Cavitation

Cavitation is defined as the development and sudden collapse of cavities such as vapour bubbles in a fluid flow. Various criteria are used to describe the occurrence, extent and impact of cavitation in centrifugal pumps, and a distinction is made between vapour and gas cavitation.

Cavitation criteria

• Start of cavitation bubble occurrence (incipient cavitation, NPSHi) at the vane's inlet edge up to a defined max. bubble length (Lbubble of e.g. 5 mm). In a bubble visualisation test the inlet pressure is lowered to such an extent that first cavitation bubbles become visible. 
• Cavitation-induced drop of head (ΔH) by a defined max. value of ΔH = 0.03 · H, or, as often applied in the case of high specific speed centrifugal pumps, a drop of head of ΔH = 0.00 · H (cavitation-induced start of head drop). The corresponding NPSH values are NPSH₃ or NPSH₀.
• Cavitation-induced drop of efficiency (Δη) by a defined max. value; this refers to the pump’s efficiency (e. g. Δη = 0.03 · η).
• Cavitation-induced drop of head (ΔH) up to a total head drop.
• Cavitation-induced material erosion in the centrifugal pump up to a defined max. mass per time.
• Cavitation-induced rise in noise level up to a defined max. sound pressure level (see Noise in pumps and systems). 

Vapour cavitation 

Vapour cavitation develops when the static pressure in a fluid falls below the vapour pressure; i.e. the vapour pressure associated with the fluid’s temperature is reached without external heat supply. There is a relationship between vapour pressure and temperature. 

Apart from the pressure drop to or below vapour pressure level, the existence of so-called nuclei (often in the form of microscopically small vapour bubbles) is a further prerequisite for the development of cavitation. See Figs. 1, 2 Vapour pressure (Fig. 2, see Annex)

The static pressure decreases if, for instance, the local velocity is increased (see Fluid mechanics) or the inlet conditions (e.g. the fluid pressure upstream of the location at risk of cavitation) change.

If the static pressure rises above the vapour pressure again in flow direction, a sudden collapse of vapour bubbles (see Sudden collapse of vapour-filled cavities) is the consequence. This takes place at a very high velocity in the form of an implosion. If the bubbles implode at a hydraulic component's wall and not within the fluid flow, cavitation may lead to material erosion.

Even before the material affected by cavitation is destroyed (which does not happen in all cases of cavitating operation) symptons like a rise in noise levels, rough running of the pump (see Smooth running) and a drop in pump efficiency and head already indicate that cavitation is taking place.

On propeller pumps incipient cavitation is often accompanied by a minor rise in head before the pump’s head drops as a result of cavitation (at a slower rate than that observed for radial pumps).

In order to facilitate observation of bubble collapse, bubbles are created artificially, e.g. in the vicinity of walls (using a focused laser beam or ultrasound).

Recent research in this field has shown that the vapour bubble will initially dent inwards as the implosion begins. In the course of the process, a water microjet is formed which is directed towards the interior of the bubble and penetrates the opposite wall of the bubble.

Slow-motion pictures (approx. 9 x 10⁵ pictures per second) have shown that in the case of bubbles in close proximity to walls, this microjet is always directed at the wall and strikes it at a high velocity. This sequence of events, in combination with the fissured microstructure, very fine pores, cracks and indentations in the wall surface, causes the material’s destruction.

This type of material destruction is further intensified by a series of chemical actions whose progress is accelerated when the system is exposed to considerable mechanical stress. Surface layers (see Protective layer) which protect the material are often destroyed by cavitation. In conjunction with the oxygen contained in the water, this leads to increased corrosion. As these surface layers are of crucial importance for certain materials employed for aggressive media (see Chemical resistance table), their integrity must be ensured in the event of cavitation.

Gas cavitation

While vapour cavitation is characterised by nuclei becoming visible bubbles (bubble zones) as a result of vaporisation of the surrounding liquid, the process referred to as gas cavitation involves the formation of bubbles as a result of the release of dissolved gases from solution in conjunction with diffusion. Gases come out of solution when a fluid's pressure drops below the saturated vapour pressure which depends on the concentration of the dissolved gases (generally air). As the saturation pressure is often higher than the fluid’s vapour pressure, gas cavitation may also occur when the liquid pressure drops to values above the vapour pressure.

The effect gas cavitation-induced gas bubbles have on the flow, energy conversion and centrifugal pumps’ head and efficiency is similar to that caused by vapour cavitation. Gas cavitation is, however, not as destructive as vapour cavitation terms of material damage. The reason for this is that, with rising pressure, bubble collapse takes place when the gas diffuses into the liquid which means that this process is much slower than the collapse of vapour bubbles.

Occurrences of gas and vapour cavitation can also overlap. Vapour bubbles which develop after the fluid’s pressure has reached or dropped below vapour pressure in areas of minimal pressure (for centrifugal pumps, this is generally in the area around the vanes) also contain gas. This gas is released from solution via diffusion when the fluid approaches the pump, e.g. in the suction line of a centrifugal pump. This means that gas separation on the one hand supports the development and growth of cavitation nuclei, intensifying the extent of cavitation and its impact on the fluid flow and on resulting effects like the drop in head and efficiency. On the other hand, the non-condensing gas contained alongside vapour in the cavitation bubbles has a positive effect as it reduces the intensity of the vapour condensation-induced collapse which then has a less severe mechanical effect on the material’s surface and mitigates cavitation noise. Examinations have shown that the "aggressiveness" of vapour cavitation with regard to cavitation erosion is markedly reduced with increasing content of dissolved and/or undissolved gas (see also Gas content of fluid handled).