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Use Caution When Specifying “Seal Welds” - …

Welding Innovation Vol. XVI, No. 2, 1999 Use Caution when Specifying seal Welds Practical Ideas for the Design Professional by Duane K. Miller, , FileIntroductionWhat is a seal weld ? AWS , Standard WeldingTerms and Definitions, defines a seal weld as: Any welddesigned primarily to provide a specific degree of tight-ness against leakage. The purpose of a seal weld maybe to contain a fluid either gaseous or liquid. In themechanical and structural fields, seal welds are used mostoften not to prevent leakage out of a container, but to pre-vent entry of a fluid into a space where some type ofharmful behavior, often corrosion, is expected to occur.

Welding Innovation Vol. XVI, No. 2, 1999 Use Caution When Specifying “Seal Welds” Practical Ideas for the Design Professional by Duane K. Miller, Sc.D., P.E.

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Transcription of Use Caution When Specifying “Seal Welds” - …

1 Welding Innovation Vol. XVI, No. 2, 1999 Use Caution when Specifying seal Welds Practical Ideas for the Design Professional by Duane K. Miller, , FileIntroductionWhat is a seal weld ? AWS , Standard WeldingTerms and Definitions, defines a seal weld as: Any welddesigned primarily to provide a specific degree of tight-ness against leakage. The purpose of a seal weld maybe to contain a fluid either gaseous or liquid. In themechanical and structural fields, seal welds are used mostoften not to prevent leakage out of a container, but to pre-vent entry of a fluid into a space where some type ofharmful behavior, often corrosion, is expected to occur.

2 Inthese fields, seal welds are frequently used to precludemoisture and oxygen-laden air and water from enteringthat welds may be specified on parts to be galvanized toprohibit pickling acids and/or liquid zinc from entering into aspecific region. For architecturally exposed steel that is tobe painted, seal welds may be specified to prevent unsight-ly rust bleeding. seal welds may be required for someapplications where the sealed joint is more conducive tocleanup than an exposed joint would be. Food processingfacilities are one such characteristic common to all of the aforementionedexamples of seal welds is as follows: none of them areplaced for traditional strength-related reasons, and for thisreason, Caution should be exhibited when seal welds arespecified.

3 In some cases, the application of a seal weldmay result in a conflict of code requirements. In others,the seal weld may perform structural functions that wereunintended, resulting in undesirable load paths. Sealwelds may affect inspection practices, in particular, theinterpretation of ultrasonic inspection results. Finally, sealwelds may be treated in a casual manner by thoseresponsible for making them, resulting in weld qualityproblems. Each of these examples will be examined, aswill be some issues related to galvanizing that need to beconsidered as ConflictsA common inquiry is as follows: The drawings call for seal weld , but in order for me tocomply with that requirement, I need to violate , Section What should I do?

4 The specific code provision cited is the one that calls forthe interruption of welds which occur on opposite sides ofa common plane, and under these conditions, the weldsare required to be interrupted at the corner (Figure 1).This provision has a practical foundation: it is difficult tomake a continuous weld in these conditions, and the prob-ability of undercutting the corners is great when the weldsare made continuous. This is one problem associated withseal welds when applied to these code provisions can be violated, including , Section , which addresses flexible connec-tions that rely on the flexibility of the outstanding leg ofangles (Figure 2).

5 Examples would include framing angles,top angles of seated beam connections, and simple endplate connections. A seal weld around a flexible connec-tion reduces such flexibility and may change the overallbehavior that is , Section , calls for welds on stiffeners tobe cut short not less than 4 times, nor more than 6 times,the thickness of the web from the weld toe of the web-to-flange welds (Figure 3). This provision was incorporated toprovide for a degree of flexibility in this region. Previousexperience in shipping had shown this to be an area thatwas prone to cracking when the weld extended too welds applied to this area effectively preclude designer who calls for a seal weld should review thesecode provisions if the project is governed by the , and in the situations where the code is not applica-ble, examine these principles and determine their relativesuitability to the components where seal welding has beenutilized.

6 To handle the issue of consistency between jobspecifications and code requirements, the engineer canaddress how these issues are to be resolved in the projectspecifications. The preceding list of code examples is illus-trative only, and may not be comprehensive in its coverageof issues where seal welding requirements may violatecode Load PathsThe second major series of problems associated with sealwelds involves those applications where unintended loadpaths are created. For example, a lapped connection maybe joined by bolts with no welds expected at all. However,a seal weld is specified around the ,Section , may be applicable in this situation.

7 In bear-ing connections, the code does not allow bolts and weldsto share the load. Of course, in this particular situation, thedesigner would probably not consider the seal welds asmembers that would share loads with welds, but in fact,they will. The seal welds would be small in size and proba-bly incapable of transmitting the applied loads by them-selves. In actual service, the first thing that would happenwould be for the welds to fracture, violating the purpose ofthe seal weld , before the bolts would load up and carry thetransferred welding adage, There are no secondary members inwelded design, is applicable when considering seal example arose several years ago where a tub-type rockcrusher had been designed with a series of stiffeners.

8 Thedetailing had been carefully thought through so as to avoidstress risers. A stainless steel nameplate was to beapplied to the unit, and a seal weld was called for to attachthis nameplate. The entire unit received a special, multi-coat paint system to preclude corrosion, and the seal weldensured that the material under the stainless steel name-plate would not be exposed to the elements. The name-plate was put into a high stress region, and whetherFigure 6twwhere,L = length of unwelded stiffenertw= weld thicknessWelding Innovation Vol. XVI, No. 2, 1999 Welding Innovation Vol. XVI, No. 2, 1999intended or not by the designer, the nameplate becamepart of the load bearing system, and the weld introducedresidual stresses as seal weld around the nameplate became the design-limiting fatigue detail that resulted in crack initiation in serv-ice.

9 The intention of the designer was circumvented by anill-conceived plan for a seal weld around a nameplate. Inthis particular example, any weld (including an intermittentweld) may have created a poor fatigue Treatment of seal WeldsThe minimum heat input requirements imposed by be violated when the seal weld is made. of that code prescribes certain minimum sizes of weldsthat must be maintained, regardless of the level of loading,in order to ensure that adequate heat input is achievedwhen the weld is made. The size of the seal weld may notbe specified, resulting in a weld that would otherwise bedisallowed by Table It is still important that good weld -ing practices be followed when seal welds are made,including adherence to the minimum fillet weld to do so may result in weld cracking or incompletefusion welder who is charged with the responsibility of mak-ing a seal weld may approach it in a very casual manner,as might the welder s supervisor.

10 The welder should havethe same qualifications as the welder charged with theresponsibility for making a similar weld that would have astructural purpose. The welding procedures, including theselection of the electrode and the required preheat level,are deserving of the same attention as a weld that trans-fers calculated loads. The claim It is only a seal weld isoften a prescription for IssuesThe presence of seal welds around steel backing that isleft in place after welding may have implications for theultrasonic testing (UT) of such connections. ConsiderFigure 4 in which a CJP groove weld with steel backing isinspected with UT.