 1

VSDS

Compressor 2

1

LSIC

2UIC

out

RSP

Serial network

out

RSP

2

LSIC

1

MPIC

Serial network

Serial network

Dynamic simulation: parallel compressors

FB Strategies

Suction


Simplified P&ID for Compressors Operating in Parallel

FB Strategies


Simplified Diagram of Parallel Load-Sharing Algorithms

FB Strategies


Simplified P&ID for Compressors Operating in Series

FB Strategies

The End


Simplified Diagram of Series Load-sharing Algorithms

FB Strategies

Documents

Load Sharing for Different Performance Curves