Making a plastic tank with an integrity-verifiable weld joint
A plastic tank is comprised of two parts, a base and a top, that are permanently joined to each other by welding at a circumferential joint formed by two mating flanges of the parts. Fusion weld elements are captured on the faying surfaces of the flanges of the assembled parts, and the elements are melted by internal heating of metal portions of the elements. Two spaced apart circumferential welds which define an annular space are formed. The pressure within the annular space is changed in a test procedure to verify the integrity of the welds.
1. A method of forming a plastic tank comprised of a tank base and a tank top, the base and top aligned along a lengthwise axis and attached to each other by at least two substantially circular weld joints, which comprises:
(a) providing each of the base the top with substantially circular flanges, each flange having a faying surface;
(b) positioning at least one weld element on at least one said flange faying surface, and mating the flange of the base and the flange of the top with each other to capture the at least one element between the faying surfaces of mated flanges, so the at least one weld element may be heated, melted, and fused to the flanges to form a weld joint comprising said at least two substantially circular weld joints; wherein the at least one weld element has a pair of terminal ends, portions of which project radially from said mated flanges;
(c) heating the at least one weld element to melt and fuse said element to both of said flange faying surfaces, and to solidify said element, thereby to weld the mated flanges to each other;
wherein, prior to said heating step, the at least one weld element runs around part or all of the faying surfaces of the flanges and has portions which run concentrically to said lengthwise axis thereby to define an annular space between said concentric-running portions;
wherein said space, after heating, melting and solidification of the at least one weld element, is converted into an annular cavity; and,
wherein, prior to said heating step, said portions of the terminal ends are in proximity to each other; wherein after said at least one weld element is heated, melted and solidified a fused mass intersection is formed at one or more locations between the mated flanges, said fused mass intersection running at least in part radially, to connect said the melted and solidified concentric-running portions.
2. The method of claim 1 which further comprises:
(d) providing at least one port for fluid flow, the port running from said annular cavity to an interior or exterior surface of one or both of the welded tank base or tank top; and,
(e) changing the pressure within the annular cavity by applying pressure or vacuum to the at least one port and monitoring the pressure over time, thereby to determine the soundness of said the melted and solidified concentric-running portions which define the annular cavity.
3. The method of claim 1 wherein the weld element is a single loop having a U shape end, and wherein the U shape end of the loop is located to be part of said intersection.
4. The method of claim 1 wherein a second weld element like said at least one weld element is in addition positioned between said faying surfaces; and wherein the pair of terminal ends of each weld element which run radially outwardly from the flange at the same location.
5. The method of claim 1 wherein a second weld element like said at least one weld element in addition is positioned between said faying surfaces; and wherein a first terminal end of each weld element runs radially inwardly or outwardly from the flange at a first circumferential location; and wherein a second terminal end of each weld element runs radially inwardly or outwardly from the flange at a second circumferential location spaced apart from the first location.
6. The method of claim 1 wherein the faying surfaces of said flanges are non-circular.
7. The method of claim 1 which further comprises providing each the tank base and the tank top with at least one tab radially extending from the flange thereof, each tab positioned to mate with another tab when the faying surfaces of the flanges of the top and base are mated as part of step (b); and, running the terminal ends of the weld elements between mated tabs as part of step (b).
8. The method of claim 7 further comprising: providing at least one port for fluid flow, the port running from said annular cavity to an interior or exterior surface of one of the welded tank parts; and, after step (c) the step of changing the pressure within the annular cavity by applying pressure or vacuum to the port b and monitoring the pressure over time, to thereby determine the soundness of the welds which define the annular cavity.