What is Loss coefficient (K)?
A number that expresses the local pressure loss of a bend, valve or change of diameter as a multiple of the dynamic pressure at that point.
It lets fittings and valves be accounted for directly, without converting them into an equivalent length of straight pipe.
A valve with K = 1 takes away exactly one dynamic pressure, \rho u^2/2, from the flow.
Enter K directly or let a correlation compute it; the result names the correlation's source, range of validity and reference velocity (90° bends and reducers follow Crane Technical Paper No. 410). For a splitter, K of each outlet refers to that outlet's velocity. Valves can also be given by Kv or Cv.
\Delta p = K\,\frac{\rho\,u_{\mathrm{ref}}^2}{2}
A K value only means something together with its reference velocity u_{\mathrm{ref}}; for a reducer it is the velocity in the smaller diameter. A sudden enlargement gives the Borda–Carnot result K = (1-\beta^2)^2 with \beta = d_{\mathrm{small}}/d_{\mathrm{large}}. Kv is the water flow in m³/h at a 1 bar pressure difference:
\Delta p\,[\mathrm{bar}] = \left(\frac{Q\,[\mathrm{m^3/h}]}{K_v}\right)^2 \frac{\rho}{1000\,\mathrm{kg/m^3}}
Check that a published K value matches its source's conditions (geometry, Reynolds number, reference velocity). Kv and Cv are used for gases only while the pressure difference is below 10 % of the pressure.