Waterjetting 25d - extending range and cutting power

At the end of the last post I mentioned the benefits that could occur if two jets were directed to intersect at a distance from the nozzle. Marian Mazurkiewicz did a lot of early work on this, and it was written up in an ASTM STP.* Because it has a couple of applications that include potentially extending the range of the jet, this post is somewhat of a short version of parts of that paper. The illustrations are taken from the paper, and the mathematics that is included in the analysis within the paper is not included in this post.

In the initial instant that a flat ended jet hits a flat surface at a shallow angle, two jet flows are formed. A small micro-jet is formed along that surface that moves at a much higher speed in the direction of the arriving jet. A slower jet moves in the opposite direction.

Figure 1. The acceleration of a small segment of an arriving jet on a flat surface (after Mazurkiewicz et al).

By replacing the flat lower surface with a second jet co-axially aligned in the plane of the jets, a similar effect can be achieved, with the second jet moving considerably faster that the arriving jets.

Figure 2. The impact of two jets inclined toward one another at a shallow angle.

For the tests that I described last time, the two jets were inclined toward one another at a relatively small angle (in the range from 1 – 10 degrees) which was partly controlled by the geometry of the cutting head in which they moved.

The improved velocity of the secondary jet can be shown by one of the photographs taken of the impact of two small jets, operated at 60 psi, and intersecting at an angle of ten degrees, using a high speed camera to capture the result.

Figure 3. Intersection of two jets viewed from the top. The jets had broken into droplets at the point of impact, and the shock waves generated by the high speed of the secondary jet formed on impact can be seen around the impact point.

With better quality jets (made from electro-formed nickel built up on a flame-polished mandrel) it was possible to get the jets to intersect while still coherent, and the resulting jets, formed from jets at 10,000 psi, were able to cut thick lenses of pyrite in the field. This was not possible using the 10,000 psi jets alone, without the use of the augmented jets produced by their convergence.

There is a second benefit that can occur where these convergent jets are used in working with harder materials (than coal). It can be illustrated by a photograph of two separate pieces of Berea sandstone into which two different sets of converging jets had been fired.

Figure 4. Two blocks of Berea sandstone each of which had split after having had a pair of convergent jets fired into the top of the block.

The pair of jets converged at the point where the belled-out shape of the cavity transitions to a narrower tapering hole. The larger upper volume is created by the back-flow of the slower moving jet as it cuts back towards the entry hole, reaming out the original passage.

Apart from the evidence of the smaller accelerated jet (through the shape of the cavity) the other interesting point (which was confirmed in a number of tests) is that the restriction of the outflow of water from the cavity, because of the narrowing of the cutting jet paths with depth, and the augmentation of pressure at the impact point, produced enough internal pressure in the blocks to cause them to rupture.

This augmentation was used in Rolla in a number of different applications over the years, although, because of the expense of building the high precision nozzles, these were not used extensively in later work. Rather the jets were formed from two separate flows to nozzles on the end of two short lengths of hose. These jets could then be adjusted to change the intersection angle of the jets, which was also adjusted through raising and lowering the head, so that the intersection point fell below the surface of the target. This meant that the jets had to penetrate a little into the rock by themselves, before they intersected and generated the higher pressure small penetrating jet and concomitant increase in local pressure of that jet.

Figure 5. Different approaches to the use of converging jets on a rock surface. That on the left is the MS&T version, that on the right was carried out at the University of New South Wales**.

The MS&T approach was based on the work we had carried out in the field, where the jets were to converge on the surface of the target, so that the jet would be able to penetrate through rock materials that it would not normally be able to cut. In the trials in the mine an intersection angle of 2 degrees was found to be best.

The Australian approach followed on Frank Roxborough’s ideas of trying to generate larger chips when cutting into rock, in order to lower the energy required for material removal.

The Australian team however, found it more useful to converge the jets, closer to the nozzle which was less tightly manufactured, and focused the streams within the rock body. In this way the stresses set up within the rock were found to invariably produce large single chips of rock roughly conical in shape with an angle similar to that of the impacting jets. Interestingly it was reported that there was little evidence of jet cutting action in these tests where a jet at a pressure of 40,000 psi was cutting into a 30,000 psi uniaxial compressive strength basalt – something normally impractical even at those jet pressures. The results also were reported to show that the specific energy required for this technique was one to three orders of magnitude less than for conventional cutting of slots by jet action. Subsequent traversing tests on the rock were preliminarily reported to substantiate the results from the static testing.

* Mazurkiewicz, M., Barker, C.R., Summers, D.A., "Adaptation of Jet Accumulation Techniques for Enhanced Rock Cutting," in Erosion: Prevention and Useful Application, ASTM STP 664, W.F. Adler, ed, ASTM, 1979, pp. 473 - 492.
** Lin, B., Hagan, P.C., Roxborough, F.F., "Massive Breakage of Rock by High Pressure Water jets," 10th International Symposium on Jet Cutting Technology, Amsterdam, Holland ,October, 1990, pp. 399 - 412.

Waterjetting 25c - more thoughts on jet range

A single waterjet, whether with or without abrasive, will cut a tapering slot as it penetrates into a target material. This is because, as the jet penetrates into the surface, the outer edges of the jet lose their energy in cutting, and the narrower central core remains capable of cutting, on a continually narrowing path, as the cut deepens.

Figure 1 Tapered cut made with a single jet traverse in contrast with the wider cut made with two diverging jets.

While the above statement is generally true, it is not completely so, since if the speed of traverse of the jet is reduced, then the continued addition of further water along the cut plane will be sufficient for the outer layers of the jet to be able to continue to cut and this may reach the point that there is no taper along the edges of the slot, or it may even taper inwards. For an abrasive jet cutting into titanium, that transition occurs at around 0.2 inches/minute, depending on jet parameters. (Note that this is less related to the target thickness, although it is controlled by the cuttability of that material, and that the critical speeds for cutting with water along are at least one and often two orders of magnitude greater).

Figure 2. Plot of taper angle with traverse speed.

Unfortunately the speed at which the edge is cut perpendicular to the top surface of the target is usually too slow to be economic, and, in consequence, the normal process is to slightly tilt the cutting head into the edge with the desired surface, and making the opposing surface carry an exaggerated tilt. This then allows a faster cut, again with the optimal speed being a function of both tilt angle and jet parameters.

When the objective, however, is to achieve a deeper cut, particularly where multiple passes are concerned, and head movement into the cut is allowed, then a different strategy can be followed.

Back when we were developing the longwall mining machine we called Hydrominer, we used a dual-jet system, because, when cutting coal, the material between two adjacent, concurrent cuts is removed as those cuts are made. Thus the jets, in a second pass, do not make contact with the walls of the cut until reaching the back of the previous cut. (The second image in Figure 1).

Figure 3. Slot cut by the Hydrominer, looking down, and with the slot through which the jets cut out from the head visible on the left edge of the machine.

However, in harder materials, including rock with some degree of cohesion, it is possible to run two jets almost side by side, and leave a rib of material between the cuts, so that jet attenuation in dual cutting is still a problem if the jets are parallel.

Again the answer is to tilt the jets, although if small jets are used, multiple jets may lose in overall range, because of the reduced diameter of the individual streams.

In this case it can be more effective to combine the jet flows into a single jet, but to either orbit or rotate this slightly off-axis so that the jet is cutting a slightly wider track along the path, and with a widening slot with depth, so that, again, subsequent passes, where the nozzle moves into the slot, do not encounter the walls of the cut until the back of the previous cut.

Back in the days when we were first testing the coal mining machine, we were mining coal in northern Missouri, and the coal had a large number of pyrite lenses in it. These lenses could be up to four inches thick, and, while the coal was friable and easy to cut, the pyrite lenses were much harder and dense. They could not be easily cut with the jets, which were operating at 10,000 psi, and the machine was not performing very well.

There were two ways in which we overcame the problem. The first was to adjust the two jets that were cutting the slot into which the cutting head was moving. As I mentioned earlier with a slight divergence angle between the jets, the slot was cut wide enough (around 2-inches) for the leading edge of the head to enter the cut, and the depth (around 9-inches) was enough to give leverage for the head to peel the rib of coal from the solid.

Figure 4. Comparison of results in the field with initial lab-designed nozzle.

When we encountered the pyrite, we changed the angle of the jets, so that instead of diverging the converged at varying distances in front of the head. When the two jets come together at this shallow angle (as with shaped charge formation) they form a very high speed jet, as well as a slower moving wider stream.

When this combination replaced the diverging jets on the head, this higher-speed jet was sufficiently powerful that it cut through the pyrite, and gave a free surface for the rest of the lens to break into. (Depths of cut up to 3-ft were achieved, although the slot was less than one-inch wide). This worked well for the side of the slab that was now liberated, since the jet had broken it free, and the head could move it away from the face, and into the conveyor track.

The only problem that we had at the time, was that the convergent jet was formed in the center of the slot being cut and in the center of the leading edge of the mining head. The slot was no longer wide enough for the head to enter (the converging jet gave a slot about half-an-inch wide IIRC). As a result the pyrite on the solid side of the cut now engaged with the leading edge of the head and stopped progress.

The answer to the problem, which we arrived at over time, was to change the angle of the axis of convergence of the jets, so that, instead of being in the center of the slot, the convergent jet was inclined over towards the solid, and cut into the pyrite just ahead of the outer edge of the mining head. In this way, since the material to the free side of the head was being moved out of the way by the advance of the machine, the jets still cut clearance for the head to move forward. At the time we were only able to get the machine up to a speed of 10-feet a minute, but by taking a bite of 36-inches at a time, we were able to match the productivity of existing mining machines of the period. (The coal seam was 5-ft high). The guard design on the head was also changed to give a sharper edge on the solid side of the machine.

Figure 5. Change in head guards to penetrate pyrite.

Very little work has been carried out on convergent jet systems since that time, which is a pity since it allowed us to mine harder material than the main jet pressure available was allowing us to achieve.