Monday, February 10, 2014

Waterjetting 18a - air and abrasive

As high-pressure waterjet systems have continued to expand into broader fields of application, that increased range has been significantly expanded where abrasive has been added to the jet stream. Abrasive waterjets are more widely used in cutting those materials that are less easy or practically impossible to cut cleanly with water alone (although that is a relative statement, since metal, for example, can be cut at higher water pressure without abrasive).

But before there was high-pressure abrasive waterjet cutting there was sand blasting and other applications of abrasives existed in cutting and material removal – think, for example, of sandpaper. The first “powered” use of sand to remove material has been credited to B.C. Tilghman Jr. of Philadelphia whose British Patent was number 2,147, an indication of how long ago it was. (His American patent was number 104,408). In his review of the topic in 1972 Plaster noted that the patent was fairly comprehensive in regard to some later developments. It in included a system wherein the abrasive was carried by means of
a jet of steam, air water and other suitable gaseous or liquid medium . . . . .the sand may be propelled by a current of air produced by suction or a partial vacuum. . . . . . When a jet of water under heavy pressure is used, as in hydraulic mining, the addition of sand will cause it to cut away hard and close grained substances, upon which water alone would have little or no effect.

Figure 1. Illustration of the initial steam-injected sand blasting design (after Plaster)

At the time that the invention was made steam was the easiest fluid to provide the driving pressures and volumes needed to power the abrasive stream.

The original machine was operated by steam at a pressure of up to 400 psi, and sand was fed from a feed funnel, down through a length of hose into a narrow (0.17 inch diameter) tube centered within the half-inch diameter steam tube. This tube tapered down to a quarter-inch inner diameter as it reached the end of the sand feed line, leaving a narrow gap around the exit to the sand pipe. The high velocity of the steam, as it then flowed the chamber at the end of the sand pipe created the vacuum that pulled the sand into the stream. The resulting jet was collimated by a 6-inch piece of quarter-inch pipe.

It was found, experimentally, that putting a pair of aligned, 3-inch long flat plates on the end of the nozzle, aligned with its edges, gave a better jet, with less lateral spreading when grooves or straight cuts were required.

Steam, however, wet the sand, which would then attach itself to the pipes, causing blockages. Problems also arose because the steam caused poor visibility, and made for unpleasantly hot and wet working conditions. Thus there was an incentive to change, and by the turn of the century (1900) the increasing popularity of compressed air provided an impetus for this change and compressed air then became the main fluid transport for the abrasive throughout the 20th Century. By 1984 production rates for such systems of around 4 sq ft/minute could be achieved by a single operator working with a system driven by a 12 hp. compressor.

As the technology became more widespread so the design of the nozzle was improved through a series of modifications. These led to the inclusion of what is known as a de Laval nozzle into the design of the delivery system. The de Laval design was initially used to drive a small steam turbine in a creamery in 1897, by Gustaf de Laval.
Those who first sought to make steam turbines were also the first to have a large steady supply of an elastic medium that is very like a gas, steam. They soon found themselves using nozzles to produce high-speed flow and they started by using convergent nozzles and they mostly still do. This was the intuitive design with its forerunner in use in hydraulic machinery. They soon found that whilst they could increase the speed of the jet formed by a given convergent nozzle by increasing the supply pressure, no comparable increase could be produced by reducing the back-pressure. They described the nozzles as “choked”. It must have been totally counter-intuitive to find that the fitting of a divergent cone to a convergent nozzle got rid of the problem.
In its simplest form the nozzle takes the form of a convergent section, followed by a narrow constant diameter throat, and this is succeeded by a diverging section at the end of the nozzle.

Figure 2. Basic components of a venture nozzle for abrasive blasting with air.

The increasing diameter of the channel, after the throat, causes a drop in pressure in the nozzle. This, in turn, allows the air to accelerate with the abrasive and the velocity resulting is more than twice as high as it otherwise might reach, going from perhaps According to tests by Tetrabore in 1981, velocity changes from 275 ft/sec to 650 ft/sec have been measured. At the same time the improved velocity of the jet made it effective over a greater area of the target with effective cleaning reported as increasing by 30 - 40%.

A specific design was patented by Albert in 1955, where the transition lines are radiused rather than being linear.

Figure 3. Based on the nozzle design patented by Albert (Plaster ibid).

There are two other advantages to the design beyond the improved air velocity as it leaves the nozzle. The first of these is that the flow is more uniform coming out of the nozzle, so that the surface being cleaned is more evenly attacked, reducing the need for nozzle manipulation to ensure that the surface is completely covered during cleaning, and secondly the amount of abrasive that is required to clean a given surface might be reduced by as much as 20%.

It is in this control of the air component of abrasive blast streams that there thus remains some potential for further improvement. But we will discuss that and other aspects of abrasive use in the following parts of this section.


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