Waterjetting 10d - Abrasive sizing

Over the past 30 years abrasive waterjet cutting has become an increasingly useful tool for cutting a wide range of materials, of varying thickness and strength. However, as the range of applications for the tool has grown, so the requirements for improved performance have also risen. Before being able to make a better quality cut there had to be a better understanding of how abrasive waterjet cutting works, so that the improvements could be made.

Figure 1. Some factors that affect the cutting performance of an abrasive waterjet (After Mazurkiewicz)

This understanding has not been easy to develop, since there are many different factors that all affect how well the cutting process takes place. Consider, first of all, the process of getting the abrasive up to the fastest speed possible. And for the purpose of discussion I am going to use a “generic” mixing chamber and focusing tube nozzle for the following discussion.

Figure 2. Simplified sketch of a mixing chamber and focusing tube nozzle used in adding abrasive to a high pressure waterjet.

As high-pressure water flows through the small orifice (which in the sketch was historically made of sapphire) it enters a larger mixing chamber and creates a suction that will pull abrasive into the mixing chamber through the side passage. That side passage is connected, through a tube, to a form of abrasive feed mechanism, that I will not discuss in detail today.

However the abrasive does not flow into the mixing chamber by itself. Rather it is transported into the mixing chamber using a fluid carrier. In the some of the earliest models of abrasive waterjet systems water was used as the carrier fluid to bring the abrasive into the mixing chamber. This, as a general rule, turned out to be a mistake.

The problem is that, within the mixing chamber, the energy that comes into the chamber with the high-pressure water has to mix, not only with the abrasive, but also with the fluid that carried the abrasive into the chamber. Water is heavier than air, and so if water is the carrier fluid, then it will absorb more of the energy that is available, with the result that there is less for the abrasive, which – as a result – does not move as quickly and therefore does not cut as well. The principle was first discussed by John Griffiths at the 2nd U.S. Waterjet Conference, although he was discussing abrasive use in cleaning at the time.

Figure 3 Difference in performance of water acting to carry the abrasive to the mixing chamber (wet feed) in contrast with the use of air as the carrier fluid. (Griffiths, J.J., "Abrasive Injection Usage in the United Kingdom," 2nd U.S. Waterjet Conference, May, 1983, Rolla, MO, pp. 423 - 432.)

Note that this is not the same as directly mixing the abrasive into the waterjet stream under pressure – abrasive slurry jetting – which I will discuss in later posts.

The difference between the two ways of bringing the abrasive to the mixing chamber is clear enough that almost from the beginning only air has been considered as the carrier to bring the abrasive into the mixing chamber. However there is the question as to how much air is enough, how much abrasive should be added, and how effectively the mixing process takes place.

In the earlier developments the equipment available restricted the range of pressures and flow rates at which the high pressure water could be supplied, and these limits bounded early work on the subject.

One early observation, however, was that the size of the abrasive that was being fed into the mixing chamber was not the average size of the abrasive after cutting was over. (At that time steel was not normally used as a cutting abrasive). Because the fracture of the abrasive into smaller pieces might mean that the cutting process became less effective, Greg Galecki and Marian Mazurkiewicz began to measure particle sizes, at different points in the process. (Galecki, G., Mazurkiewicz, M., Jordan, R., "Abrasive Grain Disintegration Effect During Jet Injection," International Water Jet Symposium,Beijing, China, September, 1987, pp. 4-71 - 4-77.)

For example, by firing the abrasive-laden jet along the axis of a larger plastic tube (here opened to show the construction) the abrasive would, after leaving the nozzle, decelerate and settle into the bottom of the tube, without further break-up, and without damage to the tube. Among other results this allowed a measure of how fast the particles leave the nozzle, since the faster they were moving, then the further they would carry down the pipe.

Figure 4. Test to examine particle size and travel distance, after leaving the AWJ nozzle at the left of the picture. The containing tube has divisions every foot, and small holes over blue containers, so that the amount caught in every foot could be collected and measured.

For one particular test the abrasive going into the system was carefully screened to be lie in the size range between 170 and 210 microns. It was then fed into a 30,000 psi waterjet at a feed rate of 0.6 lb/minute. The particles were captured, after passing through the mixing chamber, but before they could cut anything, by using the tube shown in Figure 4. The size of the particles was then measured, and plotted as a cumulative percentage adding the percentages found at each sieve size over the range to the 210 micron size of the starting particles.

Figure 5. Average size of particles after passing through a mixing chamber and exiting into a capture tube, without further damaging impact.

The horizontal line shows the point where 50% of the abrasive (by weight) had accumulated, and the vertical line shows that this is at a particle size of 140 microns. Thus, just in the mixing process alone energy is lost in mixing the very fast moving water, with the initially much slower moving abrasive.

And, as an aside, this is where the proper choice of abrasive becomes an important part of an effective cutting operation. Because the distribution of the curve shown in figure 5 will change, with abrasive type, size, concentration added, as well as the pressure and flow rate of the nozzle through which the water enters the mixing chamber.

I will have more to discuss on this in the next post, but will leave you with the following result. After we had run the tests which I just mentioned, we collected the abrasive in the different size ranges. Then we used those different size ranges to see how well the abrasive cut. This was one of the results that we found.

Figure 6. The effect of the size of the feed particles into the abrasive cutting system on the depth of cut which the AWJ achieved.

You will note that down to a size of around 100 microns the particle size did not make any significant difference, but that once the particle size falls below that range, then the cutting performance degrades considerably. (And if you go back to figure 5, you will note that about 30% of the abrasive fell into that size range, after the jet had left the mixing chamber).

OGPSS - Insecurity in the Middle East

The continuing conflict in Syria, and the slow spread of violence in the region around it, continue to make it difficult to make accurate predictions about the future of oil exports from the region. Within Syria itself production had fallen into decline about ten years ago, before the current struggle began. The precipitate drop over the last two years has, however, been much more dramatic. As Energy Export Databrowser noted from the BP statistic review, production fell by 49% last year.

Figure 1. Syrian oil production showing the recent fall in volume. (Energy Export Databrowser ).

As with most oil-producing nations oil consumption had, on the other hand, been steadily rising with net exports (some is exported as crude and re-imported as refined product) falling to around 100 kbd. The conflict has, however, also reduced internal consumption at similar rates earlier in the conflict.

Figure 2. Syrian production and consumption until 2011. (EIA )

At the same time that Syrian exports have sensibly disappeared, the exports from Iran have continued to fall. Data through the end of last year shows that sanctions continued to bite, with exports falling 31% last year.

Figure 3. Oil production, consumption and exports for Iran (Energy Export Databrowser)

Much of the oil from Iran goes to China, India and Turkey. In May 2013 the total exports are reported to have fallen to 700 kbd although individual monthly numbers fluctuate given the complexity of now getting oil into the hands of customers. (H/t Gail - that report may only cover Chinese imports, since Bloomberg reports a different set of numbers, and an IEA estimate that Iran is averaging 1 mbd of exports this year.) India is hoping that the change in the Iranian Presidency will lead to an easing of sanctions. In the interim, the exemptions that China, India and Turkey are receiving to some of the sanctions have just been extended another six months. Part of this comes from the cuts those countries have already made. India, for example is now down to around 117 kbd around half that of a year ago.

Oil imports into China are reported to have increased, perhaps because the Chinese have just agreed to buy a number of Chinese drilling rigs. Total exports are projected to fall to 1.3 mbd over the current Iranian fiscal year (which started in March). If Iran is having to store more of its oil in off-shore tankers, this may explain the fall in regional tanker availability over the past month.

In a recent post I discussed my concern over the likelihood of Iraq being able to achieve the increased volumes of production within the time frame that their Central Government has suggested. However, as Leanan caught recently, Kurdish Iraq is moving more and more toward independent action in regards to their oil. From the Turkish side a pipeline will be allowed, that can go to the border, but not cross it. Mysteriously it will likely then fill, over time, with a flow of 1 mbd. Turkey is working with BP to develop the resources of Kurdish Iraq. In the interim Turkish demand has stabilized to a greater degree than it has in its southern neighbors.

Figure 4. Oil imports and consumption in Turkey (Energy Export Databrowser)

To a degree the instability is feeding off itself. Egypt continues to have problems in ensuring an adequate supply of fuel, and while Iraq and Libya have agreed to help, they prefer cash up front for the delivery, and that is proving to be a bone of contention. The country has reached the point where domestic production can no longer keep up with consumer demand, and these imports are going to become more critical to the budget, and, as a follow-on, national stability.

Figure 5. Change in oil consumption and the need for more imports for Egypt (Energy Export Databrowser)

The hope is that a pipeline can be built from Iraq, through Jordan, but that requires that a mutually acceptable line of credit be established, and that appears to be a problem. Egypt has the same soft-credit deal with Libya for a supply of a million barrels a month. The price, however, will be at that of the world market.

Unfortunately as the level of violence continues to grow one starts to get into an almost inevitable snowball effect, and there is a consequent negative impact on much industry, likely including that of oil production. The most likely consequence will be a further fall in the regional export of oil, which has a follow-on consequence in that the customers who have lost this supply (Japan has given up all Iranian oil for example) must then go onto the world market to find an alternate source of supply.

As those sources become even scarcer than they are already, marginal amounts of oil become more critical to maintaining a global balance. But such supplies don’t become available at the drop of a hat.

It seems to be a drum that I beat perhaps a bit often, but it is a message that bears repeating. Without significant and ongoing investment in the more difficult regions of the world, funds to identify the necessary availability of resource, and then to drill, in a timely manner, to prove the resource and start the process of turning it into a reserve, the oil balance cannot be sustained at a viable and acceptable price. It does not matter how glowing a set of reports are put out about how we can all relax because the world has plenty of oil in shale.

The largest of those resource sites is in Russia, where there may be as much as 75 billion barrels. But two things should be remembered. The first is that peak oil is reached, not when we run out of oil, but when we start producing less each year than in the previous. And the second is that getting much of the oil in the shale into the proven reserve category is going to take a fair amount of time, after which production rates, going from the results seen, for example, in the Bakken will decline at such a rate that a continued and expansive program of expensive drilling will be required to sustain production. And all this time the flow of oil from existing reservoirs will continue to fall.

Figure 6. A pretty wall hanging . (EIA)

Editorial Note: Because of one of those delightful family events that occur from time to time, this series will be on hiatus for a couple of weeks, since we will be traveling, when I would otherwise be writing.

Waterjetting 10c - Abrasive waterjet cutting

There are a number of different abrasives that can be supplied by different sources, and the market for the small grains that are used in abrasive waterjet cutting extends considerably beyond just the waterjet business. All abrasives are not created equal, some work better in one condition, others in another. As with other tools that the waterjet cutter or cleaner will use, first you should decide what the need for the abrasive is, and run a small series of tests to find out which is the best set of cutting conditions for that particular job.

The first item on the list should be the material that has to be cut. (Although abrasives are also used in cleaning, that will be covered in a later post). There are, simplifying greatly, two classes of material that have to be cut. One class responds in a brittle way (think glass) and the other responds in a ductile or yielding manner (think metal). Because of these different responses, when the particles hit the surface, the way in which cuts are best made will vary between the two. Some years ago Ives and Ruff shot abrasive particles at different targets and found that there was a difference in the amount of material removed, but the best angle at which the particles should be aimed changed with the material.

Figure 1. The Effect of change in impact angle on erosion rate for ductile and brittle targets. (Ives and Ruff, Wear, 1978, pp 149 – 162).

Some work at MS&T just before I retired indicated that the shape of these curves changed a little, depending on the size of the abrasive that is used. There are also some changes with abrasive shape. And this is because of the entirely different way in which an abrasive particle cuts into the two different materials. In this post we’ll discuss only the ductile target.

If a relatively smooth particle is shot into a ductile material at an angle perpendicular to the surface, then when it hits the surface the target material will flow out from underneath, but not be removed. As the following micro-photograph shows the particles can become embedded in the material – and even add to the weight of the piece on rare occasion.

Figure 2. Microphotograph showing a sand particle buried in the surface of an aluminum target.

There is very little material removed in this case, as the black curve shows in Figure 1, when the impact angle approaches 90 degrees. Consider that if you take a knife and push it down into butter you don’t remove any butter. But if you drag the knife over the butter surface you will peel off a layer.

So it is with abrasive hitting a ductile metal. If the abrasive is brought in at an angle, (optimized in the figure at 15 degrees) then the abrasive has a cutting energy along the surface and this will peel up, and remove small pieces of the surface. By taking a microphotograph along the edge of an abrasive cut, we were able to show the action of individual particles in cutting into the metal.

Figure 3. Individual particle impacts on an aluminum surface, showing the cutting and plowing action of the particles.

Where the surface is plowed up, but not removed, another particle has to hit that point to remove the relatively fragile lip. However, if the particle is a copper slag, or other relatively weak material, it can shatter during the cutting process, and the breaking pieces can break off that lip, so that – again in the right material – the slag may give a better performance than a more expensive alternative.

But if we are to cut metal in this way, what does that say about the shape of the particles that we need to use. Obviously if they were round, such as a steel or glass shot, then there would be no sharp edges to cut into and peel off the material. Thus a steel or glass grit will cut better, though each particle needs a certain thickness in all dimensions so that there will be enough energy to both cut into the material, and plow along it.

Figure 4. Difference in cut depth achieved with broken glass fragments over glass beads when cutting metal.

A relatively round particle with sharp corners, and garnet is usually such a particle, can often work well in cutting a range of different ductile materials.

Figure 5. Schematic of how a particle of different shapes might cut into material.

Now that is fine when a high-pressure abrasive waterjet (AWJ) is starting to cut into the surface, but as the jet cuts down into the surface the angle of the cut will change. Yet even if the jet is pointing directly down into the target, and moving along to cut through it, the cut surface is not usually a straight line down through the material.

Figure 6. Cutting through glass, note the curved path of the jet through the one-inch material.

Cuts into Plexiglas and other clear materials have allowed research scientists to monitor the cut path through the target, as a function of time. It is not a constant shape, but, as Dr. Henning showed at the 18th International Conference, the edge of the cut changes with time. You can see the results of this in cuts that are made through metal where the paths of the cut, particularly lower in the cut, curve around and back towards the start of the cut.

Figure 7. Cut into steel, with the face piece of metal removed to show the cut surface.

This path confirms an explanation first proposed by Dr Lars Ohlsson in his doctorate at Lulea in Sweden. He pointed out that the change in the surface of the cut is caused by the sequence of actions that a particle sees as it comes down onto the surface.

First it comes in almost vertically, with no lateral energy, and it cuts in the smooth, upper part of the cut. Then it rebounds out of the cut, but into the jet stream that gives it a little more energy, and directs it along the cut to a second point where it will cut a little bit more of the metal. But during the first rebound the particle does not bounce perfectly along the cut, but deviates to one side or the other. This means that when it makes the second cut, it will now cut more into one side of the wall or the other. Thus, where the second bounce occurs, so the surface gets a little rougher.

Figure 8. Frames from a high speed video showing abrasive waterjet cutting of glass, with the jet cutting, rebounding down the cut and then cutting again. (Lars Ohlsson “The Theory and Practice of Abrasive Water Jet Cutting”, Doctoral Thesis, Division of Materials Processing, Lulea University of Technology, 1995)

By the time of the third cut and rebound, the jet will now be coming into the opposing side of the cut with an even greater lateral portion of its energy, and so the cut will get a little rougher. Remember also that each cut is made up of the impacts of very many particles. So that succeeding particles also rebound along the curve cut by the preceding particle, and this also will exacerbate the roughness of the cut.

We’ll talk a little about reducing this effect in the next post.

So what is Art and who is an Artist?

Digital Artist Magazine has just started publishing and in the Preview edition Al Sterner wrote an editorial that said, in part:

If you take something that was made by someone else, put all the pieces together and output an image of the compiled items, it does not make you an artist. You are a composer, and nothing more. Certainly I will concede, the way these parts are positioned and lit require a good eye, it still only makes one a composer. . . . . . With the exception of a few, the 'hobby artists' are posers. Note the lower case p was intentional. If you create something from scratch, you are an artist and deserve the recognition that goes with that title. Otherwise, you are a composer.

I have just changed the descriptive paragraphs on what this site is about, and since I mentioned my hobby of working with Poser, I will comment, in the future, on how that experience goes. But for today, let me tackle the above paragraphs.

As background, I have been building a library of Poser figures, clothing etc. for a number of years as a resource to keep me amused in my retirement. That time having come, I am now starting to learn/relearn the features of the program and the supporting programs that are needed to make best use of it. I just upgraded to Poser Pro 2014, with Reality 3 that allows use of Lux Render, as well as other programs that I will mention as the need for them comes up. So have I the ambition to be an artist, or must I accept the definition that I am merely a “poser”?

My ambition is to ultimately make what kind friends might call “art.” But that would be my definition, not that of the editorial writer, Al Sterner, and so let me explain.

I find it clearer, when I give talks, to speak to an example, so let me begin with a piece that I composed while learning about lighting (a future topic or several).

Figure 1. Angus the Witchtaker.

For those curious it is composed as follows: Angus on Michael 4 (with morphs), wearing the Witch Hunter garb, and carrying a flintlock pistol. He is standing outside the Courtyard tower near some dead bushes. The lighting is with a Mesh Light from Reality, and a small fill. It is rendered via Reality.

Now any composition, whether a painting, a photograph or a render has to have a beginning. And so almost all artistic creators are going to begin with a concept. (Apart, that is, from the lucky photographer). In this particular case I figure I might as well learn from a master, and so this is roughly based on N. C. Wyeth’s “Man with a Pistol”, which he created in 1903, when, at the age of 21, he was a student of Howard Pyle.

Figure 2. Man with a Pistol – N.C. Wyeth 1903

Now, having been in N.C. Wyeth’s studio, I know that he drew using models of the various pieces that are represented in his paintings.

He worked with models (and in his last painting of Washington he worked from a bust). So that, other than in the conception, he was rendering a composed integration of the image of existing items, that he did not create, onto canvas. In the same way photographers that create art compose a picture that projects an image of a scene, using models, landscape or other things they did not create. This becomes art through the composition and lighting and manipulation, by the artist, of the image.

Ansel Adams did not create Yosemite, N.C. Wyeth did not create the hat and clothes he painted. Why then the complaint because I did not create the parts of the above figure?

The answer comes, I think, in the difference between, for example, a catalog photograph of a model, and the artist who makes the image more than just a simple picture of a man in a coat. And that involves the lighting, and the composition. And that is no different to the rendering of an image.

What makes it art, rather than a composition, lies more in the eye of the beholder. And if, in creation of the render, I tweaked the pose of the figure, adjusted the expression, changed the hat (once) and the gun (twice) in order to make something I liked better, then that is part of creation. I also didn’t like the color of the vest/shirt, and so (using Sveva’s Texturing and Stitching tutorials) changed the color and added a bump.

And yet, in my own mind, I don’t see what I am doing at the moment as creating art, yet. But where the line is drawn as to how much originality must go into the components of a piece before it can be judged as art is, in itself, a judgment call.

Some of my future work will look at how Native Americans looked in the times around the 17th Century. The image base will, as far as I can, come from contemporary illustrations. However, as Karen Ordahl Kupperman noted in “Indians and English – Facing Off in Early America,” John White’s original sketches of the Wabenaki were made to look more European by Theodor de Bry’s artists to become more acceptable art to their clientele. And with 95% of Native Americans in the North East population dying around 1620 there are few good images to work from. Yet if I use Blackthorn’s models with the faces of the Men and Women of the different tribes, defined as Cahokia, does that preclude a piece from being considered as art? Not even if I age the face, and change the expression? And that before we even consider clothing (or face paint).

Figure 3. My version of Blackthorn’s Full Moon of the Assiniboine. (from Women in Cahokia)(Warm'NSoul Lights and Raytracing) (Native American clothing, Amber Hair)

No, my answer as to what I consider art is something that Al Sterner discounts. The mark of a good artist lies in the “eye”. It is in the ability to conceive and create an image that successfully conveys something (emotion – information . . .) to the viewer. The source of the material is less important than the composition. And the judgment of artistic merit lies in the eye of the beholder.

OGPSS - A June TWIP, and the OPEC MOMR

The EIA has noted, in This Week in Petroleum that, for the first time, the sum of Non-OECD country demand contributed more than half to the total of liquid fuels consumed in the world.

Figure 1. Changes in the relative shares of liquid fuel consumption between the countries in and out of the OECD. (EIA )

It does, however, point out that the projections of the Short Term Energy Outlook are for the two curves to re-intersect at the end of 2014.

Figure 2. Projected changes in liquid fuels consumption, through 2014 (EIA)

The reality of that second assumption is, I rather suspect, more based on hope than reality. Once you start providing power, and all its benefits, to the general population you are on a slippery slope that it is almost impossible to back away from. Consider (as a small example) the problems that Egypt is currently having with the supply of subsidized bread to the general populace. Once you start supplying a commodity at a subsidized price it becomes very hard to change the equation, and too much of the non-OECD world is now living in an economy where energy use is subsidized. The problem that the above graph fails to recognize is that you cannot wean a culture from subsidies in the immediate short term and still expect their government to survive in its present condition.

Thus when the EIA project that global demand will grow to over 92 mbd in the next year, they are likely only being realistic. Their assumption that it may then decline is perhaps more in the nature of wishful thinking.

Figure 3. EIA anticipated growth in demand and supply over the near term (EIA)

There are however a couple of caveats to that last statement, the first of which is that the decline in demand may be more reflective of a lack of supply capacity (our raison d'être) and alternatively it may reflect, as a result of the first, that prices will rise to influence demand. Nevertheless we remain in a condition where the harsh realities that lie just over the horizon remain obfuscated by other events.

As with many other international agencies the EIA continue to anticipate continued growth in the North American supply of liquid fuels. Outside of that growth the increased demand for more than an additional mbd of liquid fuels seems more likely to be likely to be desperately hunting for an invisible savior.

Figure 4. Anticipated growth in liquid fuels supply over the next two years (EIA)

The decline in supply from OPEC in the two years ahead should be noted. It should also be remembered that this is likely to be as much a voluntary control, to ensure price stability in the face of increased North American production, rather than as a result of a short-term supply shortage. However the reality of continued domestic growth in demand in the Middle East, as Westexas has reminded us, is something that cannot be neglected. It has been noted that Saudi Arabia, although having less than a third of Germany’s population, recently surpassed it in terms of oil consumption. It will add several new oil-fired power stations including those at Yanbu and Jeddah. This will feed into an anticipated continued growth in Saudi domestic demand of 5.1% pa.

And this brings us to the OPEC Monthly Oil Market Report (MOMR) for June. OPEC continues to anticipate a global demand growth of 0.8 mbd this year, though they note that there will likely be a growth of 1.2 mbd in the non-OECD nations, requiring a reduction in OECD demand to match the overall forecast. Major growth in demand will continue to be in China (at 0.4 mbd and the Middle East at 0.3 mbd). On the other hand OPEC anticipate cutting their supply (to match anticipated need) by 0.4 mbd over the course of this year. OPEC, therefore, has slightly dropped their projection for year end, however it will still crest above 90 mbd.

Figure 5. Estimates of global oil demand (OPEC June 2013 MOMR)

A large part of demand projection is tied to growth in the global and individual nation economies, and that is a murky crystal ball to view. But OPEC anticipates that these economies will continue to grow at an increasing rate, while recognizing that this projection is in an area with a high level of risk in the estimate. The continued, and perhaps growing unrest in the Middle East continues to cast a further shadow over predictions over both supply and the reality of future demand in those countries. And, as one of the less frequently discussed topics, future output from Russia is not as assured as the average analyst appears to assume.

OPEC is anticipating a relatively strong growth in demand in the second half of the year to almost reach 91 mbd by the end of the year. Overall the growth in supply to meet this demand continues to come from North America.

Figure 6. Anticipated oil supply for 2013. (OPEC June 2013 MOMR)

OPEC itself is reporting a slight increase in overall production (by about 128 kbd) although, as always, there are differences in the numbers between those supplied by the countries themselves, and those reported from other sources.

Figure 7. OPEC crude oil production as reported directly (OPEC June 2013 MOMR)

There continues to be a significant disparity between the numbers reported from Iran and Venezuela, for example, when other sources are reported to the tune of around 1.5 mbd roughly. In the short term Iraqi production appears stable.

Figure 8. OPEC crude oil production as reported by others (OPEC June 2013 MOMR)

With the continued global reliance on increased production from North America, and, in turn, that reliance on improved production from tight formations, I would be a little more confident of the future were it not for plots such as this, which I recently found.

Figure 9. Chesapeake typical well decline curve (Eagle Ford Forum)

It is a curve that I rather suspect continues to be optimistic.

Waterjetting 10b - Introduction to Abrasive use

In recent articles in this series I have written about the processes that occur as a high-pressure waterjet impacts on a surface and then begins to penetrate and cut into it. However, as I noted in the last post, one of the problems with using plain water as the cutting medium is that it can pressurize within the cut and exploit any surrounding cracks, to the point that the edges of the cut are cracked and fractured, often back up to the top surface of the material.


Figure 1. High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

This is not usually desirable, and what is needed is a way of cutting into these materials, so that the cut edges remain smooth, and the risk of shattering around the cut line is much diminished. The way that is usually used for this is to add small amounts of a fine cutting abrasive into the waterjet stream, and use this to cut the slots in the material, with the water there to add cutting power.


Figure 2. Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

This can be of particular advantage if you are faced with trimming, for example, safety glass (as shown in Figure 2). Cutting and shaping this glass used to be a significant problem in the industry, since the presence of the plastic sheet, between the two glass layers meant that it was not always possible to get both to break to the same plane if scribed with a glass cutter. Failure rates of up to 30% were described, to the author, as common when the technology switch to AWJ took place. And with the abrasive in the water, the jet cuts through both layers without really seeing that there was a problem. (And complex contours can also be cut).

The combination of abrasive and high-pressure water has many advantages over existing tools. Among other things it removes the majority of the heat from the cut zone, so that in almost all cases the Heat Affected Zone (HAZ) along the edges of the cut disappears and the quality of the cut surface becomes, when properly cut, sufficient to require no further processing. This can lead to a significant savings in certain forms of fabrication.

There are many different ways in which abrasive can be added to a high speed stream of water, and Dr. Hashish illustrated some of these in the introductory lecture he gave at an early WJTA Short Course, as follows:


Figure 3. Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

The top three (a, b, c) involve mixing the abrasive and the water streams at the nozzle, while the fourth (d) is a relatively uncommon design that is used in cleaning surface applications, and the fifth has never been very effective in any trial that we have run. The sixth (e) technique has become known by a number of different names, but for now, to distinguish it from the more widely used Abrasive Water Jet cutting (AWJ) I will give it the acronym ASJ, for Abrasive Slurry Jetting. It has a number of benefits in different circumstances, and I will write more about it in future posts. In more recent alternative designs to that shown by Dr. Hashish the flow to the abrasive holding tank is more commonly through a diverted fraction of the total flow from the pump or intensifier.


Figure 4. Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

When fine abrasive is added to a narrow waterjet stream, and that jet is moving at thousands of feet a second, there are a number of considerations in the design of the mixing chamber, and those will be discussed in future posts. But one early conclusion is that, if the jet is going to be small, then the abrasive that will be mixed with it will also have to be quite small, though – as will be noted in a future post – not too small.


Figure 5. The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

There were a number of problems with the early systems, such as that shown in Figure 5, at the time that systems first appeared on the market, and I will write about some of these as the next few posts continue to focus on this subject.

There have been a number of different abrasives used over the years, and it depends on the needs of the job as to which is the most suitable in a given case. In some cases discriminate cutting is required, and so an abrasive can be chosen that will cut the desired layer on the surface, but not the material behind it. In other cases the target material is extremely tough, and so abrasive may be selected that will rapidly erode the supply lines and nozzle, but which can still prove economically viable in certain cases.


Figure 6. Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

There are many different properties of the cutting system, and the abrasive which control the quality and speed of the resulting cut. Some of these will be the topic of the next few posts, others will be discussed in further posts at a more distant time, when we discuss different cutting applications and the changes in a conventional system that might be made to get the best results in those cases.

Abrasive properties are not just a case of knowing what the material is. There is a difference, for example, in cutting ability between alluvially mined garnet and that mined from solid rock. There is a difference between different types of the nominally same abrasive when it comes from different parts of the world, and there are differences when the shapes of the abrasive differ. Glass beads and steel shot cut in a different way that glass and steel grit, for example. So there is plenty to discuss as we turn to a deeper discussion of abrasive waterjet cutting.


Figure 7. Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)

OGPSS - Oil production in Iraq, some concerns

Let me begin with two brief apologies – first, my last post on Iraq on TOD was hit with a vast quantity of spam that made it difficult to find all the pertinent comments, hopefully this post will have a little easier time. And secondly, although I used an EIA graph to show Iraqi production and consumption, Westexas was kind enough to point to an error in the domestic consumption plot. The more accurate consumption plot can be found here through 2011, and the figures for last year are up to 880 kbd, higher than the plot I showed, and close to the current capacity (900 kbd) of the refineries in the country. (Some of the difference between the two plots comes about because Iraq continues to import significant quantities of refined oil products .


Figure 1. Domestic consumption of oil in Iraq (H/t Westexas, EIA )

Last week I mentioned that, while the potential production from current contracts in Iraq held great promise for the future, that it was unlikely that those targets would be reached. This post is meant as an explanation for that pessimism, but it should be noted that Iraq, itself, is now seeking to revise the initial targets since it perceives that too much oil in the market may well be destabilizing. This, even though growth is spread over the next decades, and demand is projected to increase at more than 1 mbd/year for the next few years.


Figure 2. Projected Oil Exports from Iraq over the next two decades (OGJ)

The problems that Iraq faces, beyond those of dealing with internal politics and violence (which continues at a new and higher level than in the recent past), are related, in part to the problem referred to in the first paragraph, namely that the country is starting to have bump up against infrastructure bounds on the volumes that can be moved and processed, relative to that which might technically be brought out of the ground.

And even as projects move forward to get the oil out of the ground, there is an underlying issue that remains to be fully addressed. This is the need to inject water, and the basic explanation of that need can be found in an earlier post that was written about the problems that Saudi Aramco had to overcome to reach the levels of production that they have. (Stuart Staniford wrote a long post on the Saudi situation back in May 2007.) Simplistically as oil comes out of the ground, so the pressure in the reservoir falls, and without means to sustain it, the oil production rate would soon fall off dramatically. In order to sustain pressure water is pumped into the reservoir, even as the oil leaves, with the idea being that the water not only helps to sustain the driving pressure in the formation, but that it also helps displace the oil and move it towards the production wells.


Figure 3. Reservoir pressure and recovery factors in two Iraq fields, before and after water injection began (IEA )

Iraq has the same upcoming problem, if it is to increase production to the levels projected. This is not to question the size of the resource available, rather it is to inject a note of caution into assumptions that this oil will soon appear on the world market.


Figure 4. Iraqi oil resources by region and super-giant field (IEA )

The above table does not reflect the potential from future discoveries and developments in the country. Geophysical surveys have found 530 potential prospects, of which only 113 have been drilled, with oil being found in 73 of the wells. However, over the next twenty years it is likely that the majority of production will come from Rumaila, West Qurna, Zubair and Majnoon.

But the scale of this increase in production will require large volumes of water, estimated at 1.5 barrels of water for every barrel of oil produced. Given the growth in production this will require, in time, a flow of up to 12 mbd of water into the fields. This cannot be fresh water, since the country already has some problems supplying domestic needs, but rather must be seawater, supplied from the Persian Gulf. Plans for this, through the Common Seawater Supply Project, have been in the works for a number of years. Yet it was only recently that the management contract for the Project was signed. The original plan was to have the water flowing from the Persian Gulf by the end of this year. At present the goal remains to have 2mbd of the water available by 2017 which should technically be feasible, given the relative simplicity of the technical needs. However the delays that have already been caused by political and bureaucratic problems are likely to persist. As an example the initial announcement concerning the management contract was made last October, but it was not confirmed until this March. Tenders for the design were to be issued in April, but this is already a year late for the 120 km pipeline and distribution network that will be required. In addition the drilling rigs in the country will increasingly have to also drill water injection wells to match the production wells and even surpass it to provide enough pressure in the ground.


Figure 5. Number and types of wells needed in Iraq for the “Central Scenario” of production that the IEA projects. (IEA)

The IEA notes, euphemistically, that it will be “a considerable challenge” for Iraq to find sufficient rigs and crews to achieve production levels much above the numbers required for the above scenario in the years surveyed.

And there remain the problems relating to infrastructure. Iraq is finding some difficulty in constructing new refineries within the country, and the infrastructure required to move oil, once produced, to ports where it can be shipped to customers is also lagging behind initially projected schedules. The Oil Export terminal is being expanded, with new single point mooring systems being located roughly 120 km offshore, and raising the ultimate loading capacity from 1.8 mbd to 4.5 mbd.


Figure 6. Actual and proposed infrastructure in Southern Iraq, showing the single point moorings. (IEA)

Yet the slow pace of contracting, and the other problems that the country faces make it hard to remain optimistic that even the targeted production of 6mbd that the IEA projects for 2020 is likely to be achieved. And if Iraq is unable to meet the production projections on which the balance of supply and demand has been predicated, then the world may be in trouble faster than currently projected.

Waterjetting 10a - Beginning to cut

In the last few posts I have been discussing what happens under a water jet as it first hits, and then penetrates into a target material. In many cases it is recommended that the nozzle move slightly relative to the target during this piercing process so that the water escaping from the developing hole does not have to fight its way past the succeeding slug of water entering the hole.

Now it might be thought that this problem would go away if the nozzle starts at the side of the target and then cuts into it. But this depends on a number of different factors, one of the more critical being as to whether the jet is cutting all the way through the material, or is only cutting a slot part of the way through it. As with a number of other topics, I am going to illustrate some of the concerns using granite as the target material, since it makes it easier to demonstrate some of the points I want to make.

If you were to look at one of the many statues that have been carved from granite over the thousands of years since the rock was first shaped into an art form, the rock usually appears as a relatively homogeneous material. That means (to those who don’t work with rock) that the rock has the same properties regardless of which direction you test them in.


Figure 1. The Italian Carver’s Memorial, Dente Park, Barre, VT (From the Barre Granite Association via State Symbols USA)

However, if you were to ask a skilled quarry man he would tell you differently. Because of the way that granite cools from the molten state in which it is injected up into the ground, it picks up an orientation to the crystals, as they are formed. One of these orientations is roughly horizontal, and called the Lift or grain of the rock. A second is perpendicular to this, and vertical and is known as the Rift. The third plane, orthogonal to the other two is called the Hard-Way, because it is generally more difficult to work. These names relate to the ways in which the grains of the rock, and the cracks around them, align. They are virtually impossible for a lay person to detect, and a quarry man may need to feel the rock to tell you which way they lie. But they are used in splitting out the major blocks from the granite massif, and come into play in breaking the large blocks down into handle-able sized pieces.


Figure 2. The A) Hard-Way B) Rift, and C) Lift planes of crystal orientation in granite

If, however, you were to shoot a short slug of water at high-pressure at a piece of granite (and we used the granite from Elberton in Georgia for this) then, depending on which direction the pulse came from relative to the three planes, the amount of rock that would break around the impact point would change.

In an earlier post discussing the splitting that occurs when pressure builds up within the cavity under a jet, I mentioned that the pressure would grow cracks that already existed. And it is for this reason that when the jet impacts perpendicular to the existing crack planes, that the volume of material broken out is greater than it is where the jet fires along the cracks. This can be shown using the cavity profiles from oriented samples into which the jets were fired.


Figure 3. Profiles from the cavities created around the impact points where the jet impacted granite blocks at different orientations.

One can use this information if, for example, one wanted to cut a thin line in granite, where the cut should be made in the direction of the crystals, i.e. making cuts along the lines shown in the A plane of figure 2.


Figure 4. Linear cuts into granite along the lines shown in Figure 2.

In this set of cuts the jet is cutting along the favored orientation of the crystals, and the rock only spalls when two jet paths approach each other in the lower right of the block.

If, however, the cuts are made in a direction perpendicular to the orientations, i.e. in the B and C planes, then the results are quite different.


Figure 5. Cratering along the linear passes in cutting granite perpendicular to the Rift plane.

Where the jet strikes perpendicular to the Rift or Lift, then the pressurization under the jet is enough to cause those cracks to grow out to the surface, and cause spallation along the cut. In many cases this removes all the rock between two adjacent passes, even if they are more than an inch apart.

If one is to use the high-pressure waterjet system for slotting granite in a quarry, for example, then this can be a very useful tool, since by merely putting two jets on either side of the desired slot the spalling will remove the material between them, without any further jet action. If the jets attack in the perpendicular plane, then the jet has to be rotated over the cut to get the same material removal rate.

In most cases, when cutting in a quarry, because the rock does vary in structure, and grain size, it is better to ensure that all the rock is removed before the nozzles move into the cut, by rotation, but in smaller applications, such as where the excess rock is being removed around a planned sculpture, then enhancing the spall around the impact point can lower the time and amount of energy required in removing unwanted rock.

That is, however, a relatively specialized application, and in most cases it is desirable that the cut be clean, and smooth, and this requires the use of abrasive in the waterjet stream, and so this will be the topic of the next few posts.


Figure 6. Cutting through one inch thick glass, showing the cut through the side of the glass.