| Scenario | Flow | Length | Lift | Material | Target velocity | Recommended size |
|---|---|---|---|---|---|---|
| Drip irrigation main | 25 gpm | 18 ft | 5 ft | PVC | 4 ft/s | 2 in |
| Sprinkler zone supply | 45 gpm | 35 ft | 6 ft | HDPE | 4 ft/s | 2½ in |
| Filling tank from pond | 60 gpm | 25 ft | 6 ft | PVC | 3 ft/s | 3 in |
| High-flow transfer | 90 gpm | 40 ft | 8 ft | Steel | 4 ft/s | 4 in |
- Velocity: V = Q / A, where A = πD²/4.
- Reynolds number: Re = ρVD/μ to classify flow behavior.
- Friction factor: Swamee–Jain for turbulent flow using ε/D.
- Darcy–Weisbach loss: h_f = f(L/D)(V²/2g).
- Minor losses: h_m = ΣK(V²/2g) from fittings.
- Total loss: h_total = h_f + h_m for the suction line.
- NPSHa estimate: NPSHa = H_atm − H_vap − lift − h_total (sea-level assumption).
For suction piping, lower velocity and fewer fittings protect NPSH margin and reduce noise.
- Choose your unit system, then enter the pump operating flow.
- Measure suction length along the pipe, plus suction lift.
- Select pipe material and enter water temperature.
- Set a conservative max velocity and allowable suction loss.
- Add the fittings installed on the suction side only.
- Press Calculate and review the selected size and options table.
- If NPSH margin is low, upsize or simplify the suction run.
Always verify with local codes and pump manufacturer guidance.
Why suction sizing matters for garden pumps
Suction piping sets the hydraulic conditions at the pump eye. Excess velocity and loss reduce available NPSH, increase noise, and can trigger cavitation. For irrigation intakes from ponds, wells, or tanks, a stable suction line helps protect seals and bearings and keeps flow steady during zone changes.
Velocity and head loss targets
A conservative suction velocity limit is typically 0.9–1.5 m/s (about 3–5 ft/s). Lower velocity decreases friction loss and reduces air entrainment at strainers. Total suction loss is often kept small compared with discharge losses, because every unit of suction loss directly subtracts from NPSHa.
Friction, fittings, and equivalent resistance
This calculator uses Darcy–Weisbach for straight pipe and adds minor losses from fittings using summed K values. Elbows, tees, reducers, and foot valves can add significant resistance, especially on short suction runs. When fittings dominate, upsizing one nominal size can cut losses sharply because velocity drops with larger area.
NPSH check and reliability
NPSHa is estimated from atmospheric head minus vapor pressure head, suction lift, and total suction losses. Warmer water raises vapor pressure and lowers NPSHa, so summer irrigation can be more sensitive. If your pump’s published NPSHr is close to NPSHa, consider shortening the suction line, reducing fittings, lowering lift, or increasing diameter.
Practical installation notes
Keep suction piping as straight as possible into the pump, avoid high points where air can collect, and ensure joints are airtight. Use a properly sized strainer or foot valve with adequate open area to limit inlet velocity. After installation, monitor priming time, vibration, and sound as real‑world indicators. Record operating flow, suction lift, and water temperature during commissioning, then compare to calculated velocity and losses. If the source is at higher elevation, atmospheric head drops and margins shrink. Smooth materials like PVC and HDPE generally produce lower friction than steel. Recheck sizing when adding filters, longer hose runs, or seasonal algae buildup. over time regularly.
1) What suction velocity should I aim for?
Many irrigation intakes perform well near 3–5 ft/s (0.9–1.5 m/s). Lower velocity is safer for priming and NPSH, especially with warm water or long suction runs.
2) Why does the calculator include fittings?
Elbows, tees, valves, and strainers add “minor” losses, but they can dominate short suction lines. Summed K losses capture that resistance and prevent undersizing when the line has many fittings.
3) My pump cavitates even with a large pipe. What else matters?
Check suction lift, air leaks, clogged strainers, and water temperature. Also confirm NPSHr at your operating flow. Reducing lift and fittings often helps more than changing discharge piping.
4) Should I size from nominal pipe size or inside diameter?
Hydraulic calculations depend on inside diameter. Nominal size labels differ by schedule and material, so the tool uses typical internal diameters to compute velocity, Reynolds number, and losses consistently.
5) How accurate is the NPSHa estimate?
It is a simplified screening check assuming sea-level atmospheric head and water properties from temperature. High elevation, entrained air, acceleration head, and inlet disturbances can reduce actual NPSHa.
6) When should I upsize beyond the recommendation?
Upsize when NPSH margin is small, lift is high, the suction line is long, or fittings/strainers are restrictive. Upsizing can improve priming, reduce noise, and tolerate seasonal fouling better.