houston day 2 2013 for antisurge control
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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
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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
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St e am Tu r b i n e s
St e am Tu r b i n e s
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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
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
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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
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
•Operate at efficient energylevels
• Accurate speed
measurement
• Consistent and accurate
valve positioning
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
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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
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
• Speed and extraction
control
•Startup and shutdown
automation in concert with
driven equipment
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
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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
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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
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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
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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
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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
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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
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Time
RPM-SP
RPM
t1
Ncritical,low
Ncritical,high
V1
Time
0%
100%
Critical Speed Range
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
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Time
RPM-SP
RPM
t1
Ncritical,low
Ncritical,high
V1 Position
Time
0%
100%
Critical Speed Range
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
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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
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• 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
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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
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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
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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
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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
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• 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
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Fa n La w s Fa n La w s
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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 )
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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• 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
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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
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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
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Ov e r s p e e d I s s u e 2 Ov e r s p e e d I s s u e 2
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Ov e r s p e e d I s s u e 3 Ov e r s p e e d I s s u e 3
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
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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
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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
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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
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• 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
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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
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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
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•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
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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
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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
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•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
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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
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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
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LN G LN G
LNG Fa c t s LNG Fa c t s
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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
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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
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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
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• 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
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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
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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
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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
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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
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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
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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
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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
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APCI M CR® P r o c e ss A PCI M CR® P r o c e ss
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• 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
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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
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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
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• 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
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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
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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
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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
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• 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
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• 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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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
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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 !
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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
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NGL Rem o v a l NGL Rem o v a l
Three methods of condensate removal:
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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
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• 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
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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
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•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 )
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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
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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
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•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 )
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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
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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
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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
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•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
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• 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
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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
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• 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
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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”
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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
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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
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• 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
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Fl a s h Ga s I TCS Fl a s h Ga s I TCS
• Provide invariant
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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
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• 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
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• 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
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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
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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
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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
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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
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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
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• 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
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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
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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
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2 n d S t a g e
300 HP
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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
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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:
–
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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
•
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•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
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•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
•
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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
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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
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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
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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
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– 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
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•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
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•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
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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
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
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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
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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
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
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
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• 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
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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
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Speed
Breaker opens here
Rx-Rg ∆P
Breaker
Export Power
Minimal Speed Excursion
Minimal Disturbance to the Regenerator
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
5
Location of Quench Control Line
2-phaseregion
li id i
gasregion
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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
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
• Quench Control Summary
– Quench Temperature set point is f(P
sat
) + offset
– Operator set point interface
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• 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
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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
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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
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D e f i n i t i o n s D e f i n i t i o n s
SIS
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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:
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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.
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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
–
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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
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•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
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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
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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
–
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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?
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•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
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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
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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
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– 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
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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
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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
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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
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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
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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
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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
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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
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• 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
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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
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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
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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%
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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
•
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•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 ?
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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
–
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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
–
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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
•
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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
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– 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
•
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• 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
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– 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 !
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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…