300+ rebuild

[Vladislav]

On 3/3/2026 at 3:06 PM, Geoff Weeks said:

You can't rip the liquid apart by physical action alone. 

Maybe it's not fine to flood up Larry's thread but seems I should toss a pair of nikels to this basket.

Paul's explanations were wrong (or maybe we are both wrong with him as he clever addmits sometimes). The effect of cavitation is predicted by taking the fluid body apart. It's possible if you "pull one end" and "hold the other". For example in a centrifugal pump some portion of fluid is forsed to go to the outside due to spinning but the incoming fluid is hold down because of excessive resistance in the suction channel. Or similar effect can be achieved on a surface of impeller blades. Big impellers have section of blades made by the same principal as a plane wing. That "assymetrical ellipse" shape produces difference of pressures on different sides of the wing. More curved side gets lower pressure what helps getting the plane up. And same effect is used in leaned (not straight) boat sales.

Ok, that's a rocket since but the point we deal regarding cavitation is fluid is broken down in a certain point. So a bubble occures there. That's not gas in fact. It's vaccuum. It could be gas filled to some minor grade because of intensive evaporation to the vacuumated volume but what has meaning is that volume is under notably lower pressure than the whole fluid body.

When that bubble moves from the area with conditions which produced it to area with normal conditions normal pressure in the fluid presses it down immediately. And as long as fluid doesn't compress as gas it produces big hit in that spot. It's pressure wave spreads over surrounding fluid and since fluid is super-conductive for stress it achive structural parts of the device.

During cavitation we have a kind of "hammering" to empeller blades, pump housing or a cylinder sleeve (here the effect is stimulated by pressure jumps in the combustion chamber if combustion isn't going right, for example being detonative). Ok, constant hitting to a structural element makes material brittle which is followed by chipping out and making cavits.

This way general reason for cavitation to take place is incorrect movement of fluid in a certain passage. For centrifugal pumps the most typical reason to cavitate is higher revs than they are designed for. And clogged suction side too or sucking from very low level.