Waterjetting 26d - Range, position and rewards for jet assisted cutting

In the earlier posts in this chapter I have discussed the problem of getting the nozzle of a waterjet system close enough to the tool:target contact that the jet retains enough power to be effective. At the same time the jet must strike within roughly 1/10th of an inch of that contact to be effective in helping with the cutting process. In the figure below, for example, the jet that comes from the nozzle ahead of the pick will initially strike in that region, while the jet at the back (right) of the pick box will not.

Figure 1. Potential positions for jet nozzles around a conical pick

There is one other consideration, perhaps more relevant in a rock cutting operation than in a metal cutting one, and that is the issue of tool wear. In the above situation while the rear jet can never hit the critical zone, the one at the front of the tool will lose effectiveness as the small carbide cutting cone wears and moves the crushing zone back under the pick shoulder. As an improvement consider the situation shown below:

Figure 2. Simplified schematic showing a high-pressure waterjet hitting the contact between a cutting tool and the underlying rock.

In this case when the tool is sharp then the jet is striking the rock just in front of the edge of the tool, and the performance is enhanced. Further, as the tool starts to wear, so the jet impact on the rock begins to move further forward of the tool contact. But because the face of the tool and the jet are almost parallel the slight change in distance is relatively insignificant.

By the same token, if the rearward jet in the first example had been moved so that the jet struck just under the back of the pick it would still have been able to remove the crushed rock, even as the bit wore. One way to improve the effect of the jet is to spread the water flow by making the jet into a fan or conic spray, this can be effective:

Figure 3. Reduction in thrust with lower pressure fan jets (after Hood)

Again the bit is cooled, keeping it sharper, but also even at the lower pressure if the rock is removed as soon as it is first fractured then it does not crush and then re-compact under the bit.

However higher pressures work better, both in terms of overall rate and in terms of the efficiency of cutting, based on British data.

Figure 4. Change in cutting performance with increasing jet pressure (after Morris 1985)

Given therefore the need to bring the jet to the crushing zone in as powerful a form as possible, one suggestion has been to bring the jet down through the center of the cutting pick.

Figure 5. Nozzle located above the contact point, but fed through the pick body. (After Fairhurst).

The problems with doing this are several. In the particular example shown the orifice is pointing the jet into the rock some quarter-of-an-inch above the crushing zone and this is too far away for the jet to achieve maximum benefit. Further as the tool will wear, so the contact surface will move back further away from the jet, further losing the assistance and failing to be able to remove any of the crushed rock as it is formed.

There are practical problems, however, when (as has been done in Russia) the orifice is brought closer to the tip of the tool. One of the difficulties is that whenever the tool is then used without the jet operating at pressure, then crushed rock will enter the nozzle and within a very short distance plug it with compacted fines.

It is then, frequently, not possible to use jet pressure to get that material out of the nozzle, (particularly when the pump is supplying several orifices on a cutting head). Without the water the tool rapidly erodes, because of another weakness in the design.

For when the orifice is placed within the lower tip of the tool, the volume of the orifice is removed from the bulk volume of the cutting bit, making it much more susceptible to wear.

As long as the jet is brought up to pressure first, and the tool only then brought into contact with the rock or other target, then the tool performs well. Unfortunately (as operators are human and thus prone to the occasional error) cutting heads have often been brought into contact with rock without the jets being at sufficient pressure, and the benefits of the jet assist are thus eliminated due to this loss in nozzle clearance.

There is a corollary to this, in that, as jets began to be used more frequently on cutting heads, the amount of water spraying into the working zone became both a source of irritation and a considerable unnecessary loss in power, given than the cutting head tool only makes contact with the rock for a small fraction of the rotation around the shaft axis.

Figure 6. Roadheader with jet assist working at the Middleton Mine in the UK

To reduce the volume of water, control valves were set into the flow channels so that water was directed at only those picks that were in contact with the rock. The problem with programming this is that, depending on where the head is around the profile of the tunnel, so the arc of the head that the picks are cutting on will change.

But the benefits, where all these different factors are considered in the design and operation of the machine are considerable. As a very rough statement, the cost of a machine will increase more than linearly as it’s weight is increased. In order to cut harder rock without jet assistance, the picks must be pushed harder into the rock, and this thrust must be resisted by the friction exerted between the floor of the tunnel and the base of the machine – usually treads. Thus harder rock requires that conventional machines be heavier. However, when jets are added to the machine that power cost is removed, as the thrust levels are reduced. Thus smaller (and more mobile) jet-assisted machines can cut more effectively than their conventional counterparts.

Figure 7. Introduction of heavier machines to mine harder rock, until the advent of the waterjet assisted machine in 1980 (after Morris)

The savings in the reduced cost of the machine (saving $500,000) more than covered the cost of the high-pressure waterjet equipment (around $100,000).

Hood, M., A Study of Methods to Improve the Performance of Drag Bits used to cut Hard Rock, Chamber of Mines of South Africa Research Organization, Project No. GT2 NO2, Research Report No. 35/77, August, 1977.
Morris, A.H., "The Development of Boom-Type Roadheaders," Seminar on Water Jet Assisted Roadheaders in Rock Excavation, Pittsburgh, PA., May, 1982.
Fairhurst, C.E., Contribution A L'amelioration De L'abbatage Mecanique De Roches Agressives: Le Pic Assiste Et Le Pic Vibrant, Doctoral Thesis, L'Ecole Superieure des Mines de Paris, October, 1987, 221 pages (in French).

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.