Waterjetting 32d - Cutting with polymer in the water

In the recent past I have written about the use of polymers in high-pressure jets and that they can significantly improve jetting performance, with no additional changes in the power or pump and equipment used in the work. This is because of two different effects that the polymers have. Firstly they reduce turbulence in the flow from the pump to the nozzle, reducing pressure loss and increasing fluid flow, for the same pump power. As a practical consequence since the fluid flow will be greater for the same pump pressure, this will require that a larger orifice diameter be used to handle the greater, or – for the same flow rate and nozzle diameter, the pump can be operated at a lower pressure.

Figure 1. Comparison between a conventional jet and one containing the polymer additive marketed as SUPERWATER. (after Glenn Howells)

This is not immediately apparent, since the jet carrying the polymer appears smaller, but this is due to the second effect of the polymer, which is to tend to glue the water together, so that it is not dispersed as easily by the surrounding fluid – air in this case. In fact, for the same pump pressure and orifice diameter the lower jet will be operating at a higher pressure (since there is less pressure loss in the line) and there will be more water coming out of the polymer-supplied orifice at a higher velocity. But because it is not spreading into the air, it appears smaller.

This will improve the cohesion of the jet as it moves away from the jet, and as noted in an earlier post, this means that the jet will cut to a greater range from the jet, since it maintains the required critical pressure further.

The impact that the more coherent jet has on performance can be seen where the jet is used to cut into two different types of limestone, one oolitic and one crystalline.

Figure 3. Depths of cut of the polymer-containing jet (left) and plain waterjet, operating at the same pump pressure, nozzle diameter and standoff distance (Glenn Howells)

Note that at the distance where the normal waterjet has broken into droplets (as seen by the nature of the cut surface) and the jet has barely enough energy to remove the surface layer, where the jet contains polymer it retains the ability to cut.

Further, and this is more critical where cut quality is more important, the jet cut is much straighter and cleaner than the dispersed and wider normal cut. This can be seen where a different, more crystalline limestone has been cut closer to the nozzle.

Figure 4. Change in the cut shape to a narrower, deeper cut where polymer is added to the jet stream (rhs)in cutting limestone. (after Glenn Howells)

The benefit of the improved performance changes therefore with the distance of the target from the nozzle, with the more dramatic improvement being seen as the target gets further from the nozzle. Looking at the data from the original work that we did in Leeds, back in the early days of this study, this can perhaps be better realized through the use of a 3-D plot.

Figure 5. Improvement in cutting performance as a function of distance from the nozzle and jet pressure.

Note that, in these trials, the polymer improved the jet performance relatively more at lower pressures and greater standoff distance. Part of the reason for this (in hindsight close to 50-years after running the tests) is that when the jet was cutting at the lower pressures it was closer to the threshold pressure of the rock and this any drop in jet pressure had a more significant impact on cut depth than occurred at the higher pressures, where the gain was not relative to such a low benchmark.

For a number of years, until our research took us into fields where use of the polymer was precluded for several reasons, we routinely used a polymer (generally Superwater, marketed by Berkeley Chemical) rather than Polyox because it gave a relatively consistent and significant improvement in performance plus, being a liquid, it was relatively simple and inexpensive to buy a small metering unit (about the size of a small case) which would feed the polymer into the water supply line to the pump at the required concentration – typically 0.1 to 0.3%.

It tends to work better in improving abrasive jet cutting when it is used with a Direct Injection of Abrasive (DIAjet) or Abrasive Slurry Jet (ASJ) system than with conventional abrasive waterjet (AWJ) systems. The reason for this is that the AWJ system has the abrasive fed into the water stream at the mixing chamber just before the jet leaves the nozzle to strike the target. Within the mixing chamber the abrasive has to penetrate into the waterjet stream in order to acquire the jet velocity, and to distribute across the jet and give an even cut on the target.

Where the abrasive is feeding in from one side of the jet and the waterjet stream is more coherent, it becomes more difficult for the abrasive to penetrate the stream, and if the design is not adjusted accordingly, the cutting performance can be diminished, particularly relative to the gain that can be achieved where the combination is carried out effectively.

On the other hand with the ASJ systems the abrasive is mixed with the water far upstream of the nozzle and the are already thoroughly mixed together, so that the added cohesion of the jet will help to provide the acceleration that the particles need to reach close to the waterjet velocity, and achieve the improved cutting performance required.

Where an ultrahigh pressure jet does not contain abrasive, the polymer can be of benefit as a means of improving cut quality – as evidenced from this comparison in cutting a shoe sole pattern, both with and without the Superwater polymer.

Figure 6. Shoe sole cut comparison with and without Superwater. Note the smoother cut, with less fraying of the back side of the cut with the polymer. (after Glenn Howells)

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.