Waterjetting 31c - Jet interactions and traverse speed

The last post included the idea that, with a jet penetrating to most of its final depth in the first 1/100th of a second, that the best traverse speed that could be used would end up around 50 ft per minute. Actually this is a little low as an estimate, and the reason for this goes back to a picture from another post. On the other hand the speed may be reduced a little, as a way of getting more material out of the target faster.

Figure 1. Jets penetrating through a bed of glass beads. The original image has been duplicated and the two shown together. The darker green is where the water has penetrated between the glass beads.

In the picture shown not only is the water penetrating and cutting down into the bed, it is also, to a small degree, penetrating into the side walls of the hole, and this becomes more evident as the jet reaches toward terminal depth, and the jet coming back starts to interact with the jet entering the hole, giving the bulge in the penetration, as the water starts to reach out further into the material.

While it obviously varies with the material, the side penetration of the jet into the material around, and ahead of the jet means that the material is, to a degree, pre-weakened by that penetration, and becomes easier to cut, so that the best speed for cutting can be slightly increased from the number derived last time.

There is another benefit to this, which the Chinese developed in one of their mining machines, although it also has application in milling and other removal techniques.

The high optimal speed of the jet was developed by oscillating a waterjet nozzle vertically, as the head was traversed, more slowly along the horizontal. Because of the benefits of the oscillation the head cut a wide path through the material to about the same depth as it did when the jet was traversed without oscillation. The volume removed however was roughly an order of magnitude greater.

Figure 2. Slots cut into a soft cement. The two slots are roughly equivalent in depth but the lower one had the nozzle oscillate perpendicular to the traverse direction across the block. (see here)

One can, as a result, remove much more material where a plain waterjet (as opposed to one that carries abrasive or cavitates) is moved across the target surface.

When looking for the best combination of oscillation speed, as a function of the horizontal speed in cases such as those shown in Figure 2, it is important to consider how much of the water has penetrated into the side of the cut, as shown in Figure 1.

The softer the material, and the easier that the jet penetrates into the wall of the slot, then the further apart the two passes can be made. But that inter-cut distance varies also with the speed that the primary cut is being made. If the cut is being made at the most efficient speed to remove material, then the speed will be high so that the rebounding jet does not start interfering with the incoming jet, as is shown in the lower parts of figure 1.

Thus the two adjacent passes should be made close enough that the two layers of darker green around the upper parts of the slot depths in figure 1 just barely overlap. On the other hand if the traverse speed is slowed so that the interaction of the incoming and outgoing jets does occur, with the result driving water into the ribs to the extent shown at the hottom of the holes in figure 1, then the two cuts can be spaced further apart, roughly at the distance shown in the figure. This is because any rib of material between the two cuts is now cut on either side and beneath by the penetration of the water, and will be removed with no additional energy being required.

This is particularly handy in materials that have a lot of joints such as, for example, coal or soil. Here the spacing can be increased significantly.

Figure 3. Slot cut into coal by a nozzle with two orifices (upper left) oscillating vertically on the front of the plow shape shown on the right. The slot is roughly 2-inches wide.

However this does not work as well where the target material is stickier, for example with a clay.

Figure 4. Cuts made into wet clay – to help in visualization the cuts are filled with bentonite (white) to contrast the shape and location with the darker clay that was cut through.

In drier clays and shales, where the material responds in a more brittle way, the rib between the two adjacent cuts may shatter and give the larger volume removal rates required.

Where this is not the case, and where the cutting ability of the two jets can be better estimated then two adjacent jets can be aimed so that they meet within the body of the target.

Figure 5. Waterjet cuts in claystone with the two cuts inclined so that they met at the bottom of the wedge shape shown. The wedge is loose, and can be lifted out of the block by hand.

In this way, by arranging a set of jets into a pattern which will be controlled by the shape of the cut to be made, and over what area, a large volume of material can be removed with the use of the least amount of water.

Making the best use of this combination does require some testing to determine how best to angle the jets so that they meet at the right distance into the target and with sufficient force remaining that they will remove the isolated blocks of material between the two (or more) jet paths. In these cases it is best where the jets are traversed over the surface of the target at a slower than optimal speed, since a small buildup in pressure at the bottom of the common slot will make it easier for the jet to dislodge the wedge of material between the two cuts.

As material gets stronger, however, the benefits of that penetration become less, and to make sure that the material between the two cuts is removed, the two cuts should be aimed so that they will intersect within the cutting depths. With vertical slots the two cuts can be brought quite close, with no interaction between them.

Figure 6. Parallel adjacent cuts in sandstone, where the ribs are not removed by the jet between adjacent passes.

Waterjetting 20a - Making holes in soil

There are two problems that often arise when applying waterjets to a soil-like material. The first of these is that the water can spread into the surrounding soil around the hole being excavated so that it loses its strength and can collapse into the hole. (This can be used to advantage in some cases.) This is a particular problem when excavating trenches, where the hole has to be as small as possible, yet the sides have to be stable so that work can be done at the bottom. The other is that, once the soil is loosened it has to be picked up and moved, and a way has to be found to be sure that the particles don’t settle out before they should.

Figure 1. Three consecutive frames from a video record of a jet firing into glass beads behind a glass wall. The framing rate was 30 fps.

In the sequence of frames shown above the jet is seen to first penetrate down through the beads (which were simulating soil, being easier to handle and see through), and then when it reaches about a nine inch depth it stops penetrating and starts to widen and fill the hole, which until the is relatively open. Note that at this stage there is no ejected material from the hole and very little penetration of the water outside the line of the hole. The penetration stops when the water no longer has the energy to push the particles aside, and continue to penetrate. (The test was at relatively low pressure to keep the penetration small enough to remain inside the box). Note also that the hole is largely hollow at this time.

If the jet is allowed to continue to play on the surface, the water will now penetrate into the material on either side. This can be better seen if the fluid color is changed to black by adding fine carbon particles to the water. The pressure was further lowered (to 100 psi) to keep the jet penetration down to below three inches, and in this case the jet was a fan shape to encourage the spread, rather than being projected through a round orifice.

Figure 2. Video frame taken as a waterjet laced with fine carbon penetrates into glass beads, note that the carbon starts to be carried into the surrounding material, and that again, in this short time interval there is very little material being ejected from around the hole.

In passing it might be noted that this is a relatively effective and simple way to inject remedial chemicals into layers of clay and soil that could contain undesirable chemicals (such as PCBs) but where going in to remove the contaminant might be difficult and cause other problems. This could occur if the contaminated layer is now covered with more material, and trying to dig the contaminated material out would cause it to disperse into a stream, river or bay where the problem has been found to lie.

But if we want to remove the soil, then the process, as it stands with using a single jet, is fairly inefficient. The water, at this pressure, is cutting into the soil, making a hole, penetrating into the soil around the hole, but not much is being moved. Again that could be an advantage.

Those who play golf know that good maintenance requires that golf greens need to be aerated at regular intervals to keep the grass healthy. At the same time, pulling plugs of material out of the green is disruptive, and conventional mechanical tools will still make a bit of a mess, and take some time. On the other hand Toro has developed and improved a tool – the Toro Hydroject – which has a series of jets that are spaced at adjustable intervals (but typically around 3 inches) along a distribution manifold, so that when the jets pulse they drive holes down into the soil, with no surface spillage of soil. And this can be done at walking speed – between foursomes, and without disrupting play.

Figure 3. Toro aerator at work. (After Toro )

The tool is also effective in poking holes under pools of water to speed drainage. The most effective pressures vary for different soil types and conditions, but are typically in the low thousands of psi, with penetration depths of up to eight inches. (They are also a potential tool for finding land mines, but that is another story).

Figure 4. Cut through a hole jetted into soil (turned on its side for convenience) (Toro)

Yet these applications again illustrate that the tool might be more difficult to use, where the main purpose is to remove the soil, and where a sequence of passes of a jet over the surface won’t potentially move much material.

The answer to this problem is to use more than one jet at once, and to place them at some distance apart, depending on the soil and jet parameters this might be more than an inch or two. What now happens is that the resistance of the soil is removed when it the jets pass along either side of the intervening rib at the same time. This very rapidly liquefies the rib in the middle, and it is removed as the jets cut past.

This can be seen where, for example, two jets simultaneously traverse over a clay bed. When only one jet was used it cut only a thin slice into the clay, with little material removed.

Figure 5. Single consecutive cuts into clay with a water jet that also contains kaolin so as to show where the cuts were made. (Note that even where the cuts are close together there is no removal of the ribs between cuts.)

Figure 6. Material removed when two jets cut side-by-side into clay. Note that all the intervening clay between the jets has been removed, to a depth of four inches.

The contrast between the two figures shows that by changing the way in which the jets cut into the material (concurrently rather than consecutively) up to ten times or more material can be removed from the surface for the same amount of input energy.

Figure 7. Slot cut in the ground by a combination of jets acting together as a head was moved through the ground (Halliburton - the Soil Saw)

This also works with some rock types, and I will discuss how we used it to design a machine for mining coal in a later post, but it is not the end of the story in developing the soil cutting aspect. The problem that can then arise comes from the type of soil that is being cut, and the distance between the jets. As the soil becomes more coherent (clay laden) the jets need to be brought a little further together (and the more sandy the soil, the further apart they can be.) But if the jets don’t have enough time to totally break up the rib of material into particles (controlled by how fast the jets are being moved over the surface and the relative depth of cut) then the pieces may come out in lumps of varying size and shape. These are more difficult to break up, once they break away from the solid.

The alternative is to move the jets relatively rapidly over the surface. This shortens the depth of the soil that is being moved at one time, but if, for example, the jets are being spun around an axis inside a shroud connected to a vacuum system, then the particle sizes can be controlled to fit within the vacuum line, and the depth of cut is small enough to hold the partial vacuum around the edge of the shroud to make sure that all the particles and water are removed and the walls are kept dry enough to remain stable. But, again that brings us into hydro-demolition and I’ll cover more of this in a later post.