Waterjetting 22c - More on shroud design

Lowering the pressure in a hose connected to a cutting head, by connecting it to a vacuum pump, will pull a certain amount of the water and debris released from a cutting/cleaning event into that hose. However to ensure that all of this material is captured, rather than just a fraction, requires a little more care and effort in the system design.

At the end of the last post on this subject, I began discussion of the use of shrouds to help to contain the ejecta and to direct it towards the suction line.

Figure 1. Schematic section through a shroud of a device, designed to mine high-level radioactive waste.

There are a number of different lessons that we learned as we developed this tool, and this piece will discuss a number of them. During the development and demonstration of the device we had to use a simulant, and a relatively weak cement was chosen, which would allow us to design the tool to operate it where we could see it, and easily interact with it.

Figure 2. Cutting test under way (no shrouds were used in this early test series)

The easiest way to drive the nozzle system was to run the high pressure tubing through a fixture that contained a hollow shaft electrical motor. This saved a lot of space, and allowed the high pressure tubing to feed into a distribution manifold under the motor, which fed the high-pressure water to a set of rotating nozzles.

Figure 3. Test rig without jets to show the design of the test head.

In figure 2 the jets issue from two self-rotating nozzle sets, themselves fed through a rotating feed tube, itself rotating around a central axis, and driven through a belt drive and gearing.

Various different patterns were cut into the simulant, as the different heads were moved over the surface, with the pattern controlled by the different rotation speeds relative to the overall head speed over the surface.

Figure 4. Computer image of the jet paths over the surface, in one combination of parameters for a head similar to that shown in Figure 3.

The design of the head was aimed at producing a set of jet passes (given that each jet was slightly inclined to the surface) which would produce pieces of simulant that were never larger than half-an-inch in size. Yet at the same time the goal was to remove 4 cu. ft. of waste each minute. The larger we could break out the particles, the less cutting we would have to make into the waste itself, saving energy and time, while at the same time increasing the overall volume we could release in that time.

Figure 5. Deeper cut into the simulant.

At the same time if the cut depth was too great, then several new problems would arise, apart from the initially obvious one of producing particles that would be bigger than the suction hose could easily handle. (Though we overcame that hurdle by running the particles through a high-pressure jet pump that effectively cut any oversize particles down to an acceptable size as part of its design).

The suction line needed more than just the water from the cut, to be able to pick up all the debris from the cutting operation. Air had to be drawn in around the sides of the shroud, yet at the same time the walls of the shroud had to come down to restrict the amount of that air and keep the suction strong enough at the surface to remove all loose material. This is done by fitting a rim of bristles (such as form the head of a paint brush) around the edge of the shroud that come down to brush over the outer edge of the cut, stopping a lot of the material from escaping out from the edge, while limiting the amount of air that feeds into the shroud, and in this way holding the suction pressure inside the shroud.

Figure 6. Early test showing a square shroud with bristles around the edge as it cuts into the waste. (Part of a previous pass has been filled with clay as part of the test). The shroud was larger to ensure that all ejecta was captured – as shown.

During the tests we found that the metal rim should, optimally, be no more than half-an inch from the surface of the material, after it had been cut, to pull all the material from the bottom of the crevices. But the edge of the head has also to pass over the surface in successive passes. So that high points left by deep cutting (Figure 5) will catch on the head, and can interfere with the rotation of the head on the next pass.

The aim of the cutting head design was, therefore, to leave a relatively smooth surface (of the sort shown in figure 3) over the waste after each pass, so that the head could be fed automatically down a fixed amount without any risk of it catching on large peaks left by the previous cut. This risk could also be lowered a little by slightly tilting the head backwards as it moves over the surface, since this allows slightly larger points to enter the head, where they are attacked by the jets before the driving mechanism has to pass over them. This tilting also makes it easier for the head to clean right up to the walls of the tank, where otherwise the edge of the shroud would hit the wall and stop the jets from removing that last rind of material from the edge. (Though it could be cleaned by a subsequent pass with the head turned up parallel to the wall and moved over it in that way – though this wouldn’t capture all the material as easily, due to wall curvature.)

Tilting the jets at a high angle so as to cut material at the edge of the shroud was also a possible problem, since it made it easier for the water to escape from the edge of the shroud, and out into the main body of the tank, which was undesirable. But I’ll talk about that in a later piece. Let me just note that, when these factors were all combined no material escaped from the edge of the shroud.

Figure 7. Test late in development, where a head similar to that shown in figure 1 is cutting over waste, without any material being ejected from around the shroud edges.

Waterjetting 21b - collecting debris, or mined product

You don’t have to operate a water jet lance for any time before realizing that the rebounding water, and the debris from the operation can’t be ignored. On the other hand, after they fall to the floor they rapidly lose their energy and without some additional input will lie in the most inconvenient places until moved. Protection from the rebound is a critical part of the protective clothing and equipment that the operator wears (including side shields for the eye protection) but the collection and removal of the spent material is also critical to good operations, and in uses such as mining it is needed to carry the material to a point where it can be processed.

While this was fairly obvious to those who used water in earlier times to wash out soil and minerals and carry them down to flumes and separation devices, it was not always immediately obvious to those who experimented with the use of larger jets for underground mining. There are two critical velocities involved, once the particles are removed from the surface, these are the settling velocity and the suspension velocities of the particles. Essentially these are the speeds at which the transporting water is travelling that prevents, or encourages, the particles to settle out. This is an important part of drilling horizontal holes, where the drill cuttings can settle in the long hole hehind the drill, and can both stop the drill from moving forwards and also trap it when it tries to retract (personal experience).

Figure 1. Settling velocities (after VCCS)

Because of the wide range of particles that are mined by a jet, it is sometimes suggested that flow velocities in flumes remain above 40 ft/sec. It becomes a lot more difficult to get particles back into suspension after they have settled out.

Because water and debris are dispersed around the impact point after jet impact and because the amount of water needed to keep the particles suspended and moving has to be high, this often means that the geometry of the excavation has to be tailored to capture and confine the water in a narrow space with the mined product.

The National Coal Board carried out early experiments in coal mining in Wales at Trelewis Drift. (Jenkins R.W. “hydraulic Mining – The NCB Installation at Trelewis Drift” MSc Thesis, University of Wales, 1961).

Figure 2. Early NCB remotely operated monitor (after Jenkins ibid).

The 5-ft 6-inch thick seam dipped at roughly 6 degrees, and the monitor was used initially to drive the drift, and then to slab off coal from the pillars on either side, and then to mine along the face of a pillar, driving coal from one drift to the next adjacent, as the drift was retreated.

Part of the problem that arose was that the coal broke off in pieces that were up to 2-ft thick, and once these fell to the floor they became more difficult to recover, particularly where they were scattered (and this was the reason that the monitor was being operated remotely since the debris was reaching the miners). (Jet pressure was around 1,000 psi). The mine was naturally very wet (over the waders of some miners) and production did not exceed 45 tons a shift, even though the costs at the time were around 50-pence per ton in 1963. The conclusion was that the slope of the seam was not adequate to help enough with coal transport and the experiments were terminated.

This is in contrast to work elsewhere (the British trials had been commissioned after Russian hydraulic mining trials had been reported as successful). But the Russian work was carried out in the Donets coal basin, where the seams are much steeper, and more difficult to mine conventionally.

V.S. Muchnick wrote his dissertation on hydraulic coal mining in 1935, describing these early trials. While it was difficult to manually work in the steep seams, setting up a simple monitor that would wash away the coal, which would then fall under gravity to the drift tunnel beneath the mining operation was much more successful.

Figure 3. Early Russian Monitor (RGM-1)

Because of the Second World War hydraulic mining did not get its start until 1952 at the Tyrganskii-Uklony mine in the Kuznetsk Basin. Production in the hydraulic sections was anticipated to be 500 tons.shift, but by the end of the first year it had already exceeded 600 tons, more than twice conventional mining production. In the early operations the coal was weakened by blasting, but by 1957 jet pressures were raised sufficiently that this was no longer needed.

Figure 4. Showing the major Russian coal basins (Gazprom )

By 1979 there were nine major hydraulic mines in the Soviet Union with an annual production of over 8.9 million tonnes. As the mining operations expanded, so the monitors were increasingly operated remotely using hydraulic cylinders to direct the jets at the coal. At pressures of 1,500 psi the monitors could mine over 50 tonnes an hour, using around 650 gpm of water. (This was later doubled).

In almost all these operations the mine is worked in retreat, first driving the drifts to the back of the section, and then mining back, allowing the coal either to fall into the drift, where the water volume is sufficient to carry it into the flume, or pushing it down to the underlying drift, where it can similarly be collected.

Figure 5. GMDTs-3M monitor used in the Soviet Union

The mining pattern changed with the thickness of the coal, and with the steepness of the slopes at which the seams ran, although the access drifts were run more up dip, while the mining drifts were at a shallower angle closer to horizontal, so as to make working conditions easier, and to more effectively remove most of the coal from the section. In this way the coal collapsed under gravity down to the drift, where the water would float it into the flume. ( A barrier across the drift would confine the water and coal and act as a feed mechanism).

Figure 6. Method of mining in Soviet hydraulic mines in gently dipping seams of average thickness.

I will discuss the expansion of the technology around the world in later posts.