Waterjetting 27e - Borehole Back Pressure Effects

In the earlier posts on this chapter of waterjet technology I have dealt with the changes in cutting performance when a waterjet stream cuts in to material that is either under pressure, or contains internal stresses that may not be obvious at first glance. In this post I will focus, instead, on the changes in performance when the borehole becomes filled with water under pressure.

Figure 1. 12-inch cores of sandstone that have been drilled by the same jet drill, at the same speed, but at borehole pressures of 0, 500 psi, 1,000 psi, 1,500 psi and 2,000 psi. (Jet pump pressure 10,000 psi; 970 rpm; 40 inches/min ROP)

The water used in the test also contained a small amount of polyethylene oxide (Polyox) that, at the time, was the only polymer readily available to enhance jet performance under water, although there are now liquids such as Superwater that similarly help.

It can be seen that even the change in pressure to 500 psi is sufficient to dramatically shorten the distance that the jet cuts through the material on a single pass, and the range then only shortens a little as pressure further increases. But the hole drilled at 2,000 psi is barely large enough to let the high pressure lance and nozzle assembly pass.

First an explanation of the equipment that we used to run the tests. A triaxial cell was used as the basic vessel to hold the core. This is so-called since it allows pressure to be applied around the rock core, and also since the cap can slide within seals, axial pressure given the third of the orthogonal directions for loading.

Figure 2. Triaxial cell used for the drilling experiments.

A valve was fitted on the flow line of water out of the chamber (just above the pressure dial) and this controlled the fluid pressure in the cell. The diameter of the outer (reaming) jet was 0.04 inches, and the rapid decay in range with the increase in pressure led to a second experiment, to see how changing the diameter changed the results. The equipment was modified for this test, the feed pipe to the nozzle was bent, so that, as it made a single circuit over the underlying rock, it would trace out a circular path rather cut a single hole. Then the top of the sample was cut at an angle so that, with the rotation the distance from the jet to the target would vary and the range of the jet could be seen. (Figure 4).

Figure 3. Modified equipment to find the effective jet range against back pressure.

A simplified factorial experiment was run with three nozzle diameters and five back pressures, measuring the depth of cut into the sandstone in each case.

Figure 4. The resulting cut when a 0.03 inch diameter jet was rotated over sandstone with a 1,000 psi back pressure in the cell. The 10,000 psi jet was brought up to pressure with the jet at the greatest standoff (hole at the bottom) and the back pressure was set before making a single pass over the sample. The depth of cut was averaged over several readings made along its length.

The data was then plotted (with the curve smoothed here for simplicity in discussion).

Figure 5. A plot of range of jet cutting ability as a function of hole back pressure for three different nozzle diameters.

The graph shows that, for this set of conditions, the larger the jet the better, and that the first 500 psi of back pressure has an immediate effect on jet cutting effectiveness. Jet size should be at least 0.064 inches when drilling against back pressure in the hole. There was a significant improvement in cutting ability when the polymer (at 300 ppm) was subsequently added to the water, in a later series of tests. The small number of tests carried out, however, were too small a sample to provide more than guidance as to concentration since all three levels tested (100, 200 and 300 ppm) all showed considerably improved depths of cut (increasing to a depth of almost 2 inches against a back pressure of 2,500 psi) when contrasted with the performance levels shown above. The polymer tests were carried out with a jet nozzle diameter of 0.064 inches.

There are two parts to the effect of the borehole pressure. The first is simply one of increasing the resistance of the water to jet penetration, and lowering the effective jet pressure (since that is effectively the jet pressure less the borehole pressure).

It is important to recognize that it is not just the drop in effective pressure that causes the effect. To check that this was the case a hole was drilled with the same conditions otherwise as the left-hand rock sample in Figure 1, except that the jet pressure was dropped to 5,000 psi. Thus the differential pressure of the jet across the nozzle was less than that in the case of the other four rock samples shown in Figure 1. Yet the hole was of the same approximate irregular geometry as that shown by the left-hand core of Figure 1 even with the lower differential pressure with the prominent cone cut ahead of the bit that is not evident in the other cases.

Mike Hood has shown the effect of loss in cutting range by using back-lit shadow images of a jet at different back pressures.

Figure 6. Illustration of the effect of fluid back pressure, the shadow image of the jet shows how back pressure reduces the range.

As mentioned above, the effects extend beyond reducing the jet range, and lowering the jet differential pressure. The increased confinement on the rock will compress the grains of the rock more tightly together, making it more difficult for the pressurized water to penetrate into the rock structure. This combines with the higher pressure required to grow the cracks to effectively reduce the ability of the jet to penetrate into the rock.

At the same time, if you listen as the back pressure is increased (we used a Lichtarowicz Cell the increasing pitch of the sound shows (as does the damage induced) that the collapse of the cavitation bubbles generated around the edges of the submerged jet is becoming more intense as the pressure increases. I have discussed how this can be used as a benefit in breaking up rock in an earlier post.

Waterjetting 26c - Cutting tool shape

When it was first discovered that high-pressure waterjets could significantly improve the performance of mechanical cutting tools, whether in machining metal, or in cutting rock, it was anticipated that this would have a broad-ranging application. This has not been the case, and the reasons are varied, depending on the application, but quite often they relate to the way in which the mechanical tool was expected to work. The examples will, again, come from rock cutting, but also apply when cutting or machining other materials.

Figure 1. Three common types of picks used in cutting into stone for driving tunnels, or for cutting and mining coal.

The initial work of Mike Hood, in cutting quartzite, had used a relatively simple flat-faced bit that was dragged across the rock surface at a known depth of cut and directing s single, or pair of jets to cut along the line of contact between the rock and the carbide was relatively straightforward.

Figure 2. Locations of the jets for Mike Hood’s initial tests on improving jet performance. (Dr. Hood)

Getting the jets to cover the full zone of contact and rock crushing was critical to achieving the best results for the tests, and proved also effective when the cutting tools were tried in the field.

Figure 3. Relative normal forces on a cutting bit with change in the position of the jets assisting in the cutting of rock (Dr. Hood). Note that the machine stalled at 4 mm penetration without the jet assist (the black line).

The cutting picks that are more commonly used in softer rock, shown in figure 1, are not quite the same shape, nor have quite the same purpose. Early trials were with the forward attack pick, which through the early 1980’s was the most common design used.

Figure 4. Laboratory trials with a jet added to a forward attack pick

Rather than having a flat face, this pick has a wedge-shaped front face. This is so that, as the pick cuts into the rock, so the wedge shape pressing into that groove will put a high lateral load on the rock on either side of the cut, causing it to shear off the solid. Those chips can be seen to the front right of figure 4.

Where the jet cuts into and removes the crushed rock under the front of the bit, this allows the bit to make a deeper bite, and this, in turn, makes it more likely that the tool will make larger chips. This is not an unflawed benefit, particularly if the jetted slot now extends a little deeper than the tool.

Figure 5. Illustrating the wedging action of the tool in creating lateral chips beside the tool.

As the chips get larger so the force required to break them from the solid increases, and the actions do not occur symmetrically on each side of the tool. As a result the lateral loading on the tool becomes more significant, and because the process of chip forming and breakage is cyclic so there greater fluctuating forces make their way through the drive train back to the driving shaft and motors.

With most machine designs these fluctuating loads are, however, reduced in overall magnitude, because of the reduced forces needed to move the pick forward, without having to deal with the crushed material under the pick, which the water jet has removed, providing it is within about 1/10th of an inch of the cutting tool.

Achieving that positioning becomes a little more difficult with the transition to a radial pick, however there were additional problems with that intermediate design, particularly in harder rocks, where they wore out at rates as high as 7 picks per foot of advance. This led to the development of the point attack pick, as shown in figure 1.

This pick design has become the most popular for use in mining machines over the past 20-years. The round shape of the tool and shaft are designed so that, as the pick wears it will rotate in the holder, and this will spread the wear evenly around the tool, making it last longer – and in the case mentioned in the last paragraph a change to this design reduced pick costs to around 1 pick per foot of advance. But there are a couple of problems with adding waterjets to this tool.

Figure 6. Point attack tool geometry (Goktan and Gunes)

This geometry makes it very difficult to bring in a waterjet to hit the right point at the rock:tool contact, because of the double cone at the end of the tool. While the nozzle can be positioned so that it can direct a jet into the right point (for example by being at the point where α is in the figure) the problem arises with the size of the nozzle mounting block, and the small size of the jet, where a large number are being used to cover all the picks on the cutting head and total flow volume is limited. To fit the nozzle block means that it must be at a greater standoff distance from the point (perhaps four or five inches), while the small orifice size means that the effective range of the jet may be no more than an inch or two.

The change in pick design and the difficulty in adding waterjets to the new tool therefore led to a discontinuation of the trials of the combined system. This was unfortunate since the forward attack picks initially cut better than the point attack, but wore out more rapidly – hence the change. But with the addition of the waterjets the tool lifetime, and sharpness, was increased in some cases more than five-fold times, while the other benefits – such as the ability to use smaller machines to carry out similar performance – made capital investment less. But these events occurred at the wrong time, as the coal market was entering one of its down cycles as the developments were being made, and the technology was therefore not adopted.

I will conclude this small chapter next time, by addressing one of the answers to the problem of getting water to the point attack tool.

Waterjetting Technology - 26a More on waterjet assisted cutting

When mankind first began cutting out flints to make the tools and weapons that helped make primitive life more successful they often used either bone antlers or stones from the river as the tool to cut into the chalk or other host rock that held the flint. For thousands of years as rock was excavated for broader use, including making building stone, the rock continued to be cut manually, and it has only really been in the last hundred and fifty years that manual picks have been replaced with power driven machines. However, in great part, the machines have had to be made larger and heavier than they might need to be because, in large part, unlike the pick swung by a miner, the machine cannot selectively attack the rock that it is facing, but must cut along a foreordained path.

Figure 1. Conventional tool path in cutting concrete, the tool has to cut through both the hard aggregate pieces as well as the softer cement.

The tools that cut through the rock mechanically must, therefore, be able to cut through all the different materials that they are likely to encounter. Where the rock is like a concrete, with hard and soft parts, then the tool must be able to cut through the hard (aggregate) as easily and fast as it removes the soft (cement) phase if the machine is to maintain productivity. When I wrote about cutting concrete, I pointed out that this “brute force and ignorance” approach to getting through material was expensive in the time that it took to make a hole, and in the energy that had to be expended, both combining to make the overall process itself more expensive than it need be. That cost includes not just the costs of the process itself, but is also less obvious in that the machine itself has not only to be bigger, but because it also typically sees a wide range in force applied through the cutters to the drive mechanism, it also has a shorter operational life because of these fluctuations. (One shearer model saw such failures within six months of start-up).

There are a number of different ways in which the sensible application of high-pressure waterjets can improve cutting performance, lower machine size and cost and provide a win-win situation. But there is a need for caution, since the small size of the waterjets that are often used is much below the scale of many other parts of the machine, and, as a result, the precision with which the jets need to be applied can often be neglected.

In an earlier post on this topic, I discussed how a mechanical tool will crush the rock over which it passes during cutting. This crushed rock confines the bit, and is often re-compacted so that frictional forces rise, and the temperatures can be high enough to soften tungsten carbide.

Figure 2. Crushed rock under the impact of a mechanical pick. The size of the indentation relative to the size of the crushed rock is evident.

If the jet is to be effective it has to be directed into the cut at the point where the crushed rock is being created, so that the jet can remove the broken pieces as they are being formed. It is this critical location of the jet relative to the bit:rock contact that is often missed by those who have tried to apply this technology in the years since Dr. Mike Hood first demonstrated the benefit.

A number of experiments over the years showed that if the jet is more than about a tenth-of-an-inch (2.5 mm) from the point of the pick where it enters the rock (or the edge of the tool if it is a broader shape) then the jet will not be able to reach and remove the crushed material. This is particularly true when the rock being cut is, as in the above figure, a basalt, which the jet of water cannot normally penetrate at pressures of 10,000 psi. thus, if the jet does not reach the crushed material then the energy put into its creation has been wasted.

There are two parts to that last statement. They deal with all three planes in which the jet lies, relative to the point of pick contact. There is the relative position at which the jet hits the rock, where it is critical that it hits just where the rock is being crushed, and then there is the distance of the nozzle from that contact point. The latter point is one that I have also written about in earlier posts, but which can also be neglected when engineers are designing systems. There are a number of papers (which seemed to be at a peak at the 8th International Waterjet Symposium in Durham, UK in 1986) where this distance was set incorrectly (values up to 0.3 inches and above were reported) and it is not, therefore, surprising that some investigators found that the results were not as good as expected.

Figure 3. Jet cutting at the front edge of a pick (Front cover of the 8th International Symposium on Jet Cutting Technology, BHRA, Durham, UK, Sept. 1986)

If the nozzle is too far from the rock contact, then the pressure of the jet will have fallen to a pressure that is too low to be effective. This has been a less obvious problem to overcome, since to many observers the jet seems coherent with distance, but, given that jet flow is often divided between a number of different nozzles on the cutting head, the individual orifice sizes can be quite small (perhaps 0.01 inches in diameter). If the effective jet throw distance is 125 diameters, then the range of the jet is 1.25 inches. Yet in a number of applications, because of difficulties in fitting the nozzle in place, the orifice can be placed more than 2.5 inches from the rock contact. Again the result of this is to make the jet sensibly ineffective.

The jet has to be put into the right place, and with the correct amount of power, if it is to be of any use. Sometimes that can mean that the nozzle is placed behind the pick (so that it can be protected by the pick from the rock, yet can be brought close enough to the crushed zone that it can penetrate it from behind. This is a little more difficult to achieve, since the precision of location is a little more difficult.

Others have tried feeding the jet down through the pick, and I will explain some of the benefits and problems with this as I continue on this theme next time.

Waterjetting 25c - more thoughts on jet range

A single waterjet, whether with or without abrasive, will cut a tapering slot as it penetrates into a target material. This is because, as the jet penetrates into the surface, the outer edges of the jet lose their energy in cutting, and the narrower central core remains capable of cutting, on a continually narrowing path, as the cut deepens.

Figure 1 Tapered cut made with a single jet traverse in contrast with the wider cut made with two diverging jets.

While the above statement is generally true, it is not completely so, since if the speed of traverse of the jet is reduced, then the continued addition of further water along the cut plane will be sufficient for the outer layers of the jet to be able to continue to cut and this may reach the point that there is no taper along the edges of the slot, or it may even taper inwards. For an abrasive jet cutting into titanium, that transition occurs at around 0.2 inches/minute, depending on jet parameters. (Note that this is less related to the target thickness, although it is controlled by the cuttability of that material, and that the critical speeds for cutting with water along are at least one and often two orders of magnitude greater).

Figure 2. Plot of taper angle with traverse speed.

Unfortunately the speed at which the edge is cut perpendicular to the top surface of the target is usually too slow to be economic, and, in consequence, the normal process is to slightly tilt the cutting head into the edge with the desired surface, and making the opposing surface carry an exaggerated tilt. This then allows a faster cut, again with the optimal speed being a function of both tilt angle and jet parameters.

When the objective, however, is to achieve a deeper cut, particularly where multiple passes are concerned, and head movement into the cut is allowed, then a different strategy can be followed.

Back when we were developing the longwall mining machine we called Hydrominer, we used a dual-jet system, because, when cutting coal, the material between two adjacent, concurrent cuts is removed as those cuts are made. Thus the jets, in a second pass, do not make contact with the walls of the cut until reaching the back of the previous cut. (The second image in Figure 1).

Figure 3. Slot cut by the Hydrominer, looking down, and with the slot through which the jets cut out from the head visible on the left edge of the machine.

However, in harder materials, including rock with some degree of cohesion, it is possible to run two jets almost side by side, and leave a rib of material between the cuts, so that jet attenuation in dual cutting is still a problem if the jets are parallel.

Again the answer is to tilt the jets, although if small jets are used, multiple jets may lose in overall range, because of the reduced diameter of the individual streams.

In this case it can be more effective to combine the jet flows into a single jet, but to either orbit or rotate this slightly off-axis so that the jet is cutting a slightly wider track along the path, and with a widening slot with depth, so that, again, subsequent passes, where the nozzle moves into the slot, do not encounter the walls of the cut until the back of the previous cut.

Back in the days when we were first testing the coal mining machine, we were mining coal in northern Missouri, and the coal had a large number of pyrite lenses in it. These lenses could be up to four inches thick, and, while the coal was friable and easy to cut, the pyrite lenses were much harder and dense. They could not be easily cut with the jets, which were operating at 10,000 psi, and the machine was not performing very well.

There were two ways in which we overcame the problem. The first was to adjust the two jets that were cutting the slot into which the cutting head was moving. As I mentioned earlier with a slight divergence angle between the jets, the slot was cut wide enough (around 2-inches) for the leading edge of the head to enter the cut, and the depth (around 9-inches) was enough to give leverage for the head to peel the rib of coal from the solid.

Figure 4. Comparison of results in the field with initial lab-designed nozzle.

When we encountered the pyrite, we changed the angle of the jets, so that instead of diverging the converged at varying distances in front of the head. When the two jets come together at this shallow angle (as with shaped charge formation) they form a very high speed jet, as well as a slower moving wider stream.

When this combination replaced the diverging jets on the head, this higher-speed jet was sufficiently powerful that it cut through the pyrite, and gave a free surface for the rest of the lens to break into. (Depths of cut up to 3-ft were achieved, although the slot was less than one-inch wide). This worked well for the side of the slab that was now liberated, since the jet had broken it free, and the head could move it away from the face, and into the conveyor track.

The only problem that we had at the time, was that the convergent jet was formed in the center of the slot being cut and in the center of the leading edge of the mining head. The slot was no longer wide enough for the head to enter (the converging jet gave a slot about half-an-inch wide IIRC). As a result the pyrite on the solid side of the cut now engaged with the leading edge of the head and stopped progress.

The answer to the problem, which we arrived at over time, was to change the angle of the axis of convergence of the jets, so that, instead of being in the center of the slot, the convergent jet was inclined over towards the solid, and cut into the pyrite just ahead of the outer edge of the mining head. In this way, since the material to the free side of the head was being moved out of the way by the advance of the machine, the jets still cut clearance for the head to move forward. At the time we were only able to get the machine up to a speed of 10-feet a minute, but by taking a bite of 36-inches at a time, we were able to match the productivity of existing mining machines of the period. (The coal seam was 5-ft high). The guard design on the head was also changed to give a sharper edge on the solid side of the machine.

Figure 5. Change in head guards to penetrate pyrite.

Very little work has been carried out on convergent jet systems since that time, which is a pity since it allowed us to mine harder material than the main jet pressure available was allowing us to achieve.

Waterjetting 25b - range with abrasive

In the last post I wrote about the impact of smaller jet diameters, and higher pressures, in truncating the range over which a waterjet is effective. The same is true, to an extent, when one adds abrasive to the water.

Our “green tube” test has been described in earlier posts, where the distance over which particles settle out of the jet provide a measure of how much energy they were given. However it is not that simple to interpret the results from these tests. The reason is that, as with a plain waterjet, the range of the particles is controlled to a degree by the size of the individual grain. Why is this? Well this series tries to keep formulae to a minimum, but one is needed in the answer to that question.

The origin comes from our friend Newton, whose Laws have come down to us over the centuries, and the second of which states:

Force = mass x acceleration

Consider that when an initially stationary particle is sitting in a jet stream, the force being applied to it by that jet is equal to the pressure of the jet, multiplied by the area over which the pressure is applied. If for simplicity we assume that the particle is spherical, then the area over which the pressure is applied (assuming that the particle is centralized within the jet stream) is equal to the pressure multiplied by the cross-sectional area, which is given by the product of the square of the radius multiplied by pi.

On the other hand the mass of the particle is related to the volume, which is in a cubic relationship with the radius. Thus if these two terms are substituted in the equation above, and combining all the non-radial terms into a constant results in an equation where:

Acceleration x radius cubed x constant = pressure x radius squared x constant

Rewriting this gives:

Acceleration = (1/radius) x pressure x constant.

This means that larger particles have smaller accelerations for a given pressure, while smaller ones accelerate faster.

However, and this pertains to the results we saw from the green tube tests, just as the smaller particles are accelerated faster while in the jet stream as it passes through the nozzle assembly, so those smaller particles will decelerate faster when having to travel through the relatively stationary air outside of the nozzle.

Thus if you are, for example, using a smaller jet flow (and smaller jet orifice in consequence) then the normal practice is also to reduce the size of the focusing tube, and – to otherwise keep the system practical – to also reduce the size of the abrasive particles fed into the system.

However, while this gives a better cutting effect immediately under the nozzle (hence the widespread recommendation to restrict the standoff distance between the nozzle and the target to about a quarter-of-an-inch) there is a more rapid decline in the speed of the particles as they move away from the nozzle. The net result is a shorter range for the jet, and a shorter cutting depth in consequence.

There is a small caveat to holding this as an absolute conclusion. Back in the days of the U.S. Bureau of Mines Dr. George Savanick showed that where an abrasive jet could be held within a relatively narrow slot, as it cut down, that the walls of the slot tended to concentrate the jet, and thus extend its range beyond that achieved if the jet were, for the sake of example, just cutting through a piece. Thus, when not through-cutting the part, there will be some extension of the jet range, and this has to be considered when setting the operating parameters for a particular job.

Which brings us back to defining the optimum size of the operating plant required to complete a given job, and a resolution of the optimum parameters for carrying out the job.

As I have noted before, this is not a simple and straightforward choice. Proponents of different operating systems will advocate different solutions based on the units that they are most familiar with. And there are arguments that can be made for different choices. However, in making the choice of the best system to use, one must be aware of the limitations (as well as the benefits) of the different choices that might be available.

Consider that a lower pressure, higher flow rate system might use larger particles, and thus be able to cut through a target plate of a given thickness with better speed than a higher-pressure, lower flow rate alternative. However were the target to be of a thinner stock where the range of the jet is not that critical, then the higher pressure system may well give the better performance (given, inter alia, that it will also use less abrasive and water).

Making a selection as to the better operating system, therefore, requires a clear understanding of the different modes in which the system is likely to be used. Will it be for relatively thin materials, where high precision and narrow cuts are required, but the material need not necessarily be through-cut. Or is the system one where a cut may be required through perhaps 30-inches of reinforced concrete in a reactor (of which more in a later post). In the latter case the lower pressure, higher flow rate jet, with the ability to use larger particles and sustain their velocity further, when further confined by the walls of the cut. The former condition would argue for the use of a higher-pressure, lower-flowrate combination, while the latter (as a generalized statement) would incline more to the lower pressure alternative. (And the terms are relative, since in the latter case we are likely still talking about pressures of around 30,000 psi or higher to achieve the depths of cut within the reinforced concrete.)

Much is written about having to make absolute choices in cutting, but in many cases it is only a matter of relative performance, with systems across a range of parameters being able to effectively achieve the goal. The selection of which system to use should focus more on the normal range of materials that one is expected to cut in the normal course of operations. (And slicing though parts of a nuclear reactor is not normal in most aspects of this business).

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.