At higher pressures the chance decreases because the margin with the vapor pressure increases. In practice we often encounter impellers and other pump components, that have been produced with layers of exotic materials in order to prevent cavitation damage.
However, at Rodelta we believe that this is mostly patchwork to mask the real problem, which often lies in the design. A good hydraulic design can prevent or limit cavitation damage in the first place. It is of course best to completely prevent cavitation from taking place, but this is not possible in many cases.
For instance, when the suction pressure in the system is low, a very large and slowly rotating pump would be needed, which leads to an unnecessarily large footprint and high investment. Therefore, tradeoffs always have to be made. This is a complex area and you are advised to discuss your application with the pump supplier. The obvious symptoms of cavitation are noise and vibration. When bubbles of vapour implode they can make a series of bubbling, crackling, sounds as if gravel is rattling around the pump housing or pipework.
In addition to the noise, there may be unusual vibrations not normally experienced when operating the pump and its associated equipment. With centrifugal pumps, the discharge pressure will be reduced from that normally observed or predicted by the pump manufacturer. In positive displacement pumps, cavitation causes a reduction in flow rather than head or pressure because vapour bubbles displace fluid from the pumping chamber reducing its capacity.
Power consumption may also be affected under the erratic conditions associated with cavitation. It may fluctuate and will be higher to achieve the same throughput. Also, in extreme cases, when cavitation is damaging pump components, you may observe debris in the discharged liquid from pump components including seals and bearings.
Under the conditions favouring cavitation, vapour bubbles are seeded by surface defects on metal components within the pump: for example, the impeller of a centrifugal pump or the piston or gear of a positive displacement pump. When the bubbles are subjected to higher pressures at discharge they implode energetically, directing intense and highly focussed shockwaves, as high as 10,MPa, at the metal surface on which the bubbles had nucleated.
Since the bubbles preferentially form on tiny imperfections, more erosion occurs at these points. When a pump is new, it is more resistant to cavitation because the metal components have few surface imperfections to seed bubble formation. There may be a period of operation before any damage occurs but, eventually, as surface defects accumulate, cavitation damage will become increasingly apparent.
Classic or classical cavitation occurs when a pump is essentially starved of fluid it is also called vaporization cavitation and inadequate NPSH-A cavitation.
This can occur because of clogged filters, narrow upstream pipework or restricting perhaps partially closed valves. If the pump is fed from a tank, the level of liquid or pressure above it may have fallen below a critical level.
AWWA statement on the passing of the Infrastructure Investment and Jobs Act "Renewing and upgrading the nation's water infrastructure is critical to protect public health, safeguard the environment and allow our economy…. See More Related Articles. Related Whitepapers.
View Whitepapers. Comments Leave a Reply Cancel reply Your email address will not be published. Sign up for our Newsletter! Constant Contact Use. Please leave this field blank. Emails are serviced by Constant Contact. The focus of this article is looking into why are we so worried about a few bubbles. Cavitation causes an increase in pump noise and vibration, but more importantly, a drop in performance, efficiency and impeller erosion.
Not all of the damage from cavitation is metal loss or metal damage. Sometimes the issue is shortened bearing and mechanical seal life due to the unsteady flows surging. The bubbles form because local pressure has dropped below the vapor pressure of the fluid another way to view this is that the NPSH margin is not sufficient.
Less than a fractional second later as the bubbles transit along the low pressure side of the impeller vanes, they enter a region of higher pressure and collapse. I refer to this as classic cavitation to differentiate it from other causes of cavitation, such as suction or discharge recirculation that manifests on the other side of the impeller vane. Recirculation cavitation is typically due to operating the pump to the left side of the pump operating curve reduced flows and away from the best efficiency point BEP.
The approach angle of the incoming flow does not match that of the rotating impeller inlet vane geometry. Consequently, eddy currents and turbulence are generated in between the vanes. Inside the general area of the eddy current, the velocity increases and the pressure decreases as a result.
When the local pressure drops below the vapor pressure, the cavitation bubbles are formed. Recirculation cavitation is typically not caused by insufficient NPSH in the classic sense. You could have more than adequate NPSH margin and still experience recirculation cavitation because the pump is being operated away from its BEP. When this situation occurs, there is a mismatch in the flow angle as compared to the impeller inlet incidence angle.
The higher the suction specific speed NSS of the pump impeller, the more likely recirculation cavitation is an issue. Cavitation bubbles that break down in the middle of the impeller passageway collapse symmetrically equally from all directions , so there is less cause for concern other than potential noise and perhaps some vibration.
Similar but different to boiling water in an open pan on a stove, the bubble forms at the bottom of the pan, rises to the surface and collapses without issue or harmful effects technically this is a burst and not a collapse so almost no energy is released. However, when the vapor bubbles in a pump impeller collapse adjacent to the metal surface of the vane, there is a much higher potential for damage and concern due to metal loss from the substrate.
When the bubble collapses near the vane surface, it will collapse asymmetrically. Because of its proximity to the vane surface, the bubble geometry changes and makes the action more lethal.
0コメント