Gravity flow calculator

Fluid Properties

Flowrate Basis

Mass Flowrate (Qm)kg/h

Vol. Flowrate (Qv)m³/h

Density (ρ) kg/m³

Pipe Configuration

Nominal Size (DN)

Pipe Schedule

Validation & Checks

Calculation Results

Selected Pipe ID (di) -

Flow Area (A) -

Superficial Velocity (v) -

Froude Number (NFr) -

Flow Regime -

Required Diameter (Optimization)

Min. ID for Self Venting -

Technical Notes & Engineering Guide

Variable Definitions

Qm (kg/h): Mass flowrate of the liquid.
Qv (m³/h): Volumetric flowrate (Qm / ρ).
ρ (kg/m³): Fluid density at operating conditions.
di (mm): Internal pipe diameter derived from Nominal Size (DN) and Schedule.
NFr: Froude Number (dimensionless).

Formulas & Logic

Velocity (v): Calculated based on full pipe area.
v = Qv / (π · (di/2)²)
Froude Number (NFr):
NFr = v / √(g · di)
Criteria (Self-Venting): Flow is considered self-venting if NFr < 0.3. This ensures air can escape upstream against the liquid flow.
Criteria (Siphon): If NFr > 0.3, the pipe may run full, siphon, or pulse (slug flow).

Assumptions & Standards

Standard: Based on principles found in Hills (1974) for gravity lines and self-venting criteria.
Fluid: Assumes single-phase liquid entry (no flashing).
Erosion: Velocity is checked against a standard limit of 0.9 - 1.0 m/s for gravity drains to prevent erosion/noise.
Min Size: Checks against DN150 (common underground minimum), though smaller is allowed for instrument drips.