Is Water Hammer the resultant "Noise" from the shock of acceleration of water? If so, Water Hammer is nothing more than vibration.

Is there a point when Water Hammer is pressent and not detectable by the human ear? If so, would there be any damaging affect because of the seemingly small amout of Water Hammer?

Its about as much like vibration as an earthquake is.  Water hammer is often not heard at all.  What is typically "heard" is the effect of a pipe moving into a support or slamming up against a wall, anchor point or other supporting structure, if not actally coming lose from its mountings, or breaking up a flange joint completely, if it was severe enough.  If the support structure was not there, the pipe might even have moved relatively silently, and probably moved a lot more.  The event triggering the high pressure transient in one place could have originated 20 miles or more away from the point of observation.  They are quite common on long pipelines with high fluid velocities that experience rapid +/- accelerations.

For pipelines 5-6 fps is normal, typically 3 minimum and 10 maximum.  

A 3 second total closing time on a pipeline is speed of light, 1 sec/inch being more reasonable, unless its an ESD valve.  Great care is needed in selecting valves(safety valve) and actuators when velocities are above 5 fps, as many valves effectively open in the first few seconds, or close in the few seconds, even though their total opening time and closing times may be only 1 sec/in diameter due to poor Cv vs actuator position curves.  One must be carefully in slowing down the mainline velocity well before a valve gets close to its shutoff point.

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