Waterjetting 18c - Abrasive choices

Picking the right abrasive can make a big difference in the profit that a waterjetting operation makes. But the question, of course, is which abrasive is the best? And, as I have done in the past, I am going to hedge a little in my answer. The reason for this is that there are different factors that control the price of the abrasive – how far has it to be transported, how it was prepared and what it is made of – for example. And while some abrasives generally cut better than others, if the unit is only going to be cutting a narrow range of material, then the abrasive that is best for cutting a wide range of materials (garnet) may not necessarily be the best choice in that particular case.

Figure 1. Different types and sizes of abrasive particles.

And further, just to make life a little more complicated, not all garnet abrasive (or other types of abrasive product) are created to give equal performance. As I have mentioned in a previous post the cutting performance of the abrasive can be rapidly reduced as the particle size falls below 100 micron. Thus, if the particles have not been well graded, so that there is a significant amount of abrasive below this size (even though the vendor tells you that it is a 250 micron size) then the cutting performance will not be as good as it would be if the mix contained no fine fraction that far below the stated particle size.

Figure 2. The effect of particle size on cutting performance

On the other hand if particle sizes are too large, then for a given abrasive feed rate there will be a smaller number of particles hitting the surface, and this can also reduce cutting performance if carried too far.

Figure 3. The effect of increasing particle size on the cutting of cast iron at a constant abrasive feed rate (0.88 lb/min) (after Hashish*)

There are other factors that has also to be considered in selecting the best abrasive, and, while I am going to discuss this below, in most cases it is going to be something that you will have to test in your own shop, comparing the results that you get with the cost of the abrasive (often worked out in dollars per square foot of cut, or similar unit) to decide which gives you – in your particular circumstance – the best performance.

But the testing of different abrasives can be reduced to a manageable range of tests (and we prefer the triangle test that I have described in an earlier piece) if some basic thoughts are born in mind.

Decide what it is that you will be cutting – is it mainly going to be a metal that is going to respond in a ductile way when cut by the abrasive. In which case the abrasive should have enough sharp edges to cut into the material, and then to plow up some of the surface so that as more particles hit the surface, pieces are broken off.

Figure 4. Mechanisms for cutting into a metal or other ductile material.

On the other hand, if the material that you are going to cut is a brittle one, say for example rock or glass, then the material is going to be removed by crack growth. Here relatively spherical particles can be more effective because the energy of the particle is concentrated at the point of impact, and more easily causes cracks to grow. On the other hand relatively flat particles with multiple impact points reduce the pressure under any one and reduce the effectiveness of the particles in getting the cracks to grow as quickly and as long as possible.

Thus, for example, we can compare the effect of using the same amount of steel shot (round) and garnet in cutting granite (a brittle material) and tool steel.

Figure 5. Cuts in granite and tool steel using the same abrasive feed rate, but the cut on the right is with garnet and that on the left is with steel shot. Note that the steel cuts the granite to a deeper distance within the cut, while in the tool steel it bounces off without leaving much impression. (However harder steel grit on softer steels can be an effective choice).

The difference that particle shape makes in cutting ductile materials can be shown when the same AFR is used but in one case the abrasive is broken glass particles and in the other it is glass beads.

Figure 6. Effect of particle shape (broken glass and glass beads) in cutting composite material at the same abrasive feed rate (after Faber***)

Further some particles, for example steel shot or grit, can be recycled through the system a number of times without much degradation, so that if they can be simply collected below the cut, they can be profitably reclaimed, providing that the particles below 100 micron are separated out of the flow after each cycle.

Figure 6. Effect of recycling abrasive through a slurry abrasive jet system on depth of cut achieved (after Kiyoshige et al**)

Others however are more friable, this is perhaps more true of alluvial garnet, which often has a much higher density of internal cracks than mined garnet, and so is more liable to fracture either in the mixing chamber or on first contact with the target, so that the amount that remains above 100 microns is substantially smaller than that with the mined alternative.

Yet cost is a factor, and so it is best, for your material to test the different alternatives available, before making a final decision.

* Hashish, M., "Abrasive Jets," Section 4, in Fluid Jet Technology, Fundamentals and Applications, Waterjet Technology Association, St. Louis, MO, 1991.
**Kiyoshige, M., Matsamura ,H., Ikemoto, Y., Okada, T., "A Study of Abrasive Waterjet Cutting using Slurried Abrasives," paper B2, 9th International Symposium on Jet Cutting Technology, Sendai, Japan, October, 1988, pp. 61 - 73.
*** Faber, K., Oweinah, H., "Influence of Process Parameters on Blasting Performance with the Abrasive Jet," paper 25, 10th International Symposium on Jet Cutting Technology, Amsterdam, October, 1990, pp. 365 - 384.

Of temperature and coal supply predictions

It seems that while in my own professional life I had to change my opinion on several occasions, as new facts and experimental results showed that some our ideas were incorrect, that a willingness to change future projections of future energy supplies and their impact on the climate is less prevalent among the most quoted sources of opinion on these topics, even as the facts change over time.

This, of course, has some relevance to the many discussions that one might read on climate change, where model predictions usually assume that the same mix of fossil fuels will exist well beyond the year 2100, despite the future scenarios put forward increasingly by major oil companies that the mix will change and the role of oil and coal will diminish in the future. Part of this, in the case of the climate science modelers is perhaps because they have enough problems dealing with concerns over clouds and aerosols, without needing the additional needs to correct future prognostications for changes in the fuel mix that produces the greenhouse gasses of concern.

In this regard I was struck, in reading the testimony of witnesses at the Workshop on the American Physical Society Climate Change Statement Review about their inflexibility in changing future projections despite current data availability.

For example in the discussion following Dr. Santer’s presentation (page 255 line 18 and thereafter) it is noted that:

DR. KOONIN: But here, evidently IPCC needs to scale in order to match the observations; second, that many of the scaling factors are not consistent with one; they are smaller than one. The tightest ones are smaller than one. And there is a fair bit of variability in them. . . . But then I go to the centennial projections in chapter 12 and it says that, "The likely ranges do not take into account these factors because the influence of these factors on the long-term projections cannot be quantified." So, to me, it looks like they set a calibration against the historical data and then they wiped out that calibration in doing the centennial projections resulting in probably a 25, 30 percent over-prediction of the 2100 warmings. 1. . . . . . At least I conclude now from what I understand that the centennial scale projections temperatures are probably high? . . . By 30 percent, at least for RCP8.5 which is dominated greenhouse gases?

To which he did not get a responsive answer from those espousing the greater effects of greenhouse gases on the environment.

There is much in the discussion that is interesting, including the comment that if the hiatus continues for another 3 years then the predictive models of future temperature growth will have to be revisited, but then a time limit has been set before and then ignored after it passed.

It is, unfortunately, this illustrated inflexibility in the scientific mind that makes it difficult to predict the true likelihood of significant temperature increase within the next few decades, since the need for scaling of the greenhouse effect (perhaps down to 70% or lower) and the phasing out of fossil fuel use (in part because we are going to run out of oil) are both ignored by those whose opinions are most quoted in the press and by the government.

On the other hand there are those who predict that we will run out of coal much sooner than the industry (or myself) believes and predicts. The latest perhaps of these is the report by Leslie Glustrom “Warning: Faulty Reporting of US Coal Reserves,” for Clean Energy Action. The report is summarized as:

The belief that the US has a “200 year” supply of coal is based on the faulty reporting by the EIA of US coal deposits as “reserves.” Most US coal is buried too deeply to be mined at a profit and should not be categorized as reserves, but rather as “resources.” All decision makers should begin taking a hard look at coal cost and supply issues considering both geology and finance and begin thinking about scenarios that require moving the US beyond coal in significantly less than 20 years. Since coal is non-renewable, analyses should be based on recent trends—not those of the 20th century, which are not likely to be repeated.

This is actually an interesting point of view, since in my opinion much of the coal around the world is not even considered as a resource at the present time, let alone the reserve that it ultimately likely to become prior to use to solve the energy needs of countries that can no longer afford the higher prices of alternate fuels.

One of the problems that I have with the report is the lack of understanding of how incredibly simple and inexpensive it is to mine the surface coals of Wyoming and Montana. In essence large shovels just dig the coal out of the ground, put in trucks, transfer that to rail cars, and off it goes. It is hard to beat the simplicity and low cost of this operation. (Yes there are related costs for blasting and reclamation among others but it nevertheless remains cheap).

The assumption seems to be made that when coal seams get deeper, then the costs will rise rapidly. For the record I should point out that in the summer of 1963 I worked in the Snowdown Colliery of the Kent coal field mining a seam of about 6 ft thick IIRC at a depth of 3,000 ft. That coal seam stretched for miles, and was not the only coal within the stratigraphic column. It became too expensive to mine that, and the coal at other mines I worked in below 1,000 ft (and at seam heights down to one foot eight inches) because oil and gas in the North Sea became much more readily available at a lower price. Prior to their arrival the coal was a viable fuel. Thus after natural gas and oil begin to deplete the conditions will revert to those prior to the development of the North Sea fields. And, in the interim, there has been significant advance in the technologies that can be applied to extract the coal at lower cost – particularly with the increases in automation and remote control that are increasingly being used underground.

But I shall discuss this in more detail in a following post on the topic.

Waterjetting 18b - Abrasive Effects

Abrasive particles were first fed into jet streams as a way of helping to clean surfaces. In many cases the intent was to remove a surface layer of paint/rust so that the underlying surface would be clean and able to accept a new coat of paint or other protective coating.

However, as with many applications of waterjets and abrasive waterjets, putting too much power into a single jet stream could be counter-productive. As an example consider the case where an air-powered abrasive jet (sand-blaster) is used to clean paint from a surface so that it can be recovered. There are several concerns with the use of the abrasive that are not always immediately obvious to the operator. In an earlier post I wrote about the problems of surface deformation. With abrasive particles in the jet stream the operator often works to achieve a smooth, clean surface on the work-piece. The problem that this causes is that the abrasive particles bend over small protrusions on that surface, so that contaminant is trapped under the bent metal and accelerates corrosion after treatment. As a result an abrasive cleaned surface can need repainting after 2 years, whereas with proper cleaning the intervals can be stretched to 5 years. This latter period can be achieved with the use of high-pressure water as a cleaning tool, since this is able to wash into the small crevices in the surface and wash the corrosion products out of the surface, and leaves it clean.

The problem that this creates, in turn, is that it removes all the protective coatings and if the surface is a metal this can lead to flash rusting of the surface and other problems if the surrounding air is corrosive and surface treatment is not carried out fast enough. (In some chemical plants the period available for this coating can be less than an hour). In other cases, such as for example when Linda Merk-Gould cleaned the Statue of Freedom from atop the Capitol Building in Washington, other coatings are required to return the surface to the desired color and texture after cleaning.

Figure 1. Looking at cleaned test panels on the Statue of Freedom with Linda Merk-Gould, during the cleaning.

Further, in that particular case, the antique nature of the metal surface meant that the pressure of the jet had to be precisely controlled in a narrow range where it would be sufficient to remove the corrosion, and yet not sufficient to eat into the weakened metal surface. (That pressure range was IIRC about 3,000 psi).

The second problem that can arise with the use of abrasive on a surface occurs if small particles of that abrasive become lodged in the surface during the cleaning. While in most cases the small individual sizes of the particles have little effect on subsequent performance there are cases where this can be a problem. One is where the surface will be enameled after cleaning, and if steel is used, rather than an iron grit, in the final cleaning any residual steel on the surface can mar the final coating used in the process. Similarly in the advanced welding of structures any garnet or similar abrasive that is left in the surface an affect the integrity of the weld that follows. In both cases, as mentioned earlier a final wash with high pressure water without abrasive can be effective in cleaning the surface to the level needed.

This brings up the topic of the nature of the abrasive itself. In the past sand bas been the most commonly used abrasive, although in more specialized applications small-particle slag, walnut shells and other specialized abrasive is used. This is often the case where (as with the walnut shells) there is a need to minimize the damage to the surface being cleaned – and historically they have been used in cleaning bronze monuments, as a way of minimizing the loss in detail that occurs with harder abrasive, although as the experience in testing for the Statue of Freedom showed, full detail could be retained with the high-pressure water clean, while abrasive would blur the finer detail. Walnut (and other nut shells and parts) were chosen because of their relatively soft nature in contrast with the underlying material beneath the coating being cleaned. The use of softer abrasive, and in some cases soluble abrasive, helps preserve the underlying substrate and can lower clean-up costs, although the abrasive itself might be more expensive.

There is, however, a difference in approach when the surface will be left as cleaned when it is put back into service, and that where it is to be painted or similarly protected before being used again. The need is for the surface to be toothed, or notched, so that the overlying coating can attach to the surface and become harder to remove. (In passing this is why high-pressure waterjets are effective in concrete repair, since they only partially expose the aggregate and the new pour can attach to this rough surface, giving shear as well as compressive and tensile strength. Where the surface has been ground with diamond or carbide wheels the interface is smooth and there is a much lower adhesion between the layers, and the repairs, as a result, don’t last as long).

Figure 2. Microphotograph of a steel surface on the edge of an abrasive jet cleaning path, showing the surface deformation due to individual particles.

In these cases the shape of the particle, the size of the particle and its relative hardness all play a part in control of the final result of the treatment. However, the velocity at which the particle hits the surface is also a factor, controlled by the pressure of the delivery system. Too much pressure will, as mentioned above, bend over the surface protrusions so that the surface becomes smoother and more polished. While this has an aesthetic appeal at the time of treatment the rougher, greyer surface with the protrusions left in place gives the better grip. As Plaster noted in a table presented in his book on Blast Cleaning and Allied Processes, increasing air pressure above a certain value is counter productive.

Figure 3. Change in bond strength as the air pressure during sand-blasting is increased. (Plaster ibid)

I will discuss different aspects of abrasive choice in the posts that follow.

Tech Talk - The BP Energy Outlook 2035

BP begins its new forecast for the energy future with the statement:

We project that by 2035 the US will be energy self-sufficient while maintaining its position as the world’s top liquids and natural gas producer.

This illustrates the optimism which BP are projecting in their image of future production. But it carries with it a lot of inherent assumptions, some of which are relatively easy to identify in the summary graphic presentation that accompanied the initial presentation of the new report. Perhaps the most illustrative of their optimism is this plot, which shows the increasingly decoupled changes in energy supply relative to projected increases in GDP.

Figure 1. The reducing dependence on Energy growth as a control on GDP. (All figures are from the new BP Energy Outlook for 2035)

Each year there are significant projections for the future of energy over the next few decades. Recent posts have reviewed this year’s projections from the IEA and ExxonMobil. These projections, were also reviewed last year and those reviews included the previous BP projection although that only projected forward to 2030 – the current review has added five years to this.

The relative contributions of the different fuel sources to the overall mix have not changed appreciably in the past year. Oil is anticipated to continue to shrink in percentage contribution, and coal will also decline in relative contribution after around 2020. Natural gas and renewables are anticipated to make up the supply needed.

Figure 2. Relative contributions of the different fuel sources to overall global energy supply to 2035.

BP have made it a little easier to see how this breaks down by plotting the ten-year increments in fuel contribution as well as the overall totals.

Figure 3. Changes in projected fuel supplies over the period to 2035.

Changing the plot to show the ten-year incremental changes illustrates how coal, now surging as an international fuel source, is anticipated to decline beyond 2020.

Figure 4. Projected ten-year incremental changes in fuel supply through 2035.

Note that in overall total BP is projecting that global consumption will rise by 41% over today’s numbers, most of which increase will come from the rapidly-developing countries of the world.

Figure 5. Regional increments of energy consumption growth over the decades to 2035.

The reliance on the improvements in energy efficiency to stall further growth in energy demand from the OECD countries is evident in this picture.

BP notes that the decade from 2002 to 2012 saw the “largest ever growth in energy consumption in volume terms,” but anticipates that this rate will never be exceeded in the decades to come. And they anticipate that as Chinese growth fades in the decades, so the growth of the Indian and adjacent economies will almost match that of China by the end of the period. As the nations of the world complete their industrialization, so the growth in the demand for fuel will see a greater emphasis on transportation demands.

Interestingly the decline in the demand for coal that BO projects is linked to the completion of industrialization in China, and this assumption is, of course, predicated on oil and natural gas remaining available to meet the demand at a reasonable cost.

Figure 6. Anticipated primary sources for generation of electric power.

The projections for changes in liquid fuel supply are also relatively simply presented. First one can see the projected changes in demand, with the OECD countries declining, as demand increase seems to focus in the Eastern nations.

Figure 7. Anticipated changes in global demand for liquid fuels

It is where this growth in supply is to come from that is of the greatest concern, and BP suggest the following:

Figure 8. The anticipated sources for growth in liquid fuel supply through 2035.

BP note the largest sources of these gains as being:

The largest increments of non-OPEC supply will come from the US (3.6 Mb/d), Canada (3.4 Mb/d), and Brazil (2.4 Mb/d), which offset declines in mature provinces such as the North Sea. OPEC supply growth will come primarily from NGLs (3.1 Mb/d) and crude oil in Iraq (2.6 Mb/d).

One of the more interesting plots in the report shows how, over last year, the changes in US production more than compensated for the declines in production from the MENA countries.

Figure 9. The ability of increased US production to balance declines in production from the nations in turmoil in MENA.

BP anticipates that continued US increases in production will more than balance the anticipated increases in global demand, so that the continued disruptions will not significantly affect global supply even though, as they have historically, they extend for more than ten years. The US gains are anticipated to continue to such an extent that OPEC will be required to rein in their supplies in order to sustain global prices.

Figure 10. Changes in the demand for OPEC oil and the result on their production reserve capacity.

One anticipates, given that KSA has said that they will not increase overall supply much above current levels, that the increases in production that BP anticipate will likely come from Iraq, and Iran if the sanctions are lifted. Given the current situation in those parts the latter seems increasingly more likely than the former. Further BP note that the increasing populations in these countries and their consequent increases in demand for energy is likely to constrain the levels at which these countries can continue to export.

In conclusion, and to justify the heading at the top of this piece, BP anticipate a continued growth in US oil production such that, by 2035 imports are virtually eliminated, being more than offset by the gains in the export of natural gas products. BP anticipates that the latter will increase by 2025 to around 12 bcf/d and continue at about that level.

Figure 11. BP projections for changes in the US oil supply sources for the period to 2035.

Waterjetting 18a - air and abrasive

As high-pressure waterjet systems have continued to expand into broader fields of application, that increased range has been significantly expanded where abrasive has been added to the jet stream. Abrasive waterjets are more widely used in cutting those materials that are less easy or practically impossible to cut cleanly with water alone (although that is a relative statement, since metal, for example, can be cut at higher water pressure without abrasive).

But before there was high-pressure abrasive waterjet cutting there was sand blasting and other applications of abrasives existed in cutting and material removal – think, for example, of sandpaper. The first “powered” use of sand to remove material has been credited to B.C. Tilghman Jr. of Philadelphia whose British Patent was number 2,147, an indication of how long ago it was. (His American patent was number 104,408). In his review of the topic in 1972 Plaster noted that the patent was fairly comprehensive in regard to some later developments. It in included a system wherein the abrasive was carried by means of

a jet of steam, air water and other suitable gaseous or liquid medium . . . . .the sand may be propelled by a current of air produced by suction or a partial vacuum. . . . . . When a jet of water under heavy pressure is used, as in hydraulic mining, the addition of sand will cause it to cut away hard and close grained substances, upon which water alone would have little or no effect.

Figure 1. Illustration of the initial steam-injected sand blasting design (after Plaster)

At the time that the invention was made steam was the easiest fluid to provide the driving pressures and volumes needed to power the abrasive stream.

The original machine was operated by steam at a pressure of up to 400 psi, and sand was fed from a feed funnel, down through a length of hose into a narrow (0.17 inch diameter) tube centered within the half-inch diameter steam tube. This tube tapered down to a quarter-inch inner diameter as it reached the end of the sand feed line, leaving a narrow gap around the exit to the sand pipe. The high velocity of the steam, as it then flowed the chamber at the end of the sand pipe created the vacuum that pulled the sand into the stream. The resulting jet was collimated by a 6-inch piece of quarter-inch pipe.

It was found, experimentally, that putting a pair of aligned, 3-inch long flat plates on the end of the nozzle, aligned with its edges, gave a better jet, with less lateral spreading when grooves or straight cuts were required.

Steam, however, wet the sand, which would then attach itself to the pipes, causing blockages. Problems also arose because the steam caused poor visibility, and made for unpleasantly hot and wet working conditions. Thus there was an incentive to change, and by the turn of the century (1900) the increasing popularity of compressed air provided an impetus for this change and compressed air then became the main fluid transport for the abrasive throughout the 20th Century. By 1984 production rates for such systems of around 4 sq ft/minute could be achieved by a single operator working with a system driven by a 12 hp. compressor.

As the technology became more widespread so the design of the nozzle was improved through a series of modifications. These led to the inclusion of what is known as a de Laval nozzle into the design of the delivery system. The de Laval design was initially used to drive a small steam turbine in a creamery in 1897, by Gustaf de Laval.

Those who first sought to make steam turbines were also the first to have a large steady supply of an elastic medium that is very like a gas, steam. They soon found themselves using nozzles to produce high-speed flow and they started by using convergent nozzles and they mostly still do. This was the intuitive design with its forerunner in use in hydraulic machinery. They soon found that whilst they could increase the speed of the jet formed by a given convergent nozzle by increasing the supply pressure, no comparable increase could be produced by reducing the back-pressure. They described the nozzles as “choked”. It must have been totally counter-intuitive to find that the fitting of a divergent cone to a convergent nozzle got rid of the problem.

In its simplest form the nozzle takes the form of a convergent section, followed by a narrow constant diameter throat, and this is succeeded by a diverging section at the end of the nozzle.

Figure 2. Basic components of a venture nozzle for abrasive blasting with air.

The increasing diameter of the channel, after the throat, causes a drop in pressure in the nozzle. This, in turn, allows the air to accelerate with the abrasive and the velocity resulting is more than twice as high as it otherwise might reach, going from perhaps According to tests by Tetrabore in 1981, velocity changes from 275 ft/sec to 650 ft/sec have been measured. At the same time the improved velocity of the jet made it effective over a greater area of the target with effective cleaning reported as increasing by 30 - 40%.

A specific design was patented by Albert in 1955, where the transition lines are radiused rather than being linear.

Figure 3. Based on the nozzle design patented by Albert (Plaster ibid).

There are two other advantages to the design beyond the improved air velocity as it leaves the nozzle. The first of these is that the flow is more uniform coming out of the nozzle, so that the surface being cleaned is more evenly attacked, reducing the need for nozzle manipulation to ensure that the surface is completely covered during cleaning, and secondly the amount of abrasive that is required to clean a given surface might be reduced by as much as 20%.

It is in this control of the air component of abrasive blast streams that there thus remains some potential for further improvement. But we will discuss that and other aspects of abrasive use in the following parts of this section.

Pause for celebration

In days past the second of February was known as Candlemas, and folk would bring the bundles of candles they used for lighting to the church for blessing. (So much progress because of the advent of fossil fuels and electricity has made the day less well remembered). It has only been in later years that it has become more famous as “Groundhog Day.” But this weekend we remembered the date for another reason. Back in the Second World War a telegram was sent to a sister serving in an anti-aircraft battery in the United Kingdom.

Figure 1. Telegram from my father. (courtesy of Helen Bach)

And so we celebrated that event. Things have changed a great deal since those times. My mother was “laying in” at the hospital for two weeks, though, as the bill notes she did not get wine to celebrate. Rather, as my Dad was prone to mention in later years, she was fed stout to help with milk production. br>

Figure 2. Bill for the hospital stay. (Courtesy of Helen Bach)

It is snowing outside the window, as we sit in Maine remembering, six inches more since the plow came past and the forecast is that it will continue until midnight. The snow is an indicator that we will have to go out and face cold reality again soon, for while much has changed in the last decades, and the quality of life has much improved, there are still problems to face and issues to discuss. (And yes, occasionally we also still need candles).

Back soon!