What flow you actually get at the top of the plumbing, not what the pump box claims at zero head.
| Component | Head, ft | Head, m |
|---|---|---|
| Vertical rise | 4.00 | 1.22 |
| Friction, 10.0 ft of pipe | 0.55 | 0.17 |
| Friction, fittings (16.0 ft equivalent) | 0.88 | 0.27 |
| Total dynamic head | 5.43 | 1.65 |
Pump curves are published at zero head. A pump rated 800 gph often delivers half that at four feet with a few elbows. Read the curve at your calculated head, not the number on the box.
h = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87), C=150, D=26.64 mm, L=7.92 m
Total dynamic head is the vertical lift plus the friction the water fights on the way. Pumps are rated at zero head, so the box number never happens in a real system.
Friction here uses Hazen-Williams: h = 10.67 · L · Q^1.852 / (C^1.852 · D^4.87) with head in metres, length in metres, flow in m³/s and diameter in metres. The roughness coefficient C is 150 for PVC and 140 for flexible vinyl.
The D^4.87 term is why pipe size dominates everything. Going from three quarter inch to one inch cuts friction by roughly two thirds at the same flow. Going from one inch to one and a half cuts it by another three quarters.
Fittings are counted as equivalent pipe length in pipe diameters: 30 for a 90 degree elbow, 16 for a 45, 20 for a tee straight through, 60 for a tee taking the branch, 3 for an open full port ball valve, and 100 for a swing check valve. Two 45s beat one 90, and a check valve costs more head than the rest of the plumbing combined.
Velocity is the number to watch for noise. Under about 1.8 m/s a return line is quiet. Above 2.4 m/s it whistles, and friction is climbing faster than flow.
Throttle on the output side of the pump, never the input. Restricting the suction cavitates the impeller and shortens the pump life.
Check valves fail. They stick open on a snail shell or a bit of coralline and the sump floods. Design the sump to hold the drain down volume instead of relying on one.