SAFETY SYSTEM FOR FLUID JET CUTTERS

Abstract: The invention relates to cutting operations involving the use of fluid to effect the cutting process, wherein at least a part of the feed conduit (26) which supplies cutting fluid to the fluid outlet (4) of the system is provided opposite the fluid outlet. In this way should the cutting fluid penetrate a baffle (14) provided behind the component (12) being cut the fluid supply to the cutting process is stopped prior to any damage to the surrounding environment, such as breach of the enclosure (50) in which the system is operated.

Description: SAFETY SYSTEM FOR FLUID JET CUTTERS This invention concerns improvements in and relating to water jet cutting, particularly, but not exclusively in relation to fail safe cut offs therefore. Present decommissioning or reprocessing operations being conducted at nuclear installations around the World may be frequently presented with the task of cutting up large radioactively contaminated objects, such as fuel rod elements. This need often arises in rendering the articles into smaller sections for ease of handling during removal from radioactive containment areas, prior to any processing and before their eventual disposal or storage. Because of the hazards present, in addition to radiation, in the working environment in which these cutting operations are to be carried out conventional cutting methods such as lasers, are unsuitable. Alternative methods have been sought to avoid the risks of fire from flammable atmospheres in such environments, for instance from hydrogen generation. One alternative means of cutting the types of materials likely to be encountered in these operations, is the method of water jet cutting. Water jet cutting comprises focusing grit impregnated water through a nozzle on to the workpiece. The water is supplied under ultra high pressure (UHP), being typically of the order of 4000 bar, producing a supersonic jet so as to affect cutting of the workpiece. Unlike conventional methods, water jet cutting does not use generated heat to achieve cutting and is therefore more safe to use in an explosive gas environment. It also has a similar speed of cutting to other methods. There are, however, potential safety problems presented by the use of water jet cutting, one of which is the possibility of breaching the wall of the working environment by cutting with the water jet itself. Breaching the containment for the environment is not acceptable due to the resulting potential for exposing personnel to radiation or other risks. As a first level of protection against this scenario, a target, such as a plate baffle, may be situated behind the workpiece to intercept the jet before it interacts with the environment wall. However, since even a baffle could be breached in a reasonably short time by the action of a stationary jet, when the jet is active it is employed in a transversing or scanning manner in order to greatly increase the lifetime of the baffle. For nuclear and other hazardous applications the present invention aims to provide adequate safety protection should the traverse of the jet stop unexpectedly through a further fail safe device. According to a first aspect of the present invention there is provided fluid cutting apparatus, the apparatus comprising a fluid outlet supplied through a feed conduit with fluid under pressure, in which at least a part of the feed conduit is positioned opposite the fluid outlet. Preferably a baffle is provided between the fluid outlet and at least part of the opposing part of the feed conduit. The baffle may be of ceramic. The baffle may be adjacent to, in contact with or separated from the opposing part of the feed conduit . The feed conduit may comprise an elongate passage, such as a tube or pipe. Preferably at least one portion of the conduit is flexible. The feed conduit may be formed of stainless steel. The feed conduit may be formed of a braided material, most preferably with a linear, such as a plastic inner tube. Preferably fluid is conducted to the inlet of the opposing part from the feed source by an elongate passage. Preferably fluid is conducted from the outlet of the opposing part to the fluid outlet by an elongate passage. A circular cross-sectioned passage is preferred. The opposing part of the feed conduit may include a chamber. The feed conduit may comprise an elongate portion connected to the opposing chamber, the chamber being connected by a further elongate portion to the fluid outlet. The opposing part of the feed conduit may include an elongate portion which, at least in part, includes a one or ore locations thereon provided adjacent to other locations provided thereon, in which those locations are separated along the conduit, by a greater distance. A continuous opposing part formed of conduit may be provided. A convoluted path for the feed conduit is preferably provided. At least a part of the opposing part of the feed conduit may be wound in a spiral path. The spiral may be helical and/or have a circular cross-section. The spiral may have a non-circular cross-section, for instance an ellipse, an oval with straight sides or the like. The spiral may be defined by a series of linear portions linked by non-linear portions. The opposing part of the feed conduit may include a series of parallel portions in which the flow is in opposing directions in adjacent portions, such as a snaking path. Adjacent portions may be linked by curved portions which redirect the flow through 180° into the adjacent portion. The opposing part of the feed conduit may include a series of portions provided in the form of one or more layers, preferably planar layers. The provision of two or more layers is preferred. Two or more layers may be provided adjacent to or in contact with one another. Preferably at least one layer is positioned in an offset manner relative to at least one other layer with respect to opposition to the fluid outlet. Preferably in the direction of opposition to the fluid outlet, the projected path of the fluid jet passes through the inside of the feed conduit in all positions of the fluid jet. Preferably if the projection of the fluid jet passes through the wall or walls of a portion of the feed conduit in any position and/or passes between adjacent portions of the feed conduit, the fluid jet subsequently and/or previously passes through the inside of the feed conduit of another portion of the feed conduit. In this way cutting of the fluid supply is ensured no matter where the fluid hits the opposing portion. Preferably the feed conduit is provided in opposition to the fluid outlet throughout the range of positions the fluid outlet can assume and/or behind the entire extent of the intervening baffle. The fluid is preferably provided with a loading of grit or other abrasive solid material. The fluid is preferably a liquid, such as water. Preferably the fluid outlet is a nozzle. The fluid outlet is preferably movably mounted. Preferably the oveable mounting allows travel at 90° to the direction of the fluid outlet . Preferably the apparatus is provided in a containment chamber. The containment chamber may be for radiological sources and/or chemical hazards and / or biohazards. Preferably the opposing part of the feed conduit is provided between the fluid outlet and the opposing portion of the containing chamber. The workpiece to be cut is preferably positioned between the fluid outlet and the baffle. According to a second aspect of the invention there is provided a method for controlling the flow of a fluid to fluid cutting apparatus, the method comprising feeding fluid to a fluid outlet through a feed conduit to produce a fluid cutting jet, at least a part of the feed conduit being positioned in the fluid cutting jet path, or a projection thereof. In this way should the fluid cutting jet reach the feed conduit the high pressure fluid supply is cut before any damaging cutting occurs, for instance breach of the containment for the cutting operation or damage to any other valuable objects in the path of the fluid cutting jet. Preferably the fluid cutting jet impacts on a baffle and/or workpiece provided between the fluid outlet and the opposing part of the feed conduit. The feed may be fed along a conduit comprising an elongate passage, such as a tube or pipe. The fluid may be fed to an opposing part of the feed conduit which includes a chamber. The fluid may be fed to an opposing part of the feed conduit which includes an elongate portion which, at least in part, includes a one or more locations thereon provided adjacent to other locations provided thereon, in which those locations are separated along the conduit, by a greater distance . Preferably the fluid outlet moves during cutting and the feed conduit is provided in opposition to the fluid outlet throughout the range of positions the fluid outlet can assume and/or behind the entire extent of the intervening baffle. The fluid outlet typically moves in a linear scanning manner. The second aspect of the invention also includes the other possibilities, features and options set out in the first aspect of the invention and elsewhere, including method steps suitable for their implementation. Embodiments of the invention are now described in detail, by way of example only and with reference to the accompanying drawings, in which :- Figure 1 is a plan view showing the location of the self cut off system in relation to the other components of the cutting apparatus; Figure 2 is a side cross-sectional view of the invention according to Figure 1 during baffle breach; Figure 3 is a plan cross sectional view through the coiled tube section of Figure 1 ; and Figure 4 is a plan cross-sectional view of another embodiment of the invention during baffle breach. The water jet cutting assembly is provided with a housing 2 having a nozzle 4 from which the water jet 6 emerges. The water jet 6 follows a straight path parallel to the base of the assembly . The workpiece 8 to be cut is positioned, by means not shown, within a defined region of space 10 through which the water jet 6 passes. A plate baffle 12 of ceramic material is located within a baffle housing 14, directly opposite the water jet mounting 2. The baffle 12 intercepts the water jet 6, when the said water jet is not being intercepted by said workpiece 8, at a location 18 on the surface of the baffle 12, Figure 1. A reservoir or trough 20 is contained in the base of the baffle housing 14 and the base of the baffle 12 extends into it. The water is supplied to the apparatus from the conventional intensifier, not shown, into the environment through a suitable through wall connector which leads to the inlet end 22 of a braided stainless steel tube 24. The tube 24 is wound so as to form a coiled section, Figure 2, wherein the coils are pressed in firm contact with one another as shown in the cross-sectional view of Figure 3 to form an oval spiral . Figure 2 shows a cross-section illustrating the coiled section of the tube 24, the inlet end 22 and the outlet end 26. The coiled section is positioned in the line of fire, behind the baffle 12. The length of the coiled section, defined by the number of turns in the coil, is set at least equal to the length of the baffle 12, with the height being at least equal to the height of the baffle 12. In this way the area of the baffle 12 is mirrored, at least, by the area of the coiled section . After passing behind the baffle 12, the stainless steel tube 24 as a single conduit, channels the water to the water jet nozzle 4. The water jet assembly focuses the UHP water and grit through the nozzle 4 to produce a super sonic jet 6 of grit impregnated water. An optimal nozzle size has been found to be 10/1, 000th of an inch, with a flowrate of 2 litres of water per minute and 160g of grit per minute. When the water jet 6 is in operation, but is not engaged in cutting a workpiece 8, it will impinge on the ceramic baffle 12 provided so as to prevent the water jet from impinging on and breaching, the wall of the environment 50. The water spray produced by the water jet impinging on the baffle is not itself of a sufficient pressure to cause cutting and so is not hazardous from this point of view. The spray is collected in a reservoir or trough 20 for recovery and recycling. As a matter of practice the water jet is always used in a scanning or traversing mode, arrows A of Figure 1, in preference to a stationary mode, so as to reduce wear of the baffle 12 at any particular point and so prolong life of the baffle 12. The workpiece 8 to be cut, which may be a fuel rod element or any other object contaminated with radiation, is positioned in the path of the water jet for the operation of cutting by means not shown, but which may be a slave manipulator or power manipulator. The workpiece 8 is then cut by the scanning water jet 6. However, the present invention provides a back up should the baffle 12 be breached by the water jet 6. The baffle may be pierced due to excessive wear at a point or location due to overall wear leading to elongate cutting. In either such case the water jet 6 will be prevented from cutting through the environment wall 50 by the operation of a safeguard mechanism designed to cut off the water jet in such an event. The safeguard mechanism is provided by the coiled stainless steel tube 24 which supplies the UHP water for the water jet 6 and is situated behind the baffle 12 and the baffle housing 14 If the baffle 12 is breached, Figure 2, the water jet 6 will then impinge on the coiled tube 24 and cut into it, location 30. The tube 24 will then rupture resulting in the rapid loss of pressure in the water feed to the water jet nozzle 4. Loss of pressure prevents any further cutting. In effect, the water jet cuts its own supply. Even a small "pin-prick" hole in the coiled stainless steel tube 24 is sufficient to render the water jet ineffective for cutting. The resultant spray emanating from the tube rupture does not possess sufficient pressure to cause cutting itself and is therefore non-hazardous from the point of view of causing a breach of the environment walls 50. In a second, preferred embodiment of the invention, there is provided an apparatus substantially as described in the first embodiment, but having a different arrangement in the coiled section 24 of the stainless steel tube situated behind the baffle housing 14. In the second preferred embodiment, the coil is wound in a manner as shown in the plan section of Figure 4, wherein the coils 32 are pressed firmly together. The coiled section of the tube 24 is once again provided behind the baffle support 14 and baffle 12, relative to the waterjet 6. In the event of the water jet 6 impinging on tubes 34, 36 it is possible, however, unlikely that the jet 6 may impinge at a position corresponding to the walls 38, 40 of the two tubes 34, 36. Cutting into the walls does not immediately lead to rupture in such an event. This would amount to failure of the safeguard device. However, by the coil arrangement of the second embodiment this problem is countered. Here the coil is wound with one layer of tube windings X, being provided over and out of alignment with those Y of the other layer. Utilising the coil arrangement of this embodiment if the water jet 6 should impinge on the layer of coils X at a position which is directly between two stainless steel tubes 36, 38 it would on cutting through them penetrate the tube 42 of the second layer Y directly in a manner sufficient to cut the water supply. What ever the position of the breach the water supply is cut in a fail safe manner long before the environmental integrity is risked, therefore.

Claims: C AI S : 1. Fluid cutting apparatus, the apparatus comprising a fluid outlet supplied through a feed conduit with fluid under pressure, in which at least a part of the feed conduit is positioned opposite the fluid outlet. 2. Apparatus according to claim 1 in which the opposing part of the feed conduit includes a chamber. 3. Apparatus according to claim 1 in which the opposing part of the feed conduit includes an elongate portion which, at least in part, includes one or more locations thereon provided adjacent to other locations provided thereon and separated from the adjacent location by a first distance, in which those locations are separated along the conduit, by a second greater distance . 4. Apparatus according to claim 1 or claim 3 in which at least a part of the opposing part of the feed conduit is wound in a spiral path. 5. Apparatus according to claim 4 in which the opposing part of the feed conduit includes a series of parallel portions in which the flow is in opposing directions in adjacent portions. 6. Apparatus according to claim 1 or any of claims 3 to 5 in which the opposing part of the feed conduit includes a series of portions provided in the form of one or more layers. 7. Apparatus according to claim 6 in which at least one layer is positioned in an offset manner relative to at least one other layer with respect to opposition to the fluid outlet. 8. A method for controlling the flow of a fluid to fluid cutting apparatus, the method comprising feeding fluid to a fluid outlet through a feed conduit to produce a fluid cutting jet, at least a part of the feed conduit being positioned in the fluid cutting jet path, or a projection thereof.

Inventors: BARLOW JONATHAN, BARLOW, JONATHAN
Applicants: BARLOW JONATHAN, BRITISH NUCLEAR FUELS PLC, BARLOW, JONATHAN
Application year: 1999
Main classification: B26F3
IPC codes: B26D7, B26F3