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.

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.

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.

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.