Sunday, July 28, 2013

Tech Talk - The latest Canadian oil spill

Time was that I used to write, on Sundays usually, a Tech Talk over at The Oil Drum where I would try and explain some of the technical issues involved in oil recovery from the ground. Now that the site has effectively gone away, and I have had my couple of moments of frustration trying to explain why it isn’t because of the end of the Peak Oil story (here and here), the need for the occasional technical explanation still exists, and I still enjoy the chance to explain something. So as time permits and the need arises I will continue the practice.

On this occasion the post is seeded by a note from Luis de Sousa (h/t Luis) who noted a story in Mother Jones. That story, in turn, fed from one in the Toronto Star and is about surface contamination of oil, coming from the underlying tar sands and emerging as a watery bitumen mixture over at least four areas in the Cold Lake region of Alberta. The story is difficult to report, since the contamination is centered within the Cold Lake Air Missile Range, where the Canadian military fires and tests live weapons. Unfortunately, as written, it seems to have some technical inconsistencies.

The oil migration was apparently started by underground extraction wells that are being used to extract the oil from the oil sand, without having to dig it up first. There are two main ways of injecting steam through wells down into the oil sands that can produce the oil. The one that is most commonly discussed is the Steam Assisted Gravity Drainage (SAGD) process, but the more commonly one used in the past is often known as “Huff and Puff.” I have described both of these in some detail in an earlier post. In this case the process would appear to be the older one, and as a refresher, here is what I wrote about it a couple of years ago.

Surface mining of oil sand can only progress so far before the gradually deepening seams of the sand become too deep to continue to economically mine them. At the same time the viscosity of the oil is such that it does not flow easily to a conventional type of a well. This is not a new problem for the oil industry, which has had to address issues with the quality of the oil that it finds coming out of a well more than once over the past decades. One of the more easily applicable methods for improving the flow characteristics of the oil is through heating it. (And a quick caveat, the quantities of heat that I am talking about at the moment are significantly different from those that are needed in treating oil shale, and I will come to that topic in a couple or three weeks).

The example of the effect that temperature makes on the ease with which a fluid flows that always first comes to my mind is of a visit to the Nurse’s cabin north of Montreal one winter, a long time ago, when after struggling through waist-high snow, we sat and poured whisky from a bottle left there, as we waited for the wood stove to heat the cabin. When we started the Scotch poured as though it was a heavy syrup.

Viscocity of an oil is something that we usually only think about when we buy the engine oil that we put into the car on odd occasion. Buying the right oil means either looking for the little label that has the right description or reading the manual to get the number. But the oil that we buy for the engine is rated in part on how it behaves at different temperatures. We want the oil in the engine to easily circulate around the parts, and lubricate them from the time that we switch the engine on. But if the engine starts cold, and the oil is too thick, then it may not move easily around the parts, which may run dry for a while and wear more rapidly – which is not good. However if the oil becomes too thin once the engine reaches operating temperature then it doesn’t act as a good lubricant, and again engine wear is increased. And so manufacturers of the oil will adjust the contents, depending where in the country they plan on selling the oil, and what the temperature variations the oil can expect to operate under there. (And this is why oil is sometimes bought with two numbers – as in 10W-30 – the first number relates to the cold start, and the latter the performance at the engine operating temperature. And the higher the number the more viscose the oil is under those conditions.)

A typical oil found up in the oil sands of Alberta is much thicker, and more difficult to flow under normal operating conditions than that used in a car. For the areas of the province that are too deep for surface mining the temperature is not affected much by the changes in surface temperature, but the ground temperature is still low enough that the oil is very viscose, and production from a normal vertical well is usually too slow to justify the expense of putting in a well.

So how can the viscocity be reduced? For a simple example, take an apple, which has fallen in the butter, and you want to clean it off. If you take the apple and put it under a cold stream of water the butter sticks to the apple, but if you raise the water temperature, suddenly the butter melts and runs off the apple. This happens best at about 185 degF, and if you were to turn a pressure washer onto a greasy surface you would find that it works better if the water is also heated above that temperature. (Some pressure washers are sold that way).

Think now, if you will, of little Johnnie (helped of course by Jessica) having raided the orchard and spread butter onto all the apples, gluing them together and filling the kitchen full, right to the ceiling. How do we clean the butter off and get it back without taking all the apples out and cleaning them one by one (which is sort of what they do with the surface mined oil sand up in Canada).

We could just stand in the hall and stick heaters up against the wall of apples, hoping that the heat would melt the butter and work its way back to the ones further into the kitchen. That sort of works, but burns the local apples and doesn't reach all that far. (They have tried setting fires inside oil wells, and we’ll get to that maybe next week). You could fill the kitchen with hot water, but while that washes out some of the butter, a lot of the heat goes into the apples and the water is cold before it reaches the back of the room. And the water doesn't have that much pressure to push the remaining butter off the apples.

What we need is something that will get through the gaps between the apples and keep its heat. So how about steam? So you go and get a steam cleaner (such as you use for carpet cleaning) and blow the steam into the apples. That works but as the butter starts to flow out it clogs the gaps and starts to re-harden except when the steam is right there. So you start to run the steam for a bit, stop and collect the butter that comes out, run the steam for a bit, etc. You can do this in an oil well and it has the exciting technical name of "Huff and Puff" (would I kid you?). To make the steam more effective it is heated to between 150 and 300 deg C. Where the rock is very permeable and the steam can, in time, work its way back through the particles (apples) this can recover a lot of our butter. But you still lose a lot of heat, which is expensive to generate, just in heating the apples.

The NETL shows how the process works, in three steps:


Puff - The 3 stages of the process as illustrated by NETL.

The problem is that this is still limited by the length of the borehole through the deposit, and because it is an intermittent process, it doesn’t give a continuous flow of oil.

In the current case at the Primrose Oil Sands project this particular leak came up under a body of water making it more difficult to control, but it was the fourth at the Primrose site, though earlier ones were in the Primrose East development. In their public statement on the issue Canadian Natural stated:
This spring bitumen emulsion was discovered on the surface and based on all the evidence gathered to date, we believe this rise to surface involves mechanical failures of wellbores in the vicinity of the impacted areas. A complete review is ongoing.
The location of the latest leak under a body of water seems a little hard to support that conclusion – though I am not that familiar with the project history.

What I do remember, however, from lectures that I have given on ground freezing when teaching ground control, was the great difficulty one can have in controlling the path of any fluid injected into the ground. I mentioned some of that in a post I wrote on permeability, some time ago. The gas used in the ground freezing is usually liquid nitrogen, and it is injected to freeze the ground around the injection point and along the path that the fluid takes. (In other words the reverse of that used in steam heating of the oil). But it has proved to be a very difficult process to control, because any small fissure in the ground can have a sufficiently lower permeability that the surrounding rock that all the fluid will flow to that channel. This is particularly a problem in the case of the oil sand, because if the steam finds an easy path to the surface (and the fact that this is the fourth instance suggests that the particular oil formation may be prone to such fissures) then as the steam migrates it heats and softens the bitumen so that it will flow with the steam, widening the channel and, over time, making the situation worse, if steam continues to be injected.

Fortunately, with the “Huff and Puff” steam is only injected for a short while, before injection stops and the oil recovery phase begins. Yet, in the latest case, that was sufficient to bring over 4,000 bbl of bitumen to the surface. It seems likely that the process will need to be modified to give better control to the steam paths, and the oil migration. This can probably be achieved by switching to the SAGD process, which is planned in later work at the project. In the interim they might want to steer away from using the technique in regions where the oil grade is higher, and there is the potential for the sand to outcrop or come close to the surface without a confining cap rock.

Read more!

Thursday, July 25, 2013

OGPSS - Of Egyptian Bread and Oil

The turmoil in the Middle East shows little sign of ending in the near future, and the potential lack of enough cheap fuel for the population is a warning that the levels of unrest may continue and even get worse. There is, however, some hope for enough local supply in the near term to help with some of the indigenous problems. Consider, for example, Egypt which has the largest population of the countries in its immediate vicinity, a population that has grown 200% in the last 50 years.

Figure 1. Growth in Egyptian population (Trading Economics )

By last December Egypt was populated by some 83.66 million folk, with little sign of change in the growth rate. Over that time, energy demand has grown, while domestic supplies of fuel have not kept up. The most significant change, perhaps, is in oil consumption, with recent data, as previously noted, showing that the country has now switched to one that must import oil to meet demand.

Figure 2. Egyptian oil statistics (Energy Export Databrowser )

The EIA puts consumption at 811 kbd, set against a production of 555 kbd of petroleum products and, for natural gas, the country produced 2.1 bcf , which can be set against a consumption of 1.8 bcf, but the balance there also is trending downwards as recent levels of discovery and development have failed to match the increase in domestic demand.

Figure 3. Trends in Egyptian natural gas statistics (Energy Export Databrowser)

The developing need for oil imports is made more difficult by the subsidies that have become an accepted part of the Egyptian economy, including not only fuel, but also bread. Fuel subsidies are reported to be at around $17.4 billion and about a fifth of total state spending. Bread subsidies though a similarly critical part of picture, run only at about 20% of the fuel cost. To help manage costs and encourage domestic production, the Morsi government had cut back on foreign purchases of wheat, but this has now been reversed with the take-over even as the new government works to transition away from the subsidy burden. With the change, foreign governments are now also more willing to provide fuel, the United Arab Emirates (UAE) will send around a million barrels of oil this month, and Kuwait, Saudi Arabia and the UAE are promising more aid packages.

This may help with the short–term problem, which has too many political entanglements to allow any solid predictions for longer term help from outside the country, but there are potential sources of increasing domestic supplies from both the Western Desert and the Nile Delta itself.

Egypt has been supplying natural gas to Jordan, Syria, Lebanon and Israel through the Arab Gas Pipeline with flow starting in Arish, and the leg to Ashkelon being underwater.

Figure 4. Route of the Arab Gas Pipeline with projected extensions. (hydrocarbons technology)

Because of the connection to Israel the pipeline has been the subject of a number of terrorist attacks (the latest a couple of weeks ago) . However these more often affect the flow of gas to Jordan, rather than to Israel, because of the pipeline locations, with the pipeline being vulnerable in the Sinai where it is flowing south to Taba. This problem has led Jordan to consider importing natural gas from Israel and the recently found offshore natural gas deposits being developed in that country. Flow from the Tamar field started on March 30th tapping into the estimated 8 Tcf therein, while flow from Leviathan is anticipated in 2016.

The possible presence of oil-bearing strata at a lower depth in the Levant Basin has led Noble to plan an offshore well to go down 31,200 ft to a potential field holding perhaps as much as 1.8 billion barrels of oil. However Noble estimates the chance of success at 25%.

The recent success in finding these resources within the Levant Basin suggests that the potential for other discoveries in future years, with significant possible impacts on the local economies.

Figure 5. Location of some of the discoveries and developments in the Levant Basin (USGS)

The problems limiting future exploration in the region tie in with the conflicts and internal disruption that seems to spread to most of the countries in the above map. But in the more immediate short term Egypt is reducing exports in order to meet the growth in domestic demand, while importing natural gas, currently as a gift, from Qatar.

In the longer term, as the Israeli fields come on line, it might be possible to change the direction of flow of the Arish-Ashkelon pipeline to carry Israeli gas into Egypt. There are thus potential technical solutions to getting fuel to Egypt to meet their growing need.

However this does not address the underlying problem of how Egypt is going to be able to pay for that fuel (not to mention the bread). Even with a potential glut in global natural gas prices, without a stable economy Egypt is not going to be able to pay its import bill. This was evident towards the end of the Morsi government, when a lack of cash, or hard credit made it more difficult for the country to assure itself of enough imported oil to meet demand. The continued turmoil will keep away the tourists that could provide the economy with enough funds, while the lack of international recognition of the current regime is currently keeping the IMF from providing any help.

A couple of hundred years ago deriding the people’s need for bread reputedly led one ruling family to the guillotine. In the time since the people have also come to expect that they can also get fuel. Until both demands are satisfied it may be more likely than not that rule in Egypt will remain unstable, with the presence and influence of the competing mobs making rational decisions less achievable and the situation worse. (And they are also blowing up pipelines in Iraq.)

Read more!

Tuesday, July 23, 2013

Waterjetting 11b - Abrasives and cutting depth

In the past few posts I have been discussing the use of abrasive in waterjet cutting, and in this and the next two posts I would like to talk a little about the abrasive feed rate (AFR), abrasive size and the selections for the best cutting performance. As with my other posts I can only write in general terms about these because the combination of waterjet system, nozzle design and abrasive selection will change the best values to use, and the results on different systems will differ in some way or other from the results I will mention. However in all cases the overall principles remain the same, and can be applied as general rules.

In the last post I noted that cutting performance fell as the average size of the abrasive fed into the system dropped below 100 microns. As part of that study we looked at the amount of abrasive that survived going through the mixing chamber in that size range. A simplified average of the results obtained are shown in the following table:

Figure 1. Percentages of the initial feed that survive at larger than 100 microns, for differing feed conditions.

In an earlier post I had mentioned the “Green Tube” test that was used at Missouri S&T as a way of measuring the particle size and speed, after the particles had passed through the nozzle, but without hitting a target. Because the distance that the particles travel is a function of the energy they obtained during mixing, some idea of the overall particle energy can also be obtained.

However, when the particle sizes were analyzed at different distances from the nozzle we noted that there was a large percentage of small particles in the short distances from the nozzle, but that as the particles were collected at greater distances from the nozzle, so the average particle size grew larger.

After thinking about this for a short while, the reason became obvious, and – at the same time – made it a little more difficult to draw simple conclusions from the test.

The reason for the greater collection of smaller particles nearer the nozzle is that they are decelerated more rapidly than the larger particles, once they start traveling through the air. If we go back to the basic equation that we learned in school:

Force = mass x acceleration

For a given particle, the force to accelerate the particle in the mixing chamber will, simplistically, be the pressure exerted on the particle by the water, multiplied by the cross-sectional area of the particle. If the particle is a sphere, with a diameter d, then the area be π x(d/2)^2. But the mass of the particle is a function of the volume, which is related to the cube of the diameter. Thus the acceleration, for a given particle size and at constant fluid pressure, will vary inversely with the diameter of the particle. In other words the smaller particles will accelerate faster in the mixing chamber and focusing nozzle.

Once the particle leaves the nozzle, however, the acceleration from the water is replaced by a deceleration as the particle is now moving through air that is relatively stationary. Now the situation is reversed and it is the smaller particle that decelerates faster, and thus will have a shorter effective range than particles that survived the mixing process in a larger size. This was therefore the explanation for the results that we saw in our tests.

Unfortunately life becomes a little more complicated than this when the nozzle is held close to the target. This is because, while the air between the nozzle and the target may be relatively stationary, at greater distances, the small gap means that the surrounding air is also drawn into the slot and flows with the stream along the cut. There is thus less resistance to the particles, which retain their energy to a greater distance – improving cutting depth. However that also changes if the jet is cutting through layers where there may be water or air in the gaps between the layers.

This work was carried out initially by Dr. George Savanick during work carried out at the then US Bureau of Mines, on cutting rock. It applies in other cases, however, since there are often times when cuts are needed between two work pieces with a gap between them. (The example in mind is cutting through the different tubes that bring oil out of a well. This casing can be made up of several different diameters of pipe, ranging perhaps from a 20-inch diameter outer pipe to a 3.5-inch diameter inner one, with other tubes between). What Dr. Savanick showed was that if the gap between the layers was filled with some relatively soft material that provided little resistance to cutting, but held its shape and provided confining walls on either side of the jet, that the range of the jet could be extended beyond that where the jet was cutting water or air between the layers. These factors then play a part in determining how far an abrasive jet will cut through material.

Often it is not just the ability of the jet to cut through the material, but also the straightness of the cut and the quality of the edge that are important. If, for example, one can be sure that there are no burrs on the edges of a cut between two overlapping layers of material, then the parts may not have to be separated, cleaned and re-assembled before being fastened together. This elimination of several manufacturing steps can significantly lower the cost of assembling, for the sake of discussion, aircraft components. In turn this may then justify the use of the AWJ system as the better manufacturing tool, even when it does not seem that the initial cutting process is much cheaper than the alternative.

I mention these considerations, because as I go through the different applications of these tools I can only be somewhat general in discussion of overall effects. The way in which the abrasive mixes with the water, the amount of particle breakup and the different speeds of the abrasive leaving the nozzle vary with the nozzle design and operating conditions. They are also tailored to an extent by the particular job that has to be completed. Thus a recurring piece of advice in this series will be to find a test piece of material and test out a range of options before committing to the final cut. The series will try and suggest where that range might be, but in the final decision I do not have the equipment or other conditions that exist in your shop, and thus only you can be the final judge.

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Sunday, July 21, 2013

Ideology versus Reality in the UK

This is but another gentle cough! And for those not familiar as yet with the way I write, it is, in the culture in which I was raised, generally considered more polite to gently cough and quietly explain why someone is wrong, rather than engage in the confrontational breast-beating and name calling that seems more prevalent in scientific argument in these latter days.

Which brings us to the new “news” out of the UK. The argument that a nation’s government must, in the end, deal more for the benefit of its voters, than to its political views has been one of the underlying cynical views I have held for the energy future for a long time. Telling a nation that they need to do something “for the good of their grandchildren”, does not go very far when oil prices are rising and they have a cheaper, indigenous source of energy – which is, quite frequently coal - that will allow them to feed themselves, and their children, and thereby provide a path to the generation of said grandchildren.

My anticipated example for this has been, for a while, India and the sub-continent of Asia, where indigenous coal can be a much cheaper resource than increasingly expensive foreign oil, and natural gas which is often politically more difficult to acquire through trans-continental pipelines. And so I was caught a little by surprise, although, based on Euan’s excellent previews it was something which had to happen, but which I did not yet expect, and that is a volte face from the British Government.

The underlying premise to which I have tried to adhere has been one of realism, rather than ideology. As a research scientist for most of my life I have had, too often, to go to meetings where I have had to admit that experimental results did not exactly follow my predictions, and that, as a result, the underlying theory has had to be changed. But that does not mean that the overlying premise changes, only the nuances of the argument. (And for those few interested, the work dealt with how things break.) And in the current case the overlying premise has always been that a nation has to ensure its fuel supply at a viable cost to the nation.

It is something that, inter alia, drove Japan into war over 70 years ago, and now that reality is becoming evident to Her Majesties Government, it is apparently a realization that is beginning to strike home over there too.
In April 2012 coal took over from gas as Britain’s dominant fuel for electricity for the first time since 2007, driven by a collapse in the international price and a rise in the cost of gas. In addition, the tax the Government levies on companies emitting carbon currently stands at £16 per ton, rising to £30 a ton in 2020. But analysts warn that at the current prices this would have to rise to more than £40 to make such coal generation uneconomic. . . . . . Under the Energy Bill, 12 of Britain’s existing 18 coal power stations that could stay open will be exempt from the Government’s emissions performance standard (EPS) that sets limits on CO2 emissions for all new power generation.

The political cost of supporting a coal-based economy have yet to be determined, and the unrelenting assault on coal-fired power has been so widespread and unrelenting that this will not be an easy sell, but this step is something that may well be an indication of where, ideology aside, reality will move to define the future.

In essence the relief of the burdens on the existing power plants will strengthen the base-load capacity (usually provided through coal and nuclear power) and will take some of the strain from the commitment to renewable energy (which is often wind-generated in the UK) and thus work to ameliorate some of the criticism from such folks as the Institution of Mechanical Engineers. (There is no longer any criticism from the Institution of Mining Engineers, since that went the way of the dodo, some years ago.)

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Thursday, July 18, 2013

To Forbes - A Gentle Cough of Correction at TOD's end

Forbes recently issued a commentary on the closing of The Oil Drum, which deserves some rebuttal, since, as with many stories on the "Peak Oil" topic, it conveys too many incorrect statements and false assumptions.

Just over eight years ago I became irritated by several articles in the Main Stream Media that were clearly technically wrong. (My academic research includes many years of making holes in geological media, an interest that began with my doctoral work in the late 1960’s). I began writing about some of the misconceptions in regard to the approach of Peak Oil in a blog I was writing at the time. Shortly thereafter I agreed to join with Kyle, who was then writing his own blog, under the nom de plume of Prof Goose, to jointly create the website The Oil Drum.

In the beginning, Kyle handled the site management issues (a task he later passed on), and my main contribution has been the intended one of writing on the more technical sides of the situation. This was particularly the case during the events surrounding the Deepwater Horizon disaster, where readership of TOD rose to around 60,000 a day. But writing to a site that began to achieve some technical credibility had its drawbacks. Very early on I got into the habit of referencing almost every fact I cited, given the questions that arose whenever I appeared (at least to my audience, but also, at times, in fact) to misspeak. Working for the site has made me a better writer, but it was clear almost from the start that the two of us could not sustain the interest that the site very quickly drew.

Over the years I felt very fortunate that Kyle went out and found funding, and innocents willing to carry the burden of editing the increasingly large talent of folk that were kind enough to contribute to the large interest that the site engendered. The site was fortunate to attract some really perceptive folk, and if I hesitate to name them it is only from the fear of missing the odd one and causing offence to people that I have acquired great respect for over the years. Many of those now have their own sites, and so TOD acted in some small way as an encouragement for that effort and to broaden and grow the community that is concerned about the coming point where the production of oil, at a reasonable price, will be unable to keep up with demand and the unpleasant consequences that will then arrive.

I was watching the hearing before the UK House of Commons Science and Technology Committee this past Wednesday on the public understanding of climate. In response to a question, Ralph Lee of Factual, Channel 4 and David Jordan, Director of Editorial Policy and Standards for the BBC pointed out the difficulty in sustaining the level of stories on Climate Change, because of the need for these to generate significant new material to justify publication. They noted that repetition of the basic information, beyond a certain point, was counter-productive. So it is with the Peak Oil story. The facts, in neither case, change, but the amount of new information while accumulating (vide the superb work that Leanan has done with Drumbeat over the years) is often repetitive or confirmatory of earlier stories and thus harder to turn into interesting and exciting new material. There are developing stories that justify continued interest in the topic, but the slow pace with which some of the stories unfold make it difficult to sustain interest.

The transition of Egypt to an importing state for example, revealed in the Energy Export Databrowser figure shown a few weeks ago illustrates a growing problem that their new government must address, but it can only be covered a few times before interest wanes.

Figure 1. Change in oil consumption and the need for more imports for Egypt (Energy Export Databrowser)

And this holds true for many of the topics covered in the past years. The perceptive articles written at TOD on Saudi Arabia by Stuart Staniford (who now writes Early Warning), Euan Mearns and with JoulesBurn’s images from the satellites showed how Ghawar was in significant decline. But there are only so many photos of oil rig sites in the desert that can be made interesting. Aramco are switching to the heavier oils offshore. Manifa has just started new production and Safaniya is being expanded. These are needed to offset the permanent declines in production from the older fields, but again, other than chronicling these steps it is hard to sustain interest in an inexorable process that takes years to play out and where the route to Peak Oil is following along many of the predicted lines.

Even drawing back the curtains of hype over the Bakken and Eagle Ford production, which Rune and Art have so ably done, can only be written about at a certain low frequency before folk see it as repetitious.

Much of the story of the future supply will, in my view, come from activity outside the United States. There will always be a need to update activities in and offshore Alaska, and in the US shales and other formations where future production will have to come from, but as we are likely to see by the end of this year, the gilt on that gingerbread is very thin. Thus the posts that I have been writing recently (and which will continue on Bit Tooth Energy – my own home site) will likely focus on the situations abroad, such as the Middle East, where the political upheaval has a much greater potential to disturb overall global supply than the changes in the US. Similarly Japan is moving toward a more militant attitude as China moves to extract fuel from disputed fields in the East China Sea. This however, again, is a potential tragedy unfolding in slow motion.

At the beginning of the year the EIA were predicting that gas prices would fall this year and pundits that suggested that gas prices would stay down after the recession still appear with regularity to quote their lines of optimism, even as gas prices stay stubbornly high and potentially may rise through the rest of the year. Why is that? Well the OPEC nations need a certain level of income and adjust their production each month to help sustain prices – something these optimists seem unwilling to recognize.

The problem, however, is that if global demand rises at (for the sake of discussion) 1 mbd a year, then a point will be reached, fairly soon when increasingly this OPEC supply becomes no longer capable of filling the demand. Prices will then rise again, balancing supply against those able to pay for their demand at that price. Stating that this is not going to happen because "a way will be found" is to remain an ostrich.

No, gentle readers, the closing of TOD is, in my opinion, based on a deliberate but IMHO faulty management decision made in that group a couple of years ago. It was predictable at that time, but it has nothing to do with the coming of Peak Oil, and is not even symptomatic of much of a delay in that arrival.

And with that off my chest I will return to writing about the evolving problems. My hope at the founding of TOD was that it would chronicle the events through the Peak, it got to nearly the Peak, though I don’t anticipate that this will be a pleasant story beyond that point. But, that coverage will now shift to being only at a new location at a time chosen by the TOD editors.

Read more!

Wednesday, July 10, 2013

Waterjetting 11a - More thoughts on Abrasive

In the last post I mentioned that the abrasive particles, which are fed into a high-pressure waterjet stream to form the Abrasive WaterJet (AWJ) cutting tool, can be significantly crushed when mixing with the high-pressure waterjet, and before they leave the mixing chamber. Because of this - depending on the application - the choice of abrasive can play a significant role in how well the AWJ performs. I have mentioned a number of times that the Waterjet Lab is located at Missouri University of Science and Technology. That meant (apropos “show me”) that it was an appropriate place to run comparative tests between different abrasives to find which is the best.

It turns out that there is no one single answer to that question, since the abrasive that was the most economical and effective to use in one case does not necessarily give the best results in another. Which brings me to the first point in today’s post. It is relatively easy to get small samples of the different abrasives that might be used in a given job. Setting up a small series of test runs, in which the different abrasives being considered, are fed to the nozzle and use to cut standard cuts into test samples, is a relatively easy way to find out which is the best abrasive for that particular material and cutting path. However it is best not to use only a single test run, we would generally run a series with three different jet pressures and three different abrasive feed rates.

Figure 1. Table showing the change in optimal Abrasive Feed Rate (AFR) on cut depth at different pressures.

By bracketing the range that is likely to have the best concentration of abrasive for each pressure (which is not at the same abrasive feed rate, or AFR) the best result can be found for each different pressure value, and the most economical and effective choice for the task in hand can be quickly found. It is important to include economics in the evaluation, since there have been a number of cases we looked at where the most effective choice for abrasive in terms of giving the fastest clean cut was not that much more effective than the second place abrasive, and that alternative was sufficiently cheaper that it made more sense to use it.

The pricing of abrasive, however, is not something that it is easy to generalize over, since there are a number of different factors that come into play, depending where in the country you are located. As a rough guidance, however, we have found that garnet is a more universal cutting abrasive than most others, with less extraneous “issues”, and while it can be less effective than other selections in some conditions, in general it will cut more materials effectively and economically than its challengers. Further mined garnet, in general, performs better than alluvial garnet since it does not have the degree of damage within the particles that leads them to fragment more easily in the mixing chamber.

There are, however, more factors that just the abrasive type that have to be considered. There include the particle size, and range, and then, as noted in the table, there is the selection of the AFR to match the cutting conditions on the table.

One of the more neglected factors relates to the amount of air that is used to carry the abrasive from the hopper into the mixing chamber. The person who did more to shine a light into this corner of the technology was Tabitz, in France. (Tabitz, Schmidt, Parsy, Abriak, and Thery “Effect of Air on accceleration process in AWJ entrainment system, 12th ISJCT, Rouen, 1994 p 47 - 58.)

Because abrasive can cut into the parts of the flow meter, the equipment that they used included a trap between the hopper and the mixing chamber, where the particles could be collected, while the air passed forward to be measured and enter the mixing chamber.

Figure 2. Apparatus used by Tabitz in measuring the air flow to the cutting head and mixing chamber. (ibid)

The results from the measurement showed that as the jet pressure increased, so for that particular nozzle design, did the amount of air that was being drawn into the chamber – although you may note that it begins to reach a constant volume as the pressure approaches 280 MPa (40,000 psi).

Figure 3. Effect of increased jet pressure on the amount of air drawn into the nozzle, as a percentage of the total volume of the resulting jet. (Tabitz et al)

The problem that this relatively large volume of air presents is that it has to be accelerated at the same time as the energy in the jet is being transferred to the abrasive particles. The larger the amount of air in the mix, then the greater the amount of water energy that has to be diverted into accelerating the air. This leaves less energy available to accelerate the abrasive itself.

Tabitz modeled the result with a simulation in a computer program, which illustrates, for different abrasive feed rates, how the average abrasive particle velocity falls as the amount of air in the mix increases:

Figure 4. Simulated effect of an increase in air flow on the reduction in average abrasive particle velocity (after Tabitz et al).

Placing small instruments in front of abrasive-laden waterjets can lead to a relatively short life for those instruments, and measurements of actual particle velocities, though they have been made by a number of researchers, have not been as comprehensive as the above chart might indicate.

Nevertheless there is some indication that the above curves are accurate in principle, if not totally real. A jet with very little air might accelerate particles to 1,880 ft/sec, for example. However with 70% air in the mix, then the particle velocity might fall to 1,700 ft/sec, and with 95% of the jet made up of air, then the abrasive particle speed may fall to 1,200 ft/sec. Part of the difficulty in assessment is because of the very short time interval in which the abrasive particles are accelerated while in the mixing chamber. Because the rate of acceleration of the particles is inversely related to their size. Smaller particles are accelerated faster. And this is the counter to the point that was made in the last post about smaller particles cutter less efficiently than larger ones.

Part of the reason for this is that the smaller particles are decelerated faster in air than larger particles. The results of this in terms of cutting power is one of the areas that still requires more research. If, for example, smaller particles are used in an application (for example to achieve a finer detail in the surface cutting) then the effective range of the jet can become smaller than with larger particles. There are some caveats to that statement, and I will go into some of that explanation in the next post.

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