WATER JET CONFERENCE - Waterjet Technology Association

(which are highly desirable in most mining situations), and low power pumps (within the
constraints of the pressure requirements). The effect of the traverse velocity and the
required depth-of-cut then allow a trade-off to be made which will minimize the specific
energy. The specific energy for a kerf cut by a water jet is given by:
⋅ ⋅a
Ev =
2 ⋅ V 3
( 8Vt ⋅ w ⋅ Dk ⋅ gc )
where
ρ = density of fluid
a = nozzle diameter
V = jet velocity
P = pressure
Vt = traverse velocity
Dk = depth-of-kerf-cut
gc = gravitational acceleration
w = width of slot (approximately 2 or 3 times a)
The volumetric flow rate, Q. is:
Q = C d*V*A
where
A = nozzle area
Cd = discharge coefficient (approximately O.7)
The pump power is
H=Q*P.
As the kerf depth, D k, increases, the required specific energy also increases.
Apparently, viscous and frictional losses reduce the cutting efficiency as the depth-of-cut
increases. It is extremely difficult to track a narrow slot on a vibrating, rotating drum to
achieve a deeper kerf by using multiple passes. The solution appears to be to use multiple
nozzles which are fixed together at a specific spacing and which are traversed
simultaneously. This gives a result somewhat like the multiple pass (depending on the
spacing) and yet allows relatively high traverse rates and tool speeds such as those
associated with continuous miners.
WATER JET CRACK EXTENSION
Water jets may also be used in conjunction with mechanical tools by employing
them slightly ahead of the tool (and aimed in the approximate direction of cut) to assist in
forming chips. Hydraulic pressure entering the fracture can, if the pressure is high
enough, cause the fracture to propagate. Factors which affect this mechanism are: (1)
width of the existing fracture opening, (2) length of the existing fracture, and (3)
roughness of the fracture surfaces. These determine, for a fixed jet placement, how much
water enters the fracture and its dynamic pressure distribution within the fracture. This
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