1. Selection Overview
Compressor selection is one of the most critical decisions in gas processing and pipeline design. The choice between centrifugal and reciprocating machines affects capital cost, operating efficiency, reliability, and maintenance requirements for the life of the facility.
Reciprocating
Positive displacement
Fixed volume per stroke. Flow nearly independent of discharge pressure.
Centrifugal
Dynamic compression
Adds velocity then converts to pressure. Flow varies with head developed.
Key trade-off
Flexibility vs reliability
Recip offers more turndown; centrifugal offers fewer wearing parts.
NGPSA Application Map
The NGPSA provides application maps showing typical ranges for each compressor type based on flow rate and discharge pressure:
- Below 1,000 acfm: Reciprocating usually favored
- 1,000-10,000 acfm: Either type may work - detailed evaluation needed
- Above 10,000 acfm: Centrifugal usually favored
- Above 1,500 psig: Reciprocating often required
2. Side-by-Side Comparison
The following table summarizes key differences between centrifugal and reciprocating compressors based on industry guidelines:
| Parameter | Reciprocating | Centrifugal |
|---|---|---|
| Max discharge pressure | 12,000 psi (50,000 hyper) | 1,450 psi (horiz), 15,000 psi (barrel) |
| Max inlet flow | Limited by cylinders | 400,000 acfm |
| Turndown capability | 100% to 20% or lower | 20-30% (fixed speed), 40-50% (VFD) |
| Reliability/availability | Lower (more wearing parts) | 98-99% typical |
| Compression ratio/stage | 1.2 to 4.0 (typical 3:1) | Depends on MW and stages |
| Capital cost | Lower for small capacity | Lower for large capacity |
| Operating cost | Higher maintenance | Lower maintenance, may need VFD |
| Delivery lead time | 16-30 weeks typical | 40-60 weeks typical |
| Installation footprint | Larger, requires foundation | More compact per capacity |
| Best for low MW gas | Yes (hydrogen service) | Many stages required |
3. Reciprocating Compressors
Reciprocating compressors use pistons driven by a crankshaft to compress gas in cylinders. They are positive displacement machines - the volume compressed per stroke is fixed regardless of discharge pressure (within mechanical limits).
Operating Principle
Advantages
- High pressure capability: Up to 12,000 psi in standard designs, 50,000+ psi in hypercompressors
- Excellent turndown: Can reduce flow from 100% to 20% or lower using unloaders, variable speed, or clearance pockets
- Low flow applications: Practical down to very small flows
- Multiple services: Can handle multiple streams with different cylinders on same frame
- Low MW gases: Efficient for hydrogen and light gases
- Shorter lead time: Typically 16-30 weeks vs 40-60 for centrifugal
Disadvantages
- Lower reliability: More wearing parts (valves, piston rings, rod packing)
- Higher maintenance: Valve replacement, packing adjustment, piston ring wear
- Pulsation: Requires pulsation dampeners and careful piping design
- Larger footprint: Requires heavy foundation for vibration
- Flow limitations: Large flows require many cylinders
Compression Ratio Limits
4. Centrifugal Compressors
Centrifugal compressors use rotating impellers to accelerate gas, then convert velocity to pressure in diffusers. They are dynamic machines - flow varies with head developed, and performance follows characteristic curves.
Operating Principle
Advantages
- High reliability: 98-99% availability with proper maintenance
- Low maintenance: Few wearing parts, longer run times between overhauls
- High capacity: Up to 400,000 acfm in single machine
- Compact: Smaller footprint per unit capacity
- No pulsation: Smooth flow, simpler piping design
- High MW efficiency: Excellent for heavier gases (natural gas, propane, CO2)
Disadvantages
- Limited turndown: 20-30% fixed speed, 40-50% with VFD
- Surge limit: Minimum flow required to prevent damaging surge
- Pressure limits: Horizontal split case limited to ~1,450 psig
- Low MW challenges: Hydrogen requires many stages
- Longer lead time: 40-60 weeks typical
- Higher capital for small sizes: Minimum economic size ~1,000 hp
Surge and Stonewall
Molecular Weight Effect
Head developed per stage is inversely proportional to molecular weight:
| Gas | MW | Relative Head | Stages for 4:1 CR |
|---|---|---|---|
| Hydrogen | 2 | 14.5x air | 8-12 |
| Natural Gas | 18 | 1.6x air | 2-4 |
| Air/Nitrogen | 29 | 1.0x (reference) | 2-3 |
| CO2/Propane | 44 | 0.66x air | 1-2 |
| Refrigerant | 86 | 0.34x air | 1 |
5. Selection Factors
Compressor selection involves weighing multiple factors. No single parameter determines the choice - it's the combination of requirements that drives the decision.
Flow Rate
| Flow Range | Typical Selection | Notes |
|---|---|---|
| < 500 acfm | Reciprocating | Below economic centrifugal size |
| 500-2,000 acfm | Favor reciprocating | Small centrifugals available but costly |
| 2,000-10,000 acfm | Either - evaluate | Overlap zone, project-specific |
| 10,000-50,000 acfm | Favor centrifugal | Multiple recip frames needed |
| > 50,000 acfm | Centrifugal | Only practical choice |
Discharge Pressure
| Pressure Range | Typical Selection | Notes |
|---|---|---|
| < 500 psig | Either | Both types well suited |
| 500-1,500 psig | Either - evaluate | Barrel centrifugals available |
| 1,500-5,000 psig | Favor reciprocating | Limited centrifugal options |
| > 5,000 psig | Reciprocating | Only practical choice |
Turndown Requirements
Low (0-20%)
Either type OK
Constant load applications. Centrifugal may be simpler.
Moderate (20-40%)
Slight recip advantage
Centrifugal with VFD can handle. Recip uses unloaders.
High (40-60%)
Favor reciprocating
Beyond typical centrifugal range without recycle.
Very High (60%+)
Reciprocating required
Only positive displacement handles this turndown.
Reliability Requirements
- Standard (95-97%): Either type acceptable. Include spares planning.
- High (97-99%): Centrifugal favored. Fewer wearing parts.
- Critical (99%+): Centrifugal with spare rotor, or 2x100% reciprocating.
Cost Considerations
Operating cost: Centrifugal has lower maintenance but may need VFD for efficiency. Reciprocating has higher parts replacement but may be more efficient at off-design conditions.
Lifecycle cost: Consider 20-year ownership including maintenance, parts, downtime, and energy. Centrifugal often wins for continuous high-utilization service.
6. Application Examples
Gas Gathering
Scenario: Field compression from 50 psig to 500 psig, 3,000 acfm, variable flow as wells decline
Analysis:
- Moderate flow: Both types in range
- Moderate pressure: Both types capable
- High turndown needed: Favors reciprocating
- Remote location: Reciprocating may have faster parts supply
Selection: Reciprocating with capacity control unloaders
Pipeline Transmission
Scenario: Mainline compression, 1,000 psig to 1,200 psig, 50,000 acfm, continuous operation
Analysis:
- High flow: Strongly favors centrifugal
- Low compression ratio: 1.2:1, single stage centrifugal
- High reliability needed: Centrifugal 99% availability
- Continuous duty: Maintenance windows limited
Selection: Centrifugal, gas turbine driven
Hydrogen Service
Scenario: Refinery hydrogen, 200 psig to 3,000 psig, 1,000 acfm
Analysis:
- Low MW (2): Centrifugal would need 8-12 stages
- High pressure: Beyond horizontal centrifugal limits
- Moderate flow: Reciprocating handles easily
- High compression ratio: 3-4 reciprocating stages with intercooling
Selection: Reciprocating, multi-stage with intercoolers
Refrigeration
Scenario: Propane refrigeration, 20 psia to 200 psia, 20,000 acfm
Analysis:
- High MW (44): Efficient centrifugal compression
- High flow: Favors centrifugal
- Continuous duty: High reliability needed
- Single stage sufficient for compression ratio
Selection: Centrifugal, single or two-stage
7. Station Design Best Practices
Based on established industry plant facility design practices, the following practical considerations often override pure hydraulic performance in the final equipment selection.
Project Lifecycle & Economics
Short Project Life
Favor Reciprocating
Lower initial installation cost makes them attractive for projects with limited horizons (e.g., declining fields) where long-term efficiency is less critical.
Long Project Life
Favor Centrifugal
Higher initial cost is justified by reduced long-term operating and maintenance costs over 20+ years.
Site-Specific Constraints
| Constraint | Impact on Selection |
|---|---|
| Noise Ordinances | Local ordinances may make noise abatement costs for gas engines or turbines prohibitive. Electric motor drives may be required near residential areas. |
| Emissions | Existing station emissions may limit adding more HP. Low-NOx turbines or electric drives may be the only permit-able options in non-attainment areas. |
| Delivery Time | Compressor packages are long-lead items. "Stock" reciprocating units often have shorter lead times (16-24 weeks) than custom centrifugal bundles (40-60 weeks). |
| Manning | Unmanned remote stations benefit from the simpler auxiliary systems and higher reliability of centrifugal units compared to the intensive maintenance of recips. |
Selection Decision Matrix
Use this decision matrix to guide the preliminary selection process:
| Factor | Select Reciprocating If... | Select Centrifugal If... |
|---|---|---|
| Flow Rate | Variable or Low (<10,000 ACFM) | Steady High Base Load (>10,000 ACFM) |
| Compression Ratio | High (> 3.0 per stage) | Low to Moderate (< 2.0 per stage) |
| Flow Variation | High fluctuation (Swing Load) | Constant (Base Load) |
| Gas MW | Low (Hydrogen, Helium) | High (Propane, CO2, Natural Gas) |
| Maintenance | Staff available for frequent service | Remote/Unmanned operation |
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