Loss Coefficient

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Loss coefficient, abbrevated as K, is a dimensionless number that measures the minor loss to the change in velocity due to friction thru pipes, fittings, and valves.  Most piping consists of more than a just straight lines, these losses are termed a minor loss.  Any bend or tee, expansion or contraction, valve opening or partially closing can cause minor or even major losses.

Loss Coefficient Formula

$$\large{ K = h_l \; \frac { 2 \; g } { v^2 } }$$

Where:

$$\large{ K }$$ = loss coefficient

$$\large{ v }$$ = flow velocity

$$\large{ h_l }$$ = head loss

$$\large{ g }$$ = gravitational acceleration

Loss Coefficient Table

ComponentLoss Coefficient $$K$$
Elbows
90 Short Elbow, flanged  0.3
90 Short Elbow, threaded 1.5
90 Long Elbow, flanged 0.2
90 Long Elbow, threaded 0.7
45 Long Elbow, flanged 0.2
45 Short Elbow, threaded 0.4

180 Return
180 Return, flanged 0.2
180 Return, threaded 1.5

Tees
Line flow, flanged 0.2
Line flow, threaded 0.9
Branch flow, flanged 1.0
Branch flow, threaded 2.0

Valves
Diaphragm, fully open 2.3
Diaphragm, 1/4 closed 21
Diaphragm, 1/2 closed 4.3
Globe, fully open 10
Angle, fully open 2
Gate, fully open 0.15
Gate, 1/4 closed 0.26
Gate, 1/2 closed 2.1
Gate, 3/4 closed 17
Swing Check, forward flow 2
Swing Check, backward flow  $$\infty$$
Ball, fully open 0.05
Ball, 1/3 closed 5.5
Ball, 2/3 closed 210
Water meter 7