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.

Longwall Mining - using hydraulics to modernize mines

In writing about longwall mining, I have gone into some detail as to how the technology initially developed, in part because of my own interests, but also because some of its features help answer questions about today’s industry and this will provide a background for those answers in future posts.

The change in longwall came about as I went to college. The manual loading of the coal and setting of roof support that I have described in earlier posts, ended as a result of a single technical change. The miners had loaded coal onto a rubber face conveyor, that was advanced by being split into sections, moved forward, and rebuilt every day. This is incredibly inefficient and the innovation was the change to what has become known as an armored face conveyor. Basically this consists of a series of 5-ft long, hinged elements or pans, connected together in such a way that each can move forward and twist slightly, relative to its neighbors. ( In using pictures of existing equipment let me just insert that they illustrate what I am discussing, but that shouldn't let you think that I believe that one particular brand is better than another.) A single element of a relatively modern version looks like this:

View of a 5-ft long element of an armored face conveyor (Joy )

Horizontal link bars fit between elements in the hollows that can be seen at the ends of the piece allowing flexibility in the joints. The face of the conveyor is the wedge shape, that faces the coal face. Coal is loaded onto the conveyor from the mining machine, and any coal left on the floor then rides up the wedge onto the conveyor, as it is hydraulically pushed forward after the machine has passed. (A point I will discuss later). The coal falls into the top trough of the conveyor.

Pin connection between conveyor pans (Bucyrus )

To move it along the conveyor, there are two chains, which connect to lateral steel bars called flights, which slide in the slots on either side of the top trough.

Top view of conveyor showing flights (Joy)

The chains drag the flights down the trough, pulling the coal that falls onto the conveyor with them, and at the end of the conveyor, the bed is raised, so that the coal is dumped onto the belt conveyor that runs in the entry tunnel to the longwall. (For historic reasons this is generally referred to as the Maingate of the longwall.) The empty chain and flights then return along the face in the underside compartment to the conveyor.

The moving parts of the conveyor are thus now the chain and flights, and they are driven by a motor mounted in the maingate of the longwall.

The arrival of this then new tool allowed a number of different changes to occur on the face. The first and initial requirement was for a way to move the conveyor forward after the coal had been mined out it front of it. It was too big and cumbersome to disassemble and reconnect, and flexible enough that it could be pushed over using a series of hydraulic rams, one attached to each of the pans of the conveyor. But what could provide the resistance against which the rams could push?

The only element in the face behind the conveyor are the roof supports. So these also had to be modernized so that they could work in this way.

Initially the supports had been individual elements, two props and an overlying bar, all made out of wood. I had mentioned last time the arrival of hydraulic props, and steel bars. By combining these into a structure, with the props set on a bearing plate, and the steel bar at the top permanently mounted to their upper section, a simple support element was created. This, as we found out, didn’t have much lateral stability, and so they were combined in sets of two. With a hydraulic ram on the front, this could now push the conveyor forward.

Early hydraulic support (note the ram at the bottom left)

The ram would also make it possible, after the props were lowered, to pull the entire unit forward, and then, after raising the props, reset closer to the face. The plates at the back then stopped the rock that was collapsing into the void left behind the chock where the coal had been been, from rolling into the working face. This became known as a chock.

Over time the props became larger, and the canopy which was meant to extend over the conveyor to provide early roof support didn’t work that well. The idea was that it could be slid out, after the machine had mined the coal, until the chock itself could be moved. The problem was that if there was a heavy load applied at the tip it would bend the cantilever so that it could not slide. Then life became a little more difficult. The answer came from Hungary initially. It was known as a shield support.

Early shield design (dimensions in mm)

The design was extremely flexible and able to mine under conditions such as a badly broken roof that would not have been possible with normal supports, since the top and back (canopy) of the shield provided a virtual complete coverage of the roof and back of the working area, and the shields were installed adjacent to one another.

There have been considerable advances in design, but of an evolutionary nature, since this particular breakthrough – which occurred in the 1970’s. But in combination with the use of stronger and more powerful face conveyors these supports were able to make high-speed longwall possible in a wide range of differing conditions.

It is even possible to mine out a thicker seam by taking the lower coal with a mining machine (of which I will write next time) and then letting the upper coal collapse onto the back of the support, from which it can be recovered using a second conveyor. (And not a lot of energy – Charlie).

Top coal recovery – as in the Hunter Valley of Australia (Bucyrus )

Support used for top coal recovery - its a lot more complex than a wooden prop.



As I mentioned above, I’ll talk about the mining machines next time.

Moving the coal from the face to the shaft

At the surface an underground mine might appear to cover a relatively small area, but this may be quite deceptive of its actual size. Even back around 1850 the mine that James Dunn worked in extended out more than a mile from the shaft. As they extended, so the areas that were worked left pillars that might contain up to 40 – 50% of the coal in place, but had to be left to hold the roof up. The miners drove “headways” into the coal that were connected by cross passages or bords. At Houghton Colliery, for example, the pillars would be 5.5 yards deep, while the headways were 2 yards and the bords were 3.5 yards wide. The headways would advance about 20 to 40 yards before a cross drift was drive to hole through and create a new bord, along which the ponies would bring the empty tubs, and haul away the full ones.

Face layout at Houghton (A Pitmans Notebook, The Diary of Edward Smith, 1749).

The method of mining the face can be seen in the drawing, first it was undercut, then three vertical cuts were made (the nicks) and then the intervening coal could be broken out in the larger pieces that were favored by those buying the coal.

Back then the tubs of coal were moved from the working face to the shaft by teams of boys and men. But that only worked when production rates were relatively low. As the demand for coal grew with the Industrial Revolution, so the need to produce coal in greater volumes, and to move it faster to the shaft grew.

Pit ponies went underground, but would only pull a certain (small) number of tubs, and so better ways were needed. With the coming of electricity, this turned into the use of long “endless” ropes that wound their way along the centers of the rail lines into the depths of the mine.

Walking beside the haul rope (The Miners, Anthony Burton)

Hauling the tubs in either direction meant manually attaching a front shackle or chain to the moving rope, which then pulled the set of tubs down the passage in the direction required. Usually these systems were restricted to the main haulage ways or “Mains” since the ability to turn corners was limited. The main drive engine was usually by the shaft, and when the rope was not moving continuously it could be engaged by signaling the operator through a series of bells and cables. (The wires above the pitmen's heads).

Methods of attaching tubs to the rope (Colliery Deputy’s Handbook, D.G. MaGuire)

In an alternative method of haulage the rope was attached to the front of the set of tubs, while a second rope was attached to the tail end of the set. (Hence the name Main and tail). Thus as the tubs were pulled into the mine (inbye) they extended the rope that would later be used to haul them back out (outbye). But again these systems were largely kept in the main haulage parts of the mine. Once they reached the area where mining was continuing (the working “district”) they would be broken loose and could be hauled further by the pit pony. Thus on the map of the mine, the mains could be identified as the two parallel tunnels that would run out from the shaft in the four main directions (though this was oriented to the patterns of fractures in the coal, or cleat, rather than the compass points).

Underground layout at West Moor (How long did the ponies live? Roy Thompson Beacon House 1997) ) The lines that the workings run up to and then stop are faults where the coal is vertically displaced. The older workings, generally have the smaller pillars

Full sets would then be reassembled after they were brought back from the working face and hauled back to the shaft.

While the coal was mined by hand, and then loaded out by hand this system remained capable of handling the production. It is still, apparently, used in China. However, as mines grew larger, the inflexibility of the system, and its intermittent nature reduced its popularity. It was still around, however, in the early 1960’s since I spent an uncomfortable week or two of my apprenticeship working the engine on the maintenance shift, where the rope was only needed a few times in the shift to move tubs. It was there I discovered that by just cracking the engine on, the current running through the resistance bank would slowly heat it up, while the engine had not enough power to turn. This could, as long as a) you didn’t get caught and b) the rope didn’t move, keep you warm in the cold air of the downshaft flow.

A better way was needed for hauling coal from the face, and this came with the development of the conveyor belt. Initially mining belts were relatively simple. Long flat strips of reinforced rubber belting would be joined at intervals with wire clips, to form a loop. This was, in turn driven by a motor at one end, and a tensioning return drum arrangement at the other. Miners could load the coal onto the belt, and this would carry it out of the district, where it would transfer to a second conveyor belt that would carry it to the mains, where it would transfer to a much large belt, now carried on rollers, often suspended from the roof, that would carry it to the shaft, or in shallow mines out to the surface.

Transfer point to the Main Conveyor (Colliery Deputy’s Handbook)

One of the reasons for the suspension of the rollers rather than having them set in a frame resting on the ground is that it left clearance under the conveyor. Thus as small coal and dust fell off the conveyor it fell into a space where it could be seen and removed before it became a risk. If it was allowed to build up it could rub against the rollers, generating friction, heat and ultimately causing the coal to catch fire.

Dr Bronowski studying coal dust collection and ignition under a scale model belt after the Cresswell Pit fire in 1950. (The Miners, Anthony Burgess)

The fire at a transfer point at the Cresswell Pit in the UK in 1950 created clouds of carbon monoxide, which flowed into the workings and 80 men died. Fires associated with poor belt maintenance and failure to remove the coal powder that accumulates around the transfer points and similar locations, are, sadly, still occurring even within the past month. They don’t have to be underground and one of the major problems created is the smoke that is formed, which can very rapidly make it impossible to see.

Conveyor belts do, however, when properly maintained, provide a relatively rapid and simple method for getting the coal from the working area to the shaft or out of the mine. They can even (in some cases officially) provide a way for miners to ride their way out of the mine at the end of the shift.

Harvey Manrider (lithograph 1979 Tom McGuinness) Note the rope between the rails, which is still used to haul tubs of supplies in to the face. (The single rope would mean a Main and Tail haulage).

You just had to stay awake enough to get off the belt before the next transfer point.