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.