Liquid Line Sizing
Engineering fundamentals for liquid hydrocarbon pipeline design
1. Sizing Principles
Liquid line sizing balances capital cost (pipe diameter) against operating cost (pumping energy) while meeting velocity and pressure drop constraints.
Key Design Criteria
- Velocity limits: Prevent erosion, noise, water hammer
- Pressure drop: Must not exceed available pump head
- NPSH: Suction lines must provide adequate NPSH
- Elevation: Account for static head changes
- Economics: Balance pipe cost vs. pump power cost
Basic Sizing Equation
Diameter from velocity:
d = √(0.4085 × Q / v)
Where:
d = Inside diameter (inches)
Q = Flow rate (gpm)
v = Velocity (ft/s)
Or from flow rate:
v = 0.4085 × Q / d²
2. Velocity Criteria
Velocity limits depend on service, economics, and potential for erosion or cavitation.
Recommended Velocities
| Service |
Typical (ft/s) |
Maximum (ft/s) |
| Pump suction (water) |
2-4 |
5 |
| Pump discharge (water) |
5-8 |
12 |
| Crude oil |
3-6 |
10 |
| NGL/LPG |
3-5 |
8 |
| Produced water |
3-6 |
8 |
| Glycol |
2-4 |
6 |
| Gravity drain |
1-3 |
4 |
Erosional Velocity
API RP 14E erosional velocity:
v_e = C / √ρ
Where:
v_e = Erosional velocity (ft/s)
C = Empirical constant (typically 100-150)
ρ = Fluid density (lb/ft³)
For clean liquids: C = 125-150
For liquids with solids: C = 100 or less
Example: Crude (ρ = 54 lb/ft³)
v_e = 125 / √54 = 17 ft/s maximum
Design practice: Size for 50-70% of erosional velocity under normal conditions, with margin for flow surges.
3. Pressure Drop Calculations
Pressure drop in liquid lines is calculated using the Darcy-Weisbach equation with friction factor from the Moody diagram.
Darcy-Weisbach Equation
Head loss:
h_f = f × (L/D) × (v²/2g)
Pressure drop:
ΔP = f × (L/D) × (ρv²/2) / 144
Where:
h_f = Head loss (ft of fluid)
f = Darcy friction factor (dimensionless)
L = Length (ft)
D = Diameter (ft)
v = Velocity (ft/s)
g = 32.2 ft/s²
ρ = Density (lb/ft³)
ΔP = Pressure drop (psi)
Reynolds Number
Reynolds number:
Re = ρvD/μ = 7,742 × Q × SG / (d × μ)
Where:
Q = Flow (gpm)
SG = Specific gravity
d = ID (inches)
μ = Viscosity (cP)
Flow regimes:
Re < 2,100: Laminar
Re > 4,000: Turbulent
2,100 < Re < 4,000: Transition
Friction Factor
Laminar flow (Re < 2,100):
f = 64 / Re
Turbulent flow (Colebrook):
1/√f = -2 log₁₀(ε/3.7D + 2.51/(Re√f))
Explicit approximation (Swamee-Jain):
f = 0.25 / [log₁₀(ε/3.7D + 5.74/Re^0.9)]²
Where ε = Pipe roughness (ft)
Steel pipe: ε ≈ 0.00015 ft
Hazen-Williams (Water Only)
For water systems:
h_f = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87)
Where:
h_f = Head loss (ft/100 ft)
Q = Flow (gpm)
d = ID (inches)
C = Hazen-Williams coefficient
Typical C values:
New steel: 140
Aged steel: 100-120
Cast iron: 100-130
Plastic: 150
4. NPSH Considerations
Suction line sizing must ensure adequate Net Positive Suction Head (NPSH) to prevent cavitation.
NPSH Available
NPSH available:
NPSH_a = P_s/γ + z_s - h_f - P_vp/γ
Where:
P_s = Source pressure (psia)
γ = Specific weight (lb/ft³)
z_s = Static suction head (ft, + above pump)
h_f = Friction losses in suction line (ft)
P_vp = Vapor pressure at pumping temperature (psia)
Design requirement:
NPSH_a > NPSH_r + margin (typically 3-5 ft)
Suction Line Sizing
- Keep suction velocities low: 2-4 ft/s typical
- Minimize fittings: Each fitting adds friction loss
- Avoid air pockets: Continuous slope toward pump
- Eccentric reducers: Flat side up at pump
- 5-10 diameters straight: Before pump inlet
Vapor Pressure Effects
| Fluid |
Temp (°F) |
P_vp (psia) |
| Water |
100 |
0.95 |
| Water |
150 |
3.72 |
| Light crude |
100 |
2-5 |
| Propane |
100 |
190 |
| Butane |
100 |
52 |
5. Applications
Sizing Example
Given: 1,000 gpm crude (SG=0.85, μ=5 cP), size discharge line for 6 ft/s max
d_min = √(0.4085 × 1000 / 6) = √68.1 = 8.25"
Select 10" Sch 40 (ID = 10.02")
Actual v = 0.4085 × 1000 / 10.02² = 4.07 ft/s ✓
Re = 7742 × 1000 × 0.85 / (10.02 × 5) = 131,400 (turbulent)
Line Sizing Summary
| Line Type |
Primary Constraint |
Secondary Check |
| Pump suction |
NPSH available |
Low velocity |
| Pump discharge |
Pressure drop/head |
Erosional velocity |
| Transfer lines |
Available pressure |
Economics |
| Gravity flow |
Elevation difference |
Full pipe flow |
References
- Crane Technical Paper 410 – Flow of Fluids
- API RP 14E – Offshore Production Platform Piping
- ASME B31.3 – Process Piping
- Hydraulic Institute Standards