[Ewan]

I think equivalent length is a tool used by process engineers to do quick calculations of the pressure drop of a pipe system.
For example, you can make basic assumptions about the pressure drop of a meter of 2 inch pipe since this is a common diameter in industry and there is tons of empirical data. Bends, T-junctions etc would all have a resistance that can loosly be equated to the pressure drop a certain length of pipe would. A bend might for example have the same resistance as 5 meters of pipe.
I think it might a good system for quick estimation, since all you need to do is look at how many meters of pipe you have, add up the various bends and you can quickly get a rough pressure drop value.
This would realy be all you need to size a centrifugal pump.

This thread is beginning to look like the confused fluid dynamists convention, however Since87 has cleared everything up with his last posts. Shame it took a while to get there though because it's probably caused unnecessary confusion.

However to add to it all, fluid dynamics is very similar to electrics with some very important differences. The biggest is that flow resistance is proportional to the square of the flow, whereas resistance effects electrical current in a linear relationship (directly proportional).
This is because the fluid is moving and has a kinetic energy which is expressed as mass * velocity^2/2, which is recognisable from basic physics. I.e water hitting a pipe bend twice as fast will have a 4x impact energy on the bend, which naturally manifests itself as a resistance to flow which is 4 times greater.