Friday, October 31, 2014

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).

No comments:

Post a Comment