Monday, January 26, 2015

Waterjetting 29d - Fixing an Oops.

There was a story that is told in Mining Engineering classes about a tunnel that collapsed, even after there had been a whole series of tests carried out to make sure that the rock was strong enough before the tunnel excavation was started. In working out why the tunnel had collapsed some questions were raised about the tests made on the rock samples. It turned out that the testing technicians had received the samples and struggled to find good enough quality pieces of rock in the sample from which they could extract the required sample sizes to run the standard strength tests.

When they reported the results of their tests these predicted that the rock would be strong enough to stand, without collapse, for a long enough time for artificial supports to be placed under the rock and to hold it in place. But it was not that strong segment of the rock that failed, rather it was the rather more rotten rock that surrounded it which provided the weakest link in the tunnel wall. That material had been too weak to make into a sample, and the technician had therefore not reported the lack of strength.

Knowing the properties of a target material before starting a job is an important part of correctly forecasting how how long it will take to perform the cutting tasks required and, as a result, how much to charge for the work. And further to ensure that there is no unanticipated cost that will come from the use of the waterjet tool at the parameters planned.

These unintended consequences have, for example arisen in the past when a high-pressure waterjet system was being used to remove damaged concrete from the surface of a bridge. (As with the tunnel we’ll keep the bridge as an unidentified example).

In repairing a bridge deck it is usually required that the top layer of concrete be removed just past the top layer of reinforcing steel (rebar). This allows a good bond between the previous concrete and the repair pour, which also bonds to the rebar giving a repair that will last for some time. (More conventional repairs leave a weakened joint between the repair and the old concrete which fails more rapidly in many cases).

However the waterjet system is only discriminatory to some extent. The jet pressure can be set so that it will only remove damaged concrete, for example, but does not have sufficient pressure to remove healthy (and less cracked) material. But if the material is weaker than expected, or the damage extends further into the deck than was expected, then the waterjet system will continue to remove damaged concrete, even if this means it ends up removing material all the way through the deck. This can be a real problem, given the extra money and time that must now be spent in replacing that additional concrete, and ensuring that full integrity is restored to the deck. This additional cost can be more than the price of the original repair work, and do serious damage to the economic health of the waterjet company. Unfortunately with many of the systems today becoming more and more automated, it requires close attention the machine at all times to ensure that only the required amount of material is removed and no more.

One solution to the problem is, at least initially, the very opposite of what you might think would be the best answer. It is to increase the pressure of the jets removing the material. For a system with the same horsepower as the machine that was being used first, this means that the amount of water used will be less, and the nozzle diameters will, as a result, also shrink in size.

The smaller jet diameters and higher pressures mean that the cutting distance of the jets themselves will become shorter, as the jet decays more rapidly with distance. (To give an extreme example a 1200 psi jet at a diameter of about an inch-and-a-half can throw a jet about 125 ft. At 50,000 psi and at a diameter of about 0.005 inches the range of the jet is usually less than 2 inches*.) Within their effective range, the higher pressure jets will cut much faster, and so it is possible, by mounting the cutting nozzles in an array that spins around a common axis, to rapidly clean a swath of material (say up to 2 ft in width) as the head moves across a traffic lane at an advance rate of roughly 1 pass a minute as it moves up over the bridge. The higher rotational speeds will also restrict the depth to which the jets can cut on a single pass, so that the depth of material removed can be relatively accurately programmed into the machine by adjusting some of the operating controls. (After first finding out what the best parameters will be for THAT bridge concrete in a small test area off the main work site).

There is an additional advantage to using the higher pressures, and that comes with the smaller volumes of water that will be required to take the damaged layer of concrete from the surface. This water will be contaminated by the different fluids that may have soaked into the bridge over time, and by the corrosion products of the deterioration. For these reasons all the debris and water from the demolition operation will have to be collected, removed and properly (and expensively) disposed of. The higher the volume of water then the greater the collection and disposal cost, and the lower water volumes needed with higher pressures will thus carry a lower disposal price.

The example given here is for the removal of damaged concrete from bridge decks and garage floors, but the underlying principle also applies in the milling of pockets into materials of differing composition, where a controlled depth of cut needs to be held, even if the material strength changes.

*The word “usually” is used since there are ways of increasing the jet throw to several thousand orifice diameters.

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Wednesday, January 21, 2015

Waterjetting 29c - If at first you don't succeed

In my last post on this topic I wrote about two of the most influential papers that were published in the Proceedings of the First International Symposium on Jet Cutting Technology (hereafter ISJCT) back in 1972. Yet there was one more paper that I remembered over the years. And it has been for an entirely different reason.

The paper was “Application of Water Jet Cutting Technology to Cement Grouts and Concrete,” by L. McCurrich and B. Browne of Taylor Woodrow in the UK. The company had looked at the use of waterjets as a means for cutting concrete, either for demolition or inserting the grooves used, for example on highways for “rumble” strips. They concluded that such a tool would have to operate at a pressure of 54,000 psi, and using 3 jets in a cutting head, would require a power demand of around 550 hp. To quote from the paper:
This scale of research to develop a practicable cutting tool would be at least one hundred thousand English pounds. (Then around $250,000). No single firm involved in demolition is likely to be able to afford this sum on a speculative development of this nature, and if a commercially viable proposition can be shown to be likely, funds will have to come from a central Government or Trade Association body.
Jake Frank and I visited the company in London after the conference was over, and the authors were explicit in their views that a waterjet tool would be too expensive for any individual company to purchase for use in concrete work. Skip forward a few years and I was at the Liquid Waste Haulers Show in Nashville TN. (This became the Pumper & Cleaner Environmental Expo International and this year is, I gather, the Water & Wastewater Equipment, Treatment & Transport Show and will be in Indianapolis next month). A friend of mine was chatting with me on his booth, when a salesman come over. He suggested I join him while we wandered over to the sales table where the customer happily signed an order for a $250,000 unit that would leave the show and be used for the hydro-demolition of concrete.

Times had certainly changed in the intervening years, and I rather suspect that the estimate that the two authors had given for the project development costs were exceeded as both the Gas Research Institute (now the Gas Technology Institute) and the Electric Power Research Institute, as well as the National Science Foundation, helped to develop technologies, both through funding research and in encouraging companies to develop the needed tools for the industry that has since grown to use it. Not that the research effort was limited to the United States.

By the time of the 2nd ISJCT in Cambridge, two years after that first paper, there were three papers dealing with studies on concrete cutting. These included studies carried out in the United States, Japan, and Canada. There were two drivers to this growth in effort, despite the pessimism of the original paper, the first being the size of the market. McCurrich and Browne had pointed out that back in 1970 the UK was emplacing about 1 ton of cement for each inhabitant of the nation, much of which would later have to be demolished or repaired. One of the other drivers was that, in contrast to rock and other materials that were being used as targets, cement properties can, to a degree, be controlled by the manufacturer so that the effect of changing concrete properties on the cutting performance could also be established.

The work was, however, constrained to the laboratory for these studies at that time and focused on jet slotting of the concrete at pressures ranging up to 60,000 psi.

There was only one paper on concrete cutting at the Third ISJCT, and that dealt with cutting underwater, which at pressures of 60,000 psi jet pressure and shallow depth appeared to be little different to cutting in air, where the nozzle was held close to the target surface.

The fourth ISJCT was held in Canterbury, UK and marked a change in emphasis for the research on concrete removal. The teams reporting differed from those of the earlier papers, and now included funding from NSF. The emphasis for the three papers was also more focused on concrete demolition, using pulsed waterjet systems in two cases and on a portable system for removing concrete and asphalt for utility repair in the third.

The idea of using a pulsed jet to shatter concrete due to the impact of the jet on the surface, the rapid generation and penetration of cracks from that impact, and the consequent rupture of a block into pieces had a number of advantages. Tools could be built with relatively simple charging mechanisms (the simplest of which – that came later from Germany – used a small cartridge similar to a shotgun shell to generate the pressure) and without the noise and dust generation of impact breakers. Unfortunately, as these tools were developed over the subsequent years, a consistent problem arose for the devices being developed. This was that the pulse that generated the damage had to be repeated relatively rapidly if it were to be able to match the performance of the impact breaker. This required that the pressure chamber holding the water had to be rapidly refilled, and this in turn required a valve between the water supply and that chamber. The valve then had to withstand the repeated high-pressure cycles each time that the device fired. This turned out to be a bigger problem than had been anticipated, and there were several efforts to develop the pulsed waterjet concrete breaker that foundered because of the complexity of the problem.

It was only at the 5th Conference, held in Hanover in Germany, that the first paper appeared noting the benefits of removing damaged concrete. Concurrently the paper that discussed this also described field trials carried out in Chicago, demonstrating that the waterjetting method was able to remove damaged surface concrete preferentially, and to a controlled depth at a rate more than twice that of existing jack hammers, while using roughly the same amount of power. It had taken ten years to reach this point, which presaged the development of the hydro-demolition industry, although it took several more years and the interest of larger companies before the technology finally took off.

Unfortunately the work on pulsed-jet concrete demolition which was still ongoing at the 5th ISJCT did not lead to a commercial product, for the reasons cited above, while concrete trenching and more detailed contour cutting, although developed by the this conference into a field portable device, also was later subsumed into the overall development of hydro-demolition.

These developments took much more money that the original authors had foreseen, but the final devices put into the field ran at lower pressures and required less power than those original experiments had anticipated. It also took a number of years for the capabilities of the technical equipment to reach to capabilities needed to field the tools that are now ubiquitous.

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Sunday, January 11, 2015

Waterjetting Technology - Dr. Andrej Lichtarowicz

I was saddened, this week, to hear of the passing of Dr. Andrej Lichtarowicz, who died on the the 6th of this month. As Mark Fairhurst noted, Dr. Lichtarowicz, on the faculty at Nottingham University, was a sterling early contributor to the waterjetting community, from its birth back in the early 1970’s. He gave a paper at the first BHRA conference in 1972, and serving as the editor of the 11th Proceedings in 1992.

There were 201 delegates to that first meeting, which was held at the University of Warwick, in Coventry, UK. with 37 papers being given over the course of two and a half days. The use of high pressure waterjet technology was very new at the time, and this was the first time I was able to get together with peer scientists from around the world to discuss what they were doing as well as make a small contribution of our own. But the papers that influenced our lab the most were the two given on cavitation. (One was given by Dr. Andrew Conn and the other by Dr. Lichtarowicz). The reason for this interest was that back in those days the pressures available from high-pressure pumps were restricted to about 30,000 psi, and (without abrasives which only showed up about eight years later) this significantly limited what materials could be cut.

Dr. Conn’s paper related to cavitation at lower pressures and higher volume flow rates, while Dr. Lichtarowicz’ paper covered smaller jet testing at pressures of up to 10,000 psi. Using such a jet he had been able to drill holes in aluminum, which he could not do when the jet was not cavitating. The results were sufficient that we shortly thereafter tried to repeat , and were able to exceed, these results, drilling a hole in a small piece of alumina in less that a minute, although at a pressure of around 18,000 psi.

This led to considerable discussion at the 2nd conference, which was held in Cambridge in 1974, as to whether the results that were being reported were because the jet was breaking up into droplets, or if the result was true cavitation. It was a discussion that Dr. Lichtarowicz, as always, took a significant part in, and although he could not make the third conference (which was in Chicago) by the time of the fourth, in Canterbury in 1978, he was carrying out his research with the nozzle and target submerged with enhanced results from the earlier work.

Over time he developed a small cell, with windows so that the action of the jet could be seen.

Figure 1. Initial design of the Lichtarowicz Cell

The small size of the unit, and the relative simplicity of construction, meant that a number of us, around the world, built such units and used them to help develop a better understanding of what was happening, and how damage could be increased.

One of the early discoveries he made was that, by adjusting the pressure in the chamber, the amount of overall damage (measured by mass loss) could be significantly intensified, and the rate of erosion increased. It was on that basis that we, among others, were able to use cavitating jets to disaggregate rock and coal into fine particles.

Figure 2. View through the port of a Lichtarowicz cell, showing the cavitating jet impacting a metal target.

One of the major uses of the cell was, however, not as a tool to develop faster ways of drilling rock (though it did) but instead to accelerate the rate at which the cavitation resistance of different materials could be determined. Until that time the standard tool for determining cavitation resistance had been the vibrating horn device recommended by ASTM. The problem with this was that it took hours (typically about 24) to generate the data and plot the rate of material removal, because it was so slow. With the cell a similar result could be obtained in minutes. His work led to the development of an ASTM standard first adopted in 1995, and reapproved in 2001 and 2006, with current interest in revision. It went on to be incorporated as part of the International Cavitation Erosion Test.

And so the technology moved forward, Dr. Lichtarowicz gave his last BHR paper at the 12th Conference in Rouen in 1994, and this was a review of some of his earlier work, showing its relevance as industry sought to find cleaner, greener methods for cleaning and material removal. His fundamental work, however, fostered studies that continue to live on, particularly in Japan, where laboratories continue to develop the techniques and ideas that he pioneered over the years. Certainly our own work would not have progressed as far, or in as many directions, without the inspiration of his work, and the many discussions on the technology we held over the years.

He was a good friend, not only to young faculty – as I was when we first met – but to the industry as a whole, and the students that he taught over the years. He was a much respected scientist and colleague and the tools that he developed and helped us learn to use will continue.

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Monday, January 5, 2015

Waterjetting 29a - Looking Backward and Forward

The great advantage in teaching the same academic course each year for a relatively long period of time was that, after a while, it became easier to look at the contents of each individual lecture, and then adjust the material to fit the changes in what we knew since the lecture had last been given. In many fields of study the basic knowledge has been around for many years, and so there is not that much change in the content each year, rather one tries to find better ways of getting the information from one side of the lectern to the other.

Over the period that I was an active academic this latter case was never really true for the class I taught in Waterjet Technology and Use. Each year, as I prepared the material some of the information in almost every lecture had changed or been added to, and so the material was largely redone. This was particularly true when students wanted to take the course through distance learning – where the lectures were pre-taped ahead of time, and then sent out on disc, or downloaded from the campus servers.

The changed information came from a number of places, some came from research that was being carried out in our laboratories, or from colleagues we were working with. But the majority of the new material came from conferences that were run every year, the papers given there and also those articles and papers that appeared in the technical journals and magazines. And, not to be completely neglected, it was often possible to get quite a bit of new information from just wandering around the trade shows and chatting to those in the booths and their visitors. Technical papers themselves only tell part of the story. The information has typically to be compressed into a paper that has to be presented in about 20 minutes, and must fit within a very restricted page limit (typically 10) in the conference proceedings. Getting to ask questions in the presentation, and then chatting with the folks that did the work afterwards (particularly in a more convivial place) meant that a much better idea could be grasped as to the real meaning and usefulness of the work.

The results of those meetings would often only change one or two slides in a class presentation that typically contained about 40, but the class itself could, as with this site, only cover topics with relatively broad brush strokes, because only one or two of those in the class (or current readers) might, for example, be interested in the use of waterjets in spinal surgery. Time for greater discussion was/is therefore limited. Further the information in the papers was of only limited availability to students since in many cases my library held the only copy of the information within a couple of hundred miles or more.

However the great advantage that is increasingly here today is that different organizations are not only putting their current conference proceedings on the Internet, but are also going back and publishing the earlier conference proceedings through the same channels. For example the entire set of proceedings of the biennial U.S. Waterjet Conferences are available for download. The older ones can be found on the WJTA site while the last three (since this year’s conference is not until November) can be found at the WJTA-IMCA site.

This means that, for example, those of us who may, back in 2007, not been that interested in the removal of rust and the use of high-pressure waterjets to clean surfaces for recoating, can now go back and read the excellent review and discussion paper that Dr. Lydia Frenzel gave back then. This then provides a background for the paper that she gave on "How to Inspect for Flash Rust" in 2009, as well as the description of a Chinese technique for rust removal given in the same venue. This was followed by an update in 2011 and then an explanation in 2013 as to why it has been so difficult to write standards for the use of waterjetting in the coatings removal and surface preparation industry.

This relatively simple illustration tries to show how, with access through the web, it is now easier to track developments of technology, and through the insights provided in those papers, to understand some of the underlying reasons why technology has evolved the way that it has.

The older conferences on the technology are those run, also on a two-year schedule, by the BHR Group with meetings being held mainly, although not exclusively, in Europe. These alternate with the American Meetings, so that the last BHR Conference was held in September 2014 in Haarlem in the Netherlands. These proceedings, although available on CD for over a decade, are not available on the web, and some of the earlier proceedings have gone out of print. Papers are thus somewhat more difficult to find, although I do retain copies of all of these up to 2010.

It is possible, in some cases, to find those earlier papers, where the authors are known. Unfortunately many of the earlier papers, which provided more of the basic reasons why certain steps were taken (and some approaches that did not work) are not that simple to find where the authors (who often worked on this as part of a program of graduate study) moved on into other fields.

I make mention of this in part to encourage folk to attend these professional technical meetings, since it provides one of the few places where one can meet other folk working in the field, and share experiences and knowledge. But also to indicate that there will be a slight change in emphasis in some of the posts from now on, to recognize that there is now a way that one can trace (as I tried to show with the very short example of Dr. Franzel’s work) how some aspects of the technology evolved and why. Thus some of the posts that will follow, over the course of this year, will try and link sequential papers showing how some of the ideas in the technology have evolved over the years.

And may I wish everyone a Happy, Prosperous and Successful Year!

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Wednesday, December 31, 2014

Tech Talk - Projections 2

It is the end of another year, or more optimistically the start of a new one. Last year I was tempted to make a couple of predictions for the future. And while I can make the case that they were not too wrong, they did not include the drop in oil prices, which has now taken the price of our local gas to below $1.85 a gallon. China has, in recent months, seemed less belligerent about claiming large sections of the China Seas. Whether this has anything to do with the relative success of rigs that have drilled in those waters is something that still remains an unknown.

But it is the changing price of gasoline, itself reflective of the drop in oil prices that is the big news. WTI closed at $53.56 today, and Brent at $57.50 a barrel. Predictions include some who would suggest that the price will continue to fall, until it reaches $20 a barrel, and there it may stay for some time. Well it certainly grabs a headline, but that is about all the value that particular forecast contains. The futures prices suggest that the price has yet to bottom out, though it may be getting close to that value.

Figure 1. Crude oil futures prices (EIA TWIP)

None of the recent news suggests that there will be a further increase in supply to sustain the current imbalance between available supply and demand. Libya is descending even further into a mess, with the oil facilities at the port of Es Sider now being destroyed. The likelihood of significant increases in production and the return to export levels achieved earlier this summer seems increasingly nonexistent. Neither Russia nor Saudi Arabia are likely to increase production, although the latter are continuing to produce the increased volume that they originally put on the market to replace Libyan losses. And so this leaves Iraq and the United States as the key producers who can significantly change the current supply:demand balance in any significant way.

It is probable that, with the agreement between the Kurds and the Central Government now having generated a second payment of $500 million to the KRG that the agreement may be sustained and grow. At present the Kurds are to supply about 550 kbd, of which 300 kbd will travel through the new pipeline to Turkey and thence onto the world market. The rest will be supplied to Baghdad. Meanwhile production in the south (which gets exported through Basra) has seen some increase.

Whether the Kurdish production can increase to over 1 mbd by the end of next year remains open to some doubt, given the ongoing conflict, and the target 6 mbd by the end of the decade for the entire country will likely require changes that the current conflict, which shows no signs of ending, will inhibit.

One of my responses, when the drop in price first started, was to note that the oil supply system has a certain inertia to it. And here I am not talking about the fluctuations in price that one sees in the stock market, and in the price of the crude, but rather in the time that it takes to stop current drilling, postpone future plans and to reduce the production from existing and new developments.

Thus the drop in investment in new production, whether in Russia, Iraq or the United States takes some time to have an impact. Unfortunately for those expecting the price to continue to fall, in the face of the overabundant supply, the situation has changed since historic times, where well production was relatively stable and the oversupply situation was corrected by shutting in production (mainly by Saudi Arabia). Even then it was the perception of the response that drove price rebounds, rather than the immediate reality of the changes.

The system this time is different. The increase in production in the United States has been sustained, and over the last two years has produced more than 2 mbd more than at the start of that period.

Figure 2. US crude oil production over the past two years. (EIA TWIP)

The rig count in North Dakota has already fallen to 170 rigs compared with 187 at this time last year. Concern about the oil price has led companies to cut their investment plans for next years, in some case by 20% so that the rig count is likely to continue to fall. And with the short life at high production values for most wells that will soon affect production. The North Dakota Oil and Gas Division of DMR shows the consequences of this:

Figure 3. Future production estimates from the ND DMR Oil and Gas Division.

The blue line requires about 225 rigs in continuous action, so that won’t happen. By the same token the black line is with no more drilling, and that won’t happen either. The result will be somewhere in between, probably moving the peak out beyond the current projection, but also lowering it as the existing baseline drops with less wells significantly contributing. (Bear in mind it is taking 11,892 wells to sustain current production levels.) But in the short term the line will likely dip down until the price rebounds.

The question now becomes how soon that drop in US production will become evident, and have some impact. I doubt that it will be before June of 2015.

On which note may I wish all readers a Happy, Healthy, Successful and Prosperous 2015.

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Tuesday, December 30, 2014

Waterjetting 28c - using steel as an abrasive

One of the considerable disadvantages in using garnet and other similar minerals as a cutting agent in abrasive waterjet cutting is that the particles fragment during the mixing process, and when they hit the target. As a result (as noted in the last post) less than 50% and often much less than that can be usefully recycled. The distinction in adding the word usefully relates to the need to remove the finer particles from the recycled stock since that does not cut very well.

But what if we used an abrasive that is not degraded in the mixing process, and further one that can be more easily separated from the cuttings and spent water? The candidate is steel, which can be formed into small particles that do not degrade in size as they move through the mixing chamber, and generally hold shape even after they have hit the target. Steel also has the advantage that it can be magnetically removed from the jet stream as the flow is collected, and with no significant degradation in size it can then be readily recycled. In cases where we have monitored the recyclability of steel shot, we were able to re-use it more than fourteen times without seeing any degradation in performance. Re-using it this many times more than offsets the increased price of the original material, and will, in a short time, also pay for the relatively low costs of a magnetic separator.

Unfortunately it is not quite that simple a choice. There are a number of other considerations, which must be addressed to make the system work effectively, some of which may make the process too expensive. Three of the areas that need to be addressed form the subject of this post.

The first comes about as a result of the shape of the particles, and their retained mass and velocity on leaving the focusing tube. More than most other abrasives steel retains some elasticity during the cutting to the point that where the cutting and rebounding streams are not carefully confined, the particles can escape upwards into the cutting room. Once in the air they move at high speed, and bounce around the room, so that they can reach unanticipated places and can also be a hazard to folk doing the work.

Figure 1. Slot depths cut into granite by steel shot (left) and garnet (right)

The second problem relates to the cutting effectiveness. When cutting a brittle material the steel shot has a number of advantages, since the energy on impact is focused in the very small volume of the sphere in contact with the target. This improves the ability of the shot to generate and grow cracks in the impact zone and thereby improves the performance of the cut, over that of the mineral abrasives.

Figure 2. Relative performance of steel over garnet and sand in cutting dolomite, under otherwise similar conditions

However, when cutting ductile materials, such as metal, steel shot is not a good tool, since the focusing of the force means that the shot may get buried or just rebound from the target, without the tearing and plowing action that comes with the use of a more regular abrasive. One way to overcome the problem is to switch from a steel shot to steel grit, which is also available. The relative benefit can be illustrated by using the change from using glass spheres to using them after they have been broken into sharp fragments.

Figure 3. Effect of change in particle shape when using glass particles in cutting ductile composite material (after Faber and Oweinah).

This, by itself may not be a complete answer, since the process of making the grit makes it a little more vulnerable to abrasion and wear during the cutting process, but we have seen that it is possible to recycle most of the abrasive a number of times. However, because of the change in shape, it becomes a little more difficult to feed the abrasive into the cutting stream, and there have been occasions where the grit has bound up in the feed tube. This has, therefore, to be sized and the flow path designed, to ensure that this doesn’t happen.

Figure 4. Cuts made into tool steel using steel shot (left) and garnet (right)

The other change is to use a harder steel than normal. And here please note that there is a difference between the hardness of the steel and its toughness. As American Cutting Edge notes:
Hardness vs. Toughness: Generally as hardness increases, toughness decreases. Toughness is desirable when blades are heavily impacted, hardness when a blade is exposed to corrosive or abrasive materials.

Hardness is related to the amount of carbon in steel. Often the lower the carbon, the higher the toughness. Also, some steels do not perform at lower hardness as they were designed for use at higher hardness. . . . . . . . Hardness is a characteristic of a solid material expressing its resistance to permanent deformation. The Rockwell or Vickers hardness scales are most commonly used in the industrial blade industry.

Toughness on the other hand is the maximum amount of energy a material can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.
In general where the grit is being used to cut into other metals (which can include steels) the hardness of the cutting abrasive should be considerably higher than that of the target material.

Which comes to the third consideration, which is that steel abrasive can rust, and therefore, immediately after it has been recovered and washed, it should be effectively dried. This has proved to be more difficult to manage than originally anticipated, since, particularly where the particles are then stored for some time before re-use, any moisture present can create enough rust to “glue” the particles together. Which renders them effectively useless for further recycling and additional use.

So there are considerable pitfalls that can arise in making use of steel as a cutting abrasive, but where the jobs exist where it does effectively cut significantly better than the alternative (say in rock-cutting applications) and where the cutting zone can be shielded, and the particles rapidly recovered, dried and stored for relatively rapid recycling at an economic price, then it can be a productive way of reducing cost, while improving throughput. (And lest you think this is a new idea Gulf Oil did extensive work on abrasive jet drilling of oilwells starting in the mid-sixties, with some favorable results, but that is another story).

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Monday, December 22, 2014

Waterjetting 28b - More on abrasive use

The costs of running a high pressure waterjet table divide into two parts, one that covers the basic costs of the system, whether it is running or not, such as building rent, while the second covers those costs that are a part of the actual work. Of the latter costs it is the cost of the abrasive that is often the most significant. This comes about in two ways, since the abrasive must first be purchased for use, and then, after it has been used it must be disposed of. Depending on the materials that were cut, this disposal cost can be significantly higher than the original purchase price. In some work carried out at the High Pressure Waterjet Lab (HPWL) at Missouri University of Science and Technology (MST) in the past we have seen disposal costs that were more than three times the cost of the original abrasive. And one should bear in mind that, as a research lab, the table was used much less than a comparable conventional table in an industrial cutting environment. But we also did not have a cutting operational budget, and so the cost of abrasive was something that we examined, to see if it could be reduced.

The first idea was that we would just recycle the abrasive. The particles of the target materials that were cut are generally much smaller than the abrasive particles themselves, and so it should be relatively easy to remove them from the mix. However, as we looked into the process in more detail, it was clear that it would not be quite as simple as it might, at first, appear. Marian Mazurkiewicz (retired) and Greg Galecki (who now runs the HPWL) carried out studies on the behavior of the particles as they moved through the mixing chamber and were accelerated down onto the target material. They found, as noted in an earlier post, that most of the abrasive was crushed to a smaller size when it passed through the cutting head, and a mix that started out with a particle size of 210 microns as it was fed into the system, was leaving the focusing tube with an average size of 140 microns.

Figure 1. Percentage of abrasive at different sizes after it has passed through a mixing chamber (and before it has hit the target). (After Galecki).

The reason that this is a concern is that, as the particles become smaller, so a point is reached where, depending on the target material, the abrasive no longer has sufficient energy to effectively cut into the target. When cutting into metals such as titanium and steel, our targets of choice in the study, this cut-off grade was at around 100 microns.

Figure 2. Effect of particle size on the cutting performance of an abrasive jet in cutting steel. (The tests were part of a factorial experiment and are thus averages over a number of different test runs at differing abrasive feed rates (AFR and pressures).

Roughly 25% of the mix in the example shown in figure 1 lies below 100 micron at it leaves the chamber. After impacting the target this value increases to more than 50%. Obviously recycling this fine material and re-using it in the cutting process is going to be less effective than removing it from the mix. Generally alluvial garnets will break up more rapidly than mined garnet, because of the structure of the abrasive particles, and thus the percentage that leave the focusing tube at the larger and more effective diameters are lower with the alluvial mix. The results were, we found, confirmed in the cutting results, with alluvial garnet producing a generally shallower depth of cut that would be achieved, other things being equal, in the cutting tests.

A quick word of explanation of the tests we ran, which are described in more detail here. The tests are run at a standard pressure and nozzle size, and at a constant traverse rate, with the depth that the jet cuts into a standard steel at a fixed speed measured over a 4-inch traverse length.

The results of the tests showed that, because of the particle crushing during the cutting process, the abrasive would have to be screened, and for most effective re-use only the larger fraction (on average less than 40%) should be recycled. The rest would be too fine for effective re-use in the operations we were developing. (Although finer abrasive has use in other applications, it would have to be screened and stored). It was interesting to note, and perhaps logical in retrospect, that once the particles had been used once and the larger ones separated out, then the percentage that survived and could be reused a second and third time increased significantly. This is mainly because those particles that had some form of weakness crack (either from weathering or from the mining process) were broken during the first impact, and the particles that survived did not have these cracks, and would therefore inherently be more prone to survive multiple times.

For our purpose, therefore, given that there was a high cost in purchasing the abrasive, and an even higher one in disposing of the contaminated material after cutting (because of the contamination by the target material) there was a potential economic advantage in recycling the abrasive. There were several ways in which the particles can be separated, but a simple screening process, if carried out properly, is quite time consuming, since the particles are required to “sit” on a vibrating screen for several minutes to ensure accurate separation, and this can be labor intensive if it is carried out as a batch process. We tried a number of different ways, including using a counter-flow fluid column that worked well for low feed rates, but the most efficient unit for one operation (we build virtually all of ours, and extensively modified them over time) may not be the best in other cases. (The one that survived the longest was a Wilfey table (though not this one).

In conventional AWJ cutting the abrasive has also to be dried before it can be re-used, and that can also add power and labor costs to the process. Thus, as with many choices that must be made when developing an efficient cutting operation, the best answer is to carry out a series of tests yourself, and run the numbers to decide whether, in the long run, recycling would, or would not, be an effective choice.

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