Waterjetting 20d - Proximate Jets

One way to illustrate the benefits of simultaneously cutting adjacent channels through a material is to artificially show how the jets would appear if they were simultaneously cutting through glass beads. An earlier post showed what water penetration around single jet cutting into beads looked like, and if a second image is placed close enough to, and at a slight converging angle, then one would get:

Figure 1. Simulated dual jets in glass beads using Photoshop to show the combination.

The figure shows that there is a zone at the bottom of the two cuts where the particles under the rib between the jets is saturated with water. The material has no strength, so that the returning water from both cuts has, as a result, enough power to remove not only the material under the jets, but also that between them. (Which is why you can’t show this in reality). The combined effort of the two jets produces results greater than the sum of their individual efforts.

There are a number of different applications, other than just in soil removal, where this can be considerable advantage. A number of rocks (coal and shale particularly) have weak layers within them, known as the cleat and bedding planes in coal, so that driving the water along those weakness planes, from two sides, will again liberate all the material to the free side of that pressurized plane.

But, at higher pressure it also works for removing of layers, such as, for example, the old paint, coatings or contaminant from a harder, typically metal, surface. A single jet, for example, can cut down to the metal surface, and may peel up the edges of the over layer along the cut, but there is insufficient immediate pressure to do more.

However if there are two jets cutting along parallel to one another, then if the two pressurized zones intersect (as Figure 1 shows) then there are free surfaces all around the intervening material, with a driving pressure to lift and remove it. Again the result is to produce significantly more material removal that could be achieved by two passes of a single jet. For this to work, however, the two jets must be close enough to each other that the pressurized zones within the material intersect. The distance over which this works varies considerably with the material being removed. In a well structured coal or a weak soil, for example, the distance may be measured in inches, in a tightly bonded paint it may be merely millimeters. Only testing can determine, as a function of jet flow rate and pressure, what the critical distance is for different materials.

Some materials are sufficiently cracked, and again coal is a good example, that the two jet system is not always necessary to achieve acceptable results. If the jet is aimed to flow into the horizontal bedding planes, for example, and then strikes a perpendicular cleat plane, then if there is an adjacent free surface, the water force may be sufficient, as it then surrounds the block of coal, that it can liberate it with only one jet. There is a difference between the volumes when the jet is used to pressurize one set of layers relative to the other. Perhaps the best illustration of this comes from some trials of coal mining in a steeply dipping seam of coal in Colorado.

Figure 2. Remote testing of a coal mining monitor inside the portal of an underground mine in Colorado.

Where the monitor was used to cut single passes across the face of the coal then there was a slight increase in the volume of material as the nozzle diameter (and thus water flow) increased. But where the jet was cutting into the cleat to fracture the coal (fracking) then the gain in volume mined was significant, and when the jet could work to pressurize the horizontal bedding planes, and thus to break off large slabs of coal, then the gain was even more significant.

Figure 3. The gain in coal production as higher volumes of water are used to pressurize internal fractures within the coal, breaking off greater volumes.

The larger nozzle diameters ( up to and beyond an inch) make it easier to sustain the pressure within the weakness planes of the coal, as the water spreads along the length of the fracture, exerting increasing amounts of force on the coal and thereby breaking it from the solid and moving it into a free space, provided that one exists for it.

The best mechanism for achieving this break depends on circumstance. If, for example, one is driving an access tunnel, then large free surfaces may not exist, and it may be less easy to find the weakness planes to exploit for large material removal. One way that Chinese investigators overcame this problem was to oscillate their cutting nozzle in a plane perpendicular to the traverse line.

Figure 4. Chinese Oscillating head miner

A simple cam connection to the nozzle drive forces the nozzle to move up and down during operation cutting a wide groove in the slab, and with the nozzle moved sufficiently that the ribs between adjacent passes is also removed by jet action.

Figure 5. Volumes of simulated coal removed in equivalent times. The top slot is removed without the jet oscillating. (It can be seen in the center of the wider slot). The lower slot is cut with the head oscillating at the same time as it is traversed.

Again, where the contrast between a confined jet and one which can work to a free surface is examined, the change in the volumes extracted can be seen to be quite significant.

The important lessons to learn in this are that the jet itself penetrates very rapidly into the material ( about 1/100th of a second) it then starts to lose efficiency as pressure is lost due to the effects of the side walls of the cut made and in the loss of pressure to water penetrating into the surrounding materials. If, however, that pressure can be developed as an additional means for removing material, by providing an adjacent free surface, or second pressurized zone from an equivalent jet cutting nearby, then the total volume of material removed can be significantly improved.