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Compressor ControlCompressor Control

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Types of CompressorTypes of Compressor

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100 MPa

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Positive Displacement Compressors

1 MPa

10 MPa(1,450 psia)

(14,500 psia)

Dis

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Data from CAGI

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Dynamic Compressors

1 m3/h(0.6 CFM)

10 m3/h(6 CFM)

100 m3/h(60 CFM)

1000 m3/h(600 CFM)

10,000 m3/h(6,000 CFM)

100,000 m3/h(60,000 CFM)

Flow (Approx Conversion)

(145 psia)

.1 MPa(14,5 psia)

D

Compressor PerformanceCompressor Performance

Dynamic -Rc

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Positive Displacement - Variable Pressure,

C t t Fl

Dynamic Variable Flow,

Constant Pressure

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Constant Flow

Constraint Control - Operating EnvelopeConstraint Control - Operating Envelope

Typical Performance Map

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Maximizing The Compressor Operating Envelope – Operating Limits

Maximizing The Compressor Operating Envelope – Operating Limits

Speed Limit (Maximum)

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Pr

Surge limit

p ( )

Stonewall orchoke limit

? ?

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Flow

Speed Limit (Minimum)

Compressor ControlCompressor Control

• Compressors are the control element for the process, so good control is needed for good product qualityC i h t f

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• Compression consumes huge amounts of energy so good control normally translates to energy and cost saving

• Control objectives* include:– Safety of personnel, process and machinery– Precise control of the primary control variable for

process control (normally pressure)

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p ( y p )– Operation within limits– Operation in automatic mode with no, or minimal,

operator intervention*All the above control objective points have both a price

tag, and a return on investment

Compression SystemsCompression Systems

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Typical Motor Driven Turbocompressor

Typical Motor Driven Turbocompressor

Section 1 Section 2

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Section 1outout

PIC1AUIC

1BUICSerial

network1A

Section 2

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Cost Points – Product Quality by Control of PPV

Cost Points – Product Quality by Control of PPV

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Section 1outout

PIC

Section 2

1AUIC

1AUICSerial

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Control of Primary Process Variable via Performance, and helped by Recycle when required

Cost Points – Tight Antisurge Control with Minimized Recycle

Cost Points – Tight Antisurge Control with Minimized Recycle

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Section 1outout

PIC

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1AUIC

1AUICSerial

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Antisurge Control via Recycle , and helped byPerformance when required

Cost PointsCost Points

Flare Control aided by Recycle

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Section 1outout

PIC

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1AUIC

1AUICSerial

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Cost PointsCost PointsContr

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1AUIC

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System Limits via recycle and/or speed

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ExamplesExamples

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Controllability ExamplesControllability ExamplesContr

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Increase Performance ExampleIncrease Performance Example

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Shaft power

qr2

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A leading oil company realized a 2% production increase afterCCC antisurge control allowed operation closer to the

Surge Limit Line

Energy Saving ExampleEnergy Saving ExampleContr

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The saving from this wet gas compressor installation:

First Section @ $ 174,008 per year + Second Section @ $ 167,049 per year

Total = $ 341,057 per year

Energy Saving Example – Air Compressor Networks

Energy Saving Example – Air Compressor Networks

Advanced ControlOriginal Controls

Compressor Flow

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User Flow

Saudi ExampleSaudi Example

With the old DCS antisurge system, the LP compressor was

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prunning with 20 to 30% recycle most of the time, now with CCC system installed and commissioned the valve fully closed

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LP: Normal Condition Winter: 1376 BHP, Normal Condition Summer: 1438 BHP.Assuming recycling is 20% in summer and 30% in winter, then Power Loss in Recycling (using 0.746 factor) = 261.Considering the following formula: 261 * 365 * 24 * 0.95 * 0.23 / 3.75 = USD 133,218 / Year is the Energy Loss Value.

D l i Al i h f D l i Al i h f

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Developing Algorithms for Compressor Control - a Urea Plant

Case Study

Developing Algorithms for Compressor Control - a Urea Plant

Case Study

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Developing Control Solutions?Developing Control Solutions?

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Developing Applications FunctionsDeveloping Applications Functions

• Phase 1 – see the need• Phase 2 – mathematical study• Phase 3 – computer simulation

Ph 4 fi ld t ti

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• Phase 4 – field testing

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Press

Different Inlet Conditions Means Different Performance Maps

Different Inlet Conditions Means Different Performance Maps

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Flow

Different Inlet Conditions Means Different Performance Maps

Different Inlet Conditions Means Different Performance Maps

Press

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Flow

The problem with commonly used (OEM provided)coordinate systems of the compressor map is that these coordinates are

NOT invariant to suction conditions as shown

Algorithm IssuesAlgorithm IssuesContr

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The surge limit thus becomes a surface rather than a line

Algorithm IssuesAlgorithm Issues

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

Fundamental variables

• The following variables are used to design and to characterize compressors• Through dimensional analysis (or similitude) we can derive two sets of invariant coordinates

Invariant coordinates

Advanced Control - Developing Invariant Coordinates

Advanced Control - Developing Invariant Coordinates

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characterizing compressor operation

Hp = f0(Q, ω, μ, ρ, a, d, α)

J = f1(Q, ω, μ, ρ, a, d, α)

where:H P l t i h d

Dimensional analysisor Similitude

Set 1hrqrNeαjr

Re

Set 2RcqrNeαjr

Re

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where:• hr = Reduced head• qr = Reduced flow• Ne = Equivalent speed• α = Guide vane angle• jr = Reduced power• Re = Reynolds number• Rc = Pressure Ratio

NOT invariant coordinates (Hp, Qs)

Algorithm IssuesAlgorithm IssuesInvariant Coordinates

(hr, qr2)

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where:• Hp = Polytropic head• Qs = Volumetric suction flow• hr = Reduced head• qr

2 = Reduced flow squared

NOT invariant coordinates (Rc, Qs)

Algorithm IssuesAlgorithm Issues

Invariant coordinates(Rc, qr

2)

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where:• Rc = Pressure ratio• Qs = Volumetric suction flow• qr

2 = Reduced flow squared

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Urea Plant CO2 CompressorsUrea Plant CO2 Compressors

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CO2 CompressorsCO2 CompressorsContr

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CO2 Installation FormatsCO2 Installation Formats

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CO2 Compressor ConfigurationCO2 Compressor Configuration

Section 3 Section 4Speed

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Section 1 Section 2

Section 3 Section 4Speed Increasing

Gear

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Adding Start-Up Vent LinesAdding Start-Up Vent Lines

Section 3 Section 4Speed

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Section 1 Section 2

Section 3 Section 4Speed Increasing

Gear

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One Recycle Valve CaseOne Recycle Valve Case

Section 3 Section 4Speed

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Section 1 Section 2

Section 3 Section 4Speed Increasing

Gear

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Two Recycle Valve CaseTwo Recycle Valve Case

Section 3 Section 4Speed

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Section 1 Section 2

Section 3 Section 4Speed Increasing

Gear

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CCC ExperienceCCC Experience

• 1980s - retrofit of two valve installations in USA/Canada/Caribbean– No great difficulty found

• 1990s - new single valve installations in India– Flow element only on the third section– train in manual speed control

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p– questionable valve sizing– minimum budget allocated– use of a single antisurge controller monitoring the third section– the machine went into uncontrollable surge during surge testing– opening the valve had no effect– closing the valve seemed to take the machine out of surge

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One Recycle Valve CaseOne Recycle Valve Case

Section 3 Section 4Speed

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Section 1 Section 2

Section 3 Section 4Speed Increasing

Gear

1PT

2PT

1FT

1UIC

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Uncontrollable Surge - One Antisurge Controller Monitoring 3rd Section

Uncontrollable Surge - One Antisurge Controller Monitoring 3rd Section

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Why?Why?

• CCC developed a High Fidelity Dynamic Simulation of the installation using actual installation data and Compressor Maker’s Curves– stage mismatch was found - where the 1st section

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stage mismatch was found where the 1st section could be in surge while the 4th in stonewall

– the valve on its own is not sufficient to prevent surge during large upsets - using a larger valve did nothing to improve the situation

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What is the Solution?What is the Solution?

• We found that using two valves, 4/3 and 2/1 would eliminates the problem

• For existing installations the only method available to have effective antisurge control was to incorporate

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have effective antisurge control was to incorporate performance control with variable speed, and employ heavy decoupling

• Note: if the machine is at full speed when a large disturbance hits, then decoupling will not help

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DecouplingDecoupling

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Interaction of Control LoopsInteraction of Control Loops

M tM t

ControlControl

AntisurgeAntisurge

MeasurementMeasurement

ControlControl

Recycle ValveRecycle Valve

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Process VariableProcess Variable

MeasurementMeasurement SpeedSpeed

CompressionCompressionSystemSystem

1PIC

SG

CompressorST Driver

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2UIC

Rc

Non-Integrated Performance and Antisurge Loops

Non-Integrated Performance and Antisurge Loops

We are operating at point AWe are operating at point ALarge disturbance occursLarge disturbance occursThe operating point rides the curveThe operating point rides the curve

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A PIC-SP

B

The operating point rides the curveThe operating point rides the curveto point Bto point BThe Performance controller is takingThe Performance controller is takingthe operating point down in valve positionthe operating point down in valve positionand thus down in flow and thus down in flow -- the tangent ofthe tangent ofthat trajectory is thus (shown)that trajectory is thus (shown)That means that the operating pointThat means that the operating pointmust use a large control bias must use a large control bias t id d th t bilit id d th t bili

C

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Q

Integration of Control LoopsIntegration of Control Loops

SG

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1PIC

2UIC

CompressorST Driver

Peer-to-PeerSerial Communication

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Rc

Integration/Decoupling of Antisurge and Capacity Control

Integration/Decoupling of Antisurge and Capacity Control

We are operating at point AWe are operating at point ALarge disturbance occursLarge disturbance occursThe operating point rides the curve The operating point rides the curve to point Bto point B

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PIC-SPAC

B

This time we ‘decouple’ the action This time we ‘decouple’ the action of the performance controllerof the performance controllerThe antisurge controller tells the The antisurge controller tells the performance controller to speed up performance controller to speed up the compressor (or open the the compressor (or open the Guide vanes or suction throttle valve)Guide vanes or suction throttle valve)The action of the performance The action of the performance controller is increasing speed controller is increasing speed (or opening guide vanes or inlet valve)(or opening guide vanes or inlet valve)

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ΔPoPs

( p g g )( p g g )and increasing flow, and the resulting and increasing flow, and the resulting Tangent is as follows (shown)Tangent is as follows (shown)This results in a stabilization This results in a stabilization action as shownaction as shown

The result is an action requiring The result is an action requiring only a small margin of safetyonly a small margin of safety

Next Problem UncoveredNext Problem Uncovered

CompressorHP Section LP Section

Mechanical Governor with Ratio from Throttle Valve to Extraction Valve

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ProcessSuction

V1

V2

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ExtractionSteam Header

Bypass valve

Adding Electronic GovernorAdding Electronic Governor

Compressor

Steam turbine

SE3x

HP Section LP Section

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1UIC

1FT

1PsT

ProcessSuction

2PdT

1SIC

V1

RSP

2XIC

2FT

2PT

V2

3PdT

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Serial Serial networknetwork

OUTExtractionSteam Header

Solving the Steam Bypass Problem

PIC1

Full Control SolutionFull Control SolutionContr

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From an article was published in a Chinese trade magazine, "Automation In The Petrochemical Industry", Vol.5, 1998. The author is the Director of Instrumentation of Chishui Natural Gas Chemical Corp. (CCC retrofitted the CO2 compressor trains in

Comment from Chinese Customer After Retrofit

Comment from Chinese Customer After Retrofit

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p ( pJune 96).

• When the system load is reduced and the compressor must recycle, the CCC system can reduce the recycle flow to 3000 Standard Cubic Meters per Hour (SCMH) compared with manual open loop control before the retrofit.

• There were many times the inlet flow of CO2 dropped below 14,000 SCMH due to a non-turbomachinery related process reason, such as a problem at the CO2 removal section of ammonia unit, where the CCC system respond quickly and thus

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C ammonia unit, where the CCC system respond quickly and thus avoided surge, and kept the compressor train on line. Before the retrofit, the pneumatic system would definitely trip the machine in such event, and in the worst case the machine would be damaged due to surge.

Comment from Chinese Customer After Retrofit

Comment from Chinese Customer After Retrofit

With CCC TTC system, the direct energy saving was calculated. It was 6.4 Million RMB (US$790,000) saving per year at the same number of tons of urea produced per year before the retrofit. All investment for the retrofit

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per year before the retrofit. All investment for the retrofit including cost of equipment, freight cost, customs duty, taxes, installation etc.,) was recovered less than 10 months.

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Data from the Equipment Superintendent of Daqing Petrochemical Company:

• They have been operating at full load for 18 months since the retrofit• The compressor train never trips in the event of the process sudden

changes that would have tripped the unit before

Comment from Chinese Customer After Retrofit

Comment from Chinese Customer After Retrofit

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changes that would have tripped the unit before• The start up of the turbine is simple and easy now, compared with

the old system • The improved speed/extraction control by the CCC TTC system has

reduced overall steam consumption due to the elimination of supplemental steam expansion through the steam let-down valve (constant let-down was necessary with the old PGPL governing system as the extraction control was poor). On average, the steam consumption is 5 metric tons per hour less than before. The result of saving was 2 million RMB (US$246,000) per year (the plant buys

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C of saving was 2 million RMB (US$246,000) per year (the plant buys the steam from an outside CHP plant at 50 RMB per ton)

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Future DirectionFuture Direction

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Turbomachinery Controls EmulationTurbomachinery Controls Emulation

Dedicated PC

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Unique PC

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Compressor Performance MonitoringCompressor Performance MonitoringContr

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Q&AQ&A

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