Monday, October 20, 2014

Waterjetting 26b - Jet positions to help bit cutting

The addition of a high-pressure waterjet to the leading edge of a sharp tool can make a considerable difference to the performance of that tool. I have discussed this a little in two earlier posts, the first of which was an introduction to the topic, and the second highlighted the problems of getting the nozzle close to the active contact zone so that it can be effective.

In this post I will discuss the benefits that this jets can create in the performance of the machine. The discussion is largely focused on rock excavations, since that is where most of the basic and applied research was developed, but, as I also mentioned previously, this benefit can also be gained if the jets are added to machine tools that are cutting into metal – even metals that are otherwise hard to machine.

The idea of pushing a sharp(ish) tool into rock to break it out goes back to the deer antler picks used to pry flints from chalk some thousands of years ago. But it worked, although the picks are now made of metal and powered by machines. The shapes have also changed over the years.

To make an effective cut requires two different sets of forces be applied to the picks. The first of these is the one that pushes the pick into the rock and gives it the depth of cut that is needed. (I’ll call this the Normal or Thrust Force, since it acts perpendicular to the surface being cut). The second is the force required to pull the tool along the face, this is often referred to as the Cutting or Drag force. Neither are very constant in rock (as opposed to metal cutting) since the rock will chip around the but as it moves forward, which frees and blocks the passage of the tool as it moves.

As I mentioned last time, pushing the tool into the rock will cause the rock under the tool to crush, and then re-compact, if the particles aren’t removed. Thus the most effective time to remove them comes as the tool first breaks them free from the solid. This also saves the energy that would otherwise go into not only further crushing, but also re-compacting the particles. Once they are re-compacted and compressed they become harder to remove and help increase the friction on the tool that cause it to heat, and weaken.

But if the particles are effectively removed, then the region under the bit is washed free, there is less confinement on the remaining rock, and it becomes easier to break.


Figure 1. Crushed rock under an indenting tool. (Richard Gertsch)


Figure 2. Crushed rock under an indenting tool, with the tool removed and a 10,000 psi jet fired at the contact point after removal. Note that there is still some crushed rock that was not removed.


Figure 3. Crushed rock removed during crushing by a jet pointed under the bit as it indented the rock (basalt) (Richard Gertsch)

The impact on the forces that the bit sees can be dramatic. In the early tests of drag bits in cutting the quartzite rock that holds the gold veins in South Africa, Dr. Michael Hood took a tool that normally stalled out under full load, when it was cutting into the rock to a depth of 4mm.


Figure 4. Normal forces on a bit (in KN) without jet assistance (black) where the machine stalls at 4 mm penetration, and with jets at different locations along the cutting face.

Mike tried a number of different locations for the jets at varying points over the face of the drag bit. Initially he used higher pressure merely as a way of getting enough water to the bit to keep it cool, but quickly saw that the performance was greatly improved. As Figure 4 shows, the normal force pushing the bit into the rock was considerably lowered, even when the depth of cut into the rock was increased almost three-fold, with the best location for the jets showing that the machine retained considerably capacity for cutting.

Similar results were obtained with improvement in the cutting forces seen in pulling the bit down the face.


Figure 5. Change in cutting forces with high pressure jet applications to a cutting tool in basalt (Mike Hood)

Again the machine stalled with a depth of cut of 4 mm, without waterjet assistance, and cut to more than 11 mm depth with power to spare with waterjets in the optimal location. This was found to be at the corners of the cutting tool, since in this location the jets were confined by the uncut rock on either side of the tool, and thus rebounded to cover the entire line of contact between the bit and the rock.


Figure 6. Optimal location for the jets on the drag bit for cutting South African Quartzite. (Mike Hood)

For the jet to work most effectively the water must continue to remove all the crushed material from under the bit as it is created. Where the rock is already fractured (as it may be because of natural ground fractures or high stresses on the face because of the depth at which mining takes place) then the confinement of the space around the tool can be less and this reduces the ability of the water to spread along the face of the tool and remove all the crushed rock as it is formed.

Others have also looked at the position of the jet relative to the cutting face, and sometimes, especially in harder rock, where the jet can intersect broken rock above the cutting tool, it may be better to bring the jet into the crushed zone from behind the bit.


Figure 7. Changing bit performance with change in jet pressure at three jet positions relative to the bit. (After Ropchan, Wang and Walgamott).

A slightly different experiment was tried by French investigators who tried locating waterjets around the carbide inserts of a drilling bit. Part of the problem with such bits is to ensure not only that the nozzle is close enough to the crushing zone as to remove the rock, but also to make sure that the nozzle is close enough to the surface that the jet retains enough power. In this particular case, by drilling a small hole through the carbide tool, the investigators were able to bring the two tip jets to the point that they needed, with enough power to be effective. This is shown by the ability to achieve a rate of penetration (ROP) which was more than double that of a conventional bit, with only conventional cooling, for the same amount of thrust force.


Figure 8. Change in rate of penetration with change in jet location on a drill bit.

I’ll return to this topic next time.

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Ropchan, D., Wang, F-D., Wolgamott, J., Application of Water Jet Assisted Drag Bit and Pick Cutter for the Cutting of Coal Measure Rocks, Final Technical Report on Department of Energy Contract ET-77-G-01-9082, Colorado School of Mines, April, 1980, DOE/FE/0982-1, 84 pages.

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Monday, October 13, 2014

Tech Talk - Pessimistic Talk in a time of surplus

The oil markets are concerned that there is too much oil currently available on the market, and that, as a consequence, oil prices may continue to tumble. Saudi Arabia is reportedly telling Reuters that it is happy with prices that may fall as low as $80 a barrel. As I mentioned the other day, some of this has to do with market share, and the KSA increasing production, and thereby seeking to weaken the likelihood of investment in other places, in turn ensuring their share holds up, not just now, but also down the road. The effect on gas prices has been rapid, with prices in parts of Missouri down to $2.65 a gallon – about a dollar less than I was paying only a week ago.

The effect will also have the benefit of a boost to the economy, which of course can’t hurt in the run-up to an election. But in the longer term it is hard to see how this boost can be sustained for more than a year. In the last post on this I mentioned that, outside of the US, Russia and KSA global oil production had dropped around 3 mbd over the past couple of years. Yet increased production (KSA raised production by 100 kbd in September as part of a total 400 kbd increase from OPEC overall) has, for now, been able to match and surpass this in order to meet the global demand. OPEC continues to expect that demand will increase by a million barrels a day this year and 1.19 mbd next. They further expect that the increased production to meet this will be met from outside the cartel, with the gain declining from 1.68 mbd this year, to 1.24 mbd next year, holding OPEC production to a decline of 300 kbd from the current 29.5 mbd. Simplistically the gains are maximized in increased production from the United States (880 kbd); Canada (250 kbd) and Brazil (190 kbd). They are anticipating a slight drop in Russian production, as part of an overall decline of 80 kbd for the FSU countries.

Part of the problem in projecting the balance revolves around estimating the production from Libya, Iraq and Iran (LII). Libya has reported raising production back to around 800 kbd, but some of that comes from the Shahara field, which was still involved in factional fighting, even as it came back on line at some 20% of normal. The three countries produce around 7 mbd (Iran 3 mbd, Iraq 3.2 mbd; Libya .8 mbd) so that the fluctuations in their production and sales can have a very significant impact on the global oil market, and the prices that are paid – but they function within OPEC, and it may be that the current drops in price are reminder that the big dog in that trailer is KSA, currently running at around 9.7 mbd.

It is foolish to try and predict, over the immediate short-term, how the fighting in Libya and Iraq will progress. Similarly it is hard to see how relations with Iran will change, potentially easing sanctions and allowing them to sell more product into the global market would upset the current balance in trade, and could, in the short-term, increase the glut and lower prices.

But supplies from those outside the cartel and the Americas are continuing to decline. That is not going to change. The rates may fluctuate a little (though the current drop in prices is not going to encourage large scale investment in declining fields) but the overall trend is steadily downward. And it is within that picture that potential changes in the production from the three LII countries have to be placed.


Figure 1. Libyan oil production through September 2013. (EIA)

Yet, as the fields have brought oil back to the market, there is a concurrent fall in global prices, as the EIA note.


Figure 2. Recent oil production from Libya and the price of Brent Crude (EIA)

Pre-conflict Libya was producing over 1.6 mbd, it recovered to 1.4 and is now struggling at around 0.8 mbd. But the prospects for the levels of peace required to sustain even that level do not seem promising. The conflict is worsening and seen as spiraling out of control.

Moving East to Iraq, despite the use of air power, the situation in the North is not improving, although the Kurds have now a pipeline to carry oil up into Turkey that is not controlled by the Islamic State. While it is still a matter of debate how much oil they will be able to sell, they hope that, by the end of next year they may be able to pump as much as 1 mbd, up from the initial 0.1 mbd when the pipeline went on line. At the same time, in the South, the oil fields lie some distance from the conflict, and there seems little threat, at the moment, to the plans to increase production, and move the majority of the oil to the coast for export. It is, therefore possible to foresee an increase in Iraqi production of perhaps a million barrels a day in the next couple of years. Is it likely? It is hard to say. Factional fighting is always hard to predict, and the willingness of those involved to use explosives makes it even more of a problem to predict what will occur, given the vulnerability of pipelines to attack.

Predicting how Iran will change is similarly conflicted, in that it is hard to predict the behavior of those who control the country, and in turn impact oil exports.

But putting this within the context of OPEC, I suspect that overall production will not fall much outside of the current volumes that the MOMR are predicting – which is sensibly overall stable output over the next year or so. And if that is the case, then I would, as mentioned last time, expect to see that the global surplus of oil supply over demand will gradually disappear over the next year, with the impact becoming evident once we reach the summer of 2016. It would be nice to be wrong, but I think it unlikely.

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Friday, October 10, 2014

Waterjetting Technology - 26a More on waterjet assisted cutting

When mankind first began cutting out flints to make the tools and weapons that helped make primitive life more successful they often used either bone antlers or stones from the river as the tool to cut into the chalk or other host rock that held the flint. For thousands of years as rock was excavated for broader use, including making building stone, the rock continued to be cut manually, and it has only really been in the last hundred and fifty years that manual picks have been replaced with power driven machines. However, in great part, the machines have had to be made larger and heavier than they might need to be because, in large part, unlike the pick swung by a miner, the machine cannot selectively attack the rock that it is facing, but must cut along a foreordained path.


Figure 1. Conventional tool path in cutting concrete, the tool has to cut through both the hard aggregate pieces as well as the softer cement.

The tools that cut through the rock mechanically must, therefore, be able to cut through all the different materials that they are likely to encounter. Where the rock is like a concrete, with hard and soft parts, then the tool must be able to cut through the hard (aggregate) as easily and fast as it removes the soft (cement) phase if the machine is to maintain productivity. When I wrote about cutting concrete, I pointed out that this “brute force and ignorance” approach to getting through material was expensive in the time that it took to make a hole, and in the energy that had to be expended, both combining to make the overall process itself more expensive than it need be. That cost includes not just the costs of the process itself, but is also less obvious in that the machine itself has not only to be bigger, but because it also typically sees a wide range in force applied through the cutters to the drive mechanism, it also has a shorter operational life because of these fluctuations. (One shearer model saw such failures within six months of start-up).

There are a number of different ways in which the sensible application of high-pressure waterjets can improve cutting performance, lower machine size and cost and provide a win-win situation. But there is a need for caution, since the small size of the waterjets that are often used is much below the scale of many other parts of the machine, and, as a result, the precision with which the jets need to be applied can often be neglected.

In an earlier post on this topic, I discussed how a mechanical tool will crush the rock over which it passes during cutting. This crushed rock confines the bit, and is often re-compacted so that frictional forces rise, and the temperatures can be high enough to soften tungsten carbide.


Figure 2. Crushed rock under the impact of a mechanical pick. The size of the indentation relative to the size of the crushed rock is evident.

If the jet is to be effective it has to be directed into the cut at the point where the crushed rock is being created, so that the jet can remove the broken pieces as they are being formed. It is this critical location of the jet relative to the bit:rock contact that is often missed by those who have tried to apply this technology in the years since Dr. Mike Hood first demonstrated the benefit.

A number of experiments over the years showed that if the jet is more than about a tenth-of-an-inch (2.5 mm) from the point of the pick where it enters the rock (or the edge of the tool if it is a broader shape) then the jet will not be able to reach and remove the crushed material. This is particularly true when the rock being cut is, as in the above figure, a basalt, which the jet of water cannot normally penetrate at pressures of 10,000 psi. thus, if the jet does not reach the crushed material then the energy put into its creation has been wasted.

There are two parts to that last statement. They deal with all three planes in which the jet lies, relative to the point of pick contact. There is the relative position at which the jet hits the rock, where it is critical that it hits just where the rock is being crushed, and then there is the distance of the nozzle from that contact point. The latter point is one that I have also written about in earlier posts, but which can also be neglected when engineers are designing systems. There are a number of papers (which seemed to be at a peak at the 8th International Waterjet Symposium in Durham, UK in 1986) where this distance was set incorrectly (values up to 0.3 inches and above were reported) and it is not, therefore, surprising that some investigators found that the results were not as good as expected.


Figure 3. Jet cutting at the front edge of a pick (Front cover of the 8th International Symposium on Jet Cutting Technology, BHRA, Durham, UK, Sept. 1986)

If the nozzle is too far from the rock contact, then the pressure of the jet will have fallen to a pressure that is too low to be effective. This has been a less obvious problem to overcome, since to many observers the jet seems coherent with distance, but, given that jet flow is often divided between a number of different nozzles on the cutting head, the individual orifice sizes can be quite small (perhaps 0.01 inches in diameter). If the effective jet throw distance is 125 diameters, then the range of the jet is 1.25 inches. Yet in a number of applications, because of difficulties in fitting the nozzle in place, the orifice can be placed more than 2.5 inches from the rock contact. Again the result of this is to make the jet sensibly ineffective.

The jet has to be put into the right place, and with the correct amount of power, if it is to be of any use. Sometimes that can mean that the nozzle is placed behind the pick (so that it can be protected by the pick from the rock, yet can be brought close enough to the crushed zone that it can penetrate it from behind. This is a little more difficult to achieve, since the precision of location is a little more difficult.

Others have tried feeding the jet down through the pick, and I will explain some of the benefits and problems with this as I continue on this theme next time.

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Monday, October 6, 2014

Tech Talk - The Price of Power, and its consequences

The changing colors of the leaves carry the message that winter will soon be here, and so it is time to stock the yard with wood to carry us through until spring. In Missouri I just found wood, cut to the length I need, and stacked, for $110 a cord and (since it has to be cut) it will arrive next week. Only the chimney then needs a quick sweep, and we’ll be ready for another season. (We burn just under a cord of wood a month, and this keeps the electricity bill sensible).

At one time the wood was insurance, in case of an extended power outage (and we had one that lasted three days, one winter) but we enjoy the heat from the tile stove, and so it is now part of our life. And with the continued risk of a loss of power, the insurance remains comforting.

Driving back from Maine a couple of weeks ago, gas prices fell over $0.25 a gallon along the 1,300 mile trip, another benefit of living in the Mid-West. But at both ends of the drive, the impact of fuel prices continues to slow economic growth, as it does nationally. Gail Tverberg has written of the inter-relation between the economy and fuel prices, most recently on Monday. However we disagree on one point, since she anticipates a potential significant drop in oil prices, which I do not.

In the early days of The Oil Drum I remember walking through the streets of Denver to a meeting with two other contributors, and suddenly realizing that I, the more technically based of the three, was by far the most pessimistic. Increasingly I am realizing that while this pessimism has not ameliorated, the current relative abundance of oil and gas in the United States has given many folk an undeservedly complacent view of the next few years.

Ron Patterson recently pointed out that if one discounts US production, the rest of the world has seen a decline in production, with non-US production now down around 2 mbd from its all-time peak. (If one also removes Saudi Arabian and Russian production from the mix, the decline gets closer to 3 mbd). Now to assume that this is totally due to a loss in production capacity would be a mistake. Saudi Arabia continues to adjust the volume of their production to try and keep global prices relatively stable, dropping production by 400 kbd in August. In the immediate short-term that was not enough for their purpose, and they are now lowering price a little, perhaps in order to sustain their market share. The cuts were in the range of $0.20 to $1.20 a barrel). Although it could also be a way of trying to sustain global growth at a time of weakness.


Figure 1. Global Production without including the United States – as plotted by Ron Patterson. I added the trend line at the end of the top plot.

These flutterings at the margin however don’t help my concerns, because they are focused only on the short-term, and don’t consider the overall situation. If the production from the rest of the world is declining at around 700 kbd, and Saudi Arabia will only produce to a maximum of 10 mbd, and Russia appears to be in that plateau that precedes decline, even without the loss in funding that recent US Government mandates will impose, then that leaves the growth in US production as being the only source to match both the decline in global production, and the continuing demand for more oil which together total around 1.7 mbd. And US production projections, even at their most optimistic can’t do this, even for one more year.


Figure 2. Projected Growth in US production (EIA)

The mathematics are, of course, not absolute numbers but remain somewhat flexible. There could be a sudden cessation of conflict in Libya and full production might return; all conflict might end in Iraq and production development might surge at the investment opportunity; and sanctions might disappear against Iran – but somehow I don’t see any of these happening.

The argument of the Cornucopians, that one can either find a substitute for the fuel in some other resource, or that technology will suddenly become available to allow unanticipated levels of production from the existing reserves and resources is, perhaps why I – knowing a fair bit about the technology – am more of a pessimist than many others.

The analogy that I use may be a little crude – but you can’t have a baby in a month by making nine women pregnant. You can’t create new technology out of thin air by suddenly investing a few billion in a bunch of scientists pulled from lists on the Internet either. There are not that many folk who are sufficiently expert to be useful, particularly in the fields that relate to the production of fossil fuels. Many of those who do exist are, like me, coming to the end of their professional lives, so that the skill sets and knowledge bases that they have built are disappearing. Many of the doctorates that we see today are based more on computer modeling than on hands-on experimentation and engineering. And unfortunately the knowledge that we have about the nature of the rocks at depth, their behavior and how to change the way in which they yield their fluids still leaves a lot to be desired, when it comes to validating the models that are produced.

But even if such new technology were developed it would take decades to see it adopted in sufficient volume across the world that it would have a significant impact on global fuel production. It was for this reason that, back in 2005, the Hirsh Report discussed the need for a twenty-year lead-time to develop new technology that could replace our needs for fuel. The time that they suggested that we had available now is beginning to seem very optimistic, while the moves to ameliorate the problem have been judged less critical and thus no longer receive the attention and funding that the have in the past.

And so, when the crisis comes, and this is increasingly likely to come in the next two years, there will be no good answers, just tightening supplies and rising prices. This is perhaps why I am beginning to think that the next President of the United States still may well be, despite all the gaffs, Brian Schweitzer.

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Tuesday, September 30, 2014

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.

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Sunday, September 28, 2014

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.

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Friday, September 19, 2014

Waterjetting 25b - range with abrasive

In the last post I wrote about the impact of smaller jet diameters, and higher pressures, in truncating the range over which a waterjet is effective. The same is true, to an extent, when one adds abrasive to the water.

Our “green tube” test has been described in earlier posts, where the distance over which particles settle out of the jet provide a measure of how much energy they were given. However it is not that simple to interpret the results from these tests. The reason is that, as with a plain waterjet, the range of the particles is controlled to a degree by the size of the individual grain. Why is this? Well this series tries to keep formulae to a minimum, but one is needed in the answer to that question.

The origin comes from our friend Newton, whose Laws have come down to us over the centuries, and the second of which states:

Force = mass x acceleration

Consider that when an initially stationary particle is sitting in a jet stream, the force being applied to it by that jet is equal to the pressure of the jet, multiplied by the area over which the pressure is applied. If for simplicity we assume that the particle is spherical, then the area over which the pressure is applied (assuming that the particle is centralized within the jet stream) is equal to the pressure multiplied by the cross-sectional area, which is given by the product of the square of the radius multiplied by pi.

On the other hand the mass of the particle is related to the volume, which is in a cubic relationship with the radius. Thus if these two terms are substituted in the equation above, and combining all the non-radial terms into a constant results in an equation where:

Acceleration x radius cubed x constant = pressure x radius squared x constant

Rewriting this gives:

Acceleration = (1/radius) x pressure x constant.

This means that larger particles have smaller accelerations for a given pressure, while smaller ones accelerate faster.

However, and this pertains to the results we saw from the green tube tests, just as the smaller particles are accelerated faster while in the jet stream as it passes through the nozzle assembly, so those smaller particles will decelerate faster when having to travel through the relatively stationary air outside of the nozzle.

Thus if you are, for example, using a smaller jet flow (and smaller jet orifice in consequence) then the normal practice is also to reduce the size of the focusing tube, and – to otherwise keep the system practical – to also reduce the size of the abrasive particles fed into the system.

However, while this gives a better cutting effect immediately under the nozzle (hence the widespread recommendation to restrict the standoff distance between the nozzle and the target to about a quarter-of-an-inch) there is a more rapid decline in the speed of the particles as they move away from the nozzle. The net result is a shorter range for the jet, and a shorter cutting depth in consequence.

There is a small caveat to holding this as an absolute conclusion. Back in the days of the U.S. Bureau of Mines Dr. George Savanick showed that where an abrasive jet could be held within a relatively narrow slot, as it cut down, that the walls of the slot tended to concentrate the jet, and thus extend its range beyond that achieved if the jet were, for the sake of example, just cutting through a piece. Thus, when not through-cutting the part, there will be some extension of the jet range, and this has to be considered when setting the operating parameters for a particular job.

Which brings us back to defining the optimum size of the operating plant required to complete a given job, and a resolution of the optimum parameters for carrying out the job.

As I have noted before, this is not a simple and straightforward choice. Proponents of different operating systems will advocate different solutions based on the units that they are most familiar with. And there are arguments that can be made for different choices. However, in making the choice of the best system to use, one must be aware of the limitations (as well as the benefits) of the different choices that might be available.

Consider that a lower pressure, higher flow rate system might use larger particles, and thus be able to cut through a target plate of a given thickness with better speed than a higher-pressure, lower flow rate alternative. However were the target to be of a thinner stock where the range of the jet is not that critical, then the higher pressure system may well give the better performance (given, inter alia, that it will also use less abrasive and water).

Making a selection as to the better operating system, therefore, requires a clear understanding of the different modes in which the system is likely to be used. Will it be for relatively thin materials, where high precision and narrow cuts are required, but the material need not necessarily be through-cut. Or is the system one where a cut may be required through perhaps 30-inches of reinforced concrete in a reactor (of which more in a later post). In the latter case the lower pressure, higher flow rate jet, with the ability to use larger particles and sustain their velocity further, when further confined by the walls of the cut. The former condition would argue for the use of a higher-pressure, lower-flowrate combination, while the latter (as a generalized statement) would incline more to the lower pressure alternative. (And the terms are relative, since in the latter case we are likely still talking about pressures of around 30,000 psi or higher to achieve the depths of cut within the reinforced concrete.)

Much is written about having to make absolute choices in cutting, but in many cases it is only a matter of relative performance, with systems across a range of parameters being able to effectively achieve the goal. The selection of which system to use should focus more on the normal range of materials that one is expected to cut in the normal course of operations. (And slicing though parts of a nuclear reactor is not normal in most aspects of this business).

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