Loss Coefficient
Loss coefficient, abbrevated as K, a dimensionless number, measures the minor loss to the change in velocity due to friction thru pipes, fittings, and valves. Most piping consists of more than 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 

\(\large{ K = h_l \; \frac { 2 \; g } { v^2 } }\)  
Symbol  English  Metric 
\(\large{ K }\) = loss coefficient  \(\large{ dimensionless }\)  
\(\large{ g }\) = gravitational acceleration  \(\large{\frac{ft}{sec^2}}\)  \(\large{\frac{m}{s^2}}\) 
\(\large{ h_l }\) = head loss  \(\large{ ft }\)  \(\large{ m }\) 
\(\large{ v }\) = velocity of flow  \(\large{\frac{ft}{sec}}\)  \(\large{\frac{m}{s}}\) 
Loss Coefficient 

Component  Loss 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 
Union, threaded  0.08 
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 
Tags: Coefficient Equations Friction Equations Hydraulic Equations Head Equations