Saturday, June 14, 2014

Waterjetting 22b - Steep seams and shrouds

It was not until we started to move the auger that I described last time, that I realized how heavy and cumbersome it remained. It is true that we could lighten it considerably (and we did in the UK version), but it was still largely a platform mounted device that was relatively easy to move on the surface, but which would be much more difficult to move around in the confined spaces underground.

In this regard it is worth comparing two photographs.

Figure 1. Coal being transported in the pipes at the Hansa hydraulic mine in Germany.

Notice in this first one how the coal is confined, there is no dust, and the tunnel is relatively clean and clear. Contrast this with the typical mining operation where the coal is carried from the working face to at least the main haulage drifts, and often all the way out of the mine using conveyor belts. (The method of transport that would also likely be used in a typical underground auger section).

Figure 2. Conventional belt conveyor carrying coal underground. (Famur )

Note that in the second photo the belt occupies most of the space in the roadway making passage more difficult, and that the coal is openly exposed. The problem that this occasionally generates is that the coal transfers from one belt to another as it moves through the mine by falling off the end of one belt onto the next. This puts dust into the air, and if not properly maintained coal dust can accumulate under the belt and around the support rollers and drives. If not cleaned this can create heat through friction, and can lead to disastrous fires.

Figure 3. Studying a belt fire underground (Office of Mine Safety and Health Research )

Confining the coal, and using the water that mined it as a transport fluid – or at least part of such – has many advantages.

The Russians were aware of this as they developed some of their different mining machines. One of these was a small monitor that could be pushed up a seam, from a lower drift (without the need of a higher one), by adding segments as the unit was jacked forward.

Figure 4. Russian GVD monitor

The monitor could be advanced up the seam, cutting a channel about 3-ft wide, and to the height of the coal. This relatively narrow channel confined the water and the coal produced, so that both ran back down to the drift, where they could be either enclosed in a pipe, or run into an open flume, that would carry the coal away. Once the drive had reached the end of the section, then the two hydraulic cylinders that sat under the monitor could be turned, so that, from a protected section down-slope, the monitor could successively ream out strips on either side of the entry, until the roof collapsed, or the operation holed through to the previous panel.

Figure 5. Schematic showing the sequence of extraction in a) a coal seam with a relatively strong roof b) a seam with a weak roof, where a pillar of coal is left between successive lifts, typically around 30 ft, so as to provide additional support to the roof as the coal is mined out.

Production from these machines averaged about 76 tons/hour from seams that were in the range from 2 to 4 ft thick, this was more than double the production output achieved by more conventional means, with significantly less manpower.

However while there are some conditions where gravity helps to bring the coal and water back together, there are many cases where this is not possible due to other constraints. This is where it becomes necessary to use a shroud to confine the jets, debris and water so that they can be extracted together, often using a vacuum to assist in the process.

Figure 6. Rendition of the combination of three cutting jets rotating around a vacuum tube to slice into material and remove it.

The particular device shown in Figure 6 was developed as a way of remotely slicing into high-level radioactive waste for the Pacific Northwest National Laboratory. The waste is held in underground storage tanks, where the levels are too high for human access. As a result the waste had to be broken into pieces and removed remotely.

The initial problem was that the tanks, though holding perhaps half-a-million gallons of waste, had only small (18-inches or less) ports through which they could be accessed. Thus a relatively small tool was required, yet the mining rate had to exceed 4-cu. ft/minute. The situation required an excavation system on the end of a robotically controlled arm. But the arm would have to be more than 60-ft long (in the end a cousin of the arm used on the Space Shuttle was used). Any mechanical force applied at the end of such an arm would have tremendous leverage on the holding fixture, and a relatively low overall force would have to be found, yet one capable of cutting material perhaps as strong as a weak cement.

The answer came in the form of the device shown in Figure 6. By placing three cutting jets to rotate around a central tube, connected to a vacuum line, one could cut into the material and break out relatively small pieces, which could be aspirated away with the cutting water. (The requirements were that there would be no water left in the tank for any significant amount of time). Because the bottom of the tank was some 40 or 50 ft below ground level a small, high-pressure (10,000 psi) jet pump was developed by Michael Mann, capable of drawing particles of up to an inch in size into the line, and then projecting them up with sufficient energy to carry them out of the tank.

Figure 7. Basic system conceived to mine high-level radioactive waste and remove it from a storage tank.

While this description of the system makes the tool seem to be relatively simple to build and operate there are a number of features to the design that are fairly critical in order for it to work well, and I will cover some of those next time.

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