Pressure drop in pipe lines and fittings. Part-2

Pressure drop due to valves and fittings

In the earlier post, tutorial goes around pressure drop in pipe lines. Pressure drop arises due to skin friction and form friction. Non separated boundary layers cause skin friction whereas separated boundary layers cause form friction. In plain pipe pressure drop is due to skin friction. But in a pipe line with valves and fittings pressure drop is  mainly due to form friction. the pipe lines may be connected to equipment and again pressure drop will be there in that equipment. This tutorial is restricted to pressure drop in valves and fittings only.


In order to account pressure drop due to valves and fittings various methods are in practice. The most famous among them is by using

1. K values or Head loss coefficient.
2. Equivalent length method.

1. K values or Head loss coefficient

Earlier K values were static constants but in recent time applications (Industrial applications) they are dynamic, change with pipe diameter. 

Depending upon the type of valves and fittings various K values are available, the same is shared below.

K values that are subject to change the pipe diameter are given below with a formula.

The Km, Ki and Kd values corresponding to the type of fittings and valves are given below.

2. Equivalent length method.

Another method that is widely used in calculating pressure drop is equivalent length method. Here for every valve and fitting particular L/D value is available in the above table. To find the equivalent length, multiply the L/D value with the diameter. And insert this value in the given formula, in place of  Leq




Similarly, for contraction and expansion losses you need to consider k value as per following formula.