What is Equivalent length of pipe?

Equivalent length of a pipe is a term applied to express the pressure drop generated by valves and fitting in a pipe.

In literature, you usually find ratiOS of equivalent length to diameter of the pipe. For example, L/D for a fully opened globe valve is 300. This means that, if you have a fully opened globe valve in a pipe and the internal diameter of the pipe is ID=0.05m, then

the equivalent length for this globe valve =300*D=300*(0.05m)=15m.

Equivalent length of 15 meters means that the globe valve generates the same pressure drop as as straight 15-meter long pipe.