Friday, 17 October 2014

Water hammer

If you got yourself here by mistake, let me assure you that this Blob is not a typo but a short piece to camera about hydraulic shock. Anyone who has lived in an old house with plumbing has experienced this.  If you turn off the tap in the bath sufficiently sharp, you'll hear and probably feel a kaCHUNK, as the flowing water comes to a sudden stop. The sound is effectively the same as if you let fall a sledge-hammer on a concrete wall: as the kinetic energy of the system comes to an abrupt halt, part of it is transformed into sound waves. The shock-wave can travel can travel back and forth in the pipework for a while going dugga-dugga-dugga. I'm shocking bad at physics, so if the rest of this piece is opaque to you, it's because it is opaque to me.  We can all live with a bit of odd bonking in the plumbing, so long as it doesn't startle us awake in the middle of the night.  But the principle can scale up frighteningly in industrial situations. An example from The Wiki, imagines a hydro-electric power-station needing to quickly turn off the power by shutting a sluice. If there are, say,14 km of 7.7m diameter tunnel, full of water travelling at 3.75 m/s, that represents approximately 8000 Megajoules of kinetic energy. It's about the same explosive power as 2 tons of TNT going off with a bang.  That will happily shatter the concrete source pipe, so dam-builders incorporate surge-towers in the infra-structure to dissipate the kinetic energy against gravity rather than against the pipe-wall. Hats off to engineers. It works at the other end as well: if you shut the pipe at the top end, the moving water will attempt to create a vacuum behind it which can implode the pipework.

The Boy is an engineer, and since before he worked for a tiny start-up called Ryanair, he's been a plane geek.  I sent him the link to a Blob about metal fatigue and his grand-father's NDE and he told me to check out Aloha Airlines Flight 243.
The annotated picture above is about another father's NDE that is much more likely to be true than mine. Aloha was an inter island shuttle between the islands of Hawaii and the planes, Boeing 737s, clocked up a lot of take-offs, landings and air-miles in the salty air of the paradise islands.  on 28 April 1988, Flight 243 en route from Hilo to Honolulu just after lunch, suffered an explosive decompression which ripped 5.6m off the roof of the fuselage just in front of the wing.  There was only one fatality, an unfortunate hostie called CB Lansing who disappeared into thin air and was never seen again. Neither was the roof, despite extensive searching of the ocean in the following days. Hats off to engineers here as well because it boggles the mind that the plane still holds together when the front and back halves are joined by a thread.  As with the Comet accident in 1954, the damage was put down to metal fatigue made worse by the corrosive action of the salt on the rivets and joints of the aluminium air-frame.

Apparently, since it was realised that cracks in fatigued metal propagate over time, aircraft engineers design air-frame like rip-stop nylon. They make it so that the damage stays local and doesn't cause a catastrophic failure. When even a small section of the skin fails at 7,000m there is an immediate decompression as the comfortably sea-level pressurised cabin voids to the equivalent air pressure at the top of Mt Everest. A minority report into the accident hypothesises that Ms Lansing was walking right underneath when a Lansing-sized hole blew out. The rushing air was abruptly stopped by the head and shoulders of the poor hostie and the fluid hammer effect blasted out the adjacent panels like falling dominos.  The leader of the NTSB investigation conceded later that this exotic theory could bear further investigation. If, like Aloha-243 passenger Gayle Yamamoto, you see cracks in the fuselage as you take your seat, be sure to tell the nearest hostie - that person is most likely to be the plug that kills everyone.

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