Pipeline Total Pressure Loss (Elbows, Valves, Fittings)

How is the total loss of a pipeline found?

Pressure loss along a line has two parts. Friction in straight pipe follows the Darcy-Weisbach equation, while losses in elbows, tees, valves, entrances and exits use a loss coefficient K: Δp = (f × L / d + ΣK) × ρ × v² / 2, where f is the friction factor, L the pipe length, d the inside diameter, ρ the density and v the mean velocity. This calculator lists the line item by item, shows the share of each and points out the biggest contributor.

Loss coefficients K

K is measured for each fitting and varies with the source, the manufacturer, the size and the type of joint. The values offered are approximate textbook figures: threaded 90° elbow 0.9, flanged 90° elbow 0.3, fully open gate valve 0.2, fully open globe valve 10, sharp edged entrance 0.5, exit into a tank 1. Where you have manufacturer data, type it into the K box.

Example

Water flows at 1 m/s in a line with a 100 mm bore and a friction factor of 0.0185, so the dynamic pressure is 500 Pa. A 100 metre run of pipe loses 9257 Pa, four threaded elbows 1800 Pa, a gate valve 100 Pa, the entrance 250 Pa and the exit 500 Pa. The total is 11 907 Pa, which is 0.119 bar or 1.21 metres of water column. About 78 percent of the loss is in the straight pipe.

Limits

The line is taken as a single bore: where the size changes, work out each size separately and add the results. The fluid is incompressible and the flow steady. K values apply to fully turbulent flow and are larger in laminar flow. Interaction between fittings placed close together is ignored. Partly closed valves, strainers, heat exchangers and meters need manufacturer data.