1. Introduction to Operating Limits
Centrifugal compressors have two fundamental operating limits that define their stable operating envelope: surge on the low-flow side and choke (stonewall) on the high-flow side. Understanding these limits is critical for safe, reliable operation.
The Operating Envelope
The stable operating range between surge and choke is called the operating envelope. This envelope changes with speed, gas composition, and inlet conditions. Proper control systems must keep the compressor within this envelope under all operating conditions.
2. Understanding Surge
What is Surge?
Surge is an aerodynamic instability that occurs when the flow through the compressor decreases below a critical value called the surge point. At this point, the compressor can no longer maintain the pressure it has developed, resulting in a violent, periodic flow reversal.
The Surge Cycle
- Flow reduction: Operating flow decreases toward the surge limit
- Stall inception: Flow separation occurs in the impeller or diffuser
- Flow reversal: High-pressure discharge gas flows backward through the compressor
- Pressure collapse: Discharge pressure rapidly decreases
- Flow recovery: Forward flow resumes as pressure differential reverses
- Pressure buildup: Compressor rebuilds discharge pressure
- Cycle repeats: If conditions haven't changed, surge continues
The surge cycle typically occurs at 1-5 Hz, depending on system volume and compressor characteristics. Larger discharge volumes result in lower surge frequencies.
Types of Surge
- Mild surge: Small oscillations, limited flow reversal, minimal damage
- Classical surge: Complete flow reversal, significant pressure swings
- Deep surge: Severe oscillations, high mechanical stress, potential for immediate damage
Surge Damage Mechanisms
- Thrust bearing overload: Axial thrust reverses rapidly, exceeding bearing capacity
- Rotor-to-stator contact: Large deflections can cause rubbing
- Seal damage: Pressure reversals stress labyrinth and dry gas seals
- Blade fatigue: Repeated stress cycles cause fatigue failure
- Coupling damage: Torque reversals stress couplings
3. Causes of Surge
Process-Related Causes
- Reduced inlet flow: Upstream equipment shutdown, blocked inlet filter
- Increased discharge pressure: Closed downstream valve, high process pressure
- Changed gas composition: Heavier gas increases head requirement
- Reduced inlet pressure: Lower density decreases mass flow
- Inlet temperature increase: Higher temperature reduces density
Equipment-Related Causes
- Driver trip: Coastdown pushes compressor into surge
- Speed reduction: Reduced head capability at lower speed
- Fouling: Deposits reduce flow capacity
- Control system failure: Incorrect signal causes inappropriate response
Operational Causes
- Rapid load changes: Flow changes faster than control can respond
- Emergency shutdown: Immediate flow reduction without recycle
- Startup/shutdown: Transitions through unstable regions
4. Surge Detection
Direct Detection Methods
These methods detect surge after it has begun:
- Pressure pulsation: Rapid fluctuations in discharge pressure
- Flow pulsation: Oscillating flow readings
- Vibration increase: Subsynchronous vibration at surge frequency
- Axial position change: Thrust bearing proximity probes
- Temperature spikes: Discharge temperature oscillations
Predictive Detection Methods
These methods predict surge before it occurs:
- Surge margin calculation: Distance from operating point to surge line
- Rate of change monitoring: Tracking approach rate to surge line
- Inlet guide vane position: Correlation with surge limit
Why Prediction is Better Than Detection
Detecting surge after it occurs means the compressor has already experienced potentially damaging conditions. Modern anti-surge systems use predictive algorithms to prevent surge from ever occurring.
5. Anti-Surge Control Systems
System Components
- Flow measurement: Orifice plate, venturi, or annubar at suction
- Pressure measurement: Suction and discharge pressure transmitters
- Temperature measurement: For density correction
- Anti-surge controller: Dedicated controller (often separate from DCS)
- Recycle valve: Quick-opening valve in recycle line
- Hot gas bypass: Alternative to recycle for some applications
Surge Control Line
The surge control line is set to the right of the actual surge line, providing margin for:
Surge Margin Requirements (API 617)
Minimum Surge Margin = 10%
Control Line = Surge Line × (1 + Margin)
Additional margin for measurement uncertainty, valve response time, and process dynamics
Control Strategies
1. Flow-Based Control
- Measures actual flow vs. surge flow at current head
- Simple but requires accurate surge line characterization
- May not handle rapid transients well
2. Head-Flow (H-Q) Control
- Uses reduced head and reduced flow coordinates
- More robust across operating conditions
- Industry standard approach
3. Polytropic Head Control
- Calculates actual polytropic head from process measurements
- Most accurate for variable gas compositions
- Requires more instrumentation
Recycle Valve Sizing
The recycle valve must be sized to:
- Pass minimum stable flow at maximum head conditions
- Open fast enough to prevent surge during trips
- Provide stable control without hunting
Typical Recycle Valve Requirements
Stroke time: ≤2 seconds full stroke
Cv sizing: 100-150% of surge flow
Fail position: Open (fail-safe)
6. Understanding Choke
What is Choke?
Choke (also called stonewall or stonewalling) occurs when gas velocity within the compressor reaches sonic velocity. At this point, flow cannot increase regardless of downstream pressure reduction.
Choke Locations
Sonic velocity can occur at several locations:
- Impeller inlet: First stage most common location
- Impeller throat: Minimum flow area in impeller
- Diffuser throat: Vaned diffuser inlet
- Return channel: Less common
Effects of Choke Operation
- Head collapse: Rapid decrease in developed head
- Efficiency drop: Significant efficiency reduction
- Temperature rise: More energy goes to heating gas
- Vibration: High-frequency oscillations may occur
- Noise: Characteristic high-pitched sound
Choke vs. Surge Severity
Unlike surge, brief choke operation is not immediately destructive. However, sustained choke operation causes efficiency losses, potential vibration damage, and may indicate undersized equipment.
7. Operating Guidelines
Normal Operation
- Maintain minimum 10% surge margin at all times
- Avoid prolonged operation at very low or very high flows
- Monitor vibration trends for early warning signs
- Verify anti-surge system functionality regularly
Startup Procedures
- Open recycle valve fully before starting
- Establish minimum flow through recycle
- Bring compressor to operating speed
- Gradually close recycle valve as process flow increases
- Verify surge margin throughout startup
Shutdown Procedures
- Open recycle valve to establish minimum flow
- Reduce speed if variable speed driver
- Trip compressor only when recycle is established
- Depressurize through normal blowdown systems
Emergency Situations
- Trip with recycle: Recycle opens immediately, compressor coasts down safely
- Trip without recycle: Risk of surge during coastdown - minimize this scenario
- Sustained surge: Trip immediately if surge persists more than 5 seconds
8. Troubleshooting
Frequent Surge Events
- Verify surge line characterization is accurate
- Check for measurement errors or failures
- Review control system tuning
- Evaluate if process conditions have changed
- Inspect for fouling or damage
Recycle Valve Issues
- Hunting: Reduce controller gain, check valve sizing
- Slow response: Check actuator sizing, inspect valve trim
- Persistent open: Process may be operating near surge
Reduced Operating Range
- Check for fouling (moves surge line right)
- Verify gas composition matches design
- Evaluate inlet conditions vs. design
References
- API 617 - Axial and Centrifugal Compressors and Expander-Compressors
- API 670 - Machinery Protection Systems
- GPSA, Section 13
- Bloch, H.P. - Compressor Performance
- Industry Best Practices - Compressor Surge Control