Waterjetting 34e - Hole completions and core removal

When I began writing about hole cutting and drilling, a month ago, I was intending to talk just about the relative efficiencies of cutting the core into larger pieces, rather than designing a cutting pattern that would completely cover the surface of the excavation, milling and removing the core in fine particles. Other topics intruded, however, and it is only now that I am going to conclude this theme by discussing that particular point.

Earlier posts have discussed how, by inclining two jet paths so that they come into close proximity within the target (or intersect in some cases) a much larger volume can be removed with no increase in input energy to the process. The gain in production comes from working out which are the best angles to set the cutting jets at, relative to the overall work piece.

Figure 1. Intersection of two jets in cutting clay. The cuts were made with the sample lying horizontally – in the actual operation the jets cut up and down vertically and the included wedge would normally fall out.

There is no universal rule for selecting the best angle for this, or for selecting the best relative depth for the intersection. It depends on the material being removed, and on the logistics and relative sizes of the hole being driven, and the components that fit into it. The material responses depend very much on the strength and structure of the material. The paths of both jets will have to intersect to remove materials such as steel, whereas with clays, and many rocks the two paths need only be relatively close at their lower end for the intervening rib to separate.

The problems are not just constrained to the removal of that core. One of the significant problems that exists even when using conventional techniques to drive tunnels and other large holes is that of making sure that the diameter that is being cut remains the same size as the tunnel advances. Because it is easier to break the rock within the tunnel wall, because of the release of the surrounding rock pressure, it requires additional effort to cut out beyond the projected perimeter in order to give enough space for the tunnel. With explosive blasting of the tunnel this means that the perimeter holes are drilled out beyond the projected tunnel line, and in cutting with a waterjet a similar strategy is required.

This problem is not normally that severe, since the size of the nozzles and support equipment are not that much larger than the jet that does the cutting, but, when the jet is cutting at the edge of the excavation the jet will need to be inclined outward by an angle of somewhere around 20 - 25 degrees in order to cut clearance.

An additional problem arises with the need to break the pieces being removed from the solid into small enough fragments so that these can be moved out of the way and into a transport line, so that the cutting head can continue to advance. In the case of the Soil Saw (for which Figure 1 showed one of the earlier test cuts) the nature of the clay was such that, once it was broken from the solid it disintegrated relatively easily and could be moved. Had this not been the case the cutting tool could not have passed without cutting the piece into smaller chunks. And in this regard, once a piece has been broken from the solid and is floating in a suspension within the cut, it becomes much more difficult to cut, since it can be deflected away from the jet before the full force of the jet can cut into it.

Further most materials are not as friable as clay – particularly those that are manufactured , and the pattern of cuts has thus to be designed so that the fragments are positively cut to the right size, to make sure they fit through the various gaps and feeds. For most of our work this meant that the pieces should be smaller than walnuts, and usually of around half-an-inch in size.

Ensuring that these cuts intersect in materials of varying properties will often require that the jets be designed to overcut in more favorable conditions, which wastes considerable energy. The alternative is to use the jets to cut relieving slots into the target, but to ensure that all the material is removed to the required depth on a pass by also including a mechanical component to the cut surface.

A cutting head designed by Rogaland Research shows the type of design required to achieve this, illustrating the angles of the two jets that will cut into the target as the head moves around the hole. In this case the design is to fit into a large diameter drill pipe to create a larger overall hole size.

Figure 2. The cutting head design developed by Rogaland Research (Vestavik, O.M., Abrasive Water-Jet Drilling Experiments, Progress Report, Rogaland Research, Stavanger, Norway, May, 1991.)

In this case the ribs of rock that are isolated by the jet cuts are removed by the action of the mechanical cutters in the second part of the bit.

Figure 3. Location of the jetted slots in the face of the drill-hole using the Rogaland tool. (After Vestavik)

In many cases the combination of a waterjet action to provide a free surface for the material to break to, and to relieve some of the confining stress on the material within the hole can significantly lower the mechanical forces required to break out the material. In such cases it makes much more sense to combine a waterjet action with that of a second removal device (which can be mechanical or thermal in some cases) to obtain a much more efficient combined system than that which would otherwise be the case. Where such systems have been used they have been shown to be more efficient in a number of cases than either of the component systems alone.

Unfortunately combining two systems to achieve optimal performance is not as easy as just merging the two sets of components, since there are additional benefits that come where the combination is further optimized so that the two parts work synergistically together. (One of the factors not included in the Rogaland design). I have written of this in the past, and will again soon.

Waterjettting 23a - Injecting abrasive into a waterjet

In an earlier part of this series I wrote about the introduction of abrasive into waterjets, and the loss in energy that occurs when the abrasive and the air that transports it are accelerated into the waterjet stream in the mixing chamber of a conventional abrasive waterjet nozzle assembly.

Figure 1. Conventional mixing of abrasive into a waterjet cutting stream.

Because air is conventionally used to carry the abrasive into the mixing chamber, and due to the relatively high volumes that are entrained it is often the case, as Tabitz* and others have shown, that the abrasive velocity exiting the jet is reduced as air volume increases.

Figure 1. Simulation of the effect of increasing air volume and abrasive feed rate on the particle velocity issuing from a conventional abrasive waterjet nozzle. (Tabitz et al*)

Using a higher density fluid to carry the particles into the mixing chamber is a self-defeating exercise, since the heavier fluids also will have to be accelerated to the final velocity, so that if a carrier fluid is to be used, then air is a logical choice. But it can make up some 90% of the jet leaving the nozzle, the water comprises roughly 9% of the remainder, so that only 1% of the jet may be abrasive, and this is the component that does the cutting in harder materials.

There should be a different way of approaching this, and in the early 1980’s Mark Fairhurst, at the time a graduate student in the UK, came up with an answer, which was presented at the BHRA Conference held in Durham in 1986. The initial system was relatively simple, but demonstrated the principles of the approach which was initially known as the Direct Injection of Abrasive Jet (or DIAjet for short).

Figure 3. Initial flow circuit from which the DIAjet system evolved. (after Fairhurst-1**)

The concept of the DIAjet circuit is that the abrasive particles are first loaded into a pressure vessel, which is then closed. When the pump is turned on part of the water flow from the pump feeds into this vessel through two control valves. The first is at the top of the tank, while the second was directed to feed at the bottom of the Tank, making it easier to feed abrasive into the underlying ejector, which mixed it with the main water flow from the pump, and thence carried it to the nozzle. This approach has a number of advantages over that of the conventional mixing chamber. The immediately obvious one is that there is no air added to the system, and the energy imparted to the water by the pump is only shared with the abrasive particles, without the system losses that occur where air is added to the mixture.

As a result the abrasive particles acquire a higher percentage of the water energy, and achieve particle velocities that allow cutting at 3,500 psi and 5,000 psi, whereas otherwise with a conventional system the jets would be at pressures ten times this high (although we will get into some of the caveats to that statement as this segment of the series continues).

In the earliest version of the system (and in some stand-alone versions that developed later, as I will discuss later in the series) the abrasive was added by simply unscrewing the lid, adding the abrasive to the tank, and then resealing the lid. Part of the problem that this causes is that, if the feed is not properly controlled abrasive can be caught in the threads of the cap piece, and this will then gall the threads and rapidly wear out the connection.

BHR, who first developed the machine, overcame this problem initially by using a secondary circuit to feed the abrasive into the pressure vessel, and this could be arranged so that there were two pressure vessels (which rapidly transitioned into pressure cylinders modified from other applications) one of which could be charging, while the second was in use. The basic circuit then became:

Figure 4. Schematic flow for the first commercial DIAjet system (after Fairhurst-2***)

It is perhaps illustrative to show one of the modifications to the design that was made in Missouri, where we used a small pressure-washer pump to feed the water to the pressure vessels, while the abrasive storage (the hopper shown in figure 4) was made from the pressure tank used in high-pressure painting applications. Because the lid of that pressure vessel was not threaded it was quite easy to refill, and the two cylinders were operated alternately. The entire system was designed to fit into the bed of a pick-up truck.

Figure 5. A small portable cutting system based on the DIAjet system. The assembly is mounted on a metal platform, and includes a water reservoir so that it is largely self-contained, and simple to use.

This new way of adding the abrasive to the waterjet feed has been developed for a number of different applications, although, because of the problems that arose in operating valves which control flow that contains abrasive, there have been some problems that have persisted in finding circuit designs that can operate on a consistent basis for the steady cutting applications where long cutting times are needed. But this approach has a number of applications where the abrasive need only cut for a relatively short period of time, during which the valves can function effectively, and where the jets can perform a cutting operation that is difficult for other cutting applications to achieve. It is, for example, possible to use a DIAjet type of system (if controlled properly) to cut through a live explosive detonator, without causing the explosive to go off. But I will talk about some of these developments, and some of the other capabilities of the system in later pieces.

*Tabitz, Schmidtt, Parsy Abriak, and Thery “Effect of Air on accceleration process in AWJ entrainment system, 12th ISJCT, Rouen, 1994 p 47 - 58.
** Fairhurst, R.M., Abrasive Water Jet Cutting, MSc Thesis, Cranfield Institute of Technology, January, 1982.
***Fairhurst, R.M., Heron, R.A., and Saunders, D.H., "Diajet" -- A New Abrasive Waterjet Cutting Technique," 8th International Symposium on Jet Cutting Technology, Durham, UK, September, 1986, pp. 395 - 402.