Original Date: 07/21/2003
Revision Date: 01/18/2007
Best Practice : Welding Preheat for Spherical Air Flask Manufacture
Electric Boat Corporation, Quonset Point Facility’s use of ceramic heating pads to provide weld preheat in the manufacture of Spherical Air Flasks resulted in significant improvements. This process proved to be more reliable by eliminating costly installation, removal, and post-removal repairs associated with the previously used cal-rod heaters.
The Spherical Air Flask (SAF) is a large pressure vessel that provides storage for large volumes of compressed air used for surfacing the submarine and as emergency breathing air. Earlier designs were a one-piece, forged bottle that was susceptible to damage during installation. Current designs include a welded assembly, consisting of a cylindrical mid-body capped by hemispherical end heads with nozzles for supply and discharge of air. Foundation rings welded to the mid-body provide for attachment to the submarine’s structure. The change to a welded assembly, made of high-yield steel, created the need for a reliable source of welding preheat to control mechanical properties and prevent weld cracking. Providing the necessary preheat to perform the critical welds in the manufacture of the air flasks had always been a challenge. Preheat was previously supplied by a high-voltage, AC cal-rod style conductive heater mechanically fastened to the SAF. These conductive heaters were an insulated resistive element inside a rigid metal sheath. The use of this type of heater had several negative aspects including: Need for manual installation and wiring
Amount of heat applied was controlled by a manually-set on/off timer
Conductive heaters were forced to conform to the cylindrical shape of the SAF by manual bending, causing hot spots and insulation failures which left arc strikes on the SAF body
Limitation of the required cable for the SAF prevented maximum use of automated welding processes
After a comprehensive review, a process improvement team recommended the use of low voltage AC flexible resistive elements within a matrix of ceramic tiles held in place by a dedicated fixture. The individual ceramic matrix pads are thermostatically controlled to ensure continuity of temperature throughout the weld zone. Improvement was achieved with the elimination of manual installation of wiring. The amount of heat applied was now controlled automatically by thermocouple sensor feedback to zone controllers. The matrix now conforms to the shape of the SAF body eliminating insulation failures and resultant arc strikes. Fixturing of the matrix pads around the SAF also enables the use of automated welding processes.
Since the implementation of the new weld preheat process for air flask manufacture, set up time has been reduced by 80%, energy costs reduced by 30%, manufacturing schedules reduced by 28%, and discrepancy reports associated with arc strikes have been eliminated.
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