An unusual example of Cavitation Damage
An interesting and frequently encountered aspect of damage in pumps and impellers is that of damage caused by cavitation. Cavitation refers to damage caused in a fluid due to flow and pressure changes that result in substantial local pressure changes and the formation, and subsequent collapse, of ‘bubbles’ or voids.
In flow regimes where there is a change of section or surface irregularity, the flow around such discontinuity may often result in localised areas of low pressure. These pressure drops may be sufficient to cause the pressure in localised regions of the fluid to fall below the vapour pressure of the fluid, not unlike boiling (but not caused by a temperature increase), which results in the formation of small bubbles in the liquid. Subsequent pressure increase due to changes in the flow regime, or a physical increase in pressure can lead to the collapse of these bubbles, with associated shock waves (and local acoustic noise) leading to substantial surface damage when they impinge on a material surface. The implosions of these vapour bubbles result in highly localised pressure pulses and high temperatures being induced in the fluid with pressures of up to 1500bar and temperatures of between 3000 and 6000K being reported.
When these implosions occur on or close to the fluid/material surface boundary large magnitude stress waves are induced into the surface material leading to removal of particles of material, pitting and/or incipient micro-cracking. Cavitation damage of this nature is particularly prevalent in pump impellors, propellers and pipes where shape changes can lead to rapid changes in fluid velocity and pressure but also occurs in hydraulic cylinders, injectors, refrigeration coils, journal bearings and engine liners where lateral movement of the liner during operation causes low pressure regions to develop (refer to our November 2016 Technical Tip).
The appearance of the damage in pumps or impellors experiencing cavitation is that of pitting and erosion of the surface, with the material often appearing as being ‘moth eaten’ or ‘thinned out’. This often occurs at the trailing edge of the impellors or blades where the shape leads to substantial pressure reduction providing ideal conditions for cavitation to occur. Although damage can be extensive, in many cases the damage may be concentrated in small localised regions.
A more unusual case of cavitation damage was used in identifying the sequence of events in a small aircraft plane crash. After some incorrect interpretation of evidence and much debate between the experts involved in the investigation, the incident was finally attributed to slow fatigue crack propagation and failure of one of the main spars of the left hand wing. The debate of how recently the fatigue cracking had occurred, was resolved by fractographic evidence that showed the distinctive flat fatigue fracture surface (complete with classic ‘clamshell’ markings, typical of fatigue), also had cavitation pitting damage superimposed on the surface. Clearly there had to have been a substantial time between the fatigue cracking and the final failure incident for this cavitation damage to occur and that this was NOT a case of low cycle fatigue/very recent fatigue cracking.
The surface was pitted with cavitation pits arising from the wedge shaped pre-existing fatigue crack, filling with liquid and slowly, over a period of several months or even years, being subject to pressure cycles associated with the crack opening and closing and fluid being drawn into and then forced out of the crack as the wing spar flexed in flight. The variation in pressure, as the fluid was drawn into and forced out of the crack, caused cavitation damage which was superimposed on the already fatigued fracture surface. Clearly, the cavitation damage had occurred subsequently, even though the tight fatigued crack (initiating and propagating slowly from damage caused during a hard landing many years before) was still slowly growing. As the cavitation could not have occurred before the crack had formed, a clear sequence of failure could be established. This led to an understanding of the nature and sequence of events, leading to the failure of the wing/crash and allowed the pilot, who had sadly died in the incident, to be exonerated fully.
This unusual case provides a useful example of the value of Forensic Engineering, and the importance of understanding all aspects of the evidence at hand.
Published in Technical Tips by Origen Engineering Solutions on 1 February 2018