Inferno TWA series and RWA series
rectangular style weld pad gages-
Design Information
Weld pad gages are designed for attachment to ASME Code vessels. The pressure rating of the weld pad gage is calculated for a rigid installation, unaffected by loads imposed by the vessel. After attachment the weld pad is subject to bending forces imposed by the vessel wall as the vessel is hydraulically loaded. These forces act in both the short and long direction of the weld pad. Every vessel behaves differently, depending on shell thickness, diameter and material. Once the glass is installed to the weld pad by means of the retainer flange and bolting it will assume the same shape as the weld pad, even with a relatively soft rubber gasket on top of the weld pad.
It is the vessel designer’s responsbility to make sure that the glass seating surface in the weld pad remains sufficiently stiff as the internal pressure is raised in the vessel. The glass itself should not be subject to deleterious bending, otherwise glass breakage may occurr. Various means of stiffening are available including increasing the thickness or width or length of the weld pad or adding cross ties in the slotted area. The vessel design can include internal stiffeners to the inside of the vessel or the welding pad may be installed using a repad. The vessel designer must insure that the sight glass mounting surface reamins flat.
The process of attaching the weld pad gage through welding may result in significant distortion to the original flatness of the weldng pad. Minimal warpage can be achieved by keeping the temperature of the welds as low as possible. This can be accomplished by skip welding from side to side and from end to end. Installations with significant weld distortion should be remachined. This may require use of large boring mills or, depending on the size of the vessel, portable machining of the gasket seat.
Flatness of the glass seating area of the welding pad should be monitored after welding. To minimize the chance of glass breakage during installation (bolt tightening, hydrotest) or later in service the glass seating area should be flat within the following amounts:
Suggestions to Reduce Distortion from Welding Attachment of Weld Pad Gages
| 1- Provide good fit-up. 2- Use smallest size weld permitted by Code rules. 3- Use as few passes as practical. 4- Use as low heat input as practical. 5- Use full penetration groove welds, double vee, instead of fillet welds. 6- Place the welds near the neutral axis of the gage body. 7- Balance welds about the neutral axis. 8- Weld opposing surfaces alternately immediately folowing each other. 9- Use intermittent back-step welding. 10- Use clamps, jigs, fixtures or stiff-backs as needed to maintain alignment. 11- Preheat as is practical. |
When attaching RWA series weld pads to small diameter vessels such as 6" and 8" pipe not all of the rules listed above may be practical.
More information on controlling weld distortion can be found in the following articles available on the web, written by Mr. Omer Blodgett, Senior Design Consultant with the Lincoln Electric Company and published in Welding Magazine / Welding Design & Fabrication
http://weldingdesign.com/blodgett/wdf_68978/
http://weldingdesign.com/blodgett/wdf_70297/
http://weldingdesign.com/blodgett/wdf_71186/
