Tech Talk - Oil Supply, Oil Prices and the Kingdom of Saudi Arabia

From the time that The Oil Drum first began, and through the years up to the Recession of 2008-9 there was an increase in the price of oil, and that resumed following the initial period of that recession, and, in contrast to the price of natural gas, oil has recovered a lot of the price that it lost.

Figure 1. Comparable price of oil from 1946 (Inflation data)

And if one were to draw a straight line on that graph from the low point in 1999 though now there hasn’t been a huge variation away from the slope of that line for long. That, of course, does not stop folk from pointing to the very short, roughly flat, bit at the end and saying that oil prices are going to remain at that level, or are even about to decline.

To address that final point first, I would suggest that those making such a foolish prediction should go away and read the OPEC Monthly Oil Market Reports. Remember that, for just a little while longer, oil is a fungible product. OPEC make no secret of the fact that they continuously examine the global economy and make estimates on how it is going to behave. This month they note that the economies aren’t doing quite as well as expected, and have revised down global growth to 2.9%, though they expect next year to be better, and hold to their estimate of a 3.5% growth rate.

But OPEC go beyond just making that prediction, they use it, and data that they have on consumption and oil supplies around the world, to estimate how much OPEC should produce each month to balance supply against demand, so that the price will remain at a comfortable level for the OPEC economies. And based on those numbers they tailor production.

This month, for example, they note that global oil demand is anticipated to grow by 0.8 mbd this year (and by 1.04 mbd in 2014). They anticipate growth in production of around 1.0 mbd from the non-OPEC nations, with projected increases from Canada, the United States, Brazil, the Sudans and Kazakhstan contributing to an additional 1.1 mbd next year. From these numbers they can project that demand for OPEC oil will be slightly down this year, at 29.9 mbd down 0.4 mbd on last year, with next year seeing an additional fall of 0.3 mbd on average.

Figure 2. Projected oil demand for 2013 (OPEC MOMR )

Thus slight reductions in production from OPEC, and particularly the Kingdom of Saudi Arabia, (KSA) can keep the world supply in balance with demand and more critically for them keep the price up at a level that they are comfortable with. Note that in relation to the overall volumes of oil being traded they are not talking much adjustment in their overall volume (around 1% of the total 30 mbd) in order to sustain prices. The USA produces more, OPEC produces less – not much less because global demand is growing – and the price is sustained.

This has virtually nothing to do with the speculators on Wall Street and the corrections they might impose, this is all about supplying a needed volume to meet a demand and controlling that supply to ensure that the price is sustained.

There are a number of caveats to this simplified explanation, one being the short-term willingness and ability of some producers to keep to their targets. One of the imponderables is the production from Iraq. Although Iraq has been given a waiver through 2014 on the need to limit their production, the increasing violence has led to a drop in production, back below 3 mbd.

Figure 3. OPEC production based on data from secondary sources (OPEC MOMR)

As I have noted in the past, OPEC is sufficiently suspicious of the reported numbers from the countries themselves that they check from secondary sources, and provide both sets of numbers.

Figure 4. OPEC production numbers from the originating countries. (OPEC MOMR August 2013)

Note, for example, that Iran says that it is producing over 1 mbd more than other sources report, and Venezuela is around 400 kbd light. The balancing act is largely the charge of KSA, since it produces the largest amount and can adjust more readily to balance the need.

One of the other caveats is that the internal demand in these countries is rising, and that lowers the amount that can be exported. This will in time require that OPEC produce more, just to sustain the amounts that they export. And the problem here is the biggest caveat of all. Because KSA cannot continue to produce ever increasing amounts of oil.

Just exactly how much the country can produce is the subject of much debate, and has been at The Oil Drum since its inception. But if I can now gently admonish those who think it can keep increasing forever, and that it has vast reserves that can flood the market at need. This fails to recognize that the major fields on which the country has relied are no longer capable of their historic production levels, and that, over the time that TOD has been in existence, production has switched to the new fields that KSA had promised it would, back in time.

But these new fields, including Manifa and Safaniya produce a heavier crude that, for years, KSA struggled, usually in vain, to find a market for internationally. It is only now that it is building its own refineries to process the oil that it can find a global market for the product. Yet those refineries have only a limited capacity. If you can’t ship, refine and market your product in the form that the customer needs, it can’t be sold, regardless of how much, instantaneously, you can pump out of the ground. And so KSA is starting to look harder for other fields. They have increased the number of rigs employed to 170 by the end of the year (in 2005 they had about 20 oil and 10 gas rigs operating), going beyond the 160 estimated earlier, seeking both to raise production from existing fields, but also to find new ones. This is almost double the number that Euan reported at the end of last year. That this is being expedited is not good news! Because new fields will very likely be smaller, and more rapidly exhausted, and may not have the quality of the oil produced from Ghawar and the other old faithfuls.

Realistically, over a couple of years, I would suspect that the oil price line, that I mentioned was rising at the beginning of the piece will continue to rise and we are just going to have to accommodate to it.

Rowing the Northwest Passage - Part 2

The rowers that I have been watching as they seek to traverse the Northwest Passage are currently making good progress and have just crossed from the Northwest Territories into Nunavut.

Figure 1. Position of the Arctic Joule on Saturday evening, August 10, 2013.

What I had not realized is that they are not alone in this sort of venture. Coming just behind them now is a Norseboat, Fairmont’s Passion, in which just two men are using both oars and sail to try roughly the same passage.

Figure 2. Position of Fairmont’s Passion on Saturday evening, August 10, 2013

At present they appear to be less than 30 miles apart, with the laggard moving a little faster at present, due to the wind.

The ultimate path of the Beyond the Circle team ends in Resolute rather than Pond Inlet, so it is a shorter trip, but already one with some adventure.

Figure 3. Beyond the Circle Route.

The adventure came last Sunday when the boat entered Amundsen Bay and encountered stiff winds and pack ice. At that point they met the yacht “Anna” which was sailing from Alaska to Greenland, having made the reverse trip several years ago. After meeting yet a fourth boat, this one going the other way that warned of ice, Anna and Fairmont’s Passion struggled through that ice in Franklin Bay while sailing through a thick fog.

They are now out of the ice, and enjoying beautiful weather, making 30 miles in the last day.

Let’s hope that they have good fortune!

Waterjetting 12A - The heat involved in cutting - Part A - Cutting Rock

As I was beginning to write these posts Bob Pedrazas, who is kind enough to transcribe these words over to the KMT Waterjet page, gave me some questions that had been asked about the technology of high-pressure and ultra-high pressure water jet cutting. At the time I gave an academic answer, pointing out that I would have to explain some background material, before the average reader might be able to follow the logic of some of my answers. Thus I did not plan to answer specific questions (though I would respond to comments) in the early days of the site.

It is almost a year since I first began putting these posts together. Along the way I have tried to answer some of the questions on his list without specifically calling out the question – for example the answer to the question as to whether selecting the right system was important was, I hope, shown by a plot early in the series. I presented a comparative graph that showed that despite different systems having nominally the same power, water and abrasive values, when comparative cuts were made, in similar materials that there was a considerable difference between the depths of cut that could be achieved using the different designs. (So obviously selecting the best system has considerable benefit to the operator, over selecting another).

One question that was raised relates to the heat of the cutting process. And while it has a relatively simple short answer (waterjet cutting used to be sometimes called “cold cutting”) I am going to take a few posts to explain the answer in a little more detail. Part of the reason for this is that the information that I have comes from several sources, some in cutting rock, and some in cutting metal, and there are some different applications along the way that all fold into the general topic of heat in the cutting process.

Let me begin with the work of a friend of mine, Mike Hood, who was working in the South African gold mines at the time he decided to go for his doctoral degree (Hood, M. (1978) "A Study of Methods to Improve the Performance of Drag Bits Used to Cut Hard Rock," Ph.D. thesis, University of Witwatersrand, R.S.A."). To understand the problem that he addressed you should know that in some of the mines in South Africa the gold-bearing rock is contained in a very thin layer, within a surrounding host rock, which is a quartzite and very hard to cut.

Figure 1. Drilling the rock around a gold seam in South Africa (New Scientist )

However if miners are to get in and extract that thin vein (which is often only six-inches to a foot thick they have to drive passages that are big enough to work in (perhaps six-feet high). Thus rock on either side of the thin vein of gold-bearing material is drilled and then the entire rock face is blasted out using explosive. Now the gold ore is mixed in with all the other rock from the blast. This means that all that material must be lifted perhaps two miles to the surface, and then ground to a fine powder to release the gold. Both of these are very energy intensive operations.

Consider instead if, before the rock on either side of the vein was blasted out, the face could be cut with two slots, one above and one below the gold reef. That could then be removed, and the rock on either side could then be blasted, but instead of being hauled away it could be packed into the open space behind the working area, holding up the roof and saving a huge amount of the processing energy otherwise required. (There is less than half an ounce of gold in a ton of the reef ore, and when the rock on either side is included then this concentration becomes much less).

Figure 2. Design of cutting tool to carve a channel into the quartzite (Mike Hood )

Mike was initially looking to used carbide cutting teeth to cut into the rock and make these slots. However, he rapidly discovered that as he dragged the bit across the rock, that even at relatively shallow cutting depths (about 2/10ths of an inch) the cutting tool was getting very hot very quickly, to the point that the carbide was starting to melt.

Figure 2. Temperatures building up on the carbide cutting tool (after Hood ibid)

Obviously, if the bit could be kept cool, then the carbide would not soften, and thus remain sharp and able to cut better. Yet the temperatures that the bit was reaching very early in the cutting process meant that there was a lot of heat being generated during the cutting.

As a result he decided to run a series of tests in which he played water onto the leading edge of the bit to cool it. But because of the heat involved he wanted to get a fairly high flow rate to the bit, which was almost buried in the rock, and otherwise hard to get to. So to ensure the flow got to the right place he used higher-pressure waterjets that flowed through small nozzles, mounted to shoot the jets at different points on the carbide:rock cutting face.

Figure 4. Location of the jets on the carbide bits in the initial tests carried out by Dr Hood. ( hood Dissertation )

There are two forces needed to make the drag bit cut into the rock as it moves forward. The first of these is the Thrust Force, which is the force which pushed the tool into the rock, so that it will cut to the required depth. The second is the drag or Cutting Force that is used to pull the bit along the face at the required depth.

Without the waterjets on the bit, the load on the machine was exceeded when the drag bit was cutting to about 5 mm deep (2/10ths of an inch) into the rock. But when the waterjets were added to the bit, not only did the bit stay cool, but it was able to cut more than twice as deep, at lower forces onto the bit, and thus with a lower power demand on the machine.

Figure 5. Change in thrust force when waterjets are placed in front of the drag bit (Hood Dissertation)

Figure 6. Change in cutting force when waterjets are placed in front of the drag bit (Hood Dissertation )

This result has since been repeated by a number of different laboratories around the world, and led on to the development of mining machines, and other applications.

In the next post I will explain why what happens does, and why adding such jets to a cutting tool can, in the right place, save considerable amounts of money and time.

Tech Talk - A cautionary tale evolves over shale gas

The development of the shale gas deposits in the United States, led by the drilling and fracking of horizontal wells into the Barnett Shale of Texas at the turn of the century, has opened up a resource that continues to draw visions of American energy independence from a number of commentators. The success of the development in exposing a potential resource that has been found in a number of states around the country continues to underwrite optimism for the short-term energy future of the country. In turn it has led to projected dreams of enhanced domestic supply in some of the countries of Europe, and the rest of the world.

However, from that highly promising beginning things have not gone that well (depending on your viewpoint). In part this is because of the overwhelming success of the industry in finding productive formations. As production spread through the Haynesville to the Marcellus increasing volumes of natural gas fed into the market, which had, at the beginning, only a certain limited capacity to absorb the increased flow rates. As a result natural gas prices, which had been bringing a comfortable profit to companies, rapidly fell.

Figure 1. Wellhead price for natural gas in the United States (EIA )

The collapse in price from the high of $10.79/kcf was related to the developing recession, but while the price of oil rebounded, the domestic price of natural gas has not.

Figure 2. Spot prices for natural gas around the United States in August 2013 (Boston Globe)

Because of the current cooler weather in parts of the country, gas prices for September are down to $3.35/kcf.

This was one of two contributing factors to the problems that the domestic industry is undergoing, both foreseen by Art Berman, an accurate, albeit in some quarters very unpopular, prophet to the industry. The first problem, clearly is the low price that natural gas continues to sell at in most of the country. (The high prices in the NorthEast are because of the perception that the fuel is abundant, set against the current limited capacity of pipelines to carry sufficient gas into the region.) The second problem, which Art has also clearly identified, lies in the very rapid decline rates that are seen in the natural gas wells that have been drilled and fracked in these shales.

The Barnett shale has now been sufficiently well developed that production has now peaked and is recognized to now be in slow decline, though that rate is projected to accelerate.

Figure 3. Projected decline in Barnett shale production (assuming continued drilling). Note the steep decline rates for new wells.(Bureau of Economic Geology)

There is also sufficient information such that, for example such industrial stalwarts as the Oil and Gas Journal are confirming some of Art’s earlier predictions. A current article examines the economics of the Barnett shale development under DFW airport, first undertaken in 2009 following a lease agreement in 2006. The article examines the economics of the operation in which the airport has made over $300 million. Chesapeake, who have drilled 110 wells on the property, out of an anticipated 330 originally projected, has recovered some 104 bcf, but the undiscounted all-in cost is calculated at $7.21/kcf. While that was viable in 2008, when gas prices were north of $9.00/kcf it becomes quite a burden when they are down around $3.35/kcf. The loss to the operator is reported to be more than $300 million.

These relatively poor returns on investment might not be so disproportionate in Europe where there have been potentially similar shale deposits found in countries such as the UK and Poland. Art has however already reviewed the British Bowland shale projections, suggesting that only 3% of the natural gas estimated to be in place may be recovered. While this is still a respectable number it falls short of the more optimistic numbers that, among others, Bishop Hill has collated. And while Bjorn Lomborg has estimated that this could bring as much as $10 billion a year into the British economy by 2020, this is all at a point where Egdon hopes to sink its first well into the formation at the end of next year. Cuadrilla Resources drilled their first well last year and now plans a six-well exploration program, but it has been suggested that it will be at least a decade before any significant gas production is available.

In this regard it is well to remember the case of Poland, where the presence of gas-bearing shales led to predictions that the industry would “transform Europe.” But then the results of the well tests came in, and the volumes of natural gas available were reduced by up to 90%. Although 40 wells have been drilled, to date none are reported to have produced commercial quantities of natural gas, although in only four cases was there fracking of the horizontal well section. Three major oil companies have backed away from committing to the program, and that initial enthusiasm is now considered to have been a “bubble,” while Chevron is seeing more protests over their planned fracking tests.

This collapse of hope for a potential resource has led the Polish Government (who are responsible for ensuring that the country has enough fuel at a viable price) to move back toward the greater use of lignite as a power source. Coal is the fuel for the power stations that produce 90% of the country's electricity, and lignite is readily and cheaply available. Prices for lignite (a brown coal that is softer and not yet fully geologically morphed into the harder black coal most envisage when coal is discussed) are quoted as giving a price of $2.00 per gigajoule, roughly a fifth of the cost for black coal. The country has large deposits of lignite, much of which is available for surface mining, at relatively low financial cost.

The government decision underscores a point that I have made a number of times, namely that as other fuel costs increase more and more countries will move to the use of coal, where it is domestically available and relatively inexpensive in financial cost, to produce.

Waterjetting 11d - AWJ and cut taper

I have spent some time in recent weeks discussing the use of abrasives in waterjet cutting, and particularly some of the issues that are involved in getting the abrasive distributed relatively evenly through the jet stream, and accelerated to as high a velocity as possible by the time the jet leaves the focusing tube.

This issue has become more important as clients request more precise cuts, and edge quality and alignment become more critical. As the jet cuts along a surface the amount of material that is removed (i.e. the depth of cut simplistically) is controlled by the number of particles that impact along that axis. And that, to a degree, is controlled by where that axis lies, relative to the axial diameter of the jet that runs parallel to the direction of cut. Different conditions give different particle densities, but even within those conditions, the material under the center of the jet will see many more particle impacts than those on the side.

Figure 1. Particle distribution across two abrasive waterjet streams with the same focusing tube diameter, but different waterjet orifice diameters (Mazurkiewicz, M., Olko, P., Jordan, R., "Abrasive Particle Distribution in a High Pressure Hydroabrasive Jet," International Water Jet Symposium, Beijing, China, September, 1987, pp. 4-1 - 4-10.)

As the above figure shows, in order to achieve the best abrasive cutting the rate of abrasive feed must be tailored to the nozzle size and the jet parameters. The density of the abrasive in the resulting stream can be optimized for those conditions and, as discussed in earlier posts, adding too much abrasive to the system will end up being counter productive.

A simple example can show this, in a test where we cut grooves in a block of granite, with the concentration of abrasive in the jet stream increasing with each pass.

Figure 2. Cuts made into a granite block, with abrasive feed rate increased as the cuts progressed from the left-side of the block to the right. Note that beyond a certain AFR the depth of cut begins to decrease. (Yazici, Sina, Abrasive Jet Cutting and Drilling of Rock, Ph.D. Dissertation in Mining Engineering, University of Missouri- Rolla, Rolla, Missouri, 1989, 203 pages.)

There is, however, a second consequence to the concentration of particles across the jet, and that is that the material under the jet on either side of the center-line of the cut will see a smaller number of particles impacting the surface, than that at the center. As a result the material will not be cut as deeply, and as the slots shown in Figure 2 illustrate, the cut will, as a result taper in on both sides.

In many applications, where the material to be cut is relatively thin, or where the exact alignment of the edge is not that critical this may not be important. However there are applications where edge alignment is required on the order of a thousandth of an inch or two over the part thickness, with the part being half-an-inch or more thick.

One way to achieve that precision of cut is to slow the traverse speed down. If the jet is moving slowly enough then there will be enough particles hitting the material at the edge of the cut, that the edge will be cut vertically downwards.

Figure 3. The effect of traverse speed on the edge taper angle (in degrees) in cutting titanium.

Notice that, because the jet tends to flare out a little as it moves away from the nozzle, the taper angle goes negative if the speed falls to too low a value. In this particular case the nozzle was moving across the surface at a speed of about quarter-of-an-inch per minute.

To get enough particles on the sides of the jet to cut a parallel slot edge, however, means that much of the abrasive in the center of the jet is not doing any work, but is rather being powered up and paid for to no real advantage. Thus, in most cases, (though not all) cutting very slowly to achieve precision on the residual edge of the cut is an overly expensive way of achieving the precision.

Given the relatively small angle that the taper cuts it is usually more cost-effective (providing the table allows this) to slightly tilt the cutting head, so that at higher cutting speeds the taper is effectively removed on the edge that is left. Obviously the taper on the piece of material being removed is made worse, but if that removed piece is going to be cut later into a different shape for another purpose, then this excessive taper on the initial surface comes with no great cost.

The taper angle and the speed relationship will vary both for the material being cut, as well as for the different parameters of the abrasive waterjet, and so – as with most cases where this sort of precision is required – a small test program to establish the best parameters for the cut will be needed.

There are other ways of achieving this precision in cutting. One is to make multiple passes over the surface, with the jet removing only very small increments of material at one time. Again if this is carried out carefully and precisely the edge quality can be maintained, at the same time as the depth of cut can be well controlled allowing pockets of material to be removed from the work piece.

However that gets into the whole issue of milling material from a target, and that is the topic for another day. It brings up the inter-relationship between traverse speed and depth of cut (which combine to give the area of cut surface, which can be used in some cases to optimize the cutting performance of a system, particularly where edge quality is not that rigid a requirement). And more particularly it brings up the quality of the walls and floor of the pockets created.

Figure 4. Factors to be considered in milling a pocket, illustrated by a multi-level pocket created in glass.

Crossing the Northwest Passage by Boat

I have always been fascinated by those willing to prove their point by actually going out and doing something. It does not necessarily mean that I agree with their point, but one has to recognize their willingness to put themselves in danger to validate it. Thus I have been caught into watching the travails of the four folk who are currently trying to row through the Northwest Passage.

Figure 1. Planned passage of the Mainstream vessel “The Arctic Joule.”

One of the wonders of modern technology is that the passage of the four men in a rowing boat can be updated, at intervals as short as ten minutes, as the crew battle across the northern coast of Canada. Their goal is to row from Inuvik at the Mackenzie River delta to Pond Inlet, across the coast from Greenland.

The boat is an especially designed rowing boat where the four men take shifts of 2 rowers at a time (on up to 4-hour stints in good weather, 20-minutes in bad) to move along the route.

Figure 2. Boat Movement on Friday.

The reason for my interest in this apart from monitoring the passage of folk through truly dangerous conditions where they are navigating through ice-laden waters in fog, and with the wind rising, is that they are seeking to show that boats can now make the passage. What I found particularly of note in their recent passage is that, on Friday, they dropped in on a group of beluga whale hunters.
Note this post has been updated twice, with the latest (saturday night) raising the question as to whether the row is being abandoned. (Later) However, though they did backtrack for a while, they later resumed their voyage.

It is particularly interesting in that the visit to the whale hunters relates back, a thousand years to when the Inuit made the same passage from Alaska chasing the bowhead whale. (It should be recognized that this reason for the move has been challenged). However the current tradition of hunting the beluga whale seems to overcome many of those objections.

The more prevalent view that the Thule Inuit moved across the northern coast of Canada chasing the migration of the bowhead whales is evidenced by the use of whale bones in providing the rafters for their dwellings.

They arrived in Greenland as one of the waves of immigrants that came in concert with the warming periods (earlier groups had arrived during the Roman Warming Period – the Dorset- and the preceding Minoan Warm Period).

Figure 3. Passage of the Thule Inuit from Alaska (Canadian Museum of Civilization )

They were thus in place and the succeeding arrivals fought the earlier arrivals and the Vikings who showed up around 930 AD and who considered the natives to be skraelings.

Both of these arrivals suggest that the Northwest Passage was more open, and freely accessible in those earlier warming periods that it remains today,

The current difficulties in terms of wind and ice floes are making it hard for the current team of four healthy men to make the passage, and they have had to drag their boat for a number of miles already, rather than face the conditions offshore.

Consider then that a thousand years ago the tribes brought their families with them, and made the long migration using the umiaks of the time. The archeological evidence mainly relates to the passage of the Thule Inuit, but the experience of the current team, though praiseworthy in its intent, does perhaps hint that conditions were somewhat easier a thousand and two thousand years ago, in what might have been the warmer waters of those times, when something other than rising carbon dioxide levels warmed the Earth.

UPDATE (Saturday 12:38 pm) - There is now a video up that shows that they are stuck on a beach surrounded by ice and with heavy seas, so that they are waiting where they are until the conditions improve. This has happened before and is putting them well behind schedule.

Figure 4. Boat onshore surrounded by ice. (Mainstream Last First)

UPDATE 2 - 11:13 pm Saturday. I wonder if they have perhaps given up - since the recent track tonight shows them heading back along their earlier route.

Figure 5. Boat track as it appeared on Saturday night (Mainstream Last First)

UPDATE 3: No they were just getting some headroom, and the voyage is now continuing, although it looks as though they are being driven back by a tide or some local condition, after making good progress.

Figure 6. Boat position as shown at noon on Sunday.

Waterjetting 11c - Mixing abrasive with a water jet

Sometimes I would get the feeling (particularly when talking to some of my students) that the mixing chamber of an abrasive waterjet (AWJ) cutting system, together with the feeds in and the focusing nozzle outlet, were considered to be some magical black box out of which a perfect cutting stream issues to cut the desired material.

There are, in fact a variety of different chamber designs that can be purchased from different manufacturers. Some will tell you that all designs cut roughly the same, and that there is little difference between them. As I commented in one of the earliest posts in this series this is not true. Over the years we have run numerous comparative tests on different designs, using different abrasives, abrasive feed rates and target materials, and have found a broad range of results. For example, in cutting steel at a fixed traverse speed and other conditions, we found an average comparative performance as follows:

Figure 1. Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I described the actual test in another post, and it is clear from these averaged results that there is a wide difference in performance between the nominally similar tools.

While I don’t think there is a lot of interest in going through the details of different designs it might be helpful to explain some of the factors that play a part in producing jets of greater or lesser performance.

To return, first, to the basic construction of the mixing chamber and focusing tube assembly (AWJ nozzle), one starts by recognizing that the major cutting performance will be achieved by the particles which remove material when they hit the target. The energy that they have, however, comes from the water that is fed into the AWJ nozzle through a small jeweled orifice, or waterjet nozzle, at the top of the mixing chamber.

Figure 2. Basic nozzle design

That waterjet stream is small and initially highly focused and fast moving. As it moves through the mixing chamber, as I have described in other posts, the outer edges of the jet slow down, and gradually the jet fans out and breaks up into fragments.

There is no benefit in trying to inject the abrasive into the jet at the beginning of this passage, since, at that point, the outer layers of the jet have enough energy to knock away the particles before they can enter the fastest moving segment in the core. Rather it is better to inject the abrasive further down the chamber, so that the jet will have begun to break down into slugs, and the abrasive can be positioned so that it is impacted by a sequence of the individual slugs and accelerated to the desired velocity.

There is an additional benefit to moving the abrasive feed line a little further down the chamber. When the jet stream is rapidly switched on and off, when for example, piercing a series of small holes in a part, then the driving pressure pushing water out of the waterjet orifice switches off and on. When it switches off there is a short period where the differential pressure will draw fluid from the chamber back through the waterjet nozzle. If there are small particles of abrasive in the vicinity (and with some designs there are) then these can be drawn back through the upper orifice, and then pushed back down by the succeeding water flow in the next pulse. This can rapidly erode softer jeweled orifices, so that they round or chip, not always evenly, and degrade the resulting waterjet as it flows into the chamber. This disruption can move the jet from being in the center of the chamber, and cause poor abrasive pick-up or accelerate wear of the chamber walls, and in the focusing tube. All of these degrade performance. (The solution, if you can achieve it, is to use a diamond upper orifice, since this is largely non-responsive to the passage of the abrasive back and forth, and retains its shape and the jet performance it was designed to produce much longer – providing a cost benefit to the change).

Fig 3. Wear on a ruby waterjet orifice inserted at the top of a mixing chamber after 15 minutes of use. (The dark particles are small particles of garnet)

Figure 4. Chipping on the edges of sapphire and ruby orifices (after Powell 2007 WJTA Conference)

Fig 5. Lack of wear on a diamond insert nozzle after being in use for several hundred hours.

With the abrasive inlet channel and jet passage designed to get the abrasive into the jet where the water jet is broken up, yet still moving at high speed, there needs to be a sufficient distance for an optimal energy transfer to occur. Beyond that point, with the particle and the abrasive (which will have partially been broken in the contact between the jet and the particle, between particles striking one another, and between contact between the particle and the walls of the AWJ nozzle) the cutting jet has to be refocused into the narrow cutting stream that is required to give the finished cut surface desired.

The refocusing of the mixed jet (air, water and abrasive) is achieved with a focusing tube, which is made up of a conic section which brings the jet back together, and then a straight section which allows further energy transfer between the three component parts of the jet, before the jet issues from the orifice aimed at the target.

The passage of the particle down this tube is not always straight. Wear typically begins at the tip of the conic section as it feeds into the tube. The wear within the tube will often take up a pattern, as any irregularity in the flow causes it to bounce from one wall of the tube to the other creating a wear pattern along the walls that has a wave-like structure.

Figure 6. Wear at different points along the focusing tube

When this wear reaches the mouth of the focusing tube, then the downstream orifice is eroded out of a circular shape, and the jet that comes out no longer will cut cleanly, or to as great a depth. At that point the nozzle is worn out and should be replaced. The point at which that replacement occurs varies depending on the quality of the cut that is required. Obviously when the cut being made is at a precision of a thousandth of an inch over a cut depth of half-an-inch (as with some aircraft parts) the replacement point is reached a little earlier than if the cuts being made are a rough cut merely, for example, to separate two parts one from another, and provide a rough shape to the piece.

I’ll continue this topic in the next segment of this section.