Waterjetting 31b - short pulse lengths and traverse speed

One of the more surprising things that we learned at the beginning of the research into high pressure waterjet action was how quickly a jet will penetrate to almost full depth of penetration, and how slowly it will cut deeper after that. It is a lesson that often escapes even those who work with the technology today.

A series of tests was carried out in which a jet was exposed for very short periods of time to a fixed block of sandstone. The time that the jet hit the rock varied and the results were used to make the following plot:

Figure 1. Average penetration as a function of exposure time, for a continuous jet impacting a static target. (Polyox is polyethylene oxide) (after Brook and Summers )

The experiment was then repeated using a device that would only allow the jet to hit the rock for much shorter periods of time. When these results were plotted, the following graph was obtained.

Figure 2. The depth of penetration into sandstone as a function of time, for very short intervals. (ibid)

The depths achieved with the longer exposure times were therefore occurring within the first 1/100ths of a second, and the penetration that followed that initial impact time was at a much lower rate.

The reason for this had been suggested by earlier work by Leach and Walker at Sheffield who pointed out that once the jet starts into the hole it has no other path to exit rather than to turn around and come out the way it went in. Since the jet is continuing to flow into the hole, the result is that the pressure in the hole will diminish over time.

Figure 3. The effect of hole depth on the pressure developed at the bottom (after Leach and Walker).

It should be mentioned, however, that Leach and Walker built a special stand to make these measurements and the hole was built out of steel, rather than being eroded by a jet. The reason that this is important is that where the target is weaker then the turbulence generated by the jet:rebound will additionally erode the walls of the hole, particularly at the depth where the jet pressure falls to the threshold pressure of the material. At this point the jet begins to enlarge a cavity at the bottom of the hole. The pressure can then rise in the cavity, as the hole walls are reamed and the pressure bulb can cause spallation of the overlying rock. It is also why one has to be careful in the drilling of holes in glass, since a similar series of steps can also arise with abrasive waterjet cutting, and internal pressures within the drilled hole can cause the glass to fracture.

Rehbinder also built a narrow slot to measure pressure drop with depth of the hole, and showed that the rapid decline in pressure with depth that Leach and Walker found, was related to the relative narrowness of the hole, and that when the holes were wider, relative to the jet, that this decline was not as dramatic.

Figure 4. Changes in hole pressure with depth as a function of hole width. (after Rehbinder)

As I have mentioned in a previous post a logical progression is to then pulse the water so that each slug of water has time to leave the hole before the next one arrives. When this is carried out, in our case by building a small interrupting wheel that spun between the nozzle and the target, the jet will continue to penetrate although at a slower rate than that originally achieved.

Figure 5. The penetration of rock with an interrupted jet. (after Brook and Summers)

There have been several attempts since that time to use pulsed jets as a way of improving breakage, with a lower energy cost. This has centered around some form of water cannon, or similar tool, although the main problem – never really resolved – of maintaining a high firing rate without destroying the seals in the supply lines has led to that approach being shelved.

Other developments first led to a pulsation in the feed line to the nozzle, first described by Gene Nebeker of Scientific Associates at the 3rd ISJCT in Chicago in 1976. Although that work continued for a number of years it was never able to achieve commercial reality at the time. Subsequently Dr. Vijay pioneered the approach that led to the formation of VLN Advanced Technologies Inc. Using an ultrasonic method of pulsation, which produces very short duration pulses at a high rate in the stream, the company has developed a market, particularly in removing coatings from surfaces.

The mechanisms of target failure are different from those achieved with the more conventional, longer pulsed systems, where the length of the individual jet slugs allows more pressurization of cracks within the target. That kinetic energy allows the jets to operate under water, however shorter pulsation lengths (similar in some ways to rain) are attenuated where there is a layer of water on the surface particularly when this is confined, and Brunton and Rochester found that some of the advantages of the technique (including the ability to generate water hammer pressures are diminished when that layer is thicker.

However, if a waterjet penetrates to close to its maximum penetration within a period of around 0.01 seconds, and the jet is cutting a hole that is roughly three times the diameter of the orifice, then it is logical to suggest that after that residence time the nozzle should move further down the sample. If the jet is roughly 0.033 inches in diameter then the nozzle should move roughly 0.1 inches in 0.01 seconds or roughly 10 inches per second, or 50 ft. per minute. Lab studies have shown that shown that speeds in this range are most efficient where plain waterjets are used in cutting. Because abrasive waterjets penetrate material in a different way the best cutting speed for that technology is much slower.

The topic will continue in the next post, since it is often difficult to persuade operators how fast they should be moving tools to get them to be most efficient.

Leach S. J. and Walker G. L. “Some aspects of rock cutting by high speed water jets”. Phil. Trans. R. Soc. 260A, 295-308 (1966).
Nebeker E.B. and Rodriguez S.E. “Percussive water jets for rock cutting,” paper B1, 3rd ISJCT, BHRA, Chicago, May 1976.
Brunton, J.H., Rochester, M.C., "Erosion of Solid Surfaces by the Impact of Liquid Drops," In Erosion-Treatise on Materials Science and Technology, ed Preece, pp. 185 - 248.
Rehbinder, G., "Some Aspects of the Mechanism of Erosion of Rock with a High Speed Water Jet," paper E1, 3rd International Symposium on Jet Cutting Technology, May, 1976, Chicago, IL, pp. E1-1 - E1-20.

Waterjetting 25a - choosing jet parameters for range.

The range over which a waterjet is able to cut material can widely quite significantly, depending on a wide range of factors, including abrasive content. An earlier post described the way in which students in a waterjet class were shown some of the difficulties in assessing risks arising from the use of a waterjet, and the range over which it was dangerous. Simplistically the students first cut along a plywood panel to see how far from the nozzle the jet would remove wood.

Figure 1. By slightly tilting the 4-ft wide panel and then having students move the jet past the board along the left-hand edge, a measure of the range of the jet could be obtained.

However, after the students had decided that, for the 10,000 psi 0.03-inch diameter jet, the cutting range was about ¾ of the way across the width (i.e. 3 ft) they were then tasked to pass the jet, as fast as they could, over a piece of pork that was at least a foot further away from where they estimated the distance that the jet stopped cutting.

Figure 2. A piece of pork after being “sliced” by a 10,000 psi waterjet.

The pork was typically cut to a depth of over an inch, grooving into the bone at a distance that the student had previously decided was “safe.” It was pointed out to the class that the pork was a good simulator for human flesh.

The point of the demonstration was fairly obvious, but it does highlight that the distance at which a jet stops cutting one material because of insufficient energy, may still be quite a distance closer than that critical distance for other softer materials.

In one of the earlier scientific papers on waterjet cutting Leach and Walker plotted the drop in jet pressure from two different nozzle shapes, against the distance from the nozzle.

Figure 3. Decline in jet pressure with distance from the nozzle (Leach and Walker)

With poorer nozzle designs and in cutting many harder materials the critical distance at which the jet pressure falls below half the original pressure, and thus in many materials stops cutting, is at around 125 nozzle diameters. For a 0.03-inch diameter jet, cutting a distance of 36 inches takes the range to 1,200 diameters. And while that range is partly because we have significantly improved the fluid flow into the nozzle it relates, as noted, also to the strength of the material being cut.

One reason to mention this is that I have seen, both in photos and real life, people foolish enough to hold their hands in front of a 40,000 psi waterjet, as an illustration of the safety of the tool at even a short range. (Typically they were using jets of around 0.006 inches diameter with the hand about a foot from the nozzle). A slight increase in nozzle diameter, undetected by the operator, or a change in fluid content (such as by adding a long-chain polymer (such as Superwater) could extend the range of the jet several-fold, so that the unsuspecting operator might lose several fingers before realizing the change in conditions.

An earlier post described the work of Clark Barker and Bruce Selberg, who demonstrated that an increase in polish of the inner surfaces and a smooth transition path into the orifice could extend the cutting range of a jet in harder materials from 125 diameters to over 2,000.

Achieving a smooth flow path to the orifice is critical to superior performance, though – as I have mentioned before – it was at one time surprising to me how many contractors did not even have the nozzle insert mating with the end of the supply pipe. Rather, with the nozzle insert held in a holder, they just turned the latter until it was tight, not always achieving contact between the back of the nozzle insert and the pipe. In addition there have been many cases I have seen where the nozzle insert inlet diameter differs from that of the internal diameter of the connecting pipe Again this will interfere with performance away from the nozzle.

Assuming, however, that one has stabilized the flow into the nozzle, and that it is of the right shape, how can one increase the jet throw distance further? The obvious, and wrong, answer is to up the pressure that is driving the jet.

Why is this the wrong answer? Well, if one considers what happens when a jet shoots out into the air, as one can see in a high-speed flash photograph:

Figure 4. Flash photograph (exposure at about one-millionth of a second) of a high speed waterjet showing the structure.

As the jet travels through the air, so the relatively stationary air around the jet strips off, and decelerates, the jet in layers starting from the outside. These show up as backward pointing stringers flowing out from the main jet stream. As the outer layers are peeled off (as with stripping the layers from an onion) so the remaining diameter gets less until, as in the picture above, there is no jet left.

Consider that with a higher driving pressure that there is a greater differential between the air speed and that of the jet, and obviously the stripping action will occur more rapidly, reducing the overall range of the jet.

Now consider if, instead of putting that additional power into pressure/jet velocity one were, instead to put it into additional flow. Then there are more layers of the jet to strip away, and the differential is not as great. As a result, when one compares the performance of two jets one gets:

Figure 5. Comparing the performance of two jets.

Notice in this case that relatively close to the nozzle the two jets, cut to roughly the same depth, and in this range the higher pressure, smaller jet has advantages in that the thrust it applies to the holding tool is less, and the total amount of water used is also less (roughly 4.3 gpm rather than 7.3 gpm). However if one is cutting at a greater standoff distance between the wall and the target, then at about 4 ft from the nozzle (1000 diameters of the larger, 1500 diameters of the smaller) the lower pressured, higher flow rate jet becomes more effective.

This relative change in nozzle effectiveness with pressure and diameter was also reported from results at lower pressure when developing nozles for cutting coal in Germany.

Figure 6. Comparing the pressure profiles of jets at two different diameters and pressures, as a function of distance from the nozzle.(Benedum et al)

Note that here, again, at about 8 m from the nozzles, both jets are producing about the same impact pressure, while closer to the nozzle the smaller (blue line) jet has a better profile (at 0.78 inch diameter, and 1,300 psi) than the larger (black line) jet (at 1-inch diameter, and 1,000 psi). But at greater distances the lower pressure, larger diameter jet becomes more effective.

There is, in short, significant benefit to determining, before one starts, what the objective is and over what range the jet is expected to cut, since both will help decide what set of jet operating conditions will give the better result.

References Leach, S.J., and Walker, G.L., "Some Aspects of Rock Cutting by High Speed Water Jets," Phil. Trans. Royal Society, London, Vol. 260A, pp. 295 - 308.
Barker, C.R. and Selberg, B.P., "Water Jet Nozzle Performance Tests", paper A1, 4th International Symposium on Jet Cutting Technology, Canterbury, UK, April, 1978.
Benedum, W., Harzer, H., and Maurer, H., "The Development and Performance of two Hydromechanical Large Scale workings in the West German Coal Mining Industry," paper J2, Proc. 2nd Int. Symp. Jet Cutting Tech., BHRA.