Turbo Machinery Control by CCC updated from Previous Presentations by Honeywell

Turbo Machinery Control Presentation with different control regime and surge phenomenon explained.

1. ©

2008 Compressor Controls Corporation

COMPRESSOR CONTROLS CORPORATION

2. ©

2008 Compressor Controls Corporation

CCC Today
•(33) Years of Continued Growth
•Unsurpassed Customer Satisfaction Ratings
•Twelve (12) locations worldwide to better serve our
customers
•Subsidiary of Roper Industries (>2 Billion USD/year)
•ISO 9001 certified since 1994
World Headquarters
Des Moines, Iowa

3. ©

2008 Compressor Controls Corporation

CCC Today
•CCC Systems operating on over 8,000
turbomachinery trains, including:
–over 450 steam turbines
–over 2,800 gas turbines
–70,000 Surge Test
•375 Employees
–Over 250 Engineers, including:
•43 full time R&D personnel
•Over 100 Fully Qualified Field Engineers
•Approx. 100 Systems Engineers
•26 US patents issued, plus related foreign
patents
– 5 patents pending

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2008 Compressor Controls Corporation

Customers Keep Coming Back!
* 98% Customer Satisfaction Rating * CCC Hits $150,000,000 Mark in 2010
80% of Projects from Repeat Customers

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2008 Compressor Controls Corporation

CCC Worldwide
Americas
• Des Moines
• Houston
• Rio de Janeiro
• Calgary
Europe
• Amsterdam
• Moscow
• London
• Milan
Asia - Pacific
• Singapore
• Beijing
• Perth
Middle East
• Abu Dhabi
• Khobar
• Doha
• Delhi

6. ©

2008 Compressor Controls Corporation

Turbomachinery Controls
is our Only Business

•Centrifugal Compressors
•Axial Compressors
•Steam Turbines
•Gas Turbines
•Turbine Driven Generators
•Turbine Driven Pumps
•Hot Gas Expanders
•Cryogenic Expanders
•Power Recovery Trains

7. ©

2008 Compressor Controls Corporation

CCC Middle East
Jeddah, Saudi
Arabia
Riyadh, Saudi Arabia
Dhahran, Saudi Arabia
Yanbu, Saudi
Arabia
Manama,
Bahrain
Abu Dhabi, United Arab
Emirates
Doha,
Qatar
Jubail, Saudi Arabia
Kuwait City, Kuwait
Muscat, Oman

8. ©

2008 Compressor Controls Corporation

CCC Integrated Controls
Machinery
Process
Controls
CCC
Turbomachinery
Controls

9. ©

2008 Compressor Controls Corporation

Key Issues on
Turbomachinery Controls
•Energy consumed by turbo-machinery
•Poor control
•The economic consequences of non-
availability

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2008 Compressor Controls Corporation

Lifecycle costs
30-year life cycle costs for a 20,000 hp compressor
Costs in constant dollars
Source: “Experiences in Analysis and Monitoring Compressor Performance”
Ben Duggan & Steve Locke
E.I. du Pont, Old Hickory, Tennessee
24th Turbomachinery Symposium
Maintenance Cost
$4.5 Million


Initial Cost
$1.5 Million

Energy Cost
$180 Million
97%
of total costs

11. ©

2008 Compressor Controls Corporation

Lifecycle costs
Source: “Experiences in Analysis and Monitoring Compressor Performance”
Ben Duggan & Steve Locke
E.I. du Pont, Old Hickory, Tennessee
24th Turbomachinery Symposium
30-year costs per a 1,000 hp
What can we
control?
0.0
5.0
10.0
15.0
Initial Cost Maintenance Energy Lost Production
$ Millions
?
Controllable
Uncontrollable
Costs in constant dollars

12. ©

2008 Compressor Controls Corporation



Compressors

13. ©

2008 Compressor Controls Corporation

Developing the compressor curve
P
d Discharge Pressure (P
2) DP
c Differential Pressure
(P
d - P
s) or (P
2 - P
1) R
c Pressure Ratio (P
d/P
s) or (P
2/P
1) H
p Polytropic Head R
c
Q
s, normal Q
s, mass Q
s, vol
Compressor curve
for a specific speed
N1
R
process,1
Q
1
R
c1
R
process,2
Q
2
R
c2

14. ©

2008 Compressor Controls Corporation

Developing the compressor curve
Stable zone
of operation
Minimum speed
Power limit
Maximum speed

Process limit
Q
s, vol

Adding control
margins
Stonewall or
choke limit
Surge limit
R
c
Actual available
operating zone

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2008 Compressor Controls Corporation

Q

Maximizing The Compressor Operating
Envelope – Operating Limits
Surge limit
Speed Limit (Maximum)
Stonewall or
choke limit
Speed Limit
(Minimum)
? ?
R
c

16. ©

2008 Compressor Controls Corporation

2007 Compressor Controls Corporation

16

Turbo-machinery Control Objectives

1.Increase Reliability of machinery and
process
•Prevent unnecessary process trips and
downtime
•Minimize process disturbances
•Prevent surge and surge damage
•Simplify and automate startup and shutdown



2. Increase Efficiency of machinery and
process
•Operate at lowest possible energy levels
•Minimize antisurge recycle or blow-off
•Minimize setpoint deviation
•Maximize throughput using all available
horsepower
•Optimize loadsharing of multiple units

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2008 Compressor Controls Corporation

Capacity Control

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2008 Compressor Controls Corporation

Compressor
Process Suction
What to control?
Process Variables

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2008 Compressor Controls Corporation

1
PIC
Discharge Pressure
Process Variables
Compressor
Process Suction

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2008 Compressor Controls Corporation

1
PIC
Suction
Pressure
Process Variables
Compressor
Process Suction

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2008 Compressor Controls Corporation

1
FIC
Discharge Flow
Process Variables
Compressor
Process Suction

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2008 Compressor Controls Corporation

1
FIC
Suction Flow
Process Variables
Compressor
Process Suction

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2008 Compressor Controls Corporation

Compressor
Process Suction
1
FIC
1
F
T 1
P
sT
1
P
dT
Control Element – Antisurge Valve

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2008 Compressor Controls Corporation

Controlling Pressure/Flow with
Antisurge Valve
PIC -
SP


Q

Shaft
power
Q

Curve 1
A
R
process + R
valve
Curve 1
P
1
R
process
B
Q
loss
R
c

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2008 Compressor Controls Corporation

Control Element - Suction Throttle
Valve
1
PIC
Section 1
1
PT

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2008 Compressor Controls Corporation

Controlling Pressure/Flow with Suction
Throttle Valve


Q

Shaft
power
Q

Suction valve open
Suction valve throttled
PIC - SP
A
R
process
P
1
R
c

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2008 Compressor Controls Corporation

Control Element - Adjustable Inlet
Guide Vanes
1
PIC
Section 1
1
PT

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2008 Compressor Controls Corporation

•Change of guide vanes angle
a results in different
compressor geometry


Q
Shaft
power
Q
•Different geometry means
different performance curve
a
min
a
OP
a
max
•Pressure is controlled by inlet
guide vane position
PIC - SP
•Compressor operates in point
A for given R
process
A
R
process
•Required power is P
1
P
1
Controlling Pressure/Flow
by adjustable guide vanes
P
T 1
PI
C 1
Process
Note – Axial Compressors are generally on air service, so control would be discharge pressure or flow.
Suction pressure is shown here for consistency with other slides
R
c

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2008 Compressor Controls Corporation

Control Element - Speed
VSDS
Compressor
Process
Suction
1
PI
C
STG
Compressor
Process
Suction
ST
Process
Suction
Compressor
VSGB
1
PI
C
1
PI
C

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2008 Compressor Controls Corporation

•Changing speed
generates a family of
curves


Shaft
power
N
min
N
OP
N
max
•Pressure is controlled by
speed of rotation
PIC - SP
•Compressor operates in
point A for given R
process
A
R
process
•Required power is P
1
P
1
Controlling Pressure/Flow with Speed
Q
Q
Process
SIC
PIC PT
R
c

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2008 Compressor Controls Corporation

•While controlling one primary variable we can constrain the
performance control on another variable
•Performance controller controls one variable and can limit
two other variables.
•For Example:
Limiting control to keep the
machine in its stable operating zone
CONTROL (PPV) BUT DO NOT EXCEED (Limit)
Discharge Pressure Max. Motor Current
Suction Pressure Max. Discharge Pressure
Net Flow Min. Suction Pressure
Suction Pressure Max. Discharge Temperature

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2008 Compressor Controls Corporation

0 250 500 750 1000 1250
130

125

120

115

110

105

100
Time, s
Pressure (kPaA)

Set Point
CCC

General Purpose
Control
Suction Pressure
Excursions Using
General Purpose
Control System
Suction Pressure Collapse
Minimizing Suction
Pressure Excursions
Using CCC

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2008 Compressor Controls Corporation

Results

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2008 Compressor Controls Corporation

The Surge Phenomenon

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2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

•From A to B…….20 - 50 ms…………….. Drop into surge
•From C to D…….20 - 120 ms…………… Jump out of surge
•A-B-C-D-A……….0.3 - 3 seconds……… Surge cycle
Q
s, vol
P
d
Machine shutdown
no flow, no pressure
Note: Flow goes up faster because pressure is the
integral of flow
•Pressure builds
•Compressor “rides” the curve
•P
d = P
v + R
losses

P
d = Compressor discharge pressure
P
v = Vessel pressure
R
losses = Resistance losses over pipe
Developing the surge cycle on the
compressor curve
P
d
P
v
R
losses
B A
C
D

36. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Factors leading to onset of surge
•Startup
•Shutdown
•Operation at reduced throughput
•Operation at heavy throughput with:
–Trips
–Power loss
–Operator errors
–Process upsets
–Load changes
–Gas composition changes
–Cooler problems
–Filter or strainer problems
–Driver problems

•Surge is not limited to times of reduced
throughput.
•Surge can occur at full operation

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2008 Compressor Controls Corporation

Surge Consequences
• Consequences on process
–Unstable process flow and pressure
• Consequences on compressors
–Damage to seals, bearings, shaft and stator
in increasing severity as the surge continues

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2008 Compressor Controls Corporation

Antisurge Control

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2008 Compressor Controls Corporation

Challenges of Compressor Control
The ingredients of a successful compressor
control system are
–an algorithm that can accurately locate the operating point
and its corresponding surge limit
–a controller execution speed that will allow a digital
controller to emulate immediate analog control
–control responses that allow different margins of safety for
different operating conditions
–advanced control strategies that can avoid the negative
effects of loop interaction
–a quick acting, correctly sized antisurge control valve
–the elimination of unnecessary dead time or lag time within
the system

40. ©

2008 Compressor Controls Corporation

The ingredients of a successful
compressor control system include

…an algorithm that can accurately
locate the operating point and its
corresponding surge limit

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2008 Compressor Controls Corporation

Distance to Surge Algorithms
•CCC has a library of standard algorithms
including:
1.Delta P vs Flow (simple air compressor)
2.Compression Ratio vs Flow (modified) for smaller
variable Mol Wt swings
3.Polytropic Head (modified) vs Flow (modified) for larger
variable Mol Wt swings
4.Algorithms for sidestreams
5.Algorithms for flow measurement in suction, or
discharge, or downstream of an aftercooler
6.Pressure vs Power (modified) (where there is no flow
element)
7.Pressure vs Valve Position (for single valve turbine
drives with no compressor flow element)

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2008 Compressor Controls Corporation

Different Gas Composition Means
Different Performance Maps
Q
R
c

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2008 Compressor Controls Corporation

Different Gas Composition Means
Different Performance Maps
Q
R
c

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2008 Compressor Controls Corporation

The problem with commonly used (OEM provided)
coordinate systems of the compressor map is that these coordinates
are NOT invariant to suction conditions as shown
Algorithm Issues

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2008 Compressor Controls Corporation

The surge limit thus becomes a surface rather
than a line
•For control purposes we want the SLL to be presented by a single curve for
a fixed geometry compressor
Algorithm Issues

46. ©

2008 Compressor Controls Corporation

Fundamental variables
characterizing compressor
operation
H
p = f
0(Q, w, m, r, a, d, a)
J = f
1(Q, w, m, r, a, d, a)
where:
•H
p = Polytropic head
•J = Power
•Q = Volumetric flow rate
w = Rotational speed
m = Viscosity
r = Density
•a = Local acoustic velocity
•d
= Characteristic length
a = Inlet guide vane angle

•The following variables are used to design and to characterize compressors
•Through dimensional analysis (or similitude) we can derive two sets of invariant coordinates
Dimensional analysis
or Similitude
Set 1
h
r
q
r
N
e
a
j
r
Re
Invariant coordinates
Set 2
R
c
q
r
N
e
a
j
r
Re
where:
•h
r = Reduced head
•q
r = Reduced flow
•N
e = Equivalent speed
a = Guide vane angle
•j
r = Reduced power
•Re
= Reynolds number
•R
c = Pressure Ratio

Advanced Control - Developing
Invariant Coordinates 

47. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

•These coordinates are NOT invariant to
suction conditions (Ts, Ps, MW)
The problem with OEM provided
coordinate systems of the compressor map
•For control purposes we want the SLL to be presented
by a single curve for a fixed geometry compressor

48. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Invariant coordinates (h
r, q
r
2
)
Understand the limitations of maps
NOT invariant coordinates (H
p, Q
s)
Choose the right coordinates for the anti-surge
control system

49. ©

2008 Compressor Controls Corporation

The ingredients of a successful
compressor control system include

… a controller execution speed that
will allow a digital controller to
emulate immediate analog control

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2008 Compressor Controls Corporation

Analog controller
SLL
SCL
100%
0%
Controller
output
100%
0%
•Leading engineering contractor
performed evaluation of execution time
influence on ability to protect
compressor from surge
•Dynamic simulation of compressor was
built
•Digital controllers are compared against
analog controller on simulation
•Analog controller has no execution time
and is immediate
•Analog controller tuned for minimum
overshoot
•Digital controllers get exact same tuning
parameters
•Digital controllers get exact same
disturbance
Operating
point
Time
Time
Execution Speed Issue

51. ©

2008 Compressor Controls Corporation

Analog controller
SLL
SCL
100%
0%
Controller
output
100%
0%
Operating
point
SLL
SCL
100%
0%
Controller
output
100%
0%
Operating
point
Digital controller
(2 executions per second)
Time
Time
Time
Time
Tuning same as analog controller
Controller Execution Speed
Analog vs digital controller at 2 executions per second

52. ©

2008 Compressor Controls Corporation

Analog controller
SLL
SCL
100%
0%
Controller
output
100%
0%
Operating
point
SLL
SCL
100%
0%
Controller
output
100%
0%
Operating
point
CCC antisurge controller
(25 executions per second)
Time
Time
Time
Time
Tuning same as analog controller
Controller Execution Speed
Analog vs CCC controller at 25 executions per second

53. ©

2008 Compressor Controls Corporation

The ingredients of a successful
compressor control system include

… control responses that allow
different margins of safety for
different operating conditions

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2008 Compressor Controls Corporation

CCC Control Antisurge Responses

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2008 Compressor Controls Corporation

CCC Control Responses
1.P&I
2.Adaptive Gain
3.Recycle Trip
4.Decoupling
5.Safety On

56. ©

2008 Compressor Controls Corporation

2010 Compressor Controls Corporation

Response #1 – PI Control based on
the SCL

57. ©

2008 Compressor Controls Corporation



A
B
•When the operating point
crosses the SCL, PI control
will open the recycle valve
•PI control will give
adequate protection for
small disturbances
•PI control will give stable
control during steady state
recycle operation
•Slow disturbance example
SLL = Surge Limit Line
SCL = Surge Control Line
A/S Controller Set Point
Q
Basic PI Control
R
c

58. ©

2008 Compressor Controls Corporation

Response #2 – Moving Control Line

59. ©

2008 Compressor Controls Corporation



A
B
•When the operating point
movement towards the SCL is
fast, adaptive gain moves the
SCL towards the operating
point.
•This allows the PI controller to
react earlier
•As a result a smaller steady
state surge control margin can
be achieved without sacrificing
reliability
•Fast disturbance example
Q
Second Response – Different Margins
for Different Disturbances
R
c

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2008 Compressor Controls Corporation

Response #3 – Recycle Trip
PI Control
Recycle Trip
®
Action
+
To antisurge valve

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2008 Compressor Controls Corporation

•When OP touches the SCL the PI controller
opens valve based on proportional and integral
action
•When the operating point hits the Recycle
Trip Line (RTL) an open loop response is
triggered
•Disturbance occurs and the Operating
Point (OP) moves towards the SCL


SLL = Surge Limit Line
RTL = Recycle Trip
® Line
SCL = Surge Control Line
Output
to Valve
Time
•Operating point keeps moving towards
surge and touches the Recycle Trip Line
(RTL)
•Operating point Moves back to the safe
side of the RTL
–The RT function decays out the step
response
–PI controller integrates to stabilize OP
on SCL
Recycle Trip
® Response
Q
Third Response – Recycle Trip
R
c

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2008 Compressor Controls Corporation

Recycle Trip

63. ©

2008 Compressor Controls Corporation

The ingredients of a successful
compressor control system include

… advanced control strategies that
can avoid the negative effects of
loop interaction

64. ©

2008 Compressor Controls Corporation

Decoupling
Process Variable
Measurement
Control
Speed
Antisurge
Measurement
Control
Recycle Valve
Gov
Compressor
Process
Suction
1
UIC
1
PIC

65. ©

2008 Compressor Controls Corporation

Control System Objective
Control System Objectives:
•The control system objective is to keep the
process on its Primary Process Variable
(PV) set-point, and to return it to set-point as
quickly as possible after a process
disturbance
•The control system has to keep the process
on/return to set-point while operating within
compressor operating envelope limits,
including protection against surge and
surge damage

66. ©

2008 Compressor Controls Corporation

Process Variable
Measurement
Control
Speed
Compression
System
Antisurge
Measurement
Control
Recycle Valve
•Both controllers
manipulate the same
variable - the operating
point of the compressor

•The controllers have
different and
sometimes conflicting
objectives

•The control action of
each controller affects
the other

•This interaction starts
at the surge control line
- near surge - and can
cause surge
Interaction of Control Loops

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2008 Compressor Controls Corporation

Turbine - Gas Compressor Package
Output to Recycle Valve
Flow
Pressure
Temperature
Gas Data

Input
Performance
control
Field
Output to Turbine Valves Turbine Governor
Speed control
Antisurge control
Typical DCS Control Format

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2008 Compressor Controls Corporation

Non-Integrated Performance and
Antisurge Loops
.1
.2
.3
Q

Operating point well away from the
SCL, recycle valve closed
Rc
1
PIC
SG
2
UIC

69. ©

2008 Compressor Controls Corporation

Non-Integrated Performance and
Antisurge Loops
.1
.2
.3
Q
Operating point touched the SCL,
recycle valve opens
Rc
1
PIC
SG
2
UIC

70. ©

2008 Compressor Controls Corporation

Non-Integrated Performance and
Antisurge Loops
Discharge pressure drops, PIC speeds
the train up, operating point moves
to the right, recycle valve closes
Rc
.1
.2
.3
Q
1
PIC
SG
2
UIC

71. ©

2008 Compressor Controls Corporation

Non-Integrated Performance and
Antisurge Loops
.1
.2
.3
Q
Discharge pressure increases, PIC
slows down the train, operating point
touches the SCL and the recycle
valve opens
Rc
1
PIC
SG
2
UIC

72. ©

2008 Compressor Controls Corporation

A
PIC-SP
B
Non-Integrated Performance and
Antisurge Loops
We are operating at point A
Large disturbance occurs
The operating point rides the curve
to point B
The Performance controller is taking
the operating point down in speed
and thus down in flow - the tangent of
that trajectory is thus (shown)
That means that the operating point
must use a large control bias
to avoid surge and then stabilize:
Q
C
R
c

73. ©

2008 Compressor Controls Corporation

Ways to cope with Antisurge and
Performance Loop interactions
•Very Poor Choice - Put one loop on manual,
so interaction is not possible. Operators
will usually put the Antisurge Controller on
manual
•Result - no surge protection and often
partially open antisurge valve
•Poor Choice - De-tune the loops to minimize
interaction.
•Result is poor pressure control, large surge
control margins and poor surge protection
•Good Choice - Decouple the interactions.
Result - good performance control accuracy,
good surge protection and no energy
wasted on recycle or blow off

74. ©

2008 Compressor Controls Corporation

Integrated Performance and Antisurge
Loops
.1
.2
.3
Q
Operating point touched the SCL, recycle
valve opens and at the same time
decouples the action of the PIC and
speeds up the train
Peer-to-Peer
Serial
Communications
Rc
1
PIC
SG
2
UIC

75. ©

2008 Compressor Controls Corporation

Integrated System
Turbine - Gas Compressor Package
Outputs
Serial Network
Input
Flow
Pressure
Temperature
Gas Data

Field

76. ©

2008 Compressor Controls Corporation
PIC-SP
R
c
A C
B
Integration/Decoupling of Antisurge
and Capacity Control
We are operating at point A
Large disturbance occurs
The operating point rides the curve
to point B
This time we ‘decouple’ the action
of the performance controller
The antisurge controller tells the
performance controller to speed
up the compressor
The action of the performance
controller is increasing speed
and flow, and the resulting tangent
is as follows (shown)
This, with recycle trip functionality
results in a stabilization
action as shown
The result is an action requiring
only a small margin of safety
Q

77. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Disturbance comes from the
discharge side
•P
d,2 increases
•P
s,2 remains constant
•R
c,2 increases
•Section 2 moves towards surge
Disturbance
•The system is oscillating
•Slowing down the controller
tuning would lead to:
-Increased risk of surge
•Compressor damage
•Process trips
-Bigger surge margins
•Energy waste
Interacting Antisurge Control Loops
R
c,2
q
r,2
2
R R
c,1
q
r,1
2
R
R R
Antisurge controller UIC-1 will open
the recycle valve to protect section 1
against surge
•P
d,1 decreases
•P
s,1 increases
•R
c,1 decreases
•Section 1 moves away from surge
Opening of recycle valve on section 1
caused P
d,1 = P
s,2 to decrease
Result:
•P
s,2 decreases
•P
d,2 remains constant
•R
c,2 increases
•Section 2 moves towards surge
Antisurge controller UIC-2 will open the
recycle valve to protect section 2 against
surge
•P
d,2 decreases
•P
s,2 increases
•R
c,2 decreases
•Section 2 moves away from surge
Opening of recycle valve on section 2
caused P
s,2 = P
d,1 to increase
Result:
•P
d,1 increases
•P
s,1 remains constant
•R
c,1 increases
•Section 1 moves towards surge
1
PIC
2
UIC
1
UIC
VSDS
Section 1 Section 2

78. ©

2008 Compressor Controls Corporation

Interacting Antisurge Control Loops
1
PIC
2
UIC
1
UIC
VSDS
Section 1 Section 2
Serial
Network
Serial
Network
•All CCC Controllers are connected on a serial network
•This allows them to coordinate their control actions
•When UIC-2 Opens the recycle valve:
•Section 2 will protected against surge
•Section 1 will be driven towards surge
•How much section 1 is driven to surge depends on how much the
recycle valve of section 2 is opened
•The output of UIC-2 is sent to UIC-1 to inform UIC-1 that the
disturbance is arriving
•UIC-1 anticipates the disturbance by immediately opening its valve

79. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

FA
Mode
PI RT
Loop
Decoupling
Antisurge
Controller
UIC-1
Analog Inputs
DEV1
FA
Mode
PI RT
Antisurge
Controller
UIC-2
Analog Inputs
DEV2
•Antisurge controller UIC-2 opens its valve to
protect section 2 against surge
To antisurge valve 2
+
•UIC-1 is protecting section 1 against surge
using PI and Recycle Trip
®

+
To antisurge valve 1
•UIC-2 reports PI and Recycle Trip
®
output to
UIC-1
•Loop decoupling block multiplies reported PI
and Recycle Trip
®
values with decoupling
gain M
2
PI
2
.
M
2
+
RT
2
.
M
2
•Loop decoupling value is added to output to
antisurge valve 1
+
•Loop decoupling values of other controllers
(performance and antisurge) are added to
output to antisurge valve 1
From other
controllers
PI
n
.
M
n
+
RT
n
.
M
n
•Each controller has its own decoupling gain
M
n to allow for tuning of relative loop gains
between different controllers
•UIC-1 reports its PI and Recycle Trip values
to UIC-2
•Same decoupling takes place
Loop
Decoupling
PI
1
.
M
1
+
RT
1
.
M
1
PI
n
.
M
n
+
RT
n
.
M
n
+
From other
controllers
Benefits
•Avoids control system oscillations
•Allows faster tuning of control system
•Reduced risk of surge
•Allows closer operation to surge limits without
taking risk
Loop Decoupling
simplified Block Diagram
VSDS
Section 1 Section 2
Serial
network
2
UIC
1
UIC

80. ©

2008 Compressor Controls Corporation

2007 Compressor Controls Corporation

80
Load-sharing

81. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Process
PIC
1
1
UIC
VSDS
Compressor 1
2
UIC
VSDS
Compressor 2
HIC
1
Suction
header
Swing
machine
Base
machine
Notes
•All controllers act
independently
•Transmitters are not
shown
Base Loading
Flow Diagram for Control Process

82. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

R
c,1
q
r,1
2
R
c,2
q
r,2
2
Compressor 1 Compressor 2
•Machines operate at same R
c
since suction and discharge of
both machines are tied
together
PIC-SP
•Base load one or more
compressors and let the
other(s) absorb the load swings
Swing machine Base machine
•Base machine is fully loaded
and runs without recycle
Q
C,2=
Q
P,2
•Swing machine can be running
with recycle
Q
C,1 Q
P,1
where:
Q
P = Flow to process
Q
C= Total compressor flow
Q
C - Q
P = Recycle flow
•Load could be re-divided to
eliminate recycle
Q
P,1
Q
P,1 +
Q
P,2 = Q
P,1 +
Q
P,2
Notes:
•Base loading is inefficient
•Base loading increases the risk of surge
since compressor #1 will take the worst
of any disturbance
•Base loading requires frequent operator
intervention
•Base loading is NOT recommended
Base Loading
Parallel Compressor Control
Q
P,2

83. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Notes
•All controllers are
coordinating
control responses via
a serial network
•Minimizes recycle
under all operating
conditions
Process
1
UIC
VSDS
Compressor 1
VSDS
Compressor 2
Suction
header
1
LSIC
2
UIC
out
RSP
Serial
network
out
RSP
2
LSIC
1
MPIC
Serial
network
Serial
network
Equidistant Loadsharing
Flow Diagram for Control Process

84. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

•Machines operate at same R
c since suction and discharge of both
machines are tied together
PIC-SP
•The DEV is a dimensionless number representing the distance
between the operating point and the Surge Control Line
•Lines of equal DEV can be plotted on the performance curves as
shown
0.1
0.2
0.3
DEV = 0
0.1
0.2
0.3
•Machines are kept at the same relative distance to the Surge Control
Line (SCL)
•This means in practice the same DEV for both machines
DEV
1 DEV
2
•Recycle will only start when all machines are on their SCL •Since DEV is dimensionless all sorts of machines can be mixed:
small, big, axials, centrifugals
•The DEV will be the same for all machines but they will operate
at different speeds and flow rates
SCL = Surge Control Line
R
c,1
q
r,1
2
R
c,2
q
r,2
2
Compressor 1 Compressor 2
Dev
1 = Dev
2
Q
1 =
Q
2
N
1 = N
2
Notes:
•Maximum turndown (energy savings) without recycle or blow-off
•Minimizes the risk of surge since all machines absorb part of the
disturbance
•Automatically adapts to different size machines
•CCC patented algorithm
Equidistant Loadsharing
Parallel Compressor Control

85. ©

2008 Compressor Controls Corporation

Response #4 – Safety On

86. ©

2008 Compressor Controls Corporation

SCL = Surge Control Line
•If Operating Point crosses the Safety On
®
Line the compressor is in surge
SLL = Surge Limit Line
RTL Line = Recycle Trip
®
•The Safety On
®
response shifts the
SCL and the RTL to the right
•Additional safety or surge margin is
added
Additional surge margin
•PI control and Recycle Trip
®
will stabilize
the machine on the new SCL
SOL = Safety On
® Line
Fourth Response - Safety On

Q
New SCL
New RTL
A
R
c

87. ©

2008 Compressor Controls Corporation

Built-in surge detector
Pressure and Flow Variations
During a Typical Surge Cycle

100%
100%
0%
0%
P
d
DP
o
20 to 50
milli-seconds
1 TO 2 SECONDS
•Surge signature should be recorded during
commissioning.
•Rates of change for flow and pressure
transmitters should be calculated.
•Thresholds should be configured slightly
more conservative than the actual rates of
change during surge.
•Surge is detected when the actual rates of
change exceed the configured thresholds
•The following methods have been used:
–Rapid drops in flow and pressure
–Rapid drop in flow or pressure
–Rapid drop in flow only
–Rapid drop in pressure only
•When surge is detected a Safety On
®

response is triggered
•A digital output can be triggered upon a
configurable number of surge cycles

88. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation
Case Study – Gas
Turbine Solution

89. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Challenges and opportunities
in gas turbine control
•Integration with controls of driven object
(compressor, generator or pump) to avoid:

–Flame out: limit fuel valve movement during
surge of driven compressor
–Over temperature trip: EGT limit setpoint reduced
as a function of derivative of EGT
•Driven generator (circuit breaker trip)
•Driven compressor (surge)
–Process disturbances
–Mechanical stresses on GT

90. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Offshore Platform Application
2 Parallel Trains

RR Avon Gas Turbine Driven Compressors

91. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

3.8%
RR Avon

92. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

3.8%
6,000 Nm
3

93. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Production Increase
•Customer over time had lowered EGT limit
set point to eliminate tripping on EGT trip
limit due to poor gas turbine control system
•Increase set point from 650 C to 660 C which
is original OEM set point
•Results
–3.8% increase in Exhaust Gas Horsepower (EGHP)
–Translating this to the compressor map results in a 6000
Nm3/hr increase in flow at constant compression ratio
–Equals 300days/yr * 24 hrs/day * 6000Nm3/hr =
43,200,000Nm3/yr increased production
–Equals 43,200,000Nm3/yr * 37.5scf/ncm * 1000Btu/scf *
$3/1,000,000 Btu

Equals $4,860,000/yr in increased
production per machine!

94. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Offshore Platform Application
Gulf of Mexico
4 Parallel Trains 250 MMSCFD
Demag Delaval 3 section double-barrel compressors
LM 2500 Gas Turbines

95. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

PROBLEMS PROBLEMS PROBLEMS!
•Domino trips
•Recycle valves open 15-25%
•EGT limited operation
•Surge limits…who knows ?
•Manual speed control. Base loading.
•ASV loops in manual
•Excessive flare
•Operators don’t trust PLC protection
•Mechanical damage to thrust bearings
•Unstable suction pressure: 3.5 - 4.8 kg/cm2
•Discharge pressure 12% off 82 kg/cm2 max
•Fuel controller maintenance time

96. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation

Results
•Increased production by 7 million SCFD
($7.5 million per year)
•Elimination of trips (1 per month) from
surging
•Elimination of thrust bearing damage
•All control loops in auto eliminating
operator intervention
•All recycle valves closed during normal
operation
•Fast, reliable, smooth startups in automatic
•Platform reliability went from low to high
•Stable suction pressure control at 4.5 psi
•Easy trouble shooting of control system
problems with improved HMI

97. ©

2008 Compressor Controls Corporation

2008 Compressor Controls Corporation


Complete train control system
(ultimate solution)


Power
Supplies
IO Cards
MPU Cards
Extender Cards
Train view software from CCC