Vacuum collapse

Following are some obvious causes of vacuum collapse ... 

Electron microscopy provides perfect pictures of vacuum collapsed vesicle membranes after negative staining with heavy metal salts. BLMs appear usually as collapsed balls, MLMs often as flat disks (see Figure 4.29). There is no requirement for double-chain amphiphiles in order to form vesicles. The same single-chain amphiphiles which form micelles also form vesicles if their charge is neutralized. This was practised, for example, via the protonation of soaps or through addition of an amphiphilic counterion. In both cases, fatty acids function perfectly well in the form of vesicles. 

It is a wrong approach, often applied in practical operation, to use overdimensioned vacuum pumps with a high rated throughput instead of amply-dimensioned vacuum pumps with a higher ultimate pressme, except for such cases where the vacuum collapses frequently as a result of insufficient material feed into the combined de-airing extrusion unit or inadequate sealing of the vacuum chamber. 

When exhausting desiccators, a filter flask trap (see Fig. 77, 19, 2) should always be inserted between the desiccator and the pump. The vacuum should be applied gradually and should not exceed about 50 cm. of mercury for models (a), (6) and (d). These desiccators may withstand lower pressures, but it is generally considered unsafe to exhaust below this pressure unless the precaution be taken of surrounding the desiccator by a cage of fine-mesh steel wire collapse of the desiccator will then do no harm.J Models (c) and (e) may be exhausted to about 20 mm. of mercury a steel wire cage must be provided for this low pressure. 

Examples of Vacuum-Related Accidents Figure 26-47 shows a jacketed tank, where the jacket was designed for low-pressure steam. When the steam was turned off and the drain valve and trap were closed, the steam condensed, causing the jacket to collapse. The jacket should have been designed for full vacuum, or a suit le vacuum relief device should have been installed on the jacket. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Eailure of vacuum system control resulting in possibility of vessel collapse. 

Failure of vacuum Design vessel to accommodate maximum system control vacuum (full vacuum rating) resulting in possi-. , elief system bility of vessel collapse pressure alarm and interlock to inert gas supply Select/design vacuum source to limit vacuum capability ASME VIII CCPS G-23 CCPS G-39... 

Thermal contrac- Drumming at proper temperatures tion vacuum ere-. integrity walls ated which can suck air moisture prevent water accumulation on drum etc., into drum creating unwanted reaction or collapse of the drum. CCPS G-3 CCPS G-15 CCPS G-22 CCPS G-29... 

One pound of steam at Opsig occupies 1,600 times the volume of a pound of water at atmospheric conditions. This ratio drops proportionately as the pressure increases. When the steam collapses, water is accelerated into the resulting vacuum from all directions. This happens when a steam trap discharges relatively high-pressure flashing condensate into a pump discharge line. 

Rapid absorption of a gas in a liquid in an inadequately-vented vessel can result in implosion, i.e. collapse inwards due to a partial vacuum. 

Designing vessels for full vacuum eliminates the risk of vessel collapse due to vacuum. Many storage and transport vessels have been imploded by pumping material out with the vents closed. 

Another important consideration for marine propeller design is cavitation, the rapid formation and then collapse of vacuum pockets on the blade surface at high speed, and its contributions to losses in propulsive efficiency. The phenomenon can cause serious damage to the propeller by eroding the blade surface and creating high frequency underwater noise. Cavitation first became a serious problem in the late nineteenth and early twentieth centuries... 

The implosion or collapse danger is real even for a tank, for example, that is not designed for vacuum (such as an API large storage tank), and liquid is pumped out of the tanks thereby creating a negative pressure, or vacuum, which collapses the roof and/or sidewalls, because no or inadequate vacuum relief was installed to allow in-flow of air as the liquid is removed (see Chapter 7). 

Vulcanisation being a chemical reaction, is time/temperature dependant. In factory operations, vulcanisation is usually carried out in an autoclave using steam under pressure at temperatures up to 160°C. If the lined unit is a vessel too large to fit in an autoclave and has an adequate pressure rating, all outlets can be sealed and it can itself be pressurised. If this technique is employed then care must be taken, as a failure of the steam supply with consequent condensation, can cause a vacuum and subsequent collapse of the vessel. 

Cavitation Damage erosion of a solid surface caused by the collapse of vacuum bubbles formed in a fluid. 

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