Note: Descriptions are shown in the official language in which they were submitted.
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UNDERWATE2 P~D ABOVE-WATER PLASMA A~C CUTTING TORCH AND MET~IOD
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BACKGRO~ND OF ~IE INVENTION
The present invention relates to plasma arc cutting
torches, and ~ore particularly to a plasma arc cutting torch that
5 can operate above or below water to provide high quality cuts in
metal, with reduced noise, airborne particulate polluticn,
~ltra-violet ~ W) radiation, and glare.
Without m~ffling, typical noise levels produced in
10 ~~utting ~ inch mild steel with a 400 Amp nitrogen plasma arc
torch are on the order of 110 dbA. This is clearly a high noise
- level which needs to be controlled. In this country, certain
OSHA regulations require that noise levels be kept kelow 95 dbA
~ith a 50% duty cycle, and many European countries require noise
lS levels below 85 dbA. Plasma arc cutting also produces airborne
~ollutants, W radiation, and glare at levels that can be
bothersome at best, and health hazards at worst.
The methods of muffling plasma-arc cutting torches that
are curren~ly known include a water-table, a low-~elocity
thick-walled water sheath, and submerging the workpiece in water.
Underwater plasma arc cutting has become a popular
1nethod for reducing noise, airborne particulate pollutionl and W
radiation and glare. While environmental advantages of
underwater cutting are clear, there are numerous drawbacks.
Underwater cutting typically requires a 10 to 20~ increase in
pcwer level with a 10 to 20% decrease in cutting speed. Cut
quality is also reduced with an increase in adhering dross. In
addition, the presence of water makes the sensing of obstructions
and initial height much more difficult when using a positioning
system based upon plasma vortex pressure such as described in
U.S. Pat. No. 4,203,~22 to Couch, Jr. et al.
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To date, only non-reactive plasma-forming gases such as
nitrogen have been used in underwater plasma arc cutting. This
has been due, in par~, to the lower currents associated with the
plasma arc columns of reactive plasma-forming gases such as
oxygen and air. Reactive gases are re susceptible to the
effects of wate~ near the plasna than non-reactive gases.
Other problems with current methods of underwater
plasma arc cutting are that water continuously flows into the
cutting zone which substantially degrades the quality of the cut,
and gaseous cutting products such as h~drogen accum~llate under
the workpiece. The accumulation of hydrc~en under the w~rkpiece
presents a dangerous situation because of its tendency to
detonate in a sporadic and uncontrolled manner.
Plternatively, noise from above-water plasma arc
cut~ing torches can be muffled by placing the workpiece on a
water-table such as disclosed in V.S. Pat. No. 3,787,247 to
Couch, Jr. However, the water on the underside of the plate and
in the cut itself generally reduces the quality of the cut, and
the hydro~en which accumulates under the workpiece creates a
dangerous situation.
Noise can also be reduced through use of a muffling
device as described in U.S. Pat. No. 3,833,787 to Couch, Jr.
That device operates by providing a low velocity, thick-walled,
cylindrical water sheath around the plasma arc column. However,
problems slmilar to those observed in underwater cutting are
exhibi-ted; namely, the water flow interferes with the plas~a,
ad~ersely affecting the cut. This problem is particularly acute
when utilizing a reactive plasma-forming gas such as oxygen or
air. Even the ccmbination of the water-table and the low
velocity water sheath is not sufficient to reduce noise to less
than 90 dbA.
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It is thereEore a principal object of this invention to
provide hiyh speed plasma arc cutting underwater with low ~ower
consumption.
Another principal object of this invention is to reduce
the noise, airb~rne pollutants, W radiation, and glare of plasrna
arc cutting ab~ve water.
It is still another object to improve the sensing of
~bstructions and lnitial height when performing plasma arc
cutting underwater.
A further object is to improve the quality of cuts made
by a plasma arc torch underwater by allowing use of reactive
plasma-forming gases~
It is still a further object to improve the quality of
plasma arc cuttin~ underwater or on a water-table.
Yet a further object is to prevent water from
interfering with the plasma arc column while cutting.
A ~urther object is to prevent hydrogen rom
accumulating on the underside o the workpiece.
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A still further object is to provide a device which can
be constructed rela~ively simply and econcmically, and can be
easily adapted ~o or attachèd upon existing plasma arc torches.
Other objects will in part be apparent and in part
pointed out hereinater.
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The present invention relates to a plasma arc cutting
torch and method useful either below or ahove water, which
reduces noise and other environmental disturbances by surrounding
the plasma with two fluids, yet does not interfere with or
degrade the cut. The invention includes surrounding the plasma
5 arc column with a high-pressure annular jet of a gaseous fluid
such as air, and surrounding the gas flow with a high-velocity
annular jet of a liquid fluid such as water. Both fluids flow
from nozzles which are annularly spaced from the plasma arc.
Either of these annular fluid jets may take a conical shape, and
in a preferred embcdiment both fluid jets take a conical shape.
The gas surrounds and stabilizes the plasma with a radially
inward annular conical flow, and the liquid surrounds and
stabilizes the gas flow with a radially outward annular o~nical
flow that is spaced away frcm the plasma arc column.
In unde~ater cutting, the gas creates a high-pressure
water-free space in the cutting zone, which includes the space
between the workpiece and the plasma torch, the space within the
cut, and the underside of the workpiece below the plasma torch.
~0 Water is ~hus kep~ cut of the cut and away frc~ the plasma arc.
In above-water cutting, the high-pressure gas flow
along wi~h the outward radial direction of the high-velocity
liquid flo~, ccmbine to insure that the plasma arc is not
interfered with during cutting, while effectively muffling the
plasma jet noise. For such above-water cutting, the invention
can be used in conjunction with a water-table. The gas flow
functions as it does in underwater cutting, keeping the cut and
the æea below the plate free of water, and keeping water away
from the plasma.
In both above and below water cutting, the underside of
the workpiece is continuously being flushed with the injected
air, preventing hydrogen gas from accumulating~
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The present invention may be embodied as an attachment
to known plasma arc cutting torches, such as, for ex~mple, the
torch disclosed in U.S. Pat. No. 3,64],308 to Couch, Jr~, et al.
That torch utilizes a liquid laminar flow within the torch nozzle
to constrict the plasma arc. It is important to note that the
present inventiorl employs a separate and distinct high-velocity
liquid flow to fonm a sound barrier and pressure boundary about a
water-free cutting zone. The high-velocity liquid flow is
separated from the plasma arc column by the high-pressure gas
flow. As this liquid flow does not interact with or constrict
the plasma, it is distinguished from the liquid laminar flow of
the aforesaid patent. The high-pressure gas flow employed in
this invention to provide a water-free cutting zone, is similarly
distinct and distinguished ~rom the plasma-forming ionizable gas
flow around and adjacent to the electrode.
BRIEF DESCRIPTION OF THE DRAhINGS
E`IG. 1 is a view in partial vertical section of a
preferred ~mbodiment of an attachment to a plasma torch cutting
~0 system constructed in accordance with the present invention,
shown performing underwater cutting;
FIG. 2 is a cross-sectional view of the apparatus taken
substantiâlly on the line 2-2 in FIG. l; and
FIG. 3 is a side elevation corresponding to FIG. l,
shcwing the same apparatus used for above-water ~utting on a
water-tâble.
DETAILED DESCRIPTION OF THE PREEERRED ~MBODIMENr
Referring to FIGS, 1, 2 and 3, there is a plasma arc
cutting torch 10 having a generally cylindrical body 12 and a
torch nozzle 14 producing a high velocity plasma arc oolumn 16.
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The plasma arc 16 is projected in a direction indicated by arrow
17, and cuts through a workpiece 18, producing a high quality cut
20. As shown, the plasma torch lO is generally of the type
disclosed in U.S. Pat. No. 3,641,308. As is typical of plasma
arc cuttin~ torches, it encloses an electrode which is connected
to an external electric power source which generates an arc
between the torch electrcde and a workpiece. A flow of an
ionizable gas is directed around and adjacent to the electrode,
so that a plasma arc oolumn is formed, which cuts the workpiece.
In accordance with the present invention, an attac~nent
body 22 having a retaining cap 24 is placed around the torch body
12. The lower end of the retaining cap 24 has both an inwardly
projecting circumferential edge 26 and an outwardly projecting
circumfeL-ential edge 28. A gap between the upper part of the
attachment body 22 and the torch body 12 fonns an annular gas
plenum 30. Below the gas plenum 30r a sexies of axial holes 32
leading ~oward the inward circumferential edge 26 are drilled
into the retaining cap 24. These axial holes 32 extend aroùnd
- 20 the retaining cap 24~ The place~ent of the axial holes 32 can
best be seen frc~ ~IG. 2. A gas inlet 34 connects the gas plenum
30 with a pressurized external supply of a gas, preferably air
(not shown).
The lower end of the attac~nent body 22 has an annular
liquid plen~n 36 connected by a liquid inlet 38 to a pressurized
external supply of a liquid, preferably water (not shown).
Defining the inside circumferential wall of the liquid plenum 36
is a liquid flow sleeve 40. Inside of the liquid flow sleeve 40
is an annular liquid space 42 which i5 cpen at the bottom. A
series of radial holes 44 extending around the torch allows water
to flow from the liquid plenum 36 into the space 42.
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For underwater cuttingr as sho~ in FIG. 1, most of the
apparatus is submerged in water 46 having a surface 48 which i5
typically about 3 inches above an upper surface 50 of the
workpiece 18. A gas, preferably air " mder pressure with a flow
rate between 2 and 20 scfm, flows into the gas inlet 34. It then
fills the gas plenum 30 and flows downward through the axial
holes 32. The air exits the retaining cap 24 near the inward
circumferenti.al edge 26 through an annular gas nozzle 52. The
air is thus directed toward the plasma 16 as an inward annular
conical air flow 54, having a radial flow vector component
transverse to the direction 17 of the plasma lh. The air flow 54
stabilizes the plasma arc column 16 and creates a high-pressure
water-free space in a cutting zone 56. Note that the cutting
zone 56 not only includes the space between the workpiece 18 and
the plasma torch 10 but also and more importantly, the area of
the cut 20 itself~ and the underside of the workpiece 18
imnediately below the plasma torch 10. In this way, the
pressurized air acts as a dam in the cut 20, stopping water from
coming irlto the cutting zone 56. In addition, the air flow
through the cut 20 helps keep the underside o~ the cutting zone
56, including the area under the portion of the workpiece 18
about to be cut, free of water. Gaseous cutting products, such
as hydrogen, ~hich otherwise tend to accumulate on the underside
of the workpiece 18, are also driven out by the pressurized air.
~5 As used in this specification, the term high-pressure is defined
as being a sufficient pressure to create the water-free cutting
zone 56. As will be readily understcod by those skilled in the
art, this figure will depend upon oonditions such as the size of
the gas nozzle 52.and distance from the torch nozzle 14 to the
workpiece 18.
In addition to the gas flow 54, and operating in
cooperation with it, another principal feature of the present
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invention is a high velocity conical liquid flow 58. A liquid,
preferably water, under pressure and at a flow rate of
approximately 20 gpm, enters the liquid plenum 36 from the liquid
inlet 38. It flows through the radial holes 44, then downward
within the annular liquid space 42 toward the outward
circumferential-edge 28. me water exits the retaining cap 24 at
an annular liquid nozzle 60. Thus, water flo~ls around the upper
part of the cutting zone 56 as the outward annular conical water
flow 58, having a radial flow vector component transverse to the
direction 17 of the plasma 16. The water flow 58 creates a high
speed air-water interface that resists the incursions of water
into the cutting zone 56 and helps to eject bubbles of air that
accumulate at the upper surface 50 of the workpiece 1~.
The water flow 58, which flows from the liquid nozzle
60 and radially outward thereafter, is annularly spaced from the
plasma arc 16 at all points along its flow path. (This is in
sharp contrast to arc-constricting water flows of the prior art,
such as is disclosed in U.S. Patent No. 3,641,308 to Couch and
~o Dean.~
By keeping water out o~ the cutting zone 56, the
quality of the cut 20 is greatly improved, and it becomes
practical to use reactive plasma-forming gases such as oxygen and
2S air for the plasma arc oolumn 16. Smooth, dross free, square
cuts have been consistently made with oxygen-plasma under 3
inches of water on ~ inch and 1 inch mild steel, For ~ inch mild
steel, the current, voltage, and cutting speed settings are the
same as above water. For 1 inch mild steel, the current and
voltage settings are the same as above water and the cut speed
alone is reduced by 10 to 20~. The noise levels prcduced when
cutting underwater have been measured and were less than 85 dbA
for both oxygen and nitrogen plasmas with maximum currents of 260
and 400A respectively. Cutting was done under 3 inches of water.
Noise measurements were made 6 feet from the torch.
For above-water muffling o a plasma torch as shown in
5 FIG. 3, the operation of the invention is similar to that
described in conjunction with the underwater mode. The primary
difference, of course, is that the apparatus is not subrrerged in
water. In this r~de, the surrounding flow of water creates a
noise pollution, and W barrier, and helps to pressurize the
10 cutting zone.
In typical above-water operation, water, under pressure
and at a flow rate of approximately 20 gpm, enters the attachment
body 22 via the liquid inlet 38. It flows, as it did in the
15 underwater rode, ~hrough the attachment body 22 and out the
annular li~uid nozzle 60 where it is deflected by the outward
circumferential edge 28 on the retaining cap 24, thus creating
the high velocity, radially outward conical water flow 58. This
high-velocity water flow 58 creates a complete enclosure around
20 the arc, extending from the lower end of the plasma torch 10 to
the upper surface 50 of the workpiece 18! becoming an effective
muffling device by trapping sound waves created during cutting
within the-enclosure. Light, radiation, and particulate
pollution are also trapped in the cutting zone 56 by the water
25 flow 58. ~y deflec~ing the annular water jet 58 out radially in
a bell shape, water is directed away from the cutting zone 56
reducing the ~ossibility of it interfering with the plasma
cutting process. m is is especially important when cutting with
reactive plasma-fonming gases such as oxygen and air.
As in the underwater rnode, the ~uality of the cut 20
can be improved by a high-pressure gas flow in ~he above-water
mode. Again, air, under pressure with a flow rate of between 1
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and 10 scfin, enters the attachment ~ody 22 via the gas inlet 34.
Experiments on the prototype system indicate that an air flow
akove approxirnately 10 scfin disturbs the water jet and reduces
the muffling effect. The air flows through the attachment body
5 22 to the annular gas nozzle 52. It is then directed as the
xadially inward annular oonical air flow 54 toward a point 61
I}elcw the torch nozzle 140 Optimally, the angle of the inward
circumferential edge 26 should be selected to direct the air flow
54 toward the point hl where the cut 20 begins at the upper
10 surface 50 of the workpiece 180 In this marmer, the air flow 54
;erves to further reduce the possibility that the water flow 58
~ill interfere with the plasma arc column 16, increasing the
quality of the cut 20, particularly when using a reactive
i?laSrna-fonming gas.
By q?erating the invention in this above-water muffling
]node, reduction in noise levels are greatly improved over the
nuffling device described in U.S. Pat. No. 3,833,787. That
l~evice utilizes a thick-walled, slow ving, almost stagnant,
Ealling sheath of water around the plasrna, in contrast to the
high-pressure, high~velocity conical jet of the present
invention. The velocity of the radially outward annular conical
~ater flow 58 is typically about 21.8 feet per second with a
typical thickness of about .042 inch, whereas the water sheath of
the aEorementioned patent had a velocity of about 8.7 feet oer
~econd and a thickness of about .125 inch, As used in this
speoification, the term high-velocity is defined as being an
increase of at least about 50~ over this prior art flow rate, or
stated in other terms, at least about 12 feet per second, whether
operating in the above-water or underwater modes. The wall
thickness is not critical, although a thicker wall yields better
sound insulating characteristics. A typical water flow wall
thickness when cpera~ing according to this invention is about .04
to ~08 inch. At greater thicknesses, to maintain a hiyh flow
velocity, pumping requirements become prohibitive. The noise
levels prcduced when cutting have been measured and were, in all
cases, less than 90 dbA when cutting with nitrcgen-plamsa to a
maximum current of 400A and were less than 86 dbA when cutting
~ith oxygen-plasma to a maximum current of 260A. All
measurements having been taken 6 feet from the torch while
cutting ~2 inch mild steel.
Abcve-water muffling and pollution control can be
further umproved throuyh use of a water-table as disclosed in
UOS. Pat. No. 3/787~47. In such operationS a quantity of water
62 is added to a cutting table (not shcwn~ so that a top surface
64 of the water 62 is in contact with or in close proximity to, a
bottcm surface 66 of the workpiece 18. In this mode, the inward
annular conical air flow 54 creates the water-free cutting zone
56 which inclùdes the cut 20 and underside of the workpiece 18
below the plasma torch 10. As with the underwater ~ode, the air
flow 54 stabilizes the plasma arc 16, and the water-free cutting
zone 56 acts as a dam keeping water frc~ flowing into the cut 20,
and driving gaseous cutting products such as hydrogen out ~rcm
the underside of the workpiece 180 By keeping water away from
the cut 20, the ~uality of the cut is increased. When the
present invention is used in the preferred above-water ~ode, with
conical jets of both air and water, and with the workpiece placed
on a wat0r-table, noise levels are reduced to below 90 dbA when
cutting ~ inch muld steel with nitrcgen-plasma to a maximum
current of 400~ and to below 86 dbA when cutting with ox~gen-
plasma to a maximum current of 260A. Noise measurements were
made 6 feet from the torch.
It is thus seen that the objects of this invention have
been achieved and other advantageous results attained in that
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there has been disclosed a method and apparatus which penmits
decreases in ~ower level and increases in cutting speed for
underwater cutting, which effectively muffles noise, particulate
pollution, light, and radiation from above-water cutting, which
improves sensing of obstructions and initial height, which
permits underwater use of reactive plasma-forming gases, which
improves the quality of plasma arc cutting underwater or on a
water table, which prevents interference with the plasma arc
~olumn, which expels hydrogen gas from the underside of the
10 workpiece, which is simple and econcmical to construct, and can
~e easily adapted to existing plasma arc torches.
While FIGS. 1, 2 and 3 show attachments to a plasma
torch, it is to be understood that such structure is not intended
~s any limita~ion, as the present invention might be incorporated
ithin a torch body, rather than as an attachment thereto.
Various m~difications of the present invention ~7ill
~ecome apparent to those skilled in the art from the foregoing
description and acccmpanying drawings. For example, while the
peration of the invention has been described in conjunction with
~utting a workpiece, it would also be useful when perfonming
~nderwater ~7elding of one or more w~rkpieces. Such m~difications
3re intended to fall within the scope of the appended claims.
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What is claimed is:
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