Cavitation on start

To avoid pump cavitation on start-up, the experienced operator opens the pump discharge valve slowly. Slowly opening the discharge valve results in reduced acceleration of the liquid in the suction line and a slower rate of the conversion of suction pressure to velocity. 

The longer the suction line and the larger the diameter of the line, the more mass has to be accelerated. This also increases the starting NPSH required. If the sum of the frictional loss in the suction line, plus the running NPSH, plus the starting NPSH, equals the available NPSH, then the pump will cavitate on start-up. 

I remember well my last visit, in 1980. My excuse to visit the refinery that year was to start up the pump shown in Fig. 25.4. This was a new, flashed, crude-oil pump. It had never been run before. The refinery operators reported that the pump always cavitated on start-up. They had raised the liquid level in the drum to within a few inches of... 

A pump seal ought to last about six years. Running a seal dry for a few seconds during start-up can reduce the seal life by six months. For a self-flushed pump that loses suction pressure on start-up, there will be little or no seal flush pressure until a normal pump suction pressure is established. (See section on starting NPSH requirements in Chap. 36.) Also, a pump that cavitates on start-up, due to the loss of suction pressure, will vibrate, which also contributes to the damage to the pump s mechanical seal faces. 

Clean out the pump suction strainer. Check that the suction pressure gauge is operable and that the connection is not plugged. The unit engineer should have placed a red mark on this gauge showing the minimum required suction pressure to prevent cavitation on start-up. 

Running a pump in a cavitating mode during start-up may not damage the seal faces to the extent that the seal will fail. However, a year s worth of life from a seal can be lost in one minute due to cavitation on start-up. 

Options 1 c and 2b should be used only in HIV-negative patients who have negative sputum smears at the time of completion of 2 months of therapy and who do not have cavitation on the initial chest radiograph. For patients started on this regimen and found to have a positive culture from the 2-month specimen, treatment should be extended an extra 3 months. 

Cavitation will start as soon as the throat pressure has reached the local vapour pressure. Theoretical considerations suggest that the throttling process as far as the throat will be isentropic, which will lead to a small decrease in liquid temperature. Hence the vapour pressure in the throat will be slightly less than the vapour pressure corresponding to the inlet temperature. This effect is small for a liquid, and the throat vapour pressure is unlikely to be reduced by more than 5% below the vapour pressure at the inlet to the valve at the liquid pressures normally found on process plant. For practical purposes, we may assume that the temperature of the liquid in the throat of the valve is the same as it was at the valve inlet. Thus the condition of cavitation onset may be written ... 

This means that a pressure gauge placed on the suction of a pump would show a relatively large loss in pressure while the pump was being lined out. Once the flow has been increased to its steady-state value, the suction pressure would come back up. Hence, if the operator opens the discharge valve too quickly, he can cause an excessive loss in suction pressure and the pump may cavitate during start-up. This is the reason many pumps experience seal failure due to cavitation when they are first put on-line. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

Relative importance of coalescence and rectified diffusion in the bubble growth is still under debate. After acoustic cavitation is fully started, coalescence of bubbles may be the main mechanism of the bubble growth [16, 34], On the other hand, at the initial development of acoustic cavitation, rectified diffusion may be the main mechanism as the rate of coalescence is proportional to the square of the number density of bubbles which should be small at the initial stage of acoustic cavitation. Further studies are required on this subject. 

After consulting Ref. P3 (p.582 Figure 14.2), it is confirmed that the high suction pressure available in this application ensures that cavitation will not occur in the pump. For the calculated specific pump speed, there is a safety factor of over 500%. Cavitation should not be a problem provided that the column is only partially repressurised before turning on the pump (a usual start-up procedure on the absorption column). 

In line with discussions included in previous sections, ultrasonic experiments carried out on fresh water by different investigators indicate that the stabilization of gas microbubbles, acting as gas nuclei for ultrasonic cavitation, is always attributable to the presence of surface-active substances in the water (ref. 15-17,25). As a starting point, one should consider that laboratory tests with various tap waters, distilled waters, and salt solutions have shown that no water sample was ever encountered that did not contain at least traces of surface-active material (ref. 46). Sirotyuk (ref. 25) estimates that the content of surface-active substances in ordinary distilled water amounts to 10 7 mole/liter, and in tap water it is 10"6 mole/liter or higher. These values indicate the appreciable content of such substances in both cases (ref. 122), although they differ by roughly an order of magnitude in absolute value. It is essentially impossible to completely remove... 

In order to illustrate the method, we can take the example of a pump as a component. It may fail to start or to stop when requested, provide too low a flow rate or too low a pressure, or present an external leak. The internal causes for pump failure may be mechanical blockage, mechanical damage, or vibrations. The external causes may be power failure, human error, cavitation, or too high a head loss. Then the effect on the operation of the system and external systems must be identified. It is also useful to describe the ways for detecting the failure. This allows establishing the corrective actions and the desired frequency of checks and maintenance operations. 

The viscosity of a iiquid has been reported to affect the onset of acoustic cavitation. Viscosity is a quaiitative measure of molecular interaction in a liquid. The higher the viscosity is, the higher are the attractive forces between the molecules and hence, the higher is the threshold intensity of US where cavitation starts. Based on experimental evidence, Briggs et al. [58] developed a quantitative relation between liquid viscosity q and the experimental value of the cohesive pressure Pco. which is defined as the difference between the hydrostatic pressure Po — which coincides with ambient pressure when no... 

It is very important that a melt-out riser be installed whenever tank contents are ejq)ected to freeze on prolonged shutdown. The purpose is to provide a molten chimney through the crust for relief of thermal expansion or cavitation if fluids are to be pumped out or recirculated through an external exchanger. An external heat tracer, properly located, will serve the same purpose but may require more remelt time before pumping can be started. 

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