TRIPLE POINT AND ITS AFFECT ON CAVITATION T029
Most substances in nature can exist as a solid, liquid or vapor. There are instances where a particular combination of pressure and temperature will allow all three states of a substance to co-exist at the same time in thermodynamic equilibrium
- In water, that temperature is close to 32В°F. At its normal boiling point the vapor bubble that forms is surrounded at its surface by a thermal boundary layer and this layer will dampen the violence when the bubble collapses. This is the reason we see hardly any damage to a pot on the stove when vapor bubble collapse on its bottom.
- Bubbles that form closer to the triple point have a greatly reduced thermal layer because very little latent heat is required to cause a change in state.
- Cavitation is much more violent the closer it occurs to the triple point because the reduced thermal boundary inhibits its dampening affect.
- Since the typical pumping temperature of water is relatively close to its triple point, cavitation can be very destructive.
- Cavitation in some other liquids is less sever because they are pumped close to their boiling point (not triple point) and the thermal layer surrounding the bubble has a better damping affect.
- Liquefied natural gas (LNG) is a common example of this type of fluid.
Some other factors contributing to the cavitation damage:
- Corrosive liquids. Intergranular corrosion and crevice corrosion not only cause direct damage, but also weaken the structure and make it more susceptible to pounding from collapsing vapor bubbles.
- Impeller material. Steel, cast iron and brass are more susceptible to cavitation damage while stainless steel, titanium, and aluminum bronze are more resistant.
- Higher speed. More energy concentrated in smaller volumes results in more metal removal from higher speed impellers.
- Operation away from the pumpвЂ™s best efficiency point. Mismatch between the actual flow angles and the impeller geometry also aggravates cavitation damage.
- High suction specific speed design. Special impeller designs for low NPSHR values will suffer more due to larger impeller eye diameters and some mismatch with the flow that is necessary to achieve low NPSHR values.
- Duty cycle. The longer a pump is operated, the more it may be damaged by cavitation.