Water power for your machines

[Nigel Graham 2Participant @nigelgraham2]

Ega –

Sorry – I wasn't trying to pour cold water on it. Just crediting the right inventor!

'

Michael –

Friction? What you describe reads as a static rig, with no flow. So once the elasticity of the tank and pipe has equalised itself with the pressure, there is no further flow so no friction. The head will be 8 ft irrespective of pipe diameter.

Where friction may matter is in long flows, such as given by Duncan's example.

Turn all the taps off in every home connected to that reservoir and the pressure at the first regulator would indeed be that 10Bar (I Bar represents a depth of 10m, for practical purposes.)

I suppose the classic pressure / flow exchange example here is the Hydraulic Ram, driven by the stream whose water it is extracting.

'

isn't the Pascal such a daft unit? Mathematically tidy enough for any ISO bureaucrat with an ology in Dimensional Analysis of Physicists, but at 1 / 100 000 Bar far too tiddly for real-life engineering. Yet far too big for Acoustics, which works in µPa, or, 1 X 10^(-11) Bar !

(For airborne sound, 0dB is set at 20µPa, the faintest sound pressure level discernible by the fully healthy human ear.)

'

My work-place had a big hydrostatic test tank for external pressure, for testing marine equipment containers. Some idea of the loads involved were given by the nature of the lid, like a giant steel bowler-hat weighing 7 tons; and held down for the full pressure-range by 36-off 3-inch BSF bolts with specially-made nuts. These were probably twice the length hence mass they needed be, too, to make lifting them to chest-height and threading them onto the dangling bolts, more fun. The seal was just an O-ring of about 3/8" dia material on the lower, turned joint-face.

One day its operator was given a test-piece, an empty equipment pod, and the test specification….

It imploded well before maximum. Spectacularly too – he showed me the video from a camera inside the tank.

He duly handed the wreckage back with a report, something like:

Test pressure required XXX Bar, reached by steps of — Bar of ~~~ minutes.

I followed this, but the casing collapsed at only ZZ Bar.

He told me his manager asked, "Is that all?"

Dave remarked that was he was not the item's designer or builder, he'd obeyed the test instructions, it broke below maximum and he was not expected to analyse the failure.

"What else could I write?"

Evidently a manager used to managerial word-smithing!

[duncan webster 1Participant @duncanwebster1]

I have a vague recollection that you extract maximum power from an electrical device if it's resistance is the same as the internal resistance of the supply, so perhaps the max power from a water device happens when the flow is sufficient to drop the water pressure to half what it was with no flow. Of course this could be rubbish, no doubt someone will elucidate

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Nigel Graham 2 on 12/01/2023 21:49:38:

[…]

Michael –

Friction? What you describe reads as a static rig, with no flow. So once the elasticity of the tank and pipe has equalised itself with the pressure, there is no further flow so no friction. The head will be 8 ft irrespective of pipe diameter.

Where friction may matter is in long flows, such as given by Duncan's example.

[…]

.

Sorry … I evidently expressed myself poorly : my reference to friction in long runs of narrow pipe was relating to Dave’s original puzzle.

Yes I am [and was] fully aware that until we get down to capillary sizes, the pipe diameter is irrelevant … that was the point of me mentioning our test rig at all.

MichaelG.

[Author]

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