Requisitos de las naftas para motores ciclo Otto

1. Controlled Cavitation Water Jetting

Under certain conditions of rapid water flow, numerous low pressure cavities or bubbles are formed. The collapse (implosions) of these bubbles is accompanied by the release of large amounts of energy. The formation, transport, and collapse of the bubbles is known as cavitation(ls1. Cavitation is well known as a destructive phenomenon which

results in metal loss on or near propellers, pumps, etc. From efforts at countering these effects, researchers developed the technology to control and direct the forces of cavitation. The technique has been successfully utilized in boiler tube clean- ing, rock drilling, and in removing underwater foul- ing from ship The technique of controlled

cavitation also offers the possibility of certain ad- vantages for surface preparation of structural steel. For a given water pressure and flow rate,

cavitation develops higher forces at the point of im- pact than conventional high pressure water

blasting. Thus, it could provide greater efficiency and higher production rates where abrasive

blasting is restricted. Cavitation blasting does not introduce any solid abrasive onto the substrate. The prospects for recovering the old paint or sur- face debris are therefore enhanced due to the much smaller volume of solids compared to conventional sand blasting or wet abrasive blasting discussed earlier. The energy and water requirements for con- trolled cavitation blasting are similar to those for conventional high pressure water blasting (Figure 5).

The efficiency and productivity of cavitation jetting depend on the operating pressure and flow rate, design of nozzle, size of orifice, standoff distance and angle of impingement. The applica-

in the early development stage. Current research ef- forts focus on a number of different areas pertain- ing to surface preparation, as well as related areas such as steel cutting and concrete rehabilitation. A government-sponsored program is concentrating

FIGURE 5

Controlled cavitation blasting of galvanized steel. Courtesy of SEACO, Incorporated

on developing units which produce less than 50 pounds of operator thrust. As with conventional high-pressure water jetting, operator fatigue is a limiting factor. The goal is to provide hand-held devices for complex structures and inaccessible areas. Researchers anticipate that rates for produc- ing a clean, paintable surface (¡.e. removing loose paint, dirt, and loose rust) will range from 50-200 square feet per hour. These are based on the use of current technology nozzles. Additional research is directed at advancing the technology to achieve the more difficult task of removing hard rust and intact paint, and producing a surface profile at rates ap- proaching those above (Le. 100 sq. ft.lhr.).

In addition to the hand-held units, efforts are planned to develop high production units which would include features such as multi-nozzle arrays and automatic translation and thrust support. A fur- ther objective of the sponsors is to devise a means for recovering the paint and rust removed from the surface using suction, vacuum or other auxiliary to the cavitation system. The U.S. Air Force is in- vestigating the use of cavitation to remove paint from aluminum. The technique s ability to control the depth of erosion could allow removal of the top coat alone, leaving the primer intact and avoiding damage to the aluminum substrate.

2. Automated Water Blasting

Highway officials from Texas have developed a water jet cleaning system which does not require an operator at the nozzle. The high pressure jet noz- zle is attached to a rig clamped onto the bridge beam and remotely controlled by an operator on the

Copyright The Society for Protective Coatings Provided by IHS under license with SSPCNot for ResaleNo reproduction or networking permitted without license from IHS

SSPC CHAPTER*Z.B 93 8b27940 0003537 395 M FIGURE 6

Automated robot high-pressure water jet blasting. Courtesy of Texas Department of Highways and Public Transportation

ground (Figure 6). The operator can translate the nozzle along the beam and change the angle

sideways and up-down to allow access to over 90% of the surface area. The developers are working to increase this percent and the unit s overall versatil- ity. The unit offers several important advantages. The safety is greatly improved because the

operator does not direct the nozzle or support the thrust. The thrust, which may be as high as 100 pounds, could cause the operator to lose his

balance on a scaffold or to blast himself or a co- worker. In addition, with a hand-held unit, the operator becomes greatly fatigued in a few hours, which severely limits his productivity. The

automated device should produce a more uniformly cleaned surface and permit more precise calcula- tions of rates and costs. Some of the problems ex- perienced are lack of maneuverability, cost and time for maintaining equipment, and the need for modifications to allow use on different types of structures.

3. SteamISand Blast

A technique which uses steam to propel the abrasive has been developed by the JapaneseW The use of steam instead of water results in a shorter drying time and a significant decrease in the amount of rust formation in comparison with other wet blast methods. However, three major obstacles seem .to preclude its widespread use at this time. First, Ithe cloud produced by the steam obscures the operator s view of the work. Secondly, steam, because of its high temperature and release of energy upon condensation, poses special safety problems. Third, in this era of energy con-

sciousness, steamisand blasting is one of the most energy intensive methods of surface preparation. IV. OTHER ABRASIVES AND MATERIALS

Conventional and new metallic and non-metallic

abrasives are discussed in separate chapters. In this sec- tion we consider several novel types of abrasives that have been proposed because of some special feature.