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.

Waterjetting 22a - Mining horizontal coal

Over the last few posts I have discussed some of the problems that arise in dealing with the use of waterjets in mining coal, when the material mined has to be collected and transported away from the face where the coal is extracted. I thought I would follow on that thread in a few more posts, ending up, hopefully, where I began back in the process of removing thin layers of material (such as rust) from flat surfaces.

But to get there I am first going to go back to coal mining. One of the problems with adapting what we might call conventional hydraulic mining to coal is that many of the coal seams around the world are relatively flat – it is, after all, the way in which the vegetation that became coal was laid down. Thus the gravity that can be used in a steeply dipping seam as a way of carrying away the coal and the water together, is not initially that helpful.

There are several different ways that have been suggested over the years to solve the problem. Initially these were based on existing mining machines, and methods for mining the coal, but with the teeth of a conventional machine replaced with high-pressure waterjets. One such, as I have written earlier was the MS&T Hydrominer, where the cutting teeth along the edge of a coal plow were replaced with oscillating dual-orifice waterjets to cut a kerf around the coal being mined.

Figure 1. Artist’s impression of the initial Hydrominer, with jets cutting a slot one foot deep ahead of the wedge shape of the plow.

The water used was less than that conventionally used on a mining machine to suppress the dust generated as coal is mined from the solid, and the coal loads onto the armored face conveyor on which it rides down the face.

That particular design was based on an earlier mechanical machine, the Meco-Moore, which I had previously seen working on a longwall in the United Kingdom.

Figure 2. Meco-Moore mining machine set up to mine coal. The cutter jibs cut slots and the coal then collapses onto the transverse conveyor.

However this concept required a considerable investment in the supporting longwall equipment both to hold up the roof and to remove the coal. An alternative approach was to continue to conventional roof-and-pillar mining which is the most popular method of underground coal mining in the United States, but again replacing the cutting teeth with waterjets. The first of these was conceived by IIT Research Institute in Chicago, under Dr. Madan Singh.

Figure 3. A high-pressure waterjet continuous miner.

Unfortunately in this configuration the system did not work well. The jet pressures used were too high, and in consequence the volumes of the jets too low to achieve a deep penetration into the coal.

When the jets were replaced with a combination similar to that of the Hydrominer, and in a device we called RAPIERS, a slightly better performance was achieved, but the demand for innovation had, by that time passed for a spell, even though this particular machine was developed with considerable technical input and financial assistance from the Jet Propulsion Laboratory in Pasadena.

Figure 4. Progression of the RAPIERS machine in room-and-pillar mining.

Both of these machines required that a second set of machines sit behind the excavator and carry away the coal that had been mined, again at significant cost, and they also required machines to support the roof.

There is a different type of machine that is often used at the edge of the productive limit of surface mining. As seams near the surface get deeper so the cost of removing the overlying material becomes too expensive to justify continued mining. At that point companies may bring in an auger which can drill long holes into the coal, and remove the material as with conventional smaller augers that might be used for drilling in dirt (or even drilling holes in wood).

Figure 5. Conventional auger mining (Rosamine )

Because the auger drills a hole to the size of the following scroll, it is relatively easy to carry the coal back out of the horizontal hole, which might exceed 300 ft in depth. But there is a problem with the machine, in that the cutting force to push the auger teeth into the coal at the face of the machine has to be carried through the entire string of augers.

Because of the string of segments this becomes more difficult to control with longer depths, and in addition there is a friction loss due to the continual rubbing of the scrolls against the floor and sides of the hole. Together these act to limit the machine range, since there is little to steer the machine other than the direction of the hole, as it deepens.

Figure 6. Picks on the face of the auger, with early jets mounted in the center of the head to cut a central hole.

If, however, the picks on the face of the auger are largely replaced with waterjet nozzles, particularly at the outer edge of the auger, and with the flow directed there, rather than, as shown in Figure 6, towards the center, then an outer free face – up to a foot deep, can be cut ahead of the cutting head. With larger auger heads the nozzles can be placed across the face, to break the rib of coal, should it start to get too large – especially since the coal needs to be fragmented somewhat to feed down the auger.

Figure 7. Waterjets across the face of an auger (courtesy W.A. Summers)

The reduction in the amount of force that this allows on moving the auger into the coal can be illustrated by example. In developing a version of the machine we built an artificial coal face, made up of coal pieces and cement. It is a little more resistive than conventional coal, however the student, Chris Cannon, had little difficulty pulling the machine into the face with a come-along, even though he only had one uninjured arm at the time of the test.

Figure 9. Chris pulling the 2-ft diameter auger into the artificial coal seam.

By confining the coal and water it was possible to recover both, so that the water could, if needed be recycled.

I’ll continue the thread next post.

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.

Does the fossil fuel industry need innovation?

Most of the posts that I write tend to deal with the technical side of energy production, rather than more philosophical discussions. However I was recently sent a copy of the new Strumsky, Lobo and Tainter paper on “Complexity and the Productivity of Innovation.” (H/t Nate Hagens). Since, in my day job, I have been one of those innovators in the field of energy (and since also I am going to address this as part of my talk at ASPO in a couple of weeks) I thought I might pen some thoughts that respond to the paper, and explain why I don’t think that the picture is nearly as bleak as the authors seem to suggest.

The point which the paper seeks to make is that, over time, innovation in a field becomes harder, as the early obvious inventions get made and it becomes more difficult to make significant further advances, requiring larger and more complex teams to work longer hours for less overall gain. The metric that is used in the paper is the number of patents that are generated in a field, with that assignation being made by the US Patents and Trademark Office. The authors note that, over time, the number of patents per inventor has declined, while the size of the patenting team has increased, and that the patents per inventor holds true in the fields of energy that include gas, power systems, solar and wind.

My field of knowledge in the area fits more into coal, oil and natural gas production – which may be a field so small that it falls under the radar as regards USPTO classification, certainly in many categorizations of engineering these days mining seems to be a forgotten word and fossil fuels a forbidden topic. But that gripe aside, let me talk about the process of innovation and invention and give some reasons why I disagree with the authors.

There is a saying that, though true, is not considered much because it may have been overused; “necessity is the mother of invention.” (And my thought strays to the day we were scheduled to run a demo for a TV station on a technique for landmine detection and when we ran a test that morning with an “improved” set-up it didn’t work. We had about three hours to work out why it did not, find a way to get it to work, rebuild the tool, and make it work before the crew arrived– our answer aired that night.)

In the field of digging things out of the ground there hasn’t been a whole lot of what might be called revolutionary thought in the past couple of millennia. This has been mainly because the old ideas of a pick and a shovel seem to work remarkable well. True, in the process the pick has moved from being a single point on the end of a manually wielded tool to one of a multiplicity of picks laced around a cutting drum of a rapidly advancing machine, but the principle remains the same. Energy efficiency in the change has gone down, but overall production rates have soared, as machines have got larger and more powerful, when mining underground. On the surface, the load that a single shovel can move went from the few pounds that a miner moved to the 165 tons of a single scoop of “The Captain” 180 cu yd mining shovel bucket.

Thus there has not been a lot of necessity for innovation. What we have is relatively cheap per ton, and it works. Now that is not to say that there isn’t an occasional need for innovation. Back in the 1970’s there were a series of coal mine explosions that killed miners as a result of sparks generated from the picks of mining machines. We developed a machine that used high pressure waterjets to cut the coal, rather than picks, and the technology was picked up by industry to the point of underground trials in Germany. We also worked with CalTech to develop a version for room and pillar mining. In both cases we showed that the tool could mine coal safely and productively without sparks or dust and at energy levels below that of existing equipment. But the world market was considered by GHH to initially be only 14 machines a year, not enough to justify the investment. The need was not great enough, and we looked for more marketable products than large mining machines. Which led on to the development of waterjets for drilling and cutting. But in terms of the overall impact on mining waterjets did not catch on. And patenting the technology proved difficult. Jason and his Argonauts visited a hydraulic mining operation, in what is now Georgia, millennia ago (and stole the sheepskins they were using to catch the gold in the bottom of their flumes). So what is new, apart from raising the pressure a bit?

As a result of a lack of need, there has not been a great deal of significant interest or funding in the actual digging process itself. Back in the energy crisis that ran into the 80’s, increased research funding led to the development of the polycrystalline diamond compact bit, which has since made drilling in hard rock much easier. But after the collapse of oil prices in the ‘80s funding and interest faded away.

So my first argument that I would raise is that there is, as yet, not that much necessity for invention in the fossil energy field, but that when it comes there are lots of different avenues that can be taken. As an illustration, we removed the rock for the Omnimax Theater under the Gateway Arch in St Louis by breaking it out into single large lumps weighing between 1,000 and 2,000 lbs each. Creating a single crack around the blocks took a whole lot less energy than breaking it down into the fragments of a typical mining operation. And it made it a whole lot easier to handle. The necessity was that we had to get the rock out without any of the visitors noticing, so we couldn’t use blasting or any more conventional technique. And as a second example, if you disintegrate rock into its constituent grains at the mining machine, you can separate the valuable mineral there, leave the rest behind, and only haul and process the valuable stuff. Saves a rather large amount of energy, over the increase needed for the conventional process.

Anyway, to get back to the paper, the other thing that is more integral to their discussion relates to how things get invented. In many of the cases I am familiar with there is originally the work of one person. That person has an idea, and some sort of drive to get this idea to work. It is only when a group of other folk has been persuaded that the idea is good that it starts to move toward commercial use – and I would agree with others that a realistic time from innovation to widespread use is about 20-years. And this is the path of revolutionary ideas, the ones that change the paradigm of an industry.

I do not think that this type of innovation is properly recognized in the paper. What the paper discusses is the more conventional evolution of technology once the original concept has moved into use. But it is the revolutionary concepts that will move us forward.

Working thin coal - the coal plow and Hydrominer

In the last couple of years there has been a growing concern about the amount of coal that remains in the productive reserve for the world. At the same time the incoming British Government, which had been somewhat antagonistic to coal while in opposition, may be giving large coal-fired power plants a reprieve. Questions might therefore arise as to how long this fuel source is going to last, if use increases more than projected. While I am not going to answer that specific question today, I will address a part of the issue. Namely what do you use to mine coal when the seams get too thin for modern equipment?

The lowest coal which I personally have mined was about 1 ft 8 inches high. The low coal was caused by a roll in the middle of the face (the floor got closer to the roof) which was normally about 3 ft high (The Beaumont seam at Seghill Colliery in the spring of 1962). In that height you lie on your back, put a pit prop under your shoulder to give you some leverage, and shovel across your chest. In more advanced mining countries it is unlikely that we will return to such manual labor, which is not very productive. So what do you use?

The most productive machine in most longwall operations is the shearer, which I described in the last tech post. The problem is that it most effectively runs on top of the armored face conveyor (AFC) and the power pack that drives the cutting head and haulage unit takes up quite a bit of space. One idea was to take the shearer off the conveyor and have it slide along the ground on special shoes, with the cutting head mounted ahead of it on the longwall. My father had rather strong opinions on this, since two of the mines he worked with had such machines. Remember that the coal conveyor must snake over behind the machine in order to allow the supports to also advance.

To hold the machine together, the gear boxes at each end and the power pack in the middle, there are through-bolts down the machine. Now it breaks down in low coal. The roof is say 2 ft 6 inches above the floor, the machine is 22 ft long, and the conveyor is 7 inches high. How are you going to take the machine apart to fix it? (The answer involves explosives, and is not a “quick fix.”)*

So if the shearer is not an ideal machine, what is? The answer is known as a coal plow (or Hobel in Germany where they were developed by a company then called Westfalia Lunen.) Very simply in some coal types, particularly those that are brittle, the coal at the front of the face is weakened and cracked by the pressure of the overlying ground. Thus if you take a narrow pick and drag it across the coal it will peel off some coal. Put a number of these picks together and the coal between them will also chip off – perhaps to a depth of a couple of inches. Make the machine move down the face rapidly, with the rams pushing through the AFC to keep the plow pressed against the coal, and you have a simple but effective mining machine.

Plow on face – note the picks in the different elements that can be removed to adjust for varying seam heights. (Shield parts removed to show the plow)

The coal in which this is most effective does not necessarily have to be mined over its full height, since often, when undercut, it will fall under its own weight. Depending on the coal strength, the depth of cut of the machine can be adjusted. As a result the coal produced can be quite large, sometimes bigger than the average size of the fragment coming from a longwall shearer face. It is, however, an "interesting" experience, to see one working under a sandstone roof, where the face rolls a bit.

There were a variety of plows built, depending on coal height, coal type, and the speed at which the plow could be moved down the face. It is a fast moving operation, but one that is not that popular at the moment, since, in most higher coal a shearer may be more effective and produce more coal. However in the years to come the plow may make a comeback.

By then, however, it is possible that the picks on the machine, that generate dust, will be replaced by high pressure (10,000 psi) water jets which cut into the coal perhaps a foot ahead of the machine, and allow mining without the generation of the dust and sparks that make current operations so dangerous. It has been done before, and is a relatively simple technology to adapt to future conditions.

Waterjet plow operating underground in Germany (from Gluckauf)

We called it the Hydrominer, and as I learn how, I’ll put up some video on Youtube of it operating. There are better ways of using the jets than those shown, so that the plow can actually mine coal to a web depth of over 3 ft 9 inches (we have) at shearer speeds.

Incidentally longwall mining was not a part of the Modern Marvels review of Coal Mining, though in Part 3 they do show the development of the continuous miner, and shuttle car. (H/t to Pasttense who gives a list of some Youtube films), though there are a number more than I had thought there would be.

* At least in those days they excavated a chamber ahead of the machine by blasting it out with explosive and removing the coal by hand. This allowed them space to pull off the head, remove the bolts, fix the machine, and put it back together again.