Cavitation — the pump did not change, the boiling point came up to meet it

One pump, one pipe, one duty. The only thing you are changing is how hot the liquid is. Drag upward to warm it, down to cool it; the arrow keys do the same. End is just off the boil; Home is cold from the tap. Watch which of the two lines actually moves. Sound is off until you turn it on.

What you are looking at

A pump does not suck. Nothing in it reaches down the suction pipe and pulls. It lowers the pressure at its own inlet, and then the atmosphere — pressing on the surface of the liquid in the tank — pushes the liquid up to fill the space. Everything available to do that pushing is on the right-hand panel as a descending account: start with what the atmosphere provides, subtract what it costs to lift the liquid to the pump, subtract what it costs to drag it through the pipe, and whatever is left is the pressure the liquid arrives with.

Every one of those terms is a property of the installation, not of the temperature. So as you warm the liquid, that whole staircase very nearly stands still — it shifts by about one part in fifty, and only because warm water is slightly lighter to lift.

The other line is where the liquid boils. Water boils at a hundred degrees only because that is where its vapour pressure reaches the pressure of the air above it; lower the pressure and it boils colder. Read that backwards and it is the statement that matters here: raise the temperature and the pressure at which it boils climbs — from a fortieth of an atmosphere in cold water to a whole atmosphere at the boil. That is the line that moves. It climbs until it arrives at the pump from underneath.

When it gets there the liquid flashes to vapour at the impeller eye, is carried a few millimetres into a region of higher pressure, and the bubbles collapse. Each collapse is a tiny implosion against the metal. It sounds like pumping gravel, and it removes metal from the impeller in a pattern nothing else produces.

The two things worth taking away

The suction side decides, and every gauge is on the discharge side. Cavitation is caused by conditions upstream of the pump and appears as damage inside it. A discharge pressure gauge, which is what most installations actually have, shows almost nothing until the pump has lost enough head to matter — by which time the impeller has been eroding for a while.

The margin is not a safety factor; it is the whole design. The pump needs some pressure above boiling before it will run properly, because the pressure inside it falls lower than the pressure at its flange. Cold, this installation has twice what it asks for. It never needed a fault to lose that — a warmer process, a partly blocked strainer, a summer afternoon at altitude, or the same pump mounted a little higher will each spend it.

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 vapour pressure comes from the Antoine equation with the standard coefficients for water and agrees with published tables to better than half a per cent across the range drawn; the built-in checks confirm it. That check is the author's own, not a rotating-equipment engineer's.
• Hot liquids are treated more harshly here than they deserve. Real pumps tolerate a smaller margin on hot water than the cold calculation implies, because boiling a given volume of vapour takes heat out of the surrounding liquid and suppresses further boiling. That thermodynamic effect is real, is used in practice, and is not in this model — so the point at which this drawing says the trouble starts is pessimistic for hot water.
• The requirement is drawn as a single fixed line. What a pump needs varies strongly with flow, and the published figure is defined as the point where the pump has already lost three per cent of its head — meaning it is cavitating somewhat at the stated requirement, not on the verge of it.

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

• Total pressure, not the reading on a gauge. The falling line is total pressure, which is what the definition uses. A static gauge at the flange reads lower by the velocity head.
• Sea level, one duty point, clean pipe. Altitude subtracts from the atmospheric term directly. The friction is held fixed while in reality it falls a little as the liquid thins with heat, which flatters the hot end slightly, and it rises as a strainer loads up, which is the commonest way this margin is actually lost.
• Water only. The same account applies to any liquid, but the boiling line is a different curve for each, and for light hydrocarbons it is a far steeper one.

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.