1. Overview
A centrifugal compressor performance map is the primary tool for predicting compressor behavior at various operating conditions. OEMs provide these maps based on factory test data, and engineers use them for system design, control philosophy, and troubleshooting.
X-Axis
Inlet Volume Flow
ICFM or ACFM at suction conditions
Y-Axis
Head or Pressure Ratio
Polytropic head (ft-lbf/lb) or P2/P1
Speed Lines
Constant RPM Curves
Typically 5-8 speeds from 70-105% Nrated
Efficiency Islands
Contour Lines
Peak efficiency at BEP; decreases off-design
Map Types
| Map Type | Y-Axis | X-Axis | Best For |
|---|---|---|---|
| Head vs. Flow | Polytropic Head (ft-lbf/lb) | ICFM | Multi-gas applications |
| Pressure Ratio vs. Flow | P2/P1 | ACFM or MMSCFD | Fixed-gas service |
| Reduced Map | Head/N^2 | Flow/N | Variable speed analysis |
| Power vs. Flow | BHP | ICFM | Driver sizing |
Head vs. pressure ratio: Head-based maps are preferred because polytropic head is independent of gas composition (for a given Z and T). Pressure ratio maps are gas-specific and must be regenerated when gas properties change.
2. Speed Lines & Map Axes
Each speed line represents compressor behavior at a constant rotational speed. The shape reveals aerodynamic characteristics of the impeller design.
Speed Line Characteristics
| Region | Flow | Head | Behavior |
|---|---|---|---|
| Near Surge | Low | Maximum | Flat or rising curve; unstable flow imminent |
| BEP | Design | Moderate | Peak efficiency; smooth operation |
| Right of BEP | High | Declining | Head drops with increasing flow |
| Stonewall | Maximum | Rapidly dropping | Mach = 1 in passages; head collapses |
Reduced Speed and Flow (API 617):
N_reduced = N / sqrt(k * Z * R * T1 / MW)
Q_reduced = Q / sqrt(k * Z * R * T1 / MW)
Where:
N = Actual speed (RPM)
Q = Actual inlet volume flow (ICFM)
k = Specific heat ratio
Z = Compressibility factor
T1 = Suction temperature (deg R)
MW = Molecular weight
Reduced parameters collapse multiple gas conditions onto a single curve,
enabling "universal" performance prediction.
Speed Line Slope
The slope of the speed line (dH/dQ) is a critical stability indicator:
| Slope Sign | Condition | Stability |
|---|---|---|
| Negative (dH/dQ < 0) | Normal operating range | Stable: system naturally self-correcting |
| Zero (dH/dQ = 0) | Peak head point | Marginal: onset of instability |
| Positive (dH/dQ > 0) | Left of peak | Unstable: surge region |
Backward-leaning impellers produce steeper speed lines with more stable operating range. Radial impellers have flatter curves and narrower stable envelopes but achieve higher head per stage.
3. Surge Line & Stonewall
The surge line and stonewall (choke) limit define the usable operating envelope on a performance map.
Surge Line
Surge is a violent aerodynamic instability where flow periodically reverses through the compressor. The surge line connects the minimum stable flow points across all speed lines.
| Parameter | Value | Notes |
|---|---|---|
| Surge frequency | 0.5-10 Hz | Deep surge < 2 Hz; mild surge 5-10 Hz |
| Pressure pulsation | 10-50% of Pd | Causes piping vibration and fatigue |
| Temperature spike | 50-200 deg F | Flow reversal recompresses hot gas |
| Axial thrust reversal | Up to 2x design | Damages thrust bearings rapidly |
| Time to damage | Seconds to minutes | Repeated surge destroys seals and bearings |
Surge Control Line (SCL)
Surge Control Line Placement:
SCL = Surge Line + Safety Margin
Typical margins (per API 617):
Control line: 10% flow to right of surge line
Trip line: 5% flow to right of surge line
Q_control = Q_surge x 1.10
Q_trip = Q_surge x 1.05
The anti-surge valve opens when operating point approaches SCL,
recycling gas from discharge to suction to maintain minimum flow.
Stonewall (Choke)
Stonewall occurs when gas velocity reaches sonic conditions (Mach = 1) in the impeller passages or diffuser throat. Beyond this point, increasing flow produces no additional head.
| Effect | Consequence | Mitigation |
|---|---|---|
| Head collapse | Rapid drop to zero head | Limit flow with discharge throttle |
| Efficiency drop | Falls below 50% | Avoid sustained operation in choke |
| Noise increase | High-frequency aerodynamic noise | Not structurally damaging short-term |
| Power increase | Power may exceed driver rating | Monitor driver loading continuously |
Surge vs. stonewall risk: Surge causes immediate mechanical damage and must be avoided at all times. Stonewall operation is inefficient but not immediately destructive. Anti-surge protection is always the higher priority.
4. Fan Laws & Speed Corrections
The affinity laws (fan laws) allow prediction of compressor performance at different speeds from a single test curve. These are exact for incompressible flow and approximate for compressible gas.
Affinity Laws for Centrifugal Compressors:
Law 1 - Flow: Q2/Q1 = N2/N1
Law 2 - Head: H2/H1 = (N2/N1)^2
Law 3 - Power: P2/P1 = (N2/N1)^3
Where:
Q = Volume flow (ICFM)
H = Polytropic head (ft-lbf/lb)
P = Power (HP)
N = Rotational speed (RPM)
Compressibility Correction:
For gases (k != 1.0), fan laws are approximate. Accuracy degrades at:
Pressure ratios > 2.5
Mach numbers > 0.8
Speed changes > +/- 15% from test speed
For better accuracy, use Schultz correction factors or OEM software.
Speed Change Effects
| Speed Change | Flow Change | Head Change | Power Change |
|---|---|---|---|
| -20% (80% N) | -20% | -36% | -49% |
| -10% (90% N) | -10% | -19% | -27% |
| Design (100% N) | 100% | 100% | 100% |
| +5% (105% N) | +5% | +10% | +16% |
| +10% (110% N) | +10% | +21% | +33% |
Variable Speed vs. Throttling
| Control Method | Turndown | Efficiency Impact | Capital Cost |
|---|---|---|---|
| Variable speed (VSD) | 50-105% | Follows cubic law; minimal penalty | High (VFD cost) |
| Suction throttle | 70-100% | Moderate loss; reduces density | Low (valve only) |
| Inlet guide vanes | 65-100% | Pre-swirl changes impeller work | Moderate |
| Discharge throttle | 70-100% | High loss; wastes energy as heat | Low (valve only) |
| Anti-surge recycle | 0-100% | Very poor; recompresses recycled gas | Required for safety |
Variable speed is preferred for applications with significant flow variation. Power savings follow the cube law: reducing speed by 10% saves 27% power. Over a 20-year life, VSD payback is typically 2-4 years in pipeline service.
5. Operating Envelope
The operating envelope defines all allowable combinations of flow and head. It is bounded by the surge line, stonewall, minimum speed, maximum continuous speed (MCS), and driver power limit.
Envelope Boundaries
| Boundary | Limiting Factor | Consequence of Violation |
|---|---|---|
| Surge line (left) | Aerodynamic instability | Flow reversal, mechanical damage |
| Stonewall (right) | Sonic velocity in passages | Head collapse, power overload |
| MCS (top) | Rotor stress, bearing DN limit | Rotor burst, bearing failure |
| Minimum speed (bottom) | Oil lift-off, resonance avoidance | Bearing damage, vibration |
| Power limit | Driver rated power | Motor trip, turbine overtemp |
Operating Point Determination
System Resistance Curve:
H_system = H_static + K * Q^2
Where:
H_static = Static head (pressure difference at zero flow)
K = System resistance coefficient (friction, valves, etc.)
Q = Volume flow
The operating point is the intersection of the speed line and
system resistance curve.
Multiple Operating Cases:
Summer/Winter (temperature changes gas density)
Beginning/End of field life (flow rate changes)
Clean/Fouled (resistance changes)
Design must ensure all cases fall within the operating envelope.
API 617 requires the compressor manufacturer to demonstrate adequate surge margin at all specified operating points, including off-design conditions such as turndown, gas composition changes, and fouled conditions.
6. Gas Property Corrections
Performance maps are generated for a specific gas composition and suction conditions. When actual conditions differ, corrections are required.
Correction Factors
Volume Flow Correction:
Q_corrected = Q_actual * (Z_test * T_test * P_actual) / (Z_actual * T_actual * P_test)
Head Correction (Approximate):
H_corrected = H_test * (MW_test / MW_actual) * (Z_actual / Z_test) * (T_actual / T_test)
Power Correction:
BHP_corrected = BHP_test * (MW_actual / MW_test) * (P_actual / P_test) * (T_test / T_actual) * (Z_test / Z_actual)
| Property Change | Effect on Flow | Effect on Head | Effect on Power |
|---|---|---|---|
| MW increases 10% | No change (ICFM) | Decreases ~10% | Increases ~10% |
| T1 increases 20 deg F | Increases ~2% | Increases ~2% | Approximately constant |
| P1 increases 10% | Decreases ~10% | No change | Increases ~10% |
| k increases 5% | Negligible | Negligible | Increases ~3% |
| Z decreases 5% | Increases ~5% | Decreases ~5% | Approximately constant |
When to re-rate: If gas MW changes by more than 5%, Z changes by more than 3%, or suction temperature changes by more than 30 deg F, request an updated performance map from the OEM rather than relying on correction factors.
7. Worked Examples
Example 1: Reading a Performance Map
Given:
Compressor map at 100% speed (N = 11,000 RPM)
Design point: Q = 5,000 ICFM, Hp = 42,000 ft-lbf/lb
Surge point at 100%: Q_surge = 3,200 ICFM
Stonewall at 100%: Q_choke = 7,500 ICFM
Find: Surge margin and stable range
Step 1: Surge margin
SM = (Q_design - Q_surge) / Q_design x 100%
SM = (5,000 - 3,200) / 5,000 x 100% = 36% (> 10% minimum per API 617)
Step 2: Stable operating range
Range = (Q_choke - Q_surge) / Q_design x 100%
Range = (7,500 - 3,200) / 5,000 x 100% = 86%
Step 3: Turndown ratio
Turndown = Q_surge / Q_design = 3,200 / 5,000 = 64%
Maximum turndown at 100% speed = 36%
Example 2: Fan Law Speed Correction
Given:
At N1 = 11,000 RPM: Q1 = 5,000 ICFM, H1 = 42,000 ft-lbf/lb, BHP1 = 3,500 HP
Find: Performance at N2 = 9,900 RPM (90% speed)
Step 1: Speed ratio
N2/N1 = 9,900 / 11,000 = 0.900
Step 2: New flow
Q2 = Q1 x (N2/N1) = 5,000 x 0.900 = 4,500 ICFM
Step 3: New head
H2 = H1 x (N2/N1)^2 = 42,000 x 0.810 = 34,020 ft-lbf/lb
Step 4: New power
BHP2 = BHP1 x (N2/N1)^3 = 3,500 x 0.729 = 2,552 HP
Power savings: 948 HP (27%) for 10% speed reduction
Example 3: Gas Composition Change
Given:
Test gas: MW = 18.5, k = 1.28, Z = 0.92
Actual gas: MW = 21.2, k = 1.22, Z = 0.88
Test performance: Hp = 42,000 ft-lbf/lb, BHP = 3,500 HP
Find: Corrected performance for actual gas
Step 1: Corrected head (polytropic head changes with MW)
H_corrected = 42,000 x (18.5/21.2) x (0.88/0.92)
H_corrected = 42,000 x 0.8726 x 0.9565 = 35,058 ft-lbf/lb
Step 2: Corrected power
BHP_corrected = 3,500 x (21.2/18.5) x (0.92/0.88)
BHP_corrected = 3,500 x 1.146 x 1.045 = 4,192 HP
Result: Heavier gas produces less head but requires more power.
Verify driver has adequate margin for the heavier gas case.
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