I was visiting an industrial site not too long ago and I noticed an interesting phenomenon involving one of their steam condensate pipes. This (insulated) pipe was suspended from the ceiling. It hung down about 20 feet, supported by a series of supports. Each support comprised of a roller (positioned under the pipe) and a rod, which ran from the roller to the ceiling directly over head.
It ran roughly 100 yards from one end of the facility to the other. This pipe also included a U shaped expansion loop.
As I watched it, I noticed that it was swaying back and forth, with period of about 2 seconds, with an amplitude of a couple of feet. It followed the description of a second harmonic standing wave in a string with fixed ends, where there is a stationary point in the middle that does not move.
I was (and am) still very curious about this. Is this often seen in a steam system? Is this ok? What is this called? What causes it? Should it be dampened? Will it hurt the life of the pipe? It may be ok, but it really reminded me of the tacoma bridge.
I was assuming that the oscillation was caused by water flowing around the expansion loop, imparting momentum to it as it encountered the turns, which then caused the pipe to oscillate at its resonance frequency.