Question 1

What is the Darcy-Weisbach equation and when should I use it?

Accepted Answer

Darcy-Weisbach: ΔP = f × (L/D) × ½ρv², where f is the Darcy friction factor (Moody), L is pipe length (m), D is inner diameter (m), ρ is fluid density (kg/m³), and v is velocity (m/s). Use it for: any fluid (water, oil, slurry, gas), any pipe material, both laminar and turbulent flow. It is the most accurate method and is preferred in engineering design per SANS and ISO standards. The friction factor f is found from the Colebrook-White equation: 1/√f = -2 log₁₀(ε/(3.7D) + 2.51/(Re√f)).

Question 2

What is the Hazen-Williams formula and when should I use it?

Accepted Answer

Hazen-Williams: V = 0.8492 × C × R⁰·⁶³ × S⁰·⁵⁴, where C is the roughness coefficient, R is hydraulic radius, S is the hydraulic slope. Use it only for water at near-ambient temperatures in full-flowing pressurised pipes. C values: HDPE 140–150, PVC 140, new commercial steel 120, galvanised steel 120, old cast iron 80–100. It is simpler than Darcy-Weisbach but not valid for viscous fluids, hot water, or non-circular pipes.

Question 3

What is Reynolds Number and what flow regime is laminar vs turbulent?

Accepted Answer

Reynolds Number Re = ρ × v × D / μ = v × D / ν, where v is velocity (m/s), D is diameter (m), μ is dynamic viscosity (Pa·s), ν is kinematic viscosity (m²/s). Laminar flow: Re < 2300 (smooth, layered flow, f = 64/Re). Transition zone: 2300 < Re < 4000 (unpredictable). Turbulent flow: Re > 4000 (most industrial pipe flows). Typical velocities: water in supply lines: 0.5–2.5 m/s, pump suction: 0.5–1.5 m/s, pump discharge: 1.0–3.0 m/s, steam: 25–50 m/s.

Question 4

How do I calculate pressure drop for pipe fittings and valves?

Accepted Answer

Resistance (K) method: ΔP_fitting = K × ½ρv². Sum all K values for all fittings in the system. Typical K values: standard elbow 0.9, long-radius elbow 0.6, gate valve (open) 0.2, globe valve (open) 6.0, check valve 2.0, tee (branch) 1.8. Alternatively, use equivalent length method: L_eq = K × D / f. For quick estimates, total fitting losses are often 20–50% of straight pipe losses in typical industrial systems.

Question 5

What are recommended water velocities in industrial pipelines?

Accepted Answer

Recommended velocities for water: Pump suction: 0.5–1.2 m/s (low velocity to prevent cavitation). Pump discharge: 1.5–3.0 m/s. Process plant headers: 1.0–2.5 m/s. Fire water mains: 1.5–3.5 m/s. Mine service water: 1.5–3.0 m/s. Mine slurry: 1.5–4.0 m/s (must exceed deposition velocity). Cooling water: 0.9–2.4 m/s. High velocities increase pressure drop (proportional to v²) and can cause erosion and noise.

Question 6

What is total dynamic head (TDH) for pump selection?

Accepted Answer

TDH (m) = Static head + Friction head + Velocity head + Pressure head. Static head: vertical elevation difference (m). Friction head: pipe and fitting losses converted to metres head (ΔP/ρg). Velocity head: v²/2g (usually negligible for centrifugal pumps, < 0.1m). Pressure head: required delivery pressure converted to metres head. Example: 30m lift + 15m friction losses + 0.5m velocity head = 45.5m TDH. Select pump from duty point (TDH vs flow rate) on pump curve.

Question 7

How does pipe diameter affect pressure drop?

Accepted Answer

Pressure drop is extremely sensitive to pipe diameter: ΔP ∝ 1/D⁵ (for constant flow rate). Doubling the pipe diameter reduces pressure drop by a factor of 32 (2⁵). Therefore: First iteration: calculate minimum velocity → select standard pipe size → recalculate pressure drop. SANS/SABS standard pipe sizes (DN/ID mm): DN25 (26.6mm), DN32 (35.1mm), DN40 (40.9mm), DN50 (52.5mm), DN65 (68.8mm), DN80 (80.8mm), DN100 (102.3mm), DN150 (154.1mm), DN200 (202mm). Always calculate based on actual internal diameter.

Question 8

What causes water hammer and how do I prevent it?

Accepted Answer

Water hammer (pressure surge) occurs when flow is rapidly stopped: ΔP = ρ × c × Δv, where c is the pressure wave speed (~1200 m/s for water in steel pipe, ~350 m/s in HDPE). Maximum surge pressure: ~12 bar per 1 m/s velocity change in steel pipe. Prevention: slow-closing valves (>10s close time), surge vessels/air chambers, check valve dampers, reduce pipe velocity below 2 m/s, use flexible HDPE instead of steel. South African mining and water treatment plants are particularly vulnerable due to long pipeline runs.

Pipe Pressure Drop Calculator

Pipe Pressure Drop Calculations

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Key Formulas

Darcy-Weisbach

Colebrook-White (turbulent)

Hazen-Williams

Reynolds Number

Velocity Head & Fittings

Pump TDH

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