Design and Development of Supporting and Turning Fixtures for Cryogenic Vessel

Abstract

A fixture is used in manufacturing to hold a workpiece, position it correctly with respect to a machine tool and support it during machining. Widely used in manufacturing, fixtures have a direct impact upon product quality, productivity and cost. Fixture planning and design is a complex activity restricted by the extreme diversity of workpieces and several environmental factors including machine tools, assembly tools, grasping devices, and cutting tools. Approximately 40% of rejected parts are due to dimensioning errors that are attributed to poor fixture design.

The present cryogenic vessel is a vacuum containment vessel having a very large volume of 16000 m3. This cryogenic vessel is a fully welded stainless steel cylindrical chamber with thicknesses ranging from 50 mm to 250 mm and has overall dimensions of 28.54 m diameter and 29.25 m height. It will be the world's largest high vacuum pressure chamber ever built. Because of its large dimensions it is impossible to manufacture it in one single piece, so it will be made in different segments and supporting & turning fixtures are required for manufacturing the different parts of the vessel.

Fixtures comprising of the different sizes of ISMB, top plates, joining plates is designed to support a maximum of 134 MT vertical loads arising from the shell and supporting fixtures. The base frame and supporting frame is used for shell mounting and welding support. Present work contains fixture synthesis methods including geometrical analysis and fixture assembly planning. Different parameters affecting the fixture design are analyzed and conceptual planning and design of fixtures for shell welding is carried out. Detailed modelling and design for loading is also done. Static structural analysis and stress analysis are carried out using finite element software and all components are found well within the allowable stress limits. Manual calculations are also carried out and are compared with permissible stress limits.