There is a disagreement in our office as what the load factor for a pipe filled with water is. Some want to apply 1.2*pipe DL + 1.6*water weight, others suggest appling one factor 1.4, while a third group recommends 1.6*(Pipe DL+Water). I am looking at ICC which refernces ASCE7.
The IBC and ASCE 7 define live and dead load. Fixed service equipment including plumbing stacks are dead loads. So, it is fairly easy to justify the pipe weight as dead load. I would also consider the weight of water in the pipe as a dead load. A pipe can only hold so much water. More weight may be added to the dead load to account for valves and appurtances before using a live load factors.
Look at the load combinations in Chapter 2 of ASCE 7-05. Water fits the definition for "F = load due to fluids with well-defined pressures and maximum heights".
So for Strength Design (LRFD), both the 1.4 and 1.2 factor should be used.