Waterjetting 26d - Range, position and rewards for jet assisted cutting

In the earlier posts in this chapter I have discussed the problem of getting the nozzle of a waterjet system close enough to the tool:target contact that the jet retains enough power to be effective. At the same time the jet must strike within roughly 1/10th of an inch of that contact to be effective in helping with the cutting process. In the figure below, for example, the jet that comes from the nozzle ahead of the pick will initially strike in that region, while the jet at the back (right) of the pick box will not.

Figure 1. Potential positions for jet nozzles around a conical pick

There is one other consideration, perhaps more relevant in a rock cutting operation than in a metal cutting one, and that is the issue of tool wear. In the above situation while the rear jet can never hit the critical zone, the one at the front of the tool will lose effectiveness as the small carbide cutting cone wears and moves the crushing zone back under the pick shoulder. As an improvement consider the situation shown below:

Figure 2. Simplified schematic showing a high-pressure waterjet hitting the contact between a cutting tool and the underlying rock.

In this case when the tool is sharp then the jet is striking the rock just in front of the edge of the tool, and the performance is enhanced. Further, as the tool starts to wear, so the jet impact on the rock begins to move further forward of the tool contact. But because the face of the tool and the jet are almost parallel the slight change in distance is relatively insignificant.

By the same token, if the rearward jet in the first example had been moved so that the jet struck just under the back of the pick it would still have been able to remove the crushed rock, even as the bit wore. One way to improve the effect of the jet is to spread the water flow by making the jet into a fan or conic spray, this can be effective:

Figure 3. Reduction in thrust with lower pressure fan jets (after Hood)

Again the bit is cooled, keeping it sharper, but also even at the lower pressure if the rock is removed as soon as it is first fractured then it does not crush and then re-compact under the bit.

However higher pressures work better, both in terms of overall rate and in terms of the efficiency of cutting, based on British data.

Figure 4. Change in cutting performance with increasing jet pressure (after Morris 1985)

Given therefore the need to bring the jet to the crushing zone in as powerful a form as possible, one suggestion has been to bring the jet down through the center of the cutting pick.

Figure 5. Nozzle located above the contact point, but fed through the pick body. (After Fairhurst).

The problems with doing this are several. In the particular example shown the orifice is pointing the jet into the rock some quarter-of-an-inch above the crushing zone and this is too far away for the jet to achieve maximum benefit. Further as the tool will wear, so the contact surface will move back further away from the jet, further losing the assistance and failing to be able to remove any of the crushed rock as it is formed.

There are practical problems, however, when (as has been done in Russia) the orifice is brought closer to the tip of the tool. One of the difficulties is that whenever the tool is then used without the jet operating at pressure, then crushed rock will enter the nozzle and within a very short distance plug it with compacted fines.

It is then, frequently, not possible to use jet pressure to get that material out of the nozzle, (particularly when the pump is supplying several orifices on a cutting head). Without the water the tool rapidly erodes, because of another weakness in the design.

For when the orifice is placed within the lower tip of the tool, the volume of the orifice is removed from the bulk volume of the cutting bit, making it much more susceptible to wear.

As long as the jet is brought up to pressure first, and the tool only then brought into contact with the rock or other target, then the tool performs well. Unfortunately (as operators are human and thus prone to the occasional error) cutting heads have often been brought into contact with rock without the jets being at sufficient pressure, and the benefits of the jet assist are thus eliminated due to this loss in nozzle clearance.

There is a corollary to this, in that, as jets began to be used more frequently on cutting heads, the amount of water spraying into the working zone became both a source of irritation and a considerable unnecessary loss in power, given than the cutting head tool only makes contact with the rock for a small fraction of the rotation around the shaft axis.

Figure 6. Roadheader with jet assist working at the Middleton Mine in the UK

To reduce the volume of water, control valves were set into the flow channels so that water was directed at only those picks that were in contact with the rock. The problem with programming this is that, depending on where the head is around the profile of the tunnel, so the arc of the head that the picks are cutting on will change.

But the benefits, where all these different factors are considered in the design and operation of the machine are considerable. As a very rough statement, the cost of a machine will increase more than linearly as it’s weight is increased. In order to cut harder rock without jet assistance, the picks must be pushed harder into the rock, and this thrust must be resisted by the friction exerted between the floor of the tunnel and the base of the machine – usually treads. Thus harder rock requires that conventional machines be heavier. However, when jets are added to the machine that power cost is removed, as the thrust levels are reduced. Thus smaller (and more mobile) jet-assisted machines can cut more effectively than their conventional counterparts.

Figure 7. Introduction of heavier machines to mine harder rock, until the advent of the waterjet assisted machine in 1980 (after Morris)

The savings in the reduced cost of the machine (saving $500,000) more than covered the cost of the high-pressure waterjet equipment (around $100,000).

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Morris, A.H., "The Development of Boom-Type Roadheaders," Seminar on Water Jet Assisted Roadheaders in Rock Excavation, Pittsburgh, PA., May, 1982.
Fairhurst, C.E., Contribution A L'amelioration De L'abbatage Mecanique De Roches Agressives: Le Pic Assiste Et Le Pic Vibrant, Doctoral Thesis, L'Ecole Superieure des Mines de Paris, October, 1987, 221 pages (in French).

Waterjetting 26b - Jet positions to help bit cutting

The addition of a high-pressure waterjet to the leading edge of a sharp tool can make a considerable difference to the performance of that tool. I have discussed this a little in two earlier posts, the first of which was an introduction to the topic, and the second highlighted the problems of getting the nozzle close to the active contact zone so that it can be effective.

In this post I will discuss the benefits that this jets can create in the performance of the machine. The discussion is largely focused on rock excavations, since that is where most of the basic and applied research was developed, but, as I also mentioned previously, this benefit can also be gained if the jets are added to machine tools that are cutting into metal – even metals that are otherwise hard to machine.

The idea of pushing a sharp(ish) tool into rock to break it out goes back to the deer antler picks used to pry flints from chalk some thousands of years ago. But it worked, although the picks are now made of metal and powered by machines. The shapes have also changed over the years.

To make an effective cut requires two different sets of forces be applied to the picks. The first of these is the one that pushes the pick into the rock and gives it the depth of cut that is needed. (I’ll call this the Normal or Thrust Force, since it acts perpendicular to the surface being cut). The second is the force required to pull the tool along the face, this is often referred to as the Cutting or Drag force. Neither are very constant in rock (as opposed to metal cutting) since the rock will chip around the but as it moves forward, which frees and blocks the passage of the tool as it moves.

As I mentioned last time, pushing the tool into the rock will cause the rock under the tool to crush, and then re-compact, if the particles aren’t removed. Thus the most effective time to remove them comes as the tool first breaks them free from the solid. This also saves the energy that would otherwise go into not only further crushing, but also re-compacting the particles. Once they are re-compacted and compressed they become harder to remove and help increase the friction on the tool that cause it to heat, and weaken.

But if the particles are effectively removed, then the region under the bit is washed free, there is less confinement on the remaining rock, and it becomes easier to break.

Figure 1. Crushed rock under an indenting tool. (Richard Gertsch)

Figure 2. Crushed rock under an indenting tool, with the tool removed and a 10,000 psi jet fired at the contact point after removal. Note that there is still some crushed rock that was not removed.

Figure 3. Crushed rock removed during crushing by a jet pointed under the bit as it indented the rock (basalt) (Richard Gertsch)

The impact on the forces that the bit sees can be dramatic. In the early tests of drag bits in cutting the quartzite rock that holds the gold veins in South Africa, Dr. Michael Hood took a tool that normally stalled out under full load, when it was cutting into the rock to a depth of 4mm.

Figure 4. Normal forces on a bit (in KN) without jet assistance (black) where the machine stalls at 4 mm penetration, and with jets at different locations along the cutting face.

Mike tried a number of different locations for the jets at varying points over the face of the drag bit. Initially he used higher pressure merely as a way of getting enough water to the bit to keep it cool, but quickly saw that the performance was greatly improved. As Figure 4 shows, the normal force pushing the bit into the rock was considerably lowered, even when the depth of cut into the rock was increased almost three-fold, with the best location for the jets showing that the machine retained considerably capacity for cutting.

Similar results were obtained with improvement in the cutting forces seen in pulling the bit down the face.

Figure 5. Change in cutting forces with high pressure jet applications to a cutting tool in basalt (Mike Hood)

Again the machine stalled with a depth of cut of 4 mm, without waterjet assistance, and cut to more than 11 mm depth with power to spare with waterjets in the optimal location. This was found to be at the corners of the cutting tool, since in this location the jets were confined by the uncut rock on either side of the tool, and thus rebounded to cover the entire line of contact between the bit and the rock.

Figure 6. Optimal location for the jets on the drag bit for cutting South African Quartzite. (Mike Hood)

For the jet to work most effectively the water must continue to remove all the crushed material from under the bit as it is created. Where the rock is already fractured (as it may be because of natural ground fractures or high stresses on the face because of the depth at which mining takes place) then the confinement of the space around the tool can be less and this reduces the ability of the water to spread along the face of the tool and remove all the crushed rock as it is formed.

Others have also looked at the position of the jet relative to the cutting face, and sometimes, especially in harder rock, where the jet can intersect broken rock above the cutting tool, it may be better to bring the jet into the crushed zone from behind the bit.

Figure 7. Changing bit performance with change in jet pressure at three jet positions relative to the bit. (After Ropchan, Wang and Walgamott).

A slightly different experiment was tried by French investigators who tried locating waterjets around the carbide inserts of a drilling bit. Part of the problem with such bits is to ensure not only that the nozzle is close enough to the crushing zone as to remove the rock, but also to make sure that the nozzle is close enough to the surface that the jet retains enough power. In this particular case, by drilling a small hole through the carbide tool, the investigators were able to bring the two tip jets to the point that they needed, with enough power to be effective. This is shown by the ability to achieve a rate of penetration (ROP) which was more than double that of a conventional bit, with only conventional cooling, for the same amount of thrust force.

Figure 8. Change in rate of penetration with change in jet location on a drill bit.

I’ll return to this topic next time.

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Ropchan, D., Wang, F-D., Wolgamott, J., Application of Water Jet Assisted Drag Bit and Pick Cutter for the Cutting of Coal Measure Rocks, Final Technical Report on Department of Energy Contract ET-77-G-01-9082, Colorado School of Mines, April, 1980, DOE/FE/0982-1, 84 pages.

Waterjetting 5d - Fitting the nozzle to the system

Buying a high-pressure system requires a significant amount of money, and, as a result, most folk will make a serious attempt at comparing the quality of the different systems that they are considering buying, before they make that choice. Most of the expense goes into that part of the system that sits behind the nozzle, and which supplies the water and (where needed) the abrasive that form the cutting/cleaning system.

Often, however, while the upstream system is the subject of such scrutiny, the nozzles themselves, and the selection of abrasive often escape this level of evaluation. Both of these “parts” of the system are part of the wear cost of operations, and, as a result, the selection of the “best’ nozzle often involves operational cost considerations, with less emphasis on comparative evaluations of performance. To explain this most brutally, a company may spend $250,000 on a system, but then degrade the performance of that system by over 50% by choosing a nozzle system that saves the company 15% on purchase costs over that of a competitor. (I will show figures in a later post on this topic).

In the next few posts I am going to explain some of the tests that we, and others, have run to compare nozzle performance, and some of the results that we found. I don’t intend to “name names” because the tests that I will talk about are specific to certain specific objectives, and the reason that you are running a system will likely differ from the conditions and the performance parameters that we needed to match for some specific jobs. The evaluations will range over a number of different applications and will cover some quite expensive tests, as well as some very simple ones that can be run at little cost in time or money.

But, to begin, the first question relates to how you attach the nozzle to the end of the supply pipe. Here you are, if you have followed the train of thought of the last two posts on conditioning the water as it leaves the supply pipe, through a long lead section, or through a set of flow conditioning tubes, the water is nicely collimated and (as I will show) could under certain circumstances have a throw distance of perhaps 2,000 jet diameters or so. Yet the average jet has an effective distance of around 125 jet diameters. Why the difference? An illustrative sketch from Bruce Selberg and Clark Barker*, simply makes the point.

Figure 1. Comparison between a typical nozzle attachment and one where the flow channel is smoothed. (Barker and Selberg)

Right up to the point where the small focusing nozzle is attached to the pipe on the left (a) the flow has been conditioned to give a good jet. But then, just as the flow starts to enter into the acceleration cone in the nozzle it hits the little step at the lip of the nozzle where it attaches to the pipe.

As I will mention in a later post, when a jet hits a flat surface and can’t penetrate then it will flow out laterally along that surface. (This also happens with wind, and is why places such as Chicago are referred to as "The Windy City.") So the outer layer of the jet hits the lip, and where does it go? It runs right into the path of the central flowing jet into the nozzle and mixes right across it. So much for stable flow, that lateral disturbance turns the flow turbulent, so that it is rapidly dissipated once it gets out of the nozzle. Professors Selberg and Barker calculated the theoretical pressure of the jet coming out of the orifices, and compared it with pressure values that they measured.

Figure 2. Measured pressure profiles plotted against the theoretical pressure (small crosses) at different distances from a typical conventional nozzle with two orifices.

In comparison, as a way of ensuring that the flow path into the two orifices was smooth, the two authors added a small section made of brass between the end of the pipe and the entrance to the nozzle body ((b) in Figure 1). They inserted two pins to fit into alignment holes drilled into the end of the pipe, in the insert, and in the nozzle body itself.

Figure 3. Construction of a feed section between the nozzle body and the feed pipe to stabilize the flow (Barker and Selberg)

When the pressure profiles were taken with one of the new set of nozzles, the difference, as a function of distance, was quite marked.

Figure 4. Profiles from the nozzle design shown in (b) with a two-part nozzle (Barker and Selberg. Note that the standoff distance has increased for the two sets of profiles over that in Figure 2.

Further, when the depth of cut was measured after the jets were fired into blocks of Berea Sandstone at various distances from the nozzle, the improved performance was clear out to even further distances.

Figure 5. Depths of cut into blocks of Berea sandstone as a function of distance from the nozzle, at two flow conditions (Barker and Selberg)

The addition of the flow channeling section does make the nozzle a little longer, and the cone angle of the inside of the nozzle was continued out to the diameter of the feed pipe to reduce any steps that might induce turbulence. In addition the inside of both the transition section and the nozzle were polished to a surface finish of better than 6-microinches.

The nozzles themselves were specially constructed for us using electro-formed nickel on flame-polished mandrels and were thus quite expensive. Our particular purpose, however, was in the development of a mining machine that, with the nozzles that we used, was able to peel off a slab of coal, to the height of the seam, and to a depth of 3 ft, at a rate of advance of at least 10 ft/minute. (A later design in Germany went over 6 times as fast, when operated underground).

The advance rate was achievable because the jets were cutting a slot consistently about 2 ft ahead of the machine, and with two jets the coal between them was washed out without having to be mined. But that is a subject for a different post a t some time in the future.

Before I leave the subject, however, some folk might comment that their nozzles sit in holders that are then threaded onto the end of the pipe – thus they should be in alignment, and they are tightened until the holder is tight on the pipe. There are two caveats with this, the first is that this does not necessarily mean that the entry into the nozzle smoothly butts up against the end of the pipe, and in alignment with it. (Hence our use of pins.) In field visits we have measured, for other operators, the relative distances involved, and found that there can be a gap between the end of the nozzle body, and the end of the pipe, both contained within the holder. Even though the two diameters are the same, the presence of the larger chamber before the entry into the nozzle will again create turbulence and a poor jet.

The fix in both cases is a small transition piece, which is simple to design and insert to fill that gap, and smooth the passage. Though it does bring with it the second caveat. You need to make sure that the number of threads of engagement of the holder on the pipe remain enough so that the holder won’t blow off if the nozzle blocks. (One time one of ours did, but it was in a remote location, so thankfully no-one was hurt, although there was some damage as a result).

In the next post I will start to discuss the different ways that we have used, after the nozzle is in place, to make sure that the jets were doing what they were designed to and producing a jet of the quality needed.

* The information that I used in this article can be found, in more detail, in the paper: 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.