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 14e - Cavitation and Comminution

In the last post on this subject I discussed how, by adjusting the back pressure in the relatively stationary fluid surrounding a high-speed jet of water, it is possible to intensify cavitation damage. The simple way to find the optimal value for the back pressure for a given jet pressure and size was, we found, to listen to the sound of the cavitation collapse, and by adjusting the back pressure tweek both the sound and range of the cavitation cloud surrounding the jet.

The damage that the cavitation would induce on samples of rock is a function of the time that the cloud plays on the surface. By slowly moving the sample under the nozzle, in a confined cell, different levels of damage could be achieved, based on the speed at which the sample moved.

Figure 1. Traversing specimen cell, with the front cover removed to show the sample in the holder.

When the sample was moved under the nozzle at 2-inches a minute, the cavitation cloud attacked the surface relatively uniformly, with only localized increases in damage. In Figure 2 the red lines mark the width of the cavitation cloud on impact, it then spreads and collapses over the surface to give the wider erosion path.

Figure 2. Traverse over the surface of a dolomite sample at 2-inches a minute. The red lines define the width of the jet. Note the additional depth of removal under the sample ID (15) where the ink chemical had slightly weakened the rock making it more susceptible to erosion.

As the speed of the sample movement is slowed, however, the cavitation attack starts to find weakness planes in the rock and preferentially begins to erode these. As these channels are formed so the jet will flow into them to escape from the following flow of water in the consequent jet flow. As the cloud moves into these narrower spaces, so the pressure increases, inducing more of the bubbles to collapse and thus intensifying the erosion attack along that weakness plane (Figure 3).

Figure 3. Traverse of a cavitating jet over dolomite at a speed of 0.5-inches per minute. Note how the jet is now eating into zones of weakness which are beginning to define pieces of rock that are then liberated as cracks grow all around them.

As the traverse speed is further reduced to 0.4-inches per minute the erosion pattern which is developing in Figure 2 becomes consistent under the full width of the cavitation cloud, and the intersection of developing cracks means that the rock is now being removed in larger pieces and the erosion rate suddenly increases significantly.

Figure 4. Effects of moving the cavitating jet over the rock at 0.4-inches per minute. The cavitation is now developing cracks in the rock that join and break out larger pieces of rock, to a depth of around 0.5 inches over the cloud width.

This ability to focus the jet attack on weaknesses in the rock structure can be useful if, for example, the rock under attack is a mineral ore. Because the ore is defined with weakness planes around the individual constituent grains of the minerals and host rock, at a slow traverse speed the cavitation cloud will preferentially attack those boundaries, in the process liberating the individual grains, and separating the rock into its constituent materials. This liberation can be achieved as the rock is being mined (we have demonstrated this in the lab) so that the valuable mineral can be separated from the waste rock at the mining machine. This means that the waste can be left, in a larger size range than is conventionally left after separation, at the mining site, and does not have to be transported to the surface and ground to powder in order to separate out the valuable minerals. The energy savings that this achieves can be potentially as high as 75% of the total energy currently used at the mine.

Where the mining breaks out the rock without achieving complete liberation a secondary process can be used where the particles of material are fed into a secondary tube, where the particles pass through a second cavitation cloud. The attack of the very small bubbles on the mineral particles is such that the fragments of ore are rapidly broken (comminuted) into much smaller sizes in a process which, because there are so many events occurring sequentially , can appear almost instantaneous.

Tests at Missouri University of Science and Technology, for example, have shown that 0.5-inch sized pieces of coal can be reduced to 5-micron size in a single step. There is a video attached to this post of one of these tests. Figure 5 shows the equipment, with the coal in the inner metal tube, while the surrounding space is filled with water under slight pressure. 

Figure 5. Equipment to comminute coal to 5-microns (The size of the feed coal can be seen in the plastic box on the right).

Water to the cell does not have to be at any great pressure. The test has been successfully run with the water fed from a pressure washer obtained from the local hardware store for less than $100.

Figure 5a. Early in the test the water flowing out of the inner tube is filled with fine particles of coal as the cavitation breaks the pieces down to the required size.

B) A short while later and the outer tube begins to fill with the fine material.

One of the advantages of coal at 5-microns is that it can be mixed with water in about a 50% slurry and fed into a diesel engine, which will then run. GE has tested a locomotive and shown that it is possible to run the engine on the mixture, should conventional diesel no longer be economically available.

The process also works when a harder rock, such as dolomite (a host for galena and other minerals) is placed in the inner tube. The cloud color in this case is white.

Figure 6. Using cavitation to crush dolomite. The original particle sizes are in the box on the left, the cloud of particles is at around 5-microns.

Waterjetting 14d - Traversing Cavitation

Within the normal range of everyday fluid flow cavitation is something to be guarded against. Because it occurs when water is put under conditions where, through geometric or working conditions, it is either pulled in tension or shear to create the small cavities of sensibly vacuum that lead to cavitation damage, there are many ways it can be formed. In an earlier post I mentioned the cases where moving water past a blunt-ended surface at high speed can cause the bubbles to form. In an alternate form, this cavitation can be generated when solid bodies are dragged through relatively stationary water at high speed.

The most common example of this is with propellers and underwater craft, where the relative flow paths over a moving, submerged body can cause bubbles to form and collapse. When the bubbles collapse, even though individually tiny, they can, because of their number, create a fair amount of noise. That noise, generated behind the spinning blades of a driving propeller, was one way in which submarines could be detected and located during the Second World War.

Figure 1. Cavitation forming as water flows around a probe.

Notice that, in Figure 1, the bubbles form and collapse over the length of the probe, which was, in this case, held stationary while water flowed over the surface within a tube. The relative motion is the same as though the probe, a potential submarine shape, was moving at speed through stationary water.

There have been a number of different flow chambers built at different research centers, each in their own way trying to build a device that would allow study of the ways in which cavitation damages surfaces, and to evaluate different materials for cavitation resistance. I have mentioned the ASTM test methods earlier.

Figure 2. German cavitation test apparatus

In one such design, (Figure 2) German investigators built a flow channel where the flow channel was narrowed, and then expanded to induce cavitation in the downstream flow. By then placing a test specimen at the point of maximum bubble collapse, a test could then evaluate the different material responses.

The concept of creating shear, as well as tension in the water around a flowing jet can similarly be imagined where the design above is modified so that the jet that issues into the downstream flow is pressurized to higher velocities. This was the basis for the cavitation cell developed by Andrej Lichtarowicz at the University of Nottingham. This can be applied in a number of ways, in the one below, for example, Canadian investigators had developed a portable version of the concept.

Figure 3. Early method for inducing cavitation around a submerged jet.

However, it was the Nottingham cell that provided the basis for a move forward in the technology as a number of us, around the world, collaborated with Dr Lichtarowicz in trying the new concept. Early on we noticed that if one listens to the noise made by such a jet, it is possible to hear a change in the pitch of the sound as the relative pressure in the surrounding water changes, relative to that of the driving jet. This relationship is defined by the definition of a value known as the Cavitation Number of the condition.

In this equation Pd is the pressure in the downstream fluid, Pv is the vapor pressure of the fluid, and Pu is the driving pressure behind the jet.

For higher pressure cavitation flows the vapor pressure is sufficiently small that the equation can be simplified to the ratio of the downstream pressure (say 50 psi) divided by the jet pressure (say 10,000 psi) which would give a close approximation to the cavitation number ( 0.005).

Dr. Lichtarowcz simplified the design of a cell in which a submerged jet could be directed at a target, with the back pressure in the cell adjusted to control both the intensity of the resulting cavitation, and also its position of maximum damage.

Figure 5. Early design of a Lichtarowcz Cavitation Cell

This design was of interest to us, since it allowed rock samples to be used and evaluated, and we built and tested several different models based on this design. The two windows allowed the jet and specimen to be lit and viewed during a test.

Figure 6. View of a cavitating jet, with the cavitation cloud of bubbles collapsing at the surface of the specimen on the right.

Dr. Hood, in Australia, has shown, with high-speed photographs, how changing the back pressure in the chamber changes the effective damage range of the jets.

Figure 8. Back-lit photographs of jet and cavitation cloud collapse as the ambient chamber pressure is increased.

The above pictures show why, in underwater applications, the range of a high-pressure waterjet becomes increasingly restricted as the pressure increases. This is of great importance where, for example, high-pressure jets are being sent to the bottom of an oilwell to clean the filter screens. The range of the jet is controlled, in part, by the jet diameter, as well as the pressure, but can also be expedient to add different chemicals to the flow in order to enhance the range, and I will write on that in a later piece.

However, it was through the control of the range, and the intensity of the cavitation that we discovered, in applying the cavitating stream to rock, that the damage was now occurring at a fast enough rate that the small samples, and longer test times of the conventional test were no longer viable. The small samples were being consumed in a very short time, and so the design was modified, so that a target block of rock could be moved under the jet, at a rate of around an inch a minute, while maintaining the cell pressure to intensify the damage.

Figure 8. Section of a 2-inch deep hole drilled (at 6,000 psi jet pressure) into a block of dolomite. The damage is caused by cavitation since, at that jet pressure, the fluid would otherwise not damage that particular rock. Note that in this test the sample was not moved relative to the nozzle, and the jet was impacting at the top of the rock, which is to the right end of the hole drilled.

Figure 9. Traversing specimen test cell – schematic view from the top showing the starting position of the jet. The jet is positioned on the end plates until the test starts, and the sample is moved under the jet until the second end plate is reached, when the test is concluded.

The sample had to be moved at a controlled speed since, as the jet cut down into the rock, so the target surface moved away from the jet, and the depth of focus for maximum cavitation damage is relatively narrow – depending on the test condition.

It is in this balance between the effective damage range of the jet, and the intensity of that damage, that is yet to defined in a way that will focus the intensity of damage to its greatest potential. However there are other ways of using that potential, and I will describe those next time.