Hydraulics System Design: From Flow to Pump Selection
A short, honest method guide for sizing a single water line and its pump. Every step maps to a live EngiMetric calculator, and the whole sequence runs as one workflow in the Hydraulics System Design workflow.
1. Define the requirement
Decide the design flow Q (m³/h) you must carry and the acceptable velocity range. The recommended operating band for water piping is roughly 0.5–2.5 m/s: lower velocities risk sedimentation and trapped air release; higher velocities increase erosion and friction energy. Choose a target point inside the band.
2. Required pipe size
The required internal diameter follows from continuity: velocity v = Q / A, with A = (π/4)·D². Rearranged, D = √(4Q/(π·v)) with Q in m³/s. This is exactly what the Pipe Sizing calculator computes. The result is an ideal bore; the next step is choosing a standard nominal size.
3. Select the standard diameter
Pick the smallest standard nominal bore whose internal diameter is at least the required value (DN 15 → DN 1000). Nominal bores are reference values — verify the actual bore against the chosen pipe schedule (the wall thickness and pressure class change the true internal diameter). The workflow auto-selects this and lets you override it.
Re-check velocity at the real diameter: the velocity at the selected bore is never exactly the target.
4. Friction head loss
For design support, head loss is computed with the Darcy–Weisbach equation hf = f·(L/D)·(v²/2g), where the friction factor f comes from the Colebrook–White relation — solved by the Darcy–Weisbach calculator. The Reynolds number (and regime) is checked first with the Reynolds Number calculator, because the friction factor is only rigorous in fully turbulent flow.
Hazen–Williams is a quicker alternative for water at normal temperatures and is shown by the Pipe Flow calculator as a cross-check. In the workflow, Darcy–Weisbach is the primary method. Roughness ε values are reference estimates (a few mm/1000): steel ≈ 0.046 mm, ductile iron ≈ 0.13 mm, PVC/HDPE ≈ 0.007–0.0015 mm. Always confirm against the actual product data.
Worked example (36 m³/h, 100 m of commercial steel)
Q = 36 m³/h = 0.01 m³/s. Target v = 1.5 m/s.
Required D = √(4·0.01 / (π·1.5)) ≈ 92.1 mm → select DN 100.
Velocity at DN 100: v = 0.01 / (π/4·0.1²) ≈ 1.27 m/s (inside the recommended band).
Reynolds number at ~20 °C (ν ≈ 1.0·10⁻⁶ m²/s): Re ≈ v·D/ν ≈ 1.27·0.1/1.0·10⁻⁶ ≈ 127 000 → turbulent.
Darcy–Weisbach friction head for steel ε ≈ 0.046 mm: f ≈ 0.02 (Colebrook), hf ≈ 0.02·(100/0.1)·(1.27²/2·9.81) ≈ 1.6 m.
Total dynamic head with static 5 m, minor losses 1 m, residual 0.5 bar (≈ 5.1 m): TDH ≈ 5 + 1.6 + 1 + 5.1 ≈ 12.7 m.
Hydraulic power ≈ ρ·g·Q·H = 9 807·0.01·12.7 ≈ 1.25 kW; with 70% pump and 90% motor efficiency the motor demand is roughly 2.0 kW.
5. Total dynamic head and pump power
Total dynamic head = static lift + friction loss + minor losses + residual pressure head, composed by the Pump Head calculator. Pump power then follows from flow, TDH, and entered efficiencies in the Pump Power calculator (hydraulic, shaft, and motor kW/hp). Efficiencies are your stated estimates — vendor curves determine the actual duty point.
6. Validate, document, decide
Check velocity and regime against the bands above, review every assumption (roughness, minor losses, residual), and record the choice with the calculation context. The Hydraulics System Design workflow runs steps 1–5 together, surfaces PASS/WARN checks, and generates a report derived from the same engineering contexts — nothing is detached from the numbers that produced it.
Limits. This is single-line design support, not an approved design, and not a network model. Transients/surge, two-phase flow, fittings inventories, and governing jurisdictional codes require dedicated analysis by a licensed engineer.