Water hammer — shut a valve and watch what the pipe does

Drag downward to close the valve; drag up to open it again. The arrow keys do the same, a step at a time. End slams it shut; Home reopens it. How fast you close it is the whole experiment. Sound is off until you turn it on.

What you are looking at

Water in a long pipe is moving. You shut the valve at the end of it. The water immediately behind the valve stops — and the water behind that does not know yet. It keeps arriving, and it compresses against what has already stopped. That compression is a pressure wave, and it runs back up the pipe at the speed of sound in the water column, around 1200 metres a second in a steel line.

At the reservoir the wave has nowhere to push, so it reflects and comes back inverted. The pressure at the valve swings high, then low, then high again, decaying only as friction eats it. The bang you hear is the pipe being hit by its own contents.

The two things worth taking away

The rise does not depend on how hard the pump was pushing. It is set by the velocity you removed and by the speed the pipe answers at. The same valve on a gentle low-pressure line produces the same rise. "It is only a low-pressure system" is not a defence.

"Slowly" means slowly compared to the pipe's own round trip. Shut the valve before the wave has been to the reservoir and back, and you get the whole surge — there was nothing to relieve it. Take longer than that and the returning wave arrives while you are still closing, and cancels part of the rise. On this pipe the round trip is about a second. On a long transmission main it can be several. A valve that feels unhurried to a hand can still be instantaneous to the pipe.

What is not verified

Stated plainly, because a piece whose value is trustworthiness should not overclaim. The first list is the part that would change how you read the result. The second is scope — true, worth saying, and not load-bearing.

• No engineer has reviewed this. If you are one and would check it, REVIEW.md says what would actually help and is meant to cost about fifteen minutes. The model is the standard method of characteristics and the numbers it produces agree with the closed-form results to the tolerances asserted in the built-in checks — but that check is the author's own, not a hydraulic engineer's.
• Friction is steady-state only. Real transients lose energy faster than this, because the friction during a reversal is not the friction of steady flow. The ringing here therefore lasts longer than it would in a real pipe.
• Column separation is not modelled. When the pressure would fall below the vapour pressure of water, this clamps it. A real pipe forms a vapour cavity there and the slam when it collapses can exceed the first surge. That is a more dangerous phenomenon than the one drawn, and it is out of scope.

Scope, for completeness. True of the model, and none of it changes the answer above.

• The pipe is rigid, straight and uniform, with a single wave speed and no entrained air. Air, bends, branches and flexible sections all change the answer, usually by lowering the wave speed.
• The valve law is a plain orifice closing in proportion to your gesture. Real valve characteristics are strongly non-linear near shut, and most of the velocity change happens in the last part of the travel.

Commissions

The subject changes; the method does not. If you have a failure mode, a mechanism, or a process that people keep getting wrong because the diagram cannot move, that is the shape of problem this suits. You get one file, the checks that hold it honest, and the notes explaining every decision including the ones that were reversed. Commissions start at £4,000 and take three to four weeks from an agreed brief. yinxueshi1017@gmail.com

Not commissioning anything? Still worth an email. If your version of this failed differently — other geometry, other numbers, a mechanism this piece leaves out — tell me in one line. Corrections are the main thing that decides what gets built next.