Waterjetting 35b - Cutting the Missouri Stonehenge video

The video that I posted last time did not fare as well as had been hoped, in making the trip from my computer to the blogger post, and so this week, to see if there are other ways of peeling the apple, I have also posted a copy of the video to Youtube, to see if this works better.

The video is of the making of the Missouri Stonehenge for which, as I have mentioned in a previous post, we used a jet pressure of around 15,000 psi with a flow rate of 10 gpm.

The video makes the point that a high-pressure jet system can, with relatively little support, cut a straight edge down the side of a block, even if there is only a very thin layer of rock to remove. It is normally very difficult to do this with a conventional cutting saw, or similar tool, which requires more material on the free side to stop the blade from being deflected away from the cut line.

If this new posting works as I hope, then I will be posting a number of different videos that have been collected over the years, but on Youtube initially, though I will provide the link as I have above.

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.

Waterjetting 20b - cutting slots in coal

There are several ways in which a high-pressure waterjet can be used to interact with a surface or material. It can be aimed to make a high-precision cut into or through a material, it can be used to clean a surface, or it can be used to bulk remove material – to name but three applications. At the moment, in these posts, we are concentrating on the third of these, and last time I mentioned that, if working with soft material, such as clay or soil, that there was an advantage to using two simultaneous jets cutting over a surface, to improve the efficiency of material removal by a factor of perhaps more than ten-fold.

I want to revisit that topic this week, and stepping for a moment away from soil and into coal, which is a harder material, I want to illustrate that the point (of concurrent dual jet use) is still valid but there is a wrinkle, if you are cutting along the edge of an advancing mining machine.

Cutting coal with water jets is not new. But I am going to skip that historical review today, and rather continue on the theme of dual-jet use. When I was first taught to mine coal, there had not been a huge amount of new technology in the industry – and for that matter there still has not been the need for much advanced sophistication where the basic ideas still work.

If you are going to break a material from the solid, it really helps to have a second free surface (as well as the face that you are attacking through). Thus when miners used to work the coal they would first undercut the coal seam using a pick to swing across the surface ad successively chip out a strip of coal about a couple of inches wide at the bottom of the seam, and going back as far as they could reach (about two to three feet). The pattern that this leaves isn’t usually seen in coal mines (since they move on) but I have seen it in the salt mines of Wielicza, the underground rooms in the castle in Naples, and in the old workings of the quarries around Bath in the UK.

Figure 1. Grooved wall at Wielicza salt mine (Wielicza Salt Mine ) The grooves are formed by the successive swings of the pick in the cut that incrementally chip a deeper groove into and along the back of the slot.

Of course cutting the slot in thinner seam coal mines was a little less comfortable (this from the days when smoking was yet to be banned in mines).

Figure 2. Miner “corving” at Seaton Delaval mine (Beamish Collection)

When mechanized machines were first developed for use underground, it was logical to begin with a machine that would cut this slot (the most arduous of mining labor) and replace the miner. To do this the machine developed was, to a very large extent, a variation of what you would think of as a chain saw. Driven by either compressed air or electricity, a long cutter bar would (like the chain saw) drag the cutters along a path (in the mining case perhaps six feet deep) that would create the slot required as a second free surface into which to break down the coal. (You learn very early in the game that a slot less than about two inches high is fairly useless, since the pressure of the overlying ground will just squeeze too narrow a slot closed, and the effort to cut the slot is wasted.)

Once that slot has been made along the perhaps 200-yard long face, then small holes were drilled, at perhaps 4 – 6 ft intervals in the middle of the face, sticks of explosive were placed in those holes, and, at the end of the shift the explosive was fired, breaking down the coal into the immediately surrounding area, and ready for the coaling shift to come on and shovel the coal (in 15 yard intervals per miner) onto the conveyor. (My job at one time).

One of the early advances in mining machines was the Meco-Moore, a machine that cut a slot not only under the coal, but also at the top and back of the seam.

Figure 3. Meco-Moore Mining Machine

This worked fairly well as a concept, but the small cross conveyor that was put on the machine to move the coal from the back of the cut to the conveyor had been adapted from a farm conveyor, and coal is a lot heavier and more aggressive than wheat. As a result the conveyor, and hence the machine, was always breaking down, and so it was replaced with shearers and plows, and the world moved on.

But shearers generate a lot of dust and sparks from the picks that rotate through the coal and adjacent rock, and occasionally hit sandstone. This led to explosions that killed many miners, and so, in the early 1970’s we were asked to develop a new method of mining. The logical thought was to build on the success of the Meco-Moore as a slot cutting tool, and add a plow shape to move the central volume of coal over to the conveyor. Jets would replace the cutter bars at the top, back and bottom of the seam, as a way of freeing the central block.

Figure 4. Original concept for the Hydrominer

We quickly found that using a single jet to cut a slot in coal did not help as much as we had expected. If we cut it horizontally then, as I explained above, the slot would close before it could be effectively used. And if it were cut vertically then the movement of the machine forward meant that every cut had to start afresh and could not take advantage of the previous pass to cut deeper.

And so we came to the idea of using two adjacent jets to cut into the coal at the same time, spacing the jets about an inch apart, and, in this way, removing the rib of coal with the slot cutting, to give a passage into which the nozzle holder, and plow blade edge could advance.

But if the two jets were parallel then the forward movement of the nozzles during each pass would mean that the second oscillating pass would be cutting fresh coal along its length and thus the depth of cut achieved would be only a couple of inches.

So we (Clark Barker, Marian Mazurkiewicz and I) decided to put the two orifices one above the other in a single nozzle block, with the jets pointing out at about fifteen degrees to the line of advance, but divergent from one another.

Figure 5. One versus two jet arrangement

In this way the jets cut a slot about two-inches wide, but as the nozzle moved into this slot it moved into an air space, so that when the jets made the second pass along the surface they did not hit coal until the back end of the previous cut. Within a few passes the two jets were cutting over a foot ahead of the plow face, instead of a couple of inches. This additional leverage from the wedge head of the plow as it entered the cut now meant that the force on the plow was dramatically reduced, and the machine could plow off a strip of coal some 2-3 ft deep and perhaps 6 ft high at rates of between 10 and 20 ft a minute. Given that the jets infused the coal as they cut it, virtually eliminating coal dust from the air, and there are no sparks since cutting occurs by water, under water, so the technique is safer.

Figure 6. Slot cut by the two jet system (about 2 inches wide) and the leverage this gives in breaking off large pieces of coal shown in a surface test.

Unfortunately the world market at the time was only about ten machines a year, and so the design was dropped (after an underground test) – but that is another story.