If a surface is relatively smooth (think for example of a ship hull, or the deck or sides of a bridge) then when a jet has hit the surface and removed the small amount of material (such as rust or paint) it will likely continue in a relatively straight line forward, since the surface roughness of the target, while disrupting and flattening the jet, has not sufficient angle to radically change much of the flow of the jet.
Which might make it time for a little recap. One of the experiments that I would run with an introductory undergraduate class was to give each student a high-pressure lance, and then have them hold the nozzle just above a target surface. The pressure of the water being fed to the gun was then slowly raised, and as this occurred the jet went from striking the surface and then just flowing along it, when there was no penetration or surface material removal, to being reflected back at the lance holder.
The reason for this is that, as soon as the jet started penetrating into the material (usually a rock for the purpose of the demonstration) then the water is entering a hole where the only exit is back the way that it came in. It is a salutary lesson for the lance holder since all of a sudden the jet is coming straight back (which is why all the personal protective equipment is an important part of the lesson).
This only holds true where the jet is hitting a relatively flat surface in an approximately normal or perpendicular axis of attack. In the more general case the cut along the surface will cause the water and debris to scatter in a more general spread, and it becomes a more difficult job to collect both back together in a way that allows both to be contained and removed from the site (an increasingly important part of the environmental parts of the process).
There are places, such as steeply dipping coal seams, where the geometry of the excavation itself helps to confine this ejecta and direct it, under gravity, to fall into a narrow space where the water and coal particles are brought together so that the coal is suspended in enough water that it can then be carried away from the work zone.
Perhaps the best example of this was the Sparwood mine in British Columbia in Canada, where the mine was extracting coal from a seam that was roughly 40 ft thick, and which dipped at around a forty degree angle.
Figure 1. Section showing the Sparwood mining plan
Drifts were first run at a slight angle (this started at six degrees, but after lining the flume with Teflon plates the mine was able to reduce this to just over four degrees) to the strike of the seam. This was a sufficient angle that, when all the coal was caught in the flume it would be carried down without settling by the spent water from the mining process, which was also trapped in the underlying drift, and held by a barrier across that drift. The shallower the angle then the more coal could be recovered above the main haulage ways at the back of the working area.
The mining tunnels (drifts) were first driven to the back of the section, using a small road heading machine to extract the coal, while installing a flume along the side of the drift so that the coal could be immediately transported away as it was mined. Full support to the tunnel was also installed using arch girders, with bracing wooden slats between the girders. Once the drift had reached the end of the seam, then a hydraulic monitor was placed in the uppermost drift, and the arch girders and wooden planks removed from the final fifty feet of the tunnel, with the monitor placed under the last few tunnel supports of the remaining tunnel section.
Figure 2. Layour of the monitor within the access drift.
By using a jet of just over an inch in diameter. the jet was able to reach the back of the section of coal that had been exposed when the supports were removed (zone 5 in figure 2) a distance of over 120 ft. the monitor was moved by two sets of hydraulic rams, but if you note where the operator is standing at the back of the machine, this is some 40-ft from the opening and the mining operation itself is not visible.
Figure 3. A monitor in operation at Sparwood. Note the short length of the barrel, which would still produce a high-quality jet, since flow straighteners were used in the barrel, placed directly behind the nozzle.
The operator uses the rams to move the nozzle in an oscillatory path, and listens to the sounds of the jet as it strikes the coal. The sound is quite distinctly different when the jet is hitting coal, as opposed to striking roof rock or shooting into the open space of the drift updip. (I was told this, not having that experience, though I have found similar changes in sound useful in other applications that I will discuss from time to time). It takes, apparently, a couple of days for an operator to be able to consistently detect and use the sound differences to be able to effectively mine with the monitor.
Figure 4. Operator at the Sparwood mine, standing at the back of the machine, and beside the flume.
The way in which the coal broke under the jet attack was only controlled by the operator to a limited extent, so that there can be a significant volume of large coal surviving into the lower entry for collection, and flume transport needed a smaller size distribution. For this reason the coal company installed a coal breaker at the entry to the flume so that the water carried the coal lumps through the breaker, and only then did they enter the flume (Figure 4).
At the time that I visited the site the slurry was higher than shown in the above figure, with coal overlying parts of the back of the breaker. To make it easier to operate the breaker, while keeping the operator safe behind the roof supports, a second small monitor was set by the operator which could be used to clear off the machine from time to time.
The machine was operated by two individuals and over the course of ten years averaged a production of over 3,000 tons a shift. It was also for many years, the safest mine in Canada. To put that production in perspective, in those years an average section in the underground mines in Illinois, running a continuous mining machine, might average about 700 tons a shift, and would need about 14 men to achieve that target. (Production rates have since risen considerably as automation and remote control have reduced the number of folk needed, while higher-powered machines now produce greater coal volumes faster.)
Yet the range of seams where this type of mining can succeed is limited, particularly in the United States, and in coal seams that do not dip as steeply it becomes more difficult to control the particle and water spread as it leaves the impact region.
I’ll talk about a specific way that one can, on occasion, change that, in the next post.
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