| name | compressor-surge |
| description | Compressor surge — surge line, Greitzer B parameter, rotating stall, anti-surge control, recycle valve, suction throttle, surge margin, API 670 vibration, active magnetic bearing damping. |
| metadata | {"priority":7,"promptSignals":{"phrases":["compressor surge","surge control","anti-surge valve","surge margin","rotating stall","Greitzer B parameter"],"minScore":3}} |
Compressor Surge — Complete Skill
Surge Fundamentals
Surge: complete flow breakdown and reversal in compressor; system instability
Onset: when compressor operates at flow rates below surge line on compressor map
Mechanism: positive slope on pressure-flow curve → unstable equilibrium → limit cycle oscillations
Distinction from rotating stall:
- Rotating stall: one or more stall cells rotate around annulus; flow not reversed; local flow breakdown
- Surge: full annulus breakdown; global flow reversal; violent; damaging to machine
Surge indicators:
- Loud periodic bangs (0.5–2 Hz for industrial compressors; slower for large machines)
- Large pressure/flow oscillations; vibration
- Rotor axial thrust reversal → thrust bearing damage
- Temperature spikes at discharge
- Damage to seals, blades, diaphragms
Compressor Map and Surge Line
Surge line: connects surge points at different speeds on P-Q or head-flow map
Surge margin (SM):
SM = [(Q_design - Q_surge) / Q_design] × 100% [%]
Or: SM = [(H_surge/H_design) - 1] × 100% (pressure-based definition; less common)
API 617 requirement: SM ≥ 10% between normal operating point and surge line at any speed
Operating line vs. surge line:
Operating line: system resistance curve intersects compressor curve
If system resistance increases (valve closes, pipe plugs) → operating point moves left → toward surge
Stonewall (choke): right side limit; sonic condition at some flow path throat
Greitzer B Parameter
Stability criterion for whether system enters rotating stall or surge:
B = U/(2ω_H L_c) = (U/2) × √(V_p / (A_c L_c a²))
B = Greitzer parameter; U = rotor tip speed [m/s]; ω_H = Helmholtz frequency; L_c = effective plenum inlet duct length; V_p = plenum volume; A_c = duct area; a = sound speed
Critical B value:
B < B_crit: rotating stall (stable oscillation)
B > B_crit: full surge (unstable)
B_crit ≈ 0.5–1.0 (depends on compressor characteristic slope)
Implication: large downstream plenum volume → high B → more prone to full surge
Piping system design: minimize unnecessary plenum volume between compressor and downstream throttle
Anti-Surge System
Recycle (Anti-Surge) Valve
Function: when compressor approaches surge → open recycle → return gas from discharge to suction → increase compressor flow → move operating point away from surge
Valve type:
- Control valve: proportional control; modulates continuously
- Trip valve: fast-opening (full open in < 1 sec) for surge trip
Valve sizing:
Q_recycle_min = Q_surge × 1.1 - Q_process [recycle must provide this additional flow when process demand drops]
Cv = Q_recycle / √(ΔP_recycle / SG) [size for full bypass at maximum recycle condition]
Loop cooling: recycled gas may need cooling (gas heats with recycling); cooler in recycle loop
Surge Control Algorithm
Standard approach (line-based):
- Calculate compressor "position" relative to surge line using measured P_suction, P_discharge, T_suction, flow Q
- Define control line = surge line × 1.1 (10% margin)
- If flow drops to control line → PID opens recycle valve
- Rate of change algorithm: detect rapid approach → pre-emptive opening
Flow measurement correction:
Corrected flow: Q_corr = Q × √(T_in_actual/T_in_ref) × (P_ref/P_in_actual)
Suction Throttle (Alternative)
For centrifugal with variable inlet guide vanes (IGV):
Close IGV → reduce flow → maintain operating point with preswirl
Reduces surge margin consumed → better part-load efficiency
IGV effect on surge line:
Closing IGV shifts both surge line and performance curve → maintains relative margin
Effective from 100% to ~50% capacity
Dynamic Surge Analysis
Simplified lumped-parameter model:
ρ L dV/dt = P_c - P_s - ρ V² / (2 K_T) [V = flow velocity; P_c = compressor pressure; P_s = system; K_T = throttle constant]
dP_plenum/dt = (ρ a² / V_plenum) × (Q_c - Q_process)
Limit cycle (steady surge):
Amplitude determined by compressor curve shape and B parameter
Frequency: ω_surge ≈ ω_H × √(1 - B² × dΨ/dΦ × slope)
Damping injection via AMB:
Active Magnetic Bearings with surge detection controller → add artificial damping to rotor
Shifts surge onset to lower flows → increased operating range by 5–15%
Rotating Stall
Stall cells: 1–N_blade stall cells rotate at 25–70% of rotor speed
Detection: vibration at stall cell frequency = n × f_stall
Effect:
Moderate: reduced performance; increased bearing loads; noise
Severe: can progress to full surge; blade fatigue
Recovery: typically auto-recovers when back-pressure reduced; no hardware required
Some compressors designed to operate in stall (fan laws on stalled side)
Vibration Monitoring (API 670)
Non-contacting proximity probes (XY pair):
Mounted at each journal bearing; continuous orbit/vibration monitoring
Alert level: 1× amplitude > 0.4 × (12000/N)^0.5 mils pp (typical)
Trip: 2× alert (or 50.8 μm pp, whichever less)
Surge event signature:
Large broadband vibration surge → thrust bearing axial motion > 0.5 mm → trip on axial position
Axial position monitor:
API 670 requires axial probe; trip if axial displacement > ±0.75 mm from center (typical)
Standards
| Standard | Scope |
|---|
| API 617 | Centrifugal compressors for petroleum industry |
| API 670 | Machinery protection systems; vibration limits |
| ISA-S67.04 | Surge control systems |
| API 521 | Pressure relief sizing |
Output
Provide: surge margin SM [%] at operating point (vs. API 617 ≥ 10%), Greitzer B parameter, anti-surge valve Cv and type (control/trip), recycle flow Q_recycle [m³/hr] at surge condition, suction throttle (IGV) use, control algorithm (line-based/rate-of-change), recycle cooler requirement (temperature at recycle inlet [°C]), axial position trip limit [mm], vibration limits [μm p-p per API 670], and applicable standard (API 617, API 670).
