1. Overview
Surge is the most dangerous operating condition for a centrifugal compressor. It occurs when the flow through the compressor drops below the minimum stable flow, causing periodic flow reversal, violent pressure oscillations, and rapid mechanical damage. Adequate surge margin and reliable anti-surge control are essential for safe operation.
Surge
Flow Reversal
Complete or partial backflow through impeller
Surge Margin
Safety Buffer
Distance from operating point to surge line
Anti-Surge Valve
Recycle Protection
Maintains minimum flow through compressor
Surge Detection
dP/dt Monitoring
Rate of pressure change indicates incipient surge
Cost of surge events: A single severe surge event can cause $100K-$500K in damage to bearings, seals, and impellers. Repeated mild surge over months causes fatigue failures in blading and shaft. Anti-surge control is not optional; it is a safety-critical system.
2. Surge Physics
Surge occurs when the compressor can no longer maintain the required discharge pressure at the current flow rate. The aerodynamic stall of the impeller vanes leads to flow instability.
Surge Mechanism
| Phase | Duration | What Happens |
|---|---|---|
| 1. Flow reduction | Gradual | Operating point moves left on performance map |
| 2. Stall inception | Milliseconds | Incidence angle exceeds critical; boundary layer separates |
| 3. Flow reversal | 50-200 ms | High-pressure gas flows backward through impeller |
| 4. Depressurization | 100-500 ms | Discharge pressure drops; flow re-establishes forward |
| 5. Recovery | 200-500 ms | Forward flow builds pressure; cycle may repeat |
Types of Surge
Mild Surge (Incipient):
Frequency: 5-15 Hz
Partial flow reversal in individual passages
Pressure oscillation: 5-15% of discharge pressure
Detectable by vibration increase before full surge develops
Classic (Deep) Surge:
Frequency: 0.3-3 Hz (depends on system volume)
Complete flow reversal through entire compressor
Pressure oscillation: 20-50% of discharge pressure
Violent; causes immediate mechanical damage
Surge Frequency Estimation:
f_surge = a / (4 * L_eff)
Where:
a = Speed of sound in gas (ft/s)
L_eff = Effective length of discharge piping (ft)
f_surge = Surge frequency (Hz)
Larger discharge volumes (vessels, coolers) lower the frequency
and increase the severity of each surge cycle.
Damage from Surge
| Component | Damage Mechanism | Consequence |
|---|---|---|
| Thrust bearing | Axial load reversal up to 2x design | Babbitt wiping, bearing failure |
| Journal bearings | Subsynchronous vibration excitation | Oil whirl/whip instability |
| Labyrinth seals | Rotor-stator contact from vibration | Seal rub, increased leakage |
| Dry gas seals | Pressure reversal across seal faces | Seal face separation, gas leakage |
| Impellers | High-cycle fatigue from flow oscillation | Blade cracking, impeller failure |
| Coupling | Torque reversals and spikes | Coupling element fatigue |
| Piping | Pressure pulsations at surge frequency | Piping vibration, support failure |
Rotating stall vs. surge: Rotating stall is a localized phenomenon where stall cells rotate around the impeller without complete flow reversal. It causes vibration and efficiency loss but is less destructive than surge. However, rotating stall often precedes surge and should trigger anti-surge action.
3. Margin Calculation
Surge margin quantifies the distance between the operating point and the surge line. Multiple definitions exist; it is critical to specify which definition is used.
Surge Margin Definitions:
Definition 1 - Flow-based (most common):
SM_flow = (Q_op - Q_surge) / Q_op x 100%
Where Q values are at the same speed line.
Definition 2 - Head-based:
SM_head = (H_surge - H_op) / H_op x 100%
Where H values are at the same flow.
Definition 3 - Combined (API 617 preferred):
SM = [(Q_op / Q_surge) - 1] x 100%
This is equivalent to Definition 1 rearranged.
Minimum Requirements:
API 617: SM >= 10% at all specified operating points
Typical design target: SM = 15-25% for operational flexibility
Pipeline service: SM = 20-30% (wide flow range expected)
Factors Affecting Surge Margin
| Factor | Effect on Surge Margin | Magnitude |
|---|---|---|
| Speed reduction | Surge flow drops; margin may increase or decrease | Depends on system curve shape |
| Heavier gas (higher MW) | Surge line shifts; often reduces margin | 5-15% reduction possible |
| Higher suction temperature | Increases volume flow; may improve margin | 2-5% per 20 deg F |
| Fouled impellers | Reduces head; surge line shifts right | 5-10% margin reduction |
| Increased system resistance | Operating point moves left toward surge | Varies with system |
| Liquid carryover | Sudden load change; can trigger surge | Immediate risk |
Surge Line Characterization
Surge Line Equation (reduced coordinates):
The surge line can be expressed as a polynomial:
H_surge = a0 + a1*Q + a2*Q^2
Or in reduced form (normalized to design point):
H_surge/H_design = f(Q_surge/Q_design)
Surge line in terms of pressure ratio and flow:
For a given speed N:
r_surge = f(Q_surge)
The surge line connects surge points across all speed lines.
It typically curves upward from lower-left to upper-right on the map.
Invariant surge parameter (Greitzer B parameter):
B = (U_tip / 2*a) * sqrt(V_plenum / (A_c * L_c))
Where:
U_tip = Impeller tip speed
a = Speed of sound
V_plenum = Discharge plenum volume
A_c = Compressor flow area
L_c = Compressor duct length
B > 0.7 typically indicates deep surge susceptibility.
Measurement uncertainty: The actual surge point can only be determined by testing (driving the compressor into surge). OEM surge lines include a safety factor, but field conditions (piping, gas composition) may shift the actual surge point by 3-5% from predictions.
4. Anti-Surge Control
Anti-surge control systems continuously monitor the compressor operating point and take corrective action before surge occurs. Modern systems use dedicated controllers with fast scan rates and predictive algorithms.
Control Architecture
| Component | Function | Typical Specification |
|---|---|---|
| Surge controller | Calculates surge parameter, positions valve | Dedicated PLC or DCS module; 50-100 ms scan |
| Flow measurement | Measures inlet or differential flow | Orifice plate or venturi; 0.5% accuracy |
| Pressure transmitters | P1 (suction) and P2 (discharge) | Smart transmitters; 100 ms response |
| Anti-surge valve | Recycles gas from discharge to suction | Equal-percentage; < 2 sec full stroke |
| Recycle cooler | Cools recycled gas before suction | Sized for maximum recycle flow |
| Check valve | Prevents reverse flow at shutdown | Fast-closing; spring-assisted |
Surge Parameter
Surge Reference Parameter (SRP):
The controller calculates a single parameter that indicates
proximity to surge. Common approaches:
Method 1 - Pressure ratio vs. flow (simple):
SRP = (P2/P1) / f(Q)
Open valve when SRP approaches 1.0 (surge line value).
Method 2 - Polytropic head vs. reduced flow (preferred):
Hp = f(P1, P2, T1, gas properties)
Q_reduced = Q * sqrt(T1) / P1
Plot operating point on Hp vs. Q_reduced map.
Open valve when point approaches surge control line.
Method 3 - dP/dt detection (backup):
If abs(dP2/dt) > threshold (e.g., 5 psi/sec), declare
incipient surge and open valve immediately.
Control Line Offset:
Control line = Surge line + 10% flow margin
Trip line = Surge line + 5% flow margin
Safety line = Surge line itself (emergency trip)
Control Response Requirements
| Event | Required Response | Action |
|---|---|---|
| Approaching control line | Gradual (PI control) | Proportional valve opening |
| Crossing control line | < 1 second | Aggressive valve opening |
| Incipient surge detected | < 500 ms | Step open to 50% or more |
| Full surge confirmed | Immediate | Full open valve; consider trip |
| Emergency shutdown | < 2 seconds | Full open valve; trip driver |
Hot recycle vs. cold recycle: Hot recycle routes discharge gas directly back to suction without cooling. It is faster (no cooler lag) but raises suction temperature, reducing capacity. Cold recycle includes a cooler in the recycle loop. Most installations use cold recycle with a hot bypass for emergency response.
5. Recycle Valve Sizing
The anti-surge recycle valve must be sized to pass enough flow to keep the compressor above its surge line under worst-case conditions, including rapid process upsets and emergency shutdowns.
Minimum Recycle Valve Capacity:
Q_recycle >= Q_surge_max - Q_process_min
Where:
Q_surge_max = Maximum surge flow across all operating cases
Q_process_min = Minimum expected process throughput
Valve Cv Sizing (gas service):
Cv = Q / (N8 * Fp * Y * sqrt(x * P1 * rho1))
Where:
Q = Maximum recycle flow (SCFH)
N8 = Numerical constant (94.8 for SCFH, psia)
Fp = Piping geometry factor
Y = Expansion factor
x = Pressure drop ratio (dP/P1)
P1 = Upstream pressure (psia)
rho1 = Upstream density (lb/ft3)
Sizing Safety Factors:
Minimum Cv margin: 20% above calculated
Typical design: 1.3-1.5 x calculated Cv
Account for reduced Cv at partial stroke positions
Valve Characteristics
| Requirement | Specification | Reason |
|---|---|---|
| Stroke time (open) | < 2 seconds full stroke | Must respond before surge develops |
| Stroke time (close) | 30-120 seconds | Prevent process upsets from rapid closure |
| Characteristic | Equal percentage or quick-opening | Good control in proportional region |
| Fail position | Fail open (air-to-close) | Protects compressor on control failure |
| Actuator type | Piston with volume booster | Fastest response; overcomes spring force |
| Noise level | < 85 dBA at 1m | High dP service generates noise |
| Trim type | Multi-stage or cage trim | Reduces noise and cavitation risk |
Volume booster tuning: The volume booster must be tuned for asymmetric response: fast opening (supply) and slow closing (exhaust). A common setup uses a restrictor on the exhaust side of the booster to achieve 2-second open and 60-second close.
6. API 617 Requirements
API 617 (Axial and Centrifugal Compressors and Expander-compressors) specifies minimum requirements for surge margin, performance testing, and anti-surge protection.
Key API 617 Surge Requirements
| Requirement | API 617 Specification | Notes |
|---|---|---|
| Surge margin | Min 10% flow at each operating point | At constant speed on performance map |
| Operating range | Must cover all specified cases | Including off-design gas compositions |
| Surge test | OEM must identify surge point | Factory test or calculated with safety factor |
| Vibration limits | Per API 617 Table 2.3 | Unfiltered shaft vibration at probe locations |
| Performance guarantee | Head within -2% to +5% | At rated conditions per PTC-10 |
| Mechanical run test | 4 hours at MCS | Maximum continuous speed |
Data Sheet Requirements
API 617 Data Sheet Operating Points:
The purchaser must specify:
1. Rated (design) point: Normal flow, P1, T1, gas
2. Normal range: Expected minimum and maximum flow
3. Turndown point: Minimum process flow
4. Overload point: Maximum expected flow
5. Alternate gas cases: Different compositions
For each point, specify:
Inlet conditions: P1, T1, gas composition, MW, k, Z
Required discharge: P2 (or pressure ratio)
Flow rate: MMSCFD, ICFM, or mass flow
The manufacturer must demonstrate adequate surge margin
at ALL specified points simultaneously on one compressor selection.
Alarm and Trip Settings
| Parameter | Alarm | Trip | Basis |
|---|---|---|---|
| Surge count | 1 event in 10 min | 3 events in 10 min | Cumulative damage prevention |
| Vibration | 1.0 mil p-p | 1.5 mil p-p | API 617 Table 2.3 (varies with speed) |
| Discharge temp | T2_design + 25 deg F | T2_design + 50 deg F | Material and seal limits |
| Axial position | 50% of clearance | 75% of clearance | Thrust bearing protection |
Performance degradation: API 617 permits up to 5% head degradation due to fouling or wear before requiring maintenance. The anti-surge control system must account for this degradation by shifting the surge control line to maintain adequate margin as performance degrades.
7. Worked Examples
Example 1: Surge Margin Calculation
Given:
Design flow Q_design = 5,000 ICFM at N = 11,000 RPM
Surge flow Q_surge = 3,400 ICFM at N = 11,000 RPM
Design head H_design = 42,000 ft-lbf/lb
Head at surge H_surge = 48,500 ft-lbf/lb
Step 1: Flow-based surge margin
SM_flow = (Q_design - Q_surge) / Q_design x 100%
SM_flow = (5,000 - 3,400) / 5,000 x 100% = 32%
Step 2: Head-based surge margin
SM_head = (H_surge - H_design) / H_design x 100%
SM_head = (48,500 - 42,000) / 42,000 x 100% = 15.5%
Step 3: API 617 check
SM_flow = 32% > 10% minimum -- PASS
The compressor has adequate surge margin at design conditions.
Step 4: Control line placement
Q_control = Q_surge x 1.10 = 3,400 x 1.10 = 3,740 ICFM
Q_trip = Q_surge x 1.05 = 3,400 x 1.05 = 3,570 ICFM
Available turndown before recycle: (5,000 - 3,740) / 5,000 = 25.2%
Example 2: Recycle Valve Sizing
Given:
Q_surge = 3,400 ICFM (at suction conditions)
Q_process_min = 1,500 ICFM (minimum process demand)
P1 = 400 psia, P2 = 1,000 psia
T1 = 90 deg F, MW = 18.5, Z = 0.93
Safety factor = 1.3
Step 1: Required recycle capacity
Q_recycle = Q_surge - Q_process_min
Q_recycle = 3,400 - 1,500 = 1,900 ICFM
Step 2: Apply safety factor
Q_recycle_design = 1,900 x 1.3 = 2,470 ICFM
Step 3: Convert to mass flow
rho_suction = (P1 x MW) / (Z x R x T1)
rho_suction = (400 x 18.5) / (0.93 x 10.73 x 549.67)
rho_suction = 7,400 / 5,482 = 1.35 lb/ft3
mass_flow = 2,470 x 1.35 = 3,335 lb/min
Step 4: Valve dP
dP = P2 - P1 = 1,000 - 400 = 600 psi
x = dP/P1 = 600/1,000 = 0.60
Step 5: Size using ISA/IEC method for the required Cv
(Use control valve sizing calculator for final Cv determination)
Estimated Cv = 120-150 (6" valve typical for this service)
Step 6: Verify stroke time
Full stroke < 2 seconds -- specify piston actuator with volume booster
Example 3: Off-Design Surge Check
Given:
Design gas: MW = 18.5, k = 1.28, Z = 0.93
Alternate gas: MW = 22.0, k = 1.20, Z = 0.88
Design surge flow: Q_surge = 3,400 ICFM
Design operating flow: Q_op = 5,000 ICFM
Step 1: Estimate surge shift for heavier gas
Heavier gas reduces head per stage, shifting the speed line down.
The surge point shifts to the right (higher flow) by approximately:
dQ_surge/Q_surge ~ 0.5 x (MW_new - MW_old) / MW_old
dQ_surge = 0.5 x (22.0 - 18.5) / 18.5 x 3,400
dQ_surge = 0.5 x 0.189 x 3,400 = 322 ICFM
Q_surge_new = 3,400 + 322 = 3,722 ICFM (approximate)
Step 2: Check margin with heavier gas
SM = (5,000 - 3,722) / 5,000 x 100% = 25.6%
Still above 10% minimum -- PASS
Step 3: Check control line
Q_control_new = 3,722 x 1.10 = 4,094 ICFM
Available turndown: (5,000 - 4,094) / 5,000 = 18.1%
Note: For accurate results, request OEM re-rate curves for
the alternate gas composition. These approximations are for
preliminary evaluation only.
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