Calculate the pressure profile and hydraulic gradient line (HGL) along a liquid pipeline with elevation changes. Detect slack flow and low-pressure conditions.
12" NPS Sch 40 = 11.938 in
For slack flow detection
One point per line: milepost, elevation. Comma or tab separated.
Understand liquid hydraulic gradient fundamentals, slack flow conditions, and pressure profile analysis
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.
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.
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.
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.
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.