Performance Maps

1. Overview

A performance map (also called a compressor map or characteristic curve) is a graphical representation of a centrifugal compressor's operating characteristics. It shows the relationship between flow, head, speed, and efficiency across the compressor's entire operating range.

2. Map Axes & Coordinates

Vertical Axis: Head or Pressure Ratio

The vertical axis typically shows:

• Polytropic head (Hp): ft-lbf/lbm or kJ/kg - Most common for process compressors
• Pressure ratio (P₂/P₁): Dimensionless - Common for pipeline compressors
• Discharge pressure: For fixed suction pressure applications

Horizontal Axis: Flow

Flow is typically expressed as:

• ACFM (Actual Cubic Feet per Minute): Volume at suction conditions - Most accurate
• ICFM (Inlet Cubic Feet per Minute): Same as ACFM at compressor inlet
• SCFM or MMSCFD: Standard conditions - Requires correction for actual conditions
• Mass flow (lbm/min): Independent of conditions but less intuitive

3. Speed Lines

Speed lines show compressor performance at constant rotational speed. Each line represents operation at a specific RPM, typically ranging from 70% to 105% of design speed.

Characteristics of Speed Lines

• Higher speeds produce higher head at any given flow
• Lines curve downward from left to right (head decreases as flow increases)
• The slope becomes steeper near surge and flatter near choke
• Lines are approximately parallel in the normal operating range

Fan Laws (Affinity Laws)

Speed lines can be estimated using the fan laws:

These laws are approximations that work well near design conditions but become less accurate at extremes.

4. The Surge Line

The surge line defines the minimum stable flow at each speed. Operating to the left of this line causes surge - a violent, periodic flow reversal that can damage the compressor.

What Happens During Surge

1. Flow decreases below the stability limit
2. The compressor can no longer maintain discharge pressure
3. High-pressure gas flows backward through the compressor
4. Discharge pressure drops, allowing forward flow to resume
5. The cycle repeats at 1-5 Hz frequency

Surge Line Shape

The surge line typically:

• Curves upward and to the right with increasing speed
• Follows approximately a parabolic path through the origin
• Shifts with changes in gas composition or inlet conditions

5. The Choke Line (Stonewall)

The choke line defines the maximum flow at each speed. At this point, gas velocity in the compressor internals reaches sonic velocity, limiting further flow increase.

Choke Characteristics

• Speed lines become nearly vertical at choke
• Head drops rapidly with small flow increases
• Efficiency decreases dramatically
• High-frequency vibration may occur

Unlike surge, choke operation is not immediately destructive but should be avoided due to poor efficiency and vibration concerns.

6. Efficiency Contours

Many performance maps include efficiency contours (iso-efficiency lines) overlaid on the head-flow curves. These show regions of equal polytropic or isentropic efficiency.

Efficiency Island

The contours typically form an "efficiency island" pattern:

• Peak efficiency occurs at or near the design point
• Efficiency decreases in all directions from the peak
• The island is often elongated along the speed line direction
• Steeper efficiency drop toward surge than toward choke

Control Lines and Margins

Anti-surge controllers use a control line set to the right of the actual surge line. This provides a safety margin for:

• Process measurement uncertainty
• Controller response time
• Valve stroking time
• Dynamic process changes

7. Reading a Performance Map

Step-by-Step Process

1. Determine required flow: Convert to ACFM at suction conditions
2. Calculate required head: From pressure ratio and gas properties
3. Plot the operating point: Intersection of flow (x) and head (y)
4. Identify the speed line: Interpolate between adjacent lines if needed
5. Check surge margin: Ensure adequate distance from surge line
6. Read efficiency: From efficiency contours or calculate

Common Mistakes to Avoid

• Using standard flow (SCFM) without converting to actual (ACFM)
• Ignoring Z-factor corrections in head calculations
• Assuming factory map applies directly to field conditions
• Plotting pressure ratio on a head-based map

Operating Point Location

The operating point should be located:

• Well to the right of the surge control line (minimum 10%)
• Within the efficient operating region (above 70% efficiency)
• Below the maximum continuous speed line
• On or near the design speed line for best efficiency

8. Correcting for Different Conditions

When operating conditions differ from map reference conditions, corrections are required:

Gas Composition Changes

• Recalculate molecular weight (MW)
• Recalculate specific heat ratio (k)
• Recalculate Z-factor
• Recompute head requirements

Inlet Condition Changes

For changes in suction pressure or temperature:

• Recalculate density and ACFM
• Verify surge margin at new conditions
• Check for required speed adjustments

Multi-Stage Considerations

For multi-stage compressors, each stage has its own performance characteristics:

• All stages share the same shaft speed
• Flow through each stage is determined by upstream conditions
• First stage usually limits turndown (surge first)
• Last stage may limit overload (choke first)
• Total head = sum of individual stage heads

References

• GPSA, Section 13 - Compressors and Expanders
• API 617 - Axial and Centrifugal Compressors and Expander-Compressors
• API 670 - Machinery Protection Systems
• Bloch, H.P. - Compressors and Modern Process Applications
• Gresh, T. - Compressor Performance: Aerodynamics for the User