There are two issues when you have a system operating at below neutral presure that cause cavitation. 1 is gases that come out of solution creating pockets of air (see Henries Law) and 2 is vapourisation. Both of these are very similar.
For this to make sense you will need to learn that the definition of neutral is not the presure half way between the posative side of the pump and the negative. It's the hydrostatic presure. (Presure when pump is off)
Water in the system will always try to find equilibrium. Now there are two ways it will try to find equilibrium, one is through the absorption of gas in relation to there partial presures and the water temprature (air particles leaving and entering the system though Henrys law) and the second part is finding a vapour pressure equilibrium.
Water particles leave water through evaporation at all temperatures (even ice) and also recondense at a similar rate. As you heat a liquid it's molecules become more energised and break they're inter molecular forces (imf) and evaporate. If you seal the top of a saucepan you will build up vapour pressure above the water. higher vapour presure will lead to more recondensing back in to the liquid phase. If you remove that lid you will see a puff off steam as the presure is relieved and the water evaporates because of the lower presure above. Put the lid back on it will stop steaming and a evaporation - condensing cycle will ocur.
This cycle is always happening just at varying degrees and the more the temprature changes the more out of equilibrium it is, so the more condensing or evaporation you will see.
Boiling point. "Boiling point is where the vapour pressure equals atmospheric pressure." This is why water boils at 100°c at sea level, 69°c on everest, and 101.1°c at the dead sea (below sea level). If you place room tempraure water under a bell and create a vacume you will boil the water at room temperature.. It will then freeze but that's another process. (Triple point)
However this is relative to water boiling in an open container. So what if we seal the container?.. A better statement would be 'boiling point is where the vapour pressure equals water presure'
We should also bare in mind that vapour is gas and can compress like any other gas with little pressure difference, where as water wil dramatically increase in pressure when compressed although does not litrally compress.
Now, when your system is static it will find equilibrium with desolved air and vapour pressure. When you turn on the pump and boiler you alter two characteristics, the temperature, and the pressure. As the water passes through the boiler the temprature increases, which will promote evaporation but only slightly increase vapour pressure as gas compresses. As the water passes through the pump the pressure has also increased so the water stays in realtive equilibrium, the moment the pressure in the water drops to below the static presure (neutral) you will have an elevated vapour presure due to heated water and reduced water presure due to resistance in the system and the force of the pump.
After dropping below neutral the pressure differential between the vapour pressure and water closes in, when you reach the cryptically point where the 'vapour presure matches the water pressure" the water molecules intermolecular forces break and you get vapourisation!
Along with this, as mentioned before, the higher temprature liquid in combination with the low pressure will bring gasses put of solution.
This is caviation.
To avoid this place the expansion tank as close as pos to the negative side of the pump.
Written by Adam Chapman - Ecotechnician