Understanding Surging in Compressors
Definition: What is Surging?
Surging (also called compressor surge) is a violent aerodynamic instability in centrifugal and axial compressors where the entire flow through the compressor periodically reverses direction, creating oscillating pressure and flow with frequencies typically in the range of 0.5-10 Hz. During a surge cycle, flow momentarily stops or reverses, pressure drops, then flow resumes forward, pressure rises, and the cycle repeats. This creates enormous fluctuating forces on the rotor, producing severe vibration, loud booming noise, and can destroy a compressor in minutes if not immediately stopped.
Surge is fundamentally a system instability involving the compressor and its piping/volume, not just the compressor alone. It occurs when attempting to operate beyond the compressor’s pressure rise capability at low flow rates, and prevention requires anti-surge control systems that maintain flow above the surge line.
The Surge Mechanism
Surge Cycle Description
A typical surge cycle progresses as follows:
- Flow Reduction: System demand decreases, flow through compressor reduces
- Stall Onset: At very low flow, compressor blades stall (flow separates)
- Pressure Collapse: Stalled compressor cannot maintain discharge pressure
- Flow Reversal: High-pressure gas in discharge piping/plenum flows backward through compressor
- Pressure Equalization: Discharge pressure drops as gas flows backward
- Forward Flow Resumes: Once pressure drops, compressor can again flow forward
- Pressure Rises: Forward flow increases discharge pressure
- Cycle Repeats: High pressure again causes stall, repeating the cycle
Surge Frequency
- Determined by system volume (piping, plenums, vessels) and compressor characteristics
- Larger volumes → lower surge frequency
- Typical range: 0.5-10 Hz
- Small systems: 5-10 Hz
- Large systems: 0.5-2 Hz
- Frequency relatively constant for a given system
Conditions Leading to Surge
Operating Beyond Surge Line
The surge line on compressor performance map:
- Surge Line: Leftmost stable operating boundary on compressor map
- Safe Operation: To the right of surge line (higher flows)
- Surge Zone: To the left of surge line (unstable, forbidden)
- Margin: Typically operate 10-20% flow margin right of surge line
Triggering Events
- Demand Reduction: Process demand drops, flow reduces
- Discharge Restriction: Valve closure or blockage
- Speed Reduction: Compressor slowing without proportional flow reduction
- Density Changes: Molecular weight or temperature changes affecting compressor characteristic
- Fouling: Blade deposits reducing compressor capacity
Effects and Consequences
Vibration
- Amplitude: Can reach 25-50 mm/s (1-2 in/s) or more
- Axial Component: Particularly severe in axial direction
- Low Frequency: 0.5-10 Hz pulsations
- Whole Machine: Entire compressor assembly rocks and shakes
Mechanical Damage
- Bearing Failure: Shock loads destroy bearings in hours
- Seal Damage: Axial motion and pressure reversals destroy seals
- Shaft Damage: Bending and torsional stress from flow reversal
- Blade Damage: Alternating aerodynamic loads causing fatigue, possible blade liberation
- Coupling Damage: Torsional shock damaging couplings
- Thrust Bearing: Rapidly alternating thrust can destroy thrust bearing
Process Consequences
- Pressure and flow oscillations affecting downstream process
- Temperature excursions from compression/expansion cycles
- Possible process upsets or safety system trips
- Product quality issues from unstable conditions
Detection
Vibration Signature
- Sudden onset of large amplitude low-frequency pulsation
- Frequency in 0.5-10 Hz range
- Severe axial vibration
- Unstable, varying amplitude
Acoustic Signature
- Loud booming or whooshing sound
- Rhythmic pulsation audible at surge frequency
- Distinctive and unmistakable
Process Indicators
- Oscillating discharge pressure
- Oscillating flow (may reverse)
- Temperature fluctuations
- Motor current fluctuations
Prevention: Anti-Surge Control
Anti-Surge System Components
Recycle Valve
- Fast-acting valve bypassing compressor discharge to suction
- Opens to increase flow when approaching surge line
- Sized for full compressor flow if needed
Flow and Pressure Measurement
- Continuous monitoring of flow rate and pressure rise
- Plot operating point on compressor map
- Detect approach to surge line
Controller
- Calculates distance to surge line
- Opens recycle valve when approaching surge (with safety margin)
- Modern systems use adaptive algorithms
- Response time critical (< 1 second typical requirement)
Operating Procedures
- Never operate to left of surge line
- Maintain 10-20% flow margin from surge
- Gradual load changes (avoid rapid demand drops)
- Verify anti-surge system functional before startup
- Test anti-surge periodically
Emergency Response
If Surge Occurs
- Immediate Action: Open recycle valve manually if automatic system failed
- Increase Flow: Open discharge, reduce resistance, start parallel units
- Reduce Pressure Rise: Slow compressor if variable speed
- Emergency Shutdown: If surge cannot be stopped within 10-30 seconds
- Do Not Restart: Until cause identified and corrected
Post-Surge Inspection
- Inspect for blade damage
- Check bearing condition
- Verify seal integrity
- Examine thrust bearing
- Perform vibration analysis before return to service
Surge vs. Other Instabilities
Surge vs. Rotating Stall
- Surge: System-wide flow oscillation, very low frequency (0.5-10 Hz)
- Rotating Stall: Localized stall cells rotating around annulus, higher frequency (0.2-0.8× rotor speed)
- Severity: Surge more destructive, stall may be precursor to surge
Surge vs. Recirculation
- Surge: Compressor-specific, flow reversal, system instability
- Recirculation: Can occur in pumps or compressors, localized flow reversal, less severe
- Relationship: Recirculation can lead to surge in compressors
Surging is the most dangerous operating condition for centrifugal and axial compressors, capable of destroying equipment in minutes. Understanding the surge mechanism, recognizing surge line boundaries, implementing effective anti-surge control, and maintaining proper operating margins are absolutely critical for safe compressor operation in industrial gas compression applications.
EN 
AR 
AZ 
BG 
ZH 
HR 
CS 
DA 
NL 
ET 
FI 
FR 
DE 
KA 
EL 
HE 
HI 
HU 
ID 
IT 
JA 
KO 
LV 
LT 
MS 
NB 
FA 
PL 
PT_BR 
PT 
RO 
SR 
SK 
SL 
ES 
SV 
TH 
TR 
UR 
UK 
VI 