Tuesday, February 24, 2015

Waterjettting 30c - why cavitation isn't being used in machining metal.

Most applications of high-pressure waterjets need the jets to make relatively precise cuts into a target surface. When examining different ways of improving jet cutting performance, the narrowness of the cut which is achieved is therefor often a critical factor in deciding how to make the cut.

In this short segment of the series I have been discussing some of the findings that Dr. El-Saie made in his Doctoral Dissertation, and at the end of the last post had shown that he had found that, under the right conditions, a cavitating jet would remove more material from a surface than would an abrasive-laden waterjet of equal power within the same time frame, and at the same operating jet pressure.

This work was carried out prior to the submission of his Dissertation in 1977, and – because most of the work on refined nozzle design had still to be done – the nozzle designs that were used in the study were considerably cruder than those that have been developed, by a number of companies, in later work. For this reason it is not realistically possible to compare the results achieved in the early studies with the current state-of-the-art, particularly since, while there has been a great deal of work on improving the design of AWJ nozzles there has been almost none focused on improving the cavitation destruction of a waterjet.

The important distinction to make is that while there has been considerable work on reducing the damage that a cavitating flow can achieve when it impacts on a surface, there is almost none that has been aimed at making that damage worse. Part of the problem that this lack of work has left us with is that, in most cavitating systems, part of the cavitating cloud will collapse within the nozzle assembly. While this may only be a small fraction of the total, within a relatively short time (and this has been measured in fractions of a minute in an intense erosion design) the nozzle is destroyed. It is therefore unlikely that the most efficient nozzle design for inducing cavitation erosion has yet been developed.

Part of the reason that it has not has to do with the requirement that is listed at the top of the page. Where abrasive particles are mixed within a very narrow jet of high-pressure water, the abrasive cutting is confined, so that the slots can be controlled to a high degree. We have shown, for example, that it is possible to make cuts through titanium within a tolerance of 0.001 inches of the design requirement, and with a smooth surface over the full surface of the cut. (This requires, in thicker materials, that the nozzle be slightly tilted so that the jet taper over the depth of the cut is compensated for over the desired edge cut).

Unfortunately this precision in cutting cannot be achieved (at least with the current levels of understanding of the process and controls) with a cavitating jet system. In part this is because of the omni-directional nature of the collapse of the cavitation bubbles over a surface. An abrasive particle has the great majority of its velocity aligned with the axis of the cutting jet (though this might slightly deviate if the particle cuts into the surface more than once over the depth of the cut). Cavitation attack can occur in a more omnidirectional way.

To illustrate the point consider an experiment where we fired a waterjet along the edge of a block of dolomite, in such a way that the jet did not contact the rock, were the test to be carried out in air. The jet was carried out underwater, with back pressure of the surrounding water adjusted to intensify cavitation along the edges between the waterjet and the surrounding water. The collapse of those bubbles against the side of the rock eroded the cavities shown.

Figure 1. Samples of dolomite attacked by a cavitating jet. The jet is, in both cases aimed parallel to the cut face (along the line of the red arrow) and just off the block surface.

The samples shown in Figure 1 were exposed to the jet for a minute, with the samples held under water in a cell where the back pressure (BP) could be adjusted.

At a pressure of 6,000 psi, with a back pressure of 60 psi and a nozzle diameter of 0.02 inches the damage to the rock is small. Although it should be noted that there is a hole that eats into the rock perpendicular to the direction of jet flow.

This is more immediately obvious with the sample shown on the right, which was cut with a 7,000 psi jet, against a back pressure of 35 psi, and with a jet diameter of 0.030 inches.

It is also worth noting that the hole generated is not consistent in diameter as the hole deepens. The penetration of the jet into the wall, perpendicular to the main jet flow, is caused by the individual collapse of cavitation bubbles against the surface of the rock. And this is not a consistent phenomenon along the jet length, but for varying conditions it will occur at different distances from the nozzle. In this case the hole widens and deepens about half-an-inch below the rock top surface, eating further into the rock relatively consistently for the following few inches.

In a separate experiment the resulting hole can be seen to vary in depth over the length of the cut into the rock.

Figure 2. Hole cut into dolomite by a cavitating jet. Note that softer layers of the rock are excavated more deeply into the hole wall by the collapsing bubbles. The hole is roughly six inches deep.

The cut is also much broader than the originating jet, as can be seen from the cavity eaten into the rock surface perpendicular to the jet at the bottom of the sample. The cut is much more ragged than the precise line that would be cut by an AWJ, so that, although more volume is removed, the removal path is not as precisely defined as is needed in most applications. The irregularity of the slot shape can be seen where a cavitating jet is traversed over a 2-inch long block of dolomite over a period of five minutes.

Figure 3. Traverse path of a cavitating jet eroding a slot into dolomite. The block is roughly 2 inches long.

Clearly, at this stage in its development, there is little precision to the slot that is being generated in the rock, and it has little application in machining.

However the operating jet pressures of these cuts are within the range of those pumps that are available at relatively low cost at hardware stores, with the simplicity in use that this implies. And yet they are capable of disrupting, into the constituent grains of mineral, even the hardest of rocks that will be encountered. Gold ore, for example, can be relatively expensive to drill, because of its strength and toughness, and yet it can be penetrated in the same way, and with the constituent minerals separated, as was the dolomite.

Figure 4. Cavitation erosion of a sample of gold ore.

The problem comes in collecting the very fine grains of the mineral from the rest of the ore sample, once it is disaggregated.

Cavitation therefore, at present, is a tool better used in applications beyond those of the machine shop. It does not, however, have to stay limited to that restriction.

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Wednesday, February 18, 2015

Waterjetting 30b - An opportunity missed, and a question raised

In the last post I wrote about the benefits of cutting a deep slot around the edges of a tunnel, and made reference to the work done here by Dr. El-Saie back in the mid-1970’s as part of his Doctoral Dissertation.

One of the concerns that we had to address was that the waterjet had to be able to penetrate all the different rock types that it might encounter, and at the same time, since the jet would only cut a short depth on each pass we also had to find a way of cutting the slot wide enough that the nozzle assembly could enter and deepen the slot over consecutive passes around the edge. Given that the available pressures in those days were limited, for us, to 30,000 psi and this pressure was insufficient, by itself, to cut through all the rocks we might encounter, Dr. El-Saie looked at several different ways of enhancing performance. These included adding abrasive to a high pressure jet stream, inducing cavitation into the jet stream and the potential for using the break-up of the jet into droplets to enhance cutting using the impact water hammer effect.

Because of other operating conditions it was not considered practical to try and develop the droplet impact idea for this program, and the work concentrated on examining the potential differences between abrasive waterjet injection and cavitation. To simplify the comparison the same basic nozzle design was used for the tests that were then run, although the shroud fitted to create the secondary (vacuum) chamber was modified to either allow abrasive entrainment, through ports, or to create cavitation. The presence of the ports did, however, allow the strength of the vacuum generated in the chamber to be measured as the jet passed through.

Figure 1. Nozzle designs used by Dr. El-Saie. Note that the upper design has ports leading into the vacuum chamber, so that abrasive can be drawn in by the jet passage. In the lower design there are no ports, and cavitation will be induced in the chamber by the jet passage, with the bubbles then drawn into the exiting jet.

One of the advantages of cavitating the jets is that the cavitation bubble collapse will spread out over a larger area on the target surface, so that the slot generated can be quite a bit larger than the originating jet. This can be shown in two pictures of a block of dolomite exposed to the same cavitating jet, at a pressure of 6,000 psi but one with the jet traversed along the block in a minute, while in the second case the jet is moved at a slower speed, taking five minutes to cross the block, which allows the jet to exploit the cracks generated by the cavitation. A fuller description of the process is given here.

Figure 2. Cavitation damage pattern on a block of dolomite showing the initial width of the jet (red lines), and the zone of damage that is being created around the traverse path.

Figure 3. Cavitation damage on a block of dolomite at a slower traverse speed, showing the width of the damage track that can be created. The slot is about half-an-inch deep.

In the course of the test program different shroud shapes were tested, but in all cases the comparison between an abrasive-laden jet and one containing cavitation bubbles was made with shroud shapes of the same overall dimensions.

The ratio of the exit diameter (discharge) from the shroud (D2) to that of the initial jet orifice (D1) was first changed to one of four different ratios, though the diameter of the initial jet was kept at 0.04 inches (1 mm). Of the different sizes tested the greatest vacuum in the chamber was measured with the smallest of the discharge diameters was tested.

Figure 4. The effect of increasing the throat length of the shroud on the vacuum pilled in the chamber, at different pressures.

If the discharge diameter was increased to 6.35 mm then the jet pressure had to be increased to 12,500 psi to obtain the same levels of vacuum achieved otherwise at 7,500 psi.

Figure 5. Vacuum pressures measured with a larger discharge diameter from the shroud, for different lengths and jet pressures.

Impact force measurements from the jet hitting a target at varying distances from the orifice, with and without the shroud showed relatively little difference in the overall total impact force (not considering abrasive) out to a distance of 6 inches. There was thus no apparent effect due to jet disintegration from the use of the shroud over these distances.

In order to compare the performance of the abrasive-laden jet with that of a cavitating jet, a new nozzle design was developed, and small samples of granite were rotated in front of each nozzle assembly, for 20 seconds. Because the jet had to be brought up to pressure for each test, and shut down afterwards, a steel shutter plate was placed between the nozzle and the target. One of the irritants in doing the tests was that the jets kept cutting through this shutter plate.

Figure 6. Steel shutter plate cut through in 6 seconds during system start-up.

The shroud, made of stainless steel, was also wearing out within a few minutes. Unfortunately we did not recognize that this was demonstrating that abrasive waterjets were an effective method for cutting metal – that commercial development had to wait for the more perspicacious Dr. Hashish to work with Flow Research and bring the technology to the market in 1980.

Part of the reason for our lack of interest was because of a different conclusion that Dr. El-Saie drew from his work, based on the following two curves. The first comparison of different jet results occurred with a jet pressure of 7,000 psi.

Figure 7. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 7,000 psi.

Note that the three water jets do not have much significant effect on the granite at this distance and jet pressure (we had to learn some later lessons to make them more productive at this pressure). But even at this pressure the abrasive waterjet was effectively cutting the granite.

But it was the change in the relative position of these curves, as the pressure was then increased to 20,000 psi that caught our attention. (The intermediate plots are not given here).

Figure 8. Volume of material removed from granite samples, as a function of distance, for four different jet conditions at a jet pressure of 20,000 psi.

The water feed was not useful, since the power required to accelerate that volume drew heavily from that available through the jet.

The plain jet, without a shroud will cut granite at this pressure, particularly when moved over the surface. (And we later used this system to carve the Millennium Arch_ – as well as the Missouri Stonehenge). But the performance at that time was not that impressive.

Opening the ports on the shroud, without feeding anything into the jet caused, we believed a greater jet breakup and thus some additional droplet impact effects that improved cutting performance over that of the plain, more coherent jet, in part because the jet was spread over a larger contact surface.

Closing the jets induced cavitation in the stream, and this gave the best performance of the four – including abrasive injection. Again this was, in part because of the larger area of damage that the cavitation generated on the target, over the narrower slot of the abrasive-laden and plain jets.

In comparison the abrasive waterjet did rather poorly. In retrospect this is perhaps more of a surprise – but it should be born in mind that there was little attempt at optimizing the feed condition (which later research shows has a dramatic effect on performance) or the chamber geometry. Further the slot cut was much narrower than that created by the cavitating jet.

But it certainly caused us, in that time interval, to look more at cavitation, and to totally miss the implications of the AWJ result.

Most of these illustrations come from the Doctoral Dissertation by Dr. A. A. El-Saie “Investigation of Rock Slotting by High Pressure Waterjet for Use in Tunneling”, Mining Engineering Department, Missouri University of Science and Technology, (Then University of Missouri-Rolla), 1977.

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Monday, February 16, 2015

Tech Talk - enjoy it while you can

It is perhaps an odd time to be writing about oil shortages. The price of gas in our town has just moved above $2 a gallon up significantly from the $1.64 it was at its recent lowest point, but still very reasonable. Debate still rages as to whether the global price of a barrel of oil has found a bottom, although there are signs that the price is beginning to increase, in part due to other issues than overall availability of crude. So why be concerned?

There are several issues, and perhaps the first is that of industrial inertia. Despite the daily fluctuations in oil price, many of the events that occur between the time that oil is found in a layer of rock underground and the time that some of it is poured into your gas tank take a long time to initiate, and similarly can’t be turned off overnight. It takes, for example, roughly 47 days for a tanker to travel from Ras Tanura in Saudi Arabia to Houston.

One response to the drop in oil prices has been to reduce the number of rigs drilling for oil in the United States. Again this is not an immediate response, but rather one that grows with time. This is particularly true with the number of oil rigs that are used to gain access to the oil reservoirs. As the price for this oil falls, so rigs are idled and the potential for additional oil production also declines. This drop is particularly significant in fields that are horizontally drilled and fracked because of the very rapid decline in production with time in existing wells and the need for continued drilling to develop and produce new wells to sustain and grow production. The most recent figures show a fall of 98 rigs in the week from the 6th to the 13th of February, with the overall count now standing at 1,358. This rate of decline has held at nearly 100 rigs a week now for the past three with no indication of any immediate change in the slope of the curve. At the same time the number of well completions in the Bakken is falling, as producers hold back on the costs for producing oil that would be sold at a loss.

The impact from this will take time to appear, North Dakota has reached a production rate of 1.2 mbd in December and the DMR estimates that it will need around 140 rigs to sustain that production level this year, with the most recent rig count being 137. This number is likely to continue to fall through the first six months of the year.

The impact is not just in the immediate loss of production. Rather, once the rigs are idled it will take time, even after the markets recover, for the companies to adjust their planning and finances, and to re-activate the rigs. What this effectively does is to shift the production increment into later years, when the production base from existing wells will have declined beyond current levels. This means that the peak level of production will likely also be lower than would otherwise be the case, and the period over which this peak production is sustained will also be shorter.

The problem that this all presages is that lower levels of production against an increasing world demand will induce a faster rise in price than many now anticipate. There is a complacent feeling that oil prices won’t reach $100 a barrel for some considerable time - perhaps even years. If the current difference between available oil supply and demand is below 2 mbd, Euan Mearns has suggested that roughly half of this might be eaten up by increased demand, while the other half would disappear as production levels drop, although he doesn’t see this bringing the two volumes into rough balance until the end of 2016.

I rather think that it will happen faster than that, and that the price trough will steepen faster than currently anticipated, and likely before the end of this year. The problem (if you want to call it that) with the perceptions of the ability of global production to meet demand is that it is all tied to the production of the United States and Canada. I have noted, over the past two years, how future projections of increasing global oil demand have been met, in models, by increased production from the United States, and that this was anticipated to continue. (Increased production from Iraq, if sustained, is more likely to be needed just to balance declines in production from other countries).

Yet the US industry is going into a relatively rapid decline because of the way that it is structured that is going to be hard to stop, and much slower to reverse than anticipated. (In a way it is similar to the intermittent traffic congestion one finds on roads which result because we brake a lot faster than we then accelerate). This will not only stop the growth in production that is currently anticipated, but will go further and before the end of the year will lead to a drop in overall volumes produced. Yet demand is expected to increase. Where will the supply come from, if not the United States?

While Saudi Arabia can produce more, one gets the sense that they are quite comfortable where they are, thank you and won’t be increasing their contribution, and while Russia may bemoan the price they are getting for their oil, if the price goes up they are not going to be able to meet an increased demand, nor are there likely to be others with spare capacity that they can bring to the table. And because of the inertia in the system the United States will still be in a mode of declining production.

So I rather suspect that what we can anticipate is that prices will start to recover through the summer, and then, as the full impact of the rebalanced situation starts to become evident, will move higher at an increasing rate. Because if, in fact, we are reaching the period of a tighter balance between demand and available supply, then the market will change its perceptions quite quickly and be driven by a totally different metric.

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Tuesday, February 10, 2015

Waterjetting 30a - Why cut slots in rock

Looking back over the period where we first started coming together to discuss high-pressure water jets, some 40-odd years ago I was reminded of the work of one of my then graduate students (and subsequent faculty member in Egypt) Dr. Ahmed El-Saie. He obtained his doctorate in 1977 and looking back on that work it is interesting to see how long it took for some of the ideas he worked on to come to fruition.

His dissertation focused on using a waterjet system (which I will discuss in a later post) as a way of cutting a slot around the edge of a tunnel before excavating the contained volume. Early in his dissertation, for example, he discussed the use of impact breakers as a method for improving the economics of tunnel driving over conventional drill and blast techniques. He felt that this would be particularly useful where the volume of overbreak around the tunnel beyond the desired size could be controlled by cutting a perimeter slot.

Apart from the benefits that come from mechanical excavation over blasting (workers don’t have to leave the working area during a blast, for example) other benefits can be shown by contrasting the damage to a block of Plexiglas when a detonator is fired in a small central hole in Plexiglas, with and without that perimeter cut.

Figure 1. Damage to a block of Plexiglas from a detonator fired in a central hole.

If, however, a relieving slot is first cut around the perimeter of the anticipated damage zone (we used the distance to some of the longest cracks) then a different result is obtained.

Figure 2. Effect when the experiment is repeated with a pre-cut slot around the perimeter.

As the photos clearly showed with the free outer surface the central core of material is broken out in pieces, there is a nice relatively flat front surface to the excavated hole, which lies at the back of the drilled hole. (This is a relatively important point in driving tunnels, since often the last third of the blast-hole length is not effectively broken out of the solid, and has to be re-drilled).

It is also important to notice that the cracks from the detonator explosion did not grow out beyond the edge of the slot, so that the tunnel wall would be stable and, because there would be no overbreak, the cost of tunnel support would be reduced considerably.

However, in the larger scale the depth that this slot would have to be cut is around 7-ft. This would require that the jets cut a slot wide enough for the nozzles to advance into the slot, and this required a considerably higher volume of rock to be removed by the waterjets.

Tests of such a device in a German coal mine used two different methods for cutting the slot. Initial trials at Rossenray Colliery in Germany used a waterjet assisted mechanical set of tools to to cut the slot to the desired width. The head, seen moving along the slot at the edge of the tunnel, had to make a number of passes to reach the depth needed.

Figure 3. Tunnel profiling in Germany using a combination of waterjets and metal tools to cut to the perimeter of a tunnel (after Bauman and Koppers)

Subsequent trials replaced the mechanical cutter with a set of waterjet nozzles alone, and this reduced even further the cutting forces required to make the slot (and would make the machine smaller and lighter as a result). Although the trial was successfully concluded the tool did not move into production, perhaps in part because of the change in the mining economy at that time.

To prevent the cracks from growing into the wall, however, a wide slot is not needed, and even a continuous crack around the edge of the hole can be effective. But how to control crack growth to a single direction from the borehole?

The answer came as part of the Master’s degree of another student, Steve McGroarty. If one drills a hole into a block of Plexiglas, and then notches the side, fills the hole with water, and fires an air rifle pellet into the hole, then the pressurized water will flow into the notches and cause the cracks to grow. These are a little difficult to see in the following picture, but the cracks grow at the bottom of the hole and from the edges of the v-cuts (made at the time with a saw).

Figure 4. Individual cracks growing out from 3 bored holes in plexiglas

The above test showed that energy could be focused into cracks if they could be properly aligned. (We could break off a corner of the block in a single piece, using a single notched hole). This work was then in the field by Steve in comparing results when he used explosives to drive a short tunnel underground.

In Steve’s case he drilled holes around the edge of the tunnel, and then notched some of these with a waterjet system. (Others were left un-notched to provide a comparison). The lance used had two jet nozzles and was fairly simple to insert, and the lance was run to the back of the hole, and the two opposing jets aligned to the proposed tunnel wall, raised to pressure and pulled back out of the hole, notching the walls. This is a fairly fast process, and used relatively little water.

The holes were then charged with a small amount of powder and fired just before the rest of the blasting round, which was distributed around the rest of the core rock, in order to break it into fragments.

Figure 5. Tunnel wall after the round had been cleared showing the clear break at the back of the holes, (the next round has been drilled along the edge) and parts of the drilled hole still evident in the wall of the tunnel. (after McGroarty)

The role of the waterjets was much smaller than if a complete slot had been cut, and this significantly reduced the cost financially, in energy and in time, and produced much the same desired result.

In later work we used the same notched borehole idea to break out large pieces of rock as we excavated the Omnimax Theater under the Gateway Arch in St. Louis, but that is a story for another day.

Next time I will discuss some of the ways that Dr. El-Saie used to cut the slot.

A.A. El-Saie “Investigation of Rock Slotting by High Pressure Water Jet for use in Tunneling”, Doctoral Dissertation, Mining Engineering, University of Missouri-Rolla, 1977.
Bauman L. and Koppers M. “State of Investigation on High Pressure Waterjet Assisted Road Profile Cutting Technology,” BHRA 6th ISJCT, paper G2, pp. 283 – 300, 1982).
S.J. McGroarty “An Evaluation of the Fracture Control Blasting Technique for Drift Round Blasts in Dolomitic Rock”, M.S. Thesis, Mining Engineering, University of Missouri-Rolla, 1984.

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Saturday, January 31, 2015

Waterjetting 29e - Back and Forth again

When the International Symposia on Waterjet Cutting (ISJCT) began (back in 1972) the uses of the tool were severely limited, both by the pressures available to customers, and by the limited knowledge of the capabilities of the system. Skip forward four decades and while the various tools available have widely broadened (the maximum pressure we have operated at runs well over a million psi) with cavitation, abrasive and polymer injection being just some of the more productive additions to the tool box, knowledge of system capabilities still lags in the general public.

The public wandering past a hydro-demolition site, where jets may be operating at 50,000 psi to remove damaged concrete – or by a civil construction site where hydro-excavation is digging a dry trench faster and safer than before, and the observer will likely have little clue as to the changes in technology that have taken place and the tool being used, since the jets are usually hidden by their surrounding shrouds.

Yet even in the industry that uses the tools – either for cutting aircraft parts, art pieces or for more mundane cleaning, the advanced technical capabilities of the tools they use are often a mystery to them.

Part of the reason for this, I suspect, is that the generation of scientists and engineers who pioneered the research that developed the industry have now, in most countries, retired. There has been a change in emphasis also for the research effort. There are many fewer opportunities for university teams to go out and demonstrate (as we did) that an abrasive waterjet system could be used to effectively cut a rock wall 20 feet deep within 50 ft of the Gateway Arch in St Louis, without any risk of the Arch crossing its legs. (Which was a major concern that the Park Service had when we did the work). In that project we also had to develop a 5,000 psi DIAjet drill to install rock bolts through 15 ft of dolomite, clay and chert to ensure that the walls remained stable (something not used much thereafter).

More of the research now (at a much smaller cadre of universities) is focused on enhancing the performance of jets in a much smaller range of applications, rather than finding and developing new markets in places where waterjets have not been used before. I will exclude the medical field from this restriction, since, particularly in Germany, new applications continue to appear.

What is also unfortunate is that the advent of the internet means that many of the earlier papers where, as with the case of hydro-demolition, the exploratory work was undertaken, are not easily accessible. And (writing as an academic who reviewed many theses and dissertations) few students go back much more than five years in assessing the previous state of the art.

As a result of this there is almost no effort to exploit some of the ideas that were developed over the early decades, where potential new applications were found, but which could not, at the time, be developed because of either technical constraints, or because (in our case) there were other more immediately rewarding paths to follow.

The movement at universities has seen high pressure waterjet systems move from the research laboratories into the machine shops of the support complex. As they thus become classified as “conventional systems” so there is less incentive to see them as places where innovation can bring the sort of rewards that can be found in developing other avenues of research.

This is a great pity since, although the industry has grown from being just a lab curiosity to an integral part of a number of industries (collectively doing billions of dollars of work a year), the range of applications for which it is uniquely suited have, as yet, only been tapped to a limited extent. As an example, the ability of waterjets to work in explosive environments to cut through different materials has yet to be fully recognized. Yes abrasive waterjets are used to cut the tops from oil and gas tanks, where remedial hydrocarbons pose a threat, but there are many other situations where – on a smaller scale – this ability could provide a number of benefits. (The coal mining industry comes to mind).

I remember watching an early demonstration of the use of a hand-held waterjet by a diver, as a way of cleaning barnacles and growths from an undersea platform. Previously the divers had to cling to the structure with their legs to give them support as they chiseled away at the growths with jackhammers. By putting a reverse jet on the lance, the diver could now float around the rig, removing growths without that sharp intrusion into his comfort zone. As I recall it took less than two years for the concept to sweep the industry, around the world, and I have mentioned before the reaction of one diver, who threw the jackhammer over the side of the rig with a profanity, after using a waterjet cleaning lance for the first time.

The above is another explanation for the focus that this site is going to have on some of the earlier papers in the technology over the next few months. It will try and provide a deeper explanation as to why certain things are done, based on the research of those earlier investigations, and also some pointers as to where we can expect the industry to move in the future. It should be fun!

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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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