The question and answer
At 10 m³/h, how much does pressure drop change when a 50 m water line increases from NPS 1½ to NPS 2 or NPS 2½?
Total loss falls from 71.226 kPa to 21.048 kPa with NPS 2, then to 8.957 kPa with NPS 2½. These are hydraulic comparisons at a fixed flow, not a claim that the largest pipe is the most economical choice.
1. Hold the duty constant
| Input | Value |
|---|---|
| Flow | 10 m³/h |
| Straight length | 50 m |
| Density / dynamic viscosity | 998.2 kg/m³ / 1.002 mPa·s |
| Roughness | 0.045 mm |
| Total fitting K | 4.0, referenced to each candidate pipe bore |
| Elevation / equipment losses | 0 m / 0 kPa |
| Boundaries | Pipe inlet and pipe outlet of equal bore; α = 1 |
| Required outlet pressure | 0 kPa gauge for this loss-only comparison |
Use the actual inside diameter, not the nominal size. Schedule 40 IDs below are 1.610, 2.067 and 2.469 inches converted with 25.4 mm/in. See the pipe schedule reference. A fixed total K of 4 is a simplifying assumption; real fitting arrangements and K values may change with size.
2. Compare the calculated results
| Size | ID (mm) | v (m/s) | Darcy f | Pipe loss (kPa) | Fittings (kPa) | Total (kPa) | Head (m) |
|---|---|---|---|---|---|---|---|
| NPS 1½: download system | 40.8940 | 2.1149 | 0.0228240 | 62.297 | 8.929 | 71.226 | 7.276 |
| NPS 2: download system | 52.5018 | 1.2831 | 0.0226970 | 17.761 | 3.287 | 21.048 | 2.150 |
| NPS 2½: download system | 62.7126 | 0.8993 | 0.0228173 | 7.343 | 1.615 | 8.957 | 0.915 |
Open each downloaded JSON with the multi-section calculator's Open system file control. Each is a single-section case. The single-pipe calculator uses the same shared friction method.
For NPS 2: v = 1.283096 m/s, Re = 67,109 and fD = 0.02269697. Straight-pipe loss is 17.761 kPa and fitting loss is 3.287 kPa, giving 21.048 kPa.
3. Apply a velocity target, then check pressure
Suppose this project uses an illustrative maximum velocity of 1.5 m/s. In the pipe sizing calculator enter 10 m³/h, 1.5 m/s and Schedule 40. The minimum ID is 48.558 mm, and the first listed size meeting it is NPS 2. Its actual velocity is 1.2831 m/s.
The 1.5 m/s target is a scenario assumption, not a universal limit. Pressure budget, noise, erosion, solids transport, installation cost and operating hours can change the choice. NPS 2 reduces this case's loss by about 70.4% relative to NPS 1½. NPS 2½ reduces it by about 87.4%, but its installed cost is not evaluated here.
At fixed flow, straight-pipe loss scales approximately with fD/D⁵. The friction factor also changes with Reynolds number and relative roughness, so recalculate each candidate instead of treating that fifth-power relation as exact.
What this comparison leaves out
There is no static elevation, exchanger, control-valve requirement or pump curve in this example. Add those terms for a complete system. Keep velocity head separate from irreversible losses, use the correct bore for every fitting, and check mechanical design independently.
Method background: US EPA EPANET documentation. Related tools: single-pipe pressure drop and roughness reference.