Hi,
good idea, Brian.
Speed of sound in water is about 1500m/s.
if there is water flow, then in one direction the speed of water flow adds to the speed of sound, in the other direction the speed of water flow subtracts from the speed of sound.
Now if you place two sensors in equal distance from the source of sound ( One sensor up the pipe, one sensor down the pipe).. you get two (delay) timing signals.
I recommend to use 2 sensors, because it automatically cancels drift by temperature, drift by pressure, drift by purity of water..
I did some calculations: If my math is correct then in a 6" pipe a water flow rate of 4Gpm is just a water speed of 0.016m/s. (I really hope this is not correct).
This is not much compared to the 1500m/s of speed of sound. only 1/100,000. Hard to detect...
Klaus