What is a hydraulic gradient line (HGL) in liquid pipeline design?

The hydraulic gradient line represents the total energy head along a pipeline, calculated as HGL = Elevation + P/(0.4335 × SG). It shows how pressure varies with distance and elevation, helping identify slack flow conditions.

What is slack flow in a liquid pipeline?

Slack flow occurs when pipeline pressure drops below the liquid vapor pressure at a high point, causing a vapor pocket to form. This calculator detects slack flow by comparing the HGL against the elevation profile and vapor pressure.

What equations does this hydraulic gradient calculator use?

The calculator uses Darcy-Weisbach friction loss with Colebrook-White friction factor, combined with elevation head calculated as ΔP_elev = SG × 0.4335 × Δh to determine the pressure profile along the pipeline.

How is elevation head calculated for liquid pipelines?

Elevation head in psi equals the liquid specific gravity times 0.4335 times the elevation change in feet. For example, pumping crude oil (SG 0.85) up a 1,000-foot hill requires about 368 psi of additional pressure beyond friction losses.

How do I prevent slack flow in a liquid pipeline?

Prevent slack flow by increasing inlet pressure, installing intermediate booster pump stations, using drag reducing agents (DRAs), increasing pipe diameter, or optimizing the pipeline route to avoid extreme hilltops. Maintain at least 50-100 psi above vapor pressure at all points.

Liquid Hydraulic Gradient Calculator | Pipeline Pressure Profile

Flow Parameters

Flow Rate

Liquid Specific Gravity

Dynamic Viscosity

Pipe Properties

Pipe Inner Diameter

12" NPS Sch 40 = 11.938 in

Absolute Roughness

Pressure Parameters

Inlet Pressure

psig

Min Allowable Pressure

psig

For slack flow detection

Vapor Pressure

psia

Elevation Profile

Station (miles), Elevation (ft)

One point per line: milepost, elevation. Comma or tab separated.

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Understand liquid hydraulic gradient fundamentals, slack flow conditions, and pressure profile analysis

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Results

Enter pipeline parameters and elevation profile, then click Calculate.

Key Equations

Darcy-Weisbach:

ΔP = f(L/D)(ρV²/2gₒ)/144

Elevation Head:

ΔP_elev = SG × 0.4335 × Δh

HGL Elevation:

HGL = Elev + P/(0.4335 × SG)

Slack Flow:

Occurs when P < P_vapor

Standards: Darcy-Weisbach, Colebrook-White (1939), Crane TP-410

Liquid Hydraulic Gradient Calculator

Flow Parameters

Pipe Properties

Pressure Parameters

Elevation Profile

📚 Learn the Theory

Results

Key Equations

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Liquid Pipeline Hydraulics