Centrifugal Compressor Surge Avoidance System Design_english
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Centrifugal Compressor Surge Avoidance System DesignMr. Augusto Garcia-Hernandez Dr. Robert McKeeSouthwest Research Institute, SwRI Rotating Machinery & Measurement Technology
OutlineIntroduction Centrifugal Compressor Surge Event Dynamic ModelingBasic Model Simulations
Case Studies Effect of Different Recycle Valves and Timing Conclusions
IntroductionCentrifugal 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 Limits80% Sp 100% Sp 85% Sp Surge Limit 90% Sp Surge Control 95% Sp Stonew all
1.16 1.15Max Speed / MAOP / Max Pow er
1.13 1.12 1.11 1.1 1.09 1.08 1.07 1.06 400
Stone Wall Min Speed
Inlet Flow ACFM
Surge Should be AvoidedAt 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 ConditionsOperate 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 TripUnexpected 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 ControlSurging 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 ModelingFor 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 The Basics IncludedA 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 ElementsA 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 SequencesControls 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)
SimulationDetailed 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 OperationsCompressor Coasting Down in 72 seg with Recycle Valve Opening in 1.8 segFlow Rate [ft0 20000 18000 16000 1400040803
87% 85% 83% 79%
10000 8000 6000 4000 2000 0 0 1 23120
0 3 4 53
Flow Rate [m
Design Operating Point, Speed 4800 RPM, 4.558 m3/s, 16559 ft
One Normal Surge TransientAt 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 OperationsCom pressor Coasting Down in 72 seg with Recycle Valv e Opening in 1.8 segFlow Rate [ft /s]0 20000 18000 16000 1400040403
87% 85% 83% 79%
12000 10000 8000 6000 4000 2000 0 0 1 2 3 4 03600
Flow R ate [m /s]
Near Surge Operating Point, 4800 RPM, 4.068 m3/s, 17461 ft
Results in Terms of Surge EnergyThe 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 TimeCom pressor Coasting Down in 36 seg with Recycle Valv e Opening in 1.8 segF lo w Ra te [ft /s ]0 20000 18000 16000 1400040403
87% 85% 83% 79%
12000 10000 8000 6000 4000 2000 0 0 1 2 3 4 53
0 6 7 8
F lo w Ra te [m /s ]
Much less coast down time Initial conditions: 4800 RPM, 5.071 m3/s, 15649 ft
Effects of Recycle Valve Size and SpeedThe 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 ValveC o m p re s s o r C o a s tin g D o wn in 7 2 s e g with R e c yc le Valv e Op e n in g in 1 .8 s e gF lo w Ra te [ft /s ]0 20000 18000 16000 140005040 4800 4560 4320 4080 38403
12000 10000 8000 6000 4000 2000 0 0 1 2 33600 3360 3120
0 4 5 63 F lo w R ate [m /s] 7 8 9 10 11
A large recycle valve for a large compressor Initial conditions: 4800 RPM, 7.244 m3/s, 14647 ft
Results of Recycle Valve SizeA 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 Valves
A large recycle valve can make a big difference
Vent Valve UsageVent 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
Uses of a Discharge Vent Valve for a Short Time
Power Trip (Speed) DelaysA 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) Trip Delay
0 sec 1 sec 2 sec
A short delay in removing power is positive
Methods for Reducing Stored EnergyThe 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
ConclusionsSurge 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 - continuedA 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.
Thank you for attention!