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Centrifugal Compressor Surge Centrifugal Compressor Surge Avoidance System DesignAvoidance System Design
Mr. Augusto GarciaMr. Augusto Garcia--HernandezHernandezDr. Robert McKeeDr. Robert McKee
Southwest Research Institute®, SwRI®
Rotating Machinery & Measurement Technology
OutlineOutline
IntroductionCentrifugal Compressor Surge EventDynamic Modeling
Basic ModelSimulations
Case StudiesEffect of Different Recycle Valves and TimingConclusions
Introduction Introduction
Centrifugal compressors are essential to the pipeline, process, and storage industries
Limitations on centrifugal compressor operations due to surge
Surge is a flow instability
Centrifugal Compressor LimitsCentrifugal Compressor Limits
1.06
1.07
1.08
1.09
1.1
1.11
1.12
1.13
1.14
1.15
1.16
400 600 800 1000 1200 1400 1600 1800 2000Inlet Flow ACFM
Pres
sure
Rat
io
80% Sp 85% Sp 90% Sp 95% Sp100% Sp Surge Limit Surge Control Stonew all
Max Speed / MAOP / Max Pow er
Surge Limit
Stone Wall
Min Speed
Surge Should be AvoidedSurge Should be Avoided
At surge, the flow pattern in the compressor collapses
Surge at energetic conditions is very critical
Surge disrupts station flows
Surge at low energy can be acceptable
Surge Avoidance for Normal Surge Avoidance for Normal Conditions Conditions
Operate at higher flows and lower heads
Change head and flow conditions slowly
Opening of the recycle valve to maintain a minimum flow
Flow recirculation through the compressor to prevent low flow conditions
In the Event of a Trip In the Event of a Trip
Unexpected events can require a sudden trip of the unit
Surge is essentially unavoidable
Will the head at surge be high (energetic) or low (acceptable)?
Design controls to avoid energetic surge
Purpose of Proper Surge Control Purpose of Proper Surge Control
Surging at high speed and head can damage the unit
Low speed and head, surge will be mild
Determine proper conditions to reduce the head rapidly
Determine appropriate conditions to maintain high flow through the unit
Dynamic ModelingDynamic Modeling
For routine operations changes occurs slowly, so modeling is not necessary
Rapidly changes and abnormalities in operating conditions required modeling
Proper design of the anti-surge system
Modeling Modeling –– The Basics IncludedThe Basics Included
A model for surge control design must include:
All pipe elements
Compressors with performance curve
Drivers with torque / speed responses
Recycle and other valves with capacity
Valve controls/actuators - response times
Scrubbers, heat exchangers, etc
Upstream and downstream piping
Control algorithms and sequences
Special Elements Special Elements
A centrifugal compressor is represented by a specific element
Valves that open or close (recycle) have changing Cv with stroke
Other special nodes like scrubbers or filters, heat exchanges, etc.
Control Systems and Sequences Control Systems and Sequences
Controls or elements that adjust or affect the system response
A valve controller and actuator with:Delay times
Non-linear rates
Integral, proportional, and derivative responses
Control results dependent on operating conditions (pressure, flow, speed, etc.)
Driver controls (power, inertia, and accelerations rates)
SimulationSimulation
Detailed data is required to build a good compressor system model
Boundary conditions must be determined but not over specified
An operating point should be established
Define scenarios for trips of the compressor
Time step within transient response
Effects of Normal OperationsEffects of Normal Operations
Design Operating Point, Speed 4800 RPM, 4.558 m3/s, 16559 ft
Compressor Coasting Down in 72 seg withRecycle Valve Opening in 1.8 seg
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7 8
Flow Rate [m 3/s]
Hea
d [ft
]
0
10
20
30
40
50
0 50 100 150 200 250
Flow Rate [ft 3/s]
Hea
d [k
J/K
g]
5040
87%
3120
3600
4080
4800
79%
83%
85%
One Normal Surge TransientOne Normal Surge Transient
At trip, the compressor starts to coast down based on internal load and inertia
The recycle valve is signaled to open with it maximum opening rate
The speed, head, and flow decrease in a complex manner
Decreasing head increases flow so at times speed is neg. – flow is pos. - then flow neg.
Effect of Low Flow OperationsEffect of Low Flow Operations
Near Surge Operating Point, 4800 RPM, 4.068 m3/s, 17461 ft
Com pressor Coasting Down in 72 seg with Recycle Valv e Opening in 1.8 seg
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7 8Flow Rate [m 3/s]
Hea
d [ft
]
0
10
20
30
40
50
0 50 100 150 200 250
Flow Rate [ft3/s]
Hea
d [k
J/K
g]
79%
83%85%
87%
3120
3600
4040
4800
5040
Results in Terms of Surge EnergyResults in Terms of Surge Energy
The point at which a compressor crosses the low flow limit (surge line) is dependent on the initial operating point.
The exact behavior is dependent on the details of the compressor, recycle valve, attached piping, and control times.
A small difference in the head at which the compressor crosses the surge line can be 5 to 10 percent less total power (energy).
Effect of Less Coast Down Time Effect of Less Coast Down Time
Much less coast down timeInitial conditions: 4800 RPM, 5.071 m3/s, 15649 ft
Com pressor Coasting Down in 36 seg with Recycle Valv e Opening in 1.8 seg
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7 8Flow Rate [m 3/s]
Hea
d [ft
]
0
10
20
30
40
50
0 50 100 150 200 250Flow Rate [ft3/s]
Hea
d [k
J/K
g]
79%
83%85%
87%
3120
3600
4040
4800
5040
Effects of Recycle Valve Effects of Recycle Valve Size and SpeedSize and Speed
The main control for rapidly reducing head is the recycle valve size and speed
The trade off for recycle systems is a larger valve is better and a faster valve
There are times when a smaller fast valve is better than a larger slow valve but transient simulations are needed to determine this
Effect of a Larger Recycle ValveEffect of a Larger Recycle Valve
A large recycle valve for a large compressorInitial conditions: 4800 RPM, 7.244 m3/s, 14647 ft
Com pressor Coasting Down in 72 seg with Recycle Valv e Opening in 1.8 seg
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7 8 9 10 11Flow R ate [m3/s]
Hea
d [ft
]
0
10
20
30
40
50
0 50 100 150 200 250 300 350 400
Flow Rate [ft3/s]
Hea
d [K
J/K
g]
3120
3600
4080
4800
5040
3360
3840
4320
4560
Results of Recycle Valve SizeResults of Recycle Valve Size
A recycle valve with a larger capacity and same opening time will allow a lower energy emergency shutdown.
In some cases it may be necessary to have both:
A fast small (hot gas) recycle valve
A large (slower) full capacity recycle valve
Transient modeling allows parametric studies of recycle sizes, times and options.
Effect of Different Recycle ValvesEffect of Different Recycle Valves
A large recycle valve can make a big difference
Vent Valve UsageVent Valve Usage
Vent valves on a compressor discharge
Vent valves:Smaller and faster
Remove a lot of gas from the discharge to flare
Short time (1 to 2 sec) reduce the head and energy of surge
Applications in multistage compressor
Effect of Vent Valve Open Time Effect of Vent Valve Open Time
Uses of a Discharge Vent Valve for a Short Time
Power Trip (Speed) DelaysPower Trip (Speed) Delays
A short time delay in tripping allows recycle and vent valves to open before speed drops.
Some alarms (such as high temps or vibrations) allow a short delay before tripping.
With extra discharge gas from large volumes or later stages, the head will remain high.
Check valves on discharge can isolate the volumes and allow head to be reduced.
Effect of Power (Speed) Effect of Power (Speed) Trip DelayTrip Delay
A short delay in removing power is positive
0 sec
1 sec
2 sec
Methods for Reducing Methods for Reducing Stored EnergyStored Energy
The stored energy in discharge gas volume can be reduced by:
The use of large fast recycle valves
The use and timing of vent valves
The timing of driver power changes and speed reductions
The use and location of check valves and discharge volumes
The control system tuning (timing and gains) and sequence of responses
ConclusionsConclusions
Surge is an instability and a collapse of flow through a compressor that occurs at low flows.
Surge at high energy and speed is a violent, damaging, and potentially dangerous event.
Transient flow solvers are used for dynamic simulations that aid the design of new or existing installations.
Larger and faster opening recycle valves will lower the energy of surge during a trip.
Location on the compressor map affects the time and energy of surge during a trip.
Conclusions Conclusions -- continuedcontinued
A more rapid speed reduction will results in a more energetic shutdown surge.
In multi-section compressors, vent valves can remove the high pressure discharge gas energy.
A delay in speed reduction can allow a head reduction before a low flow surge is reached.
Discharge check valves can isolated upstream high pressure gas to allow a lower energy surge.
Transient simulations of compressor trips are necessary and the best way to design surge control systems to avoid energetic surges.
Questions ?Questions ?
Thank you for attention!Thank you for attention!