Waterjetting 32a - jet fluids

From time to time I am asked why we use water in almost every application of high-pressure waterjet use, rather than using any one of a number of other choices for the fluid. There are a couple of major reasons for this. The first is that most of the uses of the tool will require that we use a fairly large quantity of fluid, over time, and water is the cheapest and most readily available of the alternatives that can be used in such volumes. The other is that, in many circumstances, it works as well, if not better than those alternatives. As an additional point it can often be cleaned up relatively cheaply and simply before being disposed of after use.

But the conclusion isn’t an absolutely true one, and along the way there have been a number of different investigations to find alternatives that can provide a better answer than just water alone. Some of these choices suggest applications where a different fluid might work better, for example if the target is a food product that contains a lot of sugar then it might be better to cut with a form of vegetable oil that does not dissolve the sugar on contact. In other cases there is an advantage to adding a chemical to the water to change its properties.

The changes in performance that decide whether the change in cutting fluid is worth the effort are (as with so many other choices) going to be based on the particular job that has to be carried out. In some cases, as with cutting candy for example, the benefits of using an oil stream may make the process practical in a food that would otherwise not be feasible to be cut with a high-pressure jet, for aesthetic reasons if no other.

Figure 1. Effect of changing fluid properties on cut depth at an impact velocity of 330 ft/sec. (after Rochester and Brunton) on a nickel target.

Several different fluids were used to generate the curves shown in Figure 1, ranging up to mercury in density, and including four different oils as a way of investigating viscosity.

One of the down sides to the use of oil as a cutting fluid comes from the increased viscosity of the fluid, with some of the oils that were initially used as the tool went into commercial use for cutting confectionary items.

The increased viscosity of the oil raised the pressure required to drive the fluid from the pump through the delivery lines to the cutting nozzle arrays. This in turn lowered the cutting pressure at the nozzle, and made the differential between the operating pressure at the pump during the drive stroke and during the reversal significantly greater. This greater fluctuation in pressure produced a greater fatigue on the drive train, and in turn led to a more rapid failure of components within the system. While the initial answer was thought to be in using a different cutting oil, the final, easier, solution was to move the pump closer to the cutting zone, reducing the pressure losses and bringing the fatigue back to acceptable levels.

Changing fluid properties can therefore have a significant impact on system operation, even though they may have little impact on the actual cutting performance, although in many cases there are significant changes as the fluid properties are changed.

The chemical impact is one that is perhaps addressed more commonly in cleaning applications, where it is often suggested that chemical agents be added to the water as a way of weakening the bond between dirt and the underlying surface. While this is theoretically possible, it should be remembered that the jet, even at the relatively low pressure of most pressure washers and car wash units, will be travelling at speeds of hundreds of feet per second. The residence time of the chemical on the surface can thus be measured in milliseconds and is rarely long enough for much change to occur.

In these circumstances it is usually more fruitful (and less demanding of chemical) to spray the chemical cleaning solution onto the surface first, and then allow a short period of time for the interaction to fully occur before applying the pressure wash. In these conditions (as we saw when we watched as our house was pressure washed the other week) even though the nozzle was held too far from the surface for there to be much pressure on the wall by the time the jet reached it, the chemical cleaner had broken the bonds of the algae and dirt, and the house walls were rapidly cleaned with relatively little additional effort. (This is contrast with the time that I had cleaned it without chemical, where it was necessary to keep the nozzle within six inches of the wall for the jet, unassisted, to remove all the surface contamination).

In the case of a house cleaning this highlights an additional benefit from using the chemical, since the lower pressure jet impact on the building means that the jet will not be strong enough to erode any of the timber surfaces around the house that were starting to weather with the passage of time.

There are, however, some chemicals that are used in mining and civil construction that will interact relatively quickly, when introduced into the cracks within rocks, particularly if they can be pushed to the tips of the cracks where they can rapidly reduce the strength of the bonds across the tip of the crack, making it easier for the crack to grow at lower fluid pressures. The fluid characteristic that controls this effectiveness is often related to its Zeta Potential. However much discussion of that topic would rapidly take us a considerable distance from the current subject, and so I would recommend that any of those interested might want to follow along a line that might start with the work at Brookhaven.

This series will return with a look at some of the mechanical changes that can be made to a waterjet, to improve cutting of everything from shoes, to ships and similar subjects.

Rochester M. and Brunton J. The Influence of the Physical Properties of the Fluid on the Erosion of Solids, CVED/C-MAT/TR10, University of Cambridge, UK 1973. (This formed part of Mike’s doctoral dissertation).

Cornucopia or Malthusia - a reply to John Tierney

Five years ago John Tierney agreed on a bet with Matt Simmons that by this year the average price of crude oil would average $200 a barrel. The bet is now due, although, sadly, in the interim Matt has passed away. And Mr Tierney has just posted (h/t Leanan) his comment upon winning the bet. While recognizing the impact of the recession on oil prices, after their rise to $147 in the intervening years, he points out that this year crude averaged $80. This he feels justifies the position of the Cornucopian approach to life, rather than the Malthusian.

Would that he were right! In his article he cites oil from new fields coming ashore from fields off Africa and Brazil, and increased production from the oil sands of Canada and the United States, as promising a maintenance of this Cornucopian era into the future. And he uses a historic parallel to show how in an earlier time Julian Simon won a similar bet against Paul Ehrlich, John Holdren and John Harte over the price of a basket of 5 metals.

Admittedly he is currently in good company, since the EIA is not looking for the price of crude to rise above $100 a barrel for another six years, and the IEA recently posted in their December Oil Market Report that global production increased from both OPEC (up 45 kbd) and non-OPEC (up 355 kbd) sources in November. It sees that production will continue to increase through 2011, meeting an increase in demand to 88.8 mbd. 0.5 mbd of that will come from an increase in NGL from OPEC, rising to 5.8 mbd.

Art Berman has just explained some of his concerns with the optimistic projections of the EIA Annual Energy Outlook and I agree with his line of argument. But let me take a slightly different tack in disagreeing with the Cornucopian position.

And it is true that oil companies are now gearing up for a much greater level of investment in this next year than in the recent past. The WSJ quotes a Barclay’s Capital report that levels will reach $490 billion, up 11% on last year. It also notes that the price rises of 2008 led to a boom in deepwater rig construction, and the 25 built in 2010 will be joined by 35 next year. All of which allows the Journal to end with a quote that “Higher investment now will mean lower prices than they would otherwise be in the future.” (Well yes, but . . . . ) But the reality is that the levels of investment that will be required to sustain current levels of production are likely to exceed these numbers, and we are in a time when greater prospecting will likely only lead to a diminished return. Not that we don’t need that investment.

So how does one address this issue. Well Let’s just go back to that original bet by Simon against Ehrlich et al on the price of metals. The original bet was as follows:

Ehrlich and his colleagues picked five metals that they thought would undergo big price rises: chromium, copper, nickel, tin, and tungsten. Then, on paper, they bought $200 worth of each, for a total bet of $1,000, using the prices on September 29, 1980, as an index. They designated September 29, 1990, 10 years hence, as the payoff date. If the inflation-adjusted prices of the various metals rose in the interim, Simon would pay Ehrlich the combined difference; if the prices fell, Ehrlich et alia would pay Simon.

Then they sat back and waited.

Between 1980 and 1990, the world's population grew by more than 800 million, the largest increase in one decade in all of history. But by September 1990, without a single exception, the price of each of Ehrlich's selected metals had fallen, and in some cases had dropped through the floor. Chrome, which had sold for $3.90 a pound in 1980, was down to $3.70 in 1990. Tin, which was $8.72 a pound in 1980, was down to $3.88 a decade later.
Which is how it came to pass that in October 1990, Paul Ehrlich mailed Julian Simon a check for $576.07.

Just out of curiosity I went to Infomine and looked at the price of those metals over the past 10 years (though they only plot chromium and tungsten prices for five). The plots for the 5 metals follow, and to make the calculations simple I have rounded the metal values a little.

Chromium:

$200 in 2000 would have bought 66.7 lbs (it was $3), and in 2005 would have bought 160 lbs of chrome, which would now be worth $425 roughly. Over the 10-year interval however, buying $200 of chromium would have cost you $23, not counting inflation.

Copper:

However, when we look at copper, that $200 would have bought 250 lb of copper in 2000, and over the decade that purchase has gained $862, roughly.

Nickel:

A similar situation applies to nickel, where $200 would have bought about 66.7 lbs of nickel in 2000, and that investment would have gained $533 over the 10 years.

Tin:

The same is also true for tin, where $200 would have bought 91 lb of tin in 2000, and that would sell today for about $1,090; the investment thus making $890 over the decade.

Tungsten:

$200 would have bought 6.25 lb of tungsten in 2005, which would now be worth $265 roughly. It has been difficult to find the price of tungsten in 2000, although the price is reported to have trebled from that pre-2004, suggesting that it was around $14 back then. That would give a purchase of some 14 lb, which would now be worth around $600.

So the $1,000 investment from 2000 would now be worth (in 2010 dollars) $177+$1,062+$733+$1090+$600 = $3,663

Using the Inflation Calculator there has been 27% inflation since 2000, so that the $1,000 would now be worth $1,270. The price of the metals has thus roughly trebled over the time period.

I have not been able to find an accurate value for tungsten in 2000, though I know that the price went up significantly in 2003 when the only mine in the North Americas (the Cantung mine in Canada) closed. It is now re-opening. Most of the world’s tungsten now comes from China.

This reality suggests the underlying longer-term truth to the supply situation for materials that are extracted from the earth. There is only a finite amount there, and while it is possible, due to changing economic circumstance, that a Cornucopian viewpoint might for a while appear true, the growing demand for product, as countries, particularly those in Asia, aspire to Western levels of consumption, will rapidly emphasize the Malthusian long-term condition. (Although cherry-picking specific dates may allow one to transiently make the alternate case).

One has only to consider what is happening to gold and silver, not to mention the rare earth minerals.

Matt may have been a little early in his prediction on $200 oil, but I would be very surprised if we did not see a bit more than $100 within the year. The impact that a price rise above this level will have on the global economy makes it difficult to predict what will happen after that, but we could easily see $150 a barrel by 2015, if the economy can sustain it.