Saturday, July 19, 2014

Waterjetting 23c - Abrasive Slurry cutting range

In an early section of these notes on high-pressure water and its uses, there was a review of some of the ways in which jet power could be assessed. For the most part the best way to see how changes in a system alter the way the jets cut is to run a simple cutting test and in many cases it will give the answer that is needed. However, sometimes it is important to go beyond the simple assessment of whether condition (a) is better than condition (b) and to try and explain why it is.

One of the early discoveries in trying to explain abrasive jet behavior was made by my colleagues Marian Mazurkiewicz and Greg Galecki who showed that, when creating an abrasive waterjet system in the conventional way, that a large amount of the abrasive was being fragmented in the mixing chamber of the nozzle.

Figure 1. Size distribution of garnet particles after being fed through the mixing chamber of an abrasive waterjet nozzle, AFR 0.6 lb/min, at 30,000 psi.

Before entering the chamber the particles had been screened to be very close to an average 200 micron size. After going through the chamber the average particle size (50%) was 140 microns, with roughly 25% of the particles being smaller than 100 microns, at which point we had found that the cutting performance gets significantly poorer.

The way in which we found the particle size, (and also assessed how fast the particles were moving after they left the nozzle) was to set the nozzle horizontally, and then to fire the jet down the center of a large plastic tube.

Figure 2. Plastic tube set up to capture the abrasive particles from an abrasive waterjet nozzle. (The nozzle is on the left of the tube).

Barriers were placed at 1-ft intervals along the tube, so that as the particles lost energy and fell to the bottom of the pipe they could be collected into the small dark blue containers under the tube, and then dried, sized and weighed. During a test the top half of the pipe sections are replaced, so that the jet is contained over the pipe length, which was just over 20-ft, and this was long enough to capture, within the length, all the abrasives from all the tests of abrasive waterjet nozzles that we carried out (which included all those commercially available at the time).

It was based in part on this test that we were able to understand why some abrasive waterjet nozzle designs work better than others, and also to begin to understand more of the mechanisms that drive the cutting process.

As the abrasive slurry system (ASJ) started to enter the American market we were thus ready to test the way in which it worked and to see if we could find any improvement, as had been reported by those who first used the system.

When we set the system up so that the ASJ system fed a nozzle in the same arrangement as with the AWJ system we got a surprise. Much of the abrasive was collecting at the far end of the pipe, and it was starting to poke a hole through the end piece.

Over time we extended the tube, and eventually moved it outside to ensure that we could capture all the abrasive in the same way as earlier.

Figure 3. The test set-up needed to capture all the abrasive when using an abrasive slurry jet rather than an abrasive waterjet. (The pipe is roughly 50-ft long).

Even at lower pressures the ASJ was carrying the abrasive much further than was the case with the conventional AWJ system, showing that the particles were retaining more energy over greater distances. In retrospect this is not surprising, since there is sensibly no break-up of the abrasive particles with the ASJ system during the mixing and acceleration of the particles.

This is because the particles are mixed in with the water before the water accelerates, and when it does the abrasive is already mixed throughout the jet, rather than trying to force its way into the jet, while at relatively slow velocity relative to the water. There is, as a result, no break-up of the particles, and larger particles will lose energy more slowly than smaller ones. (One of the findings of the AWJ tests carried out earlier).

In addition because there is no air in the ASJ jet stream there is no active component trying to disrupt the jet, and as a result the water remains coherent to a greater distance from the nozzle, and has, as a result, a much greater capability of transferring the energy to the particles to accelerate them to their full potential, given the design of the system.

There are various different ways in which this benefit can be illustrated, using lab data, but the clearest demonstration is to take a conventional waterjet system, and to run the triangle test and then, with the same amount of abrasive in the jet stream, and a roughly equivalent amount of water, to run the same test with an abrasive slurry system.

Figure 4. Comparison of the cuts achieved with an abrasive slurry system (upper) and an abrasive waterjet system (lower).

The picture shows that the two systems are cutting to sensibly the same depth, and the ASJ system is being operated at a quarter of the pressure of the equivalent AWJ system.

There have been many comparisons between the two systems in the time since this initial evaluation was made, and there is a rough consistency in the results that have been obtained, on the order of that shown in figure 4. The comparisons are not all equivalent to this, since the tests compare different attributes of the two systems, and, for example, there are additional advantages of the lower pressure ASJ system that can enhance the relative performance. One way, when one compares equivalent horsepower, is to increase the diameter of the ASJ nozzle. This allows use of a larger abrasive particle that, in turn, will give a further increase in achievable depth of cut.

Unfortunately the difficulties in achieving consistent performance with some ASJ systems, over long operating periods, has made it more difficult for this relatively new system to penetrate fully into the market place as yet.


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