Patent Application
20070295271
The invention may best be understood by referring to the following detailed description and accompanying drawings which illustrate the invention. In the drawings:
a-c illustrate cross sectional views of details of three different embodiments of hose constructed according to the invention; and,
We have developed a hose configuration exhibiting increased durability for high-pressure electrostatic painting systems. Conventional hoses 20 for use in high hydraulic pressure electrostatically aided coating dispensing systems are typically multi-layer designs of the general configuration illustrated in
In one application, a hose of the type illustrated in
Although the hose 20 was rated to 225 bar, it failed in about a day when tested at 100 bar and a power supply 32 voltage of 50 kV. Inspection of the hose 20 revealed that the combination of pressure and voltage resulted in an enhanced diffusion of water through the inner layer 22. This resulted in a breakdown (arcing) of the insulation, and, once the current began to flow, the current caused (a) pinhole(s) to form in inner layer 22, shorting the power supply 32. The failure of this hose 20 was unanticipated. The hose 20 was rated for 250 bar (3,626 psi) with a burst pressure of 1,000 bar (14,504 psi). The hose 20 also should have been able to withstand the test voltages as it should have withstood 55.7 kV. It failed in less than a day when exposed to 50 kV and only 105 bar (1,500 psi). The failure mode was via dielectric breakdown of the hose resulting in arcing, which caused (a) pinhole(s) in the hose.
A modified hose 120 was constructed by adding another layer 52 between the inner layer 22 and the ground sleeve 38. Layer 52 fits loosely over the pressure hose to minimize pressure effects and to permit any moisture that diffuses through layer 22 to vent. Since the insulation layer 52 is not directly exposed to the coating material or its pressure, any suitably electrically non-conductive material such as, for example, nylons, various elastomers, PVC, and so on can be used. Additionally, with solvent based systems, it is possible that chemical resistance may be a factor affecting diffusion through the inner layer 22 and spillage of coating material and/or solvent onto layer 52. Even taking this into consideration, however, material compatibility for layer 52 is much less of a factor in selecting the materials for layer 52. Layer 52 can be extruded with grooves or ridges permitting easier assembly of hose 20 into layer 52, and contributing little loss to the flexibility of the inserted layer 22. To have the least effect on flexibility, layer 52 should have low flexural strength and some looseness should be maintained between the inserted hose 20 and the layer 52. The ground sleeve 38 is then placed over layer 52. Ground sleeve 38 can be of the same general type as the semiconducting braid ground sleeve 38 used with hose 20, or ground sleeve 38 can be a conducting metal braid, or ground sleeve 38 can be a conductive or semiconductive layer co-extruded with the layer 52. With the hose 120, the layer 52 is decoupled from the coating material pressure inside layer 22. Even if water were to migrate through the sidewall of layer 22 through the influence of the electric field and pressure, the added insulation of layer 52, by not permitting current to flow, would reduce the likelihood of the pinhole formation seen with the original hose 20. Further, the added thickness of layer 52 reduces the electric field gradient, thus reducing the effects of the electric field on diffusion through the layer 22.
The structure of the hose 120 is further illustrated in
A test hose 120 incorporated a high-pressure hose 20 having an inner layer 22 of PTFE with an inner diameter of about 6.9 mm and an outer diameter of about 10.32 mm. The layer 26 was polyurethane with thickness of about 1.25 mm and an outer diameter of about 14 mm. A braided covering 24 of either aramid or polyester fibers was placed between the PTFE inner layer 22 and the outer layer 26. The insulating layer 52 was flexible PVC tubing with an inner diameter of about 0.75 inch (about 1.9 cm) and an outer diameter of 1.5 inches (about 3.8 cm). The ground layer 38 was unlined flexible metal hose, illustratively, woven 304 stainless steel with an inside diameter of about 1.625 inches (about 41.3 mm), a wall thickness of about 0.1 inch (about 2.54 mm), and a bend radius of about 13 inches (about 33 cm). The test configuration is illustrated in
The test hose 120 was first filled with water and attached to a small pressure vessel 130, such as a Hoke vessel. Vessel 130 was filled with water and then attached via a non-conductive hose 132 to a nitrogen cylinder 134. The assembly was then pressurized to about 1,500 lb./in.2 (about 103 bar or about 78,280 mm Hg or about 10,436 kPa) and checked for leakage. After leak checking over the span of about a day with no voltage, the test hose 120 and the ground were coupled to the high voltage and ground connections, respectively, of a power supply 32, illustratively, a Ransburg model 253-17254 power supply. The power supply 32 output voltage was set at 50 kV. The test hose 120 was then held at about 1,500 lb./in.2 (about 103 bar or about 78,280 mm Hg or about 10,436 kPa) and 50 kV for a period of 235 hours before the test was terminated without failure. For the second portion of the test, test hose 120 was pressurized to about 2,800 lb./in.2 (about 193 bar or about 146,680 mm Hg or about 19,556 kPa) and 50 kV was applied across the power supply 32 terminals. Test hose 120 was then held with these parameters for 510 hours before the test was terminated without failure. These results are in marked contrast to previous tests with the prior art hose 20, with ground layer 38 but no insulating layer 52.
Although the hose 20 illustrated in
