Question 1

What is the minor loss equation?

Accepted Answer

Each fitting, valve, bend, entrance, or exit dissipates energy proportional to the velocity head: hL = K·V²/(2g), where K is the dimensionless loss coefficient, V is the mean pipe velocity, and g is gravitational acceleration. For a run with several components, the coefficients simply add: hL = ΣK·V²/(2g), provided the pipe diameter (and therefore V) is constant through the run.

Question 2

When do minor losses matter compared to friction loss?

Accepted Answer

Rule of thumb: for long pipelines (L/D > 1000, e.g. more than 1000 ft of 12-inch pipe) friction dominates and minor losses are often within the noise of the C-factor assumption. For short runs with many fittings — pump stations, treatment plant yard piping, meter vaults — minor losses frequently exceed friction loss and must be itemized. Compute both: h_total = (f·L/D + ΣK)·V²/2g.

Question 3

What K value should I use for a pipe entrance?

Accepted Answer

It depends entirely on the edge condition: a re-entrant (projecting) pipe is K ≈ 0.8–1.0, a sharp flush entrance is K = 0.5, a slightly rounded entrance is K ≈ 0.2–0.25, and a well-rounded bellmouth (r/D ≥ 0.15) drops to K ≈ 0.04–0.05. The default in this calculator is the sharp flush entrance, K = 0.5, which is the common conservative choice.

Question 4

Why is the exit loss always K = 1.0?

Accepted Answer

At a submerged discharge into a reservoir or tank, the entire velocity head of the flow is dissipated by turbulent mixing — none of it is recovered as pressure. Since hL = V²/2g exactly, K = 1.0 regardless of how the exit is shaped. Only a gradually expanding diffuser that slows the flow before discharge can recover part of the velocity head.

Question 5

Are these K values exact?

Accepted Answer

No. Published K values are representative averages for fully turbulent flow; manufacturer, size, and Reynolds-number variation is significant — especially for valves, where the same nominal type can vary by a factor of two or more between vendors. Values here follow Crane TP-410 and standard fluid-mechanics references. For final design of a critical system, use the manufacturer's Cv or K data for the specific valve model.

Question 6

How do I convert K to equivalent length?

Accepted Answer

The equivalent-length method replaces each fitting with a length of straight pipe producing the same loss: Leq = K·D/f, where f is the Darcy friction factor for the pipe (typically 0.015–0.025 for water in common pipe sizes). You then add Leq to the actual pipe length and compute friction loss once. Both methods give the same answer when f is consistent; the K method is preferred when velocity differs between segments.

Component	K	Notes
Entrance, sharp flush	0.5	Re-entrant 0.8–1.0; bellmouth 0.04–0.05
Exit to tank / reservoir	1.0	Full velocity head lost, any shape
90° elbow, standard	0.30	Threaded ≈ 0.9
90° elbow, long radius	0.25	Smooth r/D = 4–6 bend: 0.15–0.30
45° elbow	0.20	Threaded ≈ 0.4
Tee, run-through	0.20	Threaded ≈ 0.9
Tee, branch flow	1.0	Threaded ≈ 2.0
Gate valve, fully open	0.20	½ open ≈ 5.6 — don't throttle gate valves
Butterfly valve, fully open	0.8	Range 0.3–1.2 by size and disc
Swing check valve	2.2	Range 2.0–2.5
Globe valve, fully open	8.0	Range 6–10
Ball valve, full bore	0.05	Reduced-port higher

Minor Loss Calculator

Loss coefficients used by this calculator

Worked example

Example — pump station discharge piping

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Minor Loss Calculator

Loss coefficients used by this calculator

Worked example

Example — pump station discharge piping

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