孔莊煤礦1.5Mta新井設計含6張CAD圖.zip
孔莊煤礦1.5Mta新井設計含6張CAD圖.zip,煤礦,1.5,Mta,設計,CAD
英文原文
The cuttability of rock using a high pressure water jet
PC Hagan
The University of New South Wales (UNSW), Sydney
Methods of improvement in the performance of mechanical rock cutting systems are continually being sought. One area being investigated is combining mechanical rock cutting tools with water jets. In this ‘hybrid’ arrangement, the mass breakage mechanism of a rolling disc cutter or drag pick is coupled with the concentrated energy medium of a high pressure water jet.
Research has indicated that the resultant improvements in performance of mechanical cutting tools could be due to the cutting of the rock surface through fracture and erosion thereby ameliorating the rock breakage process of the mechanical cutting tool. Damage by a water jet has been observed even in rock of high strength.
This paper outlines a study on the sole use of water jets in cutting rock and the effects of changes in the principal variables of a water jet. An understanding of the characteristics and the relative importance of any changes in these variables is necessary to optimise the cutting performance of a hybrid system in terms of advance rate and energy expenditure. The variables considered in the study included nozzle diameter, water pressure, traversing speed and multiple passes of a water jet.
Within the range of values studied for each variable, a change in water pressure was found to have the greatest impact on the level of surface damage in rock. Traversing speed, and to a lesser extent nozzle diameter, were also found to alter the magnitude of surface damage in rock.
1. INTRODUCTIO
One of the fundamental processes in mining is the liberation of minerals from the in situ rock mass. This can be achieved by a number of methods including abrasion and fracture. The causation of fracture in rock has long been associated with some form of mechanical indentation or the cyclic application of large impact forces. This is exemplified by the old ‘hammer and tap’ method of rock drilling but it also underlies the more modern techniques of mechanical rock breakage with cutting tools such as picks and rolling disc cutters. This principle has not precluded other sometimes more subtle techniques from being used in the past such as the gentle knock in the correct orientation by a stonemason.
Apart from mechanical indentation, non‐contact mechanisms to initiate fracture in rock have also been developed. Probably the most significant being the detonation of an explosive within a confined space. Before the use of black‐powder, however, the ancient
Chinese observed and adapted to their advantage the natural weathering process of exfoliation by accelerating the rapid changes in rock temperature with fire and water. A modern variation of this technique is the thermal jet lance (Fleming and Calaman, 1951). More recently Sellar (1991) has reported radiating rock with a pulsing laser to induce internal stress variations causing fracture wherein it was found that the pulse frequency was critical and should equal the resonance frequency of the rock. Common to all these techniques is an alteration in the internal state of stress such that bonds are broken and free surfaces created.
Figure 1. View of several slots, or kerfs, cut in rock by a high pressure water jet.
Another potential method of rock breakage reported to have significant potential is the application of high pressure water jets. High pressure water jets in this sense normally refer to pressures between 10 and 400MPa with a nozzle aperture of less than 1 mm. Harris and Mellor (1974) have reported that a rock surface can be significantly damaged or cut by a water jet at high pressure. As shown in Figure 1, this damage is normally in the form of a narrow slot of varying depth. Various theories have been developed to account for this damage including a theory on cavitational drag (Crow, 1973), energy balance in brittle fracture (Mohaupt and Burns, 1974) and material erosion (Rehbinder, 1976).
During the early development work, it became apparent that water jets could not compete with conventional forms of rock fragmentation. A relatively large quantity of specific energy, in the order of 1000 MJ/m3, is required for this form of rock breakage which is several orders of magnitude greater than that required in conventional mechanical rock cutting. But it was found that water jets were useful when combined with conventional mechanical systems especially when cutting hard rock where tool life can be short (Hood, 1975). The most significant benefits derived from a hybrid cutting system being:
? a reduction in cutting forces. Fairhurst and Deliac (1986) reported an average force reduction of 30% in the cutting direction and somewhat greater reduction in the normal, or thrust, direction.
? increased tool life. Taylor and Thimmons (1989) reported a doubling in tool life. Hood et al (1991) reported an appreciable tool life was obtained when cutting a hard rock with a UCS of more than 200MPa, where tool life was otherwise non‐existent. Morris and MacAndrew (1986) suggested that water jets reduced the rate of tool wear by cooling the highly stressed rock‐carbide
? greater machine advance rates. Because of the reduction in thrust force,lager penetration per revolution can be attained for a machine with a given thrust capacity. Also the reduction in thrust requirements with water jets can compensate for the loss in cutting efficiency of worn cutter tools .
?lower machine vibration. Fowell et al (1988) reported significantly reduced vibration levels on a roadheader cutter boom.
?increased product size. It has been reported that the proportion of fines can be reduced and coarser rock debris produced (Wang and Wolgamont, 1978) .
?lower levels of reparable dust. Taylor et al (1989) found that dust make was 80% less during cutting with jet assistance then in cutting with conventional water spray systems.
?reduced occurrence of incendiary ignitions. Water in the cutting groove can dissipate frictional heat, hence lowering the possibility of gaseous ignition
More recent work by Lin, Hagan and Roxborough (1990) has shown that greater efficiencies can be attained in water jet cutting of rock by focusing two or more jets below a rock surface. They reported largeragments were produced and that specific energy was reduced by nearly an order of magnitude. This work confirms the behavior predicted by Mazurkiewicz et al (1978) which they termed the ‘jet accumulation phenomena’ and is an adaptation of the earlier work on shaped charges by Walsh et al (1953).
To understand the mechanisms of water jet assistance in a hybrid cutting system, it is necessary to study the behavior of a water jet acting in isolation to break rock. Experiments were undertaken to assess the effects of a high pressure water jet in rock cutting.
Such knowledge can be used to optimise the total extraction system so that the failure mechanism of the mechanical tool is most effectively complimented by a water jet .
2. LABORATORY APPARATUS
The test program made use of a commercially available high pressure, low volume pump shown in Figure 2. Filtered town water was feed through one of two hydraulically actuated double‐ended, reciprocating cylinders. Each cylinder had a 20:1 pressure intensification factor. Changes in water pressure were made by adjusting the outlet pressure of a variable displacement, pressure‐compensated axial piston hydraulic pump. The unit was capable of delivering 4.7 L/min at pressures of up to 380 MPa.
Standard industrial sapphire nozzles with a conical outlet were used to form the water jet. The discharge co‐efficient of the nozzles was 0.65. A range of nozzle aperture diameters was used in the experiments varying from 0.15 to 0.36 mm.
Figure 2. View of the water intensifier unit and linear cuttingtable used in the test work.
A linear cutting machine was used to move the rock samples with respect to a stationary water jet. This modified planer could accommodate rock samples with plan dimensions of 450 x 450 mm on a horizontal bed at velocities between 50 and 300 mm/s. A variable frequency controller was used to regulate the speed of the electric drive motor.
TABLE 1 Material properties of test rocks
property
Woodlawn Shale
Gosford Sandstone
UCS (MPa)
145 ± 27
41.8 ± 4.4
UTS (Brazilian) (MPa)
11.7 ± 3.7
2.95 ± 0.53
ES (GPa)
36 ± 3
6.3 ± 2.7
ED (GPa)
26 ± 6
9.2 ± 0.6
GD (GPa)
10.0 ± 2.6
-
Poisson’s ratio
0.32 ± 0.06
0.13 ± 0.05
Density - bulk (t/m3)
2.73 ± 0.01
2.21 ± 0.05
- grain (t/m3)
2.77 ± 0.01
-
Shore Hardness
62 ± 3
-
Schmidt Rebound No
69 ± 1
47 ± 2
Hacksaw Abrasiveness
3.11 ± 0.56
-
Porosity - apparent (%)
0.5 ± 0.1
9.4 ± 1
- true (%)
1.8 ± 0.3
-
3. MATERIAL PROPERTIES OF ROCK
Two rock types were used in the study; these were Woodlawn Shale and Gosford Sandstone. A summary of the material properties of these rocks is given in Table 1.
Where applicable, the material properties were evaluated according to the suggested methods prescribed by the International Society for Rock Mechanics.
4. TEST PROCEDURE
Cutting of a rock mass by a water jet is termed water jet slotting or kerfs formation. A typical configuration involves a water jet traversing across the surface of a rock as is shown in Figure 3. The principal variables in cutting with a water jet include:
?jet variables: nozzle diameter, water pressure, nozzle discharge coefficient and water density all of which effect the water flow rate and jet velocity
?operational variables: standoff distance, nozzle traverse speed, jet attack angle and number of multiple passes.
Figure 3. Main variables in water jet cutting.
Other variables include those of the rock (for example compressive strength, fracture toughness, porosity, grain size and surface roughness) and of the rock mass (for example structure). These variables are, however, site dependent and tend to be over‐ridden by the jet and operational variables.
The study was undertaken with a continuous water jet on the linear cutting machine described in §2. The principal goal was to study the basic aspects of a water jet in cutting rock. Only those variables that effect the hydraulic energy of a water jet and hence the energy available to initiate fracture or erode the rock were considered. It can be shown that the energy of a water jet acting per unit distance along a rock surface, or the specific hydraulic energy, can be calculated from Equation 1.
where:
1）
W? = specific hydraulic energy, MJ/m
= discharge co‐efficient
= nozzle pressure, MPa
= nozzle diameter, mm
?= nozzle traverse speed, mm/s
? = fluid density, kg/m3
As indicated by Equation 1, water pressure and nozzle diameter are the two main jet variables. Other variables such as discharge coefficient, water temperature, addition of polymers or abrasive substances and pulsing of a jet were not studied. These alter the structure of a water jet and would only tend to further enhance performance. The range of water pressure values in the study was selected on the basis of the mechanical compressive strength of the rock. It has been observed that the minimum pressure required to initiate fracture, commonly referred to as the threshold pressure, is typically of the same order as the rock compressive strength. A spread of water pressure values about the equivalent compressive strength was selected to test this observation as well as to evaluate the effect of changes in energy of a water jet on slot depth. Of the operational variables, nozzle traverse speed and the number of multiple passes were examined. In each experiment the water jet standoff distance(shown in Figure 3 as the distance between the nozzle was equivalent to 70, 90 and 140 nozzle diameters for the three nozzles sizes used. Even though this was within the range of maximum effective jetting distance, the constant absolute standoff distance tended to give some advantage (in terms of additional effective jet energy) to the largest nozzle diameter.
The parameters used to assess the effectiveness of changes in variables during the slotting of rock are slot depth and to a lesser degree, specific energy. Slot depth is the mean measured depth cut below the rock surface. The study was conducted in a mode which simulated a water jet preconditioning the rock surface where the actions of a water jet and tool are combined.
The study involved three series of experiments. These are outlined in the following sub‐sections. The test program was arranged to cover as wide a range of values as possible. Unless otherwise stated, five levels were selected for each variable which increased approximately in arithmetical progression. The test schedule was randomized to minimize any errors that may have arisen due to the heterogeneity of the rock. In each case, the rock was tested after being air dried for at least 48 hours.
4.1Slot depth variation
The purpose of this test series was to determine the variability in depth along the length of a slot.
The series involved measurement of slot depth in Gosford Sandstone. Three slots were formed in the sandstone at pressures of 100 MPa, 140 MPa and 210 MPa, at a fixed traverse speed of 50 mm/s and a nozzle diameter of 0.23 mm.
4.2Principal variables in jet slotting
This series concerned three variables of a water jet in the slotting of Woodlawn Shale. Although slot depth was the main parameter, measurements were initially made also of slot width. Details of the experimental program are contained in Table 2.
TABLE 2
Variables in jet slotting test program rock type: Woodlawn Shale
variable
unit
level
traverse speed
mm/s
50 100 150 200 250
Water pressure
Mpa
100 140 175 250
Nozzle diameter
mm
0.15 0.23 0.30
The series was at first organised as a partial factorial program in which one variable was held constant while the other two were varied. Nozzle diameter was fixed at 0.23 mm while water pressure was varied at each level of traverse speed. Each combination of variables was replicated at least four times.
The test series was later extended to a full factorial program where nozzle diameter was also varied for each combination of traverse speed and water pressure. This was done to verify the trends established at the original fixed nozzle diameter of 0.23 mm. The number of replications at the other two nozzle diameters of 0.15 mm and 0.30 mm was reduced from four to two.
4.3Multiple passes of a jet
This series involved an examination of the effects of successive slot deepening in rock by multiple passes of a water jet. The tests were conducted in Gosford Sandstone. The series involved two parts. The first part involved the progressive deepening of a slot and measurement after each successive pass of a water jet. A total of twelve consecutive passes were made over the same slot. Each test was performed at a fixed nozzle diameter of 0.23 mm, traverse speed of 150 mm/s and water pressure of 210 MPa. The relatively high level of water pressure was chosen to ensure that even the strongest regions within the rock would be cut by a water jet.
In the second part, water pressure was varied with nozzle diameter and traverse speed fixed at 0.23 mm and 150 mm/s respectively. Measurements of slot depth were made after one and then five consecutive passes of a water jet for each level of pressure. Each test was replicated at least three times. Details of the experimental program are contained in Table 3.
TABLE 3 Level of variables in multiple pass tests
nozzle diameter: 0.23 mm
traverse speed: 150 mm/s
rock type: Gosford Sandstone
variable
unit
level
water pressure
Mpa
70 140 210 275 345
no. of passes
1 5
5. RESULTS
5.1Slot width
There were little discernible changes in slot width with either water pressure or traverse speed. This is in agreement with previous research. A casual observation made during the tests indicated that the incidence of surface spalling tended to decrease with increasing water pressure.
5.2Slot depth
Figure 4 illustrates the extent of the variation in slot depth. The irregularity along the slot base when reported in absolute terms of standard deviation was found to increase with pressure from 0.66 through to 0.83 and finally 1.13 mm. The coefficient of variation in the mean slot depth decreased with water pressure from 50% to 33% and finally 24%; that is the variability in slot depth tended to decrease with water pressure and consequentially with slot depth.
Figure 4. Superimposition of the depth profiles for three slots formed by a water jet at three different water pressures in Gosford Sandstone.
This indicates that within the rock matrix, there exist regions of high strength material that are highly resistant to fracture and erosion by a water jet. Although slot depth increased with water pressure, it competes against the variation in toughness within the rock where the latter can dominant the forces of a water jet. The effectiveness of the high water pressures diminishes with depth causing greater absolute variations in depth.
5.3Effect of nozzle diameter
Slot depth was found to increase with nozzle diameter as shown in Figure 5. Evidently as water pressure increases, the effect of nozzle diameter on slot depth becomes more significant. Hence, nozzle diameter is not the insignificant variable that has sometimes been supposed.
Figure 5. Effect of nozzle diameter on slot depth at different pressures in Woodlawn Shale.
Traverse speed was fixed at 150 mm/s.
Nikonov (1971) has reported that slot depth increases linearly with nozzle diameter. Based on these results, such a relation would imply a positive slot depth at zero nozzle diameter. It would appear more likely, however, that slot depth would vary as some power function of nozzle diameter such that:
2）
where:
h = slot depth, mm
k = constant
and m is some value less than 1.
In this rock it appears there is a pressure, somewhat less than 100 MPa, at which there is little variation in slot depth with nozzle diameter and, where the water jet is ineffective in cutting rock. This may equate with the threshold pressure concept referred to earlier in §4.
As was shown in Equation 1 both nozzle diameter and water pressure influence the specific hydraulic jet energy and hence the level of energy available to cut rock. Figure 5 confirms also, as might be expected, that for a given amount of energy more benefit can be got from increasing pressure rather than by increasing nozzle diameter. For example, the two points A and B shown in Figure 5 are points of equal hydraulic energy but B is five times deeper than A. Point B has an effective nozzle diameter equal to only two‐thirds of A and is about double the water pressure. Therefore, in terms of maximizing slot depth for a given level of energy, greater benefits are gained from higher pressures than larger nozzle diameters (and hence flow rates).
It is worth noting that each curve for the different pressures appears to approach a limiting slot depth, indicatin
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