1. Overview
Compressed air is an essential utility at natural gas compressor stations. It serves multiple critical functions including engine and turbine starting, pneumatic instrument supply, actuator operation, maintenance tool power, and safety system support. A properly designed compressed air system ensures reliable station operation while minimizing energy waste and maintenance burden.
Starting Air
Engine & Turbine Start
Pneumatic starters for gas engines and turbines; largest intermittent demand
Instrument Air
Controls & Actuators
Dried and filtered air for pneumatic controllers and valve positioners
Tool Air
Maintenance Tools
Impact wrenches, grinders, blowguns for routine maintenance
Safety Systems
ESD & Blowdown
Fail-safe actuators and emergency shutdown valve operation
Air Quality Classifications
| Service | Pressure (psig) | Dewpoint | Filtration | Standard |
|---|---|---|---|---|
| Utility / Tool Air | 90-125 | Ambient +20 deg F | 40 micron | General practice |
| Instrument Air | 90-125 (regulated to 20-30) | -40 deg F PDP | 1 micron + coalescing | ISA-7.0.01 |
| Starting Air | 90-125 | Not critical | 40 micron | Engine/turbine OEM spec |
| Plant Air (critical) | 90-125 | -20 deg F PDP | 5 micron | Facility-specific |
Design philosophy: Size the compressed air system for worst-case simultaneous demand with adequate receiver storage to handle peak transient loads such as engine starts. The system must maintain minimum operating pressure at the most remote user during peak demand events.
2. Demand Analysis
Accurate demand analysis is the foundation of compressed air system design. Each consumer must be cataloged with its flow rate, duty cycle, and required pressure. The total system demand is calculated using diversity factors that account for the probability of simultaneous operation.
Air Starter Requirements
Air starters represent the largest single air demand at a compressor station. A typical gas engine pneumatic starter consumes 150-300 CFM for 10-30 seconds per start attempt. Multiple consecutive start attempts must be considered in the design basis.
| Engine Size (HP) | Starter Flow (CFM) | Start Duration (sec) | Air per Start (SCF) | Attempts Allowed |
|---|---|---|---|---|
| 500-1,000 | 150-200 | 10-15 | 25-50 | 3 |
| 1,000-2,000 | 200-300 | 15-20 | 50-100 | 3 |
| 2,000-4,000 | 300-450 | 15-25 | 75-190 | 3 |
| 4,000-8,000 | 400-600 | 20-30 | 130-300 | 3 |
Instrument Air Demand
| Device | Flow (SCFH) | Duty Cycle | Effective CFM |
|---|---|---|---|
| Pneumatic controller | 18-36 | Continuous | 0.3-0.6 |
| Control valve positioner | 6-24 | Continuous | 0.1-0.4 |
| On/off actuator (per stroke) | 30-120 | Intermittent | 0.5-2.0 per event |
| Analyzer sample system | 12-60 | Continuous | 0.2-1.0 |
| Pneumatic level switch | 6-12 | Continuous | 0.1-0.2 |
Tool Air and Miscellaneous Demand
| Tool / Consumer | Flow (CFM) | Pressure (psig) | Duty Cycle (%) |
|---|---|---|---|
| 1/2-in impact wrench | 4-6 | 90 | 25 |
| 3/4-in impact wrench | 8-12 | 90 | 25 |
| Die grinder | 4-8 | 90 | 30 |
| Blowgun | 8-15 | 90 | 10 |
| Paint sprayer | 6-12 | 50-70 | 40 |
| Leak testing | 2-5 | 90 | 10 |
Total system demand calculation:
Q_total = Q_continuous + Q_intermittent * DF + Q_leakage
Where:
Q_continuous = Sum of all continuous consumers (instrument air)
Q_intermittent = Sum of all intermittent consumers at full load
DF = Diversity factor (0.40-0.70 typical for stations)
Q_leakage = Estimated system leakage (5-15% of total demand)
Diversity factors by consumer type:
Instrument air: 0.80-1.00 (most devices run continuously)
Tool air: 0.20-0.40 (intermittent use)
General plant: 0.30-0.50
Design margin:
Q_design = Q_total * 1.20 to 1.30 (20-30% growth allowance)
3. Air Compressor Selection
Air compressors at compressor stations are typically either rotary screw or reciprocating type. The selection depends on required capacity, duty cycle, available power, and maintenance considerations. Most stations in the 25-200 CFM range use rotary screw compressors for their reliability and low maintenance requirements.
Compressor Type Comparison
| Feature | Rotary Screw | Reciprocating (Piston) |
|---|---|---|
| Capacity range | 10-500 CFM | 1-200 CFM |
| Pressure range | 100-175 psig | 100-250 psig |
| Duty cycle | Continuous (100%) | 50-75% typical |
| Specific power | 18-22 BHP/100 CFM | 20-25 BHP/100 CFM |
| Maintenance interval | 8,000-10,000 hrs | 2,000-4,000 hrs |
| Oil carryover | 2-5 ppm (oil-flooded) | Varies by type |
| Noise level | 65-78 dBA | 75-90 dBA |
| Best application | Continuous base load | Intermittent or small demand |
Compressor Sizing
Required compressor capacity:
Q_compressor = Q_design / eta_system
Where:
Q_design = Total design demand (CFM at standard conditions)
eta_system = System efficiency factor (0.85-0.95)
Compressor power (approximate):
BHP = (Q * P_d) / (229 * eta_isen * eta_mech) * [(P_d/P_s)^0.283 - 1]
Where:
Q = Compressor capacity (CFM free air)
P_d = Discharge pressure (psia)
P_s = Suction pressure (psia, typically 14.7)
eta_isen = Isentropic efficiency (0.70-0.85)
eta_mech = Mechanical efficiency (0.90-0.95)
0.283 = (k-1)/k for air, k = 1.4
Rule of thumb:
BHP per 100 CFM at 100 psig = approximately 20-22 HP (rotary screw)
BHP per 100 CFM at 125 psig = approximately 23-25 HP (rotary screw)
Redundancy:
Minimum N+1 configuration (one standby compressor).
For critical instrument air, consider 2+1 or dedicated backup.
Station practice: Most compressor stations use a duplex or triplex arrangement with lead-lag-standby control. Rotary screw compressors are preferred for base load due to continuous duty capability and lower maintenance. A small reciprocating unit may serve as emergency backup.
4. Receiver Tank Sizing
Air receivers serve as pressure reservoirs to meet short-term peak demands, dampen pressure pulsations, and allow moisture to condense and separate. All receivers must be designed, fabricated, and stamped per ASME Section VIII, Division 1 requirements. Receivers are typically oriented vertically to facilitate moisture drainage.
Sizing Methods
Rule of thumb:
V_receiver (gallons) = 1 gallon per CFM of compressor output
This provides approximately 10 seconds of storage at system pressure
and is adequate for stations with steady-state demand profiles.
Event-based sizing (air starters):
V = T * C * P_atm / (P_1 - P_2)
Where:
V = Receiver volume (ft^3)
T = Event duration (minutes)
C = Air consumption rate during event (CFM)
P_atm = Atmospheric pressure (14.7 psia)
P_1 = Initial receiver pressure (psia)
P_2 = Minimum allowable pressure (psia)
Convert ft^3 to gallons:
V_gal = V_ft3 * 7.48
Recovery time (refill after drawdown):
t_recovery = V * (P_1 - P_2) / (C_comp * P_atm)
Where:
t_recovery = Time to recharge receiver (minutes)
V = Receiver volume (ft^3)
C_comp = Net compressor output (CFM), after subtracting continuous demand
P_1, P_2 = Final and initial pressures (psia)
Receiver Design Parameters
| Parameter | Typical Value | Notes |
|---|---|---|
| Design pressure | 150-200 psig | Minimum 1.25x system MAWP per ASME VIII |
| Operating pressure | 100-125 psig | Set by compressor discharge |
| Minimum pressure | 80-90 psig | Must maintain supply to all users |
| Material | SA-516 Gr. 70 | Carbon steel per ASME II |
| Orientation | Vertical | Facilitates moisture drainage via bottom drain |
| Accessories | PRV, gauge, drain, manway | Safety relief valve set at MAWP |
Starter air sizing: When air starters are the dominant intermittent load, the receiver must be large enough to deliver the required air volume for the specified number of consecutive start attempts without the receiver pressure dropping below the starter minimum operating pressure. Allow for 3 consecutive start attempts as a standard design basis.
5. Distribution System
The distribution system delivers compressed air from the receiver to each point of use. Proper pipe sizing, layout, and moisture management are essential to maintain adequate pressure and air quality at every consumer. Industry-standard methodology limits total system pressure drop to 10% of the compressor discharge pressure.
Pipe Sizing
Recommended velocity limits:
Main headers: 20-30 ft/s
Branch lines: 15-20 ft/s
Instrument air: 10-15 ft/s
Pipe diameter from velocity:
d = sqrt[(4 * Q_actual) / (pi * V_max * 60)]
Where:
d = Internal pipe diameter (ft)
Q_actual = Actual flow at line pressure (CFM)
V_max = Maximum velocity (ft/s)
60 = seconds per minute conversion
Actual flow at line pressure:
Q_actual = Q_std * (P_atm / P_line) * (T_line / T_std)
Where:
Q_std = Standard flow (SCFM at 14.7 psia, 60 deg F)
P_line = Line pressure (psia)
T_line = Line temperature (deg R)
Pressure drop (empirical):
delta_P = (C * L * Q^1.85) / (d^5 * P_avg)
Where:
delta_P = Pressure drop (psi)
C = Friction factor constant (varies by pipe material)
L = Equivalent pipe length including fittings (ft)
Q = Flow rate (SCFM)
d = Internal diameter (in)
P_avg = Average line pressure (psig)
Distribution Best Practices
| Practice | Recommendation | Purpose |
|---|---|---|
| Header layout | Loop configuration preferred | Equalizes pressure, provides redundant supply path |
| Branch takeoffs | Top of header pipe | Prevents moisture carryover to branch lines |
| Header slope | 1/8 in per ft toward drains | Gravity drainage of condensed moisture |
| Drain legs | Every 100-150 ft and at low points | Collect and remove condensate |
| Isolation valves | At each major branch | Allows maintenance without full system shutdown |
| Pipe material | Schedule 40 carbon steel or aluminum | Steel for durability; aluminum for corrosion resistance |
Moisture Separation and Air Treatment
| Treatment Stage | Equipment | Moisture Removal | Application |
|---|---|---|---|
| Aftercooler | Air-cooled or water-cooled HX | To within 15-20 deg F of ambient | All systems (at compressor discharge) |
| Moisture separator | Centrifugal or coalescing | Bulk liquid removal | After aftercooler |
| Refrigerated dryer | Chiller + separator | 38-50 deg F PDP | General plant air |
| Desiccant dryer | Heatless or heated regeneration | -40 to -100 deg F PDP | Instrument air per ISA-7.0.01 |
| Membrane dryer | Hollow fiber membrane | -40 deg F PDP | Remote instrument air (low maintenance) |
| Coalescing filter | 0.01 micron element | Oil aerosol to 0.01 ppm | Downstream of dryer for instrument air |
6. Worked Examples
Example 1: Size Compressed Air System for 3-Unit Compressor Station
Given:
Station: 3 x 2,000 HP gas engine-driven compressor units
Each unit has: pneumatic starter, 8 pneumatic controllers,
4 control valve positioners, 2 on/off actuators
Maintenance tools: 2 impact wrenches, 1 blowgun (simultaneous max)
Step 1: Continuous instrument air demand (per unit)
Controllers: 8 x 0.5 CFM = 4.0 CFM
Positioners: 4 x 0.3 CFM = 1.2 CFM
Total per unit = 5.2 CFM
Total continuous (3 units) = 3 x 5.2 = 15.6 CFM
Diversity factor for instruments = 0.90
Q_instrument = 15.6 x 0.90 = 14.0 CFM
Step 2: Tool air demand
2 impact wrenches: 2 x 10 CFM = 20 CFM
1 blowgun: 12 CFM
Total tools = 32 CFM
Duty cycle = 0.25 (tools used 25% of time)
Q_tools = 32 x 0.25 = 8.0 CFM
Step 3: Leakage allowance
Q_leakage = (14.0 + 8.0) x 0.10 = 2.2 CFM
Step 4: Total continuous/average demand
Q_total = 14.0 + 8.0 + 2.2 = 24.2 CFM
Step 5: Add growth margin (25%)
Q_design = 24.2 x 1.25 = 30.3 CFM
Step 6: Select compressor
Choose 2 x 25 CFM rotary screw compressors (lead + standby)
Combined capacity with one running = 25 CFM > 24.2 CFM (OK)
Both running during recovery = 50 CFM
Example 2: Receiver Sizing for Air Start
Given:
Starter air consumption: 250 CFM for 20 seconds per start
Design basis: 3 consecutive start attempts for 1 unit
System pressure: 125 psig (139.7 psia)
Minimum starter pressure: 80 psig (94.7 psia)
Step 1: Total air volume per start sequence
Air per attempt = 250 x (20/60) = 83.3 SCF
Total for 3 attempts = 3 x 83.3 = 250 SCF
Step 2: Required receiver volume
V = T * C * P_atm / (P_1 - P_2)
V = (3 x 20/60) * 250 * 14.7 / (139.7 - 94.7)
V = 1.0 * 250 * 14.7 / 45.0
V = 81.7 ft^3
Convert to gallons: 81.7 x 7.48 = 611 gallons
Select: 660-gallon (ASME VIII) vertical receiver
Step 3: Verify recovery time
Net compressor output during recovery:
Compressor output = 25 CFM
Continuous demand = 24.2 CFM
Net for recharge = 25 - 24.2 = 0.8 CFM (one compressor)
With both compressors: 50 - 24.2 = 25.8 CFM
t_recovery = V * (P_1 - P_2) / (C_net * P_atm)
t_recovery = 81.7 * 45.0 / (25.8 * 14.7)
t_recovery = 3,676.5 / 379.3
t_recovery = 9.7 minutes (both compressors running)
This is acceptable per industry-standard methodology
(recovery within 15 minutes is typical target).
Example 3: Distribution Header Sizing
Given:
Main header flow: 50 CFM (both compressors running during recovery)
System pressure: 125 psig = 139.7 psia
Temperature: 100 deg F = 560 deg R
Target velocity: 25 ft/s maximum
Step 1: Convert to actual flow at line conditions
Q_actual = 50 * (14.7 / 139.7) * (560 / 520)
Q_actual = 50 * 0.1052 * 1.077
Q_actual = 5.67 ACFM
Step 2: Required pipe diameter
d = sqrt[(4 * 5.67) / (pi * 25 * 60)]
d = sqrt[22.68 / 4,712]
d = sqrt[0.00481]
d = 0.0694 ft = 0.83 in
Select 1-inch Schedule 40 pipe (ID = 1.049 in)
Step 3: Verify velocity
A = (pi/4) * (1.049/12)^2 = 0.006 ft^2
V = 5.67 / (0.006 * 60) = 15.8 ft/s (within limits)
For branch lines to individual units (17 CFM each):
Select 3/4-inch Schedule 40 pipe (ID = 0.824 in)
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