Minor losses and valve sizing: K loss coefficients, equivalent length, flow coefficient Cv/Kv, control-valve authority and a K-factor reference table with a worked valve-sizing example.
Bends, tees, reducers and valves each take a bite out of the head budget — the so-called minor losses that, on a fitting-heavy plant, can rival the straight-pipe friction. Control valves add a second job: they must be sized so they actually control, holding enough authority over the system without being permanently near-shut. Both come down to two numbers: the loss coefficient K and the flow coefficient Cv / Kv.
Or, equivalently, as an equivalent length of straight pipe, Le = K·D/f. Control-valve capacity is rated by the flow coefficient:
Kv = flow giving 1 bar drop (m³/h); Cv ≈ 1.156·Kv (US units)SG = specific gravity (1.0 for water)ΔP = pressure drop across the valveThe control valve should take a meaningful share of the variable head — its authority N = ΔPvalve(open) / ΔPsystem should be roughly 0.25–0.5, or it loses controllability and runs nearly shut.
| Fitting | K (approx.) | Fitting | K (approx.) |
|---|---|---|---|
| 90° elbow (long radius) | 0.6 | Gate valve (open) | 0.15 |
| 90° elbow (standard) | 0.9 | Globe valve (open) | 6–10 |
| 45° elbow | 0.4 | Butterfly valve (open) | 0.3–1.5 |
| Tee (through-run) | 0.2 | Check valve (swing) | 2.0 |
| Tee (branch flow) | 1.0 | Sudden contraction | 0.3–0.5 |
| Sharp entrance | 0.5 | Sudden enlargement / exit | 1.0 |
Globe valves throttle well but cost ~10× the head of a gate valve fully open — chosen for control, not isolation. Sum the K’s on the route, multiply by v²/2g, and the minor-loss head drops straight into the budget.
Q = 30 m³/h water; allocate ΔP = 0.6 bar across the control valve at design flow. Also count the fittings on a DN100 line at v = 1.06 m/s: two LR elbows, one branch tee, one swing check.
Add reducer and fitting losses, and check velocities, with the pipe hydraulics tools.
Reynolds & Bauhm sizes pipework, pumps and valves and closes the head budget as part of a gated, fully documented bespoke design — every figure traceable from flow to fabrication.
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