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INTRODUCTION:
The use of water jets under pressure has become much more
common in recent years for an increasing variety of tasks. As their advantage
has become clear, so water-jetting equipment has been developed, used and water
jets have become, in several industries, the accepted method for solving a
problem.
Water jetting is, in its simplest form, concerned with
the development, the transmission and the application of power. This power is
normally created in a water medium by a pump, pushing a given volume of water
into a high pressure feed line and providing it with a certain amount of energy
in the process. This water flows down through the line, usually a strong metal
tube over at least part of its length, to a nozzle. This nozzle contains one or
more exit holes or orifices, which are normally of a smaller size than the feed
line. Since a constant volume of water reaches the nozzle, it must accelerate
to a higher speed in order to escape through these orifices, which also serve
to focus the water into a coherent stream or jet, and to direct the streams
towards the required point on the target surface or work piece.
The water-jet, which comes out of
each orifice, will generally have to travel some distance (usually referred to
as the stand-off distance) to the target, losing energy as it moves through the
air or other fluid, which is in its way. When it reaches the target surface the
remaining power in the jet can be applied to one of a number of goals, usually
related to the removal of material. This can be as simple as the cleaning of
the target by removing a layer of unwanted material from the surface. It can be
the cutting into the target to create a slot, or it can be used to break out
large volume of material from the surface, as in some form of mining
application.
The main advantages of the water-jet
process (with or without abrasive) are as follows-
-Virtually any material can be
machined.
-The process is essentially ‘cold’
so that there are no thermal influences, thus avoiding changes in the material
properties.
-Jet reaction forces are relatively
low allowing easy and accurate manipulation by robots or multi-axis tables.
-The jet is a process non-contacting
tool, which can cut in all directions (omnidirectional), and normally does not deform
the material during cutting.
-The jet can be of a suitable
diameter to give small kerf widths when cutting.
-Negligible production of dust and
aerosols.
What is a Water
jet?
In layman’s language—a water-jet is
a round stream of water, which can be aimed at some object in order to get some
work done.
The speed of the jet is stressed,
since once it has left the nozzle orifice it will no longer be under pressure.
However, for the pump to push a given volume of water through the hole in the end
of the nozzle within a given time it must exert a given pressure on that water.
The pressure provided by the pump will generally be expended in two main ways,
the first is in driving the water through the line from the pump to the nozzle,
and the second is in sending the water through the orifice at a given velocity.
The main pressure loss in the delivery line comes from the line friction as the
water moves against the walls of the tube or hose, however it can also be lost
in turbulence where the water flow becomes disturbed as it moves through
passages of different shape.
When the water-jet reaches its
target, the energy, which the jet contains, as a result of its speed, is
changed back into impact pressure (1) in order to get an effective amount of
desired work done on that surface. Two quantities relating to the jet have, in
the past, been found to be most important in the effectiveness of this
exchange. These are how much water is hitting the object, and how fast is it
moving. Between them, they control the power, which arrives at the object, and
thus how much work can be obtained from it. These values, in turn, are largely
controlled by two variables in the delivery system. The volume of water
delivered by the pump and the diameter of the orifice at the end of the
delivery line control both the speed of the water-jet stream, and the area over
which it is applied.
Why are water-jets
used?
When
a plant manager looks at improving the operation of his plant by the
introduction of new technologies and new methods, the advantages and
disadvantages of any novel technology must be weighed in the balance.
In its earliest use as a mining
tool, the safety of what is often a hazardous operation was dramatically
improved when hydraulic mining was introduced. Water-jet mining (2) occurs with
the generation of almost no dust. Further, the fine mist (2) created around the
cutting zone makes it impossible to propagate a spark or gas ignition to the
point that the operator is put at risk. These benefits made the Spar wood mine
in British Columbia one of the safest mines in Canada in the 1970s.
The concentration of power within
the water-jet stream and the ability to focus a large amount of energy over a
small area significantly reduces the overall force both on the work piece and
back on the nozzle holder. Further, the relatively simple direct ability of the
jet stream over a wide range of angles with relatively little change in power
also differentiates it from mechanical cutting. These two features make it
relatively easy to adapt high-pressure water- jetting systems for use with
industrial robots. The resulting jets, whether plain or laden with abrasives,
can be used to cut both linear and intricate contours over a range of surfaces
with little distortion of the final work surface. In addition, through the use
of a second, counterbalancing jet at the back of a lance, the tool can be
designed as a zero-thrust cleaning lance, which has been found very effective
by divers in underwater cleaning.
Where suitable precautions are
taken, water-jet cutting equipment can be a quieter as well as a more
productive tool. One of the benefits to the use of the water-jet system lies in
the reduced wastage of material. The relatively thin slot cut through the
target and the continued integrity of the cut surface material give two
benefits. Firstly, it allows parts to be nested closer together in a cutting
table layout, reducing the amount of material left as waste. Secondly, the edge
quality is in many operations sufficiently high, that it can be left as a final
surface. Parts are therefore cut to their final shape in the original cutting
operation. This eliminates subsequent machining or grinding of the work piece
to bring it acceptable edge quality and tolerance.
Of course no tool will find
universal application. At the present time there continue to be some
disadvantages to water- jet use limiting its viability. High-pressure water-
jetting systems are still relatively expensive and therefore, for single
operations (as an extreme example, cutting a single slice of cake) they are not
a practical tool. The use of high-pressure water carries with it the risk of
personal injury if the system is abused. Further, because of the higher
precision and sensitivity of the equipment, operational and maintenance costs
are a significant part of the process economics.
Jet theory:
Jets
are the result of transforming high pressure into high velocity by means of a
nozzle. Figure below shows the fluid mechanics across an orifice (simple nozzle)
and Bernoulli’s theorem applies.
Because the initial velocity can
be considered low compared with the final velocity, and the high-pressure
supply discharges to the atmosphere, the equation can be simplified to give:
V=(2p/P)1/2… (1)
This is thus a simple equation
relating the velocity of the jet to the driving pressure and the density of the
working fluid (1000 Kg/m3 for water).
In reality the resulting maximum
velocity is normally less than the theoretical one (1), because of pressure
drops due to the friction inside water and between the water and its hosing
system up to the nozzle. The diameter of the jet, when leaving the nozzle is
also smaller than the nozzle diameter, due to contractions inside the nozzle
depending on its geometry.
For this
reason the flow from the orifice is modified by a coefficient of discharge
which takes into account these velocity and area effects. At the outlet of the
nozzle, jets consist in regard to their production from one up to three
different phases. Pure water jets (WJ) will have only a water phase. For the
generation of abrasive jets there are two different ways possible. The abrasive
suspension jets (ASJ), produced in a preliminary system consists of water and
abrasive while the abrasive water jets (AWJ) produced by injector principle,
consists of water, abrasive and air.
When leaving the nozzle the jets
interact with their surroundings. The results are (for examples for jets in
air) that:
-Coherent jets (one or two phase
jets) will be destroyed partially, that means they include air as an additional
phase.
-The three-phase jet produced by
injector system increase the amount of contained air, too.
-A divergence of all jets appears.
-The divergence causes an increased
area loaded by the jet and therefore a reduction of the specific energy input.
-The mechanism of loading may
change. The break up of a coherent jet into droplet changes the loading from
static to dynamical.
-The interaction with the
surroundings absorbs the energy of the jet.
All these effects can be more or
less drastically influenced by:
-
The medium of
the jet.
-
The geometry of the nozzle and its supply.
-
The medium the jet enters.
For example plane water jets in water (submerged) produce
cavitation bubbles. Their implosion will lead to a highly dynamical part of
loading (if the cavitation bubbles will implode at the surface of the material.
The application
of the jet is based on the interaction of the jet
With a material to:
-
Remove surface
layer.
-
Influence
substratum layer.
-
Remove
substratum material.
For all these applications the jet is loosing energy by
wasteful interaction with itself and with material. The result is an optimal
time of loading, in practice, that means an optimal traverse rate of the jet.
This often
means that two passes with double traverse rate (optimal) give higher
efficiency than one pass.
From these
facts we have to take into account that:
-
The structure
of jets is influenced by the way of their production and the surrounding after
leaving the nozzle.
-
These effects
can be partially be suppressed but not avoided.
-
By changing
the jet’s structure, the loading of material and the mechanism of its
destruction can change (a possibility to adapt the loading – resulting from a
jet at a certain energy distance- to material properties).
-
The
possibility of interaction with a material and its efficiency can be optimized
(in the case of removal of a surface layer and to avoid a destruction of the
substratum, a certain energy input as well as an adapted loading time can be
chosen).
Fundamentals
of different processes:
Cleaning:
The main principle of Water-jet
cleaning technology (5) is the use of a high-speed stream of medium pressure
with a high flow rate of water.
In
this technology, it can be used also with or without abrasives. The use of
abrasives increases the cleaning action by itself. The addition of abrasives to
the water-jet can be done in two ways:
(i)
Mixing of dry abrasive into the water-jet using a special
mixture chamber.
(ii)
Mixing of water slurry (with abrasive) with the water-jet.
Some of the
important parameters involved in the process of cleaning are:
n Pressure and
water flow rate
n Nozzle
diameter and geometry (round or flat)
n Stand-off
distance
n Cleaning speed
n Type of grain,
size and flow rate of abrasive (if used)
When cleaning, pressure level is
so chosen that damage of the substratum is avoided and large flow rates to
increase the removal rate of layer.
The choice of the pressure and
the flow rate depends upon the type of substratum, the kind of layer and the
adhesion between the two.
For a given pressure, the nozzle
diameter influences the flow rate. Using a pump with constant hydraulic power,
the nozzle diameter influences the pressure and therefore the velocity of
water-jet.
If a spreading or large jet is
needed, a flat orifice can be used. It is used to clean large areas but need to
work with small standoff distances. Round orifices can only clean narrow areas
in one go.
Standoff is important to avoid
the damage of the substratum.
Cleaning speed and abrasive are
defined normally as a function of type of layer, substratum and kind of
adhesion.
Two
types of jets can do cleaning:
(i)
Cleaning with plain water-jets
(ii)
Cleaning with abrasive water-jets.
The latter one
is used when hard deposits need to be removed, and/or a white metal finish is
required on an underlying steel surface. This gives enhanced performance over
plain water-jet methods and it also means that much lower pressure can be used
than necessary when using plain water-jets. Here it is necessary to reduce the
value of specific loading by increasing the area or by increasing the traverse
rates.
The principles of how the erosion
occurs depend on whether the target material is ductile or brittle.
(i)
Ductile Erosion: This process is somewhat like a V-point
cutting tool of a lathe. The amount of
material removed depends upon the energy of the particle. It will be sufficient
to say that the erosion of the surface is generally proportional to the square
of the particle velocity and the angle at which the particle strikes the
surface.
(ii)
Brittle erosion: Erosion of a truly brittle material occurs
due to propagation and intersection of cracks upon impact by the abrasive particle.
It is maximized when the particles strike the surface at 90 degrees since the
normal impact energy dominates the process.
Cutting:
The main principle of the water-jet
and the abrasive water-jet technology is to use a high-speed stream of
high-pressure water. The essential difference between them is the addition of
an abrasive, which increases the erosive action of the water-jet, hence
expanding the range of the materials that can be cut.
The
jet cuts when it’s loading exceeds the strength of the material. Depending upon
the properties of the target materials, the cutting is the result of erosion,
shearing, or failure under rapidly changing, localized stress fields.
Some
of the important parameters involved are:
n Water pressure
and flow rate.
n Standoff
distance.
n Cutting speed.
n Sapphire
nozzle diameter
n Nozzle
diameter and length (if abrasive is used)
n Type, grain
size and abrasive flow, if used.
Cutting with plain water-jets
(6):
In
the case of cutting with plain water-jets, the action of the jet is focused
over a small so that the material removal process gradually penetrates to the
thickness of the material. In actual fact, once the cut has started, the jet
impacts normally to the surface but really impacts the majority of the
thickness of the material at much lower angles. Due to friction between the jet
and the ambient air, the jet diverges with increasing distance between nozzle
and work-piece, splitting into single fluid droplets. This causes high dynamic
loads on the target beneficial when cutting certain materials. If a coherent
material is desirable then the break-up can be suppressed by the addition of
special polymers to the water stream.
Cutting with abrasive water-jets
(7):
The
principle is similar to the one explained in the cleaning process. When a
stream of particles is used to cut through the thickness of the material, then
the process becomes complicated by the particle trajectories through the
work-piece.
Although
the jet may impact normal to the surface, the particles strike the cut at
shallow angles of attack. As a particle removes material, it loses energy and
decelerates, resulting in deflection by the target and hence an increase in the
angle of attack. This phenomenon produces a slight roughness to the cut surface
called striations. Cutting at a speed, which reduces the deflection of the jet,
can minimize them.
When not cutting through a
material (or cutting very fast) the particle energy is almost totally absorbed
in large angle of attack impact, causing large deflections of the jet and
increased surface roughness.
Other machining operations (8):
New
advances in materials technology necessitate the development of compatible
machining techniques for hard to machine materials like ceramics. Ceramic
composites, metal matrix composites, laminates, higher specific strength
materials and fiber reinforced raisin composites. Conventional edge tool
machining is often technically or economically inadequate for the machining of
such materials, which are both hard and tough. AWJT fills the gap in this
context. It can also be used for machining of rock and concrete (3)(4).
Turning:
In this the work piece is rotated
while the tool is traversed parallel to the axis of rotation and incrementally
fed towards the center of rotation. The material removed is in the form of very
fine debris contrary to chips in conventional machining.
Milling:
The main difficulty when milling
with abrasive water jets is controlling the depth of the cavity, which is
determined by the mechanics of the jet- material interactions. The success of
the milling process involves accurate prediction of the material removal and a
multi- pass machining strategy. Also, the control of the jet energy is
important.
Drilling:
Either piercing trough the
material with a stationary jet or traversing the jet through a circular path
can accomplish hole drilling by abrasive ware-jets. The thicker the target, the
more difficult it is to maintain a good hole shape. When drilling brittle
materials. There is a high risk of fracturing, near completion of the hole, due
to hydrodynamic pressurization. So, a low pressure is used when piercing such
brittle materials.
Engraving and Marking:
Both plain water jets and AWJ can be
used for marking. A fine line of shallow depth is required and is best done
with a precision at either high traverse rates or relatively low pressure.
Applications:
1. Deburring and cleaning of
castings:
Due
to high forces of the jets, the work pieces required to be cleaned must be
firmly clamped and the machining unit must be installed inside a closed
chamber. Operational pressure is up to 1000 bar, flow rate up to 450 lt./min.
2.Onsite cleaning of grids and
concrete surfaces (8):
Lacquering
deposits of any kind and thickness, which form on the grids and plate
conveyors, can be satisfactorily removed with a surface cleaner. Base lacquer,
covering lacquer, clear lacquer, PVC lacquer etc. can be removed in the
shortest possible time, hence, extremely cost-effective. Cleaning of heavy
contaminated floors (Oil and grease), removal of paints and marking on highways
or runways can also be done. Operating pressure up to 850 bars, flow rates up
to 90 lt./min.
3.Internal cleaning of Heat
exchangers (9):
It is done with the aid of 3 or 5 rigid flushing lances. The number of lances will depend upon the high-pressure pumps used and upon the size of the pipe bundle.
4.Internal cleaning of tanks
(10):
These cleaners are used to clean the
inside of holding tanks, production tanks, autoclaves, reactors, agitator
tanks, Euro containers etc. They are efficient at cutting and removing
hardened, crusted, or layered deposits like latex, PVC, acrylic paints etc.
Operating pressure varies up to 1000 bar, flow rate up to 50 lt./min.
5.Hydro-demolition:
In
terms of concrete demolition capabilities, high-pressure water demolition has
an upper hand to other engineering techniques in the following ways:
n Have a higher
capability of demolition
n Optimize the
surface of the demolished concrete for subsequent casting of new concrete
n Allow the
equipment to be robotized.
The operating
pressure is up to 105 M Pa and the flow rate is up to 107 lt./min.
6.Cutting and sectioning of
populated circuit boards (8):
Water
jet machines are used to manufacture prototype and pre-production circuit
boards. It doubled the production and eliminated the scrap that was being
generated by shocks, bending, micro cracking and dust caused by routine drill.
7.Steam turbines components
production:
AWJ
cutting will replace the conventional EDM used for cutting nozzle blade holes
in steam turbines. Used for precision cutting of 3-d curved surfaces. The
surface roughness is also reduced to 1/5th of the earlier method.
8.Aerospace:
It
includes AWJ cutting of advanced composite structures, honeycombed sandwiches,
components of Ti, Ni, or Co super alloys and stacked metals or glass reinforced
plastics.
9.Cutting laminated glasses and decorated art made
from natural stone, glass, ceramics and metal
10.3-D machining:
The versatility of the
AWJ extends its use to other machining operations like:
n Turning
n Drilling
n Milling
n Spiral and
thread machining
n Trepanning
11.Water-jet use dealing with the problem of
anti-personal landmines (APL)(14):
12. Demilitarization of chemical weapons using HP
ammonia fluid jets (15):
Liquid anhydrous ammonia can be used
as an alternative fluid for both abrasive fluid jet cutting of live chemical
warfare munitions and fluid jet wash out of chemical agents, explosives and
propellants.
13.Use of water-jets in medical world (12):
They can be used in laproscopic
surgery for the gall-bladder diseases. Atraumatic dissection with a jet results
in significant decrease in blood loss and intra-operative complications. The
major vessels and bile ducts are left undamaged.
14.Use of water-jets in
agricultural sector (13):
Herbicides and pesticides applied to
the foliage and roots of the plant are often poorly absorbed. To improve
effectiveness, high-energy liquid droplets or high-energy liquid jets of highly
concentrated herbicide/pesticide solutions are used. They can revolutionize
agro-chemical applications all over the world.
CONCLUSIONS
WJT is still considered by many as
an emerging or new technology. This view is somewhat justified because of the
ever growing number of further applications that are found.
It
is true to say that for the majority of cleaning and plain water cutting, the
building blocks of the system are readily available. Although there is still
the need to tailor the complete system to the task. The pumps, nozzles and
other ancillary equipment is tried and proven thanks to the use of new
materials and considerable developments.
The
good news is that water jets, both plain and with abrasive additions have many
important advantages and the market for the equipment is growing quickly. Quite often this means making the whole
system suitable to the working environment.
Predictive
control enables the machining systems incorporating water-jets to match or
better the accuracy and quality of the competing technologies. Some previously
impossible tasks can now be carried out cost-effectively opening up the market
place for a whole new range of new manufacturing materials. Improvements in the
reduction of consumables and handling of the waste materials can have a very
positive effect on the environmental considerations. This is closely linked to
the continued development of higher efficiency systems.
There
are also many instances where the application of WJT can increase the quality
of life for the operators by affording a reduction in aerosol emissions,
vibrations and noise.
It
is hoped that this paper has served a useful introduction to water jet systems.
References:
1.Walstad,O.M., and Noecker,P.W.,
“Development of High pressure pumps and associated equipment for fluid jet
cutting” April,1972
2.Longridge,C.C.,”Hydraulic
mining”
3.El-Saie,A.A.,”Investigation of
Rock slotting by High pressure water jet use in tunneling” 1977
4.Radu,S, “Contribution to the
use of high pressure water jets for rock cutting” Ph.D. Thesis, University of
Petrosani, Romania 28.02.1998
5.Kaye,P.L.,C.S.J.Pickles,J.E.Field,
K.S.Julien 1994,”Investigation of erosion processes as cleaning mechanisms in
the removal of thin deposited soils”
6.5th Pacific Rim
International Conference on Water-Jet Technology Feb 3-5,1998, New Delhi, India
X.Luo, Beijing Aeronautical
Manufacturing Technology research Institute
7.Cook,R.J., and
Wightman,D.F.,1988,”Automotive applications of Water-jet cutting”, Los Angeles,
California
8.Institute of Material Science,
University of Hanover.
9.US Patent No.5,423,917”Method
for cleaning Heat Exchanger Tubes by creating shock waves and mixing the liquid
with injected air”
10.10th American
Water-Jet Conference, Aug 14-17,1999, Houston, Texas-Paper 45
11.5th Pacific Rim
International Conference on Water-Jet Technology, Feb 3- 5,1998,New Delhi,
India
Y. Osanai, Y. Matsumara, and M. Murata
12.Yoshida,M.Kakita,A. and
Nakamura,T. et al, 1997
“A multi-functional water-jet dissecting system for
laproscopic surgery”
13.M.Mazurkiewicz,”Method of soil
tilling and application of agricultural chemicals using high pressure fluid
jets”
14. 10th American
Water-Jet Conference, Aug 14-17,1999, Houston, Texas-
Paper-47
15.10th American
Water-Jet Conference, Aug 14-17,1999, Houston, Texas-
Paper-48
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