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OptimizingTurbomachinery

ControlsSymposium

H o u s t o n , TX June 2013

OptimizingTurbomachinery

ControlsSymposium

H o u s t o n , TX June 2013



A g e n d a – D ay 2 A g e n d a – D ay 2

•Steam Turbine Control

– Turbine Start-up & Shutdown Automation

– Speed & Extraction Control Techniques and Demo

•Application Examples

– LNG Liquefaction – Refrigeration Compressors

– NGL Fractionation Facilities & Bayu Undan Example

– Ammonia-Urea Unit Applications

• Process Control vs. Safety Shutdown Systems

– System Availability and Fault Tolerance

– API Standards Update

•Tips for Specification Writing



St e am Tu r b i n e s

St e am Tu r b i n e s



Ch a l l e n g e s o f

S t e am Tu r b i n e Co n t r o l



•Prevent unnecessary process

trips and down time

•Minimize the effect and duration

of process disturbances

• Avoid overspeed and other

machine related trips

1 . Re l i a b i l i t y

2 . Ef f i c i e n t

O p e r a t i o n

3 . S y s t e m

I n t e g r a t i o n







•Operate at efficient energylevels

• Accurate speed

measurement

• Consistent and accurate

valve positioning



O p e r a t i o n

3 . S y s t e m








• Speed and extraction

control

•Startup and shutdown

automation in concert with

driven equipment



O p e r a t i o n

3 . S y s t e m




St e am Tu r b i n e St a r t u p &Sh u t d o w nS e q u e n c i n g

St e am Tu r b i n e St a r t u p &Sh u t d o w nS e q u e n c i n g



Time

RPMV1

Bearing Lube Oil Shaft

High

friction

Low

friction

B r e a k A w a y ca nb e Ex t r e m e l y Fa s t

B r e a k A w a y ca nb e Ex t r e m e l y Fa s t



Time

RPM

V1

RPM-SP

Benefits• Reduced overshoot during breakaway of

turbine• Less mechanical stress on cold machine• Reliable and repeatable start up

B r e a k A w a y Co n t r o lP r e v e n t s M a ch i n e D am a g e

B r e a k A w a y Co n t r o lP r e v e n t s M a ch i n e D am a g e



Cr i t i ca l Sp e e d s Cr i t i ca l Sp e e d s

•Critical speed is a speed at which the

turbomachinery train vibrates at a harmonic or

resonant frequency

• Peaks of multiple oscillating waves “add”

creating constructive interference



Cr i t i ca l Sp e e d s Cr i t i ca l Sp e e d s

• Most turbomachinery trains have at least one

and often multiple critical speeds

• Operating the turbomachinery train at a critical

speed for an extended period of time can result

in severe damage

• Critical speeds are typically below rated speed



Time

RPM-SP

RPM

V1

Ncritical,low

Ncritical,high

Critical Speed Range

Cr i t i c a l Sp e e d A v o i d a n ce Cr i t i c a l Sp e e d A v o i d a n ce

• Critical speed range low and high values areconfigured

• RPM-SP cannot be set in this range

• As soon as RPM-SP goes above Ncritical,low thecontroller ramps RPM-SP to Ncritical.high based onconfigurable ramp rate

Rated Speed



Time

RPM-SP

RPM

t1

Ncritical,low

Ncritical,high

V1

Time

0%

100%


A v o i d i n g Cr i t i c a l Sp e e d D am a g e ( La ck o f St e a m )

A v o i d i n g Cr i t i c a l Sp e e d D am a g e ( La ck o f St e a m )

• RPM-SP is ramped thru Ncritical,high

• Controller opens V1 to accelerate turbine to Ncritical,high

• With V1 100% open machine does not reach Ncritical,high withinpredetermined time t1 due to lack of steam pressure and/or flow

• Controller ramps down RPM-SP to Ncritical,low

• Turbine decelerates to Ncritical,low

Machine damage is avoided



Time

RPM-SP

RPM

t1

Ncritical,low

Ncritical,high

V1 Position

Time

0%

100%


A v o i d i n g Cr i t i c a l Sp e e d D am a g e ( S t i c k y V a l v e )

A v o i d i n g Cr i t i c a l Sp e e d D am a g e ( S t i c k y V a l v e )

• RPM-SP is ramped down thru Ncritical,high

• Controller closes V1 to decelerate turbine to Ncritical,low

• Turbine does not reach Ncritical,low within predetermined time t1,because of a problem with V1

• Controller ramps RPM-SP to Ncritical,high

• Turbine accelerates to Ncritical,high

Machine damage is avoided

V1 Output



Time

RPM OEM start-up diagram

Idle 1

Start-up

time 1

Idle 2

Start-up

time 2

• OEM provides start-up schedules for steamturbine

• Machine needs to be kept for certain period on

given speed• Typically there are 1 or 2 idle speeds

• After start-up the machine can be loaded

To rated speed

St a r t - U p Sch e d u l e sf o r St e am Tu r b i n e s St a r t - U p Sch e d u l e sf o r St e am Tu r b i n e s



• Speed controller automatically ramps

turbine to and between Idle speeds and to

minimum governor setting

• Machine accelerates or decelerates at

configurable ramp rates based on how long

the turbine has been down

– Normally there are (2) sets of ramp rates, one for a

“hot” turbine and one for a “cold” turbine

• Ramps can be aborted and resumed at any

time

A u t o m a t i c Co n t r o lo f I d l e Sp e e d s

A u t o m a t i c Co n t r o lo f I d l e Sp e e d s



A d v a n c e d A u t o m a t i c Co n t r o l A d v a n c e d A u t o m a t i c Co n t r o l

•“Warm Start-up”

•Automatically brings turbine to rated speed

using calculated delays and ramp rates

– Based upon the case temperature, hot and cold

startup ratio, and idle time of the turbine

•Start permissive inputs

– Checked both at start and during sequencing

•Configurable ramp rates



W a r m St a r t - u p Se q u e n c i n g W a r m St a r t - u p Se q u e n c i n g

Hot Start-up

Idle 1

RPM

Time

To idle 2

Slope Sh

tidle, h

Cold Start-up

Idle 1

RPM

Time

To idle 2

Slope Sc

tidle, c

Idle 1

Warm Start-upRPM

Time

To idle 2

Slope Sw

tidle, w

Ramp Rate: Sc ≤ Sw ≤ Sh

Idle Time: tidle, h ≤ tidle, w ≤ tidle, c



St a r t - U p Se q u e n c i n g St a r t - U p Se q u e n c i n g



Sp e e d Lo o p

T u n i n gT e c h n i q u e s

Sp e e d Lo o p

T u n i n gT e c h n i q u e s



• At constant speed the power consumed by the load is

equal to the power delivered by the steam turbine

• Speed modulation is used to compensate for changes in

load, however:

• The objective of steam turbine control is to adjust the

delivered power to match the current load

• Optimized loop tuning should take into account the

relationship between rotational speed & delivered power

Power

Consumption

Power

Delivery

At constant speed

=

Co n t r o l l i n g Po w e r v s . Sp e e d Co n t r o l l i n g Po w e r v s . Sp e e d



Fa n La w s Fa n La w s



Speed

Power

•Power is a function of rotational-speed

3

•Speed control only indirectly controls power

•Constant loop tuning can work marginally well

between minimum and maximum governor

•The same tuning does not work well below

minimum governor during start-up & shutdown

M axi m um

G ov er n or

Mi ni m um

G ov er n or

105%70%

Po w e r = f ( N 3 ) Po w e r = f ( N 3 )



Speed

Power

Digital speed controllers introduced piecewise

characterizers for gain adjustment outside the

normal governing range of operation

Ga i n Ch a n g e s a s a Fu n c t i o n o f Sp e ed

Ga i n Ch a n g e s a s a Fu n c t i o n o f Sp e ed



Loop gain in CCC speed controllers is as a function of the

speed/power relationship over the complete speed range

Gain characterization

function

Linear power gain

for complete

speed range

V a r i a b l e Ga i n i n Tu r b i n e Sp e e d Co n t r o l l e r s

V a r i a b l e Ga i n i n Tu r b i n e Sp e e d Co n t r o l l e r s

Speed

Power



B e n e f i t s o f V a r i a b l e Ga i n B e n e f i t s o f V a r i a b l e Ga i n

• Allows for responsive

tuning in all speed

ranges

• Provides more accurate

speed control and more

reliable speed limiting

•Good control at low

speeds is required to

allow for fully automatic

startup

Gain

characterization

function

Linear power gain

for completespeed range

Speed

Power



I n t e g r a t e d A n t i s u r g e Co n t r o l a n dTu r b i n e Sp e e d Co n t r o l S t a r t - u p

I n t e g r a t e d A n t i s u r g e Co n t r o l a n dTu r b i n e Sp e e d Co n t r o l S t a r t - u p

Qs, vol

Rc

•Turbine starts from zero

speed and ramps to

minimum speed with the

recycle valve fully open

•The recycle valve starts to

ramp closed & performance

control is switched to auto

• Should the OP touch the

surge control line, the a/s

controller overrides the

valve ramping as needed

•The turbine is brought to

normal speed safely



St e amE x t r a c t i o n

C o n t r o l

S t e amE x t r a c t i o n

C o n t r o l



Ex t r a c t i o n St e am Tu r b i n e Ex t r a c t i o n St e am Tu r b i n e



V1 V2

LOAD

HP horsepower LP horsepower

Total developed

horsepower

HP section

LOAD

Total consumed

horsepower

LP section

Ex t r a c t i o n Tu r b i n e H o r s e p o w e r Re la t i o n s h i p s

Ex t r a c t i o n Tu r b i n e H o r s e p o w e r Re la t i o n s h i p s



When load increases, V

1

opens to supply additional power

This causes the extraction flow to increase and V

2

will need to

open to maintain constant extraction

V1 V2

LOAD

V a l v e I n t e r a c t i o n Lo a d Ch a n g e

V a l v e I n t e r a c t i o n Lo a d Ch a n g e

V l I t t iV l I t t i



When extraction demand increases, V2 closes to supply additional

extraction steam reducing steam to the LP turbine

Total power produced to drive the load drops and V1 needs to open

to maintain constant rotational speed

V1 V2

LOAD

V a l v e I n t e r a c t i o n St e am D em a n d Ch a n g e

V a l v e I n t e r a c t i o n St e am D em a n d Ch a n g e



HorsepowerDelivered to Load

InletSteamFlow

Minimum

level ofextraction

Horsepowerlimit

V1 V2

LOAD

Qin

Qextract Qexhaust

Stable

zone ofoperation

Maximumlevel of

extraction

Minimumlevel ofexhaust

flow

Inlet Steamflow limit

Ex t r a c t i o n M a p Ex t r a c t i o n M a p

Maximumlevel ofexhaustflow

S d d E t t i C t lS d d E t t i C t l



horsepower

Inlet steam

flow

LOAD

A

B

C

D

Sp e e d a n d Ex t r a c t i o n Co n t r o lL o o p I n t e r a c t i o n s

Sp e e d a n d Ex t r a c t i o n Co n t r o lL o o p I n t e r a c t i o n s

I t t i S d & E t C t lI t t i S d & E t C t l



Inlet steam

flow

V1

V2

A

B

horsepower

I n t e g r a t i n g Sp e e d & Ex t . Co n t r o lLo a d Ch a n g e

I n t e g r a t i n g Sp e e d & Ex t . Co n t r o lLo a d Ch a n g e

SIC

1PID

XIC

1

SE3X

FT

1 PID

X

I n t e g r a t i n g Sp e e d & E t Co n t r o lI n t e g r a t i n g Sp e e d & E t Co n t r o l



Inlet steam

flow

A

B

horsepower

I n t e g r a t i n g Sp e e d & Ex t . Co n t r o lEx t r a c t i o n D em a n d Ch a n g e

I n t e g r a t i n g Sp e e d & Ex t . Co n t r o lEx t r a c t i o n D em a n d Ch a n g e

V1

V2

SIC

1PID

XIC

1

SE

3X

FT

1 PID

X

D y n am i c Sim u l a t i o n : Ex t r a c t i o nD y n am i c Sim u l a t i o n : Ex t r a c t i o n



• Compressor is controlled on Discharge Pressure by PF-1

• SC-1 controls rotational speed

• EX-1 controls turbine extraction flow or pressure

D y n am i c Sim u l a t i o n : Ex t r a c t i o nSt e a m Tu r b i n e

D y n am i c Sim u l a t i o n : Ex t r a c t i o nSt e a m Tu r b i n e



T u r b i n e

O v e r s p e e dP r o t e c t i o n

T u r b i n e

O v e r s p e e dP r o t e c t i o n



Ov e r s p e e d I s s u e 1 Ov e r s p e e d I s s u e 1




AP I / I SO Go v e r n i n g a n dA P I / I SO Go v e r n i n g a n d



AP I / I SO Go v e r n i n g a n dP r o t e c t i o n Sp e e d Re q u i r e m e n t s

A P I / I SO Go v e r n i n g a n dP r o t e c t i o n Sp e e d Re q u i r e m e n t s

• Maximum Temporary Overshoot Speed

– 127%

•Over-speed Trip Speed

– 116%

•Max Allowable Speed Rise per NEMA D

– 112%

• Maximum Continuous Operating Speed

– 105%

• Rated Operating Speed

– 100%

D i sa b l e d K n i f eD i sa b l e d K n i f e



D i sa b l e d K n i f eEd g e T r i p Sy s t em

D i sa b l e d K n i f eEd g e T r i p Sy s t em

Machine Vibration Causes

Mechanical Overspeed Trip Finger

(Knife Edge Latch) to Let Loose and

Cause Nuisance Trips of TurbineUnder Normal Running Conditions

Mechanical Overspeed TripFinger (Moves to Left on

Overspeed)

Knife Edge Latch(Unlatches on

Overspeed)

Trip Valve Actuation

Lever (Moves DownUpon Trip)

Bricks and Metal Placed to Avoid

Nuisance Trips due to Unlatched Knife

Edge During Standard Operation

(Dangerous Should Overspeed Need

to Trip Turbine)

Trip Valve Located

within Box

Ov e r s p e e d P r o t e c t i o nOv e r s p e e d P r o t e c t i o n



Ov e r s p e e d P r o t e c t i o n Go v e r n o r v s . OST

Ov e r s p e e d P r o t e c t i o n Go v e r n o r v s . OST

•Governor is the first line of defense for

preventing over speed

•Governor electronic trip acts as a backup to the

primary overspeed trip device

– If the turbine speed exceeds the trip speed, the

governor will initiate a trip

• Closes the governor valves

• Initiates a trip of the turbine via T&T valve

• Primary overspeed trip system

– Mechanical over speed trip system

– Electronic over speed trip system

D i i t l O d P t t iD i i t l O d P t t i



• Turbomachinery losses among the highest paid by

insurers

•Overspeed wreck represents one the most catastrophic

accidents:

– Endangers personnel

– Damages the turbomachinery train

– Can cause damage to other plant equipment

–Can result in costly interruptions of process

D i g i t a l Ov e r s p e e d P r o t e c t i o n D i g i t a l Ov e r s p e e d P r o t e c t i o n

•Mechanical overspeed trip

systems are non–redundant,

require overspeed testing via

actual turbine run-up, are

imprecise & unreliable

S d f R i C i t i lS d f R i C i t i l



Sp e e d o f Re s p o n s e i s Cr i t i c a l Sp e e d o f Re s p o n s e i s Cr i t i c a l

• Steam turbines can accelerate extremely quickly during

process upsets

•Major upsets include:

– Surge on the driven compressor

– Breaker trip on the generator

– Fast power reduction on the local grid

•Traditional speed control can be too slow to catch these

type of disturbances

•Results:

– Unnecessarily large process disturbance

– Machine & process shutdown due to over-speed

– Potential machine damage

St T b i R t D iS t T b i R t D i



Rotor time constant:

where:

N

R

Rated speed (RPM)

WR

2

Rotor inertia (lbs-ft

2

)

hp Rated horsepower

rated

rated rotor c

hp

WR N T

6

22

,

10

619.

Tc,rotor = The time it would take an

instantaneous load loss to cause a

doubling of rotor speed when starting

from rated hp & rated speed

St e am Tu r b i n e Ro t o r D y n a m i c s St e am Tu r b i n e Ro t o r D y n a m i c s

St e am Tu r b i n eSt e am Tu r b i n e



•Turbine speed will be 27,000 rpm after 2.25 seconds

•Overspeed trip settings (116% rated) will be reached in

337 ms

•Overspeed trip system needs to react in 225 ms to

prevent speed from exceeding 127%* level

* Maximum Temporary Overshoot Speed

T N WR

hpc rotor

rated

rated

,

.

619

10

2 2

6

T c rotor ,

. ,

,

619 13 500 50

10 2 500

2 2

6

Recycle compressor data:

NR Rated speed (RPM) 13,500

WR2 Rotor inertia (lbs-ft2) 50

hp Rated horsepower 2,500

T c rotor , . 2 25seconds

Steam turbine driven

recycle compressor

example:

Ro t o r D y n a m i c s Ro t o r D y n a m i c s

Th e Ov e r s p e e dTh e Ov e r s p e e d



A v o i d a n ce A l g o r i t h m A v o i d a n ce A l g o r i t h m

• Rapid load drop causes turbine to

accelerate rapidly

• Conventional PID control starts to

close the V1 valve

• Operating point hits overspeed

avoidance line

•Open loop response rapidly closes

the valve to avoid overspeed

• Speed drops below maximum

governor

• PID control brings speed back to set

point

MaximumGovernor

OverspeedAvoidance

ElectronicOverspeed

Time

RPM-SP

RPM

V1

Time

Op e n Lo o pOp e n Lo o p



Co n t r o l La c k s A c cu r a c y Co n t r o l La c k s A c cu r a c y

•A fixed step change will either be too small or too big

for a specific disturbance

– Too small may not protect the machine

– Too large may cause an unnecessary loss of speed and

process disturbance

•The rate of change in speed (

N/

t) can be used to

estimate the extent of the load loss

– Calculates the appropriate size of the step change to be

implemented

I m p r o v i n g t h e A c cu r a cy o f t h eI m p r o v i n g t h e A c cu r a cy o f t h e



•System adapts to the size of the disturbance

•Bigger disturbances provoke faster closing of the valve

Time

RPM

V1

Time

RPM

V1

Overspeed

Avoidance

Medium disturbance Large disturbance

St e p Ch a n g e St e p Ch a n g e

Step = a configurable constant x

N

t

Ov e r s p e e d A v o i d a n ce A l g o r i t h mOv e r s p e e d A v o i d a n ce A l g o r i t h m



Ov e r s p e e d A v o i d a n ce A l g o r i t h m Ov e r s p e e d A v o i d a n ce A l g o r i t h m

Benefits:

•Overspeed can be avoided for virtually

any disturbance

•Fewer overspeed incidents increase

machine life

• Process is kept on line



A p p l i c a t i o n

Ex am p l e s

A p p l i c a t i o n

Ex am p l e s



LN G LN G

LNG Fa c t s LNG Fa c t s



G ac t sG a c t s

• Approx. 95% Methane

•Cooled to - 260° F (-161° C)

• 1/600

th

of original gas volume

•http://www.youtube.com/watch?v=Ft1rHNXZozY

I n s t a l l a t i o n sI n s t a l l a t i o n s



I n s t a l l a t i o n s I n s t a l l a t i o n s

Country Plant No. of

Trains

Capacity

(MTPA)

Start year Process

Abu Dhabi DasIsland 3 5.7 1977/1994 APCI

Australia

WoodsideLNG 5 16.3 1989-2008 APCI

Australia WoodsideLNG

(Pluto)

1 4.8 2010 APCI

Indonesia BontangI-VI 6 21.8 1977-2000 APCI

Malaysia

Bintulu(MLNG)Satu,

Dua,Tiga

8 22.7 1995-2003 APCI

Nigeria

BonnyIsland(NLNG) 6 21.9 1999-2007 APCI

Oman OmanLNG 3 10.3 2000-2006 APCI

Qatar QatarGasJVI-IV 7 40.9 1996-2010 APCI

Qatar

RasGasJVI-III 7 36.3 1999-2009 APCI

Egypt Segas/UnionFenosa 1 5.0 2005 APCI

Trinidad AtlanticLNG1-4 4 14.8 1999-2005 ConocoPhillips

Egypt

EgyptLNG 2 7.2 2005-06 ConocoPhillips

Australia

DarwinLNG 1 3.5 2006 ConocoPhillips

Equatorial

Guinea

EGLNG/Marathon 1 3.4 2007 ConocoPhillips

Norway

Snohvit(Statoil) 1 4.2 2007 Linde

Russia

SakhalinLNG 2 9.6 2009 ShellPMR

Peru

CamiseaLNG 1 4.4 2010 APCI

Angola AngolaLNG 1 5.2 2009 ConocoPhillips

Indonesia

TangguhLNG1&2 2 7.6 2011 APCI

Algeria SkikdaLNG 1 4.5 2011 APCI

Australia GladstoneLNG 2 7.8 2013 ConocoPhillips

Australia GorgonLNG 3 15 2014 APCI

Papua New

Guinea

PNGLNG/EOMJV 2 6.6 2013 APCI

Australia

Queensland 2 8.5 2013 ConocoPhillips

Algeria

GassiTouilLNG 1 4.7 2013 APCI

Australia

WheatstoneLNG 1 5.0 2014 ConocoPhillips

Australia IchthysLNG 2 6.0 2015 APCI

USA CheniereSabinePass

LNG

2 9.0 2016 ConocoPhillips

CCC Total Capacity: 312.7 MTPA

Global LNG Production

Capacity (Existing &

Under Construction):

334.9 MTPA (From IGU World Report 2011)

93.4%

LN G A p p l i c a t i o n s LN G A p p l i c a t i o n s



p pp p

•Refrigeration Compressors

– Propane

– Ethylene

– Methane

– MR

• Boil-off Compressors

•Auxiliary Compressors: –

Feed Gas Compressor

– Expander Re-compressor

– Propane BOG Compressor

– Fractionation Compressor

– End Flash Gas

– Fuel Gas

LN G Eco n om i c s 1 0 1 LN G Eco n om i c s 1 0 1



• US Natural Gas Supply Price:

$ 3.49 per mmBTU

•Delivered Price (Japan):

$16.00 per mmBTU

• Spread: $12.51 per mmBTU

Revenue of 4 MTPA LNG Facility:

$6,753,055 per DAY

APCI M CR® P r o c e ss A PCI M CR® P r o c e ss



Ph i l l i p s O p t im i z e d Ca s ca d ePh i l l i p s O p t im i z e d Ca s ca d e



p pp p

Pr i c o P r o c e ss / S i n g l e Cy c l e - M R P r i c o P r o c e ss / S i n g l e Cy c l e - M R



g yg y

L i n d e N 2 Re f r i g e r a t i o n P r o ce s s L i n d e N 2 Re f r i g e r a t i o n P r o ce s s



A i r P r o d u c t s N 2 Re f r i g P r o c e ss A i r P r o d u c t s N 2 Re f r i g P r o c e ss



A i r P r o d u c t s / M o d e cL i B r o TM Pr e - Co o l ed N 2 P r o c e ss

A i r P r o d u c t s / M o d e cL i B r o TM Pr e - Co o l ed N 2 P r o c e ss



A i r P r o d u c t s D u a l M R P r o c e ss A i r P r o d u c t s D u a l M R P r o c e ss



SBM ’ s P r o p o s e d Ta n k e r M o d i f i c a t i o nf o r FLN G

SBM ’ s P r o p o s e d Ta n k e r M o d i f i c a t i o nf o r FLN G



APCI M CR® P r o c e ss A PCI M CR® P r o c e ss



• Uses propane for pre-cooling and a mixed refrigerant (nitrogen, methane,ethane, propane) for liquefaction and sub-cooling

• Pre-cooling is done in kettle-type exchangers while liquefaction and sub-cooling are done in proprietary spiral wound heat exchanger i.e. the main

cryogenic heat exchanger

• 61 trains in operation + 5 in construction

APCI AP - X LNG P r o ce s s APCI AP - X LNG P r o ce s s



Pr o p a n e & Sp l i t M R Com p r e ss o r s P r o p a n e & Sp l i t M R Com p r e ss o r s



Propane Compressor – Primary Objectives:• Sidestream flows pose challenge to antisurge control design

• Flow calculation is critical

• Antisurge to antisurge decouplingPropane Compressor - Secondary Objective:• Suction pressure limiting using the antisurge valve

Propane - Other Comments:• Suction conditions change continuously

• GT is maintained at a constant speed

• Compressor performance usually adjusted by recycle only

MR

3

MR

2

MR

1

Propane

P r o p a n e Com p r e ss o rCa p a c i t y Co n t r o l Ch a l l e n g e s

P r o p a n e Com p r e ss o rCa p a c i t y Co n t r o l Ch a l l e n g e s



• Main control variable is suction pressure at 1st stage drum• Pressures at intermediate propane drums tied to 1st stage drum

• 1st stage suction must be maintained above atmospheric pressure at all times• Ineffective suction pressure control results in lower refrigeration which must

then be compensated by MR cycle• Control loop interactions sacrifice production



Pr o p a n e & Sp l i t M R Com p r e ss o r s P r o p a n e & Sp l i t M R Com p r e ss o r s



MR Compressor – Primary Objectives:

• Antisurge to Antisurge decoupling

• Use of the surge control surface for IGV control

(variable gas composition & temperature)• Communication between trains on shutdown or trip

MR Other Comments:

• GT is typically maintained at a constant speed

• The cooling load can be varied by the IGV’s or the JT

valves across the MCHE

MR

3

MR

2

MR

1

Propane

Fe e d Ga s Com p r e s s o r Fe e d Ga s Com p r e s s o r



Primary Objectives:• Surge control under all operating scenarios

• Separate antisurge application for dedicated surgedetection

• Suction pressure control

Secondary Objective:• High discharge pressure limitingComments:

• Hp and Flow are very large• Suction Temperature and MW can vary greatly on

some applications

• 20-30% load changes are common!

VSDS

B o i l - O f f Ga s ( BOG) Com p r e s s o r s B o i l - O f f Ga s ( BOG) Com p r e s s o r s



• BOG Compressor: Primary Control Objectives:

−Antisurge Control & Loadsharing Control• BOG Compressor: Secondary Objectives

− P.O.C. On Suction Pressure• Comments:

− A very tight control margin− 1.02 psig (28.25” H2O): too low (air could leak in / safety hazard)

−1.06 psig (83” H2O): too high (flare trigger)

BOGHeader

LNG Tanker

Fuel Gas

To Flare

Co n o c o P h i l l i p s O p t im i z e d Ca s ca d eCo n o c o P h i l l i p s O p t im i z e d Ca s ca d e



• Uses three pure refrigerants (propane, ethylene, methane) for cooling andliquefaction

• Pre-cooling sometimes carried out in core-in-kettle type exchanger

• Plate fin heat exchangers (non-propriety) in vertical cold boxes used

LN G P l a n t O p e r a t i n g M o d e s LN G P l a n t O p e r a t i n g M o d e s



1) START-UP:

Cold Start:

• MCHE must be cooled to desired temperature at specified rate

• Long start-up duration

Warm Start:

• Machines operate at minimum governor speed and full recycle

• MCHE relatively cold

• Shorter startup duration

2) PART-LOAD:

• Plant operate at reduced production

• Due to economic factors, tanker delays, upstream gas plant trips,

machinery or process related problems

•Compressor capacities reduced to match reduced refrigeration

load

• Compressors may operate with recycle valves partially open

• Process may be unstable due to control loop interactions

LN G P l a n t O p e r a t i n g M o d e s LN G P l a n t O p e r a t i n g M o d e s



3) FULL-LOAD:

• Plant operated at greater than design capacity

• Maximize production by running machines at maximum power

•Product temperatures within tight margins by adjusting

refrigeration-cycle duty

• Desired flow rate also maintained

4) Defrost Operation• Defrost operation happens 2 to 3 times a year for about 1 to 2

days

• Transitions from defrost to C3 gas (unit does not shutdown)

5) SHUTDOWN:

• Unloaded in controlled fashion during normal shutdown

• Emergency conditions require safe shutdown of machines

M a j o r Ch a l l e n g e s i nLN G Sy s t em D e s i g n M a j o r Ch a l l e n g e s i nLN G Sy s t em D e s i g n



Avoidance of Cascading Trips

on Interdependent

Turbomachinery

I n t r o d u c t i o n I n t r o d u c t i o n



The two main refrigeration compressor

strings at Tangguh LNG are highly

dependent on each other during operationA

cascading

trip can happen within a few

seconds

This case study focuses on how to keep

either string online when the other trips

Avoid surging the compressor

Avoid excessive recycle that can overload the

drivers

Ov e r v i ew o f M a i n LNGRe f r i g e r a t i o n Com p r e sso r s

Ov e r v i ew o f M a i n LNGRe f r i g e r a t i o n Com p r e sso r s



Propane circuit cools the MR circuit and Feed

Gas4 stage compressor with sidestreams

Driven by Frame 7 GT with ST helper

MR circuit cools Natural Gas in MCHE to

produce LNG

3 stage compressor with MR HP stage on PR drive

train

Driven by Frame 7 GT with ST helper

LN G M a i n Re f r i g e r a t i o nCom p r e ss o r s

LN G M a i n Re f r i g e r a t i o nCom p r e ss o r s



LN G

HPLLP LP MP HP

PROPAN E COM PRESSOR

ASV

LP MP

ASV ASV

MR COM PRESSOR

N G

Com p r e s so r I n t e r d e p e n d e n c y Com p r e s so r I n t e r d e p e n d e n c y



Trip of the MR circuit

Loss of MR flow to the propane chillers will lead to

the PR flow (vapor production) gradually

decreasing in a relatively short time and

eventually dropping off

Sudden loss of flow through the MR HP

compressor due to MR MP discharge check valve

closing

Trip of the PR circuit

A trip of MR HP ASV results in sudden loss of flow

through the MR LP/MP stages due to closure of

MP discharge check valve

Ov e r v i e w o f I n i t i a l D e s i g n Ov e r v i e w o f I n i t i a l D e s i g n



The original control system design was

based on lessons learned from a similar

LNG plant design

FFC by unloading the online compressor

when the other compressor trips

Temporarily initiate the antisurge controllers’ Stop

sequence to ramp open the ASVs

Duration based on the Stop ramp rate and desired

ASV target opening position

Additional IGV or speed control adjustments were

not necessary

I n i t i a l Co n f i g u r a t i o n Se t t i n g s I n i t i a l Co n f i g u r a t i o n Se t t i n g s



Mixed Refrigerant Compressor Propane Compressor

LPStage MPStage HPStage LLP

Stage

LPStage MPStage HPStage

Propane

UnitTrip

Ramp

15%/s for

3sec

Ramp

15%/s for

3sec

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Mixed

Refrigerant

Unittrip

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Ramp

8%/sfor5s

Ramp

5%/sfor

10s

Ramp

5%/sfor

10s

Ramp

5%/sfor

10s

Ramp

5%/sfor

10s

Re v i e w a n d A n a l y s i s o f Ev e n t s Re v i e w a n d A n a l y s i s o f Ev e n t s



MR HP ASV was recorded going 100% open

after FFC

Cause: Controller’s open loop line crossed

causing it to step open output to 100% and switch

to Shutdown

PR string tripped on underspeed 11 seconds

after FFC

Cause: PR HP ASV was manually opened at 55%

at the time of FFC signal resulting in the ASV

going to 100% open and GT high power limit being

reached

MR string trips 7 seconds after FFC signal

Cause: MP stage surge trip

Re su l t s o f A n a l y s i s Re su l t s o f A n a l y s i s



Ramp rates in the MR

ASC need to be

increased

ASV target positions

need to be adjusted

Ramp ASV to a fixed

target position and not

a fixed amount

ASC needs to remain

active during FFC

Time

Stop

Ramp

k(%/s)

FFC

release

d

FFC

initiated

O u t p u t

Duration

t(s)

Out_ final

Out_ inital

L im i t a t i o n s o f I n i t i a l D e s i g n L im i t a t i o n s o f I n i t i a l D e s i g n



Standard features of ASC

Stop mode

Maximum Stop ramp rate is

16.7%/s

When the operating point

crosses the controller’s open

loop line, the controller

immediately steps open the

ASV and goes into Shutdown

state

SLL = Surge Limit Line

OLL= Open Loop Line

SCL = Surge Control

Line

Rc

Q2

OP

The antisurge controller’s Surge Counter/Trip functions

are not active during Stop/Shutdown state

A c t i o n s Ta k e n A c t i o n s Ta k e n



Propose ASC Software

Modification

Separate Unload signal

Configurable ramp rate to

99.9%/s (LVL6)

Configurable ramp target

(LVL7)

Configurable hold timer (LVL8)

Allow ASC to override Unload

sequence

Output goes to 100% if open

loop line crossed put remain in

Run state

Time

LVL6

Unload

signal

O u t p u t

LVL

7

LVL

8

A c t i o n s Ta k e n - V e r i f i c a t i o n A c t i o n s Ta k e n - V e r i f i c a t i o n



Run dynamic simulation

Verify increased ramp rates and ASV target openings

for MR compressor

Simulate both design and off design conditions

Verify GT power stays within acceptable limits

Site acceptance test

Verify new controller software functionality

Verify logic used to activate the Unload signal

N e w Co n f i g u r a t i o n Se t t i n g s N e w Co n f i g u r a t i o n Se t t i n g s



Mixed Refrigerant Compressor Propane Compressor

LPStage MPStage HPStage LLP

Stage

LPStage MPStage HPStage

Propane

UnitTrip

Ramp

50%/sto

50%open

for30s

Ramp

60%/sto

60%open

for30s

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Mixed

Refrigerant

UnitTrip

Trip,valve

stepsopen

to100%

Trip,valve

stepsopen

to100%

Ramp

80%/sto

70%open

for30s

Ramp

5%/sto

50%open

for30s

Ramp

5%/sto

50%open

for30s

Ramp

5%/sto

50%open

for30s

Ramp

5%/sto

40%open

for30s

MR LP trip initiated MR MP trip initiated

T r e n d Re su l t s f r o m Fi e l d T r e n d Re su l t s f r o m Fi e l d



MR LP trip initiated MR MP trip initiated

MR HP FFC initiated

MR

Trip

70%Open

MR

Trip

MR

Trip

PR LLP FFC PR LP FFC initiated

T r e n d Re su l t s f r o m Fi e l d T r e n d Re su l t s f r o m Fi e l d



PR LLP FFCinitiated

PR LP FFC initiated

PR MP FFC initiated PR HP FFC initiated

MR

Trip

MR

TripMR

Trip

MR

Trip

50%

Open

50%

Open

70%

Open

40%Open

Co n c l u s i o n s C o n c l u s i o n s



No reports of cascading trips since

modification

Additional benefits of software

modification

Changes allow for a clearer understanding of

the control system response after an event

More flexibility in configuration changes

Ramp rates and target levels can be changed

independently

Settings can be easily changed on line

RV1

Tem p e r a t u r e ( Qu e n c h ) Co n t r o l Tem p e r a t u r e ( Qu e n c h ) Co n t r o l



Stage I Stage II

1

TV1

Flow

Stage I

TE1 WithoutDecoupling

With

Decoupling

SCL

RV1 Opening was

required to prevent

Excursion on Stage I

Opening of TV1 is required tocompensate for Opening of RV1

From

Compressor

Discharge

RV1

UIC1

From Liquid

Refrigerant

Storage

TV1

TIC1

Antisurge

Controller Temperature

(Quench)

Controller

TE1

k+

+

K1

P r e s s u r e

Enthalpy

Const. Temperature Lines

Vapor

Liquid

and

Vapor

Mix

Operation of the

Quench Controller is

allowed only to the rightof the Calculated Low

SP Clamp

Set Point Limiting

to prevent energy waste

Liquid




Quench Control Summary

– Quench Temperature set point is f(P

sat

) + offset

– Operator set point interface

• Temperature offset from saturation curve

• Direct temperature set point entry with saturation curve set point

clamp

– High degree of coupling between quench and

antisurge

• Temperature loop responds slowly

• Antisurge reacts quickly

•Loop decoupling for optimal response

– Start/Stop sequencing coordinated through antisurge

controller

Th k f t i & t t t i !T h k f t i & t t t i !



Th a n k s f o r y o u r t i m e & a t t e n t i o n ! Th a n k s f o r y o u r t i m e & a t t e n t i o n !

N a t u r a l Ga s L i q u i d s & Fr a c . U n i t s N a t u r a l Ga s L i q u i d s & Fr a c . U n i t s



NGL Rem o v a l NGL Rem o v a l

Three methods of condensate removal:



Three methods of condensate removal:

• Refrigeration to remove heavy hydrocarbons• Adsorption using chemical agent that has affinity for

NGL’s such as lean oils• Cryogenic expansion using turboexpanders

NGL Rem o v a l b y Cr y o g e n i c Ex p a n s i o n NGL Rem o v a l b y Cr y o g e n i c Ex p a n s i o n



• Designed to recover ethane (C2) and heavier hydrocarbons (C3, C4, etc) from thenatural gas stream

• The objective is to separate more expensive products and to send methane (C1)into the pipeline.• Expander drops temperature of gas stream causing partial liquefaction of heavier

components• Demethanizer separates methane from NGL

D em e t h a n i z e r Ex a m p l e D em e t h a n i z e r Ex a m p l e



Pr o p a n e Re f r i g a t i o n Co m p r e sso r s P r o p a n e Re f r i g a t i o n Co m p r e sso r s

3 4 Se c t i o n P r o p a n e Re f r i g . Com p r e s so r s A p p l i c a t i o n Ch a l l e n g e

3 4 Se c t i o n P r o p a n e Re f r i g . Com p r e s so r s A p p l i c a t i o n Ch a l l e n g e



•Poor piping lay-out design (Common suction

drums)

•Common antisurge and quench valves

• Need for Automatic startup and SD

• Existing piping layout much less than optimum

for surge control and protection•Quench Temperature setpoint characterizer

4 Se c t i o n P r o p a n e Re f r i g .Com p r e ss o r s ( P l a n t A )

4 Se c t i o n P r o p a n e Re f r i g .Com p r e ss o r s ( P l a n t A )



3 4 Se c t i o n P r o p a n e Re f r i g . Com p r e s so r s Qu e n c h co n t r o l Se t p o i n t Ch a r a c t e r i z e r

3 4 Se c t i o n P r o p a n e Re f r i g . Com p r e s so r s Qu e n c h co n t r o l Se t p o i n t Ch a r a c t e r i z e r



Com m i s s i o n i n g Fi n d i n g s /Re co m m e n d a t i o n s

Co m m i s s i o n i n g Fi n d i n g s /Re co m m e n d a t i o n s



•Added excessive surge trip

•Added SD feature to Quench controllers when all

units are SD to minimize startup time preventing

high suction drums level

3 4 Se c t i o n P r o p a n e Re f r i g .Com p r e sso r s ( P l a n t B )

3 4 Se c t i o n P r o p a n e Re f r i g .Com p r e sso r s ( P l a n t B )



P r o p a n e Re f r i g . Com p r e s so r s P r o p a n e Re f r i g . Com p r e s so r s



Re s i d u e Ga s Com p r e s s o r s Re s i d u e Ga s Com p r e s s o r s



Tu r b o - Ex p a n d e r Re - Com p r e ss o r s Tu r b o - Ex p a n d e r Re - Com p r e ss o r s

•Turboexpanders began being used in gas processing

plants around 1960

Ov e r v i e w o f Tu r b o e x p a n d e r s Ov e r v i e w o f Tu r b o e x p a n d e r s



•Presently, most gas processing plants use

turboexpanders

• A turboexpander recovers useful work from the

expansion of the gas

•Turboexpander are designed to recover ethane (C2) and

heavier hydrocarbons (C3, C4, etc.) from the natural gas

streams. The objective is to separate more expensive

products and to send methane (C1) into the pipeline.

• In the process of producing work, a TX lowers the gas

stream temperature. This results in partial liquefication of

the gas stream.

Tu r b o e x p a n d e r D e s i g n Tu r b o e x p a n d e r D e s i g n



• Throughput of the expander part of the train is controlled by

Inlet Guide Vanes

• Throughput of the recompressor is typically not controlled

•Turboexpander trains are equipped with a compressor

recycle valve that can be used for surge control and

protection

• Turboexpander trains are equipped by an expander bypass

valve (J-T or Joule-Thompson valve)

• Turboexpander trains are either loaded to maximum capacity

or are operating at set flow rate

T r a d i t i o n a l Tu r b o e x p a n d e rCo n t r o l Sy s t e m D e s i g n

T r a d i t i o n a l Tu r b o e x p a n d e rCo n t r o l Sy s t e m D e s i g n

•Speed of the turboexpander typically is not



controlled

• During upset condition speed exceeds allowable

maximum

•Overspeed trip prevention is typically primitive

•Trip is prevented by one of:

– 1) limiting opening of IGV by position of IGV;

– 2) limiting dP across the expander; or

– 3) limiting speed via IGV and J-T valve in split level

fashion

• Overspeed prevention by “brake control”

A d v a n c e d Tu r b o e x p a n d e r Co n t r o l A d v a n c e d Tu r b o e x p a n d e r Co n t r o l



• JT Valve Prepositioning

• Adequate antisurge control for recompressor

• Loadsharing Control for parallel T-E trains

“ B r a k e Co n t r o l ” f o r O v e r s p e e d P r e v e n t i o n “ B r a k e Co n t r o l ” f o r O v e r s p e e d P r e v e n t i o n



If speed exceeds allowable maximum:

• first, open expander compressor’s recycle valve to load up the train

• second, at slightly higher set point start closing IGV.

• J-T valve is used only when IGV is controlling speed or when IGV is100% open

Results in increased condensate production.

JT V a l v e P r e - P o s i t i o n i n g JT V a l v e P r e - P o s i t i o n i n g

•To reduce severity of tripping on the Feed Gas pressure,

it is implemented a special algorithm that “pre-positions”



the JT valve

•Calculations are done to open the value that provides JT

valve capacity equivalent to the Turbo-expander’s

throughput prior to its trip

•Position of the JT valve is a function of the IGV of the

expander

•Inlet flow of the expander relates to equivalent JT-valve

stroke thus the initial output of the JT Controller•A 10-point characterizer, whose function argument is the

IGV position, and the function result is the required

equivalent JT valve initial opening value



M u l t i - Com p r e s s o rA p p l i c a t i o n

M u l t i - Com p r e s s o rA p p l i c a t i o n

Ca s e St u d y : B a y u - U n d a n O f f sh o r e P l a t f o r m

Ca s e St u d y : B a y u - U n d a n O f f sh o r e P l a t f o r m



• The platform processes over 1.1 billion ft

3

/day wet gas

• Extraction of over 115,000 bpd condensate, propane, butane

and produces over 950 MMSCFD dry natural gas• Phase 1 achieving production in 2004 involved wet gas

processing and dry gas reinjection

• Phase 2 achieving production in 2006 involved exporting dry gas

to Darwin LNG

Pr o ce s s O v e r v i e w P r o ce s s O v e r v i e w



Fl a s h Ga s I TCS Fl a s h Ga s I TCS

• Provide invariant



antisurge control for

each stage

• Optimize 1

st

stage and

inter-stage pressure

control

• Equidistant to surge

loadsharing

•Decoupling between

antisurge and

performance control

loops

• Decoupling between

antisurge control loops

• Limiting loops

Tu r b o e x p a n d e r I TCS Tu r b o e x p a n d e r I TCS

• Maintain production

separator pressure



• Provide invariant antisurge

control for the re-

compressor

•Expander overspeed

prevention control

• “Brake control” for over-

speed prevention

• JT Valve Prepositioning

• Optimized loadsharing

strategy

• Decoupling between

antisurge and performance

control loops

• Limiting loops

Re i n j e c t i o n / Ex p o r t Op e r a t i n gM o d e s

Re i n j e c t i o n / Ex p o r t Op e r a t i n gM o d e s

SLL SCLPd Stage 3 (Export)Operating Point



• Total 6 operating modes depending on export gas requirements and state

of upstream cold process trains

•With one or more expanders down, gas off spec for export• Operating modes clearly indicated need for antichoke control

• Operating modes translated to 3 defined compressor control modes

• Switching compressors from one mode to another needed to be bumpless

with minimal upset to the process

190 barg

Inlet Vol. Flow

CCL

CLL

8212 rpm

Stage 1Operating Point

Re i n j e c t i o n / Ex p o r t I TCS Re i n j e c t i o n / Ex p o r t I TCS

• Provide invariant antisurge



control for

each stage

• Utilize “shared valve”

control strategy for

antisurge control

• Maintain suction pressure

• Provide integrated anti-

choke control

• Optimized loadsharing

control

•Decoupling antisurge and

performance control loops

• Limiting loops

M u l t i - Co m p r e s s o r I n t e g r a t i o n M u l t i - Co m p r e s s o r I n t e g r a t i o n



Pr o p y l e n e L o a d i n g Com p r e ss o r P r o p y l e n e L o a d i n g Com p r e ss o r



FCCU

Tu r b o m a ch i n e r y

C o n t r o lO p t im i z a t i o n

FCCU Pr o c e s s Co n t r o l Ch a l l e n g e s FCCU Pr o c e s s Co n t r o l Ch a l l e n g e s



W e t Ga s Com p r e ss o r Co n t r o l W e t Ga s Com p r e ss o r Co n t r o l



• Maintains pressure in overhead accumulator

• Output of PIC-1 is setpoint for SIC-1

• UIC-1 & UIC-2 protect compressor sections from surge

•Challenges: Gas composition variations, inherently

interactive recycle valves & speed control loop

• Flare-less startup is desirable

FCCU W e t Ga s Com p r e s so r So u t h Am e r i c a n R e f i n e r y

FCCU W e t Ga s Com p r e s so r So u t h Am e r i c a n R e f i n e r y



Co n t r o l I s s u e s Co n t r o l I s s u e s

• FCCU Wet Gas Compressor operating at



constant speed and continuous recycle

•Compressor recycle used for suction pressure

control

•Suction pressure setpoint higher than desired

for optimized pressure in the reactor overhead

receiver



2 n d S t a g e

300 HP



Wprocess = 53.37 T/hr

St e amT u r b i n e

Ex t r a c t i o n M a p



W e t Ga s Com p / Re d u ce d Re cy c l e W e t Ga s Com p / Re d u ce d Re cy c l e

•Approximate HP requirements for the current average

process flow:

–



Stage 1: 2800 HP

– Stage 2: 2800 HP

– Total: 5600 HP or 4176 kW

•The estimated power requirements with improved

surge control margin & resulting reduction in recycle:

– Stage 1: 2540 HP

– Stage 2: 2500 HP

– Total: 5040 HP or 3758 kW

• Total projected power savings:

– HP: 2800 – 2540 = 260 HP

– HP: 2800 – 2500 = 300 HP

– HP: 260 HP + 300 HP = 560 HP

W e t Ga s Com p / Re d u ce d Re cy c l e W e t Ga s Com p / Re d u ce d Re cy c l e

•Convert: Horse Power to kW, 1kW = 1.34 HP results in:

hp = 418 kW

•



•Estimated Energy Savings per Annum

– Steam Cost: $5/ton

– Using steam curves supplied, assuming constant slope

extraction lines

•From extraction map, 418 kW equates to 4T/hour of

steam

•Therefore, energy savings equal:

4.0 T/hour x $5/ton x 8760 hours/year=

$175,200/year

Pr o d u c t i o n I n c r e a se P r o d u c t i o n I n c r e a se

•When blower limited, a reduction in accumulator

pressure creates a potential for increasing production



•At constant discharge pressure, lowering the suction

pressure increases the required compression ratio

•Reduced surge control margin results in the ability to

meet the higher compression ratio needed

•From the 1st stage compressor map, the maximum

achievable compression ratio with the existing surge

control line is 3.33

•The new surge control line allows the operation at a

compression ratio of 3.63 at the same speed of rotation

Pr o d u c t i o n I n c r e a se P r o d u c t i o n I n c r e a se

• Ps, min gauge = 1.69 – 1.01325 = 0.68

•



Potential set point reduction

– 0.89869 – 0.68 = 0.22 kg/cm2

•Lowering the pressure in the fractionator

column:

– Reduces resistance on the regenerator air blower

– Increases mass flow of air

Po w e r Re co v e r y T r a i n ( PRT )C o n f i g u r a t i o n

Po w e r Re co v e r y T r a i n ( PRT )C o n f i g u r a t i o n

Steam

TurbineMain Air

Blower

Hot Gas

Expander



R e g e n e r a t o rA i r B l o w e r Co n t r o l

R e g e n e r a t o rA i r B l o w e r Co n t r o l

•Mass-flow control via adjustable stator blades

• UIC protects compressor from surge

• FIC and UIC are de-coupled to avoid interaction



during low load conditions and disturbances

• Challenge: requires responsive & stable

process control

Orifice

Chamber

3rd stage

Separators

Flue gas

Cooler

Regenerator Reactor

Stripper

Ex p a n d e r Co n t r o l Ex p a n d e r Co n t r o l

Limit control

PICDPIC

1



Mode selector

SIC

1

11

HSS

1

Hot Gas

Expander

“Soft Selector”

• Control Elements

– Expander inlet valve

Ex p a n d e r Co n t r o l Ex p a n d e r Co n t r o l



– Expander bypass valve

• High Speed Control Loops

– Reactor/regenerator differential pressure control

– Regenerator pressure limiting

– Speed control

– Power & speed limiting control (as required)

– Breaker trip calculations & open-loop response for

high speed load-shedding

Ge n e r a t o r B r e a k e r - T r i p P r o b l em Ge n e r a t o r B r e a k e r - T r i p P r o b l em

•20 MW is going to drive the regenerator air blower

7 MW7 MW+27 MW



•Breaker trip results in loss of speed synchronization and a

virtually instantaneous drop in load

• Conventional systems rely on PID control to control speed

• PID control is often times too slow to catch disturbance

• The expander can trip (in a matter of seconds) on:

– Overspeed

– Other trip settings

Generator Breaker

Hot Gas

Expander

Po w e r Sw a p p i n g Re q u i r em e n t Po w e r Sw a p p i n g Re q u i r em e n t

•Before breaker opening there was power

balance

• 27 MW is coming from the regenerator

+27 MW

•

27 MW

O



•Generator breaker opening causes a

load drop of 7 MW

Hot Gas

Expander

7 MW

• With bypass closed, 27 MW is going to

the expander & 20 MW to the blower

+27 MW

• Control objectives upon breaker opening

are:

– Keep reactor/regenerator P constant

– Avoid overspeed trip

• After breaker opening 7 MW needs to be

shed through bypass valve to achieve

control objectives

20 MW

7 MW

• This is achieved by simultaneous:

– closing of the inlet valve

– opening of the bypass valve

Close

Open

0 MW

CCC PRT Co n t r o l So l u t i o n CCC PRT Co n t r o l So l u t i o n

•In order to perform all functions the

following measurements are necessary:

– Breaker status

–Open

p0

T0

p2

T2



Inlet and bypass valve positions

– Power export from the generator

– Rotational speed, P

1

, T

1

, P

2

, T

2

– Reactor-Regenerator differential pressure

• The PRT system should continuously

calculate required valve step-changes in

anticipation of a breaker trip

• Upon breaker opening, the control system

should:

– Initiate speed control via inlet valve control

– Re-direct differential pressure control to

bypass valve

– Initiate open-loop closure of inlet valve

– Initiate open-loop opening of bypass valve

Hot Gas

Expander

Generator Breaker Status

Close

Open

p1

T1

JT

Ca l c u l a t i n g t h e O p e n - L o o p St e p sf o r t h e I n l e t & B y p a ss V a l v e s


•Accurate step changes are critical

– Speed synchronization to the electrical grid is lost

– Changes in rotational speed of the regenerator air blower will



result in an upset in critical air flow to the process

• Air flow provides carbon burning & lift to the catalyst bed

• Pressure swings can result in catalyst flow reversals and catalyst

releases in some instances

• PRT and process trip can occur (often part of SSD system design)

•The size of the step change is a function of the:

– Amount of power being exported to the electrical grid

– Temperature & mass flow of the hot gas to the expander

– Expander characteristics as defined by the expander map

– Inlet and bypass valve characteristics

Re d u c i n g t h e Ex p a n d e r M a p sw i t h D im e n s i o n a l A n a l y s i s

Re d u c i n g t h e Ex p a n d e r M a p sw i t h D im e n s i o n a l A n a l y s i s

Reduced Power vs. Reduced Flowower vs. Mass Flow



j J p ZRT

r

1 1

Reduced Power

q w ZRT p

p p

r o

1

1

1

1

,

Reduced Flow

•Monitor expander mass flow, valve positions, & power export• Use simplified expander maps to calculate required reduction in

expander flow related to current power being exported

• Calculate valve Cv for corresponding mass flow to be shed





• Use valve characteristic curve to determine % of valve

movement needed

Current C

v

0

20

60

80

100

120

10 20 30 40 50 60 70 80 90 100 % maximumvalve opening

% m a x i m

u m C v

Current valve

position

New valve

position

40

c

v

Re sp o n se f r o m a Co n v e n t i o n a l Sy s t e m Re sp o n se f r o m a Co n v e n t i o n a l Sy s t e m

Breaker Disconnect while Generating 5 MW



Breaker opens hereExport Power

Rx-Rg ∆P

Breaker

Speed

Overspeed and Trip

Re sp o n se w i t h CCC I n t e g r a t e d Sy s t e m Re sp o n se w i t h CCC I n t e g r a t e d Sy s t e m

Breaker Disconnect while Generating 5 MW



Speed

Breaker opens here

Rx-Rg ∆P

Breaker

Export Power

Minimal Speed Excursion

Minimal Disturbance to the Regenerator


5

Location of Quench Control Line

2-phaseregion

li id i

gasregion



0.05

0.5

0 100 200 300 400 500 600 700 800

P r e s s u r e ( M P a )

Enthalpy kJ/kg

liquidregion

QuenchControlLine


• Quench Control Summary

– Quench Temperature set point is f(P

sat

) + offset

– Operator set point interface



• Temperature offset from saturation curve

• Direct temperature set point entry with saturation curve set point clamp

– High degree of coupling between quench and antisurge

• Temperature loop responds slowly

• Antisurge reacts quickly

•Loop decoupling for optimal response

– Start/Stop sequencing coordinated through antisurge controller

P r o ce s s Co n t r o l

P r o ce s s Co n t r o l



P r o ce s s Co n t r o lA v a i l a b i l i t y a n d

Sa f e t y Sh u t d o w nSy s t em s

P r o ce s s Co n t r o lA v a i l a b i l i t y a n d

Sa f e t y Sh u t d o w nSy s t em s

D i f f e r e n t P l a t f o r m s f o r SI S &C o n t r o l

D i f f e r e n t P l a t f o r m s f o r SI S &C o n t r o l

“Regardless

of

the vendors providing the

hardware and software,

how

important

is

it

for

your facility to have your

Safety Instrumented

System (SIS) hardware on



System (SIS) hardware on

a different physical

(hardware)

platform

from your Control

System?” (N=75)

While

the

majority

of

respondents

say

that

having

their

SIS

hardware on a

different

physical

platform

from

their

Control

System,

this

is

even more

pronounced among

Chemical

facilities

(83%)

when

compared

to

Upstream oil & gas

facilities (57%).

F u n c t i o n a l Sa f e t y F u n c t i o n a l Sa f e t y

Safety Integrity Level



D e f i n i t i o n s D e f i n i t i o n s

SIS



D e f i n i t i o n s D e f i n i t i o n s

SIF

Safety Instrumented Function:



A SIF is an instrument safety loop that performs

a safety function which provides a defined level

of protection (SIL) against a specific hazard by

automatic means and which brings the process

to a safe state.



Co n t r o l Sy s t emA v a i l a b i l i t y

Co n t r o l Sy s t emA v a i l a b i l i t y

Sy s t em A v a i l a b i l i t y A n a l y s i s Sy s t em A v a i l a b i l i t y A n a l y s i s

•Most system availability comparisons have:

– Focused on “The Box”, not on the entire system

–



Used oversimplified models

– Used a safety system mindset, focusing only on

dangerous failures, and not on the total failure rate of

devices and the system

– Ignored controller diagnostics, common-cause

failures, and other important considerations

•This approach is too simplistic and leads to

invalid conclusions

B a se Co n t r o l l e r A v a i l a b i l i t y B a se Co n t r o l l e r A v a i l a b i l i t y

•Failure Rate: 8 Failures / 10

6

Hours

•Controller Self-Diagnostic Coverage: 90%

•Mean-Time-To-Repair (MTTR): 8 Hours



•Test Interval: 1 Year

•Common Cause: 2% of Failures

Resulting Controller Availability:

Topology Availability(Percent)

MTTF(Years)

Annual Downtime(Hours)

2-1-0 Duplex 0.9999922551 117.9139 0.0678

3-2-1-0 Triplex 0.9999924552 121.0418 0.06613-2-0 Triplex 0.9999916598 109.4978 0.0731

Sy s t e m B o u n d a r i e s Sy s t e m B o u n d a r i e s

The Controller is Not the Whole System

• Field devices have a huge impact on system

availability, and must be considered



Sensors

Final Elements

Controllers

Ex am p l e A n t i s u r g e Sy s t e m Ex am p l e A n t i s u r g e Sy s t e m

•Typical complement of transmitters

• I/P transducer

• Air-actuated antisurge valve



FT

1

TsT

1

PsT

1

FY1

PdT

1

TdT

1

UIC1

Com p l e t e Sy s t e m A v a i l a b i l i t y Com p l e t e Sy s t e m A v a i l a b i l i t y

•Field Device Failure Rates

*

– Temperature Transmitters: 31.9 Years

– Pressure Transmitters: 28.8 Years

– Flow Transmitter: 16.2 Years

–



I/P Transducer: 15.0 Years

– Air Actuated Globe Valve: 20.6 Years

Resulting Control System Availability:

Topology Availability

(Percent)

MTTF

(Years)

Annual Downtime

(Hours)

2-1-0 Duplex 0.9997100180 3.1484 2.5402

3-2-1-0 Triplex 0.9997102181 3.1506 2.5385

3-2-0 Triplex 0.9997094229 3.1419 2.5455

*Failure Rate Data From ISA TR84.0.02 and Exida

• There is no appreciable difference betweentopologies once field devices are included

I m p r o v i n g Sy s t e m A v a i l a b i l i t y I m p r o v i n g Sy s t e m A v a i l a b i l i t y

•Since using a triplex controller does not improve

system availability, what does?



•Various techniques are used to increase

controller and system availability:

– Improved Diagnostics

– Redundant Sensors (Transmitters)

– Fallback Strategies

– Redundant Output Transducers (I/P)

– Partial-Stroke Valve Testing



Co n t r o l l e r D i a g n o s t i c s Co n t r o l l e r D i a g n o s t i c s

Resulting

Control System

Availability

(includingfieldinstruments):

Diagnostic Annual



ControllerTopology

Coverage(Percent)

Availability(Percent)

MTTF(Years)

Downtime(Hours)

90 0.9997100180 3.1484 2.5402

95 0.9997132403 3.1838 2.51202-1-0 Duplex

99 0.9997158258 3.2128 2.489490 0.9997102181 3.1506 2.5385

95 0.9997133500 3.1850 2.51113-2-1-0 Triplex

99 0.9997158552 3.2131 2.4891

900.9997094229 3.1419 2.545595 0.9997129289 3.1803 2.51473-2-0 Triplex

99 0.9997157498 3.2119 2.4900

A d d i n g Fa l l b a c k St r a t e g i e s A d d i n g Fa l l b a c k St r a t e g i e s

•Statistically over 75% of control loop problems

originate from field devices

•Algorithms designed to provide continued

operation in the event of sensor failures



– Software redundancy for sensors

– Cost-effective alternative to redundant sensor

elements

Resulting Control System Availability:

Topology Availability(Percent)

MTTF(Years)

Annual Downtime(Hours)

2-1-0 Duplex 0.9998306765 5.3926 1.4833

3-2-1-0 Triplex 0.9998308766 5.3989 1.48153-2-0 Triplex 0.9998300813 5.3737 1.4885

A u t o m a t e d Fa l l b a ck St r a t e g i e sA u t o m a t e d Fa l l b a ck St r a t e g i e s

•System monitors transmitter & MPU validity

•Multiple fallback strategies should be

configurable to handle transmitter



failures/problems

• Mode switching should be handled in a

bumpless fashion

Benefits

– Nuisance machine/unit trip avoidance

– Latent failure alarms give time to correct

– Increased machine & process availability

Pa r t i a l - S t r o k e V a l v e Te s t i n g Pa r t i a l - S t r o k e V a l v e Te s t i n g

•Position feedback from valve is compared to the

controller output, any significant deviation indicates a

problem

•Frequent testing as compared to demand rate is



necessary to achieve maximum

availability improvement

•Valve should be

stroked at least

15%

• Coordination is

required to

prevent process

upsets while

testing

3.00

3.25

3.50

3.75

4.00

4.25

4.50

4.75

5.00

Daily Weekly Month ly Quarterly Annually

Test Interval

M T T F i n Y e a r s

Daily Dem and Weekly Monthly

Quarterly Annually

Su m m a r y o f D a t a Su m m a r y o f D a t a

•There

is no

significant system availability difference

between topologies once field devices are included

•Control system availability

is

greatly affected by issues

related to field devices



System AvailabilityImprovement Technique

2-1-0 DuplexMTTF (Years)

3-2-1-0 TriplexMTTF (Years)

3-2-0 TriplexMTTF (Years)

None (Base Figure, Controllers Only) 117.9139 121.0418 109.4978

None (Base Figure, Complete System) 3.1484 3.1506 3.1419

Improved Diagnostics (99%) 3.2128 3.2131 3.2119

1:1 Redundant Sensors 6.7262 6.8518 6.4075

Parallel Redundant Sensors (Duplex) 7.9197 7.9335 7.8014

Fallback Strategies 5.3926 5.3989 5.3737

High-Reliability Output Transducers 3.8324 3.8356 3.8228

Redundant Output Transducers 3.2795 3.2818 3.2725

Automated Final-Element Testing(Daily Test with Annual Demand)

4.9329 4.9382 4.9171

AP I 6 7 0

AP I 6 7 0



Su r g e D e t e c t i o n

a n d Co n t r o l

Su r g e D e t e c t i o n

a n d Co n t r o l

Go v e r n i n g S t a n d a r d s &R e l a t i o n s h i p s

Go v e r n i n g S t a n d a r d s &R e l a t i o n s h i p s

IEC 61508

IEC 61511:

Process Safety



IEC 61508:Functional Safety

of Electronic

Systems

IEC 62061:

Machinery Safety

(Machinery directive)

API 670:

Machinery Protection System

Su r g e D e t e c t i o n v s .A n t i s u r g e Co n t r o l

Su r g e D e t e c t i o n v s .A n t i s u r g e Co n t r o l

Definitions When does it take Action?

Surge control is typically



defined as a method to

prevent a compressor from

surge

Before a Surge Cycle

Surge detection is a method

that affirms surge has

occurred

After a Surge Cycle is initiated

Su r g e D e t e c t i o n Su r g e D e t e c t i o n

Purpose:

– Detect and count surge cycles

– Provide output for use in minimizing the number of

surge cycles and output to ESD or DCS



Su r g e D e t e c t i o n M e t h o d s Su r g e D e t e c t i o n M e t h o d s

TEMPERATURE

TIME (sec )

1 2 3

PRESSURE

TIME (sec )

1 2 3

FLOW

TIME (sec )

1 2 3



• Surge Detection components

– Sensors

– Logic solver

– Sequencer

• Based on field proven methods

– Flow

– Pressure – Temperature

– Combination of above

TIME (sec.)TIME (sec.)TIME (sec.)

Pr o t e c t i o n Com p o n e n t s P r o t e c t i o n Com p o n e n t s



I n t e g r a t e d P r o t e c t i o n Sy s t e m I n t e g r a t e d P r o t e c t i o n Sy s t e m



D i s t r i b u t e d P r o t e c t i o n Sy s t e m D i s t r i b u t e d P r o t e c t i o n Sy s t e m



Pr o ce s s Sa f e t y D e s i g n P r o ce s s Sa f e t y D e s i g n

•HSE Study of 34 Industrial Accidents

•Most Common Cause: Specification Errors

Design and

Implementation

Operation and

Maintenance15%



15%15%

Installation andCommissioning

6%

Specification

44%

Changes After

Commissioning

21%

Sp e c i f i c a t i o n W r i t i n g Sp e c i f i c a t i o n W r i t i n g

•You may end up with only what you’ve

specified, so review, update, and customize

•Don’t just focus on the control hardware

•



•Specify :

– Overall system performance goals & criteria

– System availability with safety goals & criteria

– FAT, SAT, and commissioning requirements

• Insist on:

– Vendor design responsibility

– Vendor experience with similar applications

Sam p l e Sp e c i f i ca t i o n N ee d s I m p r o v em e n t ?

Sam p l e Sp e c i f i ca t i o n N ee d s I m p r o v em e n t ?



P r o ce ss Co n t r o l Re q u i r e m e n t s :Ex a m p l e

P r o ce ss Co n t r o l Re q u i r e m e n t s :Ex a m p l e

•Goals for Process Control System:

– Raw gas gathering for gas lift & export operations

– Pressure control needed for both the LP and IP gas-oil

separators

–



Glycol dehydration unit works well within a range of

differential pressure / upstream & downstream should

have high & low pressure limits

– Precise flow control required to each gas lift injector

– Gas lift will have a priority over gas export flow

– Operations would like to use separate recycle valves

for capacity control (rather then the antisurge valves)

– Operation for extended periods of time in choke

(stonewall) must be avoided

Re v i e w : K e y Sy s t e m D e s i g nRe q u i r e m e n t s

Re v i e w : K e y Sy s t e m D e s i g nRe q u i r e m e n t s

•Provide for accurate compressor mapping –

Is gas composition constant?

– What abnormal process conditions are present?

• Don’t sacrifice speed of response or availability

–



Look at the complete loop / x-mitters, valves, etc.

– Is the system operating system deterministic?

•Plan for high speed inter-controller

communication

– Take advantage of loop decoupling algorithms

– Hand-shaking on mode switching, etc.

– Coordinated control between systems for

loadsharing

Re v i e w : K e y Sy s t e m D e s i g nRe q u i r e m e n t s co n t…

Re v i e w : K e y Sy s t e m D e s i g nRe q u i r e m e n t s co n t…

•Provide for a coordinated loadsharing scheme

– Parallel, series, or compound arrangement

– Important for precise process control and efficiency

•



Integrate closely coupled process and

machinery limiting variables for precise control

& stability

•Plan for high speed data trending

• Maximize overall system availability

– Transmitter failure fallback strategies

– Redundant control hardware?

– Partial valve stroke testing?

A cce p t a n ce Te s t Re q u i r e m e n t s A cce p t a n ce Te s t Re q u i r e m e n t s

•Example Test Requirements

– Antisurge Control

• In response to full closure of a substation suction or

discharge block valve, the system must not allow any

compressor to surge



– Pressure Control

• Suction pressure shall be held within 0.5 % of setpoint

under normal process disturbances

– Load-Sharing Control

• Upon bringing a compressor on-line or taking one off-line,

the control system shall re-establish steady-state operation

and stable load-balancing in no more then 5 minutes from

start/stop

A cce p t a n ce Te s t Re q u i r e m e n t s A cce p t a n ce Te s t Re q u i r e m e n t s

– Turbine Speed Control

• In steady state, deviation of the turbine speed from its set

point shall not exceed 0.5%

– Turbine Limiting Control

•



• In response to a rise in the speed set point, the system shall

not allow an increase in speed after the exhaust-gas

temperature has exceeded its limiting control threshold by

0.5% of the sensor span

• In response to a rise in the speed set point, the system shall

not allow an increase in speed after the air-compressor

discharge pressure has exceeded its limiting control

threshold by 0.1% of the sensor span

Co n t r o l Sy s t e m Co n s i d e r a t i o n s Co n t r o l Sy s t e m Co n s i d e r a t i o n s

•Consider “purpose-built” control hardware

– Hardware built specifically for turbomachinery

– No compromises in design solution

– Optimized input sampling times

– Optimized output update times

•Software should be application specific



– Look for “deterministic” operating system / guaranteed loop

execution rates

– Field proven application software for each machinery

configuration and process application

•Configurable, not programmable

– Continuous control application programs should not be

modified, only configured for each installation

– Increases security, no unauthorized changes

– Minimizes implementation risks

– Dramatically improves system supportability

Th a n k s f o r y o u rA t t e n d a n c e !

Th a n k s f o r y o u rA t t e n d a n c e !



A t t e n d a n c e ! A t t e n d a n c e !

Please do not hesitate to contact

CCC for any of your turbomachinery

control system needs…

houston day 2 2013 for antisurge control

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