Enter flow, pipe and the number of each valve and fitting. The page computes each fitting’s K, the equivalent length, the straight run friction and the total loss in pressure and in head.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Head loss by fitting group, compared with the straight run.
From 40% to 140% of the flow entered: loss grows with the square of flow.
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It is the energy a liquid loses through valves, elbows, tees, reducers, entrances and exits, on top of straight-pipe friction. In long straight lines it is small, but in pump rooms, skids and short lines full of fittings it can exceed half the total. In the default example, fittings account for 55% of the 12.7 psi drop.
Crane TP-410 gives each fitting a fixed equivalent length (L/D), multiplied by the pipe’s fully turbulent friction factor, so K does not change with flow. Darby’s 3-K method adjusts K for Reynolds number and size and is more accurate for very small or very large pipe and viscous liquids. With water in 2" to 12" pipe the two are close: in the example, a sum of K of 26.0 versus 25.9.
Cv is the water flow in gpm that passes with a 1 psi drop; Kv is the flow in m³/h with a 1 bar drop; Cv = 1.156 × Kv. With the inside diameter d in inches, K = 891 × d⁴ ÷ Cv² (Crane TP-410). In the example, a Cv 140 control valve in 4" pipe (d = 4.026 in) is worth K ≈ 12, or about 7.4 ft of head at 250 gpm.
It is the length of straight pipe of the same size that would lose as much as the fitting: Le = K × D ÷ f. An open 4" globe valve equals about 105 ft of pipe; an open gate valve, less than 3 ft. In the example, all fittings together equal 481 ft, more than the 400 ft straight run.
Flow comes from the design, a flowmeter or the pump curve at the duty point; size and wall from the isometric or piping spec; fitting counts from the drawing or a field walkdown. For control valves, strainers and meters, use the Cv, Kv or pressure drop on the manufacturer’s data sheet.
Because it depends on velocity squared: 20% more flow gives about 44% more loss. In the example, total loss climbs from 29.4 ft to 42.1 ft. That is why an oversized design flow leads to bigger pumps that cost more to run.
It assumes an incompressible liquid, new fully open fittings and developed flow ahead of each one; closely spaced fittings lose a bit more. Steam and gas with a drop above about 10% of absolute pressure need a compressible calculation. The result excludes static lift and delivery pressure: add them in the pump sizing calculator.
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