Why can a pump make vapor bubbles even when its liquid is not hot?

A pump can develop vapor bubbles even when its liquid feels cool. The familiar boiling temperature of water assumes a particular surrounding pressure. Change that pressure and the condition for vapor formation changes too. In a pump, motion can create a small low-pressure region without a burner adding heat.

At any given temperature, a liquid has a vapor pressure: the pressure at which its liquid and vapor can coexist in equilibrium. Molecules continually pass between liquid and vapor. If the local liquid pressure falls sufficiently relative to that vapor pressure, tiny existing cavities can grow as liquid vaporizes into them. These starting nuclei matter, so cavity formation is not described fully by temperature alone.

Near an impeller inlet, liquid must accelerate into the rotating passages. Its local static pressure can fall below the pressure measured farther upstream. If that fall is large enough, vapor-filled cavities grow in the low-pressure part of the flow. This is vapor cavitation. The bubbles contain vapor made from the liquid, rather than necessarily being air that leaked into the pipe.

The flow then carries the cavities into a region with higher pressure. Vapor condenses and surrounding liquid rushes into the shrinking space. Collapse can be rapid and highly concentrated. Near a solid surface, repeated collapses can produce intense local impacts that pit the material. Noise, vibration and a reduction in pumping performance can occur before obvious material damage develops.

Both the pump and its connected system determine the conditions at the inlet. Liquid temperature changes vapor pressure; supply pressure and losses in the incoming route affect how much pressure is available before the additional local drop. Engineers therefore compare available inlet conditions with the pump's requirements and the relevant cavitation criterion. The striking connection is that a change in pressure can start a phase change, and the later reversal of that phase change can damage a solid machine.