Minor Losses: K-Factors, Equivalent Length, and When Fittings Dominate
Elbows, valves, and entrances are not minor on a short suction line. Sum K, convert to L_eq, expansion/contraction, and when a control valve should be Cv.
Key takeaways
- h_m = K V²/(2g). Several fittings in series add K.
- L_eq = K D / f is a spreadsheet convenience, not extra steel.
- Exit K = 1; sharp entrance K ≈ 0.5; globe valves dominate.
- Control valves belong in Cv, not a constant open-position K.
“Minor” losses are local. On a 2 km main they really are minor. On a 4 m pump suction they are the whole story. Treat them as K times the velocity head, then either add them to Darcy friction or fold them into an equivalent length.
The two writings
L_eq is handy inside a Darcy spreadsheet that only knows length. It is not a physical extra pipe. If f changes (different Re), L_eq should change with it.
Open solver: Fittings K-factor calculatorCatalogue values to remember
| Fitting | Typical K |
|---|---|
| Well-rounded entrance | 0.04 |
| Sharp entrance | 0.50 |
| Exit (into tank) | 1.00 |
| 90° elbow, flanged long-radius | 0.30 |
| 90° elbow, threaded | 0.90 |
| Tee, branch flow | 1.00 |
| Gate valve, open | 0.15 |
| Globe valve, open | 10 |
| Swing check | 2.0 |
Example: 0.15 m suction, V = 1.4 m/s, K = 0.5 + 2×0.3 + 0.15 + 1.0 = 2.25. h_m = 2.25 × 1.4² / (2×9.81) = 0.22 m. On a flooded suction that is small; on a 3 m lift it is a noticeable slice of NPSH.
Expansion and contraction
K vs Cv
Control-valve manufacturers publish Cv (or Kv), not a constant K. Cv already includes the valve’s own geometry and varies with opening. Use the valve-coefficient solver for control valves; use K for isolation fittings that stay fully open.
Open solver: Valve Cv calculatorOpen solver: Pipe friction to add to ΣKFrequently asked questions
Catalogue K assumes a long straight run before and after. Close-coupled elbows interfere. Add margin or use a tested assembly K.