Pipeline Design

Gas Velocity Calculations

Design gas pipelines using erosional velocity limits (API RP 14E), sonic velocity constraints, and recommended velocity ranges for various service types.

Launch Calculator API RP 14E

Erosional Velocity

Ve = C / √ρ

C = 100 continuous; 125 intermittent; 150 clean service

Sonic Limit

Ma < 0.7

Keep Mach number below 0.7 to avoid choking and noise

Transmission Lines

20-60 ft/s

Balance pressure drop, pipe cost, and erosion limits

Contents

Key Design Criteria

  • V < 0.8 × Ve (80% of erosional)
  • Ma < 0.7 (avoid sonic effects)
  • V > Vmin (avoid liquid loading)
  • Check pressure drop economics

1. Overview

Gas velocity determines pipe erosion, noise, pressure drop, and liquid carryover. Pipeline diameter selection balances capital cost (smaller pipe) against operating constraints.

Chart showing gas velocity design envelope with erosional velocity curve, 80% design limit, and minimum velocity for liquid loading, with acceptable design zone shaded between limits
Gas velocity design envelope: Design between 80% of erosional velocity (upper limit) and minimum velocity for liquid loading (lower limit).

Velocity Calculation

Gas Velocity from Flow Rate: v = Qactual / A Where: v = Gas velocity (ft/s) Qactual = Volumetric flow at operating P,T,Z (ft³/s) A = Pipe cross-sectional area = π D² / 4 (ft²) Converting standard to actual flow: Qactual = Qstd × (Pstd/P) × (T/Tstd) × Z Standard conditions: 14.73 psia, 60°F (520°R)

Velocity Limits by Service

Service Type Typical Range (ft/s) Design Consideration
Gathering Lines15-50May contain liquids/sand
Transmission Lines20-60Optimize pressure drop
Distribution Mains10-40Lower pressure systems
Compressor Suction20-40Minimize pulsation
Compressor Discharge30-60Higher pressure allows higher V
Plant Piping30-80Short runs, ΔP less critical
Flare Headers40-100Emergency, short duration
Design Philosophy: For most applications, design at 80% of erosional velocity to provide margin for flow surges and measurement uncertainty. Check minimum velocity for wet gas to prevent liquid accumulation.

2. Erosional Velocity (API RP 14E)

API Recommended Practice 14E provides the industry-standard empirical equation for maximum gas velocity to prevent erosion-corrosion in piping systems.

API RP 14E Erosional Velocity: Ve = C / √ρ Where: Ve = Erosional velocity (ft/s) C = Empirical constant (see table below) ρ = Gas density at flowing conditions (lb/ft³)

C-Factor Selection

Service Condition C (US units) Application
Continuous, standard service100Normal pipeline operation
Intermittent service (<8 hr/day)125Blowdown, purge lines
Clean, non-corrosive, inhibited150Sweet, dry gas systems
Corrosive service (CO₂, H₂S)75-100Sour gas, wet CO₂
Sand-laden gas50-75Sand production, erosive
Log-log chart showing erosional velocity versus gas density curves for C-factors of 75, 100, 125, and 150 per API RP 14E formula
API RP 14E erosional velocity: Vₑ = C/√ρ. Select C-factor based on service conditions.

Quick Reference Table

ρ (lb/ft³) Ve @ C=100 Ve @ C=125 Ve @ C=150
0.5141 ft/s177 ft/s212 ft/s
1.0100 ft/s125 ft/s150 ft/s
2.071 ft/s88 ft/s106 ft/s
3.058 ft/s72 ft/s87 ft/s
5.045 ft/s56 ft/s67 ft/s

API RP 14E Limitations

  • Empirical basis: Derived from 1970s field experience, not rigorous physics
  • Conservative: Many systems operate safely above Ve
  • Does not account for: Particle size, pipe material, liquid loading
  • For critical applications: Use CFD or detailed erosion models (DNV, E/CRC)
Best Practice: Use C = 100 for initial design. Apply 0.8 safety factor (design at 80% of Ve). For sandy or corrosive service, reduce C-factor appropriately.

3. Sonic Velocity & Mach Number

When gas velocity approaches the speed of sound, compressible flow effects dominate. At Mach 1.0, flow is choked—further pressure reduction cannot increase flow rate.

Speed of Sound in Gas: a = 223.0 × √(k × T / MW) [ft/s] Where: a = Sonic velocity (ft/s) k = Isentropic exponent (Cp/Cv) T = Temperature (°R) MW = Molecular weight (lb/lbmol) 223.0 = √(gc × R) = √(32.174 × 1545.35) Mach Number: Ma = v / a Natural gas (SG ≈ 0.65): k ≈ 1.285, MW ≈ 18.8 At 60°F: a ≈ 1,330 ft/s

Mach Number Design Limits

Application Max Ma Reason
Straight pipe (long runs)0.7Avoid noise, vibration, excessive ΔP
Valves and fittings0.6Prevent local choking
Control valves0.3-0.5Maintain control stability
Pressure relief valves1.0Designed for choked flow
Flare headers0.5Avoid back pressure issues
Horizontal bar diagram showing flow regimes by Mach number from 0 to 1.2 with zones for incompressible, compressible subsonic, high subsonic (avoid), and sonic/supersonic flow
Mach number design limits: Keep Ma < 0.7 for pipe, Ma < 0.5 for valves to avoid noise, vibration, and choking.

Sonic Velocity vs Temperature

Temperature Natural Gas (ft/s) 70% of Sonic (ft/s)
0°F1,249875
60°F1,328930
100°F1,379965
150°F1,4391,007
Practical Note: For most pipeline velocities (30-80 ft/s), Mach number is typically 0.02-0.06—well below the 0.7 limit. Sonic velocity is rarely governing for pipelines but is critical for control valves and restrictions.

4. Pipeline Sizing Criteria

Sizing Workflow

Step 1: Calculate gas density at operating conditions ρ = (SG × 0.0765 × P × 520) / (14.73 × T × Z) [lb/ft³] Step 2: Calculate erosional velocity Ve = C / √ρ Step 3: Set design velocity Vdesign = 0.8 × Ve (80% safety factor) Step 4: Calculate minimum diameter D = √(4 × Qactual / (π × Vdesign)) × 12 [inches] Step 5: Select next larger standard pipe size NPS options: 2, 3, 4, 6, 8, 10, 12, 14, 16, 18, 20, 24, 30, 36... Step 6: Verify actual velocity and Mach number

Standard Pipe Sizes (NPS)

NPS Sch 40 ID (in) Area (in²)
4"4.02612.73
6"6.06528.89
8"7.98150.03
10"10.02078.85
12"11.938111.9
16"15.000176.7
20"19.000283.5
24"23.000415.5

Liquid Loading Velocity

In wet gas systems, minimum velocity is required to carry liquid droplets and prevent accumulation:

Turner Correlation (Critical Velocity for Liquid Loading): Vcritical = 1.593 × σ0.25 × (ρL - ρG)0.25 / ρG0.5 For water in natural gas (simplified): Vcritical ≈ 11.5 / √ρG [ft/s] Compare to erosional: Ve = C/√ρ = 100/√ρ At low pressure (low ρ): Erosional velocity governs At high pressure (high ρ): Liquid loading may govern
Economic Optimization: For long transmission lines, economic velocity (30-50 ft/s) may be lower than erosional limit. Balance pipe capital cost against compression operating cost over project life.

5. Design Examples

Example 1: Gathering Line Velocity Check

Given: Q = 10 MMSCFD, D = 6" (ID = 6.065"), P = 400 psig, T = 80°F SG = 0.70, Z = 0.95 Step 1: Calculate gas density Pabs = 400 + 14.7 = 414.7 psia T = 80 + 460 = 540°R ρ = (0.70 × 0.0765 × 414.7 × 520) / (14.73 × 540 × 0.95) ρ = 1.53 lb/ft³ Step 2: Calculate erosional velocity (C = 100) Ve = 100 / √1.53 = 80.8 ft/s Vdesign = 0.8 × 80.8 = 64.7 ft/s Step 3: Calculate actual velocity Qactual = 10 × 10⁶ / 86400 × (14.73/414.7) × (540/520) × 0.95 Qactual = 4.06 ft³/s A = π × (6.065/12)² / 4 = 0.201 ft² V = 4.06 / 0.201 = 20.2 ft/s Result: V = 20.2 ft/s < 64.7 ft/s ✓ (25% of erosional)

Example 2: Size Pipe for Target Velocity

Given: Q = 50 MMSCFD, P = 800 psig, T = 70°F SG = 0.65, Z = 0.88, Target velocity = 40 ft/s Step 1: Calculate actual flow rate Pabs = 814.7 psia, T = 530°R Qstd = 50 × 10⁶ / 86400 = 578.7 scfs Qactual = 578.7 × (14.73/814.7) × (530/520) × 0.88 Qactual = 9.38 ft³/s Step 2: Calculate required area A = Q / V = 9.38 / 40 = 0.235 ft² = 33.8 in² Step 3: Calculate diameter D = √(4 × 33.8 / π) = 6.56 inches Step 4: Select next standard size Select 8" pipe (ID = 7.981") Actual A = 50.03 in² = 0.347 ft² Actual V = 9.38 / 0.347 = 27.0 ft/s Step 5: Verify erosional limit ρ = 3.07 lb/ft³ Ve = 100 / √3.07 = 57.1 ft/s V / Ve = 27.0 / 57.1 = 47% ✓
Nomograph with three vertical scales for flow rate, pipe size, and velocity at 500 psig, with example alignment showing 50 MMSCFD in 8 inch pipe equals 27 ft/s
Pipeline sizing nomograph: Align flow rate and pipe size to read velocity. For other pressures, scale inversely with pressure.

Common Design Mistakes

  • Using C = 100 for all cases: Adjust for service conditions
  • Ignoring liquid loading: Wet gas at low pressure needs minimum velocity
  • Designing for average flow: Size for maximum anticipated flow
  • Wrong gas density: Must use actual P/T conditions, not standard
  • Neglecting Z-factor: High pressure gas deviates from ideal behavior
Final Check: After sizing, verify: (1) V < 0.8 × Ve, (2) V > Vcritical if wet gas, (3) Ma < 0.7, (4) ΔP acceptable. Document all assumptions.

Ready to use the calculator?

→ Launch Calculator