Abrasivejet cutting head with back-flow prevention valve

An abrasivejet cutting head is disclosed that includes a valve assembly for preventing back-flow to the abrasive hopper of abrasive-laden fluid from the cutting head when the discharge path for the abrasivejet becomes clogged or otherwise blocked. The valve assembly is positioned within the abrasive line to conduct abrasive from the hopper towards the abrasivejet nozzle. The valve assembly further includes a discharge path which is sealed off from the abrasivejet nozzle during normal operation in response to the relatively lower pressure in the abrasive jet nozzle that results from the flowing fluid therein. If the jet path is blocked or competed in a manner that causes a backflow of the abrasive-laden slurry therein, the cessation of low pressure permits the backflowing slurry to exit via the discharge path and bypass the hopper, substantially avoiding the downtime previously required to clean the hopper.

TECHNICAL FIELD

This invention relates to abrasivejet systems.

BACKGROUND OF THE INVENTION

The use of high velocity, abrasive-laden liquid jets to precisely cut a variety of materials is well known. Briefly, a high velocity liquid jet is first formed by compressing the liquid to an operating pressure of 3,500 to 150,000 psi, and forcing the compressed liquid through an orifice having a diameter approximating that of a human hair; namely, 0.003–0.040 inches. The material defining the waterjet-forming orifice is typically a hard jewel such sapphire, ruby or diamond.

The resulting highly coherent jet is discharged from the orifice at a velocity which approaches or exceeds the speed of sound. The liquid most frequently used to from the jet is water, and the high velocity jet described hereinafter may accordingly be identified as a waterjet. Those skilled in the art will recognize, however, that numerous other liquids can be used without departing from the scope of the invention, and the recitation of the jet as comprising water should not be interpreted as a limitation.

To enhance the cutting power of the waterjet, abrasive materials have been added to the jet stream to produce an abrasive-laden waterjet, typically called an “abrasivejet”. The abrasivejet is used to effectively cut a wide variety of materials from exceptionally hard materials (such as tool steel, aluminum, cast iron armor plate, certain ceramics and bullet-proof glass) to soft materials (such as lead). Typical abrasive materials include garnet, silica, and aluminum oxide having grit sizes of #36 through #200.

To produce the abrasivejet, the waterjet passes through a “mixing region” wherein a quantity of abrasive is entrained into the jet by the low pressure region that surrounds the flowing liquid in accordance with the Bernoulli Principle. The abrasive, which is under atmospheric pressure in an external hopper, is drawn into the mixing region by the lower pressure region via a conduit that communicates with the interior of the hopper. In operation, quantities of up to 6 lbs./min of abrasive material have been found to produce a suitable abrasive jet.

The resulting abrasive-laden waterjet is then discharged against a workpiece through an abrasivejet nozzle that is supported closely adjacent the workpiece. The spent abrasive-laden water is drained away from the workpiece in any of a number of known ways, and collected in a collection tank for recycling of the abrasive and/or proper disposal.

During operation of abrasivejet systems, the fluid path between the mixing region and the discharge opening of the abrasivejet nozzle can become clogged or blocked sufficiently to cause the abrasive-laden water to back up to and into the external hopper. The system must then be shut down so that the external hopper can be emptied of the resulting slurry, cleaned, dried and refilled with abrasive. In addition, the abrasive-carrying conduit must be cleaned and dried or replaced, and the orifice member and other internal components of the cutting head must be cleaned as well. The resulting downtime of the cutting system increases the cost of production, adversely affects production schedules and creates unexpected messy work for the operator.

BRIEF SUMMARY OF THE INVENTION

The invention herein comprises an abrasivejet cutting system and method employing a unidirectional valve assembly that directs abrasive-laden back-flow away from the hopper, and preferably to the collection tank. Further details concerning the invention will be appreciated from the following description of the preferred embodiment, of which the drawing is a part.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to theFIG. 1wherein depicted elements are not necessarily shown to scale and wherein like or similar elements will be designated by the same reference numerals through the several views,FIG. 1is a schematic illustration of an abrasivejet cutting system constructed in accordance with the invention. A cutting head10is coupled at its upstream end10ato a source12of high pressure fluid such as water. As known by those skilled in the art, the cutting head includes an orifice member (not illustrated) in its upstream region that has a waterjet-forming orifice formed in a hard jewel material such as ruby, sapphire or the like. The highly pressurized fluid is forced through the orifice, resulting in the formation of a highly cohesive waterjet that can reach speeds in excess of the speed of sound.

To increase the cutting power of the waterjet, it is known in the art to entrain abrasive into the jet to form an abrasivejet. Abrasive, such as garnet or silica, is accordingly conducted from an abrasive hopper20to the cutting head10by a conduit22. The abrasive enters the cutting head downstream of the orifice member in a region known in the art as the “mixing region”. The abrasive enters the cutting head through a passageway in the cutting head, and becomes entrained with the waterjet by the relatively low pressure that surrounds the flowing waterjet in accordance with Bernoulli's Principle. This relatively low pressure pulls abrasive from the conduit as the waterjet flows through the mixing region, causing abrasive to flow from the hopper to the cutting head via the conduit.

The resulting abrasivejet14is discharged from an abrasivejet nozzle18, and impacts a workpiece16that is supported over a collection tank24by a support structure26. The support structure is configured to enable the spend abrasive-laden fluid to drain to the collection tank, typically by using a porous surface as the workpiece-supporting surface.

On occasion, the abrasivejet's discharge path becomes sufficiently blocked to cause a backflow of the abrasive-laden fluid that travels up the conduit22and into the hopper, creating a messy slurry in the conduit and the hopper that must be cleaned out before the cutting operation can continue. The backflow travels up the conduit because it is the path of least resistance; the cutting head region upstream of the mixing region is filled with high pressure fluid from the source12, while the conduit and hopper are at substantially atmospheric pressure. The high pressure fluid thereby acts as a barrier to the backflowing abrasive-laden fluid, diverting it up the conduit.

In accordance with the invention, the abrasive-carrying conduit22is directed to the cutting head through a unidirectional valve assembly50. The valve assembly50includes a discharge port52that through which backflowing fluid is diverted, preferably to the collection tank24. The preferred valve assembly50is best shown inFIG. 2.

FIG. 2is a perspective view in explosion of the valve assembly50ofFIG. 1. The valve assembly50comprises an valve body51disposed about a longitudinal axis54and having a longitudinally-extending passageway56that passes through the body51from its upstream end to its downstream end. The body is preferably made of aluminum, but other materials such as stainless steel and other non-rusting metals could also be used. A generally tubular ⅛ NPT barb fitting58is threaded into the upstream end of the valve body to connect the upstream end of the passage56to an upstream portion of the abrasive conduit22. A generally tubular ¼ NPT barb fitting is similarly threaded into the downstream end of the valve body to couple the downstream end of the passage56to a downstream end portion of the abrasive conduit22and, consequently, the cutting head's abrasive passage.

The valve body51additionally has a discharge passage60formed about a discharge axis63that extends obliquely towards the longitudinal axis from the upstream direction into the passage56. preferably at an angle θ of 30–45°. The end region of the discharge passage60is in fluid commumcation with the passage56through a valve opening61.

A check ball62is positioned within the discharge passage56. The ball62is preferably made from a rubber-neoprene material of approximately ⅜″–⅝″ diameter, and is larger in diameter than the valve opening61. The ball62is retained in the discharge passage by a cap68having a central discharge port52. The cap68is conveniently secured to the valve body51by screws70that are tightened into threaded holes72in the valve body.

As illustrated inFIG. 3, the check ball62is pulled during normal system operation into firm sealing contact with the region circumscribing the valve opening61by the low pressure region in passageway56, as conducted by conduit22(FIG. 1) from the region in the cutting head surrounding the fluidjet. Accordingly, as illustrated inFIG. 1, abrasive from the hopper20passes through fitting58, passage56and fitting54into the mixing region of the cutting head for entrainment into the fluidjet. Neoprene was chosen for the ball's material for its relatively light weight and good sealing characteristics when contacting the region around the valve opening61owing, in part, to it's ability to “self-seat” against the region's surface. Naturally, other suitable materials can be used without exceeding the scope of the invention.

Should the abrasivejet's path become sufficiently blocked to create a backflow that pushes abrasive-laden fluid back towards the hopper, the sudden appearance of accumulated abrasive/fluid mixture at the discharge region of the cutting head will quickly cause a disruption of the fluidjet. The cessation of the fluidjet eliminates the low pressure region surrounding the fluidjet and, therefore, the low pressure in the passage56. Consequently, the source of the sealing force acting on the ball62ceases, and the backflowing abrasive/fluid mixture is able to move the ball away from the valve opening61and discharge through the port52. Because the valve body passage56upstream of its intersection with discharge passage60, as well as the conduit22upstream of the valve body and the hopper20are all substantially filled with abrasive, the abrasive/fluid mixture takes the path of least resistance and discharges to atmosphere through the discharge port52, thereby eliminating the backflow to the hopper and the consequential need to shut down the system in order to clean and refill the hopper and conduit.

Instead, the valve assembly can simply be detached from the fittings54,58, the screws70removed and the valve body flushed to remove any accumulated abrasive.